Photoluminescence : advances in research and applications

PHYSICS RESEARCH AND TECHNOLOGY

PHOTOLUMINESCENCE ADVANCES IN RESEARCH AND APPLICATIONS

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PHYSICS RESEARCH AND TECHNOLOGY

PHOTOLUMINESCENCE ADVANCES IN RESEARCH AND APPLICATIONS

ELLIS MARSDEN EDITOR

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Library of Congress Cataloging-in-Publication Data ISBN: H%RRN

Published by Nova Science Publishers, Inc. † New York

CONTENTS Preface Chapter 1

Chapter 2

Chapter 3

Chapter 4

vii Complex Studies on the Photoluminescence of Er-Doped GeS2-Ga2S3 Glasses for Photonic Applications Zoya G. Ivanova Rare-Earth Activated Glasses in Integrated Optical Devices with Different Geometric Shapes: Fibers, Planar Waveguides and Microspheres Helena C. Vasconcelos and Afonso Silva Pinto Photoluminescence Properties of Layered Crystals and Their Organic-Inorganic Hybrid Composites Andreea Nila and Mihaela Baibarac Recent Progress in the Photoluminescence Properties of Composites Based on Conjugated Polymers and Carbon Nanoparticles Mihaela Baibarac, Mirela Ilie, Adelina Matea, Monica Daescu and Serge Lefrant

1

53

97

133

vi Chapter 5

Chapter 6

Index

Contents Photoluminescence of Carbon-Based Nanomaterials: Fullerenes, Carbon Nanotubes, Graphene, Graphene Oxide, Graphene and Carbon Quantum Dots Svetlana Jovanović Advances in Photoluminescence Properties of Coordination Polymers Cristina Mozaceanu and Mihaela Baibarac

167

197 211

PREFACE In this collection, chalcogenide glasses doped with rare earth elements are proposed as particularly attractive materials for applications in integrated photonics. The opening chapter is dedicated to reviewing the studies on optical properties of (GeS2)100−x (Ga2S3)x (x=20, 25 and 33 mol%) glasses, doped with Er2S3 in a wide range from 1.8 to 2.7 mol%, by absorption and photoluminescence (PL) spectroscopy. The authors focus on features in absorption, emission, and local ordering and their derivatives as a function of excitation wavelength, Er3+ doping level, Ga content and temperature for the (GeS2)80 (Ga2S3)20 host composition. Next, to demonstrate the technological importance of optical devices with unique properties derived from rare-earth activated glasses, the authors reviewed some fundamental aspects of rare-earth doped optical glassy devices where the light is confined in different volumes or shapes, namely fibers, monoliths, film/coatings and microspheres. Rare-earth activated glasses are often used as components in integrated optical circuits. Later, optical characteristics of semiconducting crystals with layered structure due to quantization effects in the architecture governed by the atomic arrangements are discussed. In order to study the microscopic optical processes of these materials, the phenomenological research from photoluminescence studies (PL) was determined to be essential to those established by conventional bulk materials. Layered crystals such as

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Cs3Bi2I9, BiI3 and PbI2 have been considered for reporting the PL spectra in order to discuss relevant information concerning photo-induced charge carrier separation and also the radiative and non-radiative recombination dependent on deep or shallow trap states. Additionally, the photoluminescence properties of composites based on conjugated polymers and carbon nanoparticles of the type carbon nanotubes, reduced graphene oxide and fullerenes are analyzed. A review is presented on the photoluminescence properties of various macromolecular compounds, for example poly(para-phenylenevinylene), poly(3-hexylthiophene), poly(3,4ethylenedioxythiophene-co-pyrene), polydiphenylamine and poly(9,9dioctylfluorenyl-2,7-diyl) as well as effects induced by the carbon nanoparticles mentioned above. The following chapter focusses on fullerenes, carbon nanotubes, graphene, graphene oxide, graphene and carbon quantum dots. Firstly, the general physical and chemical properties of different carbon-based nanomaterials are presented, such as the crystalline structure, morphology and chemical composition. Additionally, the possibilities of application of carbon-based nanomaterials due to its PL properties are analyzed. The concluding chapter focuses on coordination polymers (CPs) / metal-organic frameworks (MOFs) containing metal ions from d and 4f series and a plethora of organic ligands, the resulted compounds showing remarkable photoluminescence properties with different applications in the field light emitting devices (LEDs), biosensors in medical assays, sensors for identifying certain species (molecules, ions) and so on. Chapter 1 - Chalcogenide glasses doped with rare earth elements are particularly attractive materials because of their applications in integrated photonics. The crucial parameters for many optoelectronic applications require an extensive search for materials with improved characteristics. In particular, erbium doped glasses are intensively studied due to the Er3+ intra-4f emission at the standard telecommunications wavelength ~1540 nm. Among chalcogenide glasses, Ge-S-Ga system exhibits a larger enhanced Er3+ solubility due to the specific structural modification induced by the introduction of Ga into the Ge-S glass. This chapter is dedicated to reviewing the studies on optical properties of the

Preface

ix

(GeS2)100−x (Ga2S3)x (x = 20, 25 and 33 mol%) glasses, doped with Er2S3 in a wide range from 1.8 to 2.7 mol%, by absorption and photoluminescence (PL) spectroscopy. Consequently, a special attention has been paid to the features in absorption, emission, local ordering and their derivatives as a function of excitation wavelength, Er3+ doping level, Ga content and temperature for the (GeS2)80(Ga2S3)20 host composition. The influence of excitation on the strongest PL band at ~1540 nm has been studied in more detail by deconvolution into Gaussians, resulting in the conversion of the fine structure of emission bands into schematic Stark levels energy diagram. Usually the emission bands are explicitly pronounced at low temperatures, accompanied by an enhanced narrowing effect. Basic features in the development of the emission bands of heavily doped samples have been evaluated with decreasing temperature from 300 K down to 4.2 K. The considerable role of the host composition on the luminescence efficiency is registered with the outcome that the (GeS2)75(Ga2S3)25 host appears nearly optimal with the development of all possible eight emission bands, while at 33 mol% Ga2S3 only three bands are emerging with considerably lowered intensity. This results from the nonuniform distribution of the doping atoms by increasing the number of metal Ga-Ga bonds. Besides, CsBr addition into the studied glasses results in an increase of the PL decay time, indicating their potential suitability for optical amplifiers operating in the third telecommunication window around 1540 nm. The up-converted fluorescence spectra of Er-doped (GeSGa)90(CsBr)10 glasses are discussed and a schematic energy-level diagram of the up-conversion emission in Er3+ ions is proposed. Chapter 2 - Glasses activated by rare-earths in optical devices is a potentially important area of research in developing effective optical means for the propagation of light and luminescence enhancement. Applications involving optical glasses have grown considerably in recent years focused on the development of new shapes of optical devices and improved glass compositions, but also in new processes for their preparation. The confinement of light in small objects has become an essential requirement for optical devices. Examples of this trend are provided by optical fibers, optical integrated circuits (planar waveguides or

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slab) and, more recently, optical dielectric resonators supporting the Whispering Gallery Modes (glass microspheres). To demonstrate the technological importance of optical devices with unique properties exhibited by glasses activated by rare earths, the authors review some functional aspects of such devices, where light is confined in different volumes or shapes, particularly fibers, monoliths, films/coatings and microspheres. Glasses activated by rare-earths are often used as components in optical integrated circuits for optical communication and sensor applications. Optical waveguides (planar, strip, or fiber waveguides) and spherical microresonators are some current examples of integrated glass-based optical devices. Chapter 3 - This chapter displays a focus discussion on the optical characteristics of semiconducting crystals with layered structure due to quantization effects in the architecture governed by the atomic arrangements. In order to study the optical processes of these materials, photoluminescence studies (PL) were determined to be essential to those established by conventional bulk materials. In this chapter, the PL properties of layered crystals, such as Cs3Bi2I9, BiI3 and PbI2 were reviewed, a special attention being given to the photo-induced charge carrier separation and also to the radiative and non-radiative recombination dependent on deep or shallow trap states. A superradiant PL emission, a strong oscillations strength of excitons in layered crystals and a change in the PL intensity, caused by the transition from bulk layered crystal to fewlayer or monolayer halides are some issues debated in this work. The influence of different organic/macromolecular compounds on PL properties of layered crystals is also reviewed. In this specific field, layered-crystals become an important competitive class of materials promoted for various basic and applicative research studies in optoelectronic devices and non-linear optical fields. Chapter 4 - This chapter focuses on new aspects concerning the photoluminescence properties of composites based on conjugated polymers and carbon nanoparticles of the type carbon nanotubes, reduced graphene oxide and fullerenes. A review of the photoluminescence properties of various macromolecular compounds, for example poly(para-

Preface

xi

phenylenevinylene), poly(3-hexylthiophene), poly(3,4-ethylenedioxythiophene-co-pyrene), polydiphenylamine and poly(9,9-dioctylfluorenyl2,7-diyl) as well as effects induced by the carbon nanoparticles mentioned above. A special attention will be given to the insight of different de-excitation mechanisms developed in the composite materials based on the macromolecular compounds and carbon nanotubes highly separated in semiconducting and metallic components. The anti-Stokes photoluminescence processes and anisotropic photoluminescence properties of some composites listed above will be also reviewed. Chapter 5 - Apart from unique physical, chemical, mechanical and thermal properties, carbon-based nanomaterials exhibit unusual optical behavior. Photoluminescence (PL) of these materials is particularly interesting considering that bulk carbon materials such as graphite and diamond do not show luminescent properties. The subject of the chapter is fullerenes, carbon nanotubes, graphene, graphene oxide, graphene and carbon quantum dots. In this chapter, firstly the general physical and chemical properties of different carbon-based nanomaterials are presented, such as the crystalline structure, morphology and chemicals composition. Difference between dimensionality, type of hybridization of C atoms, the level of oxidation, size and shape are explained considering that these are key characteristics which determinate the optical behavior of these materials. In the following part of chapter, the origin of PL in different carbon-based nanomaterials, mechanisms and the tunability of the photoluminescence properties are discussed: exciton pairs which lead to PL of carbon nanotubes, defect centers such as of impurities or vacancies in nanodiamonds, different size of conjugated π-domains in reduced graphene oxide, suggested mechanisms of PL for graphene quantum dots, surface chromophores and defects in carbon quantum dots, etc. The PL properties such as emission spectra, excitation wavelengths, PL lifetimes as well as PL quantum yields are compared between different carbon-based nanomaterials. Furthermore, the parameters which affect these PL properties are argued. Additionally, the possibilities of application of carbon-based nanomaterials due to its PL properties are analyzed. Taking into account other physicochemical as well as PL properties, the most

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promising applications are highlighted, such as in bioimaging, drug delivery, sensing, in organoelectronic devices and others. Chapter 6 - The forthcoming part of this chapter is focused on coordination polymers (CPs)/metal-organic frameworks (MOFs) containing metal ions from d and 4f series and a plethora of organic ligands, the resulted compounds showing remarkable photoluminescence properties with different applications in the field of light emitting devices (LEDs), biosensors (in medical assays), sensors (identifying certain species), and so on. The reader may find information about the principles underpinning the design of luminescent MOFs and plenty of examples of luminescent CPs reported in the last few years.

In: Photoluminescence Editor: Ellis Marsden

ISBN: 978-1-53613-537-4 © 2018 Nova Science Publishers, Inc.

Chapter 1

COMPLEX STUDIES ON THE PHOTOLUMINESCENCE OF ER-DOPED GES2-GA2S3 GLASSES FOR PHOTONIC APPLICATIONS Zoya G. Ivanova* Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria

ABSTRACT Chalcogenide glasses doped with rare earth elements are particularly attractive materials because of their applications in integrated photonics. The crucial parameters for many optoelectronic applications require an extensive search for materials with improved characteristics. In particular, erbium doped glasses are intensively studied due to the Er3+ intra-4f emission at the standard telecommunications wavelength ~1540 nm. Among chalcogenide glasses, Ge-S-Ga system exhibits a larger enhanced Er3+ solubility due to the specific structural modification induced by the introduction of Ga into the Ge-S glass. This chapter is dedicated to *

Corresponding Author Email: [email protected]

2

Zoya G. Ivanova reviewing the studies on optical properties of the (GeS2)100−x (Ga2S3)x (x = 20, 25 and 33 mol%) glasses, doped with Er2S3 in a wide range from 1.8 to 2.7 mol%, by absorption and photoluminescence (PL) spectroscopy. Consequently, a special attention has been paid to the features in absorption, emission, local ordering and their derivatives as a function of excitation wavelength, Er3+ doping level, Ga content and temperature for the (GeS2)80(Ga2S3)20 host composition. The influence of excitation on the strongest PL band at ~1540 nm has been studied in more detail by deconvolution into Gaussians, resulting in the conversion of the fine structure of emission bands into schematic Stark levels energy diagram. Usually the emission bands are explicitly pronounced at low temperatures, accompanied by an enhanced narrowing effect. Basic features in the development of the emission bands of heavily doped samples have been evaluated with decreasing temperature from 300 K down to 4.2 K. The considerable role of the host composition on the luminescence efficiency is registered with the outcome that the (GeS2)75(Ga2S3)25 host appears nearly optimal with the development of all possible eight emission bands, while at 33 mol% Ga 2S3 only three bands are emerging with considerably lowered intensity. This results from the nonuniform distribution of the doping atoms by increasing the number of metal Ga-Ga bonds. Besides, CsBr addition into the studied glasses results in an increase of the PL decay time, indicating their potential suitability for optical amplifiers operating in the third telecommunication window around 1540 nm. The up-converted fluorescence spectra of Erdoped (GeSGa)90(CsBr)10 glasses are discussed and a schematic energylevel diagram of the up-conversion emission in Er3+ ions is proposed.

Keywords: chalcogenide glasses, optical properties, photoluminescence, rare-earth ions, up-conversion fluorescence

1. INTRODUCTION The unique properties of chalcogenide glasses have been studied for decades, providing applications in the electronics industry, imaging and more recently in photonics. Especially, there has been a growing research interest in glasses doped with rare-earth (RE) elements because of their exhibited attractive parameters, which make them key materials for photonic devices such as lasers, optical amplifiers, frequency convertors, among many other applications in the visible, near- and mid-infrared

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spectral regions [1-5]. In comparison with different glassy hosts, chalcogenide glasses offer several advantages [6-8]. The first one is the low phonon energy, which results in the decrease of non-radiative recombination rates and consequently leads to higher radiative efficiencies of the optical transitions between RE3+ electron energy levels. Secondly, the shift of the long-wavelength absorption edge to longer wavelengths ensures enlarged transparency in the infrared region. The high refractive index of these glasses is responsible for the high spontaneous emission probabilities and therefore to enlarged emission cross-sections. The combination of low phonon energies, smaller optical band-gap and high refractive index leads to higher quantum efficiencies of the radiative electron transitions between the energy levels of RE3+ ions in sulfide glasses, which remain relatively quenched in conventional RE-doped oxide and halide glasses [6]. Besides, chalcogenide glasses are characterized by good glass-forming ability, high values of the glass transition temperature and thermal stability [9-12]. Among chalcogenide systems, it has been found that Ge-S-Ga glasses have already been tested as attractive hosts because of the Er3+ intra-4f emission at a telecommunications wavelength of around 1540 nm [5, 8, 1316]. Currently, the 4I13/2→4I15/2 transition at ~1.5 µm is the most important since Er-doped optical fibers can be used as amplifiers namely at this wavelength [5-7, 13]. The preferred bonding state of the Er3+ ion has an incomplete 4f electronic shell surrounding from the outer world by closed 5s and 5p shells, and as a result sharp optical intra-4f transitions can be realized [13]. Interest has been also focused on erbium about infrared to visible up-conversion fluorescence. In particular, for 1.06 µm excitation two-phonon and phonon-assisted energy transfer account for the 2 H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions, leading to intense green emission at 530 and 555 nm and less intense one at 670 nm, respectively [17]. For the 1.54 µm excitation, much higher up-conversion has been obtained with the red emission signal at 670 nm than with the green one. It is confirmed that choosing glassy host with the lowest phonon energy associated with non-radiative loss will result in the most efficient up-conversion [18]. Besides, the efficiency of up-conversion fluorescence

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has a strong dependence on the thermal history of the sample - annealing of the glasses and decreasing the synthesis temperature lead to higher luminescence intensity [19]. Increasing the Ga content, relative to Ge, decreases the erbium concentration quenching effect. As reported previously [20-22], the introduction of suitable modifier, such as caesium halide CsX (X = Cl, Br, I), has led to a large decrease in the multiphonon relaxation rates and results in significant improvement of the emission properties of RE3+ ions. In particular for Ge-S-Ga-CsBr glasses, as it has been pointed out [21], studies on the local environment of RE3+ ions by extended X-ray absorption fine structure (EXAFS) spectroscopy have proved that the addition of CsBr into GeGaS glass, keeping the ratio between CsBr and Ga equal to or greater than unity, is favorable for the incorporation of larger amount of RE3+ ions. It has been found that especially the Ge-S-Ga glasses are characterized by enhanced solubility of relatively large amounts of RE3+ ions, compared to other chalcogenide systems, which results from the modification of the local structure induced by the introduction of Ga into the Ge-S glass [7, 9, 12, 19]. In fact, RE3+ ions are dissolved into the Ge-S-Ga host by either breaking homopolar (Ge-Ge or Ga-Ga) bonds or changing edge-shared GaS4/2 tetrahedra to corner-shared ones. The homopolar bonds are created to compensate for the sulfur deficiency by the formation of GaS 4/2 tetrahedra from the Ga2S3 compound. As a result non-bridging sulfurs are formed and RE3+ ions take the role of charge compensators for them [12]. Goodyear and Steigman [23] and Pardo et al. [24] have reported that the crystal structure of Ga2S3 is composed of GaS4 tetrahedral structural units, whereas two of the three S atoms are coordinated by three Ga atoms and the third S atom is coordinated by two Ga atoms. The presence of trigonally coordinated S atoms decreases the flexibility of the network structure, resulting in low vitrification tendency. In this chapter, the main results from the complex studies on optical properties of the chalcogenide (GeS2)100−x (Ga2S3)x glasses, where x = 20, 25 and 33 mol% (corresponding to the ratio of [GeS2/Ga2S3] = 4, 3 and 2, respectively), doped with Er2S3 in a wide range of 0.3, 0.6, 0.9, 1.8, 2.1, 2.4 and 2.7 mol%, have been reviewed in connection with possible

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applications in optoelectronics and photonics. In our previous paper [25], for the first time the glass-forming region in the Ge-S-Ga system has been determined, together with the characterization of the synthesized glasses by basic physical parameters, which are important from a practical point of view. The research work deals with evaluation of Er3+ inner shell 4f-4f radiative transitions and relative changes in the line-shape of emission cross-section in order to reach the conditions of optimal luminescence efficiency. Consequently, a special attention has been turned to studies on the features in absorption, emission, local ordering structure and their derivatives as a function of excitation wavelength, Er3+ doping level, host composition and temperature. The influence of the doping level on photoluminescence at ~1540 nm has been investigated in detail as an important telecommunications wavelength. The enhanced erbium solubility has been specified by the peculiarities of local ordering with introduction of Ga into the Ge-S glass. In order to identify and select useful host compositions, the influence of Ga content on the luminescence efficiency of heavily Er-doped compositions has been investigated. The changes in the line-shape of the emission cross-section with temperature decreasing have been specified as well. Further, CsBr addition into the glass host has been explored for studying their potential suitability as optical amplifiers. The up-converted fluorescence spectra of Er-doped (GeSGa)90(CsBr)10 glasses have been analyzed and a schematic energylevel diagram of the up-conversion emission in Er3+ ions is proposed.

2. EXPERIMENTAL 2.1. Synthesis and Physicochemical Characterization Bulk glasses of Er-doped (GeS2)100−x (Ga2S3)x (x = 20, 25 and 33 mol%) compositions were synthesized by conventional rapid quenching in ice water of melts from GeS2, Ga2S3 and Er2S3. Glassy GeS2 was prepared by an employed stepwise regime (at 250, 550 and 750°C for 2 h). The procedure for Ga2S3 was much more complicated due to the high

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vapor pressure of S at the melting point of this compound (the details were described in Ref. [26]). Er2S3 was a product of Alfa Aesar (Johnson Matthey Company, 99.9%). The appropriate mixtures were taken into evacuated (~10−3 Pa) silica ampoules by the same stepwise heating up to 1000°C with a rate of 2-4°C min-1. The amorphous state and homogeneity of the samples were checked by X-ray diffraction and electron microscopy. The differential thermal analysis was carried out at a non-isothermal regime in the temperature range of 25-850oC by using DTA 03 (RMI) instrument. Quartz ampoules with powders were evacuated and heated with a rate of 10 K min-1. Pure Al2O3 was used as a standard. The microhardness was determined by a Vickers microindenter with 30 measurements for each composition, accounting the average value to an accuracy of ±5%. The density was measured by the Archimedes method with toluene as an immersion media, accurate to ±0.05% [27].

2.2. Optical Procedures The transmission spectra were measured on optically polished samples with a thickness of ~1 mm by a Perkin Elmer Lambda 900 UV/VIS/NIR spectrophotometer. Raman spectra were recorded in the range of 50-550 cm-1 at room temperature using FT-spectrometer IFS-55 FRA 106 (Bruker). Room temperature photoluminescence (RTPL) spectra were evaluated by various equipments: (i) with excitation line 1064 nm of Nd:YAG laser in back scattering geometry by FT-spectrometer IFS-55FRA 106 (Bruker) with a Ge-detector cooled by liquid nitrogen; (ii) excitation by laser diodes operating at 532, 644, 770 and 982 nm, using an ORIEL Cornerstone 1/8m monochromator and an ORIEL cooled InGaAs photodiode. Low temperature photoluminescence (LTPL) spectra were measured in the range of 300-4 K with the help of Fourier-Transform Photoluminescence Spectrometer (MIDAC Corp. USA) under excitation by Ar+ laser (exc = 514.5 nm) with intensity of 200 mW cm-2. The LTPL signals were detected by a liquid-nitrogen cooled Ge-photodiode and were recorded by

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averaging out 10 recorded scans by resolution of 2 meV. The 1m focal length monochromator Jobin-Yvon coupled with a cooled high purity Ge detection system enables sensitive and high resolution measurements in the spectral range of 800-1700 nm by using the lock-in technique and computer controlled data collection. The up-conversion fluorescence spectra were recorded using a 795 nm pump beam from a tunable Ti-sapphire laser excited. The spectra of emitted radiation in the visible range were measured by using the same spectrometer and the photomultiplier tube.

3. RESULTS 3.1. Physicochemical Parameters The boundaries of glass forming area in the Ge-S-Ga system have been determined and reported for the first time in Ref. [25]. The basic physicochemical parameters of the studied glasses are summarized in Table 1. The glass transition temperature (Tg) increases slightly from 364oC to 407oC with increasing Er content. The crystallization temperature (Tc) exceeds the Tg values by 139-151oC, the maximal being for the GeS2)80(Ga2S3)20 host composition. Table 1. Physicochemical properties of (GeS2)a(Ga2S3)b(Er2S3)c glasses Composition mol%

[a/b]

Tg oC

Tc oC

T

HV kg/mm2

d g/cm3

a 75 74.6 80 79.8 79.5 79.3 90 89.5

3 3 4 4 4 4 9 9

364 381 378 384 389 407 392 395

512 527 529 533 536 548 537 534

148 146 151 149 149 141 145 139

107 126 112 121 131 140 123 152

2.89 2.97 3.02 3.05 3.08 3.13 3.22 3.42

b 25 24.8 20 19.9 19.9 19.8 10 9.9

c 0 0.6 0 0.3 0.6 0.9 0 0.6

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The crystallization kinetics of heavily Er2S3-doped compositions (1.8, 2.1 and 2.4 mol% Er2S3) is studied in non-isothermal regime with heating rates of 5, 10, 15 and 20oC min-1 [28]. The enthalpy data from the twophase crystallization have shown that these glasses are relatively stable and Er atoms do not create new nucleation centers. The microhardness (HV) changes from 107 to 152 kg mm-2 and the density (d) - from 2.89 to 3.42 g cm-3, respectively. Both parameters increase with increasing GeS2 content, depending only slightly on the presence of the Er3+ ions in the studied range of 0.3-0.9 mol% Er2S3 [27].

3.2. Optical Properties of Er-doped (GeS2)80(Ga2S3)20 Glasses 3.2.1. Absorption and Emission of Er3+ Ions The absorption coefficient () of (GeS2)80(Ga2S3)20 glasses doped with 0.3, 0.6 and 1.8 mol% Er2S3 is calculated from the transmission spectra T = T() and under the approximation of negligible light scattering as 2 ln T   2 R  R  T      , a ( )    1 1  d d  2  1  R 4 

(eq. 1)

where T is the transmission coefficient, d is the sample thickness, R is the reflection coefficient [R = (n - 1)2/(n + 1)2)] and n is the refractive index (n = 2.2) [29]. Figure 1 presents the dominant features in the absorption spectra as follows: (i) the absorption edge shifts towards lower energies with introduction of Er into the host, i.e., the optical energy gap decreases; (ii) according to the energy-level diagram of Er3+ ions [30], the observed sharp absorption bands centered at around 660, 810, 980 and 1540 nm are attributed to the radiative transitions from the ground 4I15/2 level to the excited 4F9/2, 4I9/2, 4I11/2 and 4I13/2 levels, respectively. As expected, the bands are more pronounced at the higher doping of 1.8 mol% Er 2S3 and bands amplitudes are approximately proportional to Er concentration; (iii) the dependence  = f() in the range of weak absorption shows a decrease

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with increasing Er content. The absorption in this region is associated with the impurities and the defects due to bond deviations from normal valence requirements [31]. This unusual behavior can be related to the structural modification and the decrease of the number of structural defects due to introduction of Er2S3 into the glass host. Figure 2 presents the photoluminescence spectra of the studied samples that appear in Figure 1. The measurements have been implemented at room temperature under excitation by a Nd:YAG laser operating at 1064 nm. The strongest emission band at 1540 nm, related to the excitation of 4I13/2 level, significantly increases when the Er2S3 content becomes three times higher [29]. On the other hand, the photoluminescence intensity of the broad band in the range of 1120-1460 nm becomes lower at higher Er doping level. It is clearly seen in the inset that while the PL spectrum for the un-doped glass exhibits a strong PL emission from native defects of the host, the intensity of this broad band decreases with increasing Er content, which is a sign for a structure improvement.

Absorption coefficient (cm-1)

Photon energy (eV) 3

2

1

1.5

100

4

F9/2

4

4

I9/2

10

I13/2

4

I11/2

a) b)

c) 600

900

1200

1500

1800

Wavelength (nm) Figure 1. Optical absorption spectra of (GeS2)80(Ga2S3)20 glasses doped with: a) 0.3, b) 0.6 and c) 1.8 mol% Er2S3. The bands are identified by excited levels.

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4

host

PL intensity (a. u.)

PL intensity (a. u.)

5

0.6 mol% Er2S3 1.8 mol% Er2S3

3

1200

1300

1400

Wavelength (nm)

2

1

a)

b)

c)

0 1200

1350

1500

1650

Wavelength (nm) Figure 2. Emission spectra of the studied Er-doped glasses from Figure 1. The inset shows PL intensity of the host band as well, originating from native defects of the host glass.

As an example, the comparison between the strongest emission bands at ~1540 nm of pure polycrystalline Er2S3 and one sample from the above studied glasses is shown in Figure 3. It is shown that a broader PL band is obtained for the Er-doped sample, whereas the influence of erbium introduction on the glassy GeSGa matrix is mainly expressed by its weak but observable shift to the higher energies. Note that the energy diagram of Er3+ ion [30] shows that the used 1064 nm radiation cannot excite the upper levels of the Er3+ ion but only the 4I13/2 one, therefore this emission is due to the optical 4I13/2→4I15/2 transition.

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11

Figure 3. Photoluminescence bands of Er2S3 and 0.9 mol% Er2S3-doped (GeS2)80(Ga2S3)20 samples.

3.3. Evaluation of the Main PL Band at ~1540 nm A special attention has been directed to the strongest PL band at ~1540 nm for the (GeS2)80(Ga2S3)20 glassy host doped in the wide range of Er2S3 concentrations (0.3, 0.6, 0.9, 1.8, 2.1 and 2.4 mol%). The basic peculiarities in the corresponding emission cross-section as a function of various parameters such as excitation, Er-doping level, temperature and gallium content in the host have been specified.

3.3.1. Influence of Excitation and Judd-Ofelt Analysis Following the positions of the observed absorption bands in Figure 1, Er-doped (GeS2)80 (Ga2S3)20 samples have been excited by laser diodes operating at various wavelengths (ex), corresponding to intra 4f-shell transitions in Er3+ ions (exc = 644, 770, 982 and 1064 nm) and to absorption in the Urbach tail of the host glass (exc = 532 nm). The obtained results are collected in Figure 4.

12

Zoya G. Ivanova 1.0

a)

0.8

982 nm

0.4

532 nm

0.2 0.0 1.0

770 nm

(b)

Normalized PL intensity

0.6

b)

0.8

644 nm

0.6 0.4

532 nm

0.2

982 nm

0.0 1450

1500

1550

1600

1650

W a v e l e n g t h (nm )

Figure 4. Normalized PL bands of (GeS2)80(Ga2S3)20 glasses, doped with: a) 0.3 and b) 1.8 mol% Er2S3.

It has been established that the emission cross-section under the direct excitation of Er3+ ions at 982 nm almost coincides with that under excitation via the host at 532 nm (Figure 4, a), keeping the value of the full width at half maximum (FWHM ~37 nm) [29]. However, the observed PL narrowing effect under the host excitation is well pronounced at higher Er2S3 content of 1.8 mol% (Figure 4, b). This is illustrated by the presented deconvolution of the experimental data in Figure 5. Four Gaussian subbands at 1520, 1540, 1553 and 1574 nm are detected by direct excitation of Er3+ ions at exc = 982 nm (Figure 5, a), where the development of broad sub-bands at 1553 and 1574 nm with the simultaneous reduction of the amplitude of the sharp sub-band centered at 1540 nm results in a broader global emission band (FWHM = 72 nm)). In comparison, the observed PL narrowing effect under the host excitation at exc = 532 nm (FWHM ~40 nm, Figure 5, b) is mainly due to the more intense sub-band at 1540 nm and a decrease of that at 1574 nm [32].

Complex Studies on the Photoluminescence …

13

Figure 5. Deconvoluted PL bands for 1.8 mol% Er2S3-doping content at: a) 982 nm excitation and b) 532 nm excitation.

On the basis of Judd-Ofelt (J-O) analysis [33, 34], basic radiative and spectroscopic parameters have been determined from the absorption and emission bands at ~1540 nm for the glasses studied, obtained under excitation with 1064 and 514.5 nm wavelengths. The obtained results are summarized in Table 2. The J-O intensity parameters  ( = 2, 4 and 6) are derived from the least-square fit of the experimental and the calculated oscillator strengths [35, 36]. In particular, 2 is very sensitive to the structure and is associated with the asymmetry and the covalency of the lanthanide site [37]. The observed considerably higher 2 value for 0.6 mol% Er2S3 could be attributed to higher degree of covalency and smaller degree of homogeneity in comparison with the case for 1.8 mol% Er2S3. On the other hand, 4 and 6 values are related to the rigidity of the host matrix in which the Er3+ ions are situated [38].

14

Zoya G. Ivanova Table 2. Radiative and spectroscopic data of the studied glasses

Er2S3 content (mol%) Peak PL wavelength (nm) Optical energy gap, Eg (eV)

0.6 1540 2.77

1.8 1550 2.63

2

1.287

0.560

4

0.418

0.787

6

1.019

0.614

Radiative lifetime, R (ms) FWHM (nm)

2.96

3.78

exc = 1064 nm

36.7

78.1

exc = 514.5 nm

23.5

40.8

exc = 1064 nm

14.48

5.291

exc = 514.5 nm

22.47

9.933

J–O intensity parameters,  ( 10-20 cm2)

Stimulated PL cross-section, e (10-21 cm2)

The total radiative transition probability of the excited state is calculated from the obtained J-O intensity parameters and consequently the radiative lifetime (R) for the 4I13/24I15/2 transition of Er3+ ions has been determined. Based on the normalized PL bands at ~1540 nm, spectroscopic parameters such as the FWHM values and the stimulated emission crosssection e have been estimated [35, 36].

3.3.2. Influence of Er-doping and Temperature The dependence of the measured PL band ~1540 nm under 1064 nm excitation at room temperature on Er concentration in the glasses studied is illustrated in Figure 6 with the enlarged emission cross-section up to 2.1 mol% Er2S3, followed by a considerable decrease of PL intensity at 2.4 mol% Er2S3 [29, 39]. Moreover, the amplitude of the corresponding band is smaller than that at 1.8 mol% Er2S3-doping. Obviously, a nonuniform distribution of Er atoms occurs in such case, probably accompanied by Er clustering which causes the quenching effect of the Er3+ emission.

PL intensity (a. u.)

Complex Studies on the Photoluminescence … 4

3

2

3 2

1

15

mol% Er2S3 1 - 0.3 5 2 - 0.6 4 3 - 0.9 6 4 - 1.8 5 - 2.1 6 - 2.4

1

0 1450

1500

1550

1600

1650

Wavelength (nm) Figure 6. Photoluminescence band at ~1540 nm of Er-doped (GeS2)80(Ga2S3)20 glasses.

The influence of temperature on the intensity of this band for the same series of Er-doped glasses is also determined by measuring the photoluminescence at 4.2 K under 514.5 nm excitation, corresponding to the host absorption (Figure 7). Comparing the results with those at room temperature in Figure 6, a narrowing effect of the emission cross-section at ~1540 nm is clearly pronounced at higher doping levels with the same quenching conditions at 2.1 mol% Er2S3. In particular for the quenching composition 2.1 mol% Er2S3-doped (GeS2)80(Ga2S3)20, the changes in the line-shape of the emission crosssection with decreasing temperature are specified in more detail by deconvolution of this PL band (Figure 8). The observed fine features of Er3+ emission at 514.5 nm excitation are illustrated as follows: (i) at room temperature (Figure 8, a), the rather broad band can be presented by four Gaussian sub-bands, centered at ~1520, 1538, 1554 and 1570 nm, which are attributed to the transitions F21, F11, F12 and F13 according to the energy Stark splitting diagram of Er3+ ions [40, 41]. Besides, the observed PL broadening effect is mainly due to the enhanced development of the subbands at 1520 and 1570 nm; at 77 K (Figure 8, b), the global photoluminescence band becomes more narrow by the significant decrease of the sub-band at 1520 nm and the appearance of a new one at 1546 nm at

16

Zoya G. Ivanova 0.03

mol% Er2S3

5

PL intensity (a. u.)

6

4

0.02

0.01

1 - 0.3 2 - 0.6 3 - 0.9 4 - 1.8 5 - 2.1 6 - 2.4

3 2 1

0.00 1530

1560

1590

1620

Wavelength (nm) Figure 7. PL band at ~1540 nm of the glasses studied appeared in Figure 6 at 4.2 K. 0,006

a)

0,004

PL Intensity (a. u.)

0,002 0,000 b)

0,004 0,002 0,000 0,024

c)

0,016 0,008 0,000

1500

1550

1600

Wavelength (nm) Figure 8. Deconvoluted band at 1540 nm of 2.1 mol% Er2S3-doped (GeS2)80(Ga2S3)20 glass under 514.5 nm excitation at: a) room temperature, b) 77 K, c) 4.2 K.

Complex Studies on the Photoluminescence …

17

the expense of that at 1554 nm; at 4.2 K (Figure 8, c), with further temperature decreasing, three sub-bands are observed and PL narrowing effect is mainly enhanced by the significant increase of the sub-band at 1538 nm, which coincides with that at the total one. On the other hand, intensity decrease of the sub-bands at 1546 nm and 1570 nm and disappearance of that at 1520 nm are observed. Consequently, the population of the second sub-level of the 4I13/2 manifold also decreases and the Er3+ emission occurs mainly by the transition F11, assisted by F12 and F13 ones [39].

3.3.3. Local Ordering Structure The local structure of the above mentioned studied glasses is investigated by Raman scattering analysis and is shown in Figure 9. In addition, the obtained features in the Raman spectra are evaluated by comparison between the deconvoluted bands of the host and that at maximum Er-doping level (Figure 10).

Intensity (rel. units)

6 5 4 3

host 100

200

300

400

2 1 500

-1

Raman shift (cm ) Figure 9. Raman spectra of the glasses studied. The numbers correspond to those in Figures 6 and 7.

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Zoya G. Ivanova

Intensity (a. u.)

0.5

a)

0.4 0.3 0.2 0.1 0.0 100

200

300

400

500

Intensity (a. u.)

0.10

b) 0.08 0.06 0.04 0.02 0.00 100

200

300

400

500

-1 Raman shift (cm ) Figure 10. Deconvoluted Raman spectra for: a) (GeS2)80(Ga2S3)20 host, b) doped with 2.4 mol% Er2S3.

It is known that the basic structural units (s. u.) in Ge-S-Ga glasses are GeS4- and GaS4- tetrahedra connected through bridged sulfur [12, 42]. Since Ga is in trivalent bonding state, the GaS4 tetrahedron has one negative charge because of the deficiency of sulfur, resulting in the creation of “defects” such as [GaS4]1-. The well exhibited bands at ~340 cm-1 and ~120 cm-1 are related to symmetrical bending vibrations of GeS4 tetrahedra. The shoulder at ~370 cm-1 is known as a companion band, which is attributed to the vibration of two edge-shared GeS4 tetrahedra. The addition of Ga2S3 into GeS2 results in the following

Complex Studies on the Photoluminescence …

19

changes of the Raman spectra: (i) the band at 265 cm-1 is related to the vibration of two S3Ge(Ga)-(Ga)GeS3 tetrahedra with ethane-like type of bonds; (ii) the increased width of the 340 cm-1 band is due to the formation of GaS4 tetrahedra, leading to a cross-linked glassy network. Since the atomic masses and radii of Ga and Ge are similar, the frequency of GaS4 and GeS4 vibrations are very similar and the corresponding Raman bands cannot be distinguished in the range of 340-390 cm-1; (iii) the decrease in amplitudes of the bands at ~370 and 430 cm-1 results from the substitution of GeS2 by Ga2S3, i.e., the total number of connections between two GeS4 tetrahedra decreases. The broad band at ~430 cm-1 is assigned to the vibration of two corner-shared S3Ge-S-GeS3 tetrahedra through bridged sulfur. Summarizing, it is clearly seen that the intensity of all the bands decreases with increasing Er2S3 concentration. Especially, the observed lower amplitude of the bands at 260 and 340 cm-1 could be a sign for the dissociation of metal Ga-Ga bonds and the formation of corner-shared tetrahedral s. u. with non-bridged sulfurs, and introduction of Er2S3 facilitates such process. Increasing the Er2S3 content, the number of edgeshared Ga(Ge)S4 tetrahedra decreases by conversion into corner-shared ones with non-bridged sulfurs. The presence of the [GaS4]1- tetrahedra determines the role of Er3+ ions as charge compensators for these nonbridged sulfurs [12]. This feature confirms the hypothesis made on the observation of the photoluminescence spectra in Figure 2, suggesting that the introduction of erbium reduces the number of structural defects in the glassy network [29].

3.3.4. Influence of Ga Content into Host Composition In fact, the confirmed main advantage of enlarged Er-solubility in the studied Ge-S-Ga glasses is due to the presence both Ge and Ga into the composition and the consequent structural modification by addition of Ga into Ge-S glass [7, 9, 12, 19]. The variation in the normalized PL band at ~1540 nm under excitation at exc = 1064 nm for the host compositions containing 25 and 33 mol% Ga2S3, heavily doped with Er2S3 from 1.8 to 2.7 mol%, has been illustrated in Figure 11. In the case of

20

Zoya G. Ivanova

(GeS2)75(Ga2S3)25 host, the emission cross-section increases with increasing doping content from 1.8 to 2.1 mol% Er2S3, simultaneously becoming rather broad - FWHM increases from 54 to 81 nm [43]. However, the PL band becomes considerably less intense by introduction of 2.4 mol% Er2S3 keeping FWHM almost the same, i.e., a quenching effect at 2.1 mol% Er2S3-doping is reached. The observed rather lower PL signal at 2.4 mol% Er2S3 could relate to nonuniform distribution of Er atoms [44, 45]. On the other hand, the quenching effect is reached at the higher doping concentration of 2.4 mol% Er2S3 in the case of (GeS2)67(Ga2S3)33 host. Besides, the shoulder to the main peak is developed as an additional maximum at higher Er-doping. In our previous paper [29], the same broadening of this PL band with increasing Er content has been established for (GeS2)80(Ga2S3)20 glasses (Figure 6). Such effect can be also attributed to a re-absorption effect as discussed in [46, 47].

PL intensity (a. u.)

3.0 2.5 2.0

x=33 mol% Ga2S3

x=25 mol% Ga2S3

mol% Er2S3

mol% Er2S3

5-0 6 - 2.4 7 - 2.7

1-0 2 - 1.8 3 - 2.1 4 - 2.4

7 3

1.5

2

1.0 6

0.5

5

4 1

0.0 1400 1450 1500 1550 1600 1650 1700

Wavelength (nm) Figure 11. Photoluminescence spectra of heavily Er-doped GeS2)100-x (Ga2S3)x (x = 25 and 33) glasses under 1064 nm excitation.

Studies of amorphous chalcogenide layers are very valuable in connection with their applications in thin film optoelectronic technologies. Ion implantation is a convenient way to incorporate Er3+ ions into them, as the Er concentration depth profile can be tailored by varying the ion energy

Complex Studies on the Photoluminescence …

21

and fluence [13, 16]. In particular, thin films from the bulk (GeS2)67(Ga2S3)33 glass composition have been prepared by electron beam heating onto quartz substrates, which are consequently implanted by 320 keV Er3+ ions with fluences of 1015, 2×1015, 5×1015 and 1016 ions cm-2, corresponding to concentrations at a mean ion range of 0.27, 0.54, 1.3 and 2.6%, respectively [48]. The emission band at 1540 nm under excitations at 809, 791, 662 and 532 nm appears to be broadened for the highest Er3+ dose of 1016 ions cm-2. Moreover, thermal annealing at 230oC leads to an improvement of the PL intensity by ~50%. This effect is studied as well on thin films, prepared by thermal vacuum deposition from (GeS2)75(Ga2S3)25 host composition, doped with high contents of 2.1 and 2.4 mol% Er2S3 [49, 50]. The observed effects are discussed in terms of the glass structure becoming damaged during ion implantation, and the partial restoration of the structure by subsequent annealing. The structure of amorphous GeSGa thin films, prepared by the same technique of thermal evaporation, has been studied with Raman scattering by J. Fu et al. [51]. They have reported that in addition to the basic s. u. of GeS4 tetrahedra, there are some S-S and Ge-Ge homopolar bonds which exist in the films. The increase in Ge concentration leads to the replacement of S-S bonds by Ge-Ge bonds, and the isolated Ge(Ga)S4 tetrahedral units transform into corner-sharing or edge-sharing Ge(Ga)S4 tetrahedral units.

3.4. LTPL of Er-doped (GeS2)100-x(Ga2S3)x Glasses It has been found that the emission efficiency increases significantly with decreasing temperature, which is accompanied by enhanced narrowing effect of all PL bands. The complex investigation of the influence of temperature and host composition on the photoluminescence is carried out in the range from 300 K down to 4.2 K under excitation by Ar+ laser (exc = 514.5 nm) [52-56]. Since the wavelength of 514.5 nm corresponds to the host absorption, the observed PL emission is due to the absorption of light by the glass host and the subsequent transfer of this energy to the Er3+ center.

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Zoya G. Ivanova

3.4.1. Influence of Er-doping on PL Intensity of Er-doped (GeS2)80(Ga2S3)20 Glasses As an example, the development of the emission bands at 77 K with increasing Er content of the glasses studied is demonstrated in Figure 12. While at 0.3 mol% Er2S3-doping level only the main band at ~1540 nm is clearly observable, at 0.6 and 0.9 mol% Er2S3 a PL signal at ~980 nm appears. At 1.8 mol% Er2S3, a weak peak stretching vibration at ~820 nm is noticed as well. Besides, it is clearly seen that the PL spectrum at the lowest Er2S3 content of 0.3 mol% exhibits a broad intrinsic emission band in the vicinity of 850-1500 nm, corresponding to the photoluminescence from native defects of the host [52]. The further increase in the Er3+ content leads to a quenching effect of this broad band as is shown in the inset of Figure 12. Therefore, it could be assumed that Er3+-doping causes the decrease of these defects, which is consistent with the observed decrease in the absorption with increasing Er3+-doping (Figure 1, [29]). Figure 13 demonstrates the influence of temperature on the emission by comparison of the experimental PL spectra for 1.8 mol% Er2S3-doped (GeS2)80(Ga2S3)20 sample at 77 and 4.2 K (the inset shows the corresponding optical transmittance). Three PL bands at ~1540, 980 and 820 nm are exhibited at 77 K, attributed to the excitation of the 4I13/2, 4I11/2 and 4I9/2 emitting levels [30], whose intensity increases considerably by decreasing the temperature down to 4.2 K [52]. Especially, the fine structure of the most enhanced emission cross-section at ~1540 nm is specified by deconvolution of the corresponding PL band into Gaussians [53]. The obtained features can be summarized, as follows (Figure 14): (i) at 77 K, a rather broadened total band is exhibited (FWHM = 49 nm), which is due to the presence of a sub-band at 1527 nm and the enhanced height of the sub-band at 1575 nm (Figure 14, a); (ii) decreasing the temperature down to 4.2 K, the sub-band at 1527 nm disappears, the intensity of that at 1538 nm significantly increases and as a result the total peak becomes rather sharper (FWHM = 16 nm) and rather more intense (Figure 14, b).

Complex Studies on the Photoluminescence …

23

77 K PL (a. u.)

PL intensity (a. u.)

a

b c d e

1000

1200 1400 Wavelength (nm)

e) d) c) b) a) 900

1200

1500

Wavelength (nm) Figure 12. LTPL spectra of (GeS2)80(Ga2S3)20 glasses doped with: a) 0.3, b) 0.6, c) 0.9, d) 1.8 and e) 2.4 mol% Er2S3 at 77 K. The inset shows the host luminescence for the corresponding Er2S3 contents.

Transmittance (%)

PL intensity (a. u.)

60

30

0 400

800 1200 1600 Wavelength (nm)

4.2 K

77 K 400

800

1200

1600

Wavelength (nm) Figure 13. LTPL spectra of 1.8 mol% Er2S3-doped (GeS2)80 (Ga2S3)20 glass at 77 and 4.2 K. The inset shows the corresponding optical transmission spectrum.

24

Zoya G. Ivanova

0.008

a)

0.006

PL intensity (a. u.)

0.004

0.002

0.000

0.020

b)

0.015

0.010

0.005

0.000 1500

1550

1600

1650

Wavelength (nm) Figure 14. Deconvoluted PL band for 1.8 mol% Er2S3-doping at: a) 77 K and b) 4.2 K.

Figure 15. Compositional dependence of PL intensity of Er2S3-doped (GeS2)80(Ga2S3)20 glasses at 300, 77 and 4.2 K.

Complex Studies on the Photoluminescence …

25

The compositional dependences of PL intensity of 1540 nm band for the glasses studied in the whole doping range from 0.3 to 2.4 mol% Er2S3 by decreasing temperature from 300 K down to 4.2 K are summarized in Figure 15. The results show the remarkably higher values of this parameter at 4.2 K with increasing the Er doping level up to 2.1 mol% Er2S3 [54,55]. Further doping leads to the decrease of PL intensity at 2.4 mol% Er 2S3, consequently a quenching effect occurs, which is probably a sign for a nonuniform distribution of the doping atoms in the glassy matrix [53].

3.4.2. PL Features in Heavily Doped (GeS2)100-x(Ga2S3)x Glasses In order to demonstrate the role of Ga concentration into the host on the radiative efficiency of Er3+ ions, the compositional trends in the lowtemperature photoluminescence of heavily Er-doped (GeS2)100−x(Ga2S3)x glasses are evaluated [56]. Figure 16 compares the photoluminescence spectra, measured at 80 K, for x = 20, 25 and 33 mol% Ga2S3, with varying Er2S3 concentrations from 1.8 to 2.4 mol%. The obtained data show the following features: (i) at 20 mol% Ga2S3 (Figure 16, a, b, c), the development of three PL bands at around 830, 1000 and 1550 nm is observed, whose intensity increases by increasing the content of Er2S3 from 1.8 to 2.1 mol% and decreases at 2.4 mol% of Er2S3; (ii) at 25 mol% Ga2S3 (Figure 16, d, e), five new emission bands at around 670, 870, 1120, 1260 and 1350 nm are observed in addition to the above noted ones; and (iii) at 33 mol% Ga2S3 (Figure 16, f), the typical three PL bands corresponding to those at 20 mol% Ga2S3 are seen but with lower intensity, which can be caused by the stronger metallization of the bonds in the case of high gallium concentration. In other words, it means that Er3+ radiative efficiency seems to be an optimum at around 25 mol% Ga2S3. In order to demonstrate the role of temperature on the PL efficiency, the emission of heavily Er-doped (GeS2)100−x (Ga2S3)x (x = 25 and 33 mol%) glasses is studied. Figure 17 shows the temperature dependences of all PL bands observed in the temperature range from 300 K down to 10 K, together with the corresponding inner shell transitions.

26

Zoya G. Ivanova

PL intensity (a. u.)

x, mol% Er2S3 mol% a) 20; b) 20; c) 20; d) 25; e) 25; f) 33;

1.8 2.1 2.4 1.8 2.1 2.4

T=80 K

" " " " " "

f

e

d

a

760

b

c

950 1140 1330 1520 1710

Wavelength (nm) Figure 16. Low temperature PL spectra of Er2S3-doped (GeS2)100−x (Ga2S3)x glasses at 80 K.

It has been found that at 33 mol% Ga2S3 the typical 4f–4f emission bands at ~830, 1000 and 1550 nm are exhibited, which are attributed to the 4 S3/24I13/2, 4I11/24I15/2 and 4I13/24I15/2 transitions, respectively (Figure 17, a). However, at 25 mol% Ga2S3, in excess of the three basic ones, new bands are developed at 670, 870, 1120 and 1260 nm, tentatively assigned to 2H9/2→4I11/2, 4G11/2→4F9/2, 2H11/2→4I11/2 and 4F7/2→4I9/2 transitions, respectively. In addition, a very weak PL signal at ~1350 nm is also seen which can be related to the 4F3/2→4I9/2 transition. Consequently, the important role of gallium is demonstrated by the fact that all eight PL bands are manifested for (GeS2)75(Ga2S3)25 glassy host even at the lower doping level of 1.8 mol% Er2S3 (Figure 17, b).

Complex Studies on the Photoluminescence …

PL intensity (a. u.)

2.4 mol% Er2S3

4

4

I11/2- I15/2

4

4

S3/2- I13/2

600

800

1000

(a)

10 20 40 60 80 100 140 180 220 260 300

33 mol% Ga2S3

1200

27

4

4

I13/2- I15/2

1400

1600

1800

Wavelength (nm)

10 20 40 60 80 100 140 180 220 260 300

PL intensity (a. u.)

25 mol%Ga2S3 1.8 mol% Er2S3

4

4

I11/2- I15/2

4 4

4

S3/2- I13/2

2

4

H9/2- I11/2

600

4

G11/2- F9/2

800

4

b) 4

4

I13/2- I15/2

4

4

F3/2- F9/2

4

F7/2- I9/2 4 H11/2- I11/2

2

1000

1200

1400

1600

1800

Wavelength (nm) Figure 17. PL spectra at: a) 2.4 mol% Er2S3-doped (GeS2)67(Ga2S3)33 and b) 1.8 mol% Er2S3-doped (GeS2)75(Ga2S3)25 glasses.

28

Zoya G. Ivanova

3

a)

b)

T=300 K

T=10 K

4 2

PL intensity (a. u.)

PL intensity (a. u.)

Figure 18 illustrates the changes in the emission cross-section of the strongest PL band at ~1550 nm for the glasses containing 25 and 33 mol% Ga2S3 (and doped with two Er contents each) at 300 K and 10 K. In addition to the observed narrowing effect, the inherent structure of the main PL band due to “crystal field” splitting by the temperature decrease is clearly pronounced at 10 K, where the sub-band at 1580 nm is resolved for both glassy hosts. According to the “crystal field” splitting diagram of Er3+ ion [40], the glassy host splits 4I15/2 manifold into eight components and 4I13/2 one to seven components. Consequently, the bands at around 1550 and 1580 nm can be attributed to F12 and F13 transitions, respectively, and therefore, the emission is realized between the first sub-level of 4I13/2 manifold and the two ones of 4I15/2. The reflectance spectra of these samples (together with the un-doped ones), measured in the reflection mode, are presented in Figure 19.

4 3 1

2

1

1450 1500 1550 1600 1650 1700

Wavelength (nm)

1500

1550

1600

Wavelength (nm)

Figure 18. Line-shape of ~1550 nm PL band at: a) 300 K and b) 10 K; 1- x = 33, 2.4 mol% Er2S3; 2 - x = 33, 2.7 mol% Er2S3; 3 - x = 25, 1.8 mol% Er2S3; 4 - x = 25, 2.1 mol% Er2S3.

1650

Complex Studies on the Photoluminescence …

980 nm

29

1520 nm 1720 nm

Reflectance (a. u.)

2270 nm host, x=33 host, x=25

1 2 3 4

1000

1500

2000

2500

Wavelength (nm) Figure 19. Reflectance spectra of the glasses studied. The numbers correspond to the samples from Figure 18.

The observed strong and quite distinct absorption bands around 980 and 1520 nm in all Er-doped samples, irrespective of the compositional variation, are related to corresponding 4f-4f emission bands of Er3+ ions (Figure 16) and do not show any dependence on the glass composition. The absorption bands around 1720 and 2270 nm are attributed to the host glass. It follows that only the most efficient 4f-4f transitions are revealed in the reflectance spectra, and absorption bands corresponding to the new emission bands for x = 25 mol% Ga2S3 are not observed.

3.4.3. LTPL in the Near-Infrared (NIR) Spectral Range The broad luminescence band of the host glass, mediated by the presence of deep energy states in the band gap and centred at about the mid band gap energy, is strongly dependent on temperature and is usually observed at low temperatures. On the other hand, the 4f-4f inner shell transitions are little temperature dependent and could be easily observed at room temperature. The wavelength range, where this broad PL band overlaps with 4f-4f transitions of RE3+ ions is of particular interest [57-59].

30

Zoya G. Ivanova

In this range it is possible to observe simultaneously the broad band of the host with the superimposed narrow effects due to 4f-4f transitions. The NIR low-temperature PL spectra at 4.2 K of the Er3+-doped (GeS2)80(Ga2S3)20 glasses are shown in Figure 20 for two doping contents of 0.3 and 2.1 mol% Er2S3. They are measured by using external excitation into the Urbach tail of the fundamental absorption edge of the host glass at 514.5 nm. For the lower doping level of 0.3 mol% Er2S3 (Figure 20, a), the spectrum is dominated by the host broad band at ~1200 nm with a superimposed narrow emission band at ~1530 nm due to 4f-4f downtransition in the doped-in Er3+ions. For the higher 2.1 mol% Er2S3 concentration (Figure 20, b), the PL band at 1530 nm is rather more pronounced and two emission bands at around 830 and 980 nm are located, which are attributed to the 4S3/24I13/2 and 4I11/24I15/2 radiative downtransitions, respectively. The increase of the 1530 nm band at higher doping level manifests the decrease of the native structural defects with the increase of Er doping, and consequently some improvement of the glassy structure. 8

T=4.2 K

PL intensity (a. u.)

(GeS2)80(Ga2S3)20:Er2S3

6

b)

(Eems) 4

4

I13/2- I15/2

4

4

4

I11/2- I15/2

2

(Aabc) 4

4

I15/2- I13/2

4

4

S3/2- I13/2

x 4 a)

0 800 1000 1200 1400 1600

Wavelength (nm) Figure 20. LTPL spectra of (GeS2)80(Ga2S3)20 glasses doped with: a) 0.3 mol% Er2S3 (dashed line) and b) 2.1 mol% Er2S3 (solid line).

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Particularly, it should be mentioned that in the spectrum at the lower doping level a narrow dip at 1520 nm, superposed on the host broad PL band, in addition to the three emission bands, is observed, which corresponds to the electronic 4I15/24I13/2 up-transition [60, 61]. Consequently, the narrow emission band at 1530 nm and the dip at 1520 nm (marked by arrow) are labeled as Eems and Aabs in Figure 20 (a), respectively. The quenching effect of these bands for the higher of 2.1 mol% Er2S3-doped sample is evident. The broad PL band of the host is observed simultaneously with the superimposed narrow emission bands (Eems) and a dip in PL curve (Aabs) only in the case of 0.3 mol% Er2S3 doping level notwithstanding that the spectrum is displayed at 4 x higher sensitivity. The observed narrow emission (Eems) and absorption (Aabs) bands appear to correspond to transitions between the ground and the first excited states of Er3+ ions, separated by a Stokes shift. Such effect has been also observed at low temperature by Bishop and Turnbull [62-65]. It has been interpreted as due to non-radiative energy transfer between the electronic structure of the host and that of RE3+ ions, mediated by deep energy states in resonance with the 4f-levels of RE3+ ions.

3.5. Influence of CsBr Addition on the Photoluminescence of Er-doped Ge-S-Ga Glasses As reported previously [21], an improvement on radiative properties of RE ions is observed by addition of CsBr into Ge-S-Ga glasses. It has been found that the optical properties of GeGaS-CsBr glasses are strongly affected by two competing factors: CsBr and Ga concentrations in the host. Raman and EXAFS studies on the changes in the local structure and consequent environment of RE3+ ions have shown that keeping the ratio of [CsBr/Ga] equal to or greater than unity is required for the enhancement in the radiative parameters. 3+

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3.5.1. Thermal and Optical Parameters of Er-doped Chalcohalide (GeSGa)90(CsBr)10 Glasses In particular, two host glasses containing 10 and 12 at% Ga have been chosen and the role of Er introduction (0.05-0.15 at%) on their thermal and optical properties is investigated [66-68]. The compositions used in this study, the average values of basic thermal parameters, together with the optical gaps and Urbach energies, are given in Table 3. These chalcohalide glasses possess relatively high glass transition (Tg) and crystallization (Tc) temperatures, evaluated by differential scanning calorimetry (DSC) and Temperature Modulated DSC techniques. The optical gap (Eg) is estimated from the intersection of the extrapolated straight part of the absorption edge with the abscissa, and the Urbach energy (Eu) is determined by using Urbach exponential approximation in the sub-gap absorption range. It should be mentioned that the addition of CsBr seems to induce some crystallinity (ionic and rigid structure) which is partially compensated by the addition of gallium [66]. Especially, this conclusion is supported by the observed line-shape of the strongest emission band at ~1540 nm. In conventional GeGaS this band is rather smooth with only one well expressed peak, similar to other glassy materials [43, 56, 69, 70], and in contrast with crystalline materials where several peaks are typical. This feature is well illustrated in Figure 21 with the comparison of the absorption and emission cross-sections of Er-doped Ge25S65Ga10 and corresponding GeSGa-CsBr glasses under excitation with wavelength of 532 nm. The basic Ge25S65Ga10 composition is chosen from the stoichiometric tie-line between GeS2 and Ga2S3. The observed fine structure exhibits considerable narrowing and resolution of 4I13/2→4I15/2 radiative transitions in Er3+ ions for the case of CsBr-containing glass, compared to that without CsBr. Consequently, this is an indication for smaller dispersion of the crystal field values and nicely demonstrates the increasing ceramics like nature due to the addition of CsBr into the GeSGa glasses.

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Table 3. Thermal and optical parameters of the glasses studied Host composition

1 2 3 4

(Ge0.25S0.65Ga0.10)0.9(CsBr)0.1 (Ge0.25 S0.65Ga0.10)0.9(CsBr)0.1 (Ge0.23S0.65Ga0.12)0.9(CsBr)0.1 (Ge0.23S0.65Ga0.12)0.9(CsBr)0.1

PL intensity (a. u.)

Absorption (a. u.)

№

Er at% 0.05 0.1 0.05 0.1

5

Tg °C 350 337 343 330

Tc1 °C 470 462 498

Tc2 °C 505 540 565 570

Eg eV 2.81 2.86 2.85 2.85

Eu meV 120 100 90 88

Ge25Ga10S65 Host

4

without CsBr 10 mol% CsBr

3 2 1 0 1.0 0.8

x2

0.6 0.4

x1

0.2 0.0 1450

1500

1550 1600 Wavelength (nm)

1650

Figure 21. Absorption and emission bands at ~1540 nm of (Ge0.25 S0.65Ga0.10)0.9(CsX)0.1 (solid lines) and Ge0.25 S0.65Ga0.10 (dash lines) glasses, both doped with 0.15 at% Er.

One of the most important parameters for rare-earth activated optical glasses is the lifetime of the excited 4I13/2 level, which can be estimated by measuring the PL decay time (τD). The comparison of the τD values for the strongest 4I13/24I15/2 radiative transition of Er3+ at ~1540 nm, embedded in Ge0.25S0.65Ga0.10 and (Ge0.25S0.65Ga0.10)0.9(CsX)0.1 glasses, is shown in Figure 22, illustrating the increase of PL decay time due to the presence of CsBr in the glass host (τD = 8.3 ms compared to τD = 5.9 ms). These decay times show that the present Er-doped chalcohalide glasses are suitable for potential applications in optical amplifiers operating in the third telecommunication window around 1540 nm. Both the increases of Eg and τD values indicate that the addition of CsBr should lead to more efficient up-conversion as compared to the GeSGa system [67].

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The low temperature photoluminescence spectra measured at 20 K of the samples with slightly different Ga content of 10 and 12 at%, doped with 0.05 and 0.1 at% Er (listed in Table 3), are presented in Figure 23. It is established that the increase of Ga content limits the Er concentration quenching effect. In the case of (Ge0.25S0.65Ga0.10)0.9(CsBr)0.1 host the increase of Er concentration from 0.05 to 0.1 at% leads to the lower intensity of the emission band at ~1550 nm, while this is not observed in the case for (Ge0.23S0.65Ga0.12)0.9(CsBr)0.1 host [67]. Therefore, the PL band cannot be enhanced at the lower Ga content, thus the system is at the onset of concentration quenching.

Normalised intensity (a. u.)

Figure 22. PL decay curves of excited 4I13/2 level for: a) (Ge0.25S0.65Ga0.10)0.9 (CsBr)0.1:Er0.1 and b) Ge0.25S0.65Ga0.10:Er0.1 glasses. The exciting laser line 818 nm is turned off at time = 0.

1450

4I 13/2

4I 15/2

4 3 2 1 1500

1550

1600

1650

Wavelength (nm)

Figure 23. LTPL spectra excited by 514.5 nm laser line and measured at 20 K of samples 1, 2, 3 and 4 listed in Table 3. Intensities of the plotted PL bands are normalized to unity.

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3.5.2. Up-Conversion and Energy-Level Diagram of Er3+ Ions in (GeSGa)90(CsBr)10 Glasses Figure 24 presents the up-converted luminescence spectra of the studied glasses, recorded upon 795 nm excitation at room temperature. Two intense PL bands ~530 nm and 554 nm in the green spectral region and one weaker emission band at ~640 nm are observed, which are related to 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions of Er3+ ions, respectively [67]. The efficiency of up-conversion, leading to emission in the green and red spectral regions, is independent on the Ga content in the host because of its limited variation from 10 at% to 12 at%. Figure 25 illustrates a simplified energy-level diagram of Er3+ ions about the up-conversion excitation and corresponding fluorescence emission processes. The up-conversion phenomena of Er3+ ions excited at 795 nm wavelength could be considered by a combination of two mechanisms - excited state absorption (ESA) and energy transfer (ET) [71, 72]. Under pumping by 795 nm, the 4f electrons in Er3+ ions are excited to the 4I9/2 level through ground state absorption (GSA) process, and then the excited electrons decay to the 4I11/2 and 4I13/2 levels due to multiphonon relaxation (MPR) process. The excited ion can absorb further a second photon and be promoted to the 4F5/2 state by the ESA process (ESA1). Similarly, the 2H11/2 level is populated by absorption of the pump photon in the 4I13/2 state (ESA2). There is also a possibility that ET mechanism contributes to populate the 2H11/2 level. In this process, two Er3+ ions are excited by 795 nm directly to the 4I9/2 level, then an energy transfer occurs between them by which one ion loses energy and the other is promoted to the 2H11/2 level (label ET1). The initial electronic levels of the two radiative transitions observed in the green range of the spectrum are 4S3/2 and 2H11/2 levels, respectively. Consequently, they are thermally coupled by multiphonon transitions and the Boltzmann distribution predicts a probability of about 10% for an excited ion in one of these two states to be actually in the higher 2H11/2 level. Thus, the transition 4S3/2→4I15/2 (554 nm PL band) is dominating although the transition 2H11/2→4I15/2 (530 nm PL band) turns out to have a larger emission cross-section [73]. There is also a possibility of ET2 process to populate 4F9/2 level.

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Figure 24. Up-conversion luminescence spectra of samples 1, 2, 3 and 4 listed in Table 3. The curves are shifted vertically to facilitate the comparison. 4F 3/2 4F 5/2

20

ESA1

4F 7/2

ESA2 ET1

2H 11/2

Energy (103 cm-1)

4S 3/2

15

4F 9/2

ET2 4I 9/2

10

4I 11/2

4I 13/2

0

644 nm

554 nm

530 nm

795 nm

5

4I 15/2

Figure 25. Schematic energy-level diagram of the up-conversion emission in Er3+ ions. The up-conversion excitation processes (ESA1, ESA2), energy transfer (ET1, ET2) and corresponding fluorescence emission processes (530, 554 and 644 nm) are indicated by arrows. Upwards arrows indicate pump photons and downward arrows stand for fluorescence emission. Non-radiative transitions are indicated by waved arrows.

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The energy separation between 4S3/2 level and the next lower 4F9/2 one is 3000 cm-1. So, multiphonon relaxation from 4S3/2 level is not much effective due to this energy gap and low phonon energy (below 400 cm-1) in chalcohalide glasses. Consequently, the accumulation of population at this state contributes to the strong emission at 554 nm. The red PL band at 644 nm requires population of the 4F9/2 level, which can be populated by ESA1 and ESA2 processes, following by non-radiative decay from 4F5/2 and 4 S3/2 levels. ET processes are generally less effective compared to ESA ones since they require participation of two separated Er ions. Consequently, the radiative transition 4F9/2→4I15/2 at 644 nm is weak as compared to those in the green range.

4. DISCUSSION As stated above, on the basis of the experimental data from absorption and emission spectra of a series of Er-doped GeS2-Ga2S3 glasses, the observed decrease both in the absorption coefficient and host luminescence in the range of 1120-1460 nm confirms the assumption that the presence of erbium may reduce the number of native defects due to network disorder [29]. Thus, the deep electronic states in the energy gap which are responsible for the observed host photoluminescence are effectively removed at higher Er-doping levels. This is manifested by the quenching of the corresponding broadband luminescence of the host, as has been found to be realized at concentrations exceeding 2.1 mol% Er2S3 for the (GeS2)80(Ga2S3)20 glasses. Raman scattering research in the extended range of Er-doping of these glasses has shown [39] that the introduction of Er modifies the local structure by converting the edge-shared GaS4 tetrahedra into corner-shared ones with non-bridged sulfurs, which are charge compensated by Er3+ ions. This feature predetermines the larger enhanced Er3+ solubility namely by introduction of Ga into Ge-S glass, which is connected with a significant modification of the local structure and its improvement.

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The design of erbium doped broadband amplifiers requires the evaluation of the strongest PL band at ~1.54 µm in more detail as the most important due to the 4I15/2→4I13/2 radiative transition in Er3+ intra-4f emission at the same standard telecommunications wavelength. The obtained basic radiative and spectroscopic parameters from the evaluation of absorption and emission cross-sections at ~1540 nm, such as Judd-Ofelt intensity parameters, radiative lifetime, FWHM, stimulated emission crosssection, etc., are important for possible photonic applications [35, 36]. It has been found that the direct excitation of Er3+ ions at 982 nm produces broader PL band (FWHM is ~70 nm), while the host excitation at 532 nm results in a narrow band with FWHM ~24 nm [29]. The observed considerable decrease of PL intensity at 2.4 mol% Er2S3 can be leading to nonuniform distribution of Er atoms and probably to Er clustering, which causes the quenching effect of the Er3+ emission [74]. When the Er concentration exceeds a threshold limit, the mutual interaction among Er 3+ ions becomes significant and influences the carrier lifetime assigned to 4finner shell electronic levels characterizing the Er3+ state. In this way the changes in the transition rates of intra-center electronic transitions induced by increasing the Er content will be manifested in the intensity of the PL spectra. In other words, this concentration mechanism follows the simple proportion - the higher the Er content, the higher the PL peak intensity, which means that the emission is exhibited by more optically active Er 3+ ions at higher erbium content. On the other hand, it has been found [44] that PL intensity in pseudo-binary GeS2-Ga2S3 glasses increases with Ga content as more Er3+ ions become activated until 33 mol% Ga2S3 is reached. However, the quenching effect is reached at the higher doping concentration of 2.4 mol% Er2S3 in this case and the shoulder to the main peak is developed as an additional maximum and enlargement of the emission cross-section due to the enhanced charge compensation of the [GaS4]1- “defect” tetrahedra by Er3+ ions. Usually the emission bands related to the optical transitions in the energy-level diagram of Er3+ ions are strongly intensified at low temperatures. As a general tendency, it is accompanied by a considerably enhanced narrowing effect of all bands. Since the used 514.5 nm excitation

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wavelength corresponds to the host absorption, the observed emission in the studied glasses is due to the absorption of light by the glass host and the subsequent transfer of this energy to the Er3+ center. The development of the photoluminescence bands with increasing Er3+ content and temperature decreasing from room temperature down to 4.2 K is clearly pronounced [52-55, 75]. The observed PL narrowing effect especially for the strongest band at ~1550 nm is further accompanied by the resolution of well-expressed fine structure due to “crystal field” splitting of corresponding electronic terms. The role of Ga content into the glassy host on the PL line-shape and luminescence efficiency under excitation with 1064 nm has been specified for heavily doped (1.8, 2.1 and 2.4 mol% Er2S3) compositions in the temperature range from 300 K down to 10 K [56]. The considerable influence of GeGaS host composition on the efficiency of 4f-4f transitions of embedded Er3+ ions is documented with the outcome that (GeS2)75Ga2S3)25 composition appears near optimal for the emission efficiency of Er3+ ions, exhibiting all possible eight PL bands even at the lower doping level of 1.8 mol% Er2S3. Obviously, the number of metal Ga-Ga bonds increases at 33 mol% Ga2S3, which is a sign for a nonuniform distribution of the doping atoms in the glassy matrix and consequently leads to limited PL signals, together with the corresponding inner shell radiative transitions according to the Er3+ energy-level diagram [30]. Besides, the temperature dependence for two host glasses containing 25 and 33 mol% Ga2S3 with various Er-doping reveals different influence of Er-doping on PL intensity. This behavior reflects the fact that the rate of increase of PL intensity with decreasing temperature depends on both the Ga content in the host and on Er-doping. This “double dependence” is manifested at room temperature for the glass composition at 25 mol% Ga2S3 where the PL intensity exhibits inverted dependence on Er-doping. This behavior is not observed for temperatures below 80 K, and/or for the glass composition at 33 mol% Ga2S3. It should be mentioned that at room temperature the measured photoluminescence shows optimal efficiency for the 2.4 mol% Er2S3-doped (GeS2)67(Ga2S3)33 composition [44]. The NIR LTPL spectra at 4.2 K of the studied Er-doped (GeS2)80(Ga2S3)20 glasses exhibit sharp transitions at 980 nm (4I11/24I15/2)

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and 1530 nm (4I13/24I15/2) together with the broad band centred at ~1200 nm [60,61,68]. This broad PL band, due to electronic transitions in the host glass, is found to decrease steadily with increase of Er doping. In addition, a narrow dip at 1520 nm corresponding to the electronic up-transition 4 I15/24I13/2 is observed, which can be considered as a Stokes shifted absorption band related to the strongest Er3+ emission at 1530 nm [61]. The obtained increase both of optical energy gap and decay time in the studied Er-doped chalcohalide (GeGaS)90(CsBr)10 glasses indicates that the addition of CsBr should result in more efficient up-conversion as compared to GeGaS system. Indeed, the emission bands at ~530, 554 and 644 nm in the visible range excited with 795 nm wavelength manifest efficient upconversion processes [67]. The peak at 530 nm is resolved into two Stark splitting components, which is better pronounced at the higher Er doping level for the host with 10 at% Ga. Besides, this host also exhibits greater red band intensity at 644 nm as compared to that with 12 at% Ga. As reported previously [21], an improvement on radiative properties of Er3+doped GeGaS-CsBr glasses is observed when the [CsBr/Ga] ratio in the glass is equal to or greater than unity. EXAFS studies have shown that Br ions form Ga-Br bonds are not associated with Ge. In this case, new [GaS3/2Br]− s. u. are formed by substituting sulfur with the added bromine in the GaS4/2 tetrahedra and Er3+ ions are located near them to compensate for the charge balance. Maintaining the ratio [CsBr/Ga] ≥ 1, Er3+ ions are surrounded by non-bridging Br ions in the [GaS3/2Br]− tetrahedra. The major phonon mode controlling the multiphonon relaxation then changes to the Ga-Br bond vibration from that of Ga-S, which leads to an improvement in the radiative properties of Er3+ ions. At [CsBr/Ga] ≤ 1 there is not enough Br, and Ga partially remains as GaS4/2 tetrahedra, which prevents the enhancement of radiative properties. The PL efficiency of the corresponding 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions of Er3+ ions in the green and red spectral regions does not show dependence on the Ga content, probably because of its limited variation from 10 to 12 at% [67]. The simplified energy-level diagram of Er3+ ion has been proposed to identify the observed up-conversion luminescence by

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the combination of two mechanisms - excited state absorption (ESA1, ESA2) and energy transfer (ET1, ET2).

CONCLUSION This chapter gives an overview of the fundamental optical investigations of chalcogenide (GeS2)100−x (Ga2S3)x glasses (x = 20, 25 and 33 mol%), doped with Er2S3 from 0.3 to 2.7 mol%. A special attention has been attributed to the Er3+ intra-4f emission at ~1540 nm as the most important due to the 4I15/2→4I13/2 radiative transition at the same standard telecommunications wavelength. The corresponding PL band has been studied as a function of excitation, wavelength, Er-doping level and Ga content in the host in order to reach the conditions of optimal luminescence efficiency. The fine structure of this band has been considered by the deconvolution into Gaussians according to the Stark splitting of the 4I13/2 and 4I15/2 energy levels. The main radiative and spectroscopic parameters on the basis of Judd-Ofelt analysis have been determined. The changes in the line-shape of the emission cross-section with the temperature decrease have been specified as well. The up-converted fluorescence of Er-doped (GeSGa)90(CsBr)10 glasses is evaluated and a schematic energy-level diagram of the up-conversion emission in Er3+ ions has been proposed.

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[37] Yang, Y. M., Yao, B. Q., Chen, B. J., Wang, C., Ren, G. Z., Wang, X. J., (2007). Judd-Ofelt analysis of spectroscopic properties of Tm3+, Ho3+ doped GdVO4 crystals, Opt. Mater. 29, 1159-1165. [38] Weber, M. J., (1982). Fluorescence and glass lasers, J. Non-Cryst. Solids 47, 117-133. [39] Ivanova, Z. G., (2007). Photoluminescence and local structure in GeS2-Ga2S3–Er2S3 glasses, J. Optoelectron. Adv. Mater. 9 (10), 31493152. [40] Fick, J., Knystautas, E. J., Villeneuve, A., Schiettekatte, F., Roorda, S., Richardson, K. A., (2000). High photoluminescence in erbiumdoped chalcogenide thin films, J. Non-Cryst. Solids 272, 200-208. [41] Tverjanovich, A., Tver’yanovich, Yu. S., Loheider, S., (2001). Raman spectra of gallium sulfide based glasses, J. Non-Cryst. Solids 208, 49-55. [42] Wang, X. F., Gu, S. X., Yu, J. G., Zhao, X. J., Tao, H. Z., (2004). Structural investigations of GeS2-Ga2S3-CdS chalcogenide glasses usung Raman spectroscopy, Solid State Commun. 130 (7), 459-464. [43] Ivanova, Z. G., Aneva, Z., Cernosek, Z., Cernoskova, E., Vassilev, V. S., (2003). Influence of Ga on the luminescence efficiency of Erdoped Ge-S-Ga glasses, J. Mater. Sci. 14, 761-762. [44] Ivanova, Z. G., Cernoskova, E., Cernosek, Z., Vlcek, Mil., Features in the photoluminescence line-shape of heavily Er-doped Ge-S-Ga glasses, J. Non-Cryst. Solids 355, 1873-1876. [45] Tonchev, D., Koughia, K., Ivanova, Z. G., Kasap, S. O. (2007). Thermal and optical properties of erbium doped (GeS2)75(Ga2S3)25 glasses, J. Optoelectron. Adv. Mater. 9 (2), 337-340. [46] Mattarelli, M., Montagna, M., Zampedri, L., Chiasera, A., Ferrari, M., Righini, G. C., Fortes, L. M., Goncalves, M. C., Santos, L. F., Almeida, R. M., (2005). Self-absorption and radiation trapping in Er3+-doped TeO2-based glasses, Europhys. Lett. 71 (3), 394-399. [47] Munzar, M., Koughia, K., Kasap, S. O., Haugen, C., Decorby, R., McMullin, J. C., (2006). Photoluminescence properties of Er-doped Ge-Ga-Se glasses, Phys. Chem. Glasses 47 (2), 220-224.

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[48] Ivanova, Z. G., Koughia, K., Tonchev, D., Pivin, J. C., Kasap, S. O., (2005). Photoluminescence in Er-implanted amorphous Ge-S-Ga thin films, J. Optoelectron. Adv. Mater. 7 (3), 1271-1276. [49] Maeda, K., Ikuta, J., Arima, T., Sakai, T., Ikari, T., Munzar, M., Tonchev, D., Kasap, S. O., (2006). Effect of thermal annealing on the photoluminescence of Er doped Ge-Se-Ga glasses, Physics and Chemistry of Glasses: European Journal of Glass Science and Technology B 47 (2), 189-192. [50] Koteeswara Reddy, N., Devika, M., Prashantha, M., Ramesh, K., Ivanova, Z. G., Zavadil, J., (2013). Tailoring the optical properties of amorphous heavily Er3+-doped Ge-Ga-S thin films, J. Optoelectron. Adv. Mater. 15 (3-4), 182-186. [51] Fu, J., Shen, X., Wang, G., Nie, Q., Chen, F., Li, J., Zhang, W., Dai, S., Xu, T., (2012). Structure and optical properties of Ge-Ga-S films deposited by thermal evaporation, Physica B: Condensed Matter. 407 (12), 2340-2343. [52] Ivanova, Z. G., Aneva, Z., Ganesan R., Tonchev, D., Gopal, E. S. R., Rao, K. S. R. K., Allen, T. W., DeCorby, R. G., Kasap, S. O., (2007). Low-temperature Er3+ emission in Ge-S-Ga glasses excited by host absorption, J. Non-Cryst. Solids 353, 1418-1421. [53] Ivanova, Z. G., Ganesan, R., Aneva, Z., Gopal, E. S. R., (2005). Influence of temperature on the photoluminescence efficiency of chalcogenide GeS2-Ga2S3-Er2S3 glasses, Materials Science and Engineering B 122, 152-155. [54] Ivanova, Z. G., Tonchev, D., Ganesan, R., Gopal, E. S. R., Kasap, S. O., (2005). Temperature-dependent photoluminescence in Er-doped Ge-S-Ga glasses, J. Optoelectron. Adv. Mater. 7 (4), 1863-1867. [55] Ivanova, Z. G., Ganesan, R., Adarsh, K. V., Vassilev, V. S., Aneva, Z., Cernosek, Z., Gopal, E. S. R., (2005). Low-temperature luminescence quenching and local ordering study of Er-doped Ge-SGa glasses, J. Optoelectron. Adv. Mater. 7 (1), 345-348. [56] Ivanova, Z. G., Zavadil, J., Rao, K. S. R. K., (2011). Compositional trends in low-temperature photoluminescence of heavily Er-doped GeS2-Ga2S3 glasses, J. Non-Cryst. Solids 357, 2443-2446.

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[57] Kolomiets, B. T., Mamontova, T. N., Babaev, A. A., (1970). Radiative recombination in vitreous and single crystal As2S3 and As2Se3, J. Non-Cryst. Solids 4, 289-294. [58] Street, R. A., (1976). Luminescence in amorphous semiconductors, Adv. Phys. 25 (4), 397-454. [59] Tikhomirov, V. K., Iakoubovskii, K., Hertogen, P. W., G. J. Adriaenssens, G. J., (1997). Visible luminescence from Pr-doped sulfide glasses, Appl. Phys. Lett. 71 (19), 2740-2742. [60] Zavadil, J., Kostka, P., Pedlíková, J., Ivanova, Z. G., Žďánský, K., (2010). Investigation of Ge based chalcogenide glasses doped with Er, Pr and Ho, J. Non-Cryst. Solids 356, 2355-2359. [61] Kostka, P., Zavadil, J., Iovu, M., Ivanova, Z. G., Furniss, D., Seddon A. B., (2015). Low-temperature photoluminescence in chalcogenide glasses doped with rare-earth ions, J. Alloys Compounds 648, 237243. [62] Gu, S. Q., Ramachandran, S., Reuter, E. E., Thurnbull, D. A., Verdeyen, J. T., Bishop, S. G., (1995). Novel broad-band excitation of Er3+ luminescence in chalcogenide glasses, Appl. Phys. Lett. 66, 670-672. [63] Thurnbull, D. A., S. G. Bishop, S. G., (1998). Rare earth dopants as probes of localized states in chalcogenide glasses, J. Non-Cryst. Solids 223, 105-113. [64] Thurnbull, D. A., Aitken, B. G., Bishop, S. G., (1999). Broad-band excitation mechanism for photoluminescence in Er-doped Ge25Ga1.7As8.3S65 glasses, J. Non-Cryst. Solids 244, 260-266. [65] Bishop, S. G., Turnbull, D. A., Aitken, B. G., (2000). Excitation of rare earth emission in chalcogenide glasses by broadband Urbach edge absorption, J. Non-Cryst. Solids 266-269, 876-883. [66] Ivanova, Z. G., Koughia, K., Soundararajan, G., Heo, J., Tonchev, D., Jayasimhadri, M., Kasap, S. O., (2009). The influence of CsBr addition on optical and thermal properties of GeGaS glasses doped with erbium, J. Mater Sci: Mater Electron. 20: S421-S424.

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[67] Ivanova, Z. G., Jayasimhadri, M., Heo, J., Zavadil, J., (2010). Upconversion fluorescence and low-temperature emission in Er3+-doped GeGaS-CsBr glasses, J. Non-Cryst. Solids 356, 2393-2396. [68] Ivanova, Z. G., Zavadil, J., Kostka, P., Reinfelde, M., (2017). Photoluminescence properties of Er-doped Ge-In(Ga)-S glasses modified by caesium halides,” Phys. Status Solidi B 254 (6), pp. 1600662 (1-6). [69] Gu, S. Q., Ramachandran, S., Reuter, E. E., Thurnbull, D. A., Verdeyen, J. T., Bishop, S. G., (1995). Photoluminescence and excitation spectroscopy of Er-doped As2S3 glass: Novel broad band excitation mechanism, J. Appl. Phys. 77, 3365-3371, and references therein. [70] Z. G. Ivanova, E. Cernoskova, Z. Cernosek, V. S. Vassilev, (2002). Optical properties of (GeS2)80-x (Ga2S3)x:Er2S3 glasses, Proc. XIII Intern. Symp. on Non-Oxide Glasses and New Optical Glasses, Pardubice (Czech Republic), Vol. 2, 541-544. [71] da Silva, C. J., de Araujo, M. T., (2003). Thermal effect on upconversion fluorescence emission in Er3+-doped chalcogenide glasses under anti-Stokes, Stokes and resonant excitation, Opt. Mater. 22, 275-282. [72] Xu, Y., Chen, D., Zhang, Q., Wang, W., Zeng, H., Shen, C., Chen, G., (2009). Two-photon excited red upconversion luminescence of thulium ions doped GeS2-In2S3-CsI glass, Chem. Phys. Lett. 472, 104-106. [73] Zou, X., Izumitani, T., (1993). Spectroscopic properties and mechanisms of excited state absorption and energy transfer upconversion for Er3+-doped glasses, J. Non-Cryst. Solids 162, 68-80. [74] Auzel, F., Goldner, P., (2001). Towards rare-earth clustering control on doped glasses, Opt. Mater. 16, 93-103. [75] Iovu, M., E. Lupan, E., Zavadil, J., Kostka, P., Ivanova, Z. G., Seddon, A. B., Furniss, D., (2015). Photoluminescence of some chalcogenide glasses doped with rare-earth ions, Proc. of SPIE Vol. 9258, pp. 925805 (1-6).

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BIOGRAPHICAL SKETCH

Assoc. Prof. Dr. Eng. Zoya G. Ivanova Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria E-mail: [email protected] Education M. Sci., Eng., Thesis “Study on semiconducting Ge-Se-In glasses,” Higher Institute of Chemical Technology, Department of Semiconductors, Sofia PhD in Chemistry, Thesis “Determination of the glass-forming regions in the Ge-S-Ga and Ge-S-In systems and investigations of their properties,” Higher Institute of Chemical Technology, Department of Semiconductors, Sofia Research and Professional Experience:  

Photoluminescence of Er-doped Ge-S-Ga glasses Photoinduced phenomena in chalcogenide glasses

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Zoya G. Ivanova     

Physicochemical studies of chalcogenide glasses Optical properties of chalcogenide glasses Local structure of chalcogenide glasses Glass formation in GeS-based systems Applications of chalcogenide glasses

Professional Appointments Associate Professor at the Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia Joint collaboration     

University of Pardubice, Czech Republic – Assoc. Prof. Eva Cernoskova Institute of Photonics and Electronics AS CR, Prague – Dr. Jiri Zavadil University of Saskatchewan, Saskatoon, Canada – Prof. Safa Kasap Institute of Science, Bangalore, India – Prof. E.S.R. Gopal University of Science and Technology, Pohang, Korea – Prof. Jong Heo

Selected publications for Er-doped Ge-S-Ga glasses 1. Z. G. Ivanova, E. Cernoskova, Z. Cernosek, V. S. Vassilev, “Optical properties of (GeS2)80-x(Ga2S3)x:Er2S3 glasses”, Proc. XIII Intern. Symposium on Non-Oxide Glasses and New Optical Glasses, Pardubice (Czech Republic), 2002, Vol. 2, p. 541. 2. Z. G. Ivanova, V. S. Vassilev, E. Cernoskova, Z. Cernosek, “Physicochemical, structural and fluorescence properties of Erdoped Ge-S-Ga glasses”, J. Phys. Chem. Solids, 64 (2003) 107.

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3. Z. G. Ivanova, Z. Aneva, Z. Cernosek, E. Cernoskova, V. S. Vassilev, “Influence of Ga on the luminescence efficiency of Erdoped Ge-S-Ga glasses”, J. Mater. Sci., 14 (2003) 761. 4. Z. G. Ivanova, R. Ganesan, Z. Aneva, E. S. R. Gopal, “Influence of temperature on the photoluminescence efficiency of chalcogenide GeS2-Ga2S3-Er2S3 glasses”, Mater. Sci. Eng. B, 122 (2005) 152. 5. Z. G. Ivanova, R. Ganesan, K. V. Adarsh, V. S. Vassilev, Z. Aneva, Z. Cernosek, E. S. R. Gopal, “Low-temperature luminescence quenching and local ordering study of Er-doped GeS-Ga glasses”, J. Optoelectron. Adv. Mater., Vol. 7, No 1 (2005) 345. 6. Z. G. Ivanova, K. Koughia, Z. Aneva, D. Tonchev, V. S. Vassilev, S. O. Kasap, “Photoluminescence of Er3+ ions in (GeS2)80(Ga2S3)20 glasses”, J. Optoelectron. Adv. Mater., Vol. 7, No 1 (2005) 349. 7. Z. G. Ivanova, K. Koughia, D. Tonchev, J. C. Pivin, S. O. Kasap, “Photoluminescence in Er-implanted amorphous Ge-S-Ga thin films”, J. Optoelectron. Adv. Mater., Vol. 7, No 3 (2005) 1271. 8. Z. G. Ivanova, D. Tonchev, R. Ganesan, E. S. R. Gopal, S. O. Kasap, “Temperature-dependent photoluminescence in Er-doped Ge-S-Ga glasses”, J. Optoelectron. Adv. Mater., Vol. 7, No 4 (2005)1863. 9. Z. G. Ivanova, Z. Aneva, R. Ganesan, D. Tonchev, E. S. R. Gopal, K. S. R. K. Rao, T. W. Allen, R. G. DeCorby, S. O. Kasap, “Lowtemperature Er3+ emission in Ge-S-Ga glasses excited by host absorption”, J. Non-Cryst. Solids, 353 (2007) 1418. 10. Z. G. Ivanova, Z. Aneva, K. Koughia, D. Tonchev, S. O. Kasap, “On the optical absorption and photoluminescence of Er-doped Ge-S-Ga glasses”, J. Non-Cryst. Solids, 353 (2007) 1330. 11. Z. G. Ivanova, E. Cernoskova, Z. Cernosek, “Er-doped Ge-S-Ga glasses: photoluminescence and thermal properties”, J. Phys. Chem. Solids, 68 (2007) 1260.

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Zoya G. Ivanova 12. Z. G. Ivanova, “Photoluminescence and local structure in GeS2Ga2S3-Er2S3 glasses”, J. Optoelectron. Adv. Mater., Vol. 9, No 10 (2007) 3149. 13. Z. G. Ivanova, K. Koughia, G. Soundararajan, J. Heo, D. Tonchev, M. Jayasimhadri, S. O. Kasap, “The influence of CsBr addition on optical and thermal properties of GeGaS glasses doped with erbium”, J. Mater. Sci.: Materials in Electronics, 20, No1 (2009) S421. 14. Z. G. Ivanova, M. Jayasimhadri, M. Kincl, “Optical properties of Er-doped GeS2-Ga2S3 glasses”, J. Optoelectron. Adv. Mater., Vol. 11, No 9 (2009) 1269. 15. Z. G. Ivanova, E. Cernoskova, Z. Cernosek, Mil. Vlcek, “Features in the photoluminescence line-shape of heavily Er-doped Ge-S-Ga glasses”, J. Non-Cryst. Solids, 355 (2009) 1873. 16. Z. G. Ivanova, M. Jayasimhadri, Jong Heo, J. Zavadil, “Upconversion fluorescence and low-temperature emission in Er3+doped GeGaS-CsBr glasses”, J. Non-Cryst. Solids, 356 (2010) 2393. 17. Z. G. Ivanova, J. Zavadil, K. S. R. K. Rao, “Compositional trends in low-temperature photoluminescence of heavily Er-doped GeS2Ga2S3 glasses”, J. Non-Cryst. Solids, 357 (2011) 2443. 18. Zoya G. Ivanova, Jiri Zavadil, Petr Kostka, Mara Reinfelde, “Photoluminescence properties of Er-doped Ge-In(Ga)-S glasses modified by caesium halides”, Phys. Status Solidi B, Vol. 254, No 6 (2017) 1600662.

Bulgarian Patents 1. Z.G. Ivanova, E. Vateva, D. Arsova, “Inorganic photoresist”, 1983, No 35877. 2. Z.G. Ivanova, N. Kirov, D. Kolev, "Material for infrared optical element", 1986, No 42809.

In: Photoluminescence Editor: Ellis Marsden

ISBN: 978-1-53613-537-4 © 2018 Nova Science Publishers, Inc.

Chapter 2

RARE-EARTH ACTIVATED GLASSES IN INTEGRATED OPTICAL DEVICES WITH DIFFERENT GEOMETRIC SHAPES: FIBERS, PLANAR WAVEGUIDES AND MICROSPHERES Helena C. Vasconcelos1,2,* and Afonso Silva Pinto1 1

Faculty of Sciences and Technology, Azores University, Portugal 2 Centre of Physics and Technological Research (CEFITEC), FCT/UNL, Portugal

ABSTRACT Glasses activated by rare-earths in optical devices is a potentially important area of research in developing effective optical means for the propagation of light and luminescence enhancement. Applications involving optical glasses have grown considerably in recent years focused *

Corresponding Author: Azores University (FCT), Rua da Mãe de Deus, 9500-321 Ponta Delgada, Açores, Portugal; Email:[email protected].

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Helena C. Vasconcelos and Afonso Silva Pinto on the development of new shapes of optical devices and improved glass compositions, but also in new processes for their preparation. The confinement of light in small objects has become an essential requirement for optical devices. Examples of this trend are provided by optical fibers, optical integrated circuits (planar waveguides or slab) and, more recently, optical dielectric resonators supporting the Whispering Gallery Modes (glass microspheres). To demonstrate the technological importance of optical devices with unique properties exhibited by glasses activated by rare earths, we review some functional aspects of such devices, where light is confined in different volumes or shapes, particularly fibers, monoliths, films/coatings and microspheres. Glasses activated by rareearths are often used as components in optical integrated circuits for optical communication and sensor applications. Optical waveguides (planar, strip, or fiber waveguides) and spherical microresonators are some current examples of integrated glass-based optical devices.

Keywords: optical glasses, sol-gel, rare-earths, fibers, microspheres, planar waveguides

INTRODUCTION In recent years, fully-optical networks have impacted enormously the long-distance telecommunications, replacing the electrical components by their optical counterparts. Thus, modern information systems, telecommunications and instrumentation depend significantly on optical and optoelectronic devices capable of processing signals efficiently in many applications [1-4]. The growing exploitation of optics, particularly in signal processing and new materials, has created an increasing need for integration of optical and electronic devices of both low cost and high performance. Integrated optics systems must have a light source that generates the signal and active and passive components, all together on a single platform, namely: waveguides to conduct the signal, repeaters to boost and directional couplers to connect and route the signal, detectors to capture de delivered signal and electro-optical modulators, filters, multiplexers and switches to control these signals [5, 6]. Optical waveguide concept is based on the electromagnetic nature of light and its

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interaction with matter [5]. In the integrated optical circuits, all components are made in compact, with low power consumption and easily connectable. The planar waveguides doping with erbium (Er3+), in turn, allows the manufacture of active devices such as lasers and signal amplifiers in integrated optics [7, 8]. The first glass fibers were developed in the 60s. Due to their qualities, especially the high performance in the transmission of huge amounts of data over long distances, low attenuation, low interference and longer service life, fiber optics have changed the paradigm of telecommunications; the efficient exploitation of bandwidth offered by fiber-optic communications contributed greatly to the considerable advances in this area. The invention of the laser, also in the 60s, gave rise to a new field of physics known as optoelectronics, which has grown exponentially in the following decades. Moreover, it was found that the laser would be useful for transmitting information along glass fibers in a manner analogous to electrical signals sent along copper wires. Nowadays many important advances in integrated optics have been made possible by the use of devices such as wavelength-division (de)multiplexers (WD(D)M), Mach-Zehnder interferometers, switches and splitters, just to name a few [9, 10]. All of these devices are required to handle the ultrafast optical signal without the need to convert it into electronic signals and then back to the optical domain. On the other hand, active devices, like rare-earth doped glasses, have been developed for important applications in optical amplifiers for telecommunication systems, mainly Er3+-doped glass fiber amplifiers and solid state lasers. These devices also display additional advantages like small size, low weight and volume, higher bandwidth, immunity to electromagnetic interference, and low power requirements [8]. While Moore’s Law says that the number of transistors per chip doubles every 18 months, the amount of data coming out of an optical fiber is doubling every 9 months [11]. Therefore, it seems that guiding, processing and control optical information takes place faster and faster. That’s why this technology and integrated optics become increasingly important. Nowadays, integrated optics combines light confinement with novel optical properties were the basis is the optical planar (or slab)

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waveguide. So, the less complex optical waveguide is perhaps the planar waveguide that consists of a thin dielectric film inserted between materials of slightly lower refractive indices [4, 12]. For a complete handling and full control of the signal, an integrated optical circuit must ensure all necessary linear optical functions (waveguides, modulators, switches and interconnected passive devices) and optically-nonlinear devices [3, 13]. Applications involving optical glasses have grown considerably in recent years with an intense level of research focused on the development of new shapes of optical devices and improved glass compositions, but also on new processes for their preparation [12]. Light confinement into tiny objects has become an essential requirement for optical devices. Examples of this tendency are provided by optical fibers, integrated optical circuits (planar or slab waveguides) [12, 14] and more recently optical dielectric resonators supporting Whispering Gallery Modes (e.g., glass microspheres) [15-17]. Integrated optics is the field in which photonics and glass science cooperate in a synergistic way to join physical and optical phenomena, to create new devices and innovative technological applications [18, 19]. Glass materials, such as oxides, fluorides, tellurides and glass boron-based microspheres and photonic structures are in the core of the scientific and technological expansion of integrated optics. Photonic glasses, optical glass waveguides and resonant microspheres are examples of optical devices based on glass, which greatly contribute to optical communication, photonics and signal processing [4]. Doping of glasses with rare-earths therefore enables these materials to exhibit special optical properties, in particular in the field of optoelectronics, both in integrated optical circuits and in the field of sensors [14]. It is therefore the handling of information by optical means and modeling of this information into light rays carrying it into glasses. information travels inside the glass at the speed of light [19, 20]. If a process is photonic it is because it involves photons (light/electromagnetic radiation) and the photons are transmitted through the glass from one point to another. In the 90s, much of the research in photonics was focused on planar waveguides. More recently, great attention was given to the development of spherical microresonadores, a

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very interesting class of photonic structures confined [16]. Therefore, the confinement of light in small [16]. It is therefore not surprising that the confinement of light in small volumes (glass microspheres) is in the base of the development of highly accurate optical sensors and biosensors [21].

SILICA-ON-SILICON INTEGRATED OPTICS The basic physical principle of integrated optics [1, 2], is that electromagnetic energy at optical frequencies can be confined and guided through channels of optical materials on a single substrate. The successful results on optical fibers for long distance transmissions has move toward planar waveguides and, so, low loss fibers with high bit carrying capacity and requirements to miniaturized optical devices, stimulated the progress of integrated optics [8, 14]. However, although this concept was first introduced by Miller in 1969 [22], it took a few decades until the photons, instead of the electrons, dominated the development of integrated circuits. Today integrated optics is almost 50 years old. Sizeable efforts over the years resulted in several advanced materials, technologies and device types that have giving rise to efficient optical configurations and shapes, some converted to current commercial products. Semiconductor systems, namely gallium arsenide/ gallium aluminium arsenide and indium phosphate/indium gallium arsenide phosphide (GaAs/GaAlAs and InP/InGaAsP), ferroelectric materials like lithium tantalate and lithium niobate (LiTaO3, LiNbO3) [3, 4, 23, 24] and glasses such as SiO2 were widely used for fabrication of integrated circuits for optoelectronic applications. LiNbO3 has been extensively used due to it some specific advantages, such as low losses, high electro-optical effect, which allows changes in the refractive index by the applied electric field, as well as relative ease in the manufacture of a surface waveguide by diffusion of Ti [3]. The qualities of the LiNbO3 substrate for manufacturing integrated optical devices have been demonstrated in the integration of modulators and electro- and acoustic-optical switches, diffraction gratings and non-linear optical frequency converters [3]. Moreover, semiconductors

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are among the main materials for manufacturing active devices for optical integration [25]. In particular, III-V semiconductors have been developed for optoelectronic applications [25]. However, their basic construction requires a complex multilayer epitaxial growth, usually by liquid phase epitaxy or metal organic vapor phase epitaxy, two processes difficult to achieve manufacturing speeds required for serial production, considering the large thicknesses (up to tens of m) required for guided wave optical devices. Glasses, on the other hand, are very attractive materials for employing optical passive, active and non-linear devices due to their specific physical and chemical properties (high thermal, chemical and mechanical stability), particularly their optical performance, which highlights the excellent transparency and consequently low loss and high threshold to optical damage [12, 19, 26-27]. Silica (SiO2) glasses are widely used because they are well matched to the optical fibers, presenting slight coupling losses. Systems based on glass materials and suitable fabrication and deposition techniques, such as the sol-gel deposition by dip or spin coating, permit the production of thick films of desired composition to be functionally integrated with electronic devices on a semiconductor substrate such as silicon (Si) [28-30]. Si is a prime material in the electronic industry and still widely used in the well-known Si-based integrated optoelectronics [31]. Nowadays, planar optical waveguides, as well as channel waveguides of diverse geometries, are fabricated on materials like SiO2 (e.g., silica-on-silicon), Si (e.g., silicon-on-insulator), LiNbO3 and GaAs (or other III-V materials) [3, 25, 28]. In particular, the “silica-on-silicon” technology is used in the field of photonic glasses, where SiO2 glass-based waveguides are fabricated on Si substrates allowing the joint integration of optical devices and the basic electronic substrate platforms. When comparing the substrates of Si with those of InP or GaAs, the Si ones are usually larger in size, allowing the realization of multifaceted optical circuits and fabrication processes such as photolithography, etching (e.g., porous Si), film deposition, etc. [28, 30, 31]. In addition, its monocrystalline structure allows it to be easily cleaved but not deformed. Therefore, Si substrates have enough strength to be used

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efficiently as motherboards for device integration by silica-on-silicon technology [32, 33], which is an attractive approach over the conventional systems of Ti in-diffused lithium niobate (Ti:LiNbO3), gallium-arsenidebased semiconductors (GaAs/AlGaAs), semiconductor materials based on alloys of gallium arsenide and indium phosphide (InGaAsP/InP) or ion exchanged glasses [34]. A typical fabrication process of silica-on-silicon is illustrated in Figure 1. A thick buffer layer, usually SiO2, is deposited on a Si wafer (substrate) in order to isolate the planar waveguide from the Si wafer, which has a high refractive index, nD ~3.86 (for the yellow sodium D-line ( = 589.3 nm) at 20 oC). Then a glassy waveguide film (denoted as wg) with a refractive index higher than that of SiO2 buffer (nD ~1.46) is deposited (Figure 1-a). The fabrication of SiO2 channel waveguides frequently employs microlithography and reactive ion-etching (RIE) [35]. An easier method to obtain channel waveguides and gratings is the direct writing of photosensitive glass with UV light [36]. It is known that during UVtreatment of germanosilicate optical fibers and glasses leads to a refractive index change. This method, usually referred as photosensitivity, is associated with oxygen-deficient GeO2 defect centers and has been widely investigated in optical fibers for production of Bragg gratings [37]. Regarding planar devices, high germania (GeO2) concentrations and hydrogenation [38] are often used to provide appropriate photosensitivity. Recently, femtosecond laser writing was used in LiNbO3 waveguides [39]. The ion-exchange technique is used to obtain optical channel waveguides in a glass substrate, were silver ions from a AgNO3-NaNO3 solution replace sodium ions in the glass substrate and increase the index of refraction [40]. Channel waveguides have been therefore fabricated in sol-gel materials by laser densification [41], UV-Vis densification [42] or reactive ion etching to remove unwanted materials [35] leaving the waveguide as a rectangular core, as shown in Figure 1-b); furthermore, a thick cladding layer identical to the buffer one, is usually deposited to bury the cores (Figure 1-c) with the aim of minimizing scattering losses. To avoid the absorption of light by the Si wafer the buffer layer thickness

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should be chosen taking into account the core refractive index [33]. Usually, it ranges from a few μm to multi-microns [28, 33]. Sol-gel process has also been used to obtain optical planar waveguides as buried channel guide [43] (graded index) (Figure 2-a), ridge (Figure 2-b) and strip-loaded channel guide [44] (step index) (Figure 2-c).

Figure 1. Typical structures of silica-on-silicon waveguides: a) planar waveguide; b) channel waveguide; c) channel/cladding waveguide.

Silica-on-silicon technology is so far a very attractive route to meet the requirements necessary for the good performance of integrated optical devices, particularly in terms of optical interface, loss and internal gains, integration of optical and optoelectronic devices and reduction of costs [28]. Low losses of absorption and scattering were achieved; efficient interfaces were obtained with SiO2 optical fibers and doping with rare earth ions allowed the increase of internal gain and improved optical amplification. Monolithic or hybrid device production can be achieved, as well as passive, optically active and electrically controlled regions [10, 14], plus Si V-grooves provide suitable alignment between optical fibers and integrated optics components [1, 6, 8, 10]. A diversity of methods can be used for deposition of SiO2 or SiO2-based glasses. However, the most common ones are flame hydrolysis deposition (FHD) and plasma enhanced chemical vapor deposition (PECVD). Moreover, direct writing of low loss single mode waveguides and directional couplers in PECVD SiO2-GeO2 films were developed [36], as well as the photosensitive response of germanium-boron-doped SiO2 obtained by FHD on oxidized Si substrates [45]. Channel waveguides were also written in Er3+-doped fluoride glass thin films grow on CaF2 by PVD [46]. Direct UV writing is a fabrication technique, where waveguides can be written directly into a planar glass

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sample with a focused UV laser beam; however, a valuable alternative to the referred fabrication methods is the sol-gel technique [47], which potentially offers some advantages in terms of avoiding multi-steps, hard equipment and high costs.

Figure 2. Optical waveguides geometries: a) buried channel; b) ridge; c) strip-loaded channel.

THE “SOL” HOST MATERIAL SiO2 and silicate glasses (e.g., borosilicate, phosphosilicate and arsenosilicate) may be fabricated in thick layers by several methods, including chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and flame hydrolysis. The films produced by these techniques are of high quality for waveguide applications, but the processes themselves are quite complex. Despite pure SiO2 can be grown thermally on Si substrates, (by oxidation, a chemical reaction of Si with O2 to form a layer of amorphous SiO2), the process is not practical for thick layers (typically ~ 2 m). An alternative is the sol-gel process, a chemical synthesis involving the hydrolysis and polycondensation of metal alkoxides, whose general formula is M(OR)n, where M is a metal atom and R is one alkyl group [47]. This process allows the preparation, at low temperatures, of highpurity and homogenous glass materials and ceramics; strict chemical control of the processing parameters is possible, resulting in the preparation of different kinds of materials: bulk solids, fibers, film waveguides and coatings for device applications. This process is a good alternative for low cost production of thin films, offering a great diversity in material compositions and structures, which is important for control of

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the functionality of integrated optic devices [47-49]. Through control of the viscosity of the deposition solution, different coating processes like dipping, spraying and spinning can be used. The high purity of the starting compounds, mixed in the liquid state, leads to gels with excellent homogeneity and with low impurity levels, which is especially adequate for photonic applications. Dopants, including rare-earth ions, in the form of salts or alkoxides can be added to the sol in order to prepare functionalized photonic materials [47], i.e., materials that can be tailored in their linear or non-linear optical properties for new device applications. The manufacturing process begins by making a sol, i.e., a stable suspension of particles in a solution. Then, the condensation of monomers occurs and thereafter the sol forms a continuous solid net permeated by liquid (a gel). Finally, the dried gel becomes a porous glass. Further, to achieve a high density and low porosity, the dried gel is annealed at temperatures which generally do not exceed 1000 °C [47]. The formation of films through the sol-gel process differs from the formation of bulk gels. The main stages of the traditional sol-gel process, namely gelation, aging and drying [47] occur at once during the deposition step for a very short time. In order to produce thin films, the sol is applied to a surface to be coated by spin-coating or dip-coating [47, 50-53] as shown in Figure 3-a) and Figure 3-b), respectively. The spin-coating is therefore a very popular deposition method; a substrate will rotate at a selected speed while the coating fluid (sol-gel solution) is dispensed onto its surface. Rotation does not top while the sol spins off the edges of the substrate, until the wanted thickness of the film is achieved. A practical example involves the fabrication of SiO2 films from tetraethyl-orthosilicate (Si(OC2H5)4, TEOS) [53]; after an ageing for a few hours, the sol is spincoating onto a flat substrate (e.g., SiO2 glass or single crystal Si wafers), by means a spin coater machine, at least 2500 rpm, during 30 s in order to spread the coating material. The refractive index, whose control is needed for the films to work as planar waveguides, can be tuned by doping SiO2 with TiO2 in the initial solution. A wide range of metal alkoxides may be used to produce diverse optical materials and particularly a wide range of dielectric films.

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Figure 3. Steps involved in: a) spin-coating and b) dip-coating. Credit Images: http://www.spincoater.com/what-is-spin-coating.php and https://commons.wikimedia.org/wiki/File:SolGel_DipCoating1.jpg, respectively for a) and b).

The sol-gel method is very interesting in optics because the refractive index can be changed by changing the composition of the sol to obtain a doped SiO2 layer. However, to obtain thick films by combining the techniques of sol-gel and spin-coating is necessary to overcome some disadvantages, namely: 1. Spin-coating generally yields thin films, typically below 200 nm thickness; 2. During these cycles, dust particles may easily be introduced into the film (Figure 4) and reduce its optical quality; 3. During drying, potential cracking may occur due to high capillary forces built up in very fine pores. The drying process also shrinks the volume of the spun layer which may causes tensile stresses. 4. The final film is usually porous (Figure 4) due to incomplete densification during the drying process; a subsequent annealing is required for full densification of the glassy film. Moreover,

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cracking may occur [51] due to stresses developed in the course of the annealing.

Figure 4. Typical defects often found in sol-gel films.

In order to obtain thicknesses higher than 2 m is therefore necessary developing a multistep deposition procedure (since each layer being 100 to 200 nm thick,); typically, a cycle of at least 10 depositions is necessary to achieve a coating of ~ 1 m thick. The process is based in an iterative sequence of spin-coating and rapid thermal annealing (RTA) [52, 53], where the Si wafer is generally annealed at temperatures slightly over 1000 °C on a timescale of a few seconds. Therefore, this approach also reduces the cycle time, making the method viable to a serial production. A suitable alternative to obtain thick sol-gel films is the incorporation of organics precursors (e.g., methyl-triethoxysilane, MTES) to prepare hybrid organic-inorganic films (organically modified SiO2: ormosil) [54]; the introduction of organics into a cross-linked inorganic system can also avoid cracking formation of bulk gels or thick films by increasing relaxation ability (Figure 5). In fact, the stresses formed by shrinkage during the densification of inorganic gels cannot be released by relaxation, unless an “organo-functional group” (e.g., MTES) is added to the network giving it a "ductile" behavior instead of brittle.

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Figure 5. Sol-gel routes: “inorganic” (TEOS) and “organic” (MTES); effect on the cracking possibility. Credit image: http://www.swst.org/wp/meetings/AM05/mai.pdf.

The synthesis of organic-inorganic films allows achieve several micrometers of thickness in films obtained with only a single dip-coating step (monolayers), using UV light and low annealing temperature. Ormosil films can achieve thicknesses highest than 1 m, which are suitable for optical waveguides. It is also allowed direct writing of waveguides in monolayers, which can facilitate the manufacture of components. Moreover, the doping with active ions, like rare-earth ions, such as: Yb3+, Er3+, Tm3+ and Nd3+, is a key practice for the realization of active optical devices [55]. Ormosil films were also studied for photochromic applications [56] and to produce coatings with thicknesses nearby 25 m by spin-coating techniques on glass and Si substrates [57]. The SiO2 based glasses obtained by organic-inorganic sol-gel synthesis can be classified as ormosils trapped organics, ormosils impregnated organics and ormosils chemically bonded organics-inorganics [55]. The first and the second categories consist of entrapped or impregnated organic types into porous gels, which have main applications on the fabrication of dye lasers and chemical sensors. The third category is prepared by mixing a Si alkoxide with a polymeric organosilane in order to promote chemical incorporation of the organic polymer in the inorganic SiO2 network. Thus, a Si alkoxide, like TEOS can be mixed together with polydimethylsiloxane PDMS or other organosilane polymer (e.g., diethoxydimethylsilane, DEDMS) [48, 55]; the organics stay trapped within the gel network oxide

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during the hydrolysis and condensation reactions of the TEOS [55]. One advantage is that the organosilane polymers exhibit analogous bond structures to that of the SiO2 matrix acquired from hydrolysis and condensation of TEOS. As already mentioned, the addition of PDMS increases the ductility of the hybrid glass, since their chains are chemically linked to the network. Moreover, the ormosils doped with rare-earth ions homogeneously dispersed can be prepared with lower content of hydroxyl (OH) groups with respect to their counterpart inorganic gels [58], which in optical amplification has a competitive advantage. Generally, sol-gel glasses exhibit fluorescence quenching by the OH groups. Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to study the microstructure of sol–gel glasses. The FTIR spectra of a SiO2TiO2 binary glass, as soon after deposition and after annealing at 900 ºC are shown in Figure 6. The broad peak around 3300 cm-1 is the fundamental stretching vibration of OH group that reveals the presence of hydroxyl groups in the gel. The OH band exhibits a pronounced reduction at the temperature of 900 °C. Therefore, this technique determines if the excess of water in the sol-gel process (hydrolysis step) [47] will cause an increase in the amount of active OH remaining in the structure of silicate glasses; this is not desirable for integrated optics, as this would cause increased absorption in the wavelength range 1.3 – 1.5 m [8, 14, 48, 55]. Additional hybrid materials for integrated optic devices are based on the hydrolysis and condensation of organic silane precursors with a photopolymerizable organic group [59] that can be photo-patterned using ultraviolet light irradiation and lithographic techniques. In particular, the incorporation of photo-responsive organic groups has been used to produce a wide range of optical materials for fabrication of diffraction gratings and channel optical waveguides. Planar and UV written channel optical waveguides supporting guided modes at 1550 nm, have been obtained based on d-U(600) hybrid films incorporating γ-methacryloxypropyltrimethoxysilane (MAPTMS) [59]. These compounds are usually added together with zirconium alkoxide [59] (or titanium alkoxide) to tailor the refractive index of the waveguide.

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Transmittance (a.u.)

Annealed at 900 ºC

As deposited 3600

3300

3000 1200 1000 800

600

400

-1

Wavenumber (cm ) Figure 6. FTIR spectra of a SiO2-TiO2 binary glass, as soon after deposition and after annealing at 900 ºC.

The sol-gel method has been proved as a simple and versatile method for the preparation of the rare-earth ions doped luminescent materials for various optical applications (tunable lasers, active waveguide sensors for environmental and biologic monitoring, waveguides and devices for nonlinear optics) [60] manly due to its advantages such as: wide range of compositions, low work temperature, easier composition control and high chemical homogeneity. Sol-gel derived glasses are especially suitable to host rare-earth ions since they can be easily introduced into the gel matrix [55]. The fluorescence properties of rare-earth ions and thus their amplification features are extremely dependent on the host material and on the way in which they are dispersed. In order to prevent no-radiative decay by cross-relaxation, rare-earth ions must be well spread. However, this spread is dependent on dopant concentration and on the solubility of the dopant in the host glass. It is also known that low temperatures avoid the occurrence of phase separation in glass materials; nevertheless, crystallization can often occur when a sol-gel glass is annealed (e.g., nanocrystallites dispersed within a glass matrix). This structure is the socalled glass-ceramics (GC) [61]. The nanocrystallites composition and average size can be determined from X-ray diffraction and AFM data.

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Figure 7 shown a AFM picture of a sol-gel silicate GC. This heterostructure, also referred as a nanocomposite, is considered a promising material for several photonic and optoelectronic applications [62]; it can be intentionally obtained by a post-heat treatment of the glass, which promotes in-situ a nanocrystals dispersion in the glass matrix. Thus, GC combine the glass properties (high transparency, etc.) with some typical advantages of rare-earth doped crystalline materials, such as high absorption/emission and long lifetimes [63].

Figure 7. AFM image of ErPO4 nanocrystallites dispersed in a thin silicate sol-gel film doped with Er3+ ions.

OPTICALLY ACTIVE GLASSES Materials that provide optical gain for amplification purposes are vital for optical and photonic technologies. This feature can be provided on glasses doped with rare-earth ions such as Yb3+, Er3+, Tm3+ and Nd3+, widely used in high power lasers and other modern optical devices such as fiber amplifiers or lasers. The first fiber laser employed a doped neodymium fiber with a core diameter of 300 μm [64]. The SiO2 fibers doped with Nd3+ ions provide fiber amplifiers capable of amplifying the spectral region 1.30-1.36 μm [65], especially because Nd3+ ions can be

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pumped using a four-level scheme, which is more advantageous compared to a three-level scheme, allowing a population inversion more instantly. Neodymium oxide is easily miscible in SiO2 glass below 1 wt% but above this concentration, ion aggregates or clusters are often formed. These clusters cause non-radiative deexcitation processes lowering the fluorescence lifetime and thus the optical gain. However, other ions, such as Er3+ and Tm3+ have gained a great notoriety due to their conversion efficiencies of more than 30%, for the pair Er3+-Yb3+ system and more 50% for fibers doped with Er3+ and Tm3+ [66]. In particular, Er3+-doped waveguide lasers and amplifiers, is being hailed as a milestone in optical amplification at 1.5 μm [14, 26]. Nowadays both optical amplification at 1.3 m and 1.55 m, in fiber or planar waveguide form, are of great importance in optical communications systems [6, 9, 66]. Pr3+ and Er3+ ions respectively, to 1.3 and 1.5 m, are currently two very popular options as active species, although the Nd3+ doping was first investigated for amplification at wavelengths of 0.9 μm, 1.06 μm and 1.3 μm wavelengths. Glass in thin film form is the basis of integrated optical components, both passive and active, in which SiO2-based ones are the most widely used. Other glasses, including low phonon energy ones, such as fluorides and chalcogenides, are also used; however, not so often due to difficulties in fabrication process. Rare-earths doped glass planar (or channel) waveguides allow the fabrication of more compact and efficient laser and amplifier devices. In particular, a great attention has been done to SiO2 codoping with P2O5 [67] and Al2O3 [68] to overcome clustering effects and short fluorescence lifetimes due to high Nd3+ and Er3+ doped concentrations required for amplification in short device lengths. The main objective is preserves optimum fluorescence properties and avoid the effects of non-radiative quenching phenomena caused by multiphoton relaxation, which is strongly favored in high vibrational energy matrices, like silicate glasses [63]. On the other hand, SiO2 glass has limited solubility for rare earth-ions; similarly, to Nd3+ ions, Er3+ ions also induce clustering, even at moderate concentrations [69], since these ions tend to cluster to achieve their favored six-fold coordination [70]. This is mainly due to the structure network of tetrahedral units in SiO2 and lack of enough

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non-bridging oxygens (NBO) to bond Er3+ ions [71]. The addition of oxides co-dopants rises the overall solubility limit of SiO2 and thus the NBO sites available to bond Er3+ ions. Additional rare-earth (Eu3+, Pr3+, Dy3+) were also investigated as dopants in sol-gel glasses [72]. Furthermore, the sol-gel materials co-doped with a combination of different rare-earth ions can be used as “colorful” emitters, in which emission wavelength tuning can be obtained by varying the excitation wavelength. The effects of GeO2 co-doping were also investigated, in particular in Eu3+-doped GeO2–SiO2 glass prepared by sol– gel method [73]. Its optical absorption and fluorescence properties were measured during heating under an H2 atmosphere; however, no fluorescence was observed from the Eu3+ ions due to the formation of the oxygen-deficiency defects in the Ge–O bonds [73]. Rare earth ions usually occur in a trivalent state. The 4f electron shell controls the optical properties of these ions, which are virtually insensitive to the surrounding atom of the host environment due to the screening by 5s and 5p electron shells. This is due to the fact that occurs a weak interaction between the optical cores (the 4f electrons), and the crystalline field. This interaction, although weak, produces a very well-resolved Stark structure of levels, which varies only slightly from host to host. The amplification, by the presence of rare-earth ions inside a solid matrix is due to the rupture of the degeneracy of the 4f levels of the ions by the Stark effect, allowing electronic transitions to take place between internal levels and with long lifetimes [66]. Optical transitions between individual Stark levels contribute to the total absorption and emission line shape. Typical spectral line width of the rare-earth in glass is approximately a few hundred wavenumbers [66]. Silicate glasses exhibit narrow spectral line widths for rareearths compared with that of fluoride based glasses. This is due to the specificities of each host material [14] as shown in Figure 8 [adapted from 74]. It is also shown that the emission peaks of Er3+ in glass and glass– ceramics with Al2O3 (raised NBO sites) are broader than those in silicate and fluoride glasses. This is because the width of Er3+ emission peak at 1.5 m in glasses, which is connected to the Stark splitting due to the magnetic dipole interaction, rises with increasing fluorine content.

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The overall optical properties of rare-earths doped glass-ceramics, such as fluorescence spectra, metastable state lifetime-fluorescence decay and waveguide loss, will depend on several factors, namely the size of nanocrystallites, their amount and distribution; and the remaining concentration of rare-earth ions in the amorphous matrix.

Figure 8. Comparative emission spectra of Er3+ at 1.5 m in Er2O3–BaF2–Al2O3–B2O3 glass and glass–ceramics with the equivalent spectra of silicate glass and hafnium based fluoride glass (HBLAN) [adapted from 74].

The fluorescence decay of the active ion presents two distinct behaviors depending on the nature of the surrounding environment (Figure 9) [adapted from 75]. Usually a two component decay curve is observed in the glass ceramic. A slower decay curve follows a very fast initial decay. The fast initial decay suggests an environment with a high rare-earth ions concentration and an increased efficiency of fluorescence quenching. This produces an efficient energy transfer between active ions and, consequently, a rapid mechanism of ion deexcitation that explains the initial short decay component.

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Upon clustering, the ion-ion energy transfer process tends to result in quenching of the luminescence through non-radiative relaxation of the ion pair, which manifests itself in a reduced fluorescence lifetime of its metastable state.

Intensity (a.u.)

1

Glass

Glass ceramic 0,1

0

0,05

0,1

0,15

0,2

Time (ms) Figure 9. Fluorescence decay curves obtained from Yb3+ ions in Pr3+-Yb3+ co-doped oxyfluoride glass and glass ceramic under excitation [adapted from 75].

On the other hand, the slower decay component can be associated to an active ion concentration notably lower, indicating that a fraction of the fluorescence is definitely emitted by active ions embedded in nanocrystals. The energy transfer rate is accordingly lower in this environment and the rare-earth lifetime becomes usually longer. As already mentioned, SiO2 glass exhibits, however, a high vibrational energy glass matrix [76], as most silicates, are not adequate to the desirable optical performance of the rare-earths, due to high non-radiative relaxation rates which may seriously reduce the lifetime of the excited states and therefore the quantum efficiency of the rare-earth luminescence processes.

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These glass matrices seriously reduce the metastable level lifetimes and, consequently, the quantum efficiency of the luminescence processes, especially in the case of Pr3+. One way to minimize this problem is to use a low vibrational energy oxide matrix such as germanate or telluride, or, rather, non-oxide glasses like fluoride or sulfide. Tellurite-based glasses, having the lowest maximum vibrational energies among oxide glasses and also a large refractive index [77], are specially interesting in photonics. Another option is to create a low vibrational energy environment around the rare-earth ions, such as a crystalline fluoride moiety inside aluminosilicates or SiO2 glasses. In order to confine the rare-earths in a low vibrational energy environment, Er3+ and Yb3+ ions were precipitated inside PbxCd1-xF2 nanocrystallites of about 20 nm in size, dispersed in an aluminosilicate matrix, in such a way that the resulting GC preserved a high degree of optical transparency [78]. This kind of structure is schematically presented in Figure 10. Oxy-fluoride glass ceramics containing Er3+-doped nanocrystals and their optical properties (including up-conversion effects) have also been studied [79, 80]. X-ray diffraction (XRD) is a powerful tool to probe and understand the structure of these nanocrystalline phases. XRD analysis revealed that heat treatments of the oxyfluoride glasses cause the precipitation of fluorite-type PbxCd1-xF2 nanocrystals in a glass matrix [81] (Figure 11). In the search for desirable optical properties of rare-earth ions in fluoride hosts, nanocomposite glass ceramics have been prepared by precipitating Pr3+ ions in CdF2-rich fluorite-like crystals, 9-18 nm in size and corresponding to volume fractions around 30%, in a aluminosilicate glass [82]. For these GC, it was observed a large unexpected degree of transparency, with a low optical scattering attributed to a uniform dispersion of the nanocrystals [82]. When doped with Pr3+, the fluorescence life at 1.3 μm was higher than that observed in fluorozirconate glass, suggesting an important fraction of the rare-earth ions preferentially segregated into the fluoride crystal phase [82].

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Figure 10. Rare-earths within a low vibration energy nanocrystalline PbxCd1-xF2 phase, also in a rare-earth doped glass matrix, resulting in transparent glass-ceramic.

Figure 11. XRD patterns of glass and glass ceramics, exhibiting β-PbF2 and CdF2 nanocrystals present glass ceramics are mixed PbF2 and CdF2 crystals with fluoritetype structures, i.e., PbxCd1-xF2 [adapted from 81].

On the other hand, -PbF2:Er3+ nanocrystals, about 13 nm in size, were dispersed in a SiO2 matrix [83]. In this case, the Er3+ ions showed highly efficient up-conversion luminescence between 500-700 nm, when excited with 800 nm light, which was attributed to the Er3+ ions being coupled to a 200 cm-1 phonon mode in the nanocrystallites, whereas those ions in the glass (before crystallization) are coupled to a vibrational mode at 930 cm-1 and showed little up-conversion luminescence. Moreover, sol-gel

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nanocomposite glass ceramic films have also been obtained by the controlled growth of crystallites such as Er2Ti2O7 or ErPO4 in SiO2–TiO2 based glass matrices [84, 85]. Although these two crystalline phases do not have a very low vibrational energy, the goal was to improve the Er3+ fluorescence behavior, precipitating this rare-earth in a crystalline environment of low clustering effects. It was also found that in these nanocomposites the lifetime for the Er3+ fluorescence at 1.55 μm increased in some cases more than 200%, exhibiting also low optical losses [85, 86]. Furthermore, it was demonstrated in Nd3+ doped TeO2-ZnO-ZnF2 glasses that the addition of ZnF2 to the TeO2-ZnO binary glass has produced an increase of 40% in the Nd3+ fluorescence lifetime, nevertheless phonon side band spectroscopy revealed that the vibrational character of the Nd3+ coordination shell did not change significantly between the two glass systems [87]. The luminescent properties of fluoride glass doped with rare earth ions were also studied and it was found that depend greatly on the type crystalline phase precipitated in the glass matrix. Sodium–gadolinium– fluoride compounds (NaxGdyFx+3y), particularly NaGdF4 and Na5Gd9F32, were identified as efficient up-conversion hosts for rare-earth ions doping [88, 89]. Er3+:Na5Gd9F32-based glass-ceramics were successfully fabricated by the melt-quenching method with subsequent crystallization processes. The evident Stark splitting, enhanced characteristic up-conversion emissions (1300 times for red emission, 1000 times for green emission) [89]. Phosphate glasses are also promising. Presently they are of great interest for the manufacturing of photonic devices because of their good chemical stability, easy processing, high rare-earth solubility and excellent optical characteristics [90]. One of their main advantages with respect to the SiO2 counterparts is the capacity to incorporate higher doping concentrations of Er3+ ions within the glass matrix. In addition, glassceramics are also allowable in phosphate glasses. For example, P2O5-SrONa2O glasses were successfully fabricated by fusion followed by a postheat treatment in order to causing controlled nucleation and crystallization of Er3+:Sr(PO3)2, displaying the longest lifetime [90].

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FIBERS AND PLANAR WAVEGUIDES Nowadays, glass optical fibers are routinely used for high-speed data transfer. There are two basic methods for optical fiber fabrication: 1) preform drawing and 2) drawing from double crucible [91]. However, the most versatile and most widely used industrially is the method of preform. This last process presents, however, some alternatives all based on vapor deposition [91]: the outside vapor deposition process (OVD), the vapor axial deposition method (VAD) and the modified chemical vapor deposition (MCVD) [92]. The fibers are made of pure SiO2 but in order to control its refractive index, in particular to increase the core index, small amounts of GeO2 or P2O5 are added to the base composition. Other alternatives, for example the addition of B2O3 to lower the cladding index, are also possible [93]. The principle of total reflection is the basis of the operation of all types of waveguides. A slab waveguide is analogous to an optical fiber except that it is a planar waveguide rather than a cylinder-shaped one, where a low refractive index platform contains a slab (or channel) of higher index material along which light is guided by total internal reflection (TIR). The refractive index is defined as the ratio of the velocity of light propagation in vacuum to the velocity of propagation in a medium other than vacuum (e.g a glassy medium). The Figure 12 illustrated the structure of a dielectric planar waveguide, were nf, ns and nc are the refractive indices of the film (the waveguide itself), substrate and cladding respectively, and d is the film thickness. If ns >nc the waveguide is denominated asymmetric; otherwise if ns = nc. is called symmetric. The light cannot be guided unless nf > ns > nc, and the thickness, d, of the guiding layer is above the critical thickness or cutoff thickness [2, 5, 6]. According to Snell’s law when light passes from an optically dense medium into a less dense one (ns > nc) the propagation vector k bends away from the normal. In particular, if the light is incident at the critical angle: c = sin-1 (nc/ns), the transmitted ray just grazes the surface and TIR occurs.

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Figure 12. Scheme of a dielectric planar waveguide; a ray entering in the waveguide and being guided by TIR.

A glassy planar waveguide can be studied by either the Geometrical Optics or Wave Optics approach. For the first one, the eigenvalue equations of the guided modes are obtained from the laws of refraction and reflection while the equations necessary for the wave optics approach are derived from the Maxwell’s equations [5, 10]. A typical glass optical fiber is a cylindrical waveguide and thus has a core/clad structure similar to that of planar waveguides; light propagates in the core of higher refractive index, by TIR at the interface with a cladding of lower index. In terms of geometrical optics, it is assumed that the light is transmitting in a linear, homogeneous, non-dispersive and isotropic medium. The mode is as the ray is traveling in the z-direction with repeated “zig-zag”, i.e., by TIR at the film/cladding interface and film/substrate interface. In view of the practical requirements necessary for the functionality of the integrated optics devices, several aspects must be taken into account, such as: 1) Coupling; characteristic related to the connection of device ports with the input and output of single mode fibers. Tight tolerances must be met in terms of matching and mode alignment, and a perfect combination of refractive index is necessary to eliminate cut-reflections; 2) Attenuation: no loss of waveguide above 1 dB/cm should occur, implying a low dispersion of materials and interfaces, as well as negligible material absorption (intrinsic and extrinsic);

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3) Gain: where necessary, the material must be able to provide internal gain in order to compensate eventual internal losses, power branching, optical amplification or laser emission. 4) Integration: where monolithic integration is not feasible, hybrid integration should be possible (integration of optical and electronic components), allowing the construction of more compact, reliable and efficient components and subsystems. Attenuation or loss of transmission may be defined as the decrease in the energy intensity of a signal as it propagates through a transmission medium. Indeed, the optical attenuation is one of the most important parameters to define the quality of a glass waveguide and depends on both intrinsic factors (inherent to the material) and extrinsic factors such as impurities, defects or contamination present in the glass. The energy of a mode transmitting through a planar waveguide can therefore be attenuated by: 1) Rayleigh scattering losses resulting from refractive index fluctuations in domains whose dimensions are small compared to the wavelength of light. These variations can be generated by fluctuations in composition or density (at atomic scale), phase separation, etc. Rayleigh scattering is responsible for the lower limit of the glass attenuation. This attenuation coefficient depends on -4, sharply decreasing to longer wavelengths. In a dielectric waveguide, this type of loss represents about 0.5 to 5 dB/cm of total losses [1]; 2) Multiphonon absorption losses (infrared absorption threshold), which result from harmonics of the vibration frequencies of the fundamental modes of the glass network, which are inversely related to the mass of the atoms and the strength of the bonds. The intensity of these losses increases exponentially with , so it becomes important at high wavelengths.

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3) Electron absorption losses (Urbach threshold), due to electronic transitions, that decrease exponentially with , limiting the transparency for smaller wavelengths. 4) Dispersion phenomena due to imperfections (roughness, cracks, defects, etc.) and particles present in the glass, such as bubbles, microcrystals or pores [1]. For particles of the order of magnitude of the wavelength of radiation, the loss varies with -2 and is referred to as Mie’s dispersion, whereas for larger particles it becomes -independent (Tyndall’s dispersion). Several techniques such as prism coupler, scattering and photothermal detection [1, 5] are available to measure the attenuation of planar waveguides. The prism coupler is, however, the more popular one. In optical communications, the most important technological limitation lies in the fact that the signal propagates through an optical fiber, over a distance of several kilometers, suffering heavy losses (attenuation), being regenerated periodically. The invention of The Erbium Doped Fiber Amplifier (EDFA) in the 80s was a milestone in the history of optical communications. This device is an optical repeater, capable of amplifying laser beams, without optoelectronic and electrooptical conversion. Nowadays is widely used in optical telecommunications, laser industries, and in applications such as temperature sensors [94]. In particular, EDFA is a segment of SiO2 fiber (co-doped with Al and Ge) a few meters long, doped with Er3+ ions. The amplification mechanism in EDFAs is the stimulated emission with a population inversion achieved by optical pumping (at 980 nm). During this process, pump photons raise Er3+ ions to higher energy states. The Er3+ ion in a vitreous SiO2 matrix behaves like a quantum system with three energy levels [8], as shown in Figure 13. From the fundamental level, the ion is excited to a higher (unstable) level by completely absorbing the energy of a photon. Then, from the upper level, after ~ 1 μs, the excited Er3+ ions will fall (almost instantly) and by a nonradiative process to an energy level called metastable since this state has an average life time of ~10 ms [8]. The erbium ions in the metastable level decay to the ground level either spontaneously within a time of the order of

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10 ms, or through stimulated emission by an incoming signal. The emitted light due to the ion decay between the metastable level and ground level corresponds to the wavelength of ~1.55 m. 4

I11/2

4

I13/2 Pump (980 nm)

4

Stimulated Emission Photon

I15/2

Figure 13. Signal amplification process in a vitreous SiO2 matrix, where the Er3+ ion behaves as a quantum system with three energy levels.

Figure 14. ET up-conversion mechanism in silicate optical fiber preforms glasses [94].

All the developed applications of optical fiber amplifiers are the result of long and careful optimization of the material properties, particularly in terms of dopant incorporation in the glass matrix, transparency, and quantum efficiency. It was reported an Yb3+ to Tm3+ energy-transfer quantum yield close to one in phase-separated yttrium–alumina silicate optical fiber preforms glasses host [94]. It was explained that two phases

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are segregated in the core glass during the manufacturing process. One of them has a yttrium-alumina silicate composition, while the other is an alumina silicate host. These results indicate that the specific rare-earths phase segregation into the yttrium-rich phase of the core glass can be advantageously exploited to populate the excited levels of Tm3+ through direct pumping of Yb3+ ions, which have a large absorption cross section, followed by the efficient sensitizer of energy-transfer (ET) up-conversion processes between Yb3+ and the Tm3+ ions [94] (Figure 14).

OPTICAL MICROSPHERE RESONATORS Optical microresonators are microscopic devices capable of confining light in micrometric spaces. A resonator consists of a waveguide that guides light along a closed circuit. During the last decades, dielectric microresonators have attracted a great attention due to their potential in applications of photonic devices. An actual interest is focused to optical resonators based on spherical dielectric (glass microspheres) configurations which allow the light confinement in circular orbits [95]. It is found that microspheres of glass, upon being excited by a source of monochromatic light (e.g., laser) exhibit resonant optical properties resulting from the interaction of incident radiation with the atomic structure of the glass; this effect of light propagation in within the microsphere is described in terms of a set of electromagnetic waves called resonant modes, known as “Whispering Gallery modes” (WGM). These modes are electromagnetic field configurations strongly confined within the sphere and traveling inside of the microsphere near the curvaceous borderline by a TIR process, due to the difference between the refractive index values of the glass and the outer medium where the microsphere is inserted (e.g., air, water or other). The term “whispering gallery” was first introduced in 1910 by Lord Rayleigh [96], to explain the sound journey through the dome of the walls of St. Paul’s Cathedral in London. It is an acoustic effect which manifests itself in circular rooms such that if a person issues a “whisper” in a side of

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the room (A) is heard clearly by another that is located on an opposite point away (B) (Figure 15). The sound runs through the perimeter of the room through successive reflections on the walls, similar to what happens to light inside the glass microspheres. Such designation is now widely used for defining the confinement of light within a spherical dielectric resonator [15, 16] (e.g., B2O3 glass doped with Nd3+ ions) [95]. When light strikes an interface, part is reflected and the other is transmitted. However, when light propagates in a medium (e.g., glass) whose refractive index is higher than that of the outer medium (e.g., air), there is an incident angle (at the glass/air interface) above which the light undergoes full reflection. This effect is very useful in reducing optical losses. According to Snell’s law, light as it propagates along a circular interface is fully reflected at each reflection, propagating with minimal losses. Due to the discontinuity of the refractive index glass/air, the light is strongly confined in the small volume of the microsphere, making millions of revolutions in the confined space before being absorbed, reason why it suffers interference with itself. Therefore, only a set of wavelength values () of the light can adjust to the boundary conditions imposed by the circular cavity. This selectivity allows the existence of only discrete modes (WGM), with minimal losses. Whenever the light ray tries to escape the microsphere, it is sent back to its circular path due to the phenomenon of TIR (Figure 16-a). Because of its ability to transport very specific wavelengths of light, spherical resonators are of particular interest to the design of optical components and systems with applications in various domains, namely sensing applications in biological environments [97-100] since the spectral position of the WGMs is very sensitive upon changes of the refractive index of the surrounding medium. The confinement, in the perspective of geometrical optics, was extended to the circulation of light waves (Figure 16-b), which also led to the well-known analogy between the excited microsphere (photonic atom) and the Bohr atom [101]: photons orbit around the interior of the sphere just as electrons do around the nucleus of a Bohr atom. Because of this similarity WGMs are often referred as “photonic modes of atoms” [15]. In the photonic atom, the wave functions that describe the guiding waves are

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subject to an effective potential, as a function of the sphere refractive index and the surrounding medium, similar to the effect of an electrostatic potential well that confines the electron to an atomic orbit. The photonic atom is a very stable object, scattering and absorption losses are minimal. The photons circulate in their orbit several times before finally leaving the microsphere due to other loss mechanisms. In practice, this means that a short pulse of light (e.g., of the order of the picoseconds), upon being trapped by the spherical glass, may remain therein confined a few time, e.g., ~ 1 nanosecond; the confined light will decay exponentially during this time. Hence, there is a parameter, the so-called quality factor (Q), that defines the quality of an optical resonator as being the ratio of the electromagnetic energy stored inside it and the energy lost at each confined mode cycle. Q indicates a lower rate of loss of energy relative to the stored energy of the resonator; the oscillations stop slower! In spectroscopic terms, the modal register contains the spectral information to be correlated with possible aspects to be monitored around the environment where the microspheres are; the WGM recording is displayed with extremely monochromatic peak, characterized by a high quality Q and visibly very sensitive to physical processes which manifest around the microspheres.

Figure 15. a) Illustration of acoustic effect corresponding to resonance modes (WGM), first observed in the b) St. Paul’s Cathedral (London) [image credits: https://en.wikipedia.org/ wiki/Whispering_gallery].

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So, one of its most promising applications is sensors (e.g., biosensors), due to the sensitivity of the optical modes to external disturbances. In general, the sensitivity of the resonant devices increases as the line width of their resonant “fingerprint” decreases. This line width is proportional to the optical losses, and is inversely proportional to the factor Q [17]: Q = λ0/Δλ

Figure 16. a) light rays (geometric optics) and b) WGM, confined in glass microspheres [adapted from 101].

where λ0 is the resonant center wavelength and Δλ is the line width. Therefore, WGM are presented as extremely monochromatic peaks, characterized by a high quality factor Q of usually 108-109 [102] which implies lines of very narrow modes. Q depends not only on the dissipation of energy in the walls of the microsphere but also depends on the optical characteristics of the dielectric medium in which light propagates (material specifications). Any perturbation affecting the microsphere morphology (shape, composition, size or refractive index) or the outer medium will cause a shift of the observed spectral resonances. Such changes, manifested either by wavelength deviations, or by broadening or narrowing of resonant peaks, and may be correlated with possible changes in physical parameters of the environment, e.g., temperature, pressure or biological processes. For example, studies have been carried out in view of the applications of these devices with environmental sensors [103] and biosensors [16, 21]. The confinement of light in small volumes is at the basis of the current development of highly accurate optical biosensors.

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Factors such as perfect sphericity, absence of structural defects, absence of roughness and fluctuations in the index of refraction, homogeneous composition and density contribute to the reduction of total optical losses, in accordance with the expression [95]: −1 −1 −1 −1 𝑄 −1 = 𝑄𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 + 𝑄𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑄𝑠𝑝ℎ𝑒𝑟𝑖𝑐𝑖𝑡𝑦 + 𝑄𝑐𝑜𝑢𝑝𝑙𝑖𝑛𝑔

The possibility of achieving Q near to 1014 is predictable by the theory of WGM for crystalline resonators. Indeed, a CaF2 WGM with Q > 1011 has demonstrated at 1.55 μm wavelength [104]. The presence of resonant modes in microspheres of borate (B2O3) glass doped with Nd3+ ions were obtained [95, 105] using the drop method for microsphere production [17, 106]. Figure 17 exhibit the emission spectrum of B2O3 microspheres, corresponding to the emission of Nd3+ at 880 nm (4F3/2 → 4I9/2), excited with a laser at 532 nm. Due to several experimental difficulties, it is not easy to produce perfect spheres, being more likely to obtain spheroid shapes (deviations from perfect sphericity) in which the spectral WGM positions depend on the specific angle of the normal vector of the optical path. The excitation of luminescent ions is strongly dependent on the location which produces the excitation (pumping) of the microsphere.

Figure 17. Emission spectrum of borate glass microspheres, corresponding to the emission of Nd3+ at 880 nm (4F3/2 → 4I9/2), excited with a laser at 532 nm [95].

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However, an efficient excitation and light detection, from resonant modes, is essential in the study of these devices. Therefore, excitation and detection can be performed by confocal microscopy [105]. This allows a reduction in the observation region (-transmission techniques and -photoluminescence) and collect the light coming out of a small area around a side edge of the sphere, reducing the spectral broadening of the modes due to a loss of sphericity. A typical experimental configuration used in these experiments is shown in Figure 18 [105].

Figure 18. Scheme of the combined -transmission and -PL setup. Top right. Image of a microsphere taken with the CCD when the monochromator is in the zero order and the slit of the monochromator widely opened. Top left. Energy level structure of Nd3+ ion [105].

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from FEDER, through Programa Operacional Factores de Competitividade − COMPETE and Fundação para a Ciência e a Tecnologia − FCT, by the project UID/FIS/00068/2013.

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[90] Lopez-Iscoa, Pablo, Salminen, Turkka, Hakkarainen, Petit, Teemu, Laeticia, Janner, Davide, Boetti, Nadia G., Lastusaari, Mika, Pugliese, Diego, Paturi, Petriina and Milanese, Daniel, Effect of Partial Crystallization on the Structural and Luminescence Properties of Er3+-Doped Phosphate Glasses, Materials (Basel). 2017 May; 10(5): 473. [91] Izawa, Tatsuo and Sudo, Shoichi, Optical Fibers: Materials and Fabrication, Springer Netherlands, 1986. [92] Karstensen, H. (1986) Fabrication Techniques of Optical Fibres (Invited. Paper), IETE Journal of Research, 32:4, 232-242, DOI: 10.1080/03772063.1986.11436602. [93] Kartalopoulos, S. V., Introduction to DWDM Technology, IEEE Press, 2000. [94] Lahoz, F., Perez-Rodrıguez, C., Halder, A., Das, S., Paul, M. C., Pal, M., Bhadra, S. K., and Vasconcelos, H. C., Complete energy transfer due to rare-earth phase segregation in optical fiber preform glasses, Journal of Applied Physics 110, 083121 (2011). [95] www.tntconf.org/2011/Presentaciones/TNT2011_Capuj.pdf?TNT= 6c3de0d1ebf40a7f21ccad2700a3106f. [96] Rayleigh, Lord, Philosophical Magazine, 20, 1001 (1910). [97] Biophotonics/Optical Interconnects and VLSI Photonics/WBM Microcavities, 2004 Digest of the LEOS Summer Topical Meeting (IEEE, Piscataway, NJ, 2004). [98] Vollmer, F., Braun, D., and Libchaber, A., “Protein Detection by Optical Shift of a Resonant Microcavity”, Appl. Phys. Lett. 80, 4057 (2002). [99] Vollmer, F., Arnold, S., Braun, D., Teraoka, I., and Libchaber, A., “Multiplexed DNA Quantification by Spectroscopic Shift of Two Microsphere Cavities”, Biophys. J. 85, 1974 (2003). [100] Nadeau, J. L., Ilchenko, V. S., Kossokovski, D., Bearman, G. H. and Maleki, L., “High-Q whispering-gallery mode sensor in liquids”, Proc. SPIE 4629, 172 (2002). [101] webmac.rowland.org/rjf/vollmer/images/vollmer.pdf. [102] Chiasera, A. et al., Laser & Photonics Rev. 4, 457 (2010).

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[103] Adamovsky, G. and Otugen, M. V., Journal of Aerospace Computing Information and Communication, 5, 409 (2008). [104] Savchenkov, A. A. et al., Opt. Express 15, 6768 (2007). [105] “Navarro-Urrios, D. et al. Local characterization of rare-earth-doped single microspheres by combined microtransmission and microphotoluminescence techniques”, Journal of The Optical Society of America B-Optical Physics 29, Nº 12, 3293-3299, (2012). [106] Elliott, G. R., D. Hewak, W., Murugan, G. S. and Wilkinson, J. S., Optics Express, 15, 17542 (2007).

In: Photoluminescence Editor: Ellis Marsden

ISBN: 978-1-53613-537-4 © 2018 Nova Science Publishers, Inc.

Chapter 3

PHOTOLUMINESCENCE PROPERTIES OF LAYERED CRYSTALS AND THEIR ORGANICINORGANIC HYBRID COMPOSITES Andreea Nila and Mihaela Baibarac Laboratory of Optical Processes in Nanostructured Materials, National Institute of Materials Physics, Bucharest, Romania

ABSTRACT This chapter displays a focus discussion on the optical characteristics of semiconducting crystals with layered structure due to quantization effects in the architecture governed by the atomic arrangements. In order to study the optical processes of these materials, photoluminescence studies (PL) were determined to be essential to those established by conventional bulk materials. In this chapter, the PL properties of layered crystals, such as Cs3Bi2I9, BiI3 and PbI2 were reviewed, a special attention being given to the photo-induced charge carrier separation and also to the radiative and non-radiative recombination dependent on deep or shallow trap states. A superradiant PL emission, a strong oscillations strength of excitons in layered crystals and a change in the PL intensity, caused by 

Corresponding Author Email: [email protected].

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Andreea Nila and Mihaela Baibarac the transition from bulk layered crystal to few-layer or monolayer halides are some issues debated in this work. The influence of different organic/macromolecular compounds on PL properties of layered crystals is also reviewed. In this specific field, layered-crystals become an important competitive class of materials promoted for various basic and applicative research studies in optoelectronic devices and non-linear optical fields.

Keywords: photoluminescence, layered crystals, composite materials, intercalation processes, charge collecting

INTRODUCTION In the field of nanotechnology, layered halide semiconductors and their associated composites based on chemical functionalization of surfaces and organic-inorganic hybrid materials have received particular interest in both phenomenological and applicative investigations due to their remarkable optical and electrical properties [1-10]. Typically, all halide-type crystals with highly polarizing cations and polarizable anions adopt a unique crystalline layer structure of the type sandwiches framework with strong intralayer covalent bonds held together by week van der Waals interactions in order to form three-dimensional networks. In general, dihalides (MX2) and trihalides (MX3) represent the majority of layered metal halides. These materials often form crystalline structures consisting of small molecular units, of the type one or twodimensional layers. This is the case of chloride, bromide and iodide materials when the specific atomic arrangement is explained on the basis of lower anionic charges and higher ionic radii. In this case, the cations are usually found surrounded by six anionic species that are separated into structural units bonded together by weak van der Waals interactions. This picture is no longer available as we move to the highest anionic charge, like in the case of fluoride, when crystals in three-dimensional networks become prevalent [11]. The experimental evidences of various layered structures are reported using common methods as scanning or transmission

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electron microscopic techniques [12-19]. Such studies have highlighted a cross-section with staking period [20-23]. From an optical and electrical point of view, compared to other conventional bulk materials, layered structures may reveal a higher optical response and a shorter photocarrier lifetime as a result of the spatial confinement effects in the quantum wells. Thus, knowing the PL properties of these materials is of particular importance. In this context, a brief overview of some layered semiconductors, e.g., PbI2 and BiI3, with a high potential for the non-linear optical processes study, is presented below [3, 6]. Due to their crystallographic orientation, a tendency of cleavage planes parallel to the layers, known as stacking faults, is generally expected. Furthermore, the search for other new types of two-dimensional (2D) nongraphene monolayers, such as transition metal dichalcogenides and halides, becomes meaningful for the fabrication of next-generation miniature optoelectronic devices with a higher charge transfer process due to a greater electronegativity difference between metallic and halogen/ chalcogenides species. Therefore, exfoliation by various chemical methods or micromechanical cleavage perpendicular to the c-crystallographic axis can produce high-quality monolayers containing individual nanosheets and nanostructures with few-layers or a large amount of thicker sheets directly obtained from their layered bulk structure [24-31]. To investigate the layer number, experimental measurements with complementary multi-techniques such as resonant and nonresonant Raman scattering [31-37], optical microscopy [37, 38], atomic force microscopy (AFM) [31, 37, 38, 39] and PL studies were performed. According to these experimental analyzes, the measurements can provide important insight structural information, such as the number of layers, stacking order of layers and distinct color optical images. However, PL spectroscopy studies are mandatory for completing the experimental results of the above techniques [24-28, 37, 40-43]. Strong enhancement and particular PL response are reported in this chapter for some layered semiconductors with stacking faults and a few layers that are particularly interesting for determining the performance of the electronic devices.

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An experimental method devoted to highlighting the anisotropic behavior of materials with layered structures is the PL. In this context, for solids with anisotropic crystalline structures, different PL characteristics can be obtained when a polarized light is used and the direction of the electric field is perpendicular or parallel to the main symmetry axis of the crystal. In literature, researchers have focused their attention on composites with layered semiconductors made from a wide range of organic-inorganic hybrid materials. In the past, researchers have explored the potential applications of layered semiconductors in optoelectronic using them as good candidates for room temperature (RT) X- and γ-radiation detection materials [4, 5] and more recently as laser action [1] or laser cooling [2]. At present, composite materials based on layer-type semiconductors, with an emphasis on layered halides and organic materials (e.g., amines) [7-10, 44-53] have been extensively investigated for wider applications in photovoltaics, light emitted diodes and field-effect transistors. Progress in the synthesis of composite materials requires an advanced knowledge of the interaction between the two components. The PL is a relevant tool to investigate such interactions. This chapter also shows a review of the PL properties of composites based on the PbI2 and BiI3 halide compounds and various organic/macromolecular compounds.

PHOTOLUMINESCENCE PROPERTIES OF LAYERED CRYSTALS A sustenable effort was reported concerning the influence of the crystalline structure of layer-type crystals on the optical and electronical properties of these materials. In this context, the crystalline structure offers an unique opportunity to compare the atomic arrangement with a quantumwell structure due to the repetitive units linked by van der Walls bonds. Moreover, the planes perpendicular to the c-crystallographic direction can be partially dislocated in different positions which give a high density of

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stacking faults in the layered crystals. These features are meaningful for explaining a strong enhancement of the PL band intensity, different spectral positions of the emission bands separated by several meV, the crystal quality and so on. Consequently, the three important features have been reported to be induced by: (a) a change of the PL properties due to defects and the quantum confinement effect of excitons between layers; (b) a strong PL anisotropy depending on the electric field direction; (c) the modification of the PL properties as a function of the layers number; and (d) non-linear effects due to the quasi-coherence of the PL.

Influence of Defects on PL Properties and Quantum Confinement Effects in Layered Crystals Many of the PL spectra in layered crystals originate in excitons that typically have a low binding energy of about 10 meV and a large Bohr radius with an electron-hole that extends over several unit cells. Therefore, there are delocalized states with a small Coulomb interaction that move freely through the entire crystal when an excitation light collides the material under resonant conditions. After a certain relaxation time, the excited luminescence center, due to its unstable thermodynamic state, loses the excitation energy of the electron and can return to its initial ground state in two ways: either as radiative transition, when the recombination of the electron and the hole emits a light whose energy is the difference between the excited state and the initial state, and/or as non-radiative transition when excitons are partially localized on lattice defects. The influence of stacking faults on excitons in layered structures was discussed for the first time in 1977 [54] when strongly localized excitons in nonperiodic potentials explains the intense absorption bands. Based on these features, in 1989 a strong oscillation strength was highlighted by Akai et al. 1989 [55] from the PL spectra of BiI3, when short decay times of excitons of few picoseconds were also observed due to the quantum wells

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Figure 1. The PL spectrum of the BiI3 crystal recorded at LNT under an excitation wavelength of 500 nm. Reprinted with permission of [3]. Copyright (2017) J. Lumin.

effects. New additional information on the complex PL spectrum of the BiI3 single crystal, grown by the Bridgman method, was reported in 2017 [3] (Figure 1). Using an excitation wavelength equal with 500 nm, the PL spectrum of the BiI3 single crystal, recorded at liquid nitrogen temperature (LNT), shows a weak resonant PL band associated to free excitons observed at 1.97 eV near the absorption band edge [3]. Depending on the crystal growth conditions, different defects in the BiI3 layered structure were reported. According to R. E. Brandt et al. 2015 [56], using an excitation wavelength of 532 nm, a PL band in the range of 1.76-1.83 eV associated with free excitons has been detected at RT, both in a thin film obtained by spin-coating and in a single crystal grown by a modified Bridgman method with an attached electrodynamic gradient technique. The influence of stacking faults on excitonic states has been revealed by two new broader and weaker PL bands with maxima at 1.79 and 1.58 eV, respectively, given by a non-radiative decay of excitons deeply trapped by various dislocations within the crystal [3]. A most intense PL band of BiI3, with an oscillation strength exceeding the band associated with free excitons, is observed at 1.88 eV, this being attributed to the bound states in the immediate vicinity of the conduction band [3]. The emission bands assigned to the free and bound excitons are significant if we relate the consequences of the quantization effects in BiI3, the reason for which the wavefunction of excitons can be confined to the unit cell [55]. This feature was confirmed

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by the magneto-optical analysis in the intense magnetic field [57, 58], and as a result, the two bands become susceptible to be studied in non-linear effects. The consequence of a strong electron screening in BiI3 allows a great degree of distortion and aggregation exposed to a wide structural diversity, known as iodobismuthates with a basic formula of A3Bi2I9 (A = inorganic or organic species) [59]. A review of a wide variety of BiI3 derivates with various optical properties and extended applications has been detailed by Wu et al. 2009 [60]. Until now, only a few research studies have focused on the optical properties of layered bismuth-based halide-like semiconductors using an inorganic A-site cation (e.g., K+, Cs+, Rb+) [6163]. In this context, significant PL results were discussed in Cs3Bi2I9, when the main band at ~2.08 eV, assigned to free excitons, was reported at LNT under an excitation wavelength of 500 nm. In addition, a strong peak was observed down-shifted against the free excitonic band at 1.9 eV, which was attributed to the bound excitons trapped on different energetic levels inserted in the forbidden gap as non-radiative recombination centers [61]. The occurrence of a similar excitonic PL band in certain organic-inorganic bismuth halides such as (CH3NH3)3Bi2I9, (C6H14N)3Bi2I9 and (N-ethyl-4methyl-pyridinium)3Bi2I9 [63-66], was explained based on the electronic states that hold only the anionic cluster of Bi2I93- without any contribution from the cations. Similar positions of the PL band were also expected in Cs3Bi2I9, Rb3Bi2I9 and K3Bi2I9, as a dependence on the nature of the chemical bond. Lehner et al. [62] report their absorption edges similar to that of Cs3Bi2I9 when the connectivity of the BiI6 octahedra to the cations has little influence on the band gap. So far, two studies based on Cs3Sb3I9 and Rb3Sb3I9 compounds [67, 68] suggest a maximum PL band similar with that of Cs3Bi2I9. This fact was reported to indicate a low contribution of the central metal to optical properties. Among the layered crystals, the PbI2 compound was reported to be of a considerable interest. After its efficient use as X- and γ-radiation detection material [4, 5], a lot of studies have focused on its spectacular optical and electronic properties. The first PL investigations in PbI2 at different low temperatures [69, 70] reported information related to the short lifetime of

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excitonic PL (sub-nanosecond regime) due to the extremely easy cleavage. Subsequently, the emphasis was placed on various PL studies based on the anisotropic polariton model [71, 72], the dynamic of recombination processes, when the idea of superradiant decay of excitons in PbI2 [73] and the effect of annealing treatment on PL spectra [23, 74, 75] were revealed. Regarding the low-temperature PL analysis, PbI2 exhibits a strong and narrow excitonic E band situated at 2.5 eV followed by a strong and broad G band near of 2.07 eV and a weak D band down-shifted against the E band centered at 2.44 eV. The G and E bands were attributed to the nonradiative recombination of excitons trapped on different levels of the surface and volume defects [73, 74, 76]. All these peaks display a complex recombination of excitons described by a component with a fast decay time of free excitons (pico-nanoseconds range) due to PL quenched of excitons in a quantum sphere. By increasing the temperature, the following changes of the PbI2 PL spectra were reported: i) a decrease in the relative intensities of the E and D bands, and ii) a down-shift of the G and D bands [73]. For different power lights, only the E and D bands have a rapid change in intensity, while the G band is almost unchanged at a strong laser power. All these modifications were interpreted as significant evidences demonstrating that the G band is identified with surface defects due to the impurities and the synthesis conditions of the sample [73]. The doping procedure is well-known to control and give the desired properties of a material. In this context, many papers have been devoted to modifying the PL properties of the PbI2 doped with different metals. Thus, varying the type of impurity and the density of defects, different positions and intensities of the PbI2 PL bands were reported. While for the PbI2:Ag+ system only an increase in the intensity of all PL bands with an increase in the concentration of Ag+ ions was observed [77], in the case of PbI2 doped with Fe2+ or Ni2+ ions a high concentration of stacking faults that activates the defects in the crystal was reported. As a consequence, an intense PL band assigned to defects was observed as the cations concentration increases, while the excitonic peak is broadened and weakened [78]. According to Derenzo et al. 2013 [79], in a doped PbI2 system with La3+ and Cu+, the excitonic band associated with free excitons shifts

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towards lower energies, while the PL intensity increases with the La 3+ concentration. The situation was reported to be different when Cu+ is replaced with Ag+ or Te2+ species in PbI2 crystal. Compared to the undoped PbI2, the excitonic band remains in the same position but the intensity is suppressed by increasing the dopant concentration concomitant with a significant increase of the PL band of defects. Similar behavior is observed for PbI2:Mn2+ [80] and is explained as a consequence of the high acceptability of electrons by the metallic species. A different role of impurity was observed in PbI2:Nd3+ [81], when a small concentration of Nd3+ causes a high increase in the PL band intensity and for a high amount of impurities the PL spectra are quenched as a consequence of crystal defects. In the last decade, quantum confinement effects in low-dimensional materials as quantum dots (colloids or clusters), wells (thin films, nanoplatelet or layered materials) and wires (nanowires or nanorods), have received a great attention for potential applications in the field of the next generation optoelectronic devices. The different atomic sizes, shapes and atomic structures of PbI2 give distinct optical and electrical properties with respect to its corresponding 3D or bulk system, most of them being due to the quantum manifestation of charge carriers. Therefore, various research works have been reported on the chemical and physical synthesis methods of low-dimensional PbI2 structures with different shapes and wellcontrolled morphologies, e.g., hydrothermal method - for nanoplates and nanorods systems [82, 83], reverse micelle and laser ablation methods - for the synthesis of quantum dots of PbI2 [84, 85], chemical vapor deposition and physical vapor transport - for nanowires and nanoplatelets, and different chemical solution routes in various solvents as water, methanol, ethanol and acetonitrile - for colloids or wires shapes [76, 88]. Following the last procedure, the successful synthesis of micro- and nanometric particles of PbI2 has been achieved from the colloidal suspension of Pb(NO3)2 and KI in the variety of polar solvents above indicated [76, 88]. From the scanning electron microscope (SEM) images it was reported that changing the solvent of the reaction medium and the stoichiometry of the reactants, different sizes of PbI2 particles and secondary reaction products

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were obtained as follows: i) rods (KPbI3) and platelets (PbI2) particles in methanol and ethanol, ii) only platelets shape of PbI2 particles with various sizes in water and iii) an initial mixture of platelets and rods in acetonitrile, which were transformed by the storage time of one day into rod-like particles [76]. The SEM images of the particles synthesized in water and acetonitrile showed hexagonal and irregular platelets in water solvent, while in a host liquid of acetonitrile, small rod-like particles were revealed. According to the study reported by M. Baibarac et al. 2004 [76], the PL spectrum of PbI2 platelets was characterized of two main emission bands observed at ~2.5 eV (the E band) and ~2 eV (the G band), assigned to free and trapped excitons on defects, respectively. Interesting is that the E band is decomposed into two excitonic bands assigned to the radiative recombination and the stacking faults labeled as EF and ET, respectively. Some differences have been reported between PL spectra with associated platelets and rods [76]. The PL spectrum of PbI2 rods shows two intense bands at 2.5 eV and 2 eV, labeled as AT and Ax, respectively, whose intensities vary in opposite direction to the E and G bands of PbI2 platelets. This behavior is similar to the case of the Pb2+ ions embedded in the KI or NaI crystalline structure [76].

Anisotropic PL Properties in Layer-Type Crystals In contrast to typical optical, electrical, magnetic and mechanical properties of materials, for layered semiconductors with anisotropic crystal structures, different characteristics can be achieved when the polarized light is used by dividing it into two components, according to the Maxwell equations: the s- and p-eigenmode, with the electric field perpendicular and parallel, respectively, to the incident plane. At present, there are few works that have been focused on the anisotropic PL of the layered structures of the type graphene/phosphorene, transition-metal dichalcogenides, metal halides and their composites with organic and macromolecular compounds [6, 23, 71, 89-92]. The main conclusions of the anisotropic PL studies performed on the above layered structures were in the case of: i) a phosphorene monolayer with the orientation of the armchair and zig-zag

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type, when the presence of an emission band with maximum at 1.3 eV regardless of the polarized light, showed a highest PL intensity for the armchair direction due to the reduced symmetry and screening of electrons [89]; ii) a poly (para-phenylenevinylene) (PPV) functionalized reduced graphene oxide (RGO), when the wrapping angle of RGO sheets with macromolecular chains varies as increasing the RGO weight percentage concentration [93]; and iii) the layered crystals of the type ReS2, WeSe2, WS2, and MoSe2 when different manifestations of excitons with favored linear polarization were reported [90, 92]. Returning to metal halides, the idea of the anisotropic layered structure of PbI2 was reported in 1974 and 1981 [71, 72], when the light polarization parallel and perpendicular to the c-axis of the crystalline structure, was used to illustrate the PL dependence on the light polarization. Thereafter, most studies were based on the anisotropy of the lattice vibrations of PbI2 [6, 23, 94, 95], its thermal conductivity [96, 97] and its electrical properties such as carrier transport, resistivity, dark current and x-ray induced electricity [98]. Regarding the anisotropic PL analysis, a modification of PL properties in terms of two incident polarization lights (in plane and perpendicular to the measured IPb-I plane) was reported by I. Baltog et al. 2009 [23] based on the general spectrum of PbI2 recorded at LNT, with the following bands: a) an excitonic E band at ~2.5 eV; b) a D band situated in the immediate vicinity of the E band attributed to the stacking faults of the crystal and c) the G band at ~2 eV assigned to the surface defects. In this regard, a negligible change in the position of the G band, which was accompanied by a significant change in the band profile and the intensities recorded under polarized light conditions were reported. [23] According to Ref. [23], the D band occurs in the case of p-polarized light parallel to the I-Pb-I plane due to the revealing of the stacking faults defects, while for the second measuring configuration, i.e., the s-polarized light, the D band is completely suppressed. Besides, it was reported that the G band recorded under an excitation wavelength using a p-polarization gives rise to a finite structure of narrow bands as a signature of quantum confinement effects in the PbI2 layered structures obtained after annealing treatment at 5000C. No significant changes regarding the E band in the two measuring

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configurations were observed [23]. The strongly anisotropic PL character of PbI2 has also been observed by M. Baibarac et al. 2015 [6], where three different orientations of a single crystal sample were chosen for the PL measurements (Figure 2). At an excitation wavelength of 501.5 nm, a stronger excitonic PL band was observed in the O1 configuration of the PbI2 crystal at temperatures of 300K and 88K, whereas PL intensities significantly decrease in the O2 and O3 configurations. These anisotropic PL analyses were well supported by the polarized Raman scattering in the three measuring configurations [6, 23].

Figure 2. Emission spectra at temperatures of 300K (a) and 88K (b) in different crystalline orientations of PbI2 relative to the electric field (E) of the incident laser light (501.7 nm). In the inset is shown the Raman spectra recorded at 88K overlapping the PL background. The three measuring configurations (O 1, O2, O3) are shown in the figure top. Reprinted with permission of [6]. Copyright (2015) Mater. Res. Bull.

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The Sheets Number Identification in Layer-Type Semiconductors by PL Two-dimensional materials are competitive for applications in the field of electronic and optoelectronic devices, due to their interesting and remarkable physical and chemical characteristics. Although graphene is the most studied two-dimensional material due to its outstanding electrical, thermal and optical features [99-101], its zero band-gap that limits applications in optoelectronic fields, leads to the use of two-dimensional alternative semiconductors such as transition-metal dichalcogenides or halides. Although several articles have been published on the theoretically issues of two-dimensional nanosheets materials [102-105], a limited number of such materials have been experimentally studied. The most studied PL properties in mono and few-layers, experimentally obtained from their bulk counterparts, were on transitionmetal dichalcogenides. Among these, MoS2 opens up opportunities to generate atomically layers with outstanding characteristics. The PL features on this material were studied in 2010 by Splendiani et al. 2010 [25] and Mak et al. 2010 [26], when single-layer and few-MoS2 layers were successfully obtained using the mechanical exfoliation technique. It has been demonstrated that the PL intensity increases strongly. In singlelayer, the PL increases as decreasing number of layers, which is otherwise in contrast with a no detectable PL response emerged in bulk MoS2. A supporting point of this presumption is given by a review of theoretical analysis in single-layer, few-layer and bulk MoS2 [103], from which an indirect band gap was highlighted in bulk and few-layer, while a direct gap is expected with decreasing number of the layers. The quantum confinement of d-electronic states and the absence of interlayer hybridization in single-layer have explained the PL quantum efficiency. To accurately identify the MoS2 layers number, PL studies were carried out with complementary techniques as AFM, optical microscope and Raman scattering. In the same category of transition-metal dichalcogenides, PL properties of the MoTe2 [28] and WS2 [40, 41] single and few-layers were studied. The measured PL spectra of MoTe2, mechanically exfoliated and

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deposited onto a SiO2/Si substrate, indicate an increase in the emission band intensity and a significant blue-shift with the decreasing number of layers from bulk and 5L to 1L. According to Ref. [28], a strong PL band peaked at 1.08 eV is observed in a single-layer, which is the consequence of the direct band-gap transition and quantum confinement effects. A transition from indirect band gaps of few-layers to the direct-gap in singlelayer was also observed in WS2, of which the strong PL intensity at RT is revealed in the triangular morphology of single-layers [40, 41]. In spite of their exceptional optical properties, transitional-metal dichalcogenides exhibit a low mobility of carriers, which limits their use in different optoelectronic devices and especially as field-effect transistors and photodetectors. In this regard, metal halides, in particular PbI2, have begun to be studied theoretically and experimentally recently. A highly localized exciton was expected in few-layers structures and a study of this case was reported by Toulouse et al. (2015) [106] when a Frenkel-like exciton manifests itself in atomic thin films of PbI2 and determines a great binding energy and a strong Coulomb interaction between carriers. In this context, PL experiments are essential to describe the excitonic processes. To date, there are only two papers that have experimentally reported the PL properties in ultrathin flakes, single-layer, and few-layers of PbI2 [42, 43]. PbI2 flakes with a thickness of 23 to 395 nm were obtained, according to Ref. [42] by a growth method in solution followed by mechanically exfoliations in order to deposit the thin flakes onto SiO2/Si substrates. PL spectra are presented as a function of the flakes thickness, temperature, and laser power, but the study only focuses on the recombination processes of carriers. The transition from a direct band-gap in bulk PbI2 to an indirect band-gap in single-layer was verified by Zhong et al. (2017) [43]. An intense PL band peaked at 2.41 eV is observed in bulk, so that while the thickness decreases to few and single-layers, the PL bands become weak and slightly blue-shifted. Unlike MoS2, this behavior is explained as having the origin on changes induced by the band-gap, and this hypothesis was supported by the first principle calculations from which the band electronic structures indicate indirect band gaps in thinnest layers [42].

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Non-Linear Effects Supported by the PL Studies Due to their quantum well structure, layered-crystals become an important competitive class of materials promoted for various fundamental and applicative research studies in optoelectronic devices and substantially in non-linear optical fields. In this regard, PL is the key condition for generating non-linear optical processes. The laser action in PbI2 observed under low temperatures [1] and the superradiant decay of excitons in PbI2 [73] were the first motivations that made PbI2 a good candidate for studying non-linear optical processes. In this context, a complex work involving an enhancement of the vibrational modes of the PbI2 Raman spectra has been investigated and interpreted based on a non-linear effect [6]. Under low temperatures and resonant light conditions, a strong enhancement of the Raman scattering has been observed and interpreted as an exciton-phonon interaction corresponding to a non-linear optical effect of the type Stimulated Raman Scattering (SRS). Adjusting the incident excitonic light was performed considering the overlap of the laser light over the luminescent profile until the vibrational modes are more enhanced. In this context, two light sources, the incident laser light, and the strong PL light interact together to produce a non-linear effect as SRS process. For a better understanding of this behavior, a study of BiI3 was also reported [3]. BiI3 is also a great candidate for such non-linear effects, as long as its intense PL band has been shown to behave as a quasimonochromatic light due to a high binding energy and the strong oscillation strength, features that were previously presented in detail. [57, 58] Therefore, the enhancement of the Ag band, the most intense vibrational mode in the Raman spectrum, was reported. [3] A brief view of the process is given in Figure 3 and presents the overlapping of the threeincident laser light (676, 660 and 647 nm) on the BiI3 PL profile.

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Figure 3. (left) (a) The PL band of BiI3 with the superposition of three incident lights (676, 660 and 647 nm); (b) The ratio of the A g band intensity recorded in the Stokes branch of the Raman spectrum, I(T)/I(300K), at a temperature T and 300 K; (right) The superposition of the incident light over the PL band of BiI3. Reprinted with permission of [3]. Copyright (2017) J. Lumin.

According to Figure 3 (right), when the incident wavelength of 647 nm one overlaps the states associated with the right side of the PL band maximum, a more anti-Stokes enhancement was expected. In the reverse situation, a more Stokes enhancement was predicted at the excitation wavelength of 676 nm, while overlapping the maximum of the PL band will give the more enhancement Raman scattering (Figure 3(b)). Therefore, the non-linear effect implies the coherent coupling of the laser light with the quasi-monochromatic PL light shifted Stokes or anti-Stokes. This fact was considered a consequence of a super-radiant emission of the PL band and the short decay time of several ns was correlated with the coherence of light.

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PHOTOLUMINESCENCE PROPERTIES IN COMPOSITE MATERIALS BASED ON LAYERED CRYSTALS AND ORGANIC/MACROMOLECULAR COMPOUNDS Layer-type iodide semiconductors are the most studied materials as composites that are characterized by two effective areas susceptible to functionalization/intercalation processes and chemical reactions: a) the rigid repetitive sequence of layers with a chemically reactive region due to the electronic surface states given by the defects, dislocates or dangling bonds of the metal and iodide species; b) the feeble van der Waals layer built between saturated iodide species and which are relevant for the insertion of the organic species. Since dihalides represent the majority of layered metal-halides, a large number of papers describe the formation of a new class of organicinorganic hybrid materials using these halides as hosts and starting materials. The combination of the unique properties of this halide layered materials with various organic/macromolecular compounds makes their composite materials interesting as multifunctional systems. Photovoltaic cells, photodetectors, memory and advanced optoelectronic devices are among the most promising applications 107-110. In this context, knowledge on the molecular structure of these compounds and their excitonic transitions are essential for understanding the PL properties. Organic-inorganic hybrid materials form naturally quantum wells structures built from inorganic layers intercalated with organic species. Their general crystalline structure consists of inorganic sheets of [MX6]4octahedrons, where M is the metal and X is the halide, connected with the organic species (usually alkylammonium-type) through the hydrogen, π-π or van der Waals interactions in structures that are of the type two- or three-dimensional. Therefore, the flexibility of modifying the excitonic effects depending on the interlayer dielectric separation of organic species and the metal-type can lead to various optical properties that make them very attractive [7, 8, 109]. Typically, a large length of the organic compound causes the atoms to be arranged in a two-dimensional structure,

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while a small-chain alkylammonium builds the three-dimensional structures. Although three-dimensional structures (e.g., [RNH3]MX3, where R, M, and X correspond to the alkyl chain, the metal, and the halogen, respectively) have received a special attention since they can be used as a source of material synthesized for photovoltaic cells, only twodimensional layered compounds ([RNH3]2MX4) will be presented at the present stage. After successful intercalation of PbI2 and CdCl2 layered materials with different organic species [10], the desire to know in detail the excitonic processes in each member of the (CnHn+1NH3)2MX4 compounds increased significantly. In this context, the excitonic effects in organic-inorganic hybrids of two-dimensional semiconductors were reported in 1989 by Ishihara et al. (1989) [9] using the (C10H21NH3)2PbI4 case study and one year later the (CnHn+1NH3)2PbI4 compound, where n = 4, 6, 8, 9, 10, 12 [8]. From the absorption, reflectivity and PL studies it has established a strong oscillation strength, a small Bohr radius and a binding energy of excitons that was 12 times stronger than in the case of PbI2. Due to the wide band gap of organic species, a low dielectric constant was expected, while the Coulomb interaction between electrons and holes was less screened. These unique physical properties were understood as effects of the dielectric confinement of the organic layer and were correlated with the quantum wells structures, when confined excitons were very stable even at RT. The idea of stable excitons at RT in layered semiconductors was supported by various PL studies 7, 9, 44-47. In this context, by varying the dielectric environment (i.e., the type of organic compound and its length), a high PL efficiency and a long-term stability were reported to be obtained 7. Zhang et al. (2009) [7] successfully synthesized 11 novel organic-inorganic hybrid materials based on lead halides via spin-coating of the PbI2 with different cyclic alkyl amines: phenylmethaneamine (PM), 2-phenylethaneamine (PE), 3-phenylpropane-1-amine (PP), cyclohexaneamine (C), cyclohexylmethaneamine (CM), 2-cyclohexylethaneamine (CE), 1-adamantanamine (A), 1-adamantanemethylamine (AM), myrtanylamine (M), cyclooctanemine (CO) and 2-cyclohexenylethaneamine (CHE). They studied the influence of the organic chain on optical properties and found strong

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excitonic PL bands peaked in the range of 2.34-3.07 eV. [7] Due to a slight change of several nanometers in the PL band position, the authors correlated the localization of excitons only with the inorganic part. Only three organic-inorganic compounds based on PbI2 with PP, A and M exhibit a broader PL band which was explained by the volume of the organic part. A supporting proof of this presumption was given by three PbI4-based layered materials ([C10NH3]2PbI4, [C6H5-C2H4NH3]2Pbl4 and [C6H5-C2H4NH3]2[CH3NH3]Pb217), where the effect of the dielectric environment on the confinement effects of excitons provides different corresponding binding energies [45]. Furthermore, Ahmad et al. (2014) [44] found that the PL efficiency can also be related to the conformation of the organic part, when a slow intercalation rate of the long organic chain like C12H25NH3 compared to the C6H9C2H4NH3 into the layers of PbI2 has been related. Subsequent investigated studies reported a successful synthesis of new organic-inorganic compound [4-FC6H5C2H4NH3]2PbI4 when needle-like crystals were grown from a solution mixture of organic and inorganic compound in dimethylformamide [46]. A stable exciton established on the basis of the PL vs. temperature dependence and a strong excitonic band at 2.35 eV due to the band-to-band recombination of excitons is reported. The origin of the later band was attributed to the PbI42quantum wells and was consistent with the bands observed in the following two-dimensional lead-iodide-based compounds: (C10H21NH3)2PbI4 (2.37 eV) [9], (C6H5CH2NH3)2PbI4 (2.35 eV) [7], (C6H5C2H4NH3)2PbI4 and (C6H11C2H4NH3)2PbI4 (2.37eV) [7, 48]. In addition, low-dimensional [4-FC6H5C2H4NH3]2PbI4-type crystals as 0D, 1D, and 2D were stabilized in acetonitrile and carbon tetrachloride solution [47]. A comparable position of the excitonic band recorded at RT under an excitation wavelength of 300 nm was observed to be followed by another exciton peaked at 2.27 eV. A similar PL band was observed in the most frequently used semiconductor perovskite of (C6H5C2H4NH3)2PbI4, which have been investigated by Gauthron et al. (2010) [48].

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Figure 4. PL spectra recorded at LNT of: a) PbI2 micro-crystalline powder, b) PbI2/PANI-EB and PbI2/PANI-ES as a function of the irradiation time. Reprinted with permission of [116]. Copyright (2009) Journal of Solid State Chemistry.

An exact related linear increase of the two PL bands, labeled as S1 and S2, in the laser power range implies an intrinsic origin that cannot be attributed to excitons trapped on different states. Taking into account the same intensity and shape of the PL and photoluminescence excitation (PLE) bands, the authors deduced a connection of the two PL bands to the same excited state. As decreasing the temperature, a strong increase in the relative intensity of the PL spectrum of (C6H5C2H4NH3)2PbI4 was reported by K. Gauthron et al. (2010) 48, the fact which was explained to have the origin in the dielectric environment of the organic species. Therefore, in addition to a strong confinement effect of carriers in the quantum

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structures represented by the inorganic layer, it has been demonstrated that the choice of organic compounds with a lower dielectric constant induces an additional increase in the exciton binding energy. A change in the excitonic transitions associated with the PL spectra of the above organicinorganic compounds was carried out by halogen substitution. As will be shown below, a sustained effort was devoted to the replacement of iodine with bromine in the above organic-inorganic hybrid compounds. The first attempt to partially substitute iodine ions in lead-iodide-based compounds was carried out in 1997 [49] when Kitazava has synthesized (C6H5C2H4NH3)2Pb(BrxI4-x) crystals with a strong PL emission peaked at 3.16 eV observed at RT and shifted to blue region. In addition, maintaining the same organic species, a total replacement of the iodine atom has been accomplished with obtaining the (C6H5C2H4NH3)2PbBr4 compound characterized by an excitonic PL band peaked at 2.99-3.08 eV and a strong exciton binding energy (250-430 meV), which was 10 times larger than those of a PbI2 crystal [50, 51, 53]. (C5H10N3)PbBr4, (C2H2N4)PbBr3 [52] and other lead-bromide layered materials with cyclic alkylamines [7]. These were successfully synthesized by a solution-phase grown method having a PL peak position in the range of 2.73-3.12 eV [50-53]. From an applicative point of view, both the high binding energy and the excitonic states shifted in the blue region make these organic-inorganic lead-bromide great candidates for light-emitted devices. The applications in the field of the photovoltaic devices [110] and the solar cells [111] were also reported on Pb2+ coordination polymers containing anthracene units as well as CH3NH3PbI3 perovskites whose crystallinity and morphology were controlled by the adding of a small weight of polyvinylidene fluoridetrifluoroethylene to the PbI2 solution. All these applications were possible only after detailed knowledge of optical properties of PbI2 microcrystalline embedded in different macromolecular compounds of the type ethylenemethacrylic acid copolymer [112], polyvinyl alcohol [113], polyacrylamide [114] and polyaniline [115, 116]. In all above cases [112116], variations induced in PbI2 PL spectra were understood as an intercalation process of macromolecular compounds between PbI2 layers, when either various thicknesses of nanoparticles including two until nine

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monolayers were reported. Besides, the mechanical-chemical interaction of polyaniline in the semiconducting form, i.e., of polyaniline-emeraldine base (PANI-EB), with PbI2 was demonstrated to result in a functionalization of PbI2 layers with polyaniline-emeraldine salt (PANIES) [116]. Depending on the semiconducting or conducting state of the macromolecular compounds used at the functionalization/intercalation process of PbI2 and the irradiation time, a different behavior of the emission band with the maximum at 2.0 eV was reported [116]. Decreasing the PbI2 PL intensity (Figure 4) until its disappearance was explained as a partial or total charges collection effect [116]. A PL study worth mentioning, recently published, is the work reported by A. N. Usoltev et al. (2017) [117] which demonstrates that the generation of new compounds of the type polymeric hybrid iodoplumbates and iodobismuthates with derivatives of 1, 2-bis(4-pyridyl)ethylene show a PL band at 670 nm, for which applications in the solar cells design are expected in the next period.

CONCLUSION In this chapter, the progress recorded on the photoluminescence (PL) of semiconducting crystals with a layered structure of the type PbI2 and BiI3 as well as their composites with various organic/macromolecular compounds were reviewed. Five aspects were shown in this chapter as follows: i) influence of defects on PL of layered crystals and their quantum confinement effects; ii) anisotropic PL properties in layer-type crystals, iii) the use of PL as a valuable tool for identifying the number of layers in inorganic semiconductors; iv) the highlighting of a non-linear optical effect of the type Stimulated Raman Scattering (SRS) supported by a strong PL of semiconducting crystals with layered structure; v) the emphasizing of PL properties in composite materials based on layered crystals and organic/macromolecular compounds. New PL studies are expected in the next period concerning the functionalization/intercalation processes of the layered semiconducting crystals with different insulating and conducting

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polymers as well as to assessing of the wrapping angle of the semiconducting crystals layers with various organic/ macromolecular compounds in order to improve performances of energy conversion devices.

ACKNOWLEDGMENT This work was financed by Core Program 2016–2018, project PN16480101.

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[102] Zhou, M., Duan, W., Chen, Y., Du, A. Nanoscale 2015, 7, 1516815174. [103] Zhao, W., Ribeiro, R. M., Toh, M., Carvalho, A., Kloc, C., Neto, A. H. C., Eda, G. Nano Lett. 2013, 13, 5627−5634. [104] Dong, N., Li, Y., Feng, Y., Zhang, S., Zhang, X., Chang, C., Fan, J., Zhang, L., Wang, J. Sci. Rep. 2015, 5, 14646. [105] F. Ma, M. Zhou, Y. Jiao, G. Gao, Y. Gu, A. Bilic, Z. Chen, A. Du, Sci. Rep. 2015, 5, 17558. [106] Toulouse, A. S., Isaacoff, B. P., Shi, G., Matuchová, M., Kioupakis, E., Merlin, R. Phys. Rev. B 2015, 91,165308. [107] Dou, L., Yang, Y., You, J., Hong, Z., Chang, W.-H., Li, G., Yang, Y. Nature Commun. 2014, 5, 5404. [108] Hwang, B., Lee, J.-S. Sci. Rep. 2017, 7, 673. [109] Tabuchia, Y., Asaia, K., Rikukawab, M., Sanuib, K., Ishigurea, K. J. Phys. Chem. Solids 2000, 61, 837–845. [110] Dong, D., Yu, N., Zhang, W., Cong Y., Zhao, H., Li, Z., Liu, D., Liu, J., Liu, D. Inorg. Chem. Commun. 2016, 70, 99-102. [111] Sun, C., Guo, Y., Fang, B., Yang, J., Qin, B., Duan, H., Chen, Y., Li, H., Liu, H. J. Phys. Chem. C 2016, 120, 12980–12988. [112] Goto, T., Saito, S. J. Lumin. 1996, 70, 6435-447. [113] Savchuk, A. I., Fediva, V. I., Kandyba, Y. O., Savchuk, T. A., Stolyarchuk, I. D., Nikitin, P. I. Mater. Sci. Eng. C. 2002, 19, 5962. [114] Preda, N., Mihut, L., Baibarac, M., Baltog, I. J. Optoelectron. Adv. Mat. 2007, 9, 1358-1361. [115] Ameen, S., Lakshmi, G. B. V. S., Husain, M. J. Phys. D: Appl. Phys. 2009, 42, 105104. [116] Baibarac, M., Baltog, I., Lefrant, S. J. Solid State Chem. 2009, 182, 827-835. [117] Usoltsev, A. N., Adonin, S. A., Abramov, P. A., Korolkov, I. V., Yushina, I. V., Antonova, O. V., Sokolov, M. N., Fedin, V. P. Inorg. Chim. Acta 2017, 462, 323–328.

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BIOGRAPHICAL SKETCHES Andreea Nila Affiliation: National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials Education: 2015-present: University of Bucharest, Faculty of Physics, PhD Program: Optics, Spectroscopy, Plasma, Lasers; 2015: MSc., University of Bucharest, Faculty of Physics, Master Program: Physics of Advanced Materials and Nanostructures; 2013: Bachelor (Eng.), Polytechnic University of Bucharest, Faculty of Applied Chemistry and Materials Science, Bachelor Degree: Science and Engineering of Oxide Materials and Nanomaterials. Business Address: Atomistilor Str. 405A, P.O. Box MG-7, Magurele, R077125 Ilfov, Romania Research and Professional Experience: Semiconducting Inorganic Compounds, Photoconductivity, Raman Scattering

Material Science, Photoluminescence,

Professional Appointments: Assistant Scientific Researcher, 2013 - at present Publications from the Last 3 Years: 

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Influence of TiO2 and Si on the exciton-phonon interaction in PbI2 and CdS semiconductors evidenced by Raman spectroscopy, A Nila, I. Baltog, D. Dragoman, M. Baibarac, I. Mercioniu, Journal of Physics: Condensed Matter 67, 52-58, 2017. Exciton-phonon interactions in the Cs3Bi2I9 crystal structure revealed by Raman spectroscopic studies, A. Nila, M. Baibarac, A. Matea, R. Mitran, I. Baltog, Physica Status Solidi (B) Basic Research 254, (4), 1552805, 2017.

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The exciton-phonon interaction as stimulated Raman scattering effect supported by the excitonic photoluminescence in BiI3 layered crystal structure, A. Nila, A. Matea, M. Baibarac, I. Baltog, Journal of Luminescence 182, 166-171, 2017. Influence of single-walled carbon nanotubes enriched in semiconducting and metallic tubes on the electropolymerization of tetrabromo ortho-xylene: Insights on the synthesis mechanism of poly(ortho-phenylenevinylene), M. Baibarac, A. Nila, I. Baltog, S. Lefrant, J. Y. Mevellec, S. Quillard, B. Humbert, European Polymer Journal 88, 109-125, 2017. Exciton-phonon interaction in CdS of different morphological forms manifested as stimulated Raman scattering, M. Baibarac, A. Nila, I. Baltog, Optical Materials Express, 6, 1881-1895, 2016. Polarized Raman spectra of phosphorene in edge and top view measuring configurations, M. Baibarac, A. Nila, I. Baltog, RSC Advances, 6, 58003-58009, 2016. Ab initio investigation of optical properties in triangular graphene - boron nitride core- shell nanostructures, A. Nila, G. A. Nemnes, A. Manolescu, Romanian Journal of Physics, 60(5-6), 696-700, 2015.

Mihaela Baibarac Affiliation: National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials Education: 2002 Ph.D., Physics-Optics, Spectroscopy and Laser (with Summa cum Laude), University of Bucharest, Faculty of Physics, 1996 M. Sc., Chemistry - Thermodynamics and Applied Electrochemistry, University Politehnica of Bucharest, Faculty of Industrial Chemistry 1995 Eng., Chemistry - Polymer Science, Polytechnic University of Bucharest, Faculty of Industrial Chemistry

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Business Address: Atomistilor Str. 405A, P.O. Box MG-7, Magurele, R077125 Ilfov, Romania Research and Professional Experience: Material Science, Carbon Nanotubes, Graphene, Inorganic Compounds, Polymers, Composites, Raman Scattering, Surface Enhanced Raman Scattering (SERS), Photoluminescence, Infrared Spectroscopy, Electrochemistry, Supercapacitors, Rechargeable lithium batteries Professional Appointments: Senior research scientist Ist degree, Lab. Optical Process in Nanostructured Materials, National Institute of Materials Physics (NIMP), 2010-present Honors: Prize for Physics, C. Miculescu, of the Romanian Academy for the group of papers Raman studies on conducting polymers thin films, 2000. Publications from the Last 3 Years: 

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The spectrochemical behavior of composites based on poly (paraphenylenevinylene), reduced graphene oxide and pyrene, M. Ilie, M. Baibarac, Optical Materials 72, 140-146, 2017. The influence of single-walled carbon nanotubes on optical properties of the poly[(2,5-bisoctyloy)-1, 4 phenylenevinylene] evidenced by infrared spectroscopy and anti-Stokes photoluminescence, M. Baibarac, I. Smaranda, I. Baltog, S. Lefrant, J. Y. Mevellec, Optical Materials 67, 52-58, 2017. Exciton-phonon interactions in the Cs3Bi2I9 crystal structure revealed by Raman spectroscopic studies, A. Nila, M. Baibarac, A. Matea, R. Mitran, I. Baltog, Physica Status Solidi (B) Basic Research 254, (4), 1552805, 2017. Electrochemical characterization of the poly(2, 2'-bithiophene-copyrene) functionalized single-walled carbon nanotubes films and

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their applications in supercapacitors field, M. Baibarac, I. Baltog, M. Daescu, International Journal of Electrochemical Science 12, (3), 2013-2025, 2017. Influence of single-walled carbon nanotubes enriched in semiconducting and metallic tubes on the electropolymerization of tetrabromo ortho-xylene: Insights on the synthesis mechanism of poly(ortho-phenylenevinylene), M. Baibarac, A. Nila, I. Baltog, S. Lefrant, J. Y. Mevellec, S. Quillard, B. Humbert, European Polymer Journal 88, 109-125, 2017. Optical properties of single-walled carbon nanotubes highly separated in semiconducting and metallic tubes functionalized with poly(vinylidene fluoride), A. Matea, M. Baibarac, I. Baltog, Journal of Molecular Structure 1130, 38-45, 2017. The exciton-phonon interaction as stimulated Raman scattering effect supported by the excitonic photoluminescence in BiI3 layered crystal structure, A. Nila, A. Matea, M. Baibarac, I. Baltog, Journal of Luminescence 182, 166-171, 2017. Infrared dichroism studies and anisotropic photoluminescence properties of poly(para -phenylene vinylene) functionalized reduced graphene oxide, M. Baibarac, M. Ilie, I. Baltog, S. Lefrant, B. Humbert, RSC Advances 7, (12), 6931-6942, 2017. Aging phenomena and wettability control of plasma deposited carbon nanowall layers, S. Vizireanu, M. D. Ionita, R. E. Ionita, S. D. Stoica, C.M. Teodorescu, M.A. Husanu, N.G. Apostol, M. Baibarac, D. Panaitescu, G. Dinescu, Plasma Processes and Polymers 14 (1), e1700023, 2017. Influence of TiO2 and Si on the exciton-phonon interaction in PbI2 and CdS semiconductors evidenced by Raman spectroscopy, A. Nila, I. Baltog, D. Dragoman, M. Baibarac, I. Mercioniu, Journal of Physics: Condensed Matter 67, 52-58, 2017. Optical evidence for chemical interaction of the polyaniline/fullerene composites with N-methyl-2-pyrrolidinone, M. Baibarac, I. Baltog, M. Daescu, S. Lefrant, P. Chirita, Journal of Molecular Structure, 1125, 340-349, 2016.

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Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene), M. Baibarac, I. Baltog, M. Ilie, B. Humbert, S. Lefrant, C. Negrila, Journal of Physical Chemistry C, 120, 5694-5705, 2016. Exciton-phonon interaction in CdS of different morphological forms manifested as stimulated Raman scattering, M. Baibarac, A. Nila, I. Baltog, Optical Materials Express, 6, 1881-1895, 2016. Polarized Raman spectra of phosphorene in edge and top view measuring configurations, M. Baibarac, A. Nila, I. Baltog, RSC Advances, 6, 58003-58009, 2016. Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4-ethylenedioxythiopheneco-pyrene), I. Baltog, M. Baibarac, I. Smaranda, A. Matea, M. Ilie, J. Y. Mevellec, S. Lefrant, Optical Materials, 62, 604-611, 2016. Mechanism of the cathodic process coupled to the oxidation of iron monosulfide by dissolved oxygen, M.I. Duinea, A. Costas, M. Baibarac, P. Chirita, Journal of Colloids and Interfaces Science, 467, 51-59, 2016. Exciton-phonon interaction in PbI2 revealed by Raman and photoluminescence studies using excitation light overlapping the fundamental absorption edge, M. Baibarac, I. Smaranda, M. Scocioreanu, R.A. Mitran, M. Enculescu, M. Galatanu, I. Baltog, Materials Research Bulletin, 70, 762-772, 2015. Raman scattering and photoluminescence studies of ZnO nanowhiskers assembled as flowers in the presence of fullerene, M. Baibarac, I. Baltog, A. Matea, S. Lefrant, Journal of Crystal Growth, 419, 158-164, 2015. Anti-Stokes Raman spectroscopy as a method to identify metallic and mixed metallic/semiconducting configurations of multi-walled carbon nanotubes, M. Baibarac, A. Matea, M. Ilie, I. Baltog, A. Magrez, Analytical Methods, 7(15), 6225-6230, 2015. Anti-Stokes Raman spectroscopy as a method to identify the metallic and semiconducting configurations of double-walled

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carbon nanotubes, M. Baibarac, I. Baltog, A. Matea, L. Mihut, S. Lefrant, Journal of Raman Spectroscopy, 46(1), 32-38, 2015. Photochemical processes developed in composite based on highly separated metallic and semiconducting SWCNTs functionalized with polydiphenylamine, M. Baibarac, I. Baltog, I. Smaranda, A. Magrez, Carbon, 81, 426-438, 2015. 1D-polyaniline starting from self-assembled systems, D. Donescu, M. Ghiurea, C.I. Spataru, G. Sting, D. Anghel, M. Baibarac, I. Baltog, Colloid and Polymer Science, 293(9), 2515-2524, 2015. Optical Properties of Single-Walled Carbon Nanotubes Functionalized with Poly(2,2'-bithiophene-co-pyrene) Copolymer, I Smaranda, M Baibarac, M Ilie, A Matea, I Baltog, S Lefrant, Current Organic Chemistry, 19(7), 652-661, 2015.

In: Photoluminescence Editor: Ellis Marsden

ISBN: 978-1-53613-537-4 © 2018 Nova Science Publishers, Inc.

Chapter 4

RECENT PROGRESS IN THE PHOTOLUMINESCENCE PROPERTIES OF COMPOSITES BASED ON CONJUGATED POLYMERS AND CARBON NANOPARTICLES Mihaela Baibarac1,*, Mirela Ilie1, Adelina Matea1, Monica Daescu1 and Serge Lefrant2 1

Laboratory of Optical Processes in Nanostructured Materials, National Institute of Materials Physics, Bucharest, Romania 2 Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, Paris, France

ABSTRACT This chapter focuses on new aspects concerning the photoluminescence properties of composites based on conjugated polymers and carbon nanoparticles of the type carbon nanotubes, reduced graphene oxide and fullerenes. A review of the photoluminescence

*

Corresponding Author Email: [email protected].

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Mihaela Baibarac, Mirela Ilie, Adelina Matea et al. properties of various macromolecular compounds, for example poly(para-phenylenevinylene), poly(3-hexylthiophene), poly(3,4ethylenedioxythiophene-co-pyrene), polydiphenylamine and poly(9,9dioctylfluorenyl-2,7-diyl) as well as effects induced by the carbon nanoparticles mentioned above. A special attention will be given to the insight of different de-excitation mechanisms developed in the composite materials based on the macromolecular compounds and carbon nanotubes highly separated in semiconducting and metallic components. The antiStokes photoluminescence processes and anisotropic photoluminescence properties of some composites listed above will be also reviewed.

Keywords: photoluminescence, conjugated polymers, carbon nanotubes, graphene, fullerene

INTRODUCTION The applications of composites based on polymers and carbon nanoparticles of the type carbon nanotubes, graphene and fullerene in the photovoltaic cells field have been developed after a detailed knowledge of photoluminescence (PL) properties of these materials. In this context, the first PL studies of composites based on macromolecular compounds and carbon nanoparticles of the type fullerene, carbon nanotubes and graphene have been published in 1993 [1], 1999 [2] and 2009 [3], respectively. In the last 17 years, a sustained effort in the knowledge of PL properties of composites based on polymers and carbon nanotubes/graphene was recorded by the publication of more than 450 articles. This fact was a consequence of the development of: i) new synthesis methods at large scale of graphene oxide (GO) [4], reduced graphene oxide (RGO) [5] and carbon nanotubes (CNTs) [6, 7] and ii) new protocols for the purification of carbon nanotubes [8]. Sustained efforts were achieved for the synthesis and physical/chemical properties of the composite materials as well as their applications, an argument in this sense being the large number of review articles focused both on single-walled carbon nanotubes (SWNTs) [e.g., 9] and multi-wall carbon nanotubes (MWNTs) [e.g., 10]. The influence of

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CNTs (SWNTs and MWNTs), RGO and C60 on the conjugated polymers (CPs) PL will be reviewed also in this chapter. The progress performed in the separation of the metallic and semiconducting SWNTs [11] has constituted a challenge for the field of composites containing such carbon nanoparticles and insulating or conducting polymers. The influence of the highly separated metallic and semiconducting SWNTs on the CPs PL will be discussed in this chapter. A special attention will be given to the different de-excitation mechanisms reported so far. Beginning with 2012, a special attention has been focused on antiStokes PL [9, 12] in the case of the CPs/CNT composites, as a new optical process and a brief review of these properties will be also reported in this work. Using PL as a valuable tool in assessing the photochemical reactions in the case of composites based on CPs and carbon nanoparticles of the type SWNTs [13] and RGO [14] is a new topic developed in the last three years. The PL ability in the highlighting of such reactions will be also discussed in this work. The selective wrapping of CNTs was not completely understood four years ago, when a first article reported information about the coverage degree and wrapping angle of poly(9, 9-dioctylfluorenyl-2, 7-diyl) (PFO) around CNTs [15]. A review of the progress recorded on this topic in the case of the CPs/CNT and CPs/RGO composites will be also presented in this chapter.

PHOTOLUMINESCENCE OF COMPOSITES BASED ON CONJUGATED POLYMERS AND CARBON NANOTUBES AND ROLE OF CARBON NANOTUBES IN THE CONJUGATED POLYMERS PHOTOLUMINESCENCE QUENCHING PROCESS Three types of electronic interactions in the excited state of the CPs/CNT composites were reported by W. Ki et al. as follows: a) the hole

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transfer; b) the electron transfer and iii) the energy transfer from the polymer to nanotubes based on the electron and hole pair (i.e., the exciton) [16]. The first PL studies in the field of CPs/CNT composites were reported on poly(m-phenylene vinylene-co-dioctyloxy-p-phenylene vinylene) (PmPV), when a red-shift of the emission bands of the polymer was explained taking into account that the wrapping of SWNTs with PmPV induces a decrease of the electrons delocalization in the macromolecular chain [17]. A thorough study on the effects induced by CNTs concentration on the PL spectra of poly(para-phenylene vinylene) (PPV) was reported by F. Massuyeau et al. in 2007. 18 The PL spectrum of PPV was reported to be characterized by three emission bands peaked at 2.42, 2.27 and 2.13 eV, the first two being assigned to electronic emission transitions of macromolecular chains with 5 and 7-10 repeating units, respectively, and the last one was attributed to vibronic replica of the first order of the emission band at 2.42 eV.18 A PPV PL quenching process was highlighted in the case of the samples obtained by the annealing conversion of the PPV precursor solution, containing different SWNT concentrations, performed at 300 and 1200C [18]. Besides, the presence of different SWNTs concentrations in the PPV precursor solution was demonstrated to induce a shortening of the PPV macromolecular chains [18]. Therefore, the PPV PL quenching process in the presence of SWNTs was accompanied by a PL spectrum profile change, fact which was explained by the contribution of the electronic transitions of the macromolecular chains repeating units with lengths ranging from 4 up to 7-10 [18]. The SWNTs role as PL quenchor for the following CPs: poly[2methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) [19], poly(2,5-dimethoxy-p-phenylenevinylene) (MO-PPV) [20], poly(3hexylthiophene) [21], polyfluorene [22], and so on, was studied. Thus, in the case of the MO-PPV/SWNTs composites, a blue shift of the MO-PPV PL band situated at 590 nm was reported to be induced by SWNTs [20]. In order to explain the SWNTs influence on the PL of the MO-PPV, the timeresolved PL decays studies were reported [20, 23]. According to Ref. [23] the pristine MO-PPV PL decay lifetime was described by bi-exponential fits corresponding to 0.43 ns (93%) and 0.99 ns (7%) and an average

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lifetime of 0.52 ns for the band situated at 590 nm. The decay lifetimes for the band peaked at 630 nm were of 0.56 ns (93%) and 0.56 ns (7%) with an average lifetime of 0.56 ns [23]. The short lifetimes were explained as a consequence of the presence of multiple decay channels for excitons of the macromolecular chains. Adding SWNTs in the polymer matrix was reported to induce a relaxation process and an enhancement of the nonradiative decay rate [23]. In this last case, the PL decay lifetimes were characterized by a tri-exponential behavior with values in the interval (0.28-0.43) ns for the band peaked at 590 nm and (0.35-0.46) ns for the band situated at 630 nm, respectively [23]. These changes were interpreted as a charge separation which took place at the polymer/SWNTs interface indicating a non-radiative process for photogenerated excitons. Chu et al. have demonstrated that the backbone of the MO-PPV macromolecular chains were in contact each other, which led to a different decay pathway, interchain migration and recombination [20]. A -* interaction between the P3HT and SWNTs was invoked to explain the decrease of the P3HT PL spectrum intensity in the presence of CNTs [24]. The CPs PL quenching was reported to be also induced by MWNTs. Some examples of such studies that are worth mentioning are those which were focused on MWNTs and CPs of the type: poly(N-vinylcarbazole) [25], P3HT [26], poly(9, 9-dioctylfluorenyl-2,7-diyl) end capped with dimethylphenyl (PFO) [27], MEH-PPV [28], MO-PPV [29] and so on. In the case of the MO-PPV/MWNTs composites, depending on the synthesis method, namely i) the mixture of the two compounds, i.e., MWNTs and MO-PPV, in chloroform followed by stirring for 4h [16] and ii) in situ polymerization of the monomer in the presence of oxidized MWNTs, tetrahydrofuran and tert-butoxide [30], a decrease in the MO-PPV PL spectrum intensity as well as a down-shift of the emission band at 571 nm, was reported. This behavior was explained by taking into account an energy transfer process between the two constituents of the composite material [16] and the suppression of the direct recombination, respectively, as a result of the fact that MWNTs are electron acceptors in these composites and that they have the role to dissociate the photogenerated excitons [29]. As reported by A. Bakour et al. [31], the presence of CNTs induces the appearance of

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additional structural defects like traps which are responsible of the P3HT PL quenching process and the increase of the PL average decay times when the MWNTs weight increases in the P3HT/MWNTs composite mass. The CNTs role as PL quenching was also evidenced in the case of other macromolecular compounds. An example in this sense being the ionic polymers such as poly(Pyridinium Salt)s (PS) [32, 33]. According to Refs. [32, 33], the PL spectra of the PS/SWNTs composites with different concentrations of SWNTs (varying from 5 to 50 wt.%) show emission bands with maxima situated at 448, 383 nm and a broad band in the domain 510-530 nm, which were correlated with the  - * and cationic- interactions and the energy or electron transfer, respectively, established between the two constituents (i.e., SWNTs and PS).

Figure 1. PL spectra of the BPPV/M-SWNTs (blue curve) and BPPV/S-SWNTs (red curve) composites, under the excitation wavelength of 460 nm. Reprinted with permission of [34]. Copyright (2014) Synth. Met.

Despite these efforts, above noticed, to explain the CPs PL quenching process induced by CNTs, it was reported only in 2014 a first experimental evidence that the optical process takes place differently in the presence of SWNTs as: i) a mixture of metallic (33%) and semiconducting (66%) tubes (M+S) and ii) the highly separated metallic (98%, M) and semiconducting (99%, S) tubes [34]. As observed in Figure 1, using the same weight of S-

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SWNTs and M-SWNTs in the poly[(2,5-bisoctyloxy)-1,4-phenylenevinylene] (BPPV) mass, a smaller intensity of the BPPV spectrum was reported to be induced by the M-SWNTs presence in comparison with SSWNTs.

Figure 2. PL spectra of POPV in the absence (a) and the presence of M+S-SWNTs (b), S-SWNTs (c) and M-SWNTs (d), recorded under the excitation wavelength of 390 nm. In all four cases, from bottom to top, PL spectra are shown for the samples obtained by recording of 5, 10, 20, 25 and 30 cyclic voltammograms in the potential range (-2.5; +2.5) V vs.Ag/AgCl, when the working electrodes were immersed into a solution of 0.02 M α, α, α,’ α’-tetrabromo-o-xylene and 0.1 M tetrabutylammonium bromide in dimethyl formamide. Reprinted with permission of [36]. Copyright (2017) Eur. Polym. J.

M-SWNTs were reported to be also responsible for the PPV PL quenching, when the macromolecular compound was prepared by the

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electrochemical reduction of α, α, α,’ α’-tetrabromo-p-xylene [35]. According to Ref. 35, a decrease with 50% of the formation of PPV macromolecular chains with lengths of 4 repeating units, when the PPV/CNT composites were obtained by the annealing conversion of the PPV precursor solution containing M-SWNTs or S-SWNTs was reported. Besides, a decrease with 36% of the PPV macromolecular chains with lengths of 7-10 repeating units was also reported when the electrochemical reduction of α, α, α,’ α’-tetrabromo-p-xylene in the presence of M+SSWNTs, M-SWNTs and S-SWNTs took place [35]. In comparison with above composites, other studies have demonstrated that S-SWNTs are responsible for the PL quenching processes of the following macromolecular compounds: poly(ortho-phenylenevinylene) (POPV) [36], poly(2, 2’-bithiophene-co-pyrene) (PBTh-Py) [37] and poly(3,4-ethylenedioxythiophene-co-pyrene) (PEDOT-Py) [38]. To illustrate this behavior, Figure 2 show the PL spectra of POPV in absence and in presence of M+SSWNTs, M-SWNTs and S-SWNTs [36]. According to Figure 2, the PL quenching process was accompanied by a change in the POPV PL spectrum profile as a result of the SWNTs presence. This fact was explained taking into account that in the case of POPV, the formation of macromolecular chains with length of 3 and 7-10 repeating units takes place, while in the presence of SWNTs was demonstrated the generation with greater weight of macromolecular chains having the length of 3 repeating units [36]. In the case of the the PBTh-Py copolymer, the PL spectrum was dominated by an intense band with maximum at 535-575 nm that had a shoulder to 460 nm 37. Adding SWNTs during the radicalic polymerization of the two monomers, i.e., 2, 2’-bithiophene and pyrene, was reported to induce a significant decrease in the intensity of the emission band at 535-575 nm. Thus, the PL spectra of the PBTh-Py/SWNT composites were characterized only by the emission band with maximum at 490 nm, whose intensity decreases as increasing the CNT concentration in the composite mass [37]. In the case of the PEDOT-Py/SWNTs composite, the PL quenching process was not accompanied by any variation in the CPs PL spectra profile 38. Figure 3 is relevant in this context. According to it, the maximum of the emission

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band of the PEDOT-Py copolymer is situated at 455 nm. As increasing the CNT concentration in the composite mass, a decrease in the intensity of the emission band at 455 nm was reported by I. Baltog et al. [38]. In order to explain the CPs PL quenching processes induced by MSWNTs or S-SWNTs, were taken into account different de-excitation pathways according with the diagrams of the energy levels of the composite materials constituents [35-38]. Figure 4 shows the diagrams of the energy levels of the PEDOT-Py/SWNTs composite.

Figure 3. PL spectra of the PEDOT-Py copolymer and its composites with SWNTs synthesized according to Ref. 38. Reprinted with permission of [38]. Copyright (2016) Opt. Mat.

The explanation of PL quenching process in all above cases [35-38] was described to start with the appearance of an exciton on the backbone of the macromolecular compound which, under optical excitation, is dissociated into an electron and a hole. The electron from the polymer is collected of the LUMO levels of CNTs and by the internal conversion process, this reaches the lowest LUMO level where it can be recombined with the hole existing on the HOMO level of: i) CNTs, when a CPs PL quenching process was reported or ii) the polymer, when no change in CPs PL takes place [35-38]. According to Ref. [35-38], the recombination process of the electrons with the hole existing on the CNT HOMO level allows SWNTs to act as quenchers of the PL of the composite materials.

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Figure 4. Diagrams of the energy levels of the PEDOT-Py /SWNTs composite. The red solid lines show highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of the PEDOT-Py copolymer calculated according to Ref. 38. Black and gray curves correspond to HOMO and LUMO levels of semiconducting SWNTs (13;8) and metallic SWNTs (13;7) calculated according to the protocol described in Ref. 38. Reprinted with permission of [38]. Copyright (2016) Opt. Mat.

Figure 5. Anti-Stokes PL spectra of the BPPV films with the thickness of 25 nm (black curve), 50 nm (red curve) and 100 nm (green curve) deposited onto Au support, recorded at the excitation wavelength of 676.4 nm. Reprinted with permission of [12]. Copyright (2017) Opt. Mat.

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The interest for a better knowledge of the interaction of SWNTs with CPs was reported to be for the dispersion and separation of metallic and semiconducting SWNTs in organic solvents [39]. The main CPs used in this order were: poly(9,9-dioctylfluorene) [41], poly(9,9-dioctylfluorene)alt-(1,4-benzo-2, 10, 3-thiadiazole) (F8BT) [40, 41], block copolymers containing CPs and ssDNA blocks [42], poly[(9,9’-dioctylfluorene)-alt(9,9’-dihexylfluorene)] [43], poly(9,9-dioctylfluorene)-alt-(9, 9’-bis(6bromohexylfluorene))) [43], poly[(9,9’-dioctylfluorene)-alt-(9,9’-bis(6iodohexylfluorene))] [43], poly(9,9-dihexyl-fluorenyl-2,7-diyl)-cp-(9, 10antracene)] [44] and poly(3-dodecylthiophene-2,5-diyl) [45]. All these studies have reported the separation from the bundles of semiconducting SWNTs of various chiralities, [39-45] as well as highlighting the influence of the CPs concentration and the temperature in the selection of S-SWNTs [45]. In this context, it is worthwhile to remember the PL studies concerning the coverage of the semiconducting SWNTs of the (n,m) chirality with poly(9, 9-dioctylfluorene-2,7-diyl) (PFO), when the polymer wrapping angles of 12, 17 and 14 ± 20 for the tubes (7, 5), (8, 6) and (8, 7), respectively, were calculated according to the protocol reported by M. J. Shea et al. [46] After the best knowledge of the authors, no other information was reported regarding the assessing of the CPs wrapping angles on the M+S-SWNTs and M-SWNTs. A less studied optical process was the anti-Stokes PL of the CPs/CNT composites. At present, only two anti-Stokes PL studies are reported, focused on PPV [9] and BPPV [12]. According to Refs. 9 and 12, this optical process was explained by taking into account the absorption of a photon and the phonon absorption from the CPs lower vibronic state to upper ones. The efficiency of this optical process was reported to be increased for CPs deposited onto Au supports and in the presence of SWNTs, as shown in Figures 5 and 6, respectively [9, 12]. The higher intensity of the anti-Stokes PL spectrum of BPPV when the polymer films thickness decreases (Figure 5) was explained to be induced of surface plasmons which exists at the interface macromolecular compound/Au support [12]. The existing of the surface plasmons at the interface of metallic SWNTs with CPs was invoked as an argument for the higher

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intensity anti-Stokes PL spectra of composite materials with the lowest CNT concentration (Figure 6) [12]. A feature of anti-Stokes PL of CPs and their composites was the sub-linear variation of the intensity of anti-Stokes emission with the excitation light power [9, 12].

Figure 6. Anti-Stokes PL spectra of the BPPV/SWNTs films deposited onto Au support. Black, red, green and blue curves correspond to the weight ratios of the two constituents BPPV: SWNTs equal with 1:0.1, 1:0.25, 1:0.5 and 1:1, respectively. Reprinted with permission of [12]. Copyright (2017) Opt. Mat.

PHOTOLUMINESCENCE OF THE COMPOSITES BASED ON CONJUGATED POLYMERS AND REDUCED GRAPHENE OXYDE/GRAPHENE OXYDE The development of synthesis methods at large scale of GO 4, RGO 5 and the establish of the protocol for the transfer of graphene

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monolayers obtained by chemical vapor deposition [47, 48] have allowed the use of these 2D materials in the composites field. As shown by N. Prasad et al. graphene interacts with CPs through weak π-π stacking which extends the 2D space by the overlap between the electronic clouds of both graphene and polymer, inducing a CPs PL quenching effect [49]. The PL quenching effect was reported mainly to be static, the mechanism consisting in electron transfer at the interface of the composite constituents [49]. The following macromolecular compounds were tested in order to study the influence of graphene sheets, GO and RGO on CPs PL properties: P3HT [50], polyaniline (PANI) [51], polydiphenylamine (PDPA) [14], PPV [52], MEH-PPV [53], and so on. The introduction of graphene layers into a matrix of poly(3-hexylthiophene-2, 5-diyl) was reported to induce a P3HT PL quenching as a result of the electrons transfer process at the interface of the two constituents [50]. The new PL band situated in the visible range in the case of the aqueous dispersion of the PANI grafted GO composite was explained by P. Saha et al. taking into account the electron donor-acceptor interaction which takes place between polyaniline-emeraldine salt and GO [51]. A decrease in the intensity of the PL spectra of diphenylamine (DPA) and PDPA in the presence of RGO was reported by I. Smaranda et al. in 2014 [14]. In these last two cases, DPA/PDPA PL quenching was explained via the charge transfer processes [14]. In the case of the DPA/RGO system, the change of the PL band profile was correlated with the covalent bonding of DPA onto RGO sheets surface [14]. A first study devoted to PL properties of the composites based on RGO and PPV in un-doped or doped state was reported in 2017 [52]. Regardless of the synthesis method used for the preparation of the composites based on RGO and PPV in un-doped or doped state, i.e., annealing conversion of PPV precursor solution or electropolymerization of α, α, α,’ α’-tetrabromo-p-xylene in the presence of RGO, a CP PL quenching effect was reported. 52 This process was also observed in the case of composites based on PPV, pyrene (Py) and RGO synthetized by annealing conversion way [54]. In order to illustrate this process Figure 7 shows PL spectra of the PPV-RGO composites.

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Figure 7. PL spectra of the PPV-RGO composite with a RGO concentration equal to 0, 0.01 and 0.1 wt.%. Reprinted with permission of [54]. Copyright (2017) Opt. Mat.

The PPV PL quenching effect induced by RGO was explained i) by taking into account the formation of macromolecular chains with lengths of 5 and 7-10 repeating units, which were observed in PL spectra by emission bands with maxima at 2.42 and 2.26 eV and ii) by using anisotropic PL measurements, an increase of the wrapping angle of RGO sheets with PPV was reported only in the case of macromolecular chains with lengths of 5 repeating units [52]. Besides, depending on the synthesis method, the wrapping angle of RGO sheets with PPV in the un-doped state was reported to be changed: i) from 120 to 310, when PPV was prepared by annealing conversion of the PPV precursor solution containing RGO concentrations ranging between 0 - 0.5 wt.% and ii) from 280 to 340, when PPV in the doped state, electrochemically synthesized in the presence of a RGO solution with concentration varying from 0 to 0.5 wt.%, was successively reacted with a NH4OH solution [52]. These differences were explained taking into account the formation of distyrylbenzene as a secondary product of the electrochemical synthesis of PPV and the PPV/RGO composites [52]. Another highly studied composite was MEH-PPV/RGO [55-59]. According to S. Huang et al., MEH-PPV shows a PL band with maximum at 590 nm, which originates in the relaxation process of the excited π-electrons of CP backbone to the ground state [55]. The adding of 0.5

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mg/ml graphene to the MEH-PPV solution induces a CP PL quenching process, as a result of the charge transfer which takes place between the two components [55]. A more detailed study was reported of N. Prasad et al. [56], which assigns the two emission maxima of MEH-PPV at 600 nm and 635 nm to intra and inter-chain transitions, respectively. The incorporation of graphene into MEH-PPV matrix, at an optimized concentration (0.005 wt.%), was demonstrated to induce an enhancing of PL [56]. At a higher concentration than 0.005 wt.% of graphene, MEHPPV PL emission starts decreasing as a result of the generation of interconnected conducting paths of the two constituents, when was invoked an inhibition of the formation of excitons onto the macromolecular chain [56]. New information concerning PL properties of the composite based on MEH-PPV and graphene loaded with Ag nanoparticles (AGC) was reported by A. Ghosh et al. [57]. Using transient fluorescence studies, two hypotheses were made [57]. One regards the existence of a charge transfer between MEH-PPV and AGC and another supports the idea of a static interaction between the two components. R. Chenxin et al. [58] concluded that the loading of the AGC does not influence the fluorescence life time of MEH-PPV, the fact which was considered as an argument for the static nature of the interaction between MEH-PPV and AGC. The study of PL decay of MEH-PPV as a function of RGO concentration, carried out of Y. Wang et al. reports that at the highest percent of RGO in the macromolecular compound matrix, i.e., 50 wt.%, the decay becomes 10 times faster [59].

PHOTOLUMINESCENCE OF THE COMPOSITES BASED ON CONJUGATED POLYMERS AND FULLERENE Photoinduced electron transfer studies, reported by N. S. Sariciftci et al. have demonstrated that the charge transfer from the excited state of the CPs to those of C60 occurs in a time scale of the order of picoseconds, due to quenching of the CPs when interacting with C60 [60]. One of the most

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studied polymers in the field of the CPs/C60 composites was MEH-PPV, due to its excellent electro-optical properties [61]. The PL studies were mainly performed on films resulted from the evaporation of tetrahydrofuran or ortho-dichlorobenzene, used as solvents for the solubilization of the two constituents of the MEH-PPV/C60 composite [62]. Regardless of the solvent type used for the preparation of the MEHPPV/C60 solutions, a CP PL quenching process was reported to take place as increasing the C60 concentration in the composite mass [63, 64]. A decrease of nearly three orders in magnitude of the MEH-PPV PL intensity was reported in the presence of C60 [63, 64]. A. Ltaief et al. have explained this process taking into account the photo-induced charge transfer between the two constituents of the MEH-PPV/C60 composite [64]. In comparison with results obtained using the samples resulted by the mixture of the two compounds, a more pronounced CPs PL quenching was reported recently in the case of -CP-C60 covalent hybrids, chemical synthetized by DielsAlder reactions [65]. This behavior was reported not to depend on the solvent type used for the solvation of C60 derivatives [65]. The P3HT PL quenching in the presence of the C60 derivatives was explained by B. Yameen et al. considering the energy transfer as the main relaxation process [65].

PHOTOLUMINESCENCE AS A VALUABLE TOOL FOR ASSESSING OF THE PHOTOCHEMICAL/PHOTOPHYSICAL PROCESSES IN COMPOSITES BASED ON CONJUGATED POLYMERS AND CARBON NANOTUBES A first study concerning the use of PL as a valuable tool for assessing of photochemical processes was published in 2015 when, in the case of SSWNTs and M-SWNTs functionalized with PDPA doped with H3PW12O40 heteropolyanions, new reactions were reported to take place under UV irradiation [13]. A review of the evolution of PL spectra of PDPA doped with H3PW12O40 heteropolyanions and its composites with S-SWNTs and

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M-SWNTs is shown in Figure 8. As shown in Ref. 13, three molecular structures were known for PDPA doped with H3PW12O40 heteropolyanions, these being labeled as follows: PDPA3+(PW12O40)3-, PDPA2+(H3PW12O40)2and PDPA+(H2PW12O40)- [13]. The increase in the intensity of PL spectra of PDPA doped with H3PW12O40 heteropolyanions was interpreted by M. Baibarac et al. as a result of a photochemical reaction which induces the transformation PDPA+(H2PW12O40)- into PDPA2+(HPW12O40)2- [13]. More pronounced growth of PL spectra of S-SWNTs functionalized with PDPA doped with H3PW12O40 heteropolyanions in comparison with those of MSWNTs as well as the change of emission bands profile in Figure 8 was correlated with a shortening of the PDPA macromolecular chains [13]. A consequence of this fact was the formation of DPA dimer doped with H3PW12O40 heteropolyanions, which successively interacted with S-SWNTs [13].

Figure 8. PL spectra of PDPA doped with H 3PW12O40 heteropolyanions (a) and its composites with S-SWNTs (b) and M-SWNTs (c), when the excitation wavelength and the irradiation time was equal with 275 nm and 120 min. Reprinted with permission of [13]. Copyright (2015) Carbon.

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The developing of this topic continued by the study of photophysical properties of the poly[(9, 9-bis(3’-(N, N-dimethylamino)-propyl)-2, 7fluorene)-alt-2, 7-(9,9-dioctylfluorene) (PFN)/SWNTs composite [65]. According to the study reported by K. A. Luck et al. PFN is characterized by a complex PL spectrum consisting in two emission bands, the first one of high intensity with maximum at 420-442 nm labeled as “green band” and another of small intensity which appears as a shoulder being situated in the spectral range 500-700 nm [66]. Under UV exposure of the PFN/SWNTs composite having a CNTs concentration of ~50 wt.%, K. A. Luck et al. reported a significant decrease in the intensity of PL band at 420-442 nm as a result of the side-chain intermolecular interactions [66].

CONCLUSION AND PERSPECTIVES The PL properties of composites based on CPs and carbon nanoparticles of the type carbon nanotubes, graphene and fullerene were reviewed in this chapter. Four issues were shown as it follows: i) the role of CNTs in the CPs PL quenching process, and anti-Stokes PL process of the CPs/CNT composites; ii) the influence of RGO on CPs PL and the anisotropic PL properties of the CPs/RGO composites; iii) the PL of the CPs/C60 composites and iv) the use of PL as a valuable tool in assessing of the photochemical/photophysical processes of the CPs/CNT composites. Taking into account the progress shown above, the development of new studies of anisotropic PL as well as the use of PL in the monitoring of other photochemical/photophysical processes in the field of composites based on CPs and carbon nanoparticles is anticipated in the near future.

ACKNOWLEDGMENT This work was financed by Core Program 2016–2018, project PN16480101.

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[48] Mattevi, C., Kim, H., Chhowalla, M., J. Mater. Chem 2011, 21, 3324-3334. [49] Hope, H., Sariciftci, N. S., J. Mater. Chem 2006, 16, 45-61. [50] Bkakri, R., Kusmartseva, O. E., Kusmartsev, F. V., Song, M., Baouazizi, A., J. Lumines. 2015, 161, 264-270. [51] Saha, P., Pyne, D. K., Pal, M., Datta, S., Das, P. K., Dutta, P., Halder, A., J. Lumines. 2017, 181, 138-146. [52] Baibarac, M., Ilie, M., Baltog, I., Lefrant, S., Humbert, B., RSC Adv. 2017, 7, 6931-6942. [53] Barabara, P. F., Gesquiere, A. J., Park, S. J., Lee, Y. J., Acc. Chem. Res. 2005, 38, 602-610. [54] Ilie, M., Baibarac, M., Opt. Mater, 2017, 72, 140-146. [55] Huang, S., Ren, L., Guo, J., Zhu, H., Zhang, C., Liu, T., Carbon 2012, 50, 216-224. [56] Prasad, N., Singh, I., Kumari, A., Madhwal, D., Madan, S., Dixit, S. K., Bhatnagar, P. K., Mathur, P. C., J. Lumines. 2015, 159, 166-170. [57] Gosh, A., Jana, B., Maiti, S., Bera, R., Ghosh, H.N., Patra, A., Phys. Chem Chem Phys 2014, 16, 4561-4568. [58] Chenxin, R., Minqiang, W., Weiyin, G., Zhi, Y., Xiangyu, Z., IEEENano 2013, 1075-1079. [59] Wang, Y., Kurunthu, D., Scott, G. W., Bardeen, C. J., J. Phys. Chem. C 2010, 114, 4153-4159. [60] Sariciftci, N. S., Smilowitz, L., Heeger, A. J., Wudi, F., Science 1992, 258, 1474-1476. [61] Dhibi, O., Ltaief, A., Bouazizi, A., Fuller. Nanotub. CAR N. 2013, 21, 894-900. [62] Spitsina, N., Romanova, I., Lobach, A., Yakuschenko, I., Kaplunov, M., Tolstov, I., Triebel, M., Frankevich, E., J. Low. Temp. Phys. 2006, 142, 201-206. [63] Ltaief, A., Davenas, J., Bouazizi, A., Chaabane, R. B., Alcouffe, P., Ouada, H. B., Mater. Sci. Eng. C- Biomimetric Mater. Sens. Syst 2005, 25, 67-75. [64] Ltaief, A., Bouazizi, A., Davenas, J., Alcouffe, P., Thin Solid Films 2008, 516, 1578-1583.

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BIOGRAPHICAL SKETCHES Adelina Matea Affiliation: National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials Education: 2014 – at present PhD Student, University of Bucharest, Faculty of Physics, Department of Physics: Optics, Spectroscopy, Plasma and Laser 2011 – 2013 Master of Theoretical Physics; Department of Theoretical Physics, University of Bucharest, Faculty of Physics; Romania 2008 - 2011 Bachelor’s Degree in Physics; Department of Physics, University of Bucharest, Faculty of Physics; Romania Business Address: Atomistilor Str, 405A, Bucharest, Magurele-Ilfov, P.O. Box MG-7, code 077125, Romania Research and Professional Experience: Materials Science, Inorganic Semiconductors, Carbon Nanotubes, Fullerenes, Composites, Raman Scattering, Photoluminescence Professional Appointments: 

Research Assistant, Laboratory of Optical Processes in Nanostructured Materials (LOPNM), Optical properties in nanostructured materials, National Institute of Materials Physics (NIMP), 2012 - present

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Research Assistant, The theory of Atomic Processes in Laser Field /Atomic and Ionic Spectroscopy of Laboratory and Astrophysical Plasmas, National Institute for Laser, Plasma and Radiation Physics (NILPR), 2011 - 2012

Publications from the Last 3 Years: Exciton-phonon interactions in the Cs3Bi2I9 crystal structure revealed by Raman spectroscopic studies, A. Nila, M. Baibarac, A. Matea, R. Mitran, I. Baltog, Physica Status Solidi B, 254 (4), 2017. Optical properties of single-walled carbon nanotubes highly separated in semiconducting and metallic tubes functionalized with poly(vinylidene fluoride), A. Matea, M. Baibarac, I. Baltog, Journal of Molecular Structure, 1130, 38-45, 2017. The exciton-phonon interaction as stimulated Raman scattering effect supported by the excitonic photoluminescence in BiI3 layered crystal structure, A. Nila, A. Matea, M. Baibarac, I. Baltog, Journal of Luminescence, 182, 166-171, 2017. Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4-ethylenedioxythiophene-co-pyrene), I. Baltog, M. Baibarac, I. Smaranda, A. Matea, M. Ilie, JY. Mevellec, S. Lefrant, Optical Materials, 62, 604-611, 2016. Anti-Stokes Raman spectroscopy as amethod to identify the metallic and semiconducting configurations of double-walled carbon nanotubes, M. Baibarac, I. Baltog, A. Matea, L. Mihut, S. Lefrant, Journal of Raman Spectroscopy, 46 (1), 2015. Raman scattering and photoluminescence studies of ZnO nanowhiskers assembled as flowers in the presence of fullerene, M. Baibarac, I. Baltog, A. Matea, S. Lefrant, Journal of Crystal Growth, 419, 2015. Optical Properties of Single-Walled Carbon Nanotubes Functionalized with Poly(2,2 ‘-bithiophene-co-pyrene) Copolymer, I. Smaranda, M. Baibarac, M. Ilie, A. Matea, I. Baltog, S. Lefrant, Current Organic Chemistry, 19(7), 652-661, 2015.

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Anti-Stokes Raman spectroscopy as a method to identify metallic and mixed metallic/semiconducting configurations of multi-walled carbon nanotubes, M. Baibarac, A. Matea, M. Ilie, I. Baltog, A. Magrez, Analytical Methods, 7(15), 6225-6230, 2015.

Mirela Ilie Affiliation: Lab. Optical Processes in Nanostructured Materials, National Institute of Materials Physics, Bucharest Romania Education: 2014 to the present PhD. student at University of Bucharest, Faculty of Physics, Optics Spectroscopy Plasma and Lasers Department; 2010-2012 Faculty of Chemistry, Master in Chemistry with Specialization in Chemical Pollution and the Environment; 2007-2010 Faculty of Chemistry, Domain Environmental Science with Specialization in Environmental Chemistry. Business Address: National Institute of Materials Physics, Atomistilor street, nr. 405 A, Bucharest Magurele, P. O. Box MG-7, RO-77125, Romania Research and Professional Experience: Materials Science, TiO2, Carbon Nanotubes, Graphene, Poly(para-phenylene vinylene), Raman Scattering, Infrared Spectroscopy, Photoconductivity, Photoluminescence. Publications from the Last 3 Years: Anti-Stokes Raman spectroscopy as a method to identify metallic and mixed metallic/semiconducting configurations of multi-walled carbon nanotubes, M. Baibarac, A. Matea, M. Ilie, I. Baltog, A. Magrez, Anal. Methods-UK, 7 (15) 6225-6230, 2015. Optical Properties of Single-Walled Carbon Nanotubes Functionalized with Poly(2,2 ‘-bithiophene-co-pyrene) Copolymer, I. Smaranda, M.

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Baibarac, M. Ilie, A. Matea, I. Baltog, S. Lefrant, Curr. Org. Chem., 19 (7) 652-661, 2015. Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene), M. Baibarac, I. Baltog, M. Ilie, B. Humbert, S. Lefrant and C. Negrila, J. Phys. Chem. C, 120 (10) (2016) 5694–5705. Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4 ethylenedioxythiophene-co-pyrene), I. Baltog, M. Baibarac, I. Smaranda, A. Matea, M. Ilie, J. Y. Mevellec, S. Lefrant, Opt. Mater., 62, (2016) 604-611. Infrared dichroism studies and anisotropic photoluminescence properties of poly(para-phenylene vinylene) functionalized reduced graphene oxide, M. Baibarac, M. Ilie, I. Baltog, S. Lefrant, B. Humbert, RSC Adv. 7, (2017) 6931-6942. The Spectrochemical Behavior of Composites Based on Poly[(ParaPhenylenevinylene], Reduced Graphene Oxide and Pyrene, M. Ilie, M. Baibarac, Opt. Mater., 72 (2017) 140-146.

Serge Lefrant Affiliation: Emeritus professor at the University of Nantes Education: 1975 Ph.D. Thesis, University of Paris-Sud/Orsay (France) 1971 Master Thesis, University of Paris-Sud/Orsay (France) 1970 Master Degree in Materials Sciences, University of Paris-Sud/Orsay (France) Business Address: Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France

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Research and Professional Experience: Solid State Physics, Materials Science, Optical and Vibrational spectroscopies Raman scattering and Surface Enhanced Raman Scattering (SERS) Honors: Bronze medal of CNRS (1977) Physics Prize (D. Humuzescu) of the Romanian Academy of Science (1997) Physics Prize (C. Miculescu) of the Romanian Academy of Science (2000) Prize of the French Academy of Science (Paris) and of the National Science Council of Taiwan, for the promotion of joint researches of Nanomaterials (2006) Publications from the Last 3 Years: The influence of single-walled carbon nanotubes on optical properties of the poly[(2,5-bisoctyloy)-1, 4-phenylenevinylene] evidenced by infrared spectroscopy and anti-Stokes photoluminescence. Mihaela Baibarac, Ion Smaranda, Ioan Baltog, Serge Lefrant and Jean-Yves Mevellec. Opt. Mater. 67, 52-58, 2017. Influence of single-walled carbon nanotubes enriched in semiconducting and metallic tubes on the electropolymerization of tetrabromo orthoxylene: Insights on the synthesis mechanism of poly(orthophenylenevinylene). M. Baibarac, A. Nila, I. Baltog, S. Lefrant, J Y. Mevellec, S. Quillard and B. Humbert. Eur. Polym. J. 88, 109-125, 2017. Infrared dichroism studies and anisotropic photoluminescence properties of poly(para-phenylene vinylene) functionalized reduced graphene oxide. M. Baibarac, M. Ilie, I. Baltog, S. Lefrant and B. Humbert. RSC Adv. 7, 6931-6942, 2017. Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4-ethylenedioxythiophene-co-pyrene). I. Baltog, M.

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Baibarac, I. Smaranda, A. Matea, M. Ilie, J Y. Mevellec and S. Lefrant. Opt. Mater. 2016, 62, 604-611. Optical evidence for chemical interaction of the polyaniline/fullerene composites with N-methyl-2-pyrrolidinone. M. Baibarac, I. Baltog, M. Daescu, S. Lefrant and P. Chirita. J. Mol. Struct. 1125, 340-349, 2016. Dynamics of charge migration in poly(para-phenylene vinylene) films and nanocomposites with single walled carbon nanotubes (vol 28, 045304, 2016). E. Mulazzi, D E. Galli, S. Lefrant, J. Wery, F. Massuyeau and E. Faulques. J. Phys.-Condes. Matter 28, 109501, 2016. Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene). M. Baibarac, I. Baltog, M. Ilie, B. Humbert, S. Lefrant and C. Negrila. J. Phys. Chem. C 120, 5694-5705, 2016. Corrigendum: Dynamics of charge migration in poly(para-phenylene vinylene) films and nanocomposites with single walled carbon nanotubes (2016 J. Phys.: Condens. Matter 28 045304). E. Mulazzi, D E. Galli, S. Lefrant, J. Wery, F. Massuyeau and E. Faulques. J. Phys.: Condens. Matter 28, 109501, 2016. Effect of functionalization and charging on resonance energy and radial breathing modes of metallic carbon nanotubes. S. Oberg, J -J. Adjizian, D. Erbahar, J. Rio, B. Humbert, M. Dossot, A. Soldatov, S. Lefrant, J Y. Mevellec, P. Briddon, M J. Rayson and C P. Ewels. Phys. Rev. B 93, 045408, 2016. Dynamics of charge migration in poly(para-phenylene vinylene) films and nanocomposites with single walled carbon nanotubes. E. Mulazzi, D E. Galli, S. Lefrant, J. Wery, F. Massuyeau and E. Faulques. J. Phys.Condes. Matter 28, 045304, 2016. Anti-Stokes Raman spectroscopy as amethod to identify the metallic and semiconducting configurations of double-walled carbon nanotubes. Mihaela Baibarac, Ioan Baltog, Adelina Matea, Lucian Mihut and Serge Lefrant. J. Raman Spectrosc. 2015, 46, 32-38. Optical Properties of Single-Walled Carbon Nanotubes Functionalized with Poly(2,2’-bithiophene-co-pyrene) Copolymer. Ion Smaranda,

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Mihaela Baibarac, Mirela Ilie, Adelina Matea, Ioan Baltog and Serge Lefrant. Curr. Org. Chem. 2015, 19, 652-661. Electronic interaction in composites of a conjugated polymer and carbon nanotubes: first-principles calculation and photophysical approaches. Florian Massuyeau, Jany Wery, Jean-Luc Duvail, Serge Lefrant, Abu Yaya, Chris Ewels and Eric Faulques. Beilstein J. Nanotechnol. 2015, 6, 1138-1144. Raman scattering and photoluminescence studies of ZnO nanowhiskers assembled as flowers in the presence of fullerene. M. Baibarac, I. Baltog, A. Matea and S. Lefrant. J. Cryst. Growth 2015, 419, 158-164.

Mihaela Baibarac Affiliation: National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials Education: 2002 Ph.D., Physics-Optics, Spectroscopy and Laser (with Summa cum Laude), University of Bucharest, Faculty of Physics, 1996 M. Sc., Chemistry - Thermodynamics and Applied Electrochemistry, University Politehnica of Bucharest, Faculty of Industrial Chemistry, 1995 Eng., Chemistry - Polymer Science, Polytechnic University of Bucharest, Faculty of Industrial Chemistry. Business Address: Atomistilor Str. 405A, P. O. Box MG-7, Magurele, R077125 Ilfov, Romania Research and Professional Experience: Material Science, Carbon Nanotubes, Graphene, Inorganic Compounds, Polymers, Composites, Raman Scattering, Surface Enhanced Raman Scattering (SERS), Photoluminescence, Infrared Spectroscopy, Electrochemistry, Supercapacitors, Rechargeable lithium batteries

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Professional Appointments: Senior research scientist Ist degree, Lab. Optical Process in Nanostructured Materials, National Institute of Materials Physics (NIMP), 2010-present Honors: Prize for Physics, C. Miculescu, of the Romanian Academy for the group of papers Raman studies on conducting polymers thin films, 2000. Publications from the Last 3 Years: The spectrochemical behavior of composites based on poly (paraphenylenevinylene), reduced graphene oxide and pyrene, M. Ilie, M. Baibarac, Optical Materials 72, 140-146, 2017. The influence of single-walled carbon nanotubes on optical properties of the poly[(2,5-bisoctyloy)-1, 4 phenylenevinylene] evidenced by infrared spectroscopy and anti-Stokes photoluminescence, M. Baibarac, I. Smaranda, I. Baltog, S. Lefrant, J.-Y. Mevellec, Optical Materials 67, 52-58, 2017. Exciton-phonon interactions in the Cs3Bi2I9 crystal structure revealed by Raman spectroscopic studies, A. Nila, M. Baibarac, A. Matea, R. Mitran, I. Baltog, Physica Status Solidi (B) Basic Research 254, (4), 1552805, 2017. Electrochemical characterization of the poly(2, 2’-bithiophene-co-pyrene) functionalized single-walled carbon nanotubes films and their applications in supercapacitors field, M. Baibarac, I. Baltog, M. Daescu, International Journal of Electrochemical Science 12, (3), 2013-2025, 2017. Influence of single-walled carbon nanotubes enriched in semiconducting and metallic tubes on the electropolymerization of tetrabromo orthoxylene: Insights on the synthesis mechanism of poly(orthophenylenevinylene), M. Baibarac, A. Nila, I. Baltog, S. Lefrant, J. Y. Mevellec, S. Quillard, B. Humbert, European Polymer Journal 88, 109-125, 2017.

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Optical properties of single-walled carbon nanotubes highly separated in semiconducting and metallic tubes functionalized with poly(vinylidene fluoride), A. Matea, M. Baibarac, I. Baltog, Journal of Molecular Structure 1130, 38-45, 2017. The exciton-phonon interaction as stimulated Raman scattering effect supported by the excitonic photoluminescence in BiI3 layered crystal structure, A. Nila, A. Matea, M. Baibarac, I. Baltog, Journal of Luminescence 182, 166-171, 2017. Infrared dichroism studies and anisotropic photoluminescence properties of poly(para -phenylene vinylene) functionalized reduced graphene oxide, M. Baibarac, M. Ilie, I. Baltog, S. Lefrant, B. Humbert, RSC Advances 7, (12), 6931-6942, 2017. Aging phenomena and wettability control of plasma deposited carbon nanowall layers, S. Vizireanu, M. D. Ionita, R. E. Ionita, S. D. Stoica, C. M. Teodorescu, M. A. Husanu, N. G. Apostol, M. Baibarac, D. Panaitescu, G. Dinescu, Plasma Processes and Polymers 14 (1), e1700023, 2017. Influence of TiO2 and Si on the exciton-phonon interaction in PbI2 and CdS semiconductors evidenced by Raman spectroscopy, A. Nila, I. Baltog, D. Dragoman, M. Baibarac, I. Mercioniu, Journal of Physics: Condensed Matter 67, 52-58, 2017. Optical evidence for chemical interaction of the polyaniline/fullerene composites with N-methyl-2-pyrrolidinone, M. Baibarac, I. Baltog, M. Daescu, S. Lefrant, P. Chirita, Journal of Molecular Structure, 1125, 340-349, 2016. Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene), M. Baibarac, I. Baltog, M. Ilie, B. Humbert, S. Lefrant, C. Negrila, Journal of Physical Chemistry C, 120, 5694-5705, 2016. Exciton-phonon interaction in CdS of different morphological forms manifested as stimulated Raman scattering, M. Baibarac, A. Nila, I. Baltog, Optical Materials Express, 6, 1881-1895, 2016.

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Polarized Raman spectra of phosphorene in edge and top view measuring configurations, M. Baibarac, A. Nila, I. Baltog, RSC Advances, 6, 58003-58009, 2016. Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4-ethylenedioxythiophene-co-pyrene), I. Baltog, M. Baibarac, I. Smaranda, A. Matea, M. Ilie, J. Y. Mevellec, S. Lefrant, Optical Materials, 62, 604-611, 2016. Mechanism of the cathodic process coupled to the oxidation of iron monosulfide by dissolved oxygen, M. I. Duinea, A. Costas, M. Baibarac, P. Chirita, Journal of Colloids and Interfaces Science, 467, 51-59, 2016. Exciton-phonon interaction in PbI2 revealed by Raman and photoluminescence studies using excitation light overlapping the fundamental absorption edge, M. Baibarac, I. Smaranda, M. Scocioreanu, R. A. Mitran, M. Enculescu, M. Galatanu, I. Baltog, Materials Research Bulletin, 70, 762-772, 2015. Raman scattering and photoluminescence studies of ZnO nanowhiskers assembled as flowers in the presence of fullerene, M. Baibarac, I. Baltog, A. Matea, S. Lefrant, Journal of Crystal Growth, 419, 158164, 2015. Anti-Stokes Raman spectroscopy as a method to identify metallic and mixed metallic/semiconducting configurations of multi-walled carbon nanotubes, M. Baibarac, A. Matea, M. Ilie, I. Baltog, A. Magrez, Analytical Methods, 7(15), 6225-6230, 2015. Anti-Stokes Raman spectroscopy as a method to identify the metallic and semiconducting configurations of double-walled carbon nanotubes, M. Baibarac, I. Baltog, A. Matea, L. Mihut, S. Lefrant, Journal of Raman Spectroscopy, 46(1), 32-38, 2015. Photochemical processes developed in composite based on highly separated metallic and semiconducting SWCNTs functionalized with polydiphenylamine, M. Baibarac, I. Baltog, I. Smaranda, A. Magrez, Carbon, 81, 426-438, 2015

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1D-polyaniline starting from self-assembled systems, D. Donescu, M. Ghiurea, C. I. Spataru, G. Sting, D. Anghel, M. Baibarac, I. Baltog, Colloid and Polymer Science, 293(9), 2515-2524, 2015. Optical Properties of Single-Walled Carbon Nanotubes Functionalized with Poly(2,2’-bithiophene-co-pyrene) Copolymer, I Smaranda, M Baibarac, M Ilie, A Matea, I Baltog, S Lefrant, Current Organic Chemistry, 19(7), 652-661, 2015.

Monica-Alexandra Daescu Affiliation: National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials Education: 2013-2015 MSc., Chemistry - Micro and Nanomaterials, University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science. 2009-2013 Eng., Chemistry - Science and Engineering of Oxide Materials and Nanomaterials, University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science. Business Address: Atomistilor Str. 405A, P. O. Box MG-7, Magurele, R077125 Ilfov, Romania. Research and Professional Experience: Materials Science, Carbon Nanotubes, Fullerene, Polymers, Photoluminescence, Raman scattering, UV-VIS absorption spectroscopy. Professional Appointments: Research Assistant, Laboratory of Optical Processes in Nanostructured Materials (LOPNM) National Institute of Materials Physics (NIMP) 2013 - present

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Electrochemical characterization of the poly(2,2’-bithiophene-co-pyrene) functionalized single-walled carbon nanotubes films and their applications in supercapacitors field, M. Baibarac, I. Baltog, M. Daescu, International Journal of Electrochemical Science 12, (3), 2013-2025, 2017. Optical evidence for chemical interaction of the polyaniline/fullerene composites with N-methyl-2-pyrrolidinone, M Baibarac, I Baltog, M Daescu, S Lefrant, P Chirita, Journal of Molecular Structure, 1125, 340-349, 2016.

In: Photoluminescence Editor: Ellis Marsden

ISBN: 978-1-53613-537-4 © 2018 Nova Science Publishers, Inc.

Chapter 5

PHOTOLUMINESCENCE OF CARBON-BASED NANOMATERIALS: FULLERENES, CARBON NANOTUBES, GRAPHENE, GRAPHENE OXIDE, GRAPHENE AND CARBON QUANTUM DOTS Svetlana Jovanović* Vinča Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia

ABSTRACT Apart from unique physical, chemical, mechanical and thermal properties, carbon-based nanomaterials exhibit unusual optical behavior. Photoluminescence (PL) of these materials is particularly interesting considering that bulk carbon materials such as graphite and diamond do not show luminescent properties.

*

Corresponding Author Email: [email protected].

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Svetlana Jovanovic The subject of the chapter is fullerenes, carbon nanotubes, graphene, graphene oxide, graphene and carbon quantum dots. In this chapter, firstly the general physical and chemical properties of different carbonbased nanomaterials are presented, such as the crystalline structure, morphology and chemicals composition. Difference between dimensionality, type of hybridization of C atoms, the level of oxidation, size and shape are explained considering that these are key characteristics which determinate the optical behavior of these materials. In the following part of chapter, the origin of PL in different carbonbased nanomaterials, mechanisms and the tunability of the photoluminescence properties are discussed: exciton pairs which lead to PL of carbon nanotubes, defect centers such as of impurities or vacancies in nanodiamonds, different size of conjugated π-domains in reduced graphene oxide, suggested mechanisms of PL for graphene quantum dots, surface chromophores and defects in carbon quantum dots, etc. The PL properties such as emission spectra, excitation wavelengths, PL lifetimes as well as PL quantum yields are compared between different carbon-based nanomaterials. Furthermore, the parameters which affect these PL properties are argued. Additionally, the possibilities of application of carbon-based nanomaterials due to its PL properties are analyzed. Taking into account other physicochemical as well as PL properties, the most promising applications are highlighted, such as in bioimaging, drug delivery, sensing, in organoelectronic devices and others.

Keywords: photoluminescence, fullerenes, carbon nanotubes, graphene oxide, graphene quantum dots

INTRODUCTION The era of carbon based nanomaterials started in the 1980s when research in the field of chemistry and physics led to the discovery of the first nanostructures. With progress in microscopic techniques, the first carbon based nanostructure was discovered in 1985 (Kroto et al., 1985). This was fullerene. Afterward, carbon nanotubes were synthesized in 1991 (Iijima, 1991) and later graphene was isolated. Only after 2000th,

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the synthesis of both graphene and carbon quantum dots was started. In the structure of all these carbon nanomaterials, sp2 and/or sp3-hybridized carbon atoms are present. Bulk materials which are made of the only sp2or sp3-, graphite, and diamond do not exhibit photoluminescence. These macroscopic materials contain the effectively infinite carbon network. But when the symmetry or the size of this network is limited, the material star to exhibit PL. The phenomenon of photoluminescence is the feature of the molecules or materials to emit the light spontaneously after they absorbed the light. The PL of material or molecule is characterized by the spectral position of PL (emission) signal, dynamic, and efficiency. The information relevant to PL characteristics can be obtained by steady-state PL measurements, timeresolved PL and quantum yield (QY). In the experimental set-up for steady-state measurements, the analyzed material is excited continuously and both emission and excitation spectra were obtained providing the energy difference between ground and excited state of a material. Pulsed excitation was used in time-resolved PL measurement and these experiments are used to obtain the PL decay, which is a very important parameter for different applications of photoluminescent material – describes its duration of PL emission with a parameter named PL lifetime. The efficiency of PL emission is validated with parameter QY. This parameter is the ratio between emitted and absorbed photons. There are two different approaches for QY measurements: 1) absolute QY, measured in integration sphere 2) relative QY, measured by comparing the emission signals intensities of the standard compound with known QY and the sample In this chapter the photoluminescent properties of carbon based nanomaterials will be discussed. First, the structures and morphology will be analyzed following by PL properties and possible applications.

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Structure and Morphology of Carbon Based Nanomaterials As already mentioned, fullerenes are the first discovered carbon nanomaterial and a third allotrope modification of carbon (Kroto et al., 1985). First isolated fullerene was buckminsterfullerene (C60) with 60 carbon atoms arranged in a spherical structure. After that, fullerenes with different numbers of C atoms were synthesized such as C78 (the structure presented in Figure 1). In the molecule of C60, all carbon atoms are sp2 hybridized so each carbon atom is bonded to three adjacent C atoms (Kroto et al., 1985, Kratschmer et al., 1990). It contains 12 pentagons and 20 hexagons. Due to this structure, fullerenes are hydrophobic and insoluble in water. For their application in biomedicine, it is necessary to modify the structure or functionalized fullerene with amfifile molecules (Todorović Marković et al., 2009), preparing the nanofullerenes by solvent-exchange methods (Trpkovic et al., 2010, Trpkovic et al., 2012, Jovanović et al., 2010) or using organic chemical reactions to create water soluble fullerene derivates (Bosi et al., 2003, Tsang et al., 1994, Shih et al., 2001, Innocenzi and Brusatin, 2001, Andrievsky et al., 1999, Lin et al., 2008). The small size of fullerenes and chemical inertness as well as possibility to host drugs, fullerenes are been studied as drug delivery system. Fullerenes show interesting optical properties. Due to a high degree of symmetry in the closed shell electronic configuration, fullerene C60 shows a strong absorption in the UV region at wavelengths of 210, 256 and 328 nm. But, in the visible region absorbance is low. In order to improve the solubility of C60 in water, hydroxyl functional groups are often introduced in C60 structure (Prekodravac et al., 2017). Polyhydroxy fullerene or fullerol is a highly soluble in water and polar organic solvents. The structure of fullerol is presented in figure 2. Atomic force microscopy (AFM) is very often used in the analysis of carbon based nanostructures. Two AFM images of fullerol nanoparticles are presented in figure 2. Fullerol forms in water uniform, very small, round particles with hight below 10 nm and diameter around 50 nm (Figure 2(b-d)).

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Figure 1. Structures of fullerenes C78 (left) and C60 (right). 3D view of both fullerenes structure presented below.

As already mentioned, fullerenes show structured absorption bands in UV part of the spectrum, but these bands disappear on hydroxyl groups functionalization. In absorption spectrum of fullerenol, only high structureless absorption in UV part of spectra with extended tailing in the entire visible part of spectra can be observed. The changes in absorbance are due to loss of conjugation of pristine fullerene and non-chromophoric nature of hydroxyl groups. Carbon nanotubes were discovered shortly after fullerenes, in 1991, by Iijima et al. (Iijima, 1991). They are built from sp2 hybridized C atoms forming graphene sheet which is rolled into cylindrical structures. They can be single or multi wall carbon nanotubes (SWCNTs and MWCNTs, respectively) (Iijima and Ichihashi, 1993, Iijima, 1991). The morphology of CNTs varies depending on their type: for MWCNTs the diameter is from 0.4 nm to a few nanometers; SWCNT diameters are in the range of

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0.4-3 nm. The lengths of SWCNTs and MWCNTs are usually in the micrometer range. For morphology investigation of CNT, scanning electron (SEM), atomic force (AFM) and transmission electron microscopy (TEM) are most often used. In figure 3, TEM images of SWCNTs are presented.

Figure 2. The structure of polyhydroxy fullerenes (left) and AFM images (right), low and high magnification.

The absorption in UV Vis region of the spectrum is mostly due to the π-π* transition in the sp2 conjugated system of CNT walls and it is observed as the prominent band, located at around 250 nm. In the UV Vis spectra of HiPco SWCNTs, the phenomenon named van Hove singularities (energy levels with a significant high density of states) induce optical transitions and bands located between 400-1200 nm can be observed (Saito et al., 1992). The structured study of CNTs is usually studied with Raman spectroscopy (Markovic et al., 2012, Jovanovic et al., 2014). Structural

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features which can be analyzed with Raman measurements are disorder in the sp2 structure of CNTs, diameters, conductive properties, doping, and electron delocalization. Raman spectroscopy can be used to the determinate direction of graphene sheet rolling and diameters of particular SWCNTs (Jovanovic et al., 2014, Dresselhaus et al., 2002). Due to their structure, nanotubes are extremely hydrophobic and elastic which cause the formation of “bundles” (ropes). Bundles and water insolubility were considered as main obstacles for CNT application in medicine. Solubility issues were overcome by:    

Non-covalently functionalization (Markovic et al., 2009, Jovanovic et al., 2009), Oxidation with strong oxidation conditions (Datsyuk et al., 2008, Rosca et al., 2005), Covalent modification by 1,3-dipolar cycloaddition of azomethine (Tagmatarchis and Prato, 2004, Vazquez and Prato, 2010), Gamma irradiated (Kleut et al., 2012, Jovanovic et al., 2011).

Graphene, as a single sheet of graphite, was isolated in 2004 (Novoselov et al., 2004). It is a two-dimensional monoatomic sheet of sp2 hybridized C atoms arranged in a planar honeycomb lattice.

Figure 3. TEM micrographs of SWCNTs, low (a) and high (b) magnification.

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Figure 4. AFM images of GO (left and upper right corner) and profile measurement (lower right corner).

Due to interesting physical, chemical and electronic properties, such as high transparency, excellent conductivity, strength, stability, and thinness, graphene have attracted a great scientific attention (Hu et al., 2010, Ferrari, 2007, Marković et al., 2017). In electronic structure, due to slight overlapping of the valent and conductive bands, graphene is a non-zero band gap semiconductor. Delocalization of π electrons is responsible for this property. Depending on the synthetic procedure, graphene sheets can be from few hundreds of nm up to several microns (Prekodravac et al., 2016, Prekodravac et al., 2014, Prekodravac et al., 2015). The hight vary from 0.34 nm for a single layer to several nm for multi layers graphene. Graphene oxide is oxidized form of graphene. In the process of oxidation in structure or graphene different oxygen-containing functional groups are introduced. These groups are carboxyl, carbonyl, hydroxyl, epoxy etc. Introduced functional groups are polar which increases the hydrophilic character of graphene-like structure and results in obtaining material dispersible in water and polar organic solvent, such as methanol. Polar functional groups cause an increase in the structural disorder, an increase in the interplanar distance (i.e., the distance between adjacent sheets), disrupts the symmetry of graphene sp2 arrangement and open the bandgap making GO non-zero band-gap semiconductor. Morphology of GO is similar to graphene. Different microscopic techniques are used to study size and shape of GO and graphene such as AFM, SEM or TEM. The

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size can be up to few microns. For hight measurement usually, AFM profile analysis is employed, as presented in Figure 4 (upper and lower right images). Graphene quantum dots are nanomaterials similar to GO, but much smaller, with diameters below 100 nm. Considering their size below 100 nm in all three directions, GQDs are zero-dimensional materials with properties of graphene and semiconducting quantum dots (Ponomarenko et al., 2008). Carbon atoms in a core of GQDs are sp2-hybridized and they create an aromatic, π-conjugated system or graphene core. But also GQDs contain the large number of oxygen-containing functional groups, responsible for GQD solubility in water and polar organic solvents. In the structure of GQDs, one or few graphene layers can be present. Due to small size or graphene, GQDs are a non-zero band gap semiconductor with band gap usually less than 1.5 eV. Thus GQDs are a highly photoluminescent nanomaterial. Absorption properties of GQDs are similar to GO, with a broad band between 240-260 nm which stems from π-π transitions in the sp2 domains and shoulder band at 300 nm is assigned to the n-π* transition of C = O bonds. The size and height of GQDs are most often analyzed with AFM and TEM microscopy. AFM images of GQDs are shown in Figure 5 (a and b). Due to high water solubility, low toxicity and interesting optical properties such photoluminescence, GQDs were extensively studied for application as imaging agent (Schroeder et al., 2016), agents in photodynamic therapy (Markovic et al., 2012, Kuo et al., 2017, Ristic et al., 2014, Jovanovic et al., 2015). Carbon quantum dots (CQDs) are nanosized carbon-based nanomaterial which is different from GQDs: CQDs are nanoparticles without crystalline structure. These nanoparticles are spherical thus AFM analysis (figure 5 c and d) can be used to determine GQDs which are disks like structure or round, spherical CQDs. Absorption properties of CQDs are very similar to GQDs. Both bands observed in UV Vis spectrum of GQDs are usually detected in the spectrum of CQDs: between 240-260 nm from π-π transitions and shoulder band at 300 nm from the n-π* transition of C = O bonds.

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Figure 5. AFM images of GQDs (a and b) and CQDs (c and d).

Figure 6. PL spectra of fullerol at an excitation of 328 nm, 375, 417 and 469 nm.

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PHOTOLUMINESCENCE OF CARBON BASED NANOMATERIALS Photoluminescence of Fullerene Photoluminescence of fullerenes is strongly depended on the environment. The energy of singlet excited state of fullerene C60 is 1.7 eV above ground state which results in the emission in the boundary region of ViS-NIR wit lifetime of 10-10 or 10-9s and a low quantum yield (λ = 720 nm, quantum yield ΦF = 10-5‒10-4) (Foote, 1994). Both solvent and temperature have a large influence on PL of fullerenes. Singlet excited state of fullerene C60 is deactivated by high rate intersystem crossing to triplet while a small fraction of singlet states return to the ground state radiatively (Foote, 1994). Fullerenes are good electrophiles and they can create complexes on the ground and excited state with aromatic solvents which leads to soltochromism (Kyzyma et al., 2010, Kyzyma et al., 2013). Because they are insoluble in water and have a low PL quantum yield, fullerenes are usually functionalized. The PL properties of fullerenes can be significantly improved by perturbing the almost symmetric π-conjugated system. The higher the perturbation leads to a larger increase in the intensity of PL. Although hydroxyl functional groups are not PL active, introducing these groups in the structure of C60 cause an enhancement of PL intensity. The linear relationship between the number of hydroxyl groups and PL intensity was noticed: the higher number OH groups result in higher PL intensity due to larger symmetry perturbation. In the solid state, emission spectra of fullerol at an excitation of 350 nm, two bands are presented: a weak at 419 nm (2.98 eV) and a strong at 441 (2.83 eV). The first emission band is the result of the relaxation of photoexcited electrons from lower lying excited singlet state (S12) to ground state (hu). The second emission band is the main emission and it is the result of the electron relaxation from S11 to ground state. Fullerol does not show phosphorescence. Also, weak and broad PL spectrum was recorded for fullerol dispersed in water showing

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band without clear maximum in region 550 to 810 nm, as can be observed in Figure 6. Excitation dependent behavior can be noticed: at excitation at wavelengths from 328 to 469 nm the emission band was shifted from 434 to 460, 495 and 530 nm, respectively.

Photoluminescence of Graphene and Graphene Oxide Graphene has zero band gap and its charge carrier undergoes rapid relaxations. Due to these properties, graphene does not usually exhibit PL. Functionalization or the lowering the dimensionality of graphene lead to the opening of band gap from zero to that of benzene due to increasing quantum confinement. In both cases, PL active nanomaterials can be obtained. In the process of top-down synthesis, different chemical approaches have been used to cut or exfoliated graphene-based starting materials. Chemical cutting does not cause only oxidation at edges but also can affect the core of material forming different functional groups bonded to some of the carbon centers in the graphene plane. These groups, usually oxygen-containing, create discontinuities in π-conjugated domains. Thus, the size of the band gap is not dependent on the physical dimension of graphene, but more accurately it is dependent on the size of continual πconjugated domains. Apart from polar functional groups, long distance symmetry of graphene π-system can be disturbed and ruptured by sp3 hybridized carbon atoms. This kind of structural irregularities induces quantum-confined graphene-like states. Another important structural aspect of the size of the band gap is the configuration of edges; armchair or zigzag edges have a large impact on the electronic properties of graphene quantum dots and graphene nanoribbons. For materials with sp2 and sp3 hybridized C atoms, photoluminescent properties depend on the π states of the aromatic domains. The mechanism of PL of these materials results of radiative recombination of electron-hole pairs which are confined in sp2 regions.

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It was proved that structural changes induce by functionalization plays a more important role in tuning PL characteristics that the structure of functional groups (Gong et al., 2014). For derivates of graphene, the emitted light is dependent on the spatial confinement of the electron-hole pairs. The spatial confinement is limited by the size of the embedded aromatic islands. It was noticed that the PL of GO can be tuned from nearultraviolet to blue region through controlled hydrazine (N2H4) reduction (Xin et al., 2012). They noticed that by increasing concentrations of hydrazine in the graphene oxide caused enhance in the size of small sp2 domains in the GO sheets, and lowering the bandgap and red shifting the PL from near ultraviolet to blue. The PL behavior of GO varies depending on the synthetic approach (Dong et al., 2012, Thomas et al., 2013). It was noticed that GO can show both excitation dependent and excitation independent behavior. Graphene oxide prepared from citric acid in process of incomplete hydrothermal carbonization carbon precursor showed excitation independent PL emission (Dong et al., 2010) while excitation dependent PL emission was observed for GO produced by complete carbonization of the precursor. On the contrary, GO produced by oxidation of graphite showed excitation independent PL emission (Thomas et al., 2013). Considering that GO was produced in these studies by using two completely different procedures it is not strange to obtain same material but with opposite PL properties. These results also revealed that PL emission mechanism might engage more processes than radiative recombination of electrons and holes in differently sized aromatic domains. The PL of GO may be results of carbonyl group-related localized electronic states at the oxidation sites. Gokus et al. obtained single-layer graphene using an oxygen plasma treatment (Gokus et al., 2009). They observed a strong photoluminescence which was spatially uniform across the flakes. Gokus et al. were assigned the observed PL to carbonyl group-related localized electronic states at the oxidation sites. Galande et al. found that GO aqueous dispersions emit a structured, strongly pH-dependent visible PL (Galande et al., 2011). They were conducted both experimental and theoretical work and showed that GO PL arise from carboxylic acid groups electronic coupled with nearby

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carbon atoms of graphene quasi-molecular fluorophores, similar to polycyclic aromatic compounds. It was observed that PL of GO is largely dependent on oxygen-containing functional groups while electronic states stem from defects in graphene lattice and affect radiative recombination. Studies have shown that PL of GO can be tune by changing the chemical composition – it was observed that green emission of GO is related to carboxyl and carbonyl functional groups located at the edges of graphene sheet, and blue emission in PL spectrum was assign to defects in graphene planes such as sp2/sp3 domains and epoxy functional groups (Biroju et al., 2015). Biroju et al. showed that tunability of PL properties of GO can be achieved through selective manipulation of the functional groups at the in-plane defects and edge sites. Regardless of discussed studies, the mechanism of PL emission of graphene derivates is still unclear. Thus, in the future more studies focused on this issue can be expected.

Photoluminescence of Graphene and Carbon Quantum Dots Depending on the synthetic procedure, GQDs show different photoluminescence, such as deep ultraviolet than blue, green, yellow or red light emission (Li et al., 2012, Yan et al., 2010, Tang et al., 2012). The origin of GQD photoluminescence is still the subject of scientific debates. But it seems the most probable that fluorophores of GQDs are in fact the aromatic systems of π-conjugated sp2 hybridized C atoms which forms the clusters embedded in the matrix made of sp3 hybridized C atoms (Pal, 2015). It is accepted the opinion that PL of GQDs stems from: 



intrinsic state emission is induced by quantum size effect, zigzag edge sites or recombination of localized electron-hole pairs and/ or defect state emission or defect effect (energy traps) (Zhu et al., 2012).

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We were studied PL behavior of GQDs produced by electrochemical approach and modified by gamma irradiation at different doses (Jovanović et al., 2016, Jovanovic et al., 2015) or GQDs were produced by modified electrochemical procedure for GQDs synthesis in which graphitic rods were thermal annealing at 1700 °C in vacuum and used for electrochemical quantum dots synthesis (Jovanović et al., 2017). We noticed that under the excitation of 328 nm GQDs were emitted blue light centered at around 460 nm (Jovanović et al., 2017). This emission was assigned to quantum confinement of electrons inside the πconjugated C domains and zigzag edge effect (Xu et al., 2013). The quantum confinement effect is restricted to the size of graphene sp2 domains in the structure of GQDs while the actual physical size is not a key parameter. Our results showed that GQDs produced from modified electrodes possess the larger π-conjugated sp2 domains rather than GQDs produced from non-modified electrodes. It was established that blue photoluminescence of GQDs stems from free zigzag sites (Zhu et al., 2015). We observed that when free zigzag edges are protonated, PL was quenched due to breaking the emissive triplet carbene state (Pan et al., 2010). But PL emission was restored at neutral and basic pH. All types of GQDs which were synthesized in our group possess the surface rich oxygen-containing functional groups. These groups are responsible for the localization of electron-hole pairs, facilitating radiative recombination of small clusters (Eda et al., 2010, Loh et al., 2010, Bao et al., 2011b) and the formation of “surface states” where the energy levels between π and π* states of C = C bonds are formed. Usually, GQDs exhibit the excitation-dependent PL behavior which is a common phenomenon explained in terms of electronic conjugate structures, emissive traps, and free zigzag sites. For nanomaterials with a large amount of oxygen-containing functional groups series of emissive traps can be observed which leads to excitation at various wavelengths.

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The quantum yield of GQDs varies from 2 to 86% depending on the method chosen for synthesis as well as the surface chemical environment (Zhou et al., 2016). In our studies, we calculated the values of the relative photoluminescence quantum yields using quinine sulfate as a reference (Jovanović et al., 2017, Jovanovic et al., 2015). The increase of irradiation dose caused also increase of PL quantum yield. The highest values were recorded for GQDs irradiated at a dose of 200 kGy, for which the PL quantum yield was six times higher than for nonirradiated GQDs (4.3% and 0.7%, respectively). Our analysis showed that average lifetime of GQDs produced by electrochemical approach was τaverage = 0.45 ns under excitation of 328 nm, but if they were produced from thermally annealed electrodes the average lifetime of was much longer, 2.95 ns under the same excitation (Jovanović et al., 2017). Also, gamma irradiation significantly improves the lifetime of GQD PL emission (Jovanovic et al., 2015). With the increase in the dose of gamma irradiation, the lifetime of PL was also increased, from 0.49 ns to 1.04 ns at the dose of 200 kGy. The photoluminescence of carbon quantum dots (CQDs) is being assigned to quantum size effects, surface states, and molecules states as well as carbon core states. PL properties of CQDs are sensitive to particle size, as shown by Li et al.: 1) the particles of 1.2 nm emit UV light 2) the particles of 1.2 -3 nm emit visible light 3) the particles of 3.8 nm emit near-infrared light (Li et al., 2010) Surface state emission is a result of the presence of different functional groups with various energy levels. This may lead to a series of emissive traps. With a higher degree of oxidation or other covalent modification, more surface defects are presently leading to red-shift of emission. The quantum yield of CQDs is very high, often around 50%, which is the highest compared to other carbon based nanomaterials (Wang et al., 2017, Khan et al., 2017).

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Photoluminescence of Carbon Nanotubes Photoluminescence of carbon nanotubes was first reported in 2000 (Riggs et al., 2000). Carbon nanotubes show a strong photoluminescence only in the solution while in the solid state bundles completely block their emissions. To obtain CNTs in the form of a solution, as mentioned earlier it is necessary to modify their structure or functionalize them with suitable amphiphile molecules. Depending on electrical conductivity, CNTs can be semiconducting or metallic. Photoluminescence can be observed only in semiconducting SWCNTs, while metallic SWCNTs do not exhibit PL and quench PL which stems from semiconducting SWCNT if they present in the same bundle. Due to a wide range of geometrical and chemical arrangements, their PL properties of CNTs are different. If PL of CNTs is compared with bulk PL materials, where absorbed photon cases the interband transitions, in CNTs absorption of photons leads to excitation of electrons which movement is confined in a quasi-1D system causing the formation of mobile and strongly bound excitons. The binding energy electron-hole pairs are the range of 0.2- 0.5 eV. Thus the ironing of PL of individual SWCNTs is stemmed from excitons (Hagen et al., 2005). Analysis of PL decay showed that tubes with the same (n, m) type had monoexponential decay curve with lifetimes varying between less than 20 and 200 ps from tube to tube. For semiconducting SWCNTs typical band gap is in the order of ~1 eV and PL is the result of radiative recombination of excitons across a band gap. The maximum of PL emission is centered in NIR range and the position is dependent on the chirality of SWCNTs and on diameter (Okazaki, 2008). It was observed that the origin of the PL peak can be reliably assigned to SWCNTs with specific chiral indexes (n, m) because the emission and excitation spectra show characteristic peaks depending on the molecular structure of SWCNTs. Tan et al. studied absorption and emission of SWCNT bundles (Tan et al., 2007). They observed that SWCNT complex spectra can be interpreted considering exciton energy transfer (EET) between adjacent semiconducting tubes. Due to Förster interaction between excitons, the transfer exciton energy transfer process occurs. It was proved that this is

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highly efficient in bundles, which makes an ideal candidate for high yield optoelectronics. The efficiency of SWCNTs PL is usually very low, around 0.01 to 1% (Chen et al., 2005). Quantum yield of PL can be increased by the separation of the metallic SWCNTs from semiconducting ones and remove the oxygen from their environment (Ju et al., 2009). Also, the efficiency of SWCNTs PL is strongly affected by defects in nanotubes structure due to non-radiative recombination of excitons at defect sites (Hertel et al., 2010). Hertel et al. observed that low PL quantum yield of (6,5) SWCNTs is the consequence of high-diffusive exciton mobility and the presence of only a few quenching sites. Covalent modification of CNTs can increase the PL of SWCNTs. By introducing the defects in the structure of carbon nanotubes, it is possible to improve PL properties due to the formation of an emissive deep trap (Kilina et al., 2012). It was observed that the low concentrations of chemical functionalization caused the local alteration in the π-conjugated network of the carbon nanotube sidewalls and caused a spatial confinement of the electronically excited wave functions. These structural modifications lead to changes in PL behavior and can be used for their controlled by selective chemical functionalization. The electronic structure of CNTs can be modified by covalent modification such as: 1) 2) 3) 4)

introducing of oxygen atoms, diazonium groups, hexanoic acid, alkylation

and all these modifications actually leads to disruption of the symmetry of SWCNTs lattice (Kozák et al., 2016). The photoluminescence was also observed in MWCNTs. Their PL emission is the UV-VIS range and can be induced by different structural modifications of outer nanotube walls in the case of short MWCNTs (Zhou et al., 2012). The quantum yield of modified MWCNTs was up to 25%, and it was observed the improvement of PL properties if MWCNTs were

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covalently functionalized with the shorter aliphatic chain. The origin of PL of CNTs in the visible part of the spectrum is under the scientific debate, but it is suggested it may be induced by: 1) defects and/or covalently attached functional groups which create traping sites for exciton energy 2) sp2 hybridized C atoms clusters embedded in the sp3 matrix.

APPLICATIONS OF CARBON BASED NANOMATERIALS Photoluminescent carbon based nanomaterials are investigated for their potentials application in imaging, cancer therapy, drug delivery, sensing, optoelectronic devices and other (Hola et al., 2014, Kim et al., 2011, Cheng et al., 2016, Bao et al., 2011a). The application of carbon based nanomaterials especially GQDs and CQDs in bioimaging has attracted the great attention due to vast advantages such as high solubility and long-term stability in water, resistant to photobleaching, low toxicity, and good biocompatibility. Also, these nanodots were often studied for application in sensors, due to sensitivity to perturbations. Antibacterial therapy using graphene derivates is particularly interesting in the last few years due to the offering possibilities for the production of different antibacterial surfaces (Marković et al., 2017, Chen et al., 2014). It was noticed that GQDs possess a great potential for use in the photodynamic therapy as an agent (Jovanovic et al., 2015, Ge et al., 2014). Fullerene derivates also seem to be an effective agent in cancer therapy (Markovic and Trajkovic, 2008). All these materials were studied for application in drug delivery due to large free surfaces, the ability to bond and transport drugs to sites of interest.

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CONCLUSION In this chapter, the selected carbon based nanomaterials were analyzed with the focus on their structural, morphological and optical properties. Although the chemical composition and the basis of their structure are similar, fullerenes, carbon nanotubes, graphene, graphene oxide, graphene and carbon quantum dots show different PL behavior due to differences in mechanisms behind observed optical property. Considering the increasing interest in these nanomaterials as well as their PL activity, in the future it can be expected further investigation of PL mechanisms and the improvement of their PL efficiency.

ACKNOWLEDGMENT The research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (project no. 172003).

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In: Photoluminescence Editor: Ellis Marsden

ISBN: 978-1-53613-537-4 © 2018 Nova Science Publishers, Inc.

Chapter 6

ADVANCES IN PHOTOLUMINESCENCE PROPERTIES OF COORDINATION POLYMERS Cristina Mozaceanu and Mihaela Baibarac* Laboratory of Optical Processes in Nanostructured Materials, National Institute of Materials Physics, Bucharest, Romania

ABSTRACT The forthcoming part of this chapter is focused on coordination polymers (CPs)/metal-organic frameworks (MOFs) containing metal ions from d and 4f series and a plethora of organic ligands, the resulted compounds showing remarkable photoluminescence properties with different applications in the field of light emitting devices (LEDs), biosensors (in medical assays), sensors (identifying certain species), and so on. The reader may find information about the principles underpinning the design of luminescent MOFs and plenty of examples of luminescent CPs reported in the last few years.

Keywords: photoluminescence, coordination polymers, metal-organic frameworks, transition metal ions, lanthanide ions *

Corresponding Author Email: [email protected].

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INTRODUCTION Metal-organic frameworks (MOFs) or coordination polymers (CPs) represent metal-ligand compounds with “infinite” structures of different dimensionalities (1-D, 2-D or 3-D) and different topologies. CPs are formed of organic ligands coordinated to metal ions [1, 2]. MOFs contain coordinate bonds [3] and weak non-covalent bonds, such as electrostatic interactions [3], hydrogen interactions [3, 4], - stacking [3, 4] or van der Waals interactions [3, 4]. Metal ions in d- and f-blocks are commonly used in the synthesis of CPs, thanks to their diversity of coordination numbers and geometries [2, 5-7]. Electronic configuration of metal ions, the chemical nature of ligands, solvents and counterions, the temperature, pH, and others, represent important issues that must be taken into account in the process of the synthesis and design of MOFs [3, 4, 8]. In recent years, photoluminescent (PL) metal-organic frameworks have received great attention due to their potential applications in the field of photochromic materials, thanks to their color-tunable photoluminescence [9], in light-emitting diodes [10], sensors [11, 12], bio-imaging [13], and others. Transition metal ions with d5 or d10 electronic configurations (e.g., Cu+, Ag+ [14], Au+, Zn2+ [15, 16], Cd2+ [15, 17]), or lanthanide(III) ions [7] are often used as metal centers in CPs. PL properties of transition metal ions have been discussed in detail, starting with 1970, by P. D. Fleischauer and P. Fleischauer [18]. In addition to homometallic MOFs with PL properties, heterometallic CPs have been also synthesized [19-21]. According to Ref. [22], there are three types of transitions responsible for the PL of lanthanide(III) ions, i.e., 4f-4f, 4f-5d, and the charge transfer transitions. Ref. [23] provides important information about the energy levels of trivalent rare-earth ions. According to Pearson’s hard-soft acid – base (HSAB) concept [24], Ln(III) ions are considered to be hard Lewis acids with a strong affinity towards hard bases such as O-donor ligands. The most commonly used ligands in the design and synthesis of CPs are organic species with extended -conjugated backbones, examples of such linkers being presented in Ref. [25]. Trivalent rare-earth ions are known for their weak light absorption and emission, due to their small molar

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absorption coefficients [26]. One efficient method to enhance Ln(III) ions luminescence is by using organic ligands with high molar absorption coefficients and incorporating chromophore groups. Such organic linkers act as sensitizers for lanthanide ions by absorbing the incident radiation then transferring the excess of energy to the Ln(III) ions. This process is known as the antenna effect [11, 26, 27]. Further, we will present the progress recorded in the last four years concerning the photoluminescent properties of some CPs. Thus, our attention will be focused on 23 compounds, which will be labeled with a bolded number, starting from 1.

NEW CHALLENGES IN THE FIELD OF COORDINATION POLYMERS WITH PHOTOLUMINESCENT PROPERTIES In 2014, six three-dimensional CPs have been reported by S. T. Yue et al. [28], one homometallic, [Eu2(mpda)3(H2O)4]n (1), and other five heterometallic polymers labeled as [Ln2Cd2(mpda)2(bdc)2(SO4)2(H2O)6]n, where Ln = Sm(2), Eu(3), Gd(4), Tb(5), Dy(6), mpda = 2,6dimethylpyridine-3,5-dicarboxylic acid, and bdc = 1,4-benzenedicarboxylic acid. The structure of compound 1 was composed of alternating chains and layers containing mpda coordinated in three different modes. Studies of single-crystal X-ray diffraction showed that compounds 2-6 were isostructural, consisting of 2-D [Ln2Cd2(mpda)2(bdc)2] layers pillared by SO42− anions [28]. The solid-state PL spectra of complexes 1, 3, and 5 were recorded at room temperature when characteristic emission bands of the corresponding lanthanide ions were observed. PL studies showed that when an excitation wavelength of 287 nm was used, compound 1 displayed an intense red emission and exhibited five narrow emission bands situated in the 575 - 710 nm spectral range attributed to the 5D0→7FJ (J = 0, 1, 2, 3, 4) transitions of Eu(III) ions. The most intense PL band with a maximum at 613 nm was assigned to the 5D0→7F2 transition [28]. The ratio between the intensities of PL bands assigned to the 5D0→7F2 and 5D0→7F1 electronic

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transitions (I(5D0→7F2)/I(5D0→7F1)) with a value around 2.3 represented a strong indicative of the low symmetric coordination sphere of Eu(III) ions [28, 29]. In the case of compound 3, the PL studies performed at an excitation wavelength of 299 nm revealed a red luminescence and five emission bands similar with those of the compound 1 [28]. The 3.3 value of the I(5D0→7F2)/I(5D0→7F1) ratio indicated a lower symmetric coordination environment of Eu(III) ions [28]. Compared to complexes 1 and 3, compound 5 behaved differently. When the PL spectrum was recorded at the excitation wavelength of 320 nm, polymer 5 displayed a green luminescence characteristic for Tb(III) ions and exhibited four emission bands in the 475 – 630 nm spectral range, which were assigned to the 5D4→7FJ (J = 6, 5, 4, 3) electronic transitions specific for Tb(III) ions. 28 S. T. Yue et al. [28] have concluded that the compound 5 showed better PL properties than complexes 1 and 3, thanks to a better energy transfer from the organic ligands to Tb(III) ions. CPs with PL properties were obtained using copper(I) halide clusters. One metal-organic framework composed of copper(I) iodide clusters linked by thiophosphoramide ligands has been reported by R. Boomishankar et al. in 2014 [29]. Luminescence studies revealed a thermochromic behavior of this compound [29]. At room temperature, the complex displayed a blue color, assigned to the organic ligand emission. At temperature of 77 K it exhibited an orange-yellow color, for which the iodide-to-copper charge transfer and transitions from orbitals d to s or p of Cu (I) ions proved to be responsible [29, 30]. In 2016, P. F. Yan et al. [31] have reported a series of five 2-D lanthanide-MOFs, {[Ln4K4L6(H2O)x]·yH2O}n [Ln = Dy (6), x = 24, y = 12; Ln = Ho (7), x = 23, y = 12; Ln = Er (8), x = 24, y = 12; Ln = Yb (9), x = 24, y = 11; Ln = Lu (10), x = 24, y = 12 (11)], obtained using a deprotonated form of L-di-p-toluoyl-tartaric acid (ligand L) and different lanthanide salts. The PL studies of compounds 6-9 were performed at an excitation wavelength of 295 nm. 31 The PL spectrum of polymer 6 displayed four emission bands: one broad band at 386 nm assigned to the -* transition of the organic ligand, and three narrow bands between 482 and 662 nm, attributed to the f-f electronic transitions of Dy(III) ions [31].

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W. Y. Niu et al. used the yellow emission of Dy(III) ions and the blue emission of ligand L to generate white-light emission from a singlecomponent coordination polymer of compound 6 by modifying the excitation wavelength [31]. CPs 7-9 showed near-infrared (NIR) emissions when luminescence spectra were recorded under the excitation wavelengths of 354 nm and 356 nm. NIR photoluminescence of CPs 7-9 was assigned to the characteristic electronic transitions of the corresponding lanthanide ions [31]. G. Li et al. [32] have reported in 2016 four heterometallic MOFs containing multinuclear copper(I) iodide clusters (Cu8I8), lanthanide ions, and organic linkers, the resulting polymers being named as follows: [La2(Cu8I8)(pba)6(C4H8O2)2(H2O)2]·3C4H8O2·2H2O (11), [Ce2(Cu8I8)(pba)6 (H2O)4]·5C4H8O2 (12), [Eu2(Cu8I8)(pba)6(H2O)4]·5C4H8O2 (13), and [Tb2(Cu8I8)(pba)6(H2O)4]·5C4H8O2 (14), where Hpba = 3-(pyridin-4yl)benzoic acid. It is well known that Cu8I8 clusters with d10 electronic configuration which show an intense luminescence are used in organic light-emitting diodes (OLEDs) [32]. Lanthanide metal-organic frameworks are materials with low chemical selectivity and sensitivity. One efficient method to enhance their selectivity and sensitivity is by incorporating Cu8I8 clusters as second luminescent centers within the Ln(III)-MOFs structure [32]. In the case of compounds 11 and 12, La(III) and Ce(III) ions did not show specific emissions. Therefore, the strongest emission bands of the PL spectra were assigned to the Cu8I8 clusters. The luminescence spectra of compounds 13 and 14 revealed characteristic emission bands of the corresponding rare-earth ions [32]. G. Li et al. performed chemicalsensing studies on a calcinated powder of compound 14 to detect solvent molecules, such as benzene, toluene, p-xylene, and mesitylene [32]. PL studies were performed on different suspensions containing the desolvated form of compound 14 soaked in selected guest solvents [32]. The results revealed that using benzene, toluene, or p-xylene as guest solvents, emission bands of Tb(III) ions increased in intensity, and emission bands of Cu8I8 clusters displayed blue shifts. 32 In contrast, when using mesitylene as a guest solvent, the suspension exhibited a red emission, while the characteristic emission bands of Tb(III) ions almost disappeared.

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G. Li et al. [32] concluded that the sensing mechanism was strongly related to the structural changes of compounds when guest solvent molecules entered the MOFs pores. Other series of CPs containing multinuclear copper(I) iodide clusters have been reported in 2016 [33]. J. P. Lang et al. mixed various derivatives of ((pyridinyl)-1H-pyrazolyl)pyridine with CuI to create a diversity of luminescent CPs. In that case, the PL spectra recorded at an excitation wavelength of 300 nm revealed broad emission bands in the 470 - 800 nm spectral range, which were assigned to different types of charge transfer transitions (iodide-to-Cu(I), ligand-to-metal or iodide-to-ligand), to clustercentered transitions, or to a combination between the above mentioned [33]. M. Pan et al. 34 have reported white-light-emitting Ln(III)-MOFs based on monopodal or bipodal organic ligands of the zwitterionic types. The PL studies performed on the above CPs revealed visible and/or NIR emissions. 34 Three ternary solid solutions, namely Gd-Eu-Tb-bipodal ligand, La-Eu-Tb-monopodal ligand, and La-Dy-Sm-monopodal ligand, were found to exhibit white-light emissions [34]. Another method to obtain CPs with PL properties is by using mixed ligands [35]. There are a great number of PL-MOFs containing mixed ligands, [Zn2(btc)(biimpy)(OH)]n (15) [H3btc = 1,3,5-benzenetricarboxylic acid, biimpy = 2,6-bis(1-imdazoly)pyridine)] being one them [35]. This compound showed a 3-D structure, employing tetranuclear [Zn4(COO)4(OH)2] clusters as subunits. The solid-state PL studies of compound 15 were performed at room temperature. The results showed that the intensity of the dominant emission band decreased when an increase of the excitation wavelength from 315 to 340 nm took place [35]. [Cd2(1,4-NDC)2(dppe)] (16) and [Mn3(OH)2(1,4-NDC)2(dppe)(H2O)] (17) [1,4-NDCH2 = 1,4-naphthalene dicarboxylic acid, dppe = 1,3-di(4pyridyl)propane] represent two other examples of PL-CPs obtained by mixing two ligands, one dicarboxylic acid and one exo-bidentate diamine, with transition metal ions [36]. PL studies revealed that compound 16 exhibited two emission bands located in the blue (410 nm) and green (570 nm) regions when a change of the excitation wavelength from 350 to 360

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nm occurred. 36 The results showed that PL band at 410 nm decreased in its relative intensity as increasing the excitation wavelength. In contrast, the intensity of the PL band at 570 nm increased [36]. Compound 16 displayed white-light emission properties with potential applications in lighting [36]. Unlike 16, complex 17 exhibited only one emission band in the yellow region. The distinct luminescent properties of the above CPs relied on the different electronic configurations of the metal ions. Regarding the different luminescent properties of compounds 16 and 17, Huang X. and Wu Z. concluded that: i) Cd(II) ions were difficult to be reduced or oxidized, as a consequence of their stable electronic configurations, and ii) the partially filled 3d shell of Mn(II) ions may lead to re-absorption of the ligand-to-metal charge transfer [36]. H. Zheng et al. [37] have reported luminescent CPs containing phenylamine derivatives, terephthalate anions, and the Zn(II) or Cd(II) ions. The PL bands of the above mentioned complexes were assigned to the π*→π or π*→n electronic transitions of the ligands. Moreover, the different metal centers and coordination spheres, as well as the conjugation degree of the backbones of ligands were found to be responsible for the shifts of PL bands [37]. Schiff bases were intensively used as ligands in synthesis and design of luminescent CPs with different dimensionalities. X. Wu et al. [38] have reported three MOFs containing such linkers. The PL studies revealed emission bands situated in the 325 – 650 nm spectral range, which were ascribed to the charge transfer within the naphthalene-2,7-disulfonate counterions [38]. MOFs with luminescent properties may be synthesized by using organic ligands containing fluorine atoms, too. Examples of such CPs were two 1-D complexes containing Ag(I) or Cd(II) ions linked by deprotonated 2-fluoroisonicotinic acid, as shown in Ref. [39]. The PL studies of the above mentioned polymers showed that both compounds displayed red shifts of the emission bands [39]. Au(I) or Ag(I) cyanides may be also employed as ligands, as a consequence of their linear geometry and ability to stabilize these complexes by aurophilic or argentophilic interactions 40. D. B. Leznoff and J. S. Ovens [40] have reported few examples of the

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heterobimetallic Cu(I)/[Au(CN)2]¯ coordination polymers with luminescent properties. Another approach to design and synthesize PL-CPs is inserting rareearth ions into the void-space cavities of metal-organic frameworks 9, 4143. Examples of such complexes have been reported by H. J. Holdt et al. [41], in 2016. Above CPs were consisted from Cd(II) ions linked by derivatives of imidazolate, forming extended frameworks with Ln(III) ions trapped inside the cavities. H. J. Holdt et al. concluded that the charge transfer from ligand to the lanthanide ion (i.e., antenna effect) and the electronic transitions from f levels of Ln(III) ions were responsible for the PL properties of the above compounds [41]. In 2017, X. Feng et al. [42] have reported a series of Zn(II)-4f heteronuclear MOFs containing dicarboxylates and diamines as ligands. A mixture of Zn(II)-Eu(III) and Zn(II)-Tb(III) complexes was used to prepare a ternary system, [Eu1.35Tb1.65Zn6(bipy)2(Hmimda)7]n (18), where H3mimda = 2-methyl-1-H-imidazole-4,5-dicarboxylic acid and bipy = 4,4’bipyridine. The PL studies showed that compound 18 exhibited white-light emission at an excitation wavelength of 363 nm [42]. In addition, the results revealed that varying the molar ratio between the metal ions, the emission color was changed [42]. L. Wang et al. [9] have reported Ln(III)-doped CPs with tunable luminescence. One of the compounds, reported in Ref. [9], was used in small molecules detection studies. Other examples of PL-MOFs containing Ln(III) ions have been reported for their color tunability [43]. Luminescent coordination polymers may be designed and synthesized using tripodal ligands. According to the paper reported by J. Xie et al. [44], Gd(III)-CPs of the type {[Gd(TTTA)(H2O)2]2H2O}n (19) and [Gd(TTTA)DMF]n (20) (H3TTTA = 2,2,’2”-[1,3,5-triazine-2,4,6triyltris(thio)]tris-acetic acid), were obtained using TTTA as tripodal ligand. The PL studies revealed no characteristic PL bands of Gd(III) ions. In contrast, the emission bands of the 19 and 20 compounds were ascribed to the ligand-to-ligand charge transfer transitions [44]. X. Chen et al. [45] have reported in 2017 two CPs with luminescent properties, [Ln(btpa)(phen)2(OH)]n·nH2O [Ln = Tb 21, Pr 22, H2btpa = 5-

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(3,’4’-bis(tetrazol-5”-yl)phenoxy)isophthalic acid, phen = 1,10phenanthroline]. When the PL spectrum was recorded under excitation wavelength of 348 nm, compound 21 displayed four emission bands, ascribed to the 5D4  7FJ (J = 3, 4, 5, 6) transitions of Tb(III) ions. In contrast, when the PL spectrum was recorded under excitation wavelength of 315 nm, complex 22 showed a broad emission band attributed to the * electronic transitions of the organic ligands [45]. Four luminescent lanthanide-MOFs, Ln(TDC)(OAc)(H2O)n, [Ln = Eu, Tb, Dy or Sm, TDC = thiophene-2, 5-dicarboxylic acid, OAc = acetate] have been reported by J. Han et al. [46]. The PL spectra of the four compounds were reported to be described of the following emission bands: i) 362, 380, 394, 580 and 650 nm in the case of Eu(TDC)(OAc)(H2O)n; ii) 493, 544, 592 and 616 nm, in the case of Tb(TDC)(OAc)(H2O)n; iii) 476 and 576 nm, in the case of Dy(TDC)(OAc)(H2O)n; and iv) 557, 597 and 645 nm in the case of Sm(TDC)(OAc)(H2O)n 46. Above emission bands were assigned to the following transitions: i) 7F0 5G6, 7F05H4, 7 F0.15L6, 5D07F0, 5D07F1 in the case of Eu(TDC)(OAc)(H2O)n; ii) 5 D47FJ (J = 6, 5, 4, 3) in the case of Tb(TDC)(OAc)(H2O)n; iii) 4 F9/26H13/2 and 4F9/26H15/2 in the case of Dy(TDC)(OAc)(H2O)n; and iv) 4G5/26H5/2, 4G5/26H7/2 and 4G5/26H9/2 in the case of Sm(TDC)(OAc)(H2O)n 46. P. Mahata et al. [47] have reported the CP labeled as [Cd2.5(PDA)(tz)3] [23 PDA = 1,4-phenylenediacetate and tz = 1,2,4-triazolate] was used to detect the azinphos-methyl pesticide in the aqueous solution. In this context, authors have showed that the luminescence quenching of complex 23 occurred as increasing the pesticide concentration [47]. Other examples of PL-CPs correspond to complexes containing either Pr(III) ions [48], either copper(I) iodide cationic clusters [49]. In the context of this progress, it is worth mentioning that few papers have been published on CPs containing Pr(III) ions [48]. Summarizing all these, we conclude that much effort has been given for the synthesis of white-light-emitting MOFs [e.g., 10, 49-51] and MOFs exhibiting NIR emissions [e.g., 20, 21, 49, 52, 53] as well as for the

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development of new applications of these materials in the field of the small molecules detection [54-57] and the light-emitting diodes [50]. According to the literature, the coordination polymers exhibiting emission bands characteristic for the lanthanide ions are in a continuous process of development [10, 29, 42, 58].

CONCLUSION In this chapter, the progress recorded in the field of photoluminescent properties of coordination polymers was reviewed. Special attention has been given to white-light-emitting MOFs as well as to MOFs exhibiting NIR emissions. The growing interest concerning synthesis and design methods of new coordination polymers with PL properties in order to extend the field of applications of these materials is anticipated to develop in the near future.

ACKNOWLEDGMENT This work was financed by Core Program 2016–2018, project PN16480101.

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INDEX A absorption spectra, 8, 9 absorption spectroscopy, 165 acetonitrile, 105, 115 acid, 117, 179, 184, 187, 193, 198, 199, 200, 201, 202, 203, 204, 205 amine(s), 100, 114 amplitude, 12, 14, 19, 89 anisotropy, 101, 107 annealing, 4, 21, 46, 63, 64, 65, 66, 67, 104, 107, 136, 140, 145, 146, 181, 190, 193 atmosphere, 70, 92 atomic force microscopy (AFM), 67, 68, 99, 109, 170, 172, 174, 175, 176 atoms, ix, xi, 2, 4, 8, 14, 20, 25, 38, 39, 78, 82, 113, 168, 169, 170, 171, 173, 175, 178, 180, 184, 185, 203

B band gap, 29, 103, 109, 110, 114, 174, 175, 178, 179, 183 base, xi, 57, 69, 76, 89, 103, 118, 167, 168, 175, 190, 198

batteries, 129, 161 benzene, 178, 201 binding energy(ies), 101, 110, 111, 114, 115, 117, 183 biocompatibility, 185 biological processes, 84 biomolecules, 187 biosensors, viii, xii, 57, 84, 197 Boltzmann distribution, 35 bonding, 3, 18, 145 bonds, ix, 2, 4, 19, 21, 25, 39, 40, 70, 78, 98, 100, 113, 175, 181, 198 broadband, 37, 38, 47 bromine, 40, 117 bulk materials, vii, x, 97, 99

C cancer therapy, 185, 188 carbon, viii, x, xi, 115, 128, 129, 130, 131, 132, 133, 134, 135, 150, 156, 157, 158, 159, 160, 161, 162, 163, 164, 166, 167, 168, 169, 170, 171, 175, 178, 179, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195 carbon atoms, 169, 170, 178, 180

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Index

carbon nanotubes, viii, x, xi, 128, 129, 130, 131, 132, 133, 134, 150, 156, 157, 158, 159, 160, 161, 162, 163, 164, 166, 168, 171, 183, 184, 186, 187, 188, 189, 190, 191, 193, 194, 195 carbon tetrachloride, 115 carbonization, 179, 187 carboxyl, 174, 180 carboxylic acid, 179, 199 cathodic process, 131, 164 ceramic(s), 71, 72, 74, 75, 91 chalcogenide glasses, vii, viii, 1, 2, 42, 43, 44, 45, 47, 48, 49, 50 chalcogenide glasses, viii, 1, 42 chalcogenides, 69, 99 chalcohalide glasses, 32, 33, 37 chemical, viii, xi, 58, 60, 61, 65, 67, 75, 76, 89, 98, 99, 103, 105, 109, 113, 118, 130, 134, 145, 148, 160, 163, 166, 167, 168, 170, 174, 178, 180, 182, 183, 184, 186, 190, 193, 194, 198, 201 chemical functionalization, 98, 184, 190 chemical interaction, 118, 130, 160, 163, 166 chemical properties, viii, xi, 58, 134, 168 chemical reactions, 113, 170 chemical vapor deposition (CVD), 60, 61, 76, 105, 145 cladding, 59, 60, 76, 77 clustering, 14, 38, 48, 69, 72, 75 clusters, 69, 105, 180, 181, 185, 200, 201, 202, 205 coatings, vii, x, 54, 61, 65, 90 composite materials, xi, 98, 100, 113, 118, 134, 141, 144 composites, viii, x, 98, 100, 106, 113, 118, 129, 130, 133, 134, 135, 138, 140, 141, 143, 145, 146, 148, 149, 150, 160, 161, 162, 163, 166 composition, vii, ix, xi, 2, 5, 6, 7, 15, 19, 21, 29, 32, 33, 39, 58, 63, 67, 76, 78, 81, 84, 85, 168, 180, 186

compounds, viii, x, xii, 62, 66, 75, 98, 100, 103, 106, 113, 114, 117, 118, 134, 137, 140, 145, 148, 197, 198, 199, 200, 201, 203, 204, 205 conductivity, 107, 174, 183 configuration, 86, 107, 170, 178, 198, 201 confinement, ix, 54, 55, 56, 57, 81, 82, 84, 99, 101, 105, 107, 110, 114, 116, 118, 178, 179, 181, 184 conjugated polymers, viii, x, 133, 134, 135 conjugation, 171, 203 constituents, 137, 141, 144, 145, 147, 148 coordination, viii, xii, 69, 75, 117, 197, 198, 200, 201, 203, 204, 206 copolymer, 117, 131, 140, 141, 142, 143, 156, 158, 159, 164 copper, 55, 200, 201, 202, 205 Coulomb interaction, 101, 110, 114 covalent bond, 98, 145, 198 crystal growth, 102 crystal structure, 4, 43, 106, 127, 128, 129, 130, 156, 162, 163 crystalline, viii, xi, 32, 68, 70, 73, 75, 85, 98, 100, 106, 107, 108, 113, 116, 168, 175 crystallinity, 32, 117 crystallization, 7, 8, 32, 67, 74, 75 crystals, vii, x, 45, 73, 74, 93, 97, 98, 100, 101, 103, 107, 111, 115, 117, 118

D decay, ix, 2, 33, 34, 35, 37, 40, 67, 71, 72, 79, 83, 101, 102, 104, 111, 112, 136, 147, 169, 183, 188 decay times, 33, 101, 138 deconvolution, ix, 2, 12, 15, 22, 41 defect site, 184 defects, xi, 9, 10, 18, 19, 22, 30, 37, 64, 70, 78, 79, 85, 101, 102, 104, 106, 107, 113, 118, 138, 168, 180, 182, 184, 185

Index deficiency, 4, 18, 70 deposition, 21, 58, 60, 61, 62, 64, 66, 67, 76, 90, 105, 145 derivatives, vii, ix, 2, 5, 118, 148, 187, 192, 202, 203, 204 detection, 7, 79, 86, 100, 103, 204, 206 diamines, 195, 204 dielectric constant, 114, 117 differential scanning calorimetry (DSC), 32 diffraction, 6, 57, 66, 67, 73, 199 dimensionality, xi, 168, 178 diodes, 6, 11, 100, 198, 201, 206 disorder, 37, 173, 174 dispersion, 32, 68, 73, 77, 79, 143, 145 dissolved oxygen, 131, 164 distribution, ix, 2, 14, 20, 25, 35, 38, 39, 71 diversity, 60, 61, 103, 198, 202 dopants, 47, 70 doping, vii, ix, 2, 5, 8, 9, 11, 13, 14, 15, 17, 20, 22, 24, 25, 26, 30, 31, 37, 38, 39, 40, 41, 55, 60, 62, 65, 69, 70, 75, 104, 173, 188 drug delivery, xii, 168, 170, 185 drugs, 170, 185 drying, 62, 63

E electric field, 57, 100, 101, 106, 108 electrical conductivity, 183 electrical properties, 98, 105, 107 electrodes, 139, 181, 182 electromagnetic, 54, 55, 56, 57, 81, 83 electron microscopy, 6, 172, 186 electronic structure, 31, 110, 174, 184 electron(s), 3, 6, 21, 35, 57, 70, 82, 83, 99, 101, 103, 105, 107, 114, 136, 141, 145, 146, 147, 172, 173, 174, 177, 178, 179, 180, 181, 183, 186, 188 electro-optical properties, 148

213

emission, vii, viii, x, xi, 1, 3, 4, 5, 9, 10, 11, 12, 13, 14, 15, 17, 19, 21, 22, 25, 26, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 43, 46, 47, 48, 51, 52, 68, 70, 71, 75, 78, 79, 85, 88, 92, 97, 101, 102, 106, 107, 110, 112, 117, 136, 140, 144, 146, 147, 149, 150, 168, 169, 177, 179, 180, 181, 182, 183, 184, 187, 198, 199, 200, 201, 202, 203, 204, 205, 206 emitters, 70, 188 energy, ix, 2, 3, 5, 8, 10, 14, 15, 20, 21, 29, 31, 32, 35, 36, 37, 38, 40, 41, 48, 57, 69, 71, 72, 73, 74, 75, 78, 79, 80, 83, 84, 94, 101, 110, 111, 114, 117, 119, 136, 141, 142, 148, 160, 169, 172, 177, 180, 181, 182, 183, 185, 187, 194, 198, 200 energy stark splitting diagram of Er3+ ions, 15 energy transfer, 3, 31, 35, 36, 41, 48, 71, 72, 94, 136, 148, 183, 187, 194, 200 environment, 4, 31, 70, 71, 72, 73, 75, 83, 84, 114, 116, 177, 182, 184, 200 Er solubility into Ge-S-Ga glasses, 19 erbium, viii, 1, 3, 5, 10, 19, 37, 38, 45, 47, 52, 55, 79, 90, 93 etching, 58, 59, 89 ethylene, 117, 118, 140 evaporation, 21, 46, 148 evidence, 130, 138, 160, 163, 166, 194 EXAFS, 4, 31, 40, 43 excitation, vii, ix, xi, 2, 3, 5, 6, 9, 11, 12, 13, 14, 15, 16, 19, 20, 21, 22, 30, 32, 35, 36, 38, 41, 47, 48, 70, 72, 85, 86, 101, 102, 103, 107, 112, 115, 116, 131, 134, 135, 138, 139, 141, 142, 144, 149, 164, 168, 169, 176, 177, 179, 181, 182, 183, 199, 200, 202, 204, 205 exciton, xi, 110, 111, 115, 117, 127, 128, 130, 136, 141, 156, 163, 168, 183, 184, 185, 190, 194 exploitation, 54, 55

214

Index F

fabrication, 57, 58, 59, 60, 62, 65, 66, 69, 76, 87, 94, 99 fiber(s), v, vii, x, 3, 42, 43, 53, 54, 55, 56, 57, 58, 59, 60, 61, 68, 76, 77, 79, 80, 87, 94 films, x, 21, 43, 45, 46, 51, 54, 58, 60, 61, 62, 63, 64, 65, 66, 75, 90, 91, 93, 105, 110, 129, 142, 143, 144, 148, 160, 162, 166, 192, 193 fluorescence, ix, 2, 3, 5, 7, 35, 36, 41, 43, 44, 48, 50, 52, 66, 67, 69, 70, 71, 72, 73, 75, 147, 194, 195 fluorine, 70, 203 formation, 4, 19, 50, 62, 64, 70, 90, 113, 140, 146, 147, 149, 173, 181, 183, 184 formula, 61, 103 fullerene(s), vi, viii, x, xi, 130, 131, 133, 134, 147, 150, 151, 155, 156, 160, 161, 163, 164, 165, 166, 167, 168, 170, 171, 172, 177, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194 functionalization, 98, 113, 118, 160, 171, 173, 179, 184, 190

G gallium, 11, 25, 26, 32, 42, 45, 57, 59 gel, 54, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 70, 74, 90, 91, 92, 93 geometrical optics, 77, 82 glass transition, 3, 7, 32 glasses, v, vii, viii, ix, 1, 2, 3, 4, 5, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 65, 67, 68, 69, 70, 73, 75, 80, 88, 91, 92, 93, 94

graphene, vi, viii, x, xi, 99, 106, 109, 128, 129, 130, 133, 134, 144, 147, 150, 157, 158, 159, 161, 162, 163, 167, 168, 171, 173, 174, 175, 178, 179, 180, 181, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195 graphene oxide, viii, x, xi, 107, 129, 130, 133, 134, 158, 159, 162, 163, 168, 174, 179, 186, 187, 190, 191, 194, 195 graphene quantum dots (GQD), xi, 168, 175, 178, 180, 182, 187, 189, 190, 191, 192, 193, 195 graphene sheet, 145, 171, 173, 174, 180, 192 graphite, xi, 167, 169, 173, 179, 188, 190 gratings, 57, 59, 66 growth, 58, 75, 102, 110, 149

H halogen, 99, 114, 117 homogeneity, 6, 13, 62, 67 host, vii, ix, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 13, 15, 17, 18, 19, 21, 22, 23, 25, 26, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 43, 46, 51, 67, 70, 80, 106, 170 hybrid, 60, 64, 66, 78, 91, 92, 98, 100, 113, 114, 117, 118 hybridization, xi, 109, 168 hydrogen, 113, 198 hydrolysis, 60, 61, 66 hydroxyl, 66, 170, 171, 174, 177 hydroxyl groups, 66, 171, 177

I image(s), 65, 68, 83, 94, 99, 105, 170, 172, 174, 175, 176 impurities, xi, 9, 44, 78, 104, 105, 168 indium, 57, 59 infrared spectroscopy, 129, 159, 162

Index integrated circuits, ix, 54, 57 integrated optics, 55, 56, 57, 60, 66, 77, 88, 89, 90 integration, 54, 57, 58, 60, 78, 169 intercalation processes, 98, 113, 118 interface, 60, 77, 82, 137, 143, 145 interference, 55, 82 intermolecular interactions, 150 inversion, 69, 79 ion implantation, 21 ion-exchange, 59, 89 ionic polymers, 138 ions, viii, ix, xii, 2, 3, 4, 5, 8, 11, 12, 13, 14, 15, 19, 20, 25, 29, 30, 31, 32, 35, 36, 37, 38, 40, 41, 42, 43, 44, 47, 48, 51, 59, 60, 62, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 79, 81, 82, 85, 92, 104, 106, 117, 197, 198, 199, 200, 201, 203, 204, 205, 206 iron, 131, 164 irradiation, 66, 116, 118, 148, 149, 181, 182, 189, 190 isophthalic acid, 205 issues, x, 81, 98, 109, 150, 173, 198

L lanthanide, 13, 197, 198, 199, 200, 201, 204, 205, 206 lanthanide ions, 197, 199, 201, 206 lanthanum, 42 laser ablation, 105 lasers, 2, 45, 55, 65, 67, 68, 87 layered crystals, vii, x, 97, 98, 101, 103, 107, 118 Lewis acids, 198 lifetime, 14, 33, 38, 69, 71, 72, 75, 99, 103, 136, 169, 177, 182 ligand, 198, 200, 202, 203, 204 light, vii, ix, xii, 8, 21, 39, 53, 54, 55, 56, 59, 65, 66, 74, 76, 77, 78, 80, 81, 82, 84,

215

86, 100, 101, 106, 108, 111, 112, 117, 131, 144, 164, 169, 179, 180, 181, 182, 188, 197, 198, 201, 202, 203, 204, 205, 206 light conditions, 107, 111 light-emitting diodes, 198, 201, 206 liquid phase, 58 lithium, 57, 59, 129, 161 local order, vii, ix, 2, 5, 46, 51 low temperatures, ix, 2, 29, 38, 61, 67, 103, 111 low-temperature photoluminescence, 25, 34, 46, 47, 52 luminescence, viii, ix, xi, 2, 4, 5, 23, 29, 35, 36, 37, 39, 40, 41, 43, 45, 46, 47, 48, 51, 53, 72, 74, 93, 101, 186, 193, 199, 200, 201, 204, 205 luminescence efficiency, ix, 2, 5, 39, 41, 45, 51

M macromolecular chains, 107, 136, 140, 146, 149 magnetic field, 102 manufacturing, 57, 62, 75, 81 mass, 78, 138, 139, 140, 148 materials, vii, viii, x, xi, 1, 2, 32, 42, 54, 56, 57, 58, 59, 61, 62, 66, 67, 70, 77, 91, 92, 97, 98, 100, 105, 106, 109, 111, 113, 114, 117, 118, 134, 141, 144, 145, 155, 167, 168, 169, 175, 178, 183, 185, 194, 198, 201, 206 materials science, 194 matrix, 10, 13, 25, 39, 66, 67, 70, 71, 72, 73, 74, 75, 79, 80, 137, 145, 147, 180, 185 Maxwell equations, 106 measurement(s), 6, 7, 9, 99, 108, 146, 169, 173, 174, 175 mechanical properties, 106 metal ion, viii, xii, 92, 197, 198, 203, 204

216

Index

metal-organic frameworks, viii, xii, 197, 198, 201, 204 methanol, 105, 174 microhardness, 6, 8 microscopy, 6, 86, 99, 170, 175, 186 microspheres, v, vii, x, 53, 54, 56, 81, 82, 83, 84, 85, 87, 95 migration, 137, 160 mission(s), 36, 201, 202, 203, 205, 206 mixing, 65, 202 modifications, 104, 184 molecular orbital, 142 molecular structure, 113, 149, 183 molecules, viii, 169, 170, 182, 183, 202, 204, 206 monolayer, x, 98, 106 monomers, 62, 140 morphology, viii, xi, 84, 110, 117, 168, 169, 171 multiwalled carbon nanotubes, 187, 193

N nanocomposites, 75, 160 nanocrystals, 68, 72, 73, 74, 93 nanodots, 185, 186, 195 nanomaterials, viii, xi, 167, 168, 169, 175, 178, 181, 182, 185, 186 nanomedicine, 194 nanometers, 115, 171 nanoparticles, viii, x, 117, 133, 134, 135, 147, 150, 170, 175, 194 nanostructured materials, 155 nanostructures, 99, 127, 128, 168, 170, 190, 191 nanotube, 184, 194 naphthalene, 202, 203 NIR, 6, 29, 30, 39, 92, 177, 183, 201, 202, 205, 206 nitrogen, 6, 102, 191 non-linear optics, 67

non-radiative transition, 101 nucleation, 8, 75 nucleus, 82

O optical amplifiers, ix, 2, 5, 33, 55 optical communications, 69, 79 optical devices, vii, ix, 53, 56, 57, 58, 60, 65, 68 optical fiber, ix, 3, 42, 54, 55, 56, 57, 58, 59, 60, 76, 77, 79, 80, 94 optical gain, 68 optical glasses, ix, 33, 53, 54, 56 optical microscopy, 99 optical networks, 54 optical parameters, 33 optical properties, vii, viii, 2, 4, 31, 32, 42, 44, 45, 46, 48, 50, 52, 55, 56, 62, 70, 71, 73, 81, 91, 93, 103, 110, 113, 114, 117, 128, 129, 130, 131, 148, 155, 156, 158, 159, 162, 163, 164, 170, 175, 186 optoelectronics, 5, 55, 56, 58, 89, 184, 188 organic compounds, 117 organic solvents, 143, 170, 175 oscillation, 101, 102, 111, 114 overlap, 111, 145 oxidation, xi, 61, 131, 164, 168, 173, 174, 178, 179, 182, 187 oxidative stress, 194 oxygen, 59, 70, 131, 164, 174, 175, 178, 179, 181, 184, 188, 190, 192 oxygen plasma, 179, 188

P parallel, 99, 100, 106 permission, 102, 108, 112, 116, 138, 139, 141, 142, 144, 146, 149 pH, 179, 181, 198 phase diagram, 43

Index phosphate, 57, 75 phosphate glasses, 75 phosphorescence, 177 photobleaching, 185 photodetectors, 110, 113 photodynamic therapy, 175, 185, 188 photolithography, 58 photoluminescence, v, vi, vii, ix, x, xi, xii, 1, 2, 5, 6, 9, 11, 15, 19, 20, 21, 22, 25, 31, 34, 37, 39, 44, 45, 46, 47, 48, 49, 51, 52, 86, 88, 97, 98, 100, 113, 116, 118, 127, 128, 129, 130, 131, 133, 134, 135, 144, 147, 148, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 167, 168, 169, 175, 177, 178, 179, 180, 181, 182, 183, 184, 187, 188, 189, 190, 192, 194, 195, 197, 198, 201 photonic applications, 38, 62 photonics, vii, viii, 1, 2, 5, 41, 42, 56, 73, 187 photons, 36, 56, 57, 79, 82, 169, 183 photosensitivity, 59, 89 photovoltaic cells, 114, 134 photovoltaic devices, 117 PL spectrum, 9, 22, 101, 102, 106, 116, 136, 140, 143, 150, 177, 180, 200, 205 planar waveguides, ix, 54, 55, 56, 57, 60, 62, 77, 79 polar, 105, 170, 174, 175, 178 polycyclic aromatic compounds, 180 polymer(s), viii, x, xii, 65, 66, 117, 119, 129, 133, 134, 135, 136, 138, 141, 143, 145, 148, 161, 162, 195, 197, 198, 199, 200, 201, 203, 204, 206 population, 17, 37, 69, 79 preparation, ix, 54, 56, 61, 67, 145, 148, 186, 195 principles, xii, 161, 197 project, 86, 119, 150, 186, 206 propagation, ix, 53, 76, 81

217 Q

quantization, vii, x, 97, 102 quantum confinement, 101, 105, 107, 109, 118, 178, 181 quantum dot(s), viii, xi, 105, 168, 169, 175, 178, 181, 182, 186, 187, 188, 189, 190, 191, 192, 193, 195 quantum structure, 117 quantum well, 99, 101, 111, 113, 114 quantum yields, xi, 168, 182, 188

R radiation, 7, 10, 45, 56, 79, 81, 100, 103, 156, 199 radiative transitions in Er3+ ions, 32 radius, 101, 114 raman spectra, 6, 17, 18, 19, 45, 108, 111, 128, 131, 164 raman spectroscopy, 45, 127, 130, 131, 156, 157, 160, 163, 164, 172, 187, 188, 192, 193 rare earth elements, vii, viii, 1 rare-earth ions, 2, 42, 47, 48, 62, 65, 66, 67, 68, 70, 71, 73, 75, 198, 201, 204 rare-earths, ix, 53, 54, 56, 69, 70, 71, 72, 73, 74, 81 reactions, 66, 113, 135, 148, 170 reactive ion-etching, 59 reactive oxygen, 192 recombination, viii, x, 3, 44, 47, 97, 101, 103, 104, 106, 110, 115, 137, 141, 178, 179, 180, 181, 183, 184 recombination processes, 104, 110 red shift, 179, 203 reflectance spectra, 28, 29 refractive index, 3, 8, 57, 59, 60, 62, 63, 66, 73, 76, 77, 78, 81, 82, 83, 84 refractive indices, 56, 76

218

Index

relaxation, 4, 35, 37, 40, 64, 67, 69, 72, 101, 137, 146, 148, 177 relaxation process, 137, 146, 148 relaxation rate, 4, 72 requirement(s), ix, 9, 54, 55, 56, 57, 60, 77 resolution, 7, 32, 39 resonator, 81, 82, 83 response, 60, 99, 109 rods, 105, 181 room temperature, 6, 9, 14, 15, 16, 29, 35, 39, 100, 199, 200, 202

S salts, 62, 200 scattering, 6, 8, 17, 21, 37, 59, 60, 73, 78, 79, 83, 99, 108, 109, 111, 112, 128, 130, 131, 156, 159, 161, 163, 164, 165 semiconductor(s), 47, 57, 58, 59, 98, 99, 100, 103, 106, 109, 113, 114, 115, 118, 127, 130, 163, 174, 175, 188 sensing, xii, 82, 168, 185, 187, 195, 201 sensitivity, 31, 84, 89, 185, 201 sensor(s), viii, xii, 54, 56, 65, 67, 79, 84, 89, 93, 94, 185, 193, 197, 198 signals, 6, 39, 54, 55, 169 silica, 6, 58, 60, 88, 90, 91 silicon, 58, 60, 88, 89 single walled carbon nanotube (SWNTs), 134, 135, 136, 138, 139, 140, 141, 142, 143, 144, 148, 149, 150, 160 SiO2, 57, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 69, 70, 72, 73, 74, 75, 76, 79, 80, 92, 110 sodium, 59, 191 solar cells, 117, 118 sol-gel, 54, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 70, 74, 90, 91, 92, 93 solid state, 55, 177, 183 solubility, viii, 1, 4, 5, 19, 37, 42, 67, 69, 75, 170, 175, 185

solution, 59, 62, 105, 110, 115, 117, 136, 139, 140, 145, 146, 147, 183, 205 solvents, 105, 143, 148, 170, 175, 177, 198, 201 species, viii, xii, 69, 98, 99, 103, 105, 113, 114, 116, 192, 197, 198 spectroscopy, vii, ix, 2, 4, 42, 43, 45, 48, 75, 99, 127, 129, 130, 131, 156, 157, 159, 160, 162, 163, 164, 165, 173, 187, 188, 192, 193, 194 speed of light, 56 spin, 58, 62, 63, 64, 65, 91, 102, 114 stability, 3, 58, 75, 114, 174, 185 Stark effect, 70 structural changes, 179, 202 structural defects, 9, 19, 30, 85, 138 structural modifications, 184 structure, vii, ix, x, xi, 2, 4, 5, 9, 13, 17, 21, 22, 28, 30, 31, 32, 37, 39, 41, 43, 45, 50, 52, 58, 66, 67, 69, 70, 73, 76, 77, 81, 86, 92, 97, 98, 99, 100, 102, 106, 107, 111, 113, 118, 127, 128, 129, 130, 156, 162, 163, 168, 169, 170, 171, 172, 173, 174, 175, 177, 179, 181, 183, 184, 186, 193, 199, 201, 202 substitution, 19, 117 substrate(s), 21, 57, 58, 59, 60, 61, 62, 65, 76, 77, 110 sulfur, 4, 18, 40 symmetry, 100, 107, 169, 170, 174, 177, 178, 184 synthesis, 4, 61, 65, 100, 104, 105, 115, 128, 130, 134, 137, 144, 146, 159, 162, 169, 178, 181, 182, 193, 198, 203, 205, 206

T techniques, 32, 58, 61, 63, 65, 66, 79, 86, 87, 95, 99, 109, 168, 174

Index technology(ies), 20, 55, 57, 58, 60, 68, 89, 91, 92 temperature, vii, ix, 2, 4, 5, 6, 7, 9, 11, 14, 15, 16, 17, 21, 22, 25, 26, 28, 29, 30, 31, 34, 35, 39, 41, 46, 47, 48, 51, 52, 65, 66, 67, 79, 84, 93, 100, 102, 104, 110, 112, 115, 116, 143, 177, 193, 198, 199, 200, 202 temperature dependence, 25, 39, 115 TEOS, 62, 65 tetrahydrofuran, 137, 148 therapy, 175, 185, 188, 191 thermal analysis, 6 thermal evaporation, 21, 46 thermal history, 4 thermal properties, xi, 44, 47, 51, 52, 167 thin films, 20, 45, 46, 51, 60, 61, 62, 63, 90, 105, 110, 129, 162, 193 TIR, 76, 77, 81, 82 toluene, 6, 191, 201 total internal reflection, 76 toxicity, 175, 185 transformation, 149, 192 transition metal, 99, 197, 198, 202 transition rate, 38 transition temperature, 3, 7 transmission, 6, 8, 23, 55, 78, 86, 98, 172 transmission electron microscopy (TEM), 172, 173, 174, 175 transparency, 3, 58, 68, 73, 79, 80, 174 transport, 82, 105, 107, 185 treatment, 59, 68, 75, 104, 107, 179, 188, 194

U up-conversion fluorescence, 2, 3, 7, 43, 48, 52 UV irradiation, 148 UV light, 59, 65, 182

219 V

vacancies, xi, 168 vacuum, 21, 76, 181 vapor, 6, 58, 60, 61, 76, 105, 145 variations, 78, 117 vector, 76, 85, 190 vibration, 18, 22, 40, 66, 74, 78 vinylidene fluoride, 130, 156, 163

W water, 5, 66, 81, 105, 170, 173, 174, 175, 177, 185 wavelengths, xi, 3, 11, 13, 69, 78, 79, 82, 168, 170, 178, 181, 201 weak interaction, 70 weight ratio, 144 wells, 99, 101, 105, 113, 115 wettability, 130, 163 whispering gallery modes, x, 54, 56 wide band gap, 114 wires, 55, 105

X x-ray diffraction (XRD), 6, 67, 73, 74, 199

Y yield, 80, 169, 177, 182, 184 yttrium, 80

Z zirconia, 91 zirconium, 66 ZnO, 75, 131, 156, 161, 164