Effect of CuO doping on structural features, optical absorption and photoluminescence behavior of ZnO-based glasses

Glasses composed of ZnO–SiO2–B2O3–In2O3 with the addition of different content of CuO were prepared by a popular melt–quenching methodology. The effect of the addition of copper oxide in the glass network was investigated using density, FTIR spectroscopy, optical absorption, and photoluminescence spectroscopy. By increasing the concentration of CuO, a decrease in the density is observed, indicating structural changes in the glass network. The structural building units of glasses are BO3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{BO}}_{3}$$\end{document},BO4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{BO}}_{4}$$\end{document}, and ZnO4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{ZnO}}_{4}$$\end{document} according to the FTIR spectra. The absorption spectra of these glasses reveal a distinct broad band at 744 nm due to 2B1g\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{B}}_{1\mathrm{g}}$$\end{document} → 2B2g\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{B}}_{2\mathrm{g}}$$\end{document} characteristic transition of cupric ion in the distorted octahedral sites. The optical band gap energies (Eg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${E}_{\mathrm{g}}$$\end{document}) and refractive indices of the studied glasses were also evaluated using the ASF, DASF, and Tauc techniques which revealed that (Eg) values are in gradual decrease with a rise content of CuO. The photoluminescence analysis including emission and CIE chromaticity of the prepared samples were carried out. The emission spectra supported divalent state of copper ions in a distorted octahedral geometry of the glass network upon UV excitation at 270 nm. These results may suggest the material of glass as a potential candidate in the photonics applications.


Introduction
Zinc borate glasses are of technological importance because they can be used in a variety of electronic products since of their low melting temperatures. In the area of electric equipment, like in multi-layer ceramic capacitors, electric cathode ray tubes, modules plasma display panels, low-temperature co-fired ceramics (LTCC), low melting glasses are often used [1][2][3].
Zinc oxide is a glass modifier that breaks BOB linkages and introduces coordinated defects called as dangling bonds, as well as non-bridging oxygen (NBO) atoms into the glass matrix; in this case, Zn 2þ is octahedral coordinated. ZnO may take part in the glass matrix with ZnO 4 structural units whenever zinc is connected to four oxygen ions in a covalent bond form. The network structure in this instance is assumed to be made up of ZnO 4 and BO 4 pyramidal units connected by B-O-Zn bonds [4,5]. The addition of zinc to the borate glass system improves its durability, strength, optical functionalization, and electron emission capacity, making it suitable for a variety of applications [6,7].
Transition metal ions (TMIs)-doped glasses have received much interest in recent years due to their photoconducting and memory properties. They are also found in fiber optic communication systems, solid state lasers, and luminescent solar energy concentrators [8][9][10].
The copper atom's electron configuration is [Ar] 3d 10 4s 0 , and its stable ionic states are Cu þ and Cu 2þ ; the cuprous ion does not create coloring because its five d-orbitals are filled, whereas cupric ions produce color centers with absorption bands in the visible wavelength region [11], resulting in materials with attractive blue and sometimes green color. The color of the glasses is determined by the Cu 2þ content, its specific coordination, composition, and basicity [12].
The Cu 2þ state glass network is expected to have the most copper ions. During the melting and annealing processes, however, copper ions can be oxidized or reduced from Cu 2þ to Cu 3þ and Cu þ . Cu 2þ ions are found in octahedral sites in the glass network, while Cu þ and Cu 3þ ions are found in tetrahedral places [13,14]. Copper-containing glasses have a number of attractive features that cover a broad range of applications such as lighting [15], nonlinear optics [16], bandpass filters [17], spectral conversion in solar cells [18], and lasers [19].
The physical and optical properties of copperdoped glasses have been studied by a number of scientists. For example, Samir et al. [20] used the ternary 30 Na 2 O-20ZnO-50B 2 O 3 and added CuO up to 7 mol% at the expense of B 2 O 3 . The possibility of adjusting the filtering effect has been offered [11] based on several physical property estimations and the evaluation of band gap and Urbach energies. Additionally, Zhang et al. [21] and Verwey et al. [22] have also investigated Cu 2 O-doped B 2 O 3 /BaO/Li 2 O/La 2 O 3 /Al 2 O 3 /As 2 O 3 and SrB 2 O 7 glasses, emphasizing on the luminescence caused by the existence of Cu þ ions. The use of As 2 O 3 with reductant characteristics in these kinds of works was assumed to be advantageous in avoiding Cu þ oxidation to Cu 2þ and therefore facilitating the evaluation of Cu þ photoluminescence in borate glasses. Okasha et al. [23] recently investigated the 15NaF-5 CaF 2 -80B 2 O 3 glass, for which CuO was added to 12 wt% instead of B 2 O 3 to make tunable glasses for optical filters. After many spectroscopic characterizations, optical band gap energies were determined and reported in a similar manner, with a decreasing trend.
The goal of this paper is to present results from UV/Vis, FTIR, and PL spectroscopies, as well as density measurements, on multicomponent ZnO-SiO 2 -B 2 O 3 -In 2 O 3 glasses doped with CuO, and to study the effect of copper oxide concentration variation on them. These results pointed to glass as a possible candidate for photonics applications.

Experimental
The detailed chemical compositions of the glasses used in the present study are as follows: Appropriate amounts (all in mol%) of analytical grade reagents of ZnO, SiO 2 , H 3 BO 3 , In 2 O 3 , and CuO were used. Appropriate amounts of chemicals were grinded and mixed in an agate mortar and the batch were placed in porcelain crucibles and then melted in an electrical furnace at 1100°C for 2 h till a bubblefree liquid was formed. The resultant bubble-free melt was then poured on stainless steel plates. The samples were immediately transferred to annealing furnace at 400°C for half an hour then it was switched off and left to cool to ambient temperature. The photographs of transparent samples synthesized and without any treatments are shown in Fig. 1. All the samples contain CuO showed blue color.
Archimedes method is used to estimate the density (q) of the prepared samples at room temperature by using deionized water as immersion liquid [24].
where W 1 is the weight of sample in air, W 2 is the weight of sample in water, and q water is the density of water (q water = 0.9977 g/cm 3 ).
FTIR spectra were obtained in the wavenumber range of 400-4000 cm -1 using Nicolet TM iS TM 10 FTIR Spectrometer. Each sample was mixed with KBr by the ratio 1:100 in weight and then pressed to produce a homogeneous pellet. The infrared absorbance measurements were carried out at room temperature and immediately after preparing the pellets to avoid moisture.
UV-Vis spectra were recorded on these samples at room temperature using a Cary series UV/Vis-spectrophotometer within a wavelength of 200-1100 nm in the absorption mode.
The photoluminescence spectra were recorded under UV excitation using a fluorescence spectrometer (Type JASCO FP-6500, Japan) equipped with a xenon flash lamp at the excitation light source. The scan speed is 0.1 s/step with a step length of 0.25 nm and a slit width of 0.2 nm. The CIE1931 chromaticity diagram was applied to identify the emitted color from light sources using three dimension-less quantities x k ð Þ, y k ð Þ , and z k ð Þ. The tristimulus values for a non-monochromatic light source with spectral relative power P k ð Þ are given by where X, Y, and Z are the tristimulus values that identify the three primary light colors red, green, and blue that needed to match the color P k ð Þ and from the tristimulus values the color chromaticity coordinates x and y can be determined using the following expression [25][26][27]: 3 Results and discussion

Density
It is well known that density is a property which can be used for determining rigidity and changes in the structure of the material. Changes in geometrical structure, crosslink density, coordination number, structural softness, and interstitial spaces distances in glass matrix are all the factors that influence it [28]. The variation of density values of the studied glass samples is presented in Fig. 2. The addition of CuO to base glass sample leads to decrease the density values ( Table 1). As the amount of the CuO modifier increases, structural changes including the conversion of BO 4 tetrahedral structural units into BO 3 trigonal

FTIR analysis
Infrared spectroscopy is commonly employed to get important information on the arrangement of structural units in the prepared glasses. It is well known that the vibrations of specific structural units in the network of glass are believed to be independent of vibrations of any other units in the network [30]. The overall spectrum is made up of several absorption bands that extend from 400 to 1600 cm À1 and are centered in the mid-region. Figure 3 shows IR spectra of the examined glasses doped with various amounts of copper ion. Three traditional broad bands are identified because of borate family [31][32][33]: (1) 1st groups are ranged from 1200 to 1600 cm À1 , which are attributed to the presence of BO 3 units, i.e., the asymmetric stretching relaxation of the B-O bonds; (2) 2nd groups are ranged from 800 to 1200 cm À1 , which may be due to presence of BO 4 units, i.e., the B-O bond stretching of BO 4 groups; (3) the last groups are ranged from 625 to 725 cm À1 due to the presence of bending B-O linkages.
The infrared absorption spectrums of the glass free CuO (Fig. 3) showed five absorption bands centered around 1270, 1240, 913, 686, and 415 cm À1 . The weak band around 1270 can be attributed to the presence of meta-borate chains [30]. The appearance of absorption band at 1240 cm À1 may be due to the asymmetric stretching vibrations of B-O bonds in BO 3 units, from pyro-and ortho-borate groups, which characterized by presence of a large number of NBOs. As shown in Fig. 3, the addition of CuO leads to increase in the relative area of 1240 cm -1 band and this behavior indicates that the addition of CuO increases the amount of NBOs in great agreement with the formation of octahedral coordinated copper ion [34,35].
The appearance of the band around at 913 cm À1 may be due to the stretching vibrations of BO 4 units in different structural units [36].The absorption band *690 cm À1 may be due to the presence of B-O-B bending vibrations of borate network [37,38] or/and the vibrated oxygen which connect the two trigonal boron atoms [39]. With increasing copper ion content, the relative area of this band increases and shifts to higher wavenumbers. The Cu 2þ ions' electrostatic field could be responsible for this shift [40]. The increasing of CuO content, causing a rise in the electron cloud density surrounding the oxygen in the BO 3 unit, leads to an increase in the B-O-B band's roll-torque and, as a result, a shift toward higher wavenumbers. Because of the generated electrostatic field, which weakens the borate network, this action creates additional new B-O-Cu bridge bonds [40].

Fig. 3 Infrared absorption spectra of the prepared glasses
Generally, the boroxol ring in the borate glass network is usually attributed to the IR absorption band at 806cm À1 . The peak at 806 cm À1 is missing in this study, indicating that there is no boroxol ring in the glass network. With the addition of CuO into base glass and with increasing its concentration the band due to BO 4 units is observed to shift toward higher wavenumber and its intensity decreases, whereas the center of BO 3 groups is shifted to lower wavenumber and its intensity increases. The appearance of bands the range 400-600 cm À1 is due to the presence of ZnO which forms Zn-O tetrahedral bending vibrations in the current glass system [41]. It is well known that ZnO is consider as a glass modifier which breaks B-O-B bonds and then enters the glass network (usually, the oxygen in ZnO breaks local symmetry while Zn 2þ ions occupy interstitial sites) and introducing coordination defects known as dangling bonds, as well as NBO ions. Zinc oxide may also contribute to the glass matrix with ZnO 4 structural groups when Zn 2þ is joined to four oxygens in a covalence bond configuration [42,43]. The intensity of the band due to ZnO 4 units is highest in the spectrum of the glass code Cu 1 and lowest in the spectrum of the glass code Cu 5 . Copper ions operate as modifiers, causing bonding defects by disrupting the bonds B-O-B, B-O-Zn, etc. [44]. With the addition of CuO in glass matrix suggests an increase in copper ions content participating in network position modification and the change of BO 4 groups to BO 3 groups in the glass matrix.
The deconvoluted spectra of all glass samples are made to obtain N4. The fraction of four coordinated boron was calculated using the relative areas corresponding to BO 3 and BO 4 vibrational groups. Sample Cu 3 spectra are given as an example of the deconvoluted spectral data (Fig. 4). Calculated values of N4 are as follows: where A3 and A4 are the relative areas corresponding to BO 3

Analysis of ultraviolet and visible (UV-Vis) spectrum
The study of optical absorption in glasses has helped to understand electronic band structure and optical transitions in these materials. Depending on their valence state and coordination number, and hence their d level state, TMIs have a wide range of optical absorption spectra in the UV-Vis range. [29].
The optical absorption spectra of the prepared glasses are presented in Fig. 5. The edge of absorption (k c ) is red-shifted as the CuO content increases from * 396 nm in Cu 1 to * 410 nm in Cu 5 . With increasing amounts of CuO, the cutoff wavelength (k c ) increases, indicating an increase in octahedral Cu 2? ions. This behavior indicates that the band gap energy E g values decreased due to the doping of varying content of CuO inside the glass network. As a result, electrons can migrate from the valance to the conduction band, resulting in charge flow [45].  Further, copper can be present in glasses as Cu 2? and Cu ? ions. In the UV-Vis range, Cu ? ions exhibit no absorption, but Cu 2? ions have a significant absorption [46].
The 3d 9 electronic configuration of Cu 2? ions has a two-dimensional (2D) ground state. The 2D states are separated into e g and t 2g energy levels in the octahedron crystal field. The e g levels are divided into 2 B 1g and 2 A 1g , whereas the t 2g levels are divided into 2 B 2g and 2 E g . The ground state of these is B 1g . As a result, copper ions are expected to have three bands related to the transitions of 2 B 1g ? 2 A 1g , 2 B 1g ? 2 B 2g , and 2 B 1g ? 2 E g [47][48][49].
The overlap of the three transitions was interpreted as a single optical absorption band, as shown in Fig. 5. The absorbance peak was centered at 744 nm due to the transition of Cu 2? ions into distorted octahedral sites from 2 B 1g ?B 2g .

Determination of optical band gap energy
In the nanomaterials industries and semiconductor, measuring the band gap energy is important. Tauc [50], Mott and Davis [51] explored the energy band gaps associated to optical energy. Souri and Shomalian [52], Souri and Tahan [53], and Alarcon et al. [54] used the absorbance spectrum fitting (ASF) approach to estimate optical band gaps, besides Souri and Tahan [53] and Ali et al. [55] used the first derivation of the absorption spectrum fitting method to evaluate optical band gaps (DASF). The optical energy of the glasses under investigation was estimated using the following methods.

Tauc's method (E Tauc gap )
The Tauc method [50] has traditionally been used to estimate band gap energies in crystalline and amorphous semiconductors in most of the investigations. One of the most important parameters for estimating the band gap energy is the optical absorption coefficient (a) and whether the band structure has changed, and it is a material's ability to absorb light at a specific wavelength [56]. When evaluating materials of different thicknesses, the absorption coefficient is a good measure to use [57]: where A is the absorbance and d is the thickness of the sample. The optical band gap of glass samples can be determined using the following formula [58]: where B is a constant and hm is the energy of the incident photon. The type of optical transition (n = 2, 1/2, 3, 1/3) is represented by n for indirect allowed, direct allowed, forbidden indirect, and forbidden direct transitions [59]. For allowed direct transitions, the obtained band gap values for the produced glasses decreased from 3.263 to 3.127 eV, and for allowed indirect transitions, they decreased from 2.936 to 2.887 eV as shown in Table 2.
The addition of CuO content has been found to decrease E g values because it contributes additional defect states (such wrong bonds and color centers) into the matrix [60]. Also, there is some overlap between delocalized quasi-band states and localized d orbital of Cu states, which could result in major changes in optical excitation transfer. The concentration of these defects, and hence the CuO content, was found to be related to localized state density. Fig. 6 Variation of (a hm) 0.5 with hm for Cu 1

-Cu 5 glasses
Color centers or wrong bonds localized states overlap and expand into the mobility gap, lowering E g . Besides, BO 3 units containing NBOs atoms formed when the amount of Cu 2? ions in the borate network increases. The band gap is shown to decrease as NBOs become more excited than bridging oxygens (BOs) [61].

Absorbance spectrum fitting method (ASF)
The optical band gap can also be estimated using the absorption spectrum fitting (E ASF gap ) method, as proposed by Souri and Shomalian [52] and Alarcon et al. [54], the a (m) can be determined as a function of photon wavelength (k): where h, c, k g express to the Plank's constant, light velocity, and the wavelength corresponding to the optical band gap. Equation (10) can be rewritten as follows: where D = [C hc ð Þ rÀ1 d/2.303]. Equation (11) demonstrates that the optical energy band gap could be estimated only from absorbance values, without need for a thick glass sample. Extension of the linear region of ðAk À1 Þ r À1 versus (k À1 ) at ðAk À1 Þ r À1 = 0 can be used to determine the k g . Figures 8 and 9 show the variation of ðAk À1 Þ 0:5 and ðAk À1 Þ 2 with (k À1 ) for indirect and direct allowed transition, respectively. Moreover, Eq. (12) can be used to estimate the E ASF gap directly from k g : When the CuO content increased, the indirect E g of glass samples were decreased from 3.038 to 2.8520 eV and from 3.286 to 3.124 eV for direct E g .

Derivation absorbance spectrum fitting method (DASF)
The optical band gap energies of the examined glasses were accurately determined using the DASF method [62,63]. The band gap energy values were determined by the samples' optical absorption, which inhibited the evaluation of sample thickness, compounding the estimation error [63]. The glasses' optical absorbance (A) can be represented as [53] ln  Fig. 7 Variation of (a hm) 2 with hm for Cu 1 -Cu 5 glasses at k À1 =k g À1 . Thus, the optical energy gap (E DASF gap ) values can be determined by using the following formula: The dependence of d ln A k ð Þk À1 À Á È É on (k À1 ) for the proposed glasses (Cu 1 to Cu 5 ) is represented in Fig. 10 Table 1 shows all the optical energy band gap values for the produced glasses using various models.
Using the DASF, ASF, and Tauc techniques, the CuO content-dependent energy band gaps of the studied glasses were calculated. The results ( Fig. 11; Table 1) showed that when the CuO content in the glass network increased, the values of E g for both direct and indirect optical transitions decreased. The creation of more NBO in the highest valence band allowed the NBO to move to the conduction band, decreasing the glasses' E g values.

Refractive index
The optical refractive index (n) determines the application of the investigated glasses in various optical applications. As a result, n was determined for all glasses in this investigation. In Tauc's, ASF, and DASF techniques, the refractive index in direct  Table 2 lists the obtained n values for the current samples. The value of optical energy band gaps was inversely proportional to the refractive indices of the samples studied. There is good agreement between values of refractive indices estimated by different methods, as seen in Table 2.

Photoluminescence spectra
The emission spectra of glasses are shown in Fig. 12. There is no emission spectrum detected because the basic glass Cu 1 contains no copper ions. The excitation wavelength affects the intensity of emission peaks and the position of bands. Figure 12 shows the emission spectra of the Cu-doped glass samples when they are excited at 270 nm. The spectra reveal two main peaks for all samples at about 465 and 510 nm with two shoulders at 420 and 560 nm. It is noted that with increasing CuO concentration above 0.005, the intensity of the emission peaks decreased.
Ultraviolet excitation causes strong and broad emission peaks in copper-doped oxide glasses in the visible range [65,66], where the excitation bands of the cuprous and cupric ions are placed in UV region. Due to the UV absorption by the oxygen ligands around the cations, oxide-based glasses show charge transfer band in the ultraviolet region, according to the ligand field theory [67]. The present glass samples show emission spectra in the visible region when excited at 270 nm which is due to the charge transfer Emission peaks and shoulders at 420 and 465 nm can be attributed to 3d 9 4s 2 ? 3d 10 4s 1 triplet transitions in Cu 2? ions in the photoluminescence spectrum of CuO-doped glasses [61]. Also, based on the previous studies [68,69], in an environment with octahedral symmetry, the lowest energy level 3 E g of Cu ? ions is split in the spin-orbit components T 1g and T 2g . The emission peaks at about 510 nm and 560 nm can be attributed to the transition between these T 1g and T 2g levels to 1 A g level ground state of Cu ? ions, which may be generated from the interaction of NBOs with Cu 2? ions in the glass samples. Hence, the luminescence spectra reveal the presence of both Cu 2? and Cu ? ions, which are responsible both for the observed emission peaks. The electronic excitation energy transfer from Cu ? to Cu 2? ions is responsible for the quenching of photoluminescence intensity when CuO content is increased [68]. Another mechanism postulated by Tanaka et al. [19] is absorption caused by a charge transfer from Cu 2? to O 2ions interfering with Cu ? ions excitation. This mechanism interprets the increase of intensity of the emission peak at 465 nm due to Cu 2? ions on the expense of that of Cu ? ions at 510 nm by increasing CuO content.

Chromaticity coordinates
All glass samples had their CIE chromaticity coordinates determined using emission spectra under the excitation of 270 nm. Table 3 lists color coordinates for all glass samples, and Fig. 13 depicts the CIE color chart that corresponds to them. Its color is greenish blue.   Homogeneous glasses of zinc borate doped with copper oxide were prepared by the melt-quenching method. Density, FTIR spectroscopy, UV/Vis optical absorption, and photoluminescence spectroscopy were used to analyze the glass samples. The density decreases from 3.9318 to 3.8474 g/cm 3 with the addition of CuO. The analysis of FTIR results suggests that the glass consists of BO 3 ,BO 4 , and ZnO 4 bridge bonds, forming a large glass matrix. The transformation from [BO 4 ] tetrahedral to ½BO 3 ] triangular occurs as CuO content increases. Due to Cu 2? ion 2 B 1g ? 2 B 2g transitions, the UV-Vis absorption spectra of these glasses exhibit a single broad band. This demonstrates that the cupric ions occupy distorted octahedral positions. Tauc, ASF, and DASF methods were used to investigate changes in optical band gap energies (E g ). The conclusion is that the three techniques provide results that are extremely similar. The optical absorption spectra the E opt g have been examined glasses decrease with the addition of copper ions due to structural changes. Emission spectra displayed emission bands owing to Cu 2? ions. CIE chromaticity diagram shows glass sample Cu 2 with color coordinates x = 0.1570 and y = 0.4597 have highest emission intensity. Analysis of CIE color coordinates illustrates that the CuO codoped multicomponent glasses emit greenish blue light. These results suggest that the prepared glasses can be useful for the photonics applications.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Conflict of interest The authors have not disclosed any competing interests.
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