Triboluminescence: Materials, Properties, and Applications

*Zhaofeng Wang and Fu Wang*

## **Abstract**

Triboluminescence is one of the types of luminescence that could be activated by mechanical stress. Considering the rising research efforts and achievements in recent years, this chapter provides an overview on the study of triboluminescence. The first part gives a background description regarding the history, research status, and advantages of triboluminescence. Then, we summarize the material systems for triboluminescence in both organics and inorganics. In the third part, we review the properties of triboluminescence, particularly on the unique characteristics and their improvements. Finally, we give a comprehensive summary on the developments of triboluminescent devices for applications in various fields in terms of mechanical engineering, energy, biological monitoring, and sensors as well as lighting, imaging, and displaying.

**Keywords:** triboluminescence, crystals, spectral characteristics, cycling stability, advanced applications

## **1. Introduction**

Triboluminescence (TL) refers to the phenomenon that materials could emit light when they are mechanically stimulated, such as rubbing, grinding, impact, stretching, and compression [1–3]. TL was first recorded by Francis Bacon in 1605 when breaking the sugar crystals [4]. After that, TL has been found in many solids, such as rocks, quartz, alkaline halide, molecular crystals, and some organic materials [5]. It is estimated that nearly 50% of inorganic compounds and 30% of organic molecular solids have been confirmed to have TL [6]. Because TL could be directly activated by the widely existed mechanical activities in daily life without requiring artificial optical/electrical sources, TL shows great advantages in energy saving and environmental protection [7].

In general, TL could be classified into three types, i.e., fracture TL, plastic TL, and elastic TL [8], as illustrated in **Figure 1**. Among them, the elastic TL has gained the most attention because of its structure nondestructive characteristic which is crucial for practical applications. The present researches of TL are mainly focused on the development of novel TL materials and the performance improvement in terms of brightness, color manipulation, and cyclic stability [9–11]. Based on the efforts in the above aspects, a variety of decent applications of TL materials have been achieved in recent years, covering the fields of mechanical engineering, energy, biological monitoring, and sensors as well as lighting, imaging, and displaying.

In this chapter, we provide an overview of TL, regarding the materials, properties, and applications. Since TL covers a large range from organics to inorganics with emitting types from fracture TL to plastic TL and elastic TL, most of the

#### **Figure 1.**

*Illustration of the fracture, plastic, and elastic deformation-induced TL in organic and inorganic crystals.*

content of the chapter was focused on the elastic TL of inorganic solids in which the most significant progresses have been made during recent years. We hope that this chapter could provide a deep understanding of TL and stimulated new ideas for further researches.

## **2. TL materials**

## **2.1 Organic crystals and organometallic compounds**

Organic crystals and organometallic compounds represent an important part of TL materials. About 19% of organics and 37% of aromatic compounds are estimated to have TL [12]. According to molecular structure, the TL organic crystals could be divided into nonaromatic organic crystals and aromatic compounds. The main nonaromatic organics include sugar (e.g., D-glucose, lactose, maltose, L-rhamnose, sucrose), tartaric acid/tartrate (e.g., ammonium tartrate, sodium tartrate) and other nonaromatic organics (e.g., L-ascorbic acid, cholesteryl salicylate, cholestenol, ammonium oxalate, disodium hydrogen citrate, aniline hydrochloride) [13–15]. The main aromatic compounds are coumarin, acenaphthene, phthalic anhydride, phenanthrene, phenol derivatives, 9-anthryl carbinol, N-phenyl-substituted imides, carbazole derivatives, hexaphenylcarbodiphosphorane (Ph3P)2C, and some aggregation-induced emission compounds (e.g., tetraphenylethene compounds, N-substituted phenothiazine, aryl dioxaborolane, N-substituted dihydroacridine) [16–18]. The above aromatic compounds always possess distinctive TL characteristics because of their peculiar molecular structure, and their TL should arise from the spin-allowed/spin-forbidden electron transition of molecular excited state (π-π\* transition), likewise with their photoluminescence (PL). Moreover, impurities play special roles in TL of some compounds.

Organometallic compounds, including rare earth and transition metal complexes, have also featured TL. The typical examples are some β-diketone complexes of LnIII ion (Ln = Eu, Sm, Pr, Yb, Tb, Gd, or Nb). Among them, the europium complexes (EuD4TEA and its doped forms) generate extremely bright and daylightvisible red-orange TL, which is much stronger than that of the others [19]. But these

**27**

*Triboluminescence: Materials, Properties, and Applications*

complexes show a very sharp emission band corresponding to the f-f transition of Eu3+ ions. The transition metal-based complexes are mainly MnII, copperI

PtII complexes, such as Mn(Ph3PO)2X2 (X = Cl, Br), (MePh3P)2MnCl4, Cu(NCS) (py)2(PPh3), and Pt(ipyim)(bipz), which give a broad emission band [20].

The inorganic TL compounds are composed by hosts and doping luminescent centers. The inorganic hosts include the halides (e.g., KCl, KBr, NaF, RbBr, and RbI [21]), oxides (e.g., Al2O3 [22] and ZrO2 [23]), sulfides (e.g., ZnS [24]), oxysulfides (e.g., CaZnOS [10] and BaZnOS [25]), aluminates (e.g., SrAl2O4 [1], Sr3Al2O6 [7], and CaYAl3O7 [9]), silicates (e.g., Sr2MgSi2O7 and SrCaMgSi2O7 [26]), phosphates (e.g., Li3PO4 [27] and SrMg2(PO4)2 [28]), borates (e.g., BaB4O7 [29]), titanates (e.g., BaTiO3 and CaTiO3 [30]), niobates (e.g., Ca2Nb2O7 [31] and LiNbO3 [32]), stannates (e.g., Sr2SnO4 [33]), sulfates (e.g., BaSO4 [34]), and oxynitrides (e.g., BaSi2O2N2 [35]). Rare earth ions are the common doped ions in inorganic TL compounds, such as Eu2+, Eu3+, Pr3+, Dy3+, Ce3+, Tb3+, Er3+, and Sm3+ [4]. The other metal ions, like

, and Ti4+ ions [23, 36], are also employed as the luminescent centers in

.

inorganic TL compounds. To date, the well-recognized inorganic compounds with

In many organic and inorganic systems, the TL spectra are consistent with the PL spectra, suggesting they possess the same emitting processes. The differences between TL and PL lie in the excitation/activation processes that TL originates from the release of the trapped carries or the piezoelectric effect under mechanical stimuli. In some systems, like BaZnOS:Mn2+ [25], the compression-induced TL and rubbing-induced TL exhibit 24 nm and 48 nm blueshift, respectively, compared to that of PL (**Figure 2**). Such phenomenon could be ascribed to the conduction band

In piezoelectric materials, there is also obvious difference on the concentration quenching between PL and TL. For example, the quenching concentrations of Pr3+ in CaNb2O6, Ca2Nb2O7, and Ca3Nb2O8 for TL are 0.25 mol%, 0.1 mol%, and 0.075 mol%, respectively, while the values for PL are 0.5 mol%, 0.3 mol%, and 0.1 mol%, respectively [31]. The decreased quenching concentration of TL was attributed to the participation of piezoelectric field in delivering the energy from

The TL of organic molecules or complexes mostly originates from the fracture of crystals, and thus there is no cycling stability for such materials. For the TL along with the nondestructive structure, mainly referring to the piezoelectric effect and de-trapping-induced TL, the cycling stability is particularly important. The TL aroused by piezoelectric effects usually exhibits stable luminescence when activated by cyclic mechanical tests [37]. For example, the Pr3+-doped LiNbO3 could keep its TL intensity for more than 100 cycles [32]. ZnS:Cu/PDMS composites could maintain the TL intensity up to 30,000 cycles of stretching, and the intensity still reached 65% of the initial one without a color change even after 100,000 cycles of

bright TL are SrAl2O4:Eu2+, Dy3+ (SAOED), and ZnS:Mn2+/Cu+

and valence band tailoring by piezoelectric fields.

, and

*DOI: http://dx.doi.org/10.5772/intechopen.81444*

**2.2 Inorganic compounds**

Mn2+, Cu+

**3. TL properties**

**3.1 Spectral characteristics**

traps to quenching centers.

**3.2 Cycling stability**

complexes show a very sharp emission band corresponding to the f-f transition of Eu3+ ions. The transition metal-based complexes are mainly MnII, copperI , and PtII complexes, such as Mn(Ph3PO)2X2 (X = Cl, Br), (MePh3P)2MnCl4, Cu(NCS) (py)2(PPh3), and Pt(ipyim)(bipz), which give a broad emission band [20].

## **2.2 Inorganic compounds**

The inorganic TL compounds are composed by hosts and doping luminescent centers. The inorganic hosts include the halides (e.g., KCl, KBr, NaF, RbBr, and RbI [21]), oxides (e.g., Al2O3 [22] and ZrO2 [23]), sulfides (e.g., ZnS [24]), oxysulfides (e.g., CaZnOS [10] and BaZnOS [25]), aluminates (e.g., SrAl2O4 [1], Sr3Al2O6 [7], and CaYAl3O7 [9]), silicates (e.g., Sr2MgSi2O7 and SrCaMgSi2O7 [26]), phosphates (e.g., Li3PO4 [27] and SrMg2(PO4)2 [28]), borates (e.g., BaB4O7 [29]), titanates (e.g., BaTiO3 and CaTiO3 [30]), niobates (e.g., Ca2Nb2O7 [31] and LiNbO3 [32]), stannates (e.g., Sr2SnO4 [33]), sulfates (e.g., BaSO4 [34]), and oxynitrides (e.g., BaSi2O2N2 [35]). Rare earth ions are the common doped ions in inorganic TL compounds, such as Eu2+, Eu3+, Pr3+, Dy3+, Ce3+, Tb3+, Er3+, and Sm3+ [4]. The other metal ions, like Mn2+, Cu+ , and Ti4+ ions [23, 36], are also employed as the luminescent centers in inorganic TL compounds. To date, the well-recognized inorganic compounds with bright TL are SrAl2O4:Eu2+, Dy3+ (SAOED), and ZnS:Mn2+/Cu+ .

## **3. TL properties**

## **3.1 Spectral characteristics**

In many organic and inorganic systems, the TL spectra are consistent with the PL spectra, suggesting they possess the same emitting processes. The differences between TL and PL lie in the excitation/activation processes that TL originates from the release of the trapped carries or the piezoelectric effect under mechanical stimuli. In some systems, like BaZnOS:Mn2+ [25], the compression-induced TL and rubbing-induced TL exhibit 24 nm and 48 nm blueshift, respectively, compared to that of PL (**Figure 2**). Such phenomenon could be ascribed to the conduction band and valence band tailoring by piezoelectric fields.

In piezoelectric materials, there is also obvious difference on the concentration quenching between PL and TL. For example, the quenching concentrations of Pr3+ in CaNb2O6, Ca2Nb2O7, and Ca3Nb2O8 for TL are 0.25 mol%, 0.1 mol%, and 0.075 mol%, respectively, while the values for PL are 0.5 mol%, 0.3 mol%, and 0.1 mol%, respectively [31]. The decreased quenching concentration of TL was attributed to the participation of piezoelectric field in delivering the energy from traps to quenching centers.

## **3.2 Cycling stability**

The TL of organic molecules or complexes mostly originates from the fracture of crystals, and thus there is no cycling stability for such materials. For the TL along with the nondestructive structure, mainly referring to the piezoelectric effect and de-trapping-induced TL, the cycling stability is particularly important. The TL aroused by piezoelectric effects usually exhibits stable luminescence when activated by cyclic mechanical tests [37]. For example, the Pr3+-doped LiNbO3 could keep its TL intensity for more than 100 cycles [32]. ZnS:Cu/PDMS composites could maintain the TL intensity up to 30,000 cycles of stretching, and the intensity still reached 65% of the initial one without a color change even after 100,000 cycles of

#### **Figure 2.**

*Spectral comparison of the PL, compression-induced TL, and rubbing-induced TL in BaZnOS:Mn2<sup>+</sup> . Reproduced by permission of the Royal Society of Chemistry [25].*

tests [38]. However, for the TL aroused by the de-trapping of carriers in structure, intensity degradation would be serious during cycling tests, i.e., such materials showed poor cycling stability [39, 40]. To overcome the above issue, great efforts have been made based on the TL mechanism in terms of the de-trapping processes. Researchers proposed a strategy to improve the cycling stability of the de-trappinginduced TL by applying an extra UV irradiation source to ensure the balance between the trapping and de-trapping of carries [11]. The power density played a key role to stabilize the TL intensity, and the effective power density was determined to be 1000 mW/cm<sup>2</sup> as shown in **Figure 3** [41].

#### **3.3 Intrinsic structure-dependent TL**

The TL characteristics could be directly modulated by varying the concentration of luminescent centers. Generally, there is a concentration quenching phenomenon in terms of TL intensity. In CaZnOS:Mn2+, the increase of the doping concentration of Mn2+ could not only vary the TL intensity with a trend that increases first and then decreases but also arouse a redshift on the TL spectra with the emitting color manipulated from orange to red [10]. In addition, the chemical composition of the hosts, namely, the variation of the defect phases or traps, could also cause significant variations on the TL intensity and color. In (Ba,Ca)TiO3:Pr3+, the co-dopant of trivalent rare earth ions, such as La3+, Y3+, Nd3+, Gd3+, Yb3+, and Lu3+, could greatly improve the TL intensity, in which Gd3+ could enhance the intensity more than 61% [30]. This is because that the co-dopant of the above ions could increase the concentration of the carries in traps and thus lead to more luminescence emitted under mechanical stimuli. In Sr2MgSi2O7:Eu2+, when part of Sr2+ was substituted by Ca2+ or Ba2+, the TL intensity and emitting color could be adjusted simultaneously [26]. SrBaMgSi2O7:Eu2+ showed the lowest TL intensity compared to that of Sr2MgSi2O7:Eu2+ and SrCaMgSi2O7:Eu2+. The replacement of Sr2+ by Ca2+ or Ba2+ in Sr2MgSi2O7:Eu2+ could further manipulate the emission band in a wide range from 440 nm to 499 nm. Researches also showed that the TL performance is dependent on the crystal size. In sucrose crystals, the TL intensity significantly increased with increasing crystal size (**Figure 4**), which could be explained by piezoelectric mechanism [42].

**29**

**Figure 4.**

**Figure 3.**

*Publishing [41].*

**3.4 External factor-dependent TL**

*American Chemical Society [42].*

The TL powders could be directly stimulated by ultrasonication or impact. The ultrasonic TL is dependent on the ultrasonic power with a linear relationship [43]. The impact-induced TL is strongly affected by the impact velocity or impact energy [2, 44]. When TL powders were composited in various matrices, other mechanical

*TL integrated intensity on dependent of particle sizes of sucrose crystals. Reproduced by permission of the* 

*TL intensity of SAOED response to the cyclic load at a frequency of 1 Hz under different irradiation conditions: (a) with UV irradiation turned off; (b) under a UV irradiation with a power density of 200 mW/cm2; (c) under a UV irradiation with a power density of 1000 mW/cm2. Reproduced by permission of the OSA* 

*Triboluminescence: Materials, Properties, and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.81444*

*Triboluminescence: Materials, Properties, and Applications DOI: http://dx.doi.org/10.5772/intechopen.81444*

#### **Figure 3.**

*TL intensity of SAOED response to the cyclic load at a frequency of 1 Hz under different irradiation conditions: (a) with UV irradiation turned off; (b) under a UV irradiation with a power density of 200 mW/cm2; (c) under a UV irradiation with a power density of 1000 mW/cm2. Reproduced by permission of the OSA Publishing [41].*

#### **Figure 4.**

*TL integrated intensity on dependent of particle sizes of sucrose crystals. Reproduced by permission of the American Chemical Society [42].*

#### **3.4 External factor-dependent TL**

The TL powders could be directly stimulated by ultrasonication or impact. The ultrasonic TL is dependent on the ultrasonic power with a linear relationship [43]. The impact-induced TL is strongly affected by the impact velocity or impact energy [2, 44]. When TL powders were composited in various matrices, other mechanical

actions, such as rubbing, stretching, and compression, would be employed for TL. The intensity of the rubbing-induced TL shows relationships to both the applied normal load and the friction velocity [43]. For the stretching-induced TL, the elastic modulus plays a key role on the critical strain [45]. In addition, TL intensity varies along with the change of strain levels and stretching speeds [7]. For the compression-induced TL, it depends on the applied load as well as the deformation rate [31, 37].

## **4. Applications**

TL materials could be composited in a variety of hosts, such as polymer matrices and metal bulk materials. The as-fabricated TL composites could emit light under the stimulus of mechanical behaviors for various applications. Because SrAl2O4:Eu2+ and ZnS:Mn2+/Cu+ are the well-recognized intense TL materials, the present applications almost focus on them.

#### **4.1 Structural health monitoring**

The TL composites could be directly stimulated by the inner stress, showing application perspectives in structural health monitoring of devices, machines, and buildings [46–48]. To date, TL materials have been well employed to visualize and monitor the stress distribution as well as the fatigue crack initiation and propagation of matrices [1, 49, 50]. The sensitization of stress distribution in solids was first conducted by C-N. Xu et al. [1] They composited the green-emitting SrAl2O4:Eu2+ TL powders in epoxy resins and confirmed that the TL behaviors of the SrAl2O4:Eu2+/epoxy composites under a compressive load of 1000 N could reflect the stress distribution based on the experimental and simulative results. The SrAl2O4:Eu2+/epoxy composites were further employed to realize the measurements of instantaneous R-curves and bridging stress in a fast-propagating crack system (**Figure 5**) [51].

Based on the above pioneering achievements, researchers successfully developed the structural health monitoring applications of TL materials in steel box girders [52], hydrogen storage cylinders [53], and gas pipelines [54]. Compared with the conventional monitoring methods by electrical and magnetic signals, the approach by TL signals shows advantages of contactless, wireless, convenient, and visualization [49, 52].

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**Figure 6.**

*Reproduced by permission of the OSA Publishing [41].*

*Triboluminescence: Materials, Properties, and Applications*

When TL materials undergo loading or impact, the emitted luminescence shows one-to-one correspondence between the emission intensity and impact/loading energy, which could be utilized to develop impact/load sensors to record the related mechanical information [2]. However, for the sensors fabricated from SrAl2O4:Eu2+, the prominent problem is that the TL intensity will be decreased along with the increase of impact times or loading time, i.e., SrAl2O4:Eu2+ shows poor cycling stability that goes against for its applications as impact/load sensors [41, 44]. Researchers further found that when an ultraviolet (UV) irradiation source with a certain power density was applied, SrAl2O4:Eu2+ could keep the TL intensity stably based on the balance of trapping and de-trapping of the carriers in structure [41, 44]. The proposed SrAl2O4:Eu2+-based sensor under UV irradiation could stably sensitize the applied load both in dynamic and static states (**Figure 6**) [41]. Differing from SrAl2O4:Eu2+, ZnS:Mn/Cu showed almost no TL intensity degradation along with the increase of cycle numbers because of the piezoelectric effect, which could be directly used for impact/load sensor applications without needing

The exploited devices for lighting, imaging, and displaying are mainly fabricated from ZnS:Mn/Cu and elastomer matrices. The as-fabricated ZnS:Cu/PDMS flexible composites showed bright and durable TL under stretching with a brightness of ca.

fabricated into fabrics with patterns that could be applied for imaging and display-

*Load responsiveness of SAOED in static and dynamic states with UV irradiation turned on and turned off.* 

In addition to the stimulus of stretching and rubbing, the TL composites could also be activated by various mechanical sources, such as wind [5], magnetic field [57], and ultrasonic wave [58], which fulfill the requirements of green and sustainable developments. For practical applications in lighting and displaying, the TL flexible devices with a white light or multicolored emissions are required, and a variety of strategies have been proposed. For example, Jeong et al. employed ZnS:Cu, Mn and ZnS:Cu as the orange and green TL materials, respectively, and fabricated ZnS-based flexible composites, in which TL color manipulation including a warm white light was demonstrated by adjusting the component ratios of

and durability over 10,000 cycles [38]. The composites could be further

*DOI: http://dx.doi.org/10.5772/intechopen.81444*

**4.2 Impact/load sensor**

an extra UV irradiation [37, 55].

ing as presented in **Figure 7** [56].

120 cd/m<sup>2</sup>

**4.3 Lighting, imaging, and displaying**

#### **Figure 5.**

*Experimental R-curve and bridging stress distribution in the crack wake based on the TL of SAOED. Reproduced by permission of Elsevier B.V. [51].*

## **4.2 Impact/load sensor**

When TL materials undergo loading or impact, the emitted luminescence shows one-to-one correspondence between the emission intensity and impact/loading energy, which could be utilized to develop impact/load sensors to record the related mechanical information [2]. However, for the sensors fabricated from SrAl2O4:Eu2+, the prominent problem is that the TL intensity will be decreased along with the increase of impact times or loading time, i.e., SrAl2O4:Eu2+ shows poor cycling stability that goes against for its applications as impact/load sensors [41, 44]. Researchers further found that when an ultraviolet (UV) irradiation source with a certain power density was applied, SrAl2O4:Eu2+ could keep the TL intensity stably based on the balance of trapping and de-trapping of the carriers in structure [41, 44]. The proposed SrAl2O4:Eu2+-based sensor under UV irradiation could stably sensitize the applied load both in dynamic and static states (**Figure 6**) [41]. Differing from SrAl2O4:Eu2+, ZnS:Mn/Cu showed almost no TL intensity degradation along with the increase of cycle numbers because of the piezoelectric effect, which could be directly used for impact/load sensor applications without needing an extra UV irradiation [37, 55].

## **4.3 Lighting, imaging, and displaying**

The exploited devices for lighting, imaging, and displaying are mainly fabricated from ZnS:Mn/Cu and elastomer matrices. The as-fabricated ZnS:Cu/PDMS flexible composites showed bright and durable TL under stretching with a brightness of ca. 120 cd/m<sup>2</sup> and durability over 10,000 cycles [38]. The composites could be further fabricated into fabrics with patterns that could be applied for imaging and displaying as presented in **Figure 7** [56].

In addition to the stimulus of stretching and rubbing, the TL composites could also be activated by various mechanical sources, such as wind [5], magnetic field [57], and ultrasonic wave [58], which fulfill the requirements of green and sustainable developments. For practical applications in lighting and displaying, the TL flexible devices with a white light or multicolored emissions are required, and a variety of strategies have been proposed. For example, Jeong et al. employed ZnS:Cu, Mn and ZnS:Cu as the orange and green TL materials, respectively, and fabricated ZnS-based flexible composites, in which TL color manipulation including a warm white light was demonstrated by adjusting the component ratios of

### **Figure 6.**

*Load responsiveness of SAOED in static and dynamic states with UV irradiation turned on and turned off. Reproduced by permission of the OSA Publishing [41].*

**Figure 7.**

*TL fabrics based on the doped ZnS (a) fibers, (b) ribbons, and (c) dots; corresponding (d–f) optical and (g–i) TL photographs of the fabrics in (a–c). Reproduced by permission of the Royal Society of Chemistry [56].*

ZnS:Cu, Mn and ZnS:Cu [36]. They further presented a strategy for the TL color manipulation of doped ZnS by physically combining fluorescent dyes in PDMS elastomers based on the energy transfer between the TL of doped ZnS and the PL of dyes [59]. Hao and his co-workers also realized the remote tuning of TL color of ZnS:Al, Cu/PDMS composites by modulating the frequency of magnetic field [60]. In addition, flexible devices with dual-mode emissions, i.e., EL and TL, have also been developed for imaging and displaying [59, 61].

## **4.4 Pressure sensor**

The TL flexible composites exhibit luminescent signals dependent on the applied pressure. Based on such performance, Wang et al. developed a ZnS:Mnbased pressure sensor for both single-point dynamic pressure recording and 2D planar pressure mapping with a high spatial resolution of 100 μm and a fast response time less than 10 ms [24]. The pressure sensor was further used as a flexible handwriting device that could collect the information of both signatures and signing habits as shown in **Figure 8**, exhibiting high-level security compared with the existing technologies. They further introduced the single-electrode triboelectric nanogenerator in the ZnS:Mn-based flexible composites and obtained a full dynamic-range pressure sensor for the visualization of pressure distribution both in low pressure regimes (< 100 kPa) and high-pressure regimes (> 1 MPa) with an excellent pressure sensitivity of 6 MPa<sup>−</sup><sup>1</sup> [62]. In addition, CaZnOS:Er3+ thin-film was prepared, which possessed the pressure and temperature sensing based on its TL and upconversion luminescence [63].

**33**

**4.5 Stress/strain sensor**

**Figure 8.**

When the TL materials are introduced in elastic matrices, stress/strain sensor could be obtained. At present, the widely employed TL materials for fabricating stress/strain sensors are ZnS:Mn, ZnS:Cu, and SrAl2O4:Eu, because of their prominent TL properties as well as the one-to-one correspondence between the TL intensity and stress/strain. Yun et al. [64] further found that the co-dopant of Dy3+ in SrAl2O4:Eu could improve its performance as stress sensor based on the sensitivity. In addition to sense the stress or strain by analyzing the TL intensity, the risetime and decay time of TL during cyclic elastic deformation of SrAl2O4:Eu were also demonstrated to be suitable for evaluating the change of the strain energy [65]. Moreover, a calibration method for SrAl2O4:Eu, Dy-based thin-film sensor was proposed to enable quantitative full-field strain measurements in pixel-level resolution [66]. Qian et al. [45] prepared ZnS:Mn/Cu@Al2O3/PDMS flexible composites and adjusted the elastic modulus by introducing SiO2 nanoparticles. They finally obtained a TL stress/strain sensor that could be driven by weak mechanics of skin movements. In the very recent work [7], Sr3Al2O6:Eu with bright and tunable PL and TL was presented when it was composited in PDMS elastomers. By combining the wavelength selectivity of PL and dynamic stress responsiveness of TL, a multimode stretching/strain sensor was developed by a bilayered structure design of Sr3Al2O6:Eu/PDMS composites with coating a light-shielding layer of Au atop

*Flexible handwriting device based on the TL of ZnS:Mn for visualization of dynamic pressure distributions: (a) schematic illustration of the system; (b) visualization of 2D planar pressure distribution; (c-h)* 

*visualization of the signing process. Reproduced by permission of Wiley-VCH [24].*

*Triboluminescence: Materials, Properties, and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.81444*

#### **Figure 8.**

*Flexible handwriting device based on the TL of ZnS:Mn for visualization of dynamic pressure distributions: (a) schematic illustration of the system; (b) visualization of 2D planar pressure distribution; (c-h) visualization of the signing process. Reproduced by permission of Wiley-VCH [24].*

#### **4.5 Stress/strain sensor**

When the TL materials are introduced in elastic matrices, stress/strain sensor could be obtained. At present, the widely employed TL materials for fabricating stress/strain sensors are ZnS:Mn, ZnS:Cu, and SrAl2O4:Eu, because of their prominent TL properties as well as the one-to-one correspondence between the TL intensity and stress/strain. Yun et al. [64] further found that the co-dopant of Dy3+ in SrAl2O4:Eu could improve its performance as stress sensor based on the sensitivity. In addition to sense the stress or strain by analyzing the TL intensity, the risetime and decay time of TL during cyclic elastic deformation of SrAl2O4:Eu were also demonstrated to be suitable for evaluating the change of the strain energy [65]. Moreover, a calibration method for SrAl2O4:Eu, Dy-based thin-film sensor was proposed to enable quantitative full-field strain measurements in pixel-level resolution [66]. Qian et al. [45] prepared ZnS:Mn/Cu@Al2O3/PDMS flexible composites and adjusted the elastic modulus by introducing SiO2 nanoparticles. They finally obtained a TL stress/strain sensor that could be driven by weak mechanics of skin movements.

In the very recent work [7], Sr3Al2O6:Eu with bright and tunable PL and TL was presented when it was composited in PDMS elastomers. By combining the wavelength selectivity of PL and dynamic stress responsiveness of TL, a multimode stretching/strain sensor was developed by a bilayered structure design of Sr3Al2O6:Eu/PDMS composites with coating a light-shielding layer of Au atop

(as shown in **Figure 9**). The fabricated sensor could sense the stretching states and strain levels simultaneously, breaking the limit of static strain sensing in previous researches.

## **4.6 Mechanics-light-electricity conversion**

The TL materials could convert mechanics into light, which could be further utilized to generate electricity for various applications. When the SrAl2O4:Eu/epoxy TL composites were combined in a commercial silicon solar cell, the mechanicslight-electricity conversion could be achieved [67]. In addition to the generation of electricity by utilizing the mechanics-induced luminescence, TL materials could be combined with a nanogenerator and convert the input mechanical stimuli to electric and light simultaneously [68]. The TL materials could also be composited with a photocatalyst to realize the catalysis activity in dark under the stimuli of mechanics [69]. The above conversion systems based on TL show great perspectives for applications in dark environments, such as deep sea and polar night region.

#### **Figure 9.**

*Multimode stretching/strain sensor based on the TL and PL of Sr3Al2O6:Eu: (a) fabricating process; (b) crack opening when stretched, scale bar: 100 μm; (c–e) stretching state responses; (f–g) strain level responses; (h) corresponding color conversion based on various dynamic strain levels. Reproduced by permission of Wiley-VCH [7].*

**35**

provided the original work is properly cited.

*Triboluminescence: Materials, Properties, and Applications*

Because some of the TL materials show good biocompatibility, such as the rare earth-doped oxide ceramics, they are promising for the detection of mechanical behaviors in biological tissues/organs. The SrAl2O4:Eu TL powders was applied in the synthetic bone, and the related mechanical dynamic environment was monitored with a high-definition and high-speed visualization [70]. SAOED powders were also applied in artificial tooth for occlusal examination [71]. The composition of SAOED in the commercial denture base resin (DBR) could not only endow with bright TL but also improve its mechanical performance. As a result, an artificial tooth model with SAOED was made in which bright and sensitive TL could be directly observed to guide clinicians to purposefully adjust the occlusal surface until

In summary, we present a comprehensive overview on the study of TL. The material systems in both organics and inorganics, unique spectral characteristics, and TL performance, as well as the representative applications in various fields, are included. We hope that this chapter could help researchers in the field to gain a comprehensive and in-depth understanding of TL and stimulate continued interests

The authors thank the support from the Natural Science Foundation of Gansu

and endeavors in this area to promote more innovative applications.

Province (17JR5RA319) and the CAS Pioneer Hundred Talents Program.

The authors declare no conflict of interest.

Chinese Academy of Sciences, Lanzhou, Gansu, China

\*Address all correspondence to: zhfwang@licp.cas.cn

*DOI: http://dx.doi.org/10.5772/intechopen.81444*

**4.7 Biological applications**

a balanced occlusion established.

**5. Conclusions**

**Acknowledgements**

**Conflict of interest**

**Author details**

Zhaofeng Wang\* and Fu Wang

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

## **4.7 Biological applications**

Because some of the TL materials show good biocompatibility, such as the rare earth-doped oxide ceramics, they are promising for the detection of mechanical behaviors in biological tissues/organs. The SrAl2O4:Eu TL powders was applied in the synthetic bone, and the related mechanical dynamic environment was monitored with a high-definition and high-speed visualization [70]. SAOED powders were also applied in artificial tooth for occlusal examination [71]. The composition of SAOED in the commercial denture base resin (DBR) could not only endow with bright TL but also improve its mechanical performance. As a result, an artificial tooth model with SAOED was made in which bright and sensitive TL could be directly observed to guide clinicians to purposefully adjust the occlusal surface until a balanced occlusion established.

## **5. Conclusions**

In summary, we present a comprehensive overview on the study of TL. The material systems in both organics and inorganics, unique spectral characteristics, and TL performance, as well as the representative applications in various fields, are included. We hope that this chapter could help researchers in the field to gain a comprehensive and in-depth understanding of TL and stimulate continued interests and endeavors in this area to promote more innovative applications.

## **Acknowledgements**

The authors thank the support from the Natural Science Foundation of Gansu Province (17JR5RA319) and the CAS Pioneer Hundred Talents Program.

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Zhaofeng Wang\* and Fu Wang State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, Gansu, China

\*Address all correspondence to: zhfwang@licp.cas.cn

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[53] Fujio Y, Xu C-N, Terasawa Y, Sakata Y, Yamabe J, Ueno N, et al. Sheet sensor using SrAl2O4:Eu mechanoluminescent material for visualizing inner crack of high-pressure hydrogen vessel. International Journal of Hydrogen Energy. 2016;**41**:1333-1340

[54] Yang Y, Zheng S, Fu X, Zhang H. Remote and portable mechanoluminescence sensing system based on a SrAl2O4:Eu,Dy film and its potential application to monitoring the safety of gas pipelines. Optik. 2018;**158**:602-609

[55] Shohag MAS, Jiang Z, Hammel EC, Braga Carani L, Olawale DO, Dickens TJ, et al. Development of frictioninduced triboluminescent sensor for load monitoring. Journal of Intelligent Material Systems and Structures. 2018;**29**:883-895

[56] Zhang J, Bao L, Lou H, Deng J, Chen A, Hu Y, et al. Flexible and stretchable mechanoluminescent fiber and fabric. Journal of Materials Chemistry C. 2017;**5**:8027-8032

[57] Wong M-C, Chen L, Tsang M-K, Zhang Y, Hao J. Magnetic-induced luminescence from flexible composite laminates by coupling magnetic field to piezophotonic effect. Advanced Materials. 2015;**27**:4488-4495

[58] Terasaki N, Yamada H, Xu C-N. Ultrasonic wave induced mechanoluminescence and its application for photocatalysis as ubiquitous light source. Catalysis Today. 2013;**201**:203-208

[59] Moon JS, Seongkyu S, Hyunmin K, Kyung-Il J, Hideo T. Mechanoluminescence color conversion by spontaneous fluorescentdye-diffusion in elastomeric zinc sulfide composite. Advanced Functional Materials. 2016;**26**:4848-4858

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**Chapter 3**

**Abstract**

thermosensors.

**1. Introduction**

arsenic sulfide [4–6].

**41**

Mechanism of Photoluminescence

The monograph describes the technique of the synthesis of glasses and the method of the growth of erbium-doped single crystals. The photoluminescence spectra of Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses and glasses from the Ga2S3-La2S3-Er2S3 system have been investigated in the visible and near-infrared ranges. According to the energy transitions in the erbium ions, a radiation mechanism for conversion and up-conversion luminescence has been established. The role of structural ordering and the influence of defects on the radiation efficiency of Er3+ ions have been investigated. The spectra of photoluminescence of (Ga54.59In44.66Er0.75)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals have been studied. The efficiency of the radiation of the amorphous and crystalline materials has been compared. Also, the temperature dependence of the integral intensity of the radiation of glasses and single crystals has been studied. It is established that in a limited temperature range,

these materials can be used for the manufacture of non-contact optical

**Keywords:** glass, single crystal, photoluminescence, energy transition,

Modern scientific and technological progress requires the constant search for and introduction of new multifunctional materials. One of the directions in the search for new semiconductor materials is the study of multicomponent systems of binary and ternary phases, which are promising for practical use. This is due to increasing requirements for semiconductor materials in connection with the development of new or modernization of known technical devices. To expand the list of materials, a study of chalcogenide quasi-ternary systems was undertaken. Chalcogenide semiconductor materials were the object of research for many decades. The best studied glassy and crystalline chalcogenides are based on the Ge-X, Ga-X, As-X systems (X = S, Se, Te) modified by the admixture of Hg, Sb, Pb, Ag, etc. [1–3]. An important feature of chalcogenide glass is that the introduced admixtures are electrically and optically inactive. As a rule, they do not create localized states in the band gap. When introduced into a glass-forming matrix, admixture atoms rebuild their local environment resulting in the saturation of their valence bonds. Exceptions are the admixtures of bismuth, platinum, and gold into

radiation mechanism, visible, and near-infrared ranges

in Erbium-Doped Chalcogenide

*Volodymyr V. Halyan and Inna A. Ivashchenko*

cyclic elastic deformation using mechanoluminescence of SrAl2O4:Eu2+. Optics Express. 2014;**22**:21991-21998

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[67] Terasaki N, Xu C-N, Imai Y, Yamada H. Photocell system driven by mechanoluminescence. Japanese Journal of Applied Physics. 2007;**46**:2385

[68] Huajing F, Xiandi W, Qiang L, Dengfeng P, Qingfeng Y, Caofeng P. A stretchable nanogenerator with electric/light dual-mode energy conversion. Advanced Energy Materials. 2016;**6**:1600829

[69] Terasaki N, Zhang H, Imai Y, Yamada H, Xu C-N. Hybrid material consisting of mechanoluminescent material and TiO2 photocatalyst. Thin Solid Films. 2009;**518**:473-476

[70] Hyodo K, Xu C, Mishima H, Miyakawa S. Optical Stress Imaging for Orthopedic Biomechanics-Comparison of Thermoelastic Stress Analysis and Developed Mechanoluminescent Method. Berlin, Heidelberg: Springer Berlin Heidelberg; 2010. pp. 545-548

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## **Chapter 3**

## Mechanism of Photoluminescence in Erbium-Doped Chalcogenide

*Volodymyr V. Halyan and Inna A. Ivashchenko*

## **Abstract**

The monograph describes the technique of the synthesis of glasses and the method of the growth of erbium-doped single crystals. The photoluminescence spectra of Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses and glasses from the Ga2S3-La2S3-Er2S3 system have been investigated in the visible and near-infrared ranges. According to the energy transitions in the erbium ions, a radiation mechanism for conversion and up-conversion luminescence has been established. The role of structural ordering and the influence of defects on the radiation efficiency of Er3+ ions have been investigated. The spectra of photoluminescence of (Ga54.59In44.66Er0.75)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals have been studied. The efficiency of the radiation of the amorphous and crystalline materials has been compared. Also, the temperature dependence of the integral intensity of the radiation of glasses and single crystals has been studied. It is established that in a limited temperature range, these materials can be used for the manufacture of non-contact optical thermosensors.

**Keywords:** glass, single crystal, photoluminescence, energy transition, radiation mechanism, visible, and near-infrared ranges

## **1. Introduction**

Modern scientific and technological progress requires the constant search for and introduction of new multifunctional materials. One of the directions in the search for new semiconductor materials is the study of multicomponent systems of binary and ternary phases, which are promising for practical use. This is due to increasing requirements for semiconductor materials in connection with the development of new or modernization of known technical devices. To expand the list of materials, a study of chalcogenide quasi-ternary systems was undertaken.

Chalcogenide semiconductor materials were the object of research for many decades. The best studied glassy and crystalline chalcogenides are based on the Ge-X, Ga-X, As-X systems (X = S, Se, Te) modified by the admixture of Hg, Sb, Pb, Ag, etc. [1–3]. An important feature of chalcogenide glass is that the introduced admixtures are electrically and optically inactive. As a rule, they do not create localized states in the band gap. When introduced into a glass-forming matrix, admixture atoms rebuild their local environment resulting in the saturation of their valence bonds. Exceptions are the admixtures of bismuth, platinum, and gold into arsenic sulfide [4–6].

Special attention is devoted to the research of the luminescence properties of chalcogenide crystals and glasses due to practical application in optoelectronic technology. Their unique properties create advantages over other light-emitting materials and cause considerable interest both from the fundamental and the applied point of view. The chalcogenide materials combine high transparency in the visible, near-infrared, and medium-infrared spectral regions. By selecting the optimal component composition, it is possible to obtain wide areas of glass formation and to introduce relatively high concentration of rare earth metals (RE). Additionally, chalcogenides are characterized by high refractive index, good nonlinear optical properties, resistance to aggressive media, and easy manufacture technology. The addition of RE (Er, La, Eu, Pr, Tb, Ho, etc.) to the crystalline or amorphous chalcogenide medium creates potential opportunities for their use as high-quantum output luminophores, optical filters, active media in laser technology, in telecommunication devices, as well as non-contact optical temperature, and γ-irradiation sensors. The chalcogenide media that are activated by RE ions can also exhibit both conversion and up-conversion photoluminescence (PL) which creates the prerequisites for the design of effective converters from the infrared range to visible light.

The method and conditions for the single crystal growth of the phases (Ga55In45)2S300 and (Ga55In45)2S300:0.3 at.% Er [7] were selected from the Ga2S3- In2S3 phase diagram; the supercooling temperature was determined from the cooling curves of the sample thermograms. The solution-melt method was used; supercooling of the solution melt was 70 K. The synthesis of the initial alloys at the maximum temperature of 1200 K and the growth of crystals were combined in evacuated graphitized quartz container with a conical bottom and a 2 mm diameter neck. The growth process was performed in a vertical two-zone furnace. The maximum temperature was 1200 K; the temperature gradient at the solid-melt interface was 20 K/cm. After melting the batch, the ampoule was lowered at the maximum rate. After crystallization of 10 mm of melt along the length of the ampoule, the growth was stopped, followed by re-melting of 6.0–8.0 mm of the crystallized portion and by annealing the seed during 100 h. Further growth of the single crystal was performed at a rate of 5 mm/day. After completion of the process, both furnaces were cooled to 820 K at a rate of 50–70 K/day, and the resulting single crystal

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

*Growth unit setup and the container for the single crystal growth: (a) 1—metal flange, 2—asbestos-cement casing, 3—Pt/Pt-Rh-thermocouple, 4—growth container, 5—melt, 6—metal disk, 7—heater, 8—thermal insulator, and 9—pulley for moving ampoules; (b) 1—cylindrical part of the container, 2—necks for seed*

*Container for the synthesis of glassy alloys: (1) cylindrical part of the container; (2) neck; (3) thin-walled pear-*

**Figure 1.**

**Figure 2.**

**43**

*formation, and 3—pear-shaped chambers.*

*shaped chamber.*

Therefore, the preparation of chalcogenide glasses and crystals doped with rare earths and the study of their emission properties under different temperature and irradiation regimes are an extremely important area of modern solid-state physics and chemistry.

## **2. Synthesis of glasses and single crystal growth**

Elementary high-purity substances were used for the synthesis of samples: Ag (99.99 wt.% of the principal), Ga, In, Ge (99.999 wt.% purity), La, Er (99.9 wt.%), and S, Se (99.997 wt.%). The elements for single crystal growth were further purified by double vacuum distillation. The weight of the starting components for the glasses was 3 g; the batches for the synthesis of single crystals were 10 g.

The samples were synthesized in cylindrical quartz containers with 9–15 mm diameter. The loaded ampoules were evacuated to a residual pressure of 1.33 <sup>10</sup><sup>2</sup> Pa. The synthesis of glasses was performed in a shaft-type furnace with a temperature control system of 5 K accuracy.

The alloys for the glasses of the La2S3-Er2S3-Ga2S3 system with sample weight 2 g were pre-synthesized at 870 K and 24 h exposure. Obtained samples were ground in an agate mortar and loaded into quartz containers with a spherical bottom of 1 cm diameter. The heating was stepwise, first to 1070 K at a rate of 50 K/h and then to 1420 K at a rate of 40 K/h. After 3 h exposure at the maximum temperature, the samples were quenched into saturated NaCl solution with crushed ice.

Synthesis of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glass alloys was performed in two stages. Initially, the ampoules were heated in the flame of an oxygen-gas burner to complete bonding of elemental sulfur. Then the ampoules were placed in a shafttype furnace and heated at a rate of 20 K/h to 1273 K. The samples were kept at the maximum temperature for 10 h with periodic vibration. The alloys were cooled at a rate of 10–20 K/h to annealing temperature. The homogenizing annealing was held for 500 h at 720 K. The annealed alloys were quenched into 25% aqueous NaCl solution at room temperature.

To prevent the spatter of melt during quenching as well as to reduce losses due to the vapor phase condensing on the walls of the container, the special form of the container was used (**Figure 1**), and the upper part was thermostated by asbestos cord after the binding of sulfur in the oxygen-gas burner flame was used.

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

#### **Figure 1.**

Special attention is devoted to the research of the luminescence properties of chalcogenide crystals and glasses due to practical application in optoelectronic technology. Their unique properties create advantages over other light-emitting materials and cause considerable interest both from the fundamental and the applied point of view. The chalcogenide materials combine high transparency in the visible, near-infrared, and medium-infrared spectral regions. By selecting the optimal component composition, it is possible to obtain wide areas of glass formation and to introduce relatively high concentration of rare earth metals (RE). Additionally, chalcogenides are characterized by high refractive index, good nonlinear optical properties, resistance to aggressive media, and easy manufacture technology. The addition of RE (Er, La, Eu, Pr, Tb, Ho, etc.) to the crystalline or amorphous chalcogenide medium creates potential opportunities for their use as high-quantum output luminophores, optical filters, active media in laser technology, in telecommunication devices, as well as non-contact optical temperature, and γ-irradiation sensors. The chalcogenide media that are activated by RE ions can also exhibit both conversion and up-conversion photoluminescence (PL) which creates the prerequisites for the design of effective converters from the infrared range to

Therefore, the preparation of chalcogenide glasses and crystals doped with rare earths and the study of their emission properties under different temperature and irradiation regimes are an extremely important area of modern solid-state physics

Elementary high-purity substances were used for the synthesis of samples: Ag (99.99 wt.% of the principal), Ga, In, Ge (99.999 wt.% purity), La, Er (99.9 wt.%), and S, Se (99.997 wt.%). The elements for single crystal growth were further purified by double vacuum distillation. The weight of the starting components for the glasses was 3 g; the batches for the synthesis of single crystals were 10 g. The samples were synthesized in cylindrical quartz containers with 9–15 mm

1.33 <sup>10</sup><sup>2</sup> Pa. The synthesis of glasses was performed in a shaft-type furnace with

Synthesis of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glass alloys was performed in two stages. Initially, the ampoules were heated in the flame of an oxygen-gas burner to complete bonding of elemental sulfur. Then the ampoules were placed in a shafttype furnace and heated at a rate of 20 K/h to 1273 K. The samples were kept at the maximum temperature for 10 h with periodic vibration. The alloys were cooled at a rate of 10–20 K/h to annealing temperature. The homogenizing annealing was held for 500 h at 720 K. The annealed alloys were quenched into 25% aqueous NaCl

To prevent the spatter of melt during quenching as well as to reduce losses due to the vapor phase condensing on the walls of the container, the special form of the container was used (**Figure 1**), and the upper part was thermostated by asbestos cord after the binding of sulfur in the oxygen-gas burner flame was used.

The alloys for the glasses of the La2S3-Er2S3-Ga2S3 system with sample weight 2 g were pre-synthesized at 870 K and 24 h exposure. Obtained samples were ground in an agate mortar and loaded into quartz containers with a spherical bottom of 1 cm diameter. The heating was stepwise, first to 1070 K at a rate of 50 K/h and then to 1420 K at a rate of 40 K/h. After 3 h exposure at the maximum temperature, the

diameter. The loaded ampoules were evacuated to a residual pressure of

samples were quenched into saturated NaCl solution with crushed ice.

**2. Synthesis of glasses and single crystal growth**

*Luminescence - OLED Technology and Applications*

a temperature control system of 5 K accuracy.

solution at room temperature.

**42**

visible light.

and chemistry.

*Container for the synthesis of glassy alloys: (1) cylindrical part of the container; (2) neck; (3) thin-walled pearshaped chamber.*

The method and conditions for the single crystal growth of the phases (Ga55In45)2S300 and (Ga55In45)2S300:0.3 at.% Er [7] were selected from the Ga2S3- In2S3 phase diagram; the supercooling temperature was determined from the cooling curves of the sample thermograms. The solution-melt method was used; supercooling of the solution melt was 70 K. The synthesis of the initial alloys at the maximum temperature of 1200 K and the growth of crystals were combined in evacuated graphitized quartz container with a conical bottom and a 2 mm diameter neck. The growth process was performed in a vertical two-zone furnace. The maximum temperature was 1200 K; the temperature gradient at the solid-melt interface was 20 K/cm. After melting the batch, the ampoule was lowered at the maximum rate. After crystallization of 10 mm of melt along the length of the ampoule, the growth was stopped, followed by re-melting of 6.0–8.0 mm of the crystallized portion and by annealing the seed during 100 h. Further growth of the single crystal was performed at a rate of 5 mm/day. After completion of the process, both furnaces were cooled to 820 K at a rate of 50–70 K/day, and the resulting single crystal

#### **Figure 2.**

*Growth unit setup and the container for the single crystal growth: (a) 1—metal flange, 2—asbestos-cement casing, 3—Pt/Pt-Rh-thermocouple, 4—growth container, 5—melt, 6—metal disk, 7—heater, 8—thermal insulator, and 9—pulley for moving ampoules; (b) 1—cylindrical part of the container, 2—necks for seed formation, and 3—pear-shaped chambers.*

was annealed for 100 h. After that, the furnace was switched off. The single crystals of orange color, 14 mm in diameter and 20 mm in length, were obtained.

gap of chalcogenide glass semiconductors. Optical transitions of electrons into this band can cause luminescence with the energy of light quanta close to the half-width

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

The glassy alloy Ag0.05Ga0.05Ge0.95S2 is characterized by the largest transparency window [8] among other glasses of the AgGaSe2 + GeS2 ⇔ AgGaS2 + GeSe2 system. This alloy was doped with an Er2S3 admixture. PL spectra investigation (**Figure 4**) was performed on the glasses (100 � X)Ag0.05Ga0.05Ge0.95S2-(X) Er2S3, where X = 0.42, 0.25, and 0.18 mol.% (0.27, 0.16, and 0.12 at.% Er, respectively).

The excitation was performed by a laser with 980-nm wavelength which corre-

I15/2 transition in Er3+ ion, respectively. The PL intensity increases with Er

ð1Þ

I11/2 transition, whereas the PL emission band is due to the <sup>4</sup>

content. The position of the luminescence maximum at 1540 nm does not depend on the content of Er or other components of the glass-forming matrix. The effective width Δλeff of the spectra PL was calculated for these glasses according to the

where І(λ) is the emission intensity at wavelength λ and Іmax is the maximum

The maximum Δλeff value was found for the sample with 0.27 at.% Er (61 nm); this decreases the alloys with 0.16 at.% (52 nm) and 0.112 at.% Er (52 nm). The widening of the PL band in erbium-doped glasses was associated by the authors of [10] with the formation of clusters in the samples where Ga/Er < 10 is performed. Among the glassy alloys investigated here, the PL band widens in the sample with 0.27 at.% Er, and the above inequality is fulfilled. The microstructure study of alloys [11] showed the existence of inhomogeneities of 6–7 μm size the concentration of which increases with the amount of erbium. The generated inhomogeneities are

of the band gap.

sponds to the <sup>4</sup>

emission intensity.

I13/2 ! <sup>4</sup>

formula

**Figure 4.**

**45**

*Conversion FL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses.*

I15/2 ! <sup>4</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

The single crystal growth was performed in a setup installation shown in **Figure 2a** along with the temperature distribution along the heaters. The temperature in the growth zone was maintained by precision temperature controllers with an accuracy of 0.5 K.

A version of a container with two pear-shaped chambers connected by a neck was used for the growth of single crystals (**Figure 2b**). This increases the probability of obtaining a single crystal with its subsequent growth to a larger size in the cylindrical portion of the container.

Independent temperature control in different areas of the heater allows us to vary the gradient at the solid-melt interface within 3–5 K/mm. The growth rate was within 2 mm/day. The vertical movement of the plane of the crystallization zone was ensured by moving the container while the heater was at a fixed position.

### **3. Photoluminescence in chalcogenide glasses**

An analysis of literature sources shows that chalcogenide glasses, unless doped with REs, do not usually exhibit luminescent properties at room temperature. For a long time, this limited their use in optoelectronic technology. Investigations of REfree glasses with the substitution of selenium for sulfur revealed the existence of a broad unstructured PL band at 80 K (**Figure 3**).

The glasses of the AgGaSe2 + GeS2 ⇔ AgGaS2 + GeSe2 system excited by a laser with 532-nm wavelength exhibit at low-temperature luminescence with a single maximum in the near-infrared range (λ<sup>m</sup> ≈ 1150–1180 nm) with an emission band half-width ΔE ≈ 0.26–0.30 eV (**Figure 3a**), which is typical of the recombination luminescence in disordered systems. The intensity of the luminescence maximum depends on the content of Se (when sulfur is substituted with selenium, **Figure 3b**). The dependence is complex and to a large extent is due to the change in the level of luminescence excitation (caused by the shift of the absorption edge with increasing concentration of selenium in the alloy [8]). An important feature of the luminescence spectrum is that the position of the luminescence maximum is close to the center of the band gap (as determined by the position of optical absorption edge [8]). This is well illustrated by the model of Mott and Davis [9] stating that a band of localized states (several tenths of eV wide) is located in the center of the band

#### **Figure 3.**

*(a) Luminescence spectra of the glassy alloys of the AgGaSe2 + GeS2 AgGaS2 + GeSe2 system (at.% Se is indicated). (b) Dependence of the emission intensity at the maximum (λ<sup>m</sup> 1180 nm) on the content of Se (excitation wavelength 532 nm, temperature 80 K).*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

was annealed for 100 h. After that, the furnace was switched off. The single crystals

A version of a container with two pear-shaped chambers connected by a neck was used for the growth of single crystals (**Figure 2b**). This increases the probability of obtaining a single crystal with its subsequent growth to a larger size in the

Independent temperature control in different areas of the heater allows us to vary the gradient at the solid-melt interface within 3–5 K/mm. The growth rate was within 2 mm/day. The vertical movement of the plane of the crystallization zone was ensured by moving the container while the heater was at a fixed position.

An analysis of literature sources shows that chalcogenide glasses, unless doped with REs, do not usually exhibit luminescent properties at room temperature. For a long time, this limited their use in optoelectronic technology. Investigations of REfree glasses with the substitution of selenium for sulfur revealed the existence of a

The glasses of the AgGaSe2 + GeS2 ⇔ AgGaS2 + GeSe2 system excited by a laser with 532-nm wavelength exhibit at low-temperature luminescence with a single maximum in the near-infrared range (λ<sup>m</sup> ≈ 1150–1180 nm) with an emission band half-width ΔE ≈ 0.26–0.30 eV (**Figure 3a**), which is typical of the recombination luminescence in disordered systems. The intensity of the luminescence maximum depends on the content of Se (when sulfur is substituted with selenium, **Figure 3b**). The dependence is complex and to a large extent is due to the change in the level of luminescence excitation (caused by the shift of the absorption edge with increasing concentration of selenium in the alloy [8]). An important feature of the luminescence spectrum is that the position of the luminescence maximum is close to the center of the band gap (as determined by the position of optical absorption edge [8]). This is well illustrated by the model of Mott and Davis [9] stating that a band of localized states (several tenths of eV wide) is located in the center of the band

*(a) Luminescence spectra of the glassy alloys of the AgGaSe2 + GeS2 AgGaS2 + GeSe2 system (at.% Se is indicated). (b) Dependence of the emission intensity at the maximum (λ<sup>m</sup> 1180 nm) on the content of Se*

of orange color, 14 mm in diameter and 20 mm in length, were obtained. The single crystal growth was performed in a setup installation shown in **Figure 2a** along with the temperature distribution along the heaters. The temperature in the growth zone was maintained by precision temperature controllers with

an accuracy of 0.5 K.

**Figure 3.**

**44**

*(excitation wavelength 532 nm, temperature 80 K).*

cylindrical portion of the container.

*Luminescence - OLED Technology and Applications*

**3. Photoluminescence in chalcogenide glasses**

broad unstructured PL band at 80 K (**Figure 3**).

gap of chalcogenide glass semiconductors. Optical transitions of electrons into this band can cause luminescence with the energy of light quanta close to the half-width of the band gap.

The glassy alloy Ag0.05Ga0.05Ge0.95S2 is characterized by the largest transparency window [8] among other glasses of the AgGaSe2 + GeS2 ⇔ AgGaS2 + GeSe2 system. This alloy was doped with an Er2S3 admixture. PL spectra investigation (**Figure 4**) was performed on the glasses (100 � X)Ag0.05Ga0.05Ge0.95S2-(X) Er2S3, where X = 0.42, 0.25, and 0.18 mol.% (0.27, 0.16, and 0.12 at.% Er, respectively).

The excitation was performed by a laser with 980-nm wavelength which corresponds to the <sup>4</sup> I15/2 ! <sup>4</sup> I11/2 transition, whereas the PL emission band is due to the <sup>4</sup> I13/2 ! <sup>4</sup> I15/2 transition in Er3+ ion, respectively. The PL intensity increases with Er content. The position of the luminescence maximum at 1540 nm does not depend on the content of Er or other components of the glass-forming matrix. The effective width Δλeff of the spectra PL was calculated for these glasses according to the formula

$$
\Delta \mathcal{J}\_{\text{eff}} = \frac{\int I(\mathcal{X}) d\mathcal{X}}{\mathcal{I}\_{\text{max}}} \tag{1}
$$

where І(λ) is the emission intensity at wavelength λ and Іmax is the maximum emission intensity.

The maximum Δλeff value was found for the sample with 0.27 at.% Er (61 nm); this decreases the alloys with 0.16 at.% (52 nm) and 0.112 at.% Er (52 nm). The widening of the PL band in erbium-doped glasses was associated by the authors of [10] with the formation of clusters in the samples where Ga/Er < 10 is performed. Among the glassy alloys investigated here, the PL band widens in the sample with 0.27 at.% Er, and the above inequality is fulfilled. The microstructure study of alloys [11] showed the existence of inhomogeneities of 6–7 μm size the concentration of which increases with the amount of erbium. The generated inhomogeneities are

**Figure 4.** *Conversion FL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses.*

good drains of defects which serve as the basis for cluster formation. Higher amounts of erbium favor the formation of clusters which include it. Thus, the PL intensity in the glasses is due to the emission of erbium ions which are uniformly distributed in the glass-forming matrix, as well as those which are located near the inhomogeneities and are involved in the formation of clusters. The study of static magnetization in the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses [12] confirmed the formation of clusters. The calculated number of erbium ions in the cluster was estimated there as 1–1.5 � <sup>10</sup><sup>3</sup> .

The excitation at 980 nm yielded, in addition to the conversion PL (maximum at 1540 nm, **Figure 4**), the up-conversion PL bands in the visible and near-infrared spectral range at room temperature (**Figures 5** and **6**).

For all glasses, the luminescence is represented by three maxima at 520, 657, and 855 nm, which correspond to the radiative transitions <sup>2</sup> H11/2 ! <sup>4</sup> І15/2, <sup>4</sup> F9/2 ! <sup>4</sup> І15/2, and <sup>4</sup> S3/2 ! <sup>4</sup> І13/2 in Er3+ ions, respectively. In addition, a wide PL band in the range of 695–810 nm with an emission maximum at 765 nm was detected for the sample containing 0.27 at.% Er that cannot be interpreted by any radiative transition in erbium ion.

The intensity of the up-conversion PL bands depends on the laser excitation power (IIR) which is expressed by the formula IPL ∝ I n IR, where n is the number of infrared photons per one PL photon. The number (n) can be found by the slope of the dependence of log (IPL) on log (IIR). PL spectral dependences for the sample with 0.27 at.% Er at different laser excitation powers are shown in **Figure 6**; these are also typical of the main maxima (520, 657, and 855 nm) in samples with less erbium content. The logarithmic dependence of the PL intensity on the excitation power [13], log (IPL) from log (PIR), is plotted in **Figure 7**.

Studies show that two photons with a wavelength of 980 nm are needed for the emission of one IPL photon. An energy level diagram in erbium ions when excited by hν<sup>980</sup> quanta is shown in **Figure 8**. The absorption by the ground-state erbium ion of one hν<sup>980</sup> photon excites it to <sup>4</sup> I11/2 state. Such a transition can also take place via energy transfer (ET) from the neighboring excited erbium ion.

The excited state <sup>4</sup> F7/2 may be attained by the successive absorption of two hν<sup>980</sup> photons as well as by energy transfer to Er3+ ion in the <sup>4</sup> I11/2 state from another neighboring excited Er3+ ion [14]:

$$\mathbf{^4I\_{15/2}} + \mathbf{h}\boldsymbol{\nu\_{980}} \to \mathbf{^4I\_{11/2}} + \mathbf{h}\boldsymbol{\nu\_{980}} \to \mathbf{^4F\_{7/2}} \tag{2}$$

$$\mathbf{^4I\_{11/2}} + \mathbf{^4I\_{11/2}} \to \mathbf{^4I\_{15/2}} + \mathbf{^4F\_{7/2}} \tag{3}$$

The states <sup>2</sup>

quently, the state <sup>4</sup>

from the state <sup>4</sup>

3000 cm<sup>1</sup>

**47**

**Figure 7.**

**Figure 6.**

H11/2 and <sup>4</sup>

S3/2 to <sup>4</sup>

S3/2 arise from non-radiative relaxation of the state <sup>4</sup>

F9/2 is unlikely due to large energy gap between them (about

due to the small energy distance between them. The non-radiative relaxation of Er3+

the energy of phonons for these glasses is close to 300–400 cm<sup>1</sup> [13]. Conse-

*Dependence of the luminescence intensity on the laser excitation power for the glass with 0.27 at.% Er.*

*Up-conversion FL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses for various excitation power.*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

) and low energy of phonons. According to Raman spectroscopy study,

F9/2 may arise by the absorption of the hν<sup>980</sup> photon

F7/2

**Figure 5.** *Up-conversion FL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses.*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

**Figure 6.**

good drains of defects which serve as the basis for cluster formation. Higher amounts of erbium favor the formation of clusters which include it. Thus, the PL intensity in the glasses is due to the emission of erbium ions which are uniformly distributed in the glass-forming matrix, as well as those which are located near the inhomogeneities and are involved in the formation of clusters. The study of static magnetization in the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses [12] confirmed the formation of clusters. The calculated number of erbium ions in the cluster was estimated

The excitation at 980 nm yielded, in addition to the conversion PL (maximum at 1540 nm, **Figure 4**), the up-conversion PL bands in the visible and near-infrared

For all glasses, the luminescence is represented by three maxima at 520, 657, and

of 695–810 nm with an emission maximum at 765 nm was detected for the sample containing 0.27 at.% Er that cannot be interpreted by any radiative transition in

The intensity of the up-conversion PL bands depends on the laser excitation

infrared photons per one PL photon. The number (n) can be found by the slope of the dependence of log (IPL) on log (IIR). PL spectral dependences for the sample with 0.27 at.% Er at different laser excitation powers are shown in **Figure 6**; these are also typical of the main maxima (520, 657, and 855 nm) in samples with less erbium content. The logarithmic dependence of the PL intensity on the excitation

Studies show that two photons with a wavelength of 980 nm are needed for the emission of one IPL photon. An energy level diagram in erbium ions when excited by hν<sup>980</sup> quanta is shown in **Figure 8**. The absorption by the ground-state erbium

І13/2 in Er3+ ions, respectively. In addition, a wide PL band in the range

n

F7/2 may be attained by the successive absorption of two hν<sup>980</sup>

4I15*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> <sup>h</sup>ν<sup>980</sup> ! 4I11*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> <sup>h</sup>ν<sup>980</sup> ! 4F7*<sup>=</sup>*<sup>2</sup> (2)

4I11*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I11*<sup>=</sup>*<sup>2</sup> ! 4I15*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4F7*<sup>=</sup>*<sup>2</sup> (3)

H11/2 ! <sup>4</sup>

I11/2 state. Such a transition can also take place

І15/2, <sup>4</sup>

IR, where n is the number of

I11/2 state from another

F9/2 ! <sup>4</sup>

І15/2,

there as 1–1.5 � <sup>10</sup><sup>3</sup>

S3/2 ! <sup>4</sup>

and <sup>4</sup>

erbium ion.

.

*Luminescence - OLED Technology and Applications*

spectral range at room temperature (**Figures 5** and **6**).

855 nm, which correspond to the radiative transitions <sup>2</sup>

power (IIR) which is expressed by the formula IPL ∝ I

power [13], log (IPL) from log (PIR), is plotted in **Figure 7**.

photons as well as by energy transfer to Er3+ ion in the <sup>4</sup>

*Up-conversion FL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses.*

via energy transfer (ET) from the neighboring excited erbium ion.

ion of one hν<sup>980</sup> photon excites it to <sup>4</sup>

neighboring excited Er3+ ion [14]:

The excited state <sup>4</sup>

**Figure 5.**

**46**

*Up-conversion FL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses for various excitation power.*

**Figure 7.** *Dependence of the luminescence intensity on the laser excitation power for the glass with 0.27 at.% Er.*

The states <sup>2</sup> H11/2 and <sup>4</sup> S3/2 arise from non-radiative relaxation of the state <sup>4</sup> F7/2 due to the small energy distance between them. The non-radiative relaxation of Er3+ from the state <sup>4</sup> S3/2 to <sup>4</sup> F9/2 is unlikely due to large energy gap between them (about 3000 cm<sup>1</sup> ) and low energy of phonons. According to Raman spectroscopy study, the energy of phonons for these glasses is close to 300–400 cm<sup>1</sup> [13]. Consequently, the state <sup>4</sup> F9/2 may arise by the absorption of the hν<sup>980</sup> photon

**Figure 8.** *Energy level diagram of erbium ions (for the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses, excitation by 980 nm).*

(transition <sup>4</sup> I13/2 ! <sup>4</sup> F9/2) or through the ET from the neighboring excited erbium ion. Additionally, the state <sup>4</sup> F9/2 state may appear due to cross-relaxation (CR):

$$\mathbf{^4F\_{7/2}} + \mathbf{^4I\_{11/2}} \to \mathbf{2^4F\_{9/2}} \tag{4}$$

**Sample no. Ga2S3 La2S3 Er2S3**

1 64 35 1 2 62 35 3 3 59 40 1 4 57 40 3

**Table 1.**

**Figure 9.**

**Figure 10.**

**49**

*Component composition of the Ga2S3-La2S3-Er2S3 glasses.*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

*PL spectra of the Ga2S3-La2S3-r2S3 glasses at 80 K (visible range).*

*PL spectra of the Ga2S3-La2S3-Er2S3 glasses at 80 K (infrared range).*

**(mol. %)**

Such a mechanism of occurrence of the excited state <sup>4</sup> F9/2 would explain the decrease in the intensity of the green (520 nm) and the amplification of the red PL band with an increase in the content of Er3+ ions when the distance between them decreases and the conditions favor ET.

The increase of PL intensity in the chalcogenide glasses may be due to the increase in erbium content and the change in the composition of the glass-forming matrix. It should be noted that an increase in erbium content in glasses can lead to the concentration quenching of PL [15] as well as to the crystallization of the host. Therefore, increasing the PL efficiency requires the selection of optimal composition of the glass-forming matrix and adding into its composition the maximum concentration of erbium that will not cause PL quenching and the crystallization of glass. The choice of the components of the glass-forming matrix should take into account their glass-forming ability and the ability of sustaining large amount of REs and also maintaining a wide transparency window in the visible and infrared range when adding admixtures. With the above considerations, the Ga2S3-La2S3-Er2S3 system is of interest, in our opinion. Using the technique in paragrap. 2, we successfully introduced up to 40 mol.% La2S3 and 3 mol.% Er2S3 into the glass. The shift of the optical absorption edge does not exceed 0.13 eV with Er2S3 doping and 0.10 eV with the addition of 30–40 mol.% La2S3 [16]. Thus, the luminescence properties of the Ga2S3-La2S3-Er2S3 system glasses at room temperature and 80 K were investigated with a considerable variation in the host composition. The component composition of the glasses is listed in **Table 1**.

Photoluminescence spectra in the 480–1700 nm range upon laser excitation at 488 nm wavelength at 80 K are shown in **Figures 9** and **10**. The intense maxima at 550, 855, 985, 1100, and 1540 nm correspond to 4f intra-shell transitions 4 S3/2 ! <sup>4</sup> І15/2, <sup>4</sup> S3/2 ! <sup>4</sup> І13/2, <sup>4</sup> <sup>І</sup>11/2 ! <sup>4</sup> І15/2, <sup>2</sup> H11/2 ! <sup>4</sup> І11/2, <sup>4</sup> I13/2 ! <sup>4</sup> І15/2 in Er3+ ions.


## *Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

**Table 1.**

(transition <sup>4</sup>

**Figure 8.**

4 S3/2 ! <sup>4</sup>

**48**

І15/2, <sup>4</sup>

S3/2 ! <sup>4</sup>

I13/2 ! <sup>4</sup>

*Luminescence - OLED Technology and Applications*

decreases and the conditions favor ET.

Such a mechanism of occurrence of the excited state <sup>4</sup>

component composition of the glasses is listed in **Table 1**.

<sup>І</sup>11/2 ! <sup>4</sup>

І13/2, <sup>4</sup>

decrease in the intensity of the green (520 nm) and the amplification of the red PL band with an increase in the content of Er3+ ions when the distance between them

*Energy level diagram of erbium ions (for the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses, excitation by 980 nm).*

The increase of PL intensity in the chalcogenide glasses may be due to the increase in erbium content and the change in the composition of the glass-forming matrix. It should be noted that an increase in erbium content in glasses can lead to the concentration quenching of PL [15] as well as to the crystallization of the host. Therefore, increasing the PL efficiency requires the selection of optimal composition of the glass-forming matrix and adding into its composition the maximum concentration of erbium that will not cause PL quenching and the crystallization of glass. The choice of the components of the glass-forming matrix should take into account their glass-forming ability and the ability of sustaining large amount of REs and also maintaining a wide transparency window in the visible and infrared range when adding admixtures. With the above considerations, the Ga2S3-La2S3-Er2S3 system is of interest, in our opinion. Using the technique in paragrap. 2, we successfully introduced up to 40 mol.% La2S3 and 3 mol.% Er2S3 into the glass. The shift of the optical absorption edge does not exceed 0.13 eV with Er2S3 doping and 0.10 eV with the addition of 30–40 mol.% La2S3 [16]. Thus, the luminescence properties of the Ga2S3-La2S3-Er2S3 system glasses at room temperature and 80 K were investigated with a considerable variation in the host composition. The

Photoluminescence spectra in the 480–1700 nm range upon laser excitation at 488 nm wavelength at 80 K are shown in **Figures 9** and **10**. The intense maxima at 550, 855, 985, 1100, and 1540 nm correspond to 4f intra-shell transitions

H11/2 ! <sup>4</sup>

І11/2, <sup>4</sup>

I13/2 ! <sup>4</sup>

І15/2 in Er3+ ions.

І15/2, <sup>2</sup>

ion. Additionally, the state <sup>4</sup>

F9/2) or through the ET from the neighboring excited erbium

F9/2 state may appear due to cross-relaxation (CR):

4F7*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I11*<sup>=</sup>*<sup>2</sup> ! 24F9*<sup>=</sup>*<sup>2</sup> (4)

F9/2 would explain the

*Component composition of the Ga2S3-La2S3-Er2S3 glasses.*

**Figure 9.** *PL spectra of the Ga2S3-La2S3-r2S3 glasses at 80 K (visible range).*

**Figure 10.** *PL spectra of the Ga2S3-La2S3-Er2S3 glasses at 80 K (infrared range).*

The lower-intensity maxima at 492, 660, 810, and 1245 nm correspond to the transitions <sup>4</sup> F7/2 ! <sup>4</sup> І15/2, <sup>4</sup> F9/2 ! <sup>4</sup> І15/2, <sup>4</sup> <sup>І</sup>9/2 ! <sup>4</sup> І15/2, <sup>4</sup> F7/2 ! <sup>4</sup> І9/2 in erbium ions, respectively.

PL intensity increases in the visible and infrared ranges with Er2S3 concentration at constant content of La2S3. The best medium for PL (the most intense PL) is the glass-forming matrix with 35 mol.% La2S3 and 3 mol.% Er2S3. PL intensity decreases when La2S3 content increased to 40 mol.%. A similar effect was observed by Ivanova in [17] in the glasses of the GeS2-Ga2S3 system, explaining this by an increase in the number of homopolar metallic bonds. It is likely that the number of La-La-type bonds increases in the Ga2S3-La2S3-Er2S3 glasses with increasing La2S3 content.

Changing the component composition of glass leads to redistribution of the intensity of PL bands. The maximum at 550 nm is dominating in the visible range, and its intensity decreases with increasing La2S3 content. The greatest increase in the intensity of PL bands occurs for the infrared spectral range with an increase in erbium concentration. The changes in the PL spectra are clearly related to the mechanism of the realization of excited states in Er3+ ions.

The redistribution of the PL intensity is due to the competing ways of achieving excited states in erbium ions. **Figure 11** shows an energy level diagram of erbium ions [18] which reveals the mechanism of PL emission in the glasses of the Ga2S3- La2S3-Er2S3 system. Accordingly, erbium ions jump from the ground to the excited state of <sup>4</sup> F7/2 upon excitation by 488 nm wavelength. Intense emission of green and infrared light (550, 1100, and 855 nm) results when the majority of erbium ions non-radiatively relax to the states <sup>2</sup> H11/2 and <sup>4</sup> S3/2. Only a small fraction of erbium undergoes transitions <sup>4</sup> F7/2 ! <sup>4</sup> I15/2 and <sup>4</sup> F7/2 ! <sup>4</sup> I9/2 as PL bands at 492 and 1245 nm are characterized by low intensity. Additionally, PL with a maximum at 660 nm occurs due to the cross-relaxation process CR resulting in the excited state <sup>4</sup> F9/2.

PL spectra of the Ga2S3-La2S3-Er2S3 glasses at room temperature are shown in **Figures 12** and **13**. Compared to the spectra at 80 K, the PL intensity at 530 nm increased slightly (transition <sup>2</sup> H11/2 ! <sup>4</sup> I15/2), and the band intensity at 660 nm increased (transition <sup>4</sup> F9/2 ! <sup>4</sup> I15/2). Additionally, the intensity of the maxima of

1100 and 855 nm significantly decreased, and a weak PL band at 1050 nm was observed in the infrared spectral range. Such intensity redistribution of the PL maxima is associated with a change in the concentration of erbium ions in different excited states. This is related to the fact that when the temperature rises, the phonon subsystem of the glass-forming matrix is transformed, so the probability of the energy exchange of the neighboring erbium ions increases. As a result, the role of cross-relaxation processes increases and the number of erbium ions in the state

*PL spectra of the Ga2S3-La2S3-Er2S3 glasses at room temperature (infrared range).*

*PL spectra of the Ga2S3-La2S3-Er2S3 glasses at room temperature (visible range).*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

F9/2 increases. Thus the intensity of the PL band with a maximum at 660 nm increases. Additionally, CR affects the decrease of the number of erbium ions in

1100 and 855 nm at room temperature. The band with a maximum at 1050 nm

S3/2 leading to the decrease of the intensity of the maxima at

I9/2) is completely overlapped by the intense PL maximum at

4

**51**

the states <sup>2</sup>

**Figure 13.**

**Figure 12.**

(transition <sup>4</sup>

1100 nm at 80 K.

H11/2 and <sup>4</sup>

F3/2 ! <sup>4</sup>

**Figure 11.** *Energy level diagram of erbium ions (for the Ga2S3-La2S3-Er2S3 glasses).*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

**Figure 12.** *PL spectra of the Ga2S3-La2S3-Er2S3 glasses at room temperature (visible range).*

#### **Figure 13.**

The lower-intensity maxima at 492, 660, 810, and 1245 nm correspond to the

<sup>І</sup>9/2 ! <sup>4</sup>

PL intensity increases in the visible and infrared ranges with Er2S3 concentration at constant content of La2S3. The best medium for PL (the most intense PL) is the glass-forming matrix with 35 mol.% La2S3 and 3 mol.% Er2S3. PL intensity decreases when La2S3 content increased to 40 mol.%. A similar effect was observed by Ivanova in [17] in the glasses of the GeS2-Ga2S3 system, explaining this by an increase in the number of homopolar metallic bonds. It is likely that the number of La-La-type bonds increases in the Ga2S3-La2S3-Er2S3 glasses with increasing La2S3 content. Changing the component composition of glass leads to redistribution of the intensity of PL bands. The maximum at 550 nm is dominating in the visible range, and its intensity decreases with increasing La2S3 content. The greatest increase in the intensity of PL bands occurs for the infrared spectral range with an increase in erbium concentration. The changes in the PL spectra are clearly related to the

The redistribution of the PL intensity is due to the competing ways of achieving excited states in erbium ions. **Figure 11** shows an energy level diagram of erbium ions [18] which reveals the mechanism of PL emission in the glasses of the Ga2S3- La2S3-Er2S3 system. Accordingly, erbium ions jump from the ground to the excited

infrared light (550, 1100, and 855 nm) results when the majority of erbium ions

H11/2 and <sup>4</sup>

are characterized by low intensity. Additionally, PL with a maximum at 660 nm occurs due to the cross-relaxation process CR resulting in the excited state <sup>4</sup>

PL spectra of the Ga2S3-La2S3-Er2S3 glasses at room temperature are shown in **Figures 12** and **13**. Compared to the spectra at 80 K, the PL intensity at 530 nm

I15/2 and <sup>4</sup>

H11/2 ! <sup>4</sup>

F7/2 upon excitation by 488 nm wavelength. Intense emission of green and

F7/2 ! <sup>4</sup>

І15/2, <sup>4</sup>

F7/2 ! <sup>4</sup>

S3/2. Only a small fraction of erbium

I15/2), and the band intensity at 660 nm

I15/2). Additionally, the intensity of the maxima of

I9/2 as PL bands at 492 and 1245 nm

F9/2.

І9/2 in erbium ions,

І15/2, <sup>4</sup>

transitions <sup>4</sup>

respectively.

state of <sup>4</sup>

**Figure 11.**

**50**

F7/2 ! <sup>4</sup>

non-radiatively relax to the states <sup>2</sup>

increased slightly (transition <sup>2</sup>

undergoes transitions <sup>4</sup>

increased (transition <sup>4</sup>

І15/2, <sup>4</sup>

*Luminescence - OLED Technology and Applications*

F9/2 ! <sup>4</sup>

mechanism of the realization of excited states in Er3+ ions.

F7/2 ! <sup>4</sup>

F9/2 ! <sup>4</sup>

*Energy level diagram of erbium ions (for the Ga2S3-La2S3-Er2S3 glasses).*

*PL spectra of the Ga2S3-La2S3-Er2S3 glasses at room temperature (infrared range).*

1100 and 855 nm significantly decreased, and a weak PL band at 1050 nm was observed in the infrared spectral range. Such intensity redistribution of the PL maxima is associated with a change in the concentration of erbium ions in different excited states. This is related to the fact that when the temperature rises, the phonon subsystem of the glass-forming matrix is transformed, so the probability of the energy exchange of the neighboring erbium ions increases. As a result, the role of cross-relaxation processes increases and the number of erbium ions in the state 4 F9/2 increases. Thus the intensity of the PL band with a maximum at 660 nm increases. Additionally, CR affects the decrease of the number of erbium ions in the states <sup>2</sup> H11/2 and <sup>4</sup> S3/2 leading to the decrease of the intensity of the maxima at 1100 and 855 nm at room temperature. The band with a maximum at 1050 nm (transition <sup>4</sup> F3/2 ! <sup>4</sup> I9/2) is completely overlapped by the intense PL maximum at 1100 nm at 80 K.

## **4. Photoluminescence in erbium-doped single crystals**

The occurrence of recombination PL that is characterized by wide emission bands in the visible, rarely in the infrared range, is quite often found in singlecrystalline chalcogenide semiconductors at room temperature or low temperatures [19, 20]. From the application viewpoint of laser technology and telecommunications, the introduction of erbium to such materials has significant advantages since the emission bands of erbium-doped glasses and single crystals are usually intense and narrow. The addition of erbium is also accompanied by the extinction of PL radiation [21] which is due to the crystalline or amorphous chalcogenide host. The influence of external factors on the PL efficiency in erbium-doped chalcogenides is limited due to the shielding of radiative transitions in the 4f-shell of Er by outer shells. At the same time, the local environment of erbium ion, both in glasses and single crystals, affects the efficiency of PL emission which is why it is extremely important for the design of fluorescent materials. It should be noted that the single crystals, unlike glasses, do not permit a wide variation of the component composition since it is limited by the solid solubility of impurities and the homogeneity of the crystalline compound. It is more difficult to select the composition of the host to obtain effective emission in erbium-doped single crystals; therefore, PL is less common in these than in the corresponding amorphous media.

The investigation of the optical properties of the single crystals (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 [7] determined that the introduction of erbium does not result in a change in the bandgap energy or significant changes in the electronic structure [22]. The absorption maxima at 530, 660, 810, 980, and 1530 nm [7] recorded for (Ga54.59In44.66Er0.75)2S300 single crystal correspond to 4f intra-shell transitions from the ground state to the excited states <sup>2</sup> H11/2, <sup>4</sup> F9/2, <sup>4</sup> I9/2, <sup>4</sup> I11/2, and <sup>4</sup> I13/2 in Er3+ ions, respectively. PL spectra at room temperature in the visible and close infrared range (600–1650 nm) were studied when excited by a laser with 532 nm wavelength (**Figure 14**). Two intense PL bands with maxima at 810 and 1540 nm were recorded, which correspond to the transitions of <sup>4</sup> I9/2 ! <sup>4</sup> I15/2 and <sup>4</sup> <sup>І</sup>13/2 ! <sup>4</sup> I15/2 in Er3+ ions, respectively.

excited states <sup>4</sup>

**Figure 15.**

I9/2 and <sup>4</sup>

state <sup>4</sup>

**53**

the excited state <sup>4</sup>

Er3+ ions from excited states <sup>2</sup>

I13/2 to <sup>2</sup>

relaxation CR 2:

can excite erbium ions from the ground state <sup>4</sup>

545 nm [25]. The appearance of excited states <sup>4</sup>

2

relaxation of erbium ions to the state <sup>4</sup>

between adjacent erbium ions in the states <sup>2</sup>

І9/2 and the other into <sup>4</sup>

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

2

I9/2 involves the cross-relaxation process CR. Energy transfer

І13/2:

*Energy level diagram in erbium ions (for a (Ga54.59In44.66Er0.75)2S300 single crystal, excitation by 532 nm).*

Such a mechanism yields substantial numbers of erbium ions in excited states <sup>4</sup>

I13/2, which results in PL with maxima at 810 and 1540 nm. Similar to glasses, we also investigated single crystals in which two rare earth metals were introduced. (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals were obtained by a solution-melt method [24] selected in accordance with the phase diagram of the Ga2S3-La2S3 system. The bandgap energy of the single crystals little changes upon erbium doping at 2.01 and 1.99 eV for (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300, respectively [24]. PL of (Ga54.59In44.66Er0.75)2S300 single crystal in the 500–1700 nm range was investigated at room temperature (**Figure 16**).

Four intense PL bands with maxima at 525, 545, 980, and 1540 nm and one

H11/2, <sup>4</sup>

low-intensity band at 660 nm were recorded. They correspond to the transitions in

Energy transition diagram (**Figure 17**) shows that the wavelength of 810 nm

either by directly absorbing light quanta or through ET, can be promoted from the

I11/2, <sup>4</sup>

H11/2 resulting in PL at 525 nm. Additionally, after non-radiative

S3/2, <sup>4</sup>

H11/2 and <sup>4</sup>

H11*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I15*<sup>=</sup>*<sup>2</sup> ! 4I9*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I13*<sup>=</sup>*<sup>2</sup> (5)

I13/2, and <sup>4</sup>

F9/2 and <sup>4</sup>

H11*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I13*<sup>=</sup>*<sup>2</sup> ! 4F9*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I11*<sup>=</sup>*<sup>2</sup> (6)

I15/2 to the state <sup>4</sup>

S3/2, there is a PL band with a maximum at

І15/2 promotes one ion into

F9/2 to the ground state.

I9/2. Er3+ ions,

I11/2 occurs by cross-

Er3+ ions excited by a 532 nm laser are promoted from the ground <sup>4</sup> I15/2 to the excited state <sup>2</sup> H11/2. The emission mechanism in this case is quite simple as demonstrated at an energy transition diagram in erbium ions (**Figure 15**). Erbium ions cannot relax non-radiatively from the state <sup>4</sup> S3/2 to <sup>4</sup> F9/2 or <sup>4</sup> І9/2 due to the large energy distance and the low phonons energy (200–300 cm�<sup>1</sup> for the single crystals (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 [23]). Therefore, the appearance of

**Figure 14.** *Visible (a) and NIR (b) PL spectra of the (Ga54.59In44.66Er0.75)2S300 single crystal (excitation by 532 nm).*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

**4. Photoluminescence in erbium-doped single crystals**

*Luminescence - OLED Technology and Applications*

common in these than in the corresponding amorphous media.

1540 nm were recorded, which correspond to the transitions of <sup>4</sup>

transitions from the ground state to the excited states <sup>2</sup>

I15/2 in Er3+ ions, respectively.

cannot relax non-radiatively from the state <sup>4</sup>

<sup>І</sup>13/2 ! <sup>4</sup>

**Figure 14.**

**52**

excited state <sup>2</sup>

The occurrence of recombination PL that is characterized by wide emission bands in the visible, rarely in the infrared range, is quite often found in singlecrystalline chalcogenide semiconductors at room temperature or low temperatures [19, 20]. From the application viewpoint of laser technology and telecommunications, the introduction of erbium to such materials has significant advantages since the emission bands of erbium-doped glasses and single crystals are usually intense and narrow. The addition of erbium is also accompanied by the extinction of PL radiation [21] which is due to the crystalline or amorphous chalcogenide host. The influence of external factors on the PL efficiency in erbium-doped chalcogenides is limited due to the shielding of radiative transitions in the 4f-shell of Er by outer shells. At the same time, the local environment of erbium ion, both in glasses and single crystals, affects the efficiency of PL emission which is why it is extremely important for the design of fluorescent materials. It should be noted that the single crystals, unlike glasses, do not permit a wide variation of the component composition since it is limited by the solid solubility of impurities and the homogeneity of the crystalline compound. It is more difficult to select the composition of the host to obtain effective emission in erbium-doped single crystals; therefore, PL is less

The investigation of the optical properties of the single crystals (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 [7] determined that the introduction of erbium does not result in a change in the bandgap energy or significant changes in the electronic structure [22]. The absorption maxima at 530, 660, 810, 980, and 1530 nm [7] recorded for (Ga54.59In44.66Er0.75)2S300 single crystal correspond to 4f intra-shell

I11/2, and <sup>4</sup> I13/2 in Er3+ ions, respectively. PL spectra at room temperature in the visible and close infrared range (600–1650 nm) were studied when excited by a laser with 532 nm wavelength (**Figure 14**). Two intense PL bands with maxima at 810 and

I15/2 and <sup>4</sup>

strated at an energy transition diagram in erbium ions (**Figure 15**). Erbium ions

energy distance and the low phonons energy (200–300 cm�<sup>1</sup> for the single crystals (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 [23]). Therefore, the appearance of

*Visible (a) and NIR (b) PL spectra of the (Ga54.59In44.66Er0.75)2S300 single crystal (excitation by 532 nm).*

H11/2. The emission mechanism in this case is quite simple as demon-

S3/2 to <sup>4</sup>

Er3+ ions excited by a 532 nm laser are promoted from the ground <sup>4</sup>

H11/2, <sup>4</sup>

F9/2 or <sup>4</sup>

F9/2, <sup>4</sup>

I9/2, <sup>4</sup>

I9/2 ! <sup>4</sup>

І9/2 due to the large

I15/2 to the

**Figure 15.** *Energy level diagram in erbium ions (for a (Ga54.59In44.66Er0.75)2S300 single crystal, excitation by 532 nm).*

excited states <sup>4</sup> I9/2 involves the cross-relaxation process CR. Energy transfer between adjacent erbium ions in the states <sup>2</sup> H11/2 and <sup>4</sup> І15/2 promotes one ion into the excited state <sup>4</sup> І9/2 and the other into <sup>4</sup> І13/2:

2 H11*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I15*<sup>=</sup>*<sup>2</sup> ! 4I9*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I13*<sup>=</sup>*<sup>2</sup> (5)

Such a mechanism yields substantial numbers of erbium ions in excited states <sup>4</sup> I9/2 and <sup>4</sup> I13/2, which results in PL with maxima at 810 and 1540 nm.

Similar to glasses, we also investigated single crystals in which two rare earth metals were introduced. (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals were obtained by a solution-melt method [24] selected in accordance with the phase diagram of the Ga2S3-La2S3 system. The bandgap energy of the single crystals little changes upon erbium doping at 2.01 and 1.99 eV for (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300, respectively [24]. PL of (Ga54.59In44.66Er0.75)2S300 single crystal in the 500–1700 nm range was investigated at room temperature (**Figure 16**). Four intense PL bands with maxima at 525, 545, 980, and 1540 nm and one low-intensity band at 660 nm were recorded. They correspond to the transitions in Er3+ ions from excited states <sup>2</sup> H11/2, <sup>4</sup> S3/2, <sup>4</sup> I11/2, <sup>4</sup> I13/2, and <sup>4</sup> F9/2 to the ground state.

Energy transition diagram (**Figure 17**) shows that the wavelength of 810 nm can excite erbium ions from the ground state <sup>4</sup> I15/2 to the state <sup>4</sup> I9/2. Er3+ ions, either by directly absorbing light quanta or through ET, can be promoted from the state <sup>4</sup> I13/2 to <sup>2</sup> H11/2 resulting in PL at 525 nm. Additionally, after non-radiative relaxation of erbium ions to the state <sup>4</sup> S3/2, there is a PL band with a maximum at 545 nm [25]. The appearance of excited states <sup>4</sup> F9/2 and <sup>4</sup> I11/2 occurs by crossrelaxation CR 2:

$$\text{\textbullet }^2\text{H}\_{11/2} + \,^4\text{I}\_{13/2} \to \,^4\text{F}\_{9/2} + \,^4\text{I}\_{11/2} \tag{6}$$

**5. The influence of temperature on the photoluminescent properties of**

Doping the binary and ternary chalcogenides, particularly by rare earth metals, creates materials for the design of active and passive media for laser technology, photonic devices, light converters, and telecommunications. Special attention is given to the crystalline and amorphous environments that can exhibit high-

Additionally, modern optoelectronic industry actively researches and implements high-precision temperature sensors based on the sensitivity of PL emission to temperature changes. The design of such devices requires the investigation of the effect of temperature on the mechanism of the achievement of excited states and the processes of the relaxation of erbium ions when the PL intensity changes in various temperature ranges. This will allow optimizing the component composition of crystals and glasses to produce effective luminescent materials for the develop-

PL of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses in 600–1050 nm range was investigated in a temperature range of 80–300 K upon laser excitation at 532 nm [26]. Recorded PL bands with maxima at 660, 860, and 980 nm correspond to the

I13/2, <sup>4</sup>

I11/2 ! <sup>4</sup>

I11/2 result in the cross-relaxation processes CR1,

H11/2 state can also nonradiatively relax to the <sup>4</sup>

I15/2 in erbium ions,

S3/2

S3/2 ! <sup>4</sup>

respectively. The dependences of PL intensity on wavelength at different temperatures for the sample with 0.27 at.% Er are plotted in **Figure 18** (also typical of the glasses with 0.12, 0.16 at.% Er). The temperature increase leads to changes in the ratio of PL intensities, with the intensity of all bands decreasing at temperatures above 180 K. The PL band with a maximum at 860 nm is the most sensitive to temperature changes. The PL emission mechanism is well explained by the energy

Illumination by 532 nm wavelength promotes erbium ions to the excited state

state due to small energy gap between them. The integral intensity of the emission

bands was calculated from the results of the investigation of PL spectra.

*PL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glass (0.27 аt.% Er) at various temperatures.*

**glasses and crystals**

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

ment of non-contact optical sensors.

level diagram of Er3+ ions (**Figure 19**).

CR2, and CR3. Er3+ ions in the <sup>2</sup>

F9/2, <sup>4</sup>

I9/2, and <sup>4</sup>

radiative transitions <sup>4</sup>

H11/2. The states <sup>4</sup>

2

**Figure 18.**

**55**

intensity PL under the influence of external factors.

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

F9/2 ! <sup>4</sup>

I15/2, <sup>4</sup>

**Figure 16.** *PL of the (Ga69.75La29.75Er0.5)2S300 single crystal (excitation by 810 nm).*

**Figure 17.** *Energy level diagram in erbium ions (for a (Ga69.75La29.75Er0.5)2S300 single crystal, excitation by 810 nm).*

The transition from these states to the ground state produces PL at 660 and 980 nm. An intense PL band at 1540 nm wavelength occurs due to the formation of a significant number of Er3+ ions in the state <sup>4</sup> I13/2 by cross-relaxation CR 1:

$$\rm \rm \rm \rm \rm \rm \rm \rm I\_{11/2} + \rm \rm \rm \rm I\_{15/2} \to \rm \rm \rm I\_{9/2} + \rm \rm \rm I\_{13/2} \tag{7}$$

The excitation of the single crystal yields also an intense up-converted green PL (**Figure 16**). Therefore, (Ga54.59In44.66Er0.75)2S300 single crystal can be recommended as a material for the manufacture of light converters.

## **5. The influence of temperature on the photoluminescent properties of glasses and crystals**

Doping the binary and ternary chalcogenides, particularly by rare earth metals, creates materials for the design of active and passive media for laser technology, photonic devices, light converters, and telecommunications. Special attention is given to the crystalline and amorphous environments that can exhibit highintensity PL under the influence of external factors.

Additionally, modern optoelectronic industry actively researches and implements high-precision temperature sensors based on the sensitivity of PL emission to temperature changes. The design of such devices requires the investigation of the effect of temperature on the mechanism of the achievement of excited states and the processes of the relaxation of erbium ions when the PL intensity changes in various temperature ranges. This will allow optimizing the component composition of crystals and glasses to produce effective luminescent materials for the development of non-contact optical sensors.

PL of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses in 600–1050 nm range was investigated in a temperature range of 80–300 K upon laser excitation at 532 nm [26]. Recorded PL bands with maxima at 660, 860, and 980 nm correspond to the radiative transitions <sup>4</sup> F9/2 ! <sup>4</sup> I15/2, <sup>4</sup> S3/2 ! <sup>4</sup> I13/2, <sup>4</sup> I11/2 ! <sup>4</sup> I15/2 in erbium ions, respectively. The dependences of PL intensity on wavelength at different temperatures for the sample with 0.27 at.% Er are plotted in **Figure 18** (also typical of the glasses with 0.12, 0.16 at.% Er). The temperature increase leads to changes in the ratio of PL intensities, with the intensity of all bands decreasing at temperatures above 180 K. The PL band with a maximum at 860 nm is the most sensitive to temperature changes. The PL emission mechanism is well explained by the energy level diagram of Er3+ ions (**Figure 19**).

Illumination by 532 nm wavelength promotes erbium ions to the excited state 2 H11/2. The states <sup>4</sup> F9/2, <sup>4</sup> I9/2, and <sup>4</sup> I11/2 result in the cross-relaxation processes CR1, CR2, and CR3. Er3+ ions in the <sup>2</sup> H11/2 state can also nonradiatively relax to the <sup>4</sup> S3/2 state due to small energy gap between them. The integral intensity of the emission bands was calculated from the results of the investigation of PL spectra.

**Figure 18.** *PL spectra of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glass (0.27 аt.% Er) at various temperatures.*

The transition from these states to the ground state produces PL at 660 and 980 nm. An intense PL band at 1540 nm wavelength occurs due to the formation of

*Energy level diagram in erbium ions (for a (Ga69.75La29.75Er0.5)2S300 single crystal, excitation by 810 nm).*

The excitation of the single crystal yields also an intense up-converted green PL

(**Figure 16**). Therefore, (Ga54.59In44.66Er0.75)2S300 single crystal can be recommended as a material for the manufacture of light converters.

I13/2 by cross-relaxation CR 1:

H11*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I15*<sup>=</sup>*<sup>2</sup> ! 4I9*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I13*<sup>=</sup>*<sup>2</sup> (7)

a significant number of Er3+ ions in the state <sup>4</sup>

**Figure 16.**

**Figure 17.**

**54**

2

*PL of the (Ga69.75La29.75Er0.5)2S300 single crystal (excitation by 810 nm).*

*Luminescence - OLED Technology and Applications*

**Figure 19.** *Energy level diagram in erbium ions (for the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses, excitation by 532 nm).*

The dependence of integral PL intensity on temperature is shown in **Figure 20** for the glass with 0.27 at.% Er.

The dependence of PL intensity on temperature can be described using the probability of radiative and non-radiative processes by the formula [27]:

$$I(T) = \frac{I\_0}{1 + \frac{\alpha\_{sr}}{\alpha\_r} \exp\left(-\frac{E\_t}{kT}\right)}\tag{8}$$

materials where the recombination luminescence was recorded [28], the PL intensity decreases with increasing temperature. At the same time, the emission intensity may increase with temperature in crystals and glasses where PL is associated with transitions in the 4f-shell of erbium ions. This is due to the fact that the neighboring erbium ions are in different excited states. As the temperature increases, the phonon subsystem of the crystal changes which contributes to the cross-relaxation

*Ratio of integral PL intensity of the glasses of the Ag0.05Ga0.05Ge0.95S2-Er2S3 system (0.27 аt.% Er).*

*Temperature dependence of integral PL intensity of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glass (0.27 аt.% Er).*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

3+ ions can non-radiatively relax to the state <sup>2</sup> H11/2. Excited states <sup>4</sup>

Therefore, CR processes play greater role with increasing temperature, which

of two photons promotes erbium ion into the state <sup>4</sup>

I9/2 result from the CR process:

4I15*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> <sup>2</sup>

will contribute to a higher concentration of erbium ions in the <sup>4</sup>

3+ ions. If a crystal is excited by 980 nm wavelength, the absorption

F7/2 (**Figure 22**). Due to small

I9/2 and <sup>4</sup>

I13/2 states

H11*<sup>=</sup>*<sup>2</sup> ! 4I13*<sup>=</sup>*<sup>2</sup> <sup>þ</sup> 4I9*<sup>=</sup>*<sup>2</sup> (9)

processes of Еr

**Figure 21.**

**Figure 20.**

energy gap, Еr

and the increase in PL intensity.

I13/2 and <sup>4</sup>

**57**

where Іо is a constant, k is Boltzmann's constant, ωnr, ω<sup>r</sup> is the probability of nonradiative and radiative processes, respectively, and Et is the thermal activation energy of luminescence.

The I(T) dependence (**Figure 20**, solid line) was calculated from Eq. (8) for the band with the 860 nm maximum, and the corresponding activation energy Et was calculated as 90 � 6 meV [26]. According to the diagram in **Figure 19**, this energy determines the activation of erbium ions from the state <sup>4</sup> S3/2 в <sup>2</sup> H11/2 (i.e., the energy gap between these states). It should be noted that changes in PL intensity is complex. Therefore we calculated the logarithm of the ratio of integral PL intensities ln (І980/І660) and plotted its dependence on temperature (**Figure 21**). This dependence is linear in the range of 125–300 K. The sensitivity of the sample as the temperature sensor was calculated according to the results of these studies as 0.43 K�<sup>1</sup> .

Erbium-doped single crystals may also exhibit changes of PL intensity with temperature. PL spectra of the single crystal (Ga54.59In44.66Er0.75)2S300 were investigated at 150, 200, 250, and 300 K upon laser excitation at 980 nm wavelength (**Figures 22** and **23**). The position and shape of the maxima do not change but the intensity of PL increases with temperature. For the majority of semiconductor

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

**Figure 20.** *Temperature dependence of integral PL intensity of the Ag0.05Ga0.05Ge0.95S2-Er2S3 glass (0.27 аt.% Er).*

**Figure 21.** *Ratio of integral PL intensity of the glasses of the Ag0.05Ga0.05Ge0.95S2-Er2S3 system (0.27 аt.% Er).*

materials where the recombination luminescence was recorded [28], the PL intensity decreases with increasing temperature. At the same time, the emission intensity may increase with temperature in crystals and glasses where PL is associated with transitions in the 4f-shell of erbium ions. This is due to the fact that the neighboring erbium ions are in different excited states. As the temperature increases, the phonon subsystem of the crystal changes which contributes to the cross-relaxation processes of Еr 3+ ions. If a crystal is excited by 980 nm wavelength, the absorption of two photons promotes erbium ion into the state <sup>4</sup> F7/2 (**Figure 22**). Due to small energy gap, Еr 3+ ions can non-radiatively relax to the state <sup>2</sup> H11/2. Excited states <sup>4</sup> I13/2 and <sup>4</sup> I9/2 result from the CR process:

$$\rm {^4I\_{15/2}} + \, {^2H\_{11/2}} \to \, {^4I\_{13/2}} + \, {^4I\_{9/2}} \tag{9}$$

Therefore, CR processes play greater role with increasing temperature, which will contribute to a higher concentration of erbium ions in the <sup>4</sup> I9/2 and <sup>4</sup> I13/2 states and the increase in PL intensity.

The dependence of integral PL intensity on temperature is shown in **Figure 20** for

*Energy level diagram in erbium ions (for the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses, excitation by 532 nm).*

The dependence of PL intensity on temperature can be described using the

where Іо is a constant, k is Boltzmann's constant, ωnr, ω<sup>r</sup> is the probability of non-

The I(T) dependence (**Figure 20**, solid line) was calculated from Eq. (8) for the band with the 860 nm maximum, and the corresponding activation energy Et was calculated as 90 � 6 meV [26]. According to the diagram in **Figure 19**, this energy

energy gap between these states). It should be noted that changes in PL intensity is complex. Therefore we calculated the logarithm of the ratio of integral PL intensities ln (І980/І660) and plotted its dependence on temperature (**Figure 21**). This dependence is linear in the range of 125–300 K. The sensitivity of the sample as the temperature sensor was calculated according to the results of these studies as

Erbium-doped single crystals may also exhibit changes of PL intensity with temperature. PL spectra of the single crystal (Ga54.59In44.66Er0.75)2S300 were investigated at 150, 200, 250, and 300 K upon laser excitation at 980 nm wavelength (**Figures 22** and **23**). The position and shape of the maxima do not change but the intensity of PL increases with temperature. For the majority of semiconductor

S3/2 в <sup>2</sup>

H11/2 (i.e., the

radiative and radiative processes, respectively, and Et is the thermal activation

determines the activation of erbium ions from the state <sup>4</sup>

ð8Þ

probability of radiative and non-radiative processes by the formula [27]:

the glass with 0.27 at.% Er.

*Luminescence - OLED Technology and Applications*

**Figure 19.**

energy of luminescence.

0.43 K�<sup>1</sup>

**56**

.

**Figure 22.** *Conversion PL spectra of the (Ga54.59In44.66Er0.75)2S300 single crystal at various temperatures.*

**6. Conclusions**

**Figure 24.**

**Author details**

**59**

Volodymyr V. Halyan<sup>1</sup>

European University, Lutsk, Ukraine

provided the original work is properly cited.

The principal mechanisms of the occurrence of PL due to the transitions in the

4f-shell of erbium ions are presented on the basis of sulfide glasses and single crystals. The appearance of many emission bands in chalcogenide glasses (unlike single crystals) is due to the fact that in an amorphous medium, erbium ions may occupy several different positions in the glass-forming matrix. The change of the excitation wavelength leads to a change in the mechanism for the excited states in Er3+ ions and the emergence of some radiation bands and the extinction of others. Energy exchange processes (energy transfer, cross-relaxation) between the neighboring Er3+ ions strongly influence the intensity of the conversion and upconversion PL. The investigation of the temperature dependence of PL indicates that erbium-doped chalcogenide semiconductors can be recommended as materials

*Temperature dependence of integral PL intensity of the (Ga54.59In44.66Er0.75)2S300 single crystal.*

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide*

*DOI: http://dx.doi.org/10.5772/intechopen.81445*

\* and Inna A. Ivashchenko<sup>2</sup>

1 Department of Experimental Physics and Technologies for Information Measuring, Lesya Ukrainka Eastern European University, Lutsk, Ukraine

2 Department of Inorganic and Physical Chemistry, Lesya Ukrainka Eastern

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: halyan.volodimir@eenu.edu.ua

for the design of non-contact optical thermometers.

**Figure 23.** *Up-conversion PL spectra of the (Ga54.59In44.66Er0.75)2S300 single crystal at various temperatures.*

The integral PL intensity for both maxima was calculated from the spectra of upconversion and conversion PL of (Ga54.59In44.66Er0.75)2S300 single crystal (**Figure 24**). The plots of integral PL intensity linearly depend on temperature. From the results of the temperature dependence of the integral PL intensity, the sensitivity was calculated as 1.187 <sup>10</sup><sup>3</sup> <sup>K</sup><sup>1</sup> for PL at 805 nm and 1.818 <sup>10</sup><sup>3</sup> <sup>K</sup><sup>1</sup> for the maximum at 1540 nm. It should be noted that the sensitivity defined as the ratio of the integral intensities of the two PL bands is higher in the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses. The sensitivity in (Ga54.59In44.66Er0.75)2S300 single crystal was calculated separately for each PL band (maxima at 805 and 1540 nm), the latter being in the operating range of fiber-optic networks and telecommunication devices.

Therefore, in a limited temperature range, the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses (with 0.27 at.% Er) and the single crystals (Ga54.59In44.66Er0.75)2S300 can be used to design non-contact optical temperature sensors.

*Mechanism of Photoluminescence in Erbium-Doped Chalcogenide DOI: http://dx.doi.org/10.5772/intechopen.81445*

**Figure 24.** *Temperature dependence of integral PL intensity of the (Ga54.59In44.66Er0.75)2S300 single crystal.*

## **6. Conclusions**

The principal mechanisms of the occurrence of PL due to the transitions in the 4f-shell of erbium ions are presented on the basis of sulfide glasses and single crystals. The appearance of many emission bands in chalcogenide glasses (unlike single crystals) is due to the fact that in an amorphous medium, erbium ions may occupy several different positions in the glass-forming matrix. The change of the excitation wavelength leads to a change in the mechanism for the excited states in Er3+ ions and the emergence of some radiation bands and the extinction of others. Energy exchange processes (energy transfer, cross-relaxation) between the neighboring Er3+ ions strongly influence the intensity of the conversion and upconversion PL. The investigation of the temperature dependence of PL indicates that erbium-doped chalcogenide semiconductors can be recommended as materials for the design of non-contact optical thermometers.

## **Author details**

Volodymyr V. Halyan<sup>1</sup> \* and Inna A. Ivashchenko<sup>2</sup>

1 Department of Experimental Physics and Technologies for Information Measuring, Lesya Ukrainka Eastern European University, Lutsk, Ukraine

2 Department of Inorganic and Physical Chemistry, Lesya Ukrainka Eastern European University, Lutsk, Ukraine

\*Address all correspondence to: halyan.volodimir@eenu.edu.ua

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The integral PL intensity for both maxima was calculated from the spectra of up-

Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses. The sensitivity in (Ga54.59In44.66Er0.75)2S300 sin-

Therefore, in a limited temperature range, the Ag0.05Ga0.05Ge0.95S2-Er2S3 glasses (with 0.27 at.% Er) and the single crystals (Ga54.59In44.66Er0.75)2S300 can be used to

gle crystal was calculated separately for each PL band (maxima at 805 and 1540 nm), the latter being in the operating range of fiber-optic networks and

conversion and conversion PL of (Ga54.59In44.66Er0.75)2S300 single crystal (**Figure 24**). The plots of integral PL intensity linearly depend on temperature. From the results of the temperature dependence of the integral PL intensity, the sensitivity was calculated as 1.187 <sup>10</sup><sup>3</sup> <sup>K</sup><sup>1</sup> for PL at 805 nm and 1.818 <sup>10</sup><sup>3</sup> <sup>K</sup><sup>1</sup> for the maximum at 1540 nm. It should be noted that the sensitivity defined as the

*Up-conversion PL spectra of the (Ga54.59In44.66Er0.75)2S300 single crystal at various temperatures.*

*Conversion PL spectra of the (Ga54.59In44.66Er0.75)2S300 single crystal at various temperatures.*

*Luminescence - OLED Technology and Applications*

ratio of the integral intensities of the two PL bands is higher in the

telecommunication devices.

**Figure 22.**

**Figure 23.**

**58**

design non-contact optical temperature sensors.

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**References**

00044-5

[1] Frumar M, Wagner T. Ag doped chalcogenide glasses and their

applications. Current Opinion in Solid State and Materials Science. 2003;**7**: 117-126. DOI: 10.1016/S1359-0286(03)

*Luminescence - OLED Technology and Applications*

(Ga55In45)2S300, (Ga54.59In44.66Er0.75)2S300 single crystals. Journal of Solid State Chemistry. 2015;**227**:255-264. DOI:

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**Chapter 4**

**Abstract**

consumer electronics

Galaxy Note Edge [3].

**63**

**1. Introduction**

for OLED

*and Christopher J. Campbell*

Optically Clear Adhesives

*Joel T. Abrahamson, Hollis Z. Beagi, Fay Salmon*

Optically clear adhesives (OCA) have been used for more than a decade to bond rigid LCD and AMOLED displays for consumer electronics applications, offering optical, mechanical, and electrical performance benefits. The performance requirements of an OCA to bond cover window, touch sensors, and circular polarizers in a plastic OLED display to bent cover glass or a flexible, foldable OLED display are drastically different from a flat, rigid device. For plastic OLED to bent cover glass bonding, the adhesive needs to be strong enough to resist spring back of the flat, plastic OLED devices. For flexible, foldable OLED displays, the neutral plane needs to be managed during folding keeping strain to a minimum in critical layers of the device (e.g., touch sensor, TFT, TFE), and the OCA cannot deform (or cause other layers to deform) during the folding process. Folding also brings challenges to touch sensors that can no longer use conventional passivation layers. As a result, the OCA will be responsible for preventing corrosion of touch sensor materials such as metal mesh, silver nanowire, carbon nanotube, and graphene. The chapter will discuss OCA performance requirements for rigid, flexible, and foldable OLED bonding.

**Keywords:** adhesives, OLED, optically clear, OCA, LOCA, displays, bonding,

Optically clear adhesives (OCA) and liquid optically clear adhesives (LOCA or OCR for optically clear resins) have been used in consumer electronics, industrial and automotive bonding solutions for more than 15 years. OCA usage increased significantly as consumer electronics devices saw the transition from resistive to capacitive touch with its first implementation in the LG Prada phone in 2006 [1]. The advantage of using an OCA shows improvements in mechanical, optical and electrical performance of the display module and device. The initial application in OCAs in rigid OLED-based devices was similar to LCD devices, but as plastic OLED (pOLED) devices were introduced to the market, OCAs enabled new form factors, such as curved OLEDs in the Galaxy Round and Galaxy Gear S [2] as well as the

There are two important mechanical considerations for optically clear adhesives – does it stick (adhesive strength) and how strong is it (cohesive strength). Adhesive strength can be defined by the work of adhesion, which is the amount of work required to separate the adhesion from the adherend. Adhesion can be achieved

Moshchalkov V. Quantum yield of luminescence of Ag nanoclusters dispersed within transparent bulk glass vs. glass composition and temperature. Applied Physics Letters. 2012;**101**: 251106. DOI: 10.1063/1.4772957

[28] Agafonova D, Kolobkova E, Sidorov A. Temperature dependence of the luminescence intensity in optical fibers of oxyfluoride glass with CdS and CdSxSe1<sup>x</sup> quantum dots. Technical Physics Letters. 2013;**39**:629-631. DOI: 10.1134/S1063785013070158

## **Chapter 4**

Moshchalkov V. Quantum yield of luminescence of Ag nanoclusters dispersed within transparent bulk glass vs. glass composition and temperature. Applied Physics Letters. 2012;**101**: 251106. DOI: 10.1063/1.4772957

*Luminescence - OLED Technology and Applications*

[28] Agafonova D, Kolobkova E, Sidorov A. Temperature dependence of the luminescence intensity in optical fibers of oxyfluoride glass with CdS and CdSxSe1<sup>x</sup> quantum dots. Technical Physics Letters. 2013;**39**:629-631. DOI:

10.1134/S1063785013070158

**62**
