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The Effect of Deposition Parameters on Morphological and Optical Properties of Cu2S Thin Films Grown by Chemical Bath Deposition Technique

The Effect of Deposition Parameters on Morphological and Optical Properties of Cu2S Thin Films... hv photonics Article The Effect of Deposition Parameters on Morphological and Optical Properties of Cu S Thin Films Grown by Chemical Bath Deposition Technique Honar S. Ahmed and Raghad Y. Mohammed * Department of Physics, College of Science, University of Duhok, Duhok 42001, Kurdistan Region, Iraq; honar.salah@uod.ac * Correspondence: ssraghad@uod.ac Abstract: The chemical bath deposition technique has been used for the deposition of Cu S thin films on glass substrates. The thickness of deposited thin films strongly depends on the deposition parameters. The present study revealed that the thickness increased from 185 to 281 nm as deposition time increased and from 183 to 291 nm as bath temperature increased. In addition, the thickness increased from 257 to 303 nm with the increment of precursors concentration and from 185 to 297 nm as the pH value increased. However, the thickness decreased from 299 to 234 nm with the increment of precursors concentration. The morphology of Cu S thin films remarkably changed as the deposition parameters varied. The increase in deposition time, bath temperature, and CuSO .5H O concentration 4 2 leads to the increase in particle sizes, homogeneity, compactness of the thin films, and the number of clusters, and agglomeration, while the increase in thiourea concentration leads to the decrease in particle sizes and quality of films. Optical results demonstrated that the transmission of thin films rapidly increased in the UV–VIS region at ( = 350–500 nm) until it reached its maximum peak at ( = 600–650 nm) in the visible region, then it decreased in the NIR region. The high absorption was obtained in the UV–VIS region at ( = 350–500 nm) before it decreased to its minimum value in the visible region, and then increased in the NIR region. The energy bandgap of thin films Citation: Ahmed, H.S.; Mohammed, effectively depends on the deposition parameters. It decreased with the increasing deposition time R.Y. The Effect of Deposition (3.01–2.95 eV), bath temperature (3.04–2.63 eV), CuSO .5H O concentration (3.1–2.6 eV), and pH 4 2 Parameters on Morphological and value (3.14–2.75 eV), except for thiourea concentration, while it decreased with the increasing thiourea Optical Properties of Cu S Thin Films concentration (2.79–3.09 eV). Grown by Chemical Bath Deposition Technique. Photonics 2022, 9, 161. https://doi.org/10.3390/ Keywords: Cu S thin film; chemical bath deposition; energy bandgap; thin films; optical properties; photonics9030161 thickness measurements; deposition parameters Received: 17 January 2022 Accepted: 4 March 2022 Published: 6 March 2022 1. Introduction Publisher’s Note: MDPI stays neutral Transition metal chalcogenides have been attracting great interest due to their physical with regard to jurisdictional claims in and chemical properties. These properties are beneficial in various applications, such published maps and institutional affil- as optical sensors, solar energy conversion, sensors for low temperatures, catalysts, and iations. microelectronic devices [1–5]. The copper sulfide (Cu S) system has five different phases, including chalcocite, djurleite, digenite, anilite, and covellite. Additionally, it depends on the value of X = 2, 1.95, 1.8, 1.75, and 1, respectively [6–9]. Bulk Cu S solids are classified as Copyright: © 2022 by the authors. phase (stable above 425 C), phase (high chalcocite; stable between 105 and 425 C), and Licensee MDPI, Basel, Switzerland. phase (low chalcocite; the first solid–liquid hybrid phase and stable below 105 C) [10,11]. This article is an open access article Cu S has crucial properties, such as non-toxicity, low cost [12], and an ideal bandgap. distributed under the terms and In addition, it plays an important role in various applications, including solar energy ab- conditions of the Creative Commons sorbers [13], electroconductive coatings [14], tabular solar collectors [15], ion batteries and Attribution (CC BY) license (https:// superconductors [16,17], heterojunction photodetectors, such as Cu S/CdS, ZnO/Cu S, 2 2 creativecommons.org/licenses/by/ Cu S/ZnS, and Cu S/n–Si [18–20], and sensors [3]. Various deposition techniques are 2 2 4.0/). Photonics 2022, 9, 161. https://doi.org/10.3390/photonics9030161 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 161 2 of 13 utilized for the synthetization of Cu S thin films, such as vacuum evaporation [21], photo- chemical deposition [22], chemical bath deposition [15,23,24], sputtering [25], continuous flow microreactor [26,27], spray pyrolysis [28,29], and successive ionic layer adsorption and reaction technique (SILAR) [30,31]. The chemical bath deposition technique is one of the most popular techniques for the deposition of thin films due to its advantage over other deposition techniques, such as low cost, low pressure, ability to deposit large areas, and low-temperature requirement [15,32,33]. The properties of chemically deposited thin films strongly depend on the deposition parameters, such as deposition time [23], pH value [34,35], bath temperature [36], and substrate nature [13,33,37]. In the present study, the chemical bath deposition technique has been utilized for the synthetization of Cu S thin films. The growth, morphological, structural, and optical properties of deposited thin films have been studied as a function of deposition parameters (deposition time (t ), bath temperature (T ), pH value, precursors concentration (copper d b sulfate pentahydrate (CuSO .5H O), and thiourea concentration (SC (NH ) ). 4 2 2 2 2. Experiment 2.1. Cleaning Process Glass substrates with dimensions of 25  75  1 mm have been immersed in chromic acid for 24 h. Then, the substrates were ultrasonically cleaned with distilled water and rinsed in ethanol. Thereafter, they were maintained inside a desiccator to deposit Cu S thin films. 2.2. Deposition of Cu S Thin Films Initially, the CuSO .5H O precursor was used as a source of Cu ions by diluting the 4 2 solution in 50 mL of distilled water. Under continuous stirring, 2 mL of triethanolamine (TEA; C H NO ) acted as a complex agent. Then, 15% of ammonia solution was consec- 6 15 3 utively added to achieve the desired pH value. Subsequently, the color of the solution changed to dark blue, which indicates the formation of copper ions. Then, 50 mL of thiourea (SC (NH ) ) solution as a source of S was added to the solution. The color of the solution 2 2 changed from dark blue to dark brown during the deposition process, which indicates Photonics 2022, 9, x FOR PEER REVIEW 3 of 13 the steps of the formation of Cu S thin films, as shown in Figure 1. The reactions inside the bath can be expressed as shown in Equations (1) and (2) [36]. Thereafter, the substrate was immersed vertically in the solution. The deposition process was carried out at various deposition parameters and presented in Table 1. Δ𝑥 𝜆 (3) 𝑑= 𝑥 2 2+ CuSO .5H O + n(TEA) ! [Cu(TEA) ] +SO +5H O (1) 4 2 n 4 2 where d is the thin film thickness, He–Ne is the laser wavelength (λ = 632.8 nm), ∆x is the 2+ [Cu(TEA) ] +SC (NH ) +2OH ! Cu S + [NC_NH ] +2H O (2) distance between two f n ringes, and x2 is 2 the width of the f 2 ringe. 2 2 Figure 1. The stages of chemical bath deposition of Cu S: (a) 0 min, (b) after 2 min, (c) 3 min, (d) 5 min, Figure 1. The stages of chemical bath deposition of Cu 2 2S: (a) 0 min, (b) after 2 min, (c) 3 min, (d) 5 and (e) 6.5 min with CuSO . H O = 3 M, SC (NH ) = 1 M, bath temperature = 70 C, and pH = 10. min, and (e) 6.5 min with CuSO4.5H2O = 3 M, SC (NH2)2 = 1 M, bath temperature = 70 °C, and pH = 4 5 2 2 2 2.3. Characterization of Cu2S Thin Films The morphological properties of Cu2S thin films has been studied by the MIRA 3 TESCAN scanning electron microscopy (SEM). The optical transmission measurements of Cu2S thin films were obtained using an UV–Vis spectrophotometer (JANEWAY 6850) in the range of 350–1100 nm. The direct energy bandgap of Cu2S thin films was calculated using Tauc’s equation: (4) (𝛼ℎ𝜈) =𝐵( ℎ𝜈− 𝑔𝐸) where the bandgap energy (Eg), the transmittance (T), the absorption (A), and the absorp- tion coefficient (α) are given by (=2.303 log (T/d)). In addition, (hʋ) is the incident photon energy, n depends on the transition type (which is equal to 1/2 for the allowed direct tran- sition and 2 for the indirect one), and (d) is the film thickness [41,42]. 3. Results and Discussion 3.1. Deposition Parameters Figure 2 presents the thickness and growth rate variation of Cu2S thin films as a func- tion of deposition parameters (deposition time, bath temperature, CuSO4.5H2O concen- tration, pH value, and SC (NH2)2 concentration). Photonics 2022, 9, 161 3 of 13 Table 1. Chemical bath solutions. CuSO .5H O SC (NH ) 4 2 2 2 Bath No. t (min) T ( C) pH Value TEA (mL) (M) (M) 1 2–10 step 2 0.3 1 70 10 2 2 8 0.1, 0.5 1 70 10 2 0.4–1.2 3 8 0.3 70 10 2 step 0.2 4 8 0.3 1 70 8, 9, and 11 2 40, 50, 60, 5 8 0.3 1 10 2 80, and 90 The thickness of Cu S was measured using an optical interferometer technique [38–40]: Dx l d = (3) x 2 where d is the thin film thickness, He–Ne is the laser wavelength (l = 632.8 nm), Dx is the distance between two fringes, and x is the width of the fringe. 2.3. Characterization of Cu S Thin Films The morphological properties of Cu S thin films has been studied by the MIRA 3 TESCAN scanning electron microscopy (SEM). The optical transmission measurements of Cu S thin films were obtained using an UV–Vis spectrophotometer (JANEWAY 6850) in the range of 350–1100 nm. The direct energy bandgap of Cu S thin films was calculated using Tauc’s equation: 2 n (ahn) = B(hn Eg) (4) where the bandgap energy (Eg), the transmittance (T), the absorption (A), and the absorption coefficient ( ) are given by (=2.303 log (T/d)). In addition, (hu) is the incident photon energy, n depends on the transition type (which is equal to 1/2 for the allowed direct transition and 2 for the indirect one), and (d) is the film thickness [41,42]. 3. Results and Discussion 3.1. Deposition Parameters Figure 2 presents the thickness and growth rate variation of Cu S thin films as a function of deposition parameters (deposition time, bath temperature, CuSO .5H O con- 4 2 centration, pH value, and SC (NH ) concentration). 2 2 In Figure 2a, the thickness of deposited thin films increased almost linearly as deposi- tion time increased from 2 to 6 min. Thereafter, the thickness started to decrease when the time reached 8 min, with a significant decrease at deposition time of 10 min. This decrement can be attributed to the porous formation of an outer layer and peeling off from the glass substrate [43]. The growth rate of deposited films (Figure 2a) gradually decreased with the deposition time, which can be attributed to the precursor consumption over time [42]. The thickness increased as bath temperature increased, until it reached its saturation point at a bath temperature of 70 C. Thereafter, when the temperature reached 80 C, the thickness started to decrease due to the desorption and/or dissolution of preformed Cu S [44], as shown in Figure 2b. While the growth rate of deposited films gradually increased with the bath temperature due to additional generations of colloidal ions [42], as shown in Figure 2b. Photonics 2022, 9, x FOR PEER REVIEW 4 of 13 Photonics 2022, 9, 161 4 of 13 100 300 40 Thickness Thickness Growth rate 38 b growth rate 40 26 24 68 10 40 50 60 70 80 90 t (min) d T ( C) 310 40 40 Thickness Thickness 300 Growth rate 39 38 Growth rate c d 38 36 37 34 280 35 240 30 34 28 33 26 32 24 31 22 250 30 20 0.1 0.2 0.3 0.4 0.5 8 9 10 11 12 CuSO .H O concentration (M) pH 4 2 Thickness e Growth rate 38 0.4 0.6 0.8 1.0 1.2 SC (NH ) concentration (M) 2 2 Figure 2. Figure 2. The t Thehthickness ickness and growth rate variation of and growth rate variation of Cu Cu S 2thin S thin films films as aas a function function of of deposition deposition parameters: ( parameters:a) ( Deposition a) Deposition time, ( time, (b b)) ba bath th te temperatur mperature, ( e, (c) c CuSO ) CuS.5H O4.5H O 2 concentration, O concentration, ( (d) pH d) pH value, value, 4 2 (e) and SC (e) and SC (NH (NH 2)2 concentration. ) concentration. 2 2 In Figure 2a, the thickness of deposited thin films increased almost linearly as depo- sition time increased from 2 to 6 min. Thereafter, the thickness started to decrease when the time reached 8 min, with a significant decrease at deposition time of 10 min. This dec- rement can be attributed to the porous formation of an outer layer and peeling off from the glass substrate [43]. The growth rate of deposited films (Figure 2a) gradually d (nm) d (nm) d (nm) Growth rate (nm/min Growth rate (nm/min) d (nm) d (nm) Growth rate (nm/min) Growth rate (nm/min) Growth rate (nm/min) Photonics 2022, 9, x FOR PEER REVIEW 5 of 13 Photonics 2022, 9, 161 5 of 13 decreased with the deposition time, which can be attributed to the precursor consumption over time [42]. The thickness increased as bath temperature increased, until it reached its saturation In Figure 2c, the thickness and growth rate increased as CuSO .5H O concentration 4 2 point at a bath temperature of 70 °C. Thereafter, when the temperature reached 80 °C, the increased. This is attributed to the competition of heterogeneous nucleation on the substrate thickness started to decrease due to the desorption and/or dissolution of preformed Cu2S and homogeneous nucleation in the solution, which could modify the growth of thin films [44], as shown in Figure 2b. While the growth rate of deposited films gradually increased and increase the thickness [45]. with the bath temperature due to additional generations of colloidal ions [42], as shown The thickness of Cu S thin films almost linearly increased from 185 to 297 nm as the in Figure 2b. pH value of bath solution increased from 8 to 11, as shown in Figure 2d. In Figure 2c, the thickness This increase and g in thickness rowth ra iste incr attributed eased to as C the incr uSO ement 4.5H2of O concentration OH ions concentration in the solution that pushes the reaction of thiourea hydrolysis forward, causing the high increased. This is attributed to the competition of heterogeneous nucleation on the sub- generation of sulfide ions [46], as shown in Figure 2d. In addition, the growth rate increased strate and homogeneous nucleation in the solution, which could modify the growth of as the pH value increased (Figure 2d). thin films and increase the thickness [45]. Figure 2e shows the effect of SC (NH ) concentration on the thickness of deposited 2 2 The thickness of Cu2S thin films almost linearly increased from 185 to 297 nm as the thin films. pH value of bath solution increased from 8 to 11, as shown in Figure 2d. Notably, the thickness of deposited thin films decreased as the SC (NH ) concentration 2 2 2+ - 2 This increase in thickness is attributed to the increment of OH ions concentration in increased due to the limited concentration of Cu and S ions that was released in the solution [39]. The growth rate of Cu S thin films can be seen in Figure 2e. the solution that pushes the reaction of thiourea hydr 2 olysis forward, causing the high gen- eration of sulfide ions [46], as shown in Figure 2d. In addition, the growth rate increased 3.2. Physical Properties as the pH value increased (Figure 2d). 3.2.1. Morphological Properties Figure 2e shows the effect of SC (NH2)2 concentration on the thickness of deposited Figure 3 demonstrated the SEM images of Cu S thin films at various deposition thin films. parameters (deposition time, bath temperature, pH value, CuSO .5H O concentration, and 4 2 Notably, the thickness of deposited thin films decreased as the SC (NH2)2 concentra- SC (NH ) concentration). 2 2 2+ 2− tion increased due to the li Figuremi 3ate shows d conc that entr the atio FESEM n of Cu images andof Sthin ions t films hat at w deposition as released time in of 6 min were uniform, well covered, and homogenous without cracks. In addition, the increase in the solution [39]. The growth rate of Cu2S thin films can be seen in Figure 2e. deposition time leads to the increase in aggregations. Moreover, few cracks were found in both deposited thin films at 8 and 10 min. The number and size of the grain increased 3.2. Physical Properties from 37.25 to 99.57 nm as deposition time increased from 6 to 10 min. Figure 3a indicates 3.2.1. Morphological Properties that additional nucleation sites have formed [47]. The increase in bath temperature leads to the increase in compactness, homogeneity, and uniformity of films, as shown in Figure 3b. Figure 3 demonstrated the SEM images of Cu2S thin films at various deposition pa- The surface of films deposited at 40 C was not completely covered and had few cracks. rameters (deposition time, bath temperature, pH value, CuSO4.5H2O concentration, and However, the surface of films deposited at 70 and 90 C was homogeneous, compact, and SC (NH2)2 concentration). relatively completely covered and uniform. Figure 3. Cont. Photonics 2022, 9, x FOR PEER REVIEW 6 of 13 Photonics 2022, 9, 161 6 of 13 Figure 3. Cont. Photonics 2022, 9, x FOR PEER REVIEW 7 of 13 Photonics 2022, 9, 161 7 of 13 Figure 3. FESEM images of Cu S thin films at different deposition parameters: (a) Deposition time Figure 3. FESEM images of Cu2S thin films at different deposition parameters: (a) Deposition time (t ), (b) bath temperature (T ), (c) SC (NH ) concentration, (d) CuSO .5H O concentration, and d b 2 2 4 2 (td), (b) bath temperature (Tb), (c) SC (NH2)2 concentration, (d) CuSO4.5H2O concentration, and (e) (e) pH value. pH value. The deposited films at 0.6 and 1 M of SC (NH ) concentration are well covered, 2 2 uniform, and homogeneous with few small cracks on the surface deposited at 1 M of SC Figure 3a shows that the FESEM images of thin films at deposition time of 6 min were (NH ) concentration. The quality of thin films decreased as the SC (NH ) concentration 2 2 2 2 uniform, well covered, and homogenous without cracks. In addition, the increase in dep- increased to 1.2 M. Agglomerations and cracks existed on the film’s surface, as shown in osition time leads to the increase in aggregations. Moreover, few cracks were found in Figure 3c. Deposited thin films at various CuSO .5H O molar concentrations were well 4 2 both deposited thin films at 8 and 10 min. The number and size of the grain increased covered, uniform, homogeneous, and few small cracks existed on the surface of the films from 37.25 to 99.57 nm as deposition time increased from 6 to 10 min. Figure 3a indicates deposited at 0.3 and 0.5 M, as shown in Figure 3d. However, the average particle sizes were that additional nucleation sites have formed [47]. The increase in bath temperature leads 36.87, 55.03, and 43.98 nm for the deposited thin films at 0.1, 0.3, and 0.5 M of CuSO .5H O 4 2 molar concentration, respectively. The increase in CuSO .5H O molar concentration leads to the increase in compactness, homogeneity, and uniformity of films, as shown in Figure 4 2 to the increase in the number of clusters and agglomerated nanoparticles. The FESEM 3b. The surface of films deposited at 40 °C was not completely covered and had few cracks. images of deposited films at different pH values are shown in Figure 3e. The films were However, the surface of films deposited at 70 and 90 °C was homogeneous, compact, and homogeneous and well covered. However, the agglomerations and clusters increased as relatively completely covered and uniform. the pH value increased. The average particle sizes decreased from 65.72 to 33.28 nm as The deposited films at 0.6 and 1 M of SC (NH2)2 concentration are well covered, uni- the pH value increased from 8 to 11. The increase in the pH value leads to noticeable form, and homogeneous with few small cracks on the surface deposited at 1 M of SC changes in the morphology of deposited thin films due to the increment of OH ions. This (NH2)2 concentrat leads ion. The the reaction quality of forwar th d, inincr films dec ease there attractive ased asfor thce e S and C (NH decrease 2)2 con repculsive entration force, which allow for the growth of oriented attachment (OA). When the particles are perfectly aligned, increased to 1.2 M. Agglomerations and cracks existed on the film’s surface, as shown in the common boundary is eliminated, resulting in the formation of larger well-defined Figure 3c. Deposited thin films at various CuSO4.5H2O molar concentrations were well morphology particles [48,49]. covered, uniform, homogeneous, and few small cracks existed on the surface of the films deposited at 0.3 and 0.5 M, as shown in Figure 3d. However, the average particle sizes 3.2.2. Optical Properties were 36.87, 55.03, and 43.98 nm for the deposited thin films at 0.1, 0.3, and 0.5 M of Figure 4 shows the transmission of Cu S thin films deposited at different deposition CuSO4.5H2O molar concentration, respectively. The increase in CuSO4.5H2O molar con- parameters (deposition time, bath temperature, CuSO .5H O concentration, SC (NH ) 4 2 2 2 centration leads tconcentration, o the increaseand in th pH e nu value). mber of Notably clust , the ers transmittance and agglomerat decr ed eased nano aspart these icle parameters s. increased. However, the transmission increased as the SC (NH ) concentration increased. 2 2 The FESEM images of deposited films at different pH values are shown in Figure 3e. The In addition, the transmittance of deposited thin films in the UV–Vis region rapidly increased films were homogeneous and well covered. However, the agglomerations and clusters until it reached its maximum peak at ( = 600–650 nm). Thereafter, a slight decrease was increased as the pH value increased. The average particle sizes decreased from 65.72 to found in the visible part before the rapid decrease in the NIR region, except for thin films 33.28 nm as the pH value increased from 8 to 11. The increase in the pH value leads to deposited at (TB = 40 C ) and (pH = 8), which have high transmittance of 94% and 86.2% noticeable changes in the morphology of deposited thin films due to the increment of OH at the NIR region. This decrease in transmission is due to the increase in the thickness of ions. This leads the reaction forward, increase the attractive force and decrease repulsive deposited Cu S thin films [23]. force, which allow for the growth of oriented attachment (OA). When the particles are perfectly aligned, the common boundary is eliminated, resulting in the formation of larger well-defined morphology particles [48,49]. 3.2.2. Optical Properties Figure 4 shows the transmission of Cu2S thin films deposited at different deposition parameters (deposition time, bath temperature, CuSO4.5H2O concentration, SC (NH2)2 concentration, and pH value). Notably, the transmittance decreased as these parameters Photonics 2022, 9, x FOR PEER REVIEW 8 of 13 increased. However, the transmission increased as the SC (NH2)2 concentration increased. In addition, the transmittance of deposited thin films in the UV–Vis region rapidly in- creased until it reached its maximum peak at (𝜆 = 600–650 nm). Thereafter, a slight de- crease was found in the visible part before the rapid decrease in the NIR region, except for thin films deposited at (TB = 40 °C°) and (pH = 8), which have high transmittance of 94% and 86.2% at the NIR region. This decrease in transmission is due to the increase in Photonics 2022, 9, 161 8 of 13 the thickness of deposited Cu2S thin films [23]. 100 85 2 min 0.1 M (a) (b) 90 (c) 4 min 0.3 M 6 min 0.5 M 8 min 40 C 10 min 50 C 60 C 70 C 70 80 C 90 C 55 50 50 45 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) λ (nm) Tu= 0.4 M (d) 85 (e) 90 Tu= 0.6 M Tu= 0.8 M Tu= 1 M 75 Tu= 1.2 M pH=8 pH=9 pH=10 pH=11 65 40 30 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) Figure 4. Transmission spectrums of Cu S thin films as a function of different deposition parameters: Figure 4. Transmission spectrums of Cu2S thin films as a function of different deposition parame- (ters: ( a) Deposition a) Deposition time, ( time, (b) bath b) b temperatur ath temperature, ( e, (c) CuSO c) CuSO .5H4.5 OH concentration, 2O concentratio (d n, ( ) pH d) pH value, value and , and ( (e) SC e) 4 2 SC (NH2)2 concentration. (NH ) concentration. 2 2 Figure 5 shows the absorbance spectra of Cu S thin films deposited at different deposi- Figure 5 shows the absorbance spectra of Cu2S thin films deposited at different dep- tion parameters. The absorbance of deposited thin films increased as deposition time, bath osition parameters. The absorbance of deposited thin films increased as deposition time, temperature, CuSO .5H O concentration, and pH value increased. In contrast, it decreased bath temperature, CuSO 4 2 4.5H2O concentration, and pH value increased. In contrast, it de- when the SC (NH ) concentration increased. In general, all of the deposited thin films creased when the SC (NH2)2 concentration increased. In general, all of the deposited thin 2 2 have high absorbance at UV and NIR regions, except for thin films deposited at (T = 40 C) films have high absorbance at UV and NIR regions, except for thin films deposited at (Tb and (pH = 8), which had low absorbance at the NIR region. In addition, thin films had low = 40 °C) and (pH = 8), which had low absorbance at the NIR region. In addition, thin films absorbance at the VIS region. Low transparency in UV region provides thin films with a had low absorbance at the VIS region. Low transparency in UV region provides thin films coating quality for use in eyeglasses to protect the eye from UV radiation [23]. with a coating quality for use in eyeglasses to protect the eye from UV radiation [23]. The high absorption coefficient indicates that deposited Cu S thin films with various The high absorption coefficient indicates that deposited Cu 2 2S thin films with various deposition parameters have a direct energy bandgap [19]. This was calculated from the deposition parameters have a direct energy bandgap [19]. This was calculated from the extrapolation straight-line portion of the curve ( h) versus photon energy, as shown in Figure 6. T ( % ) T (% ) T % T (% ) T ( % ) Photonics 2022, 9, x FOR PEER REVIEW 9 of 13 extrapolation straight-line portion of the curve (αhυ) versus photon energy, as shown in Figure 6. In Figure 6a and Table 2, the energy gap of deposited thin films decreased as deposi- tion time increased due to the increase in thin films’ thickness [19] and crystallinity in the quantum size effect [43]. The decrease in energy gap with the increment of bath tempera- ture is due to the increase in crystallite size, as shown in Figure 6b. This leads to the in- creased absorption and causes a shift in optical absorption edge towards longer wave- lengths, and then the bandgap decreases [50]. In Figure 6c, the energy gap decreased lin- early with the increase in CuSO4.5H2O concentration due to the increased grain size and low sulfur ions [24]. The obtained energy gap was slightly larger than the previous report by Ismail R.A. et al., which was prepared by a different precursor (CuCl2) [24]. The energy gap of thin films prepared at different pH values decreased as the pH value of bath solu- tion increased due to the increase in thickness, as shown in Table 2. The obtained value is almost the same as the value obtained by Ahmed H.S. et al. [51], which was prepared by Cu2S thin films of a different precursor (CuCl2.2H2O). The effect of thickness on the energy gap could increase due to the height of crystalline films and large density of dislocation [21]. The energy gap increased as the SC (NH2)2 concentration increased due to the higher concentration of S ions, which leads to a decrease in the thickness of thin films. The ob- Photonics 2022, 9, 161 9 of 13 tained values are slightly different from the obtained values by Muhammed A.M. et al. [52], which was prepared by Cu2S thin films of a different precursor (CuCl2). 0.30 (c) (a) 0.30 (b) 0.32 0.25 0.25 0.28 0.20 0.20 0.24 40 C 0.15 0.20 0.15 50 C 60 C 0.16 0.10 70 C 0.10 2 min 80 C 4 min 0.12 90 C 0.05 6 min 0.1 M 8 min 0.3 M 0.05 10 min 0.08 0.5 M 0.00 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) λ (nm) 0.35 0.50 (d) Tu= 0.4 M (e) 0.45 Tu= 0.6 M 0.30 Tu= 0.8 M 0.40 Tu= 1 M Tu= 1.2 M 0.25 0.35 0.30 0.20 0.25 0.15 pH=8 0.20 pH=9 pH=10 0.15 0.10 pH=11 0.10 0.05 0.05 0.00 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) Figure 5. Absorbance of Cu2S thin films as a function of different deposition parameters: (a) Depo- Figure 5. Absorbance of Cu S thin films as a function of different deposition parameters: (a) Depo- sition time, (b) bath temperature, (c) CuSO4.5H2O concentration, (d) pH value, and (e) SC (NH2)2 sition time, (b) bath temperature, (c) CuSO .5H O concentration, (d) pH value, and (e) SC (NH ) 4 2 2 2 concentration. concentration. In Figure 6a and Table 2, the energy gap of deposited thin films decreased as deposition time increased due to the increase in thin films’ thickness [19] and crystallinity in the quantum size effect [43]. The decrease in energy gap with the increment of bath temperature is due to the increase in crystallite size, as shown in Figure 6b. This leads to the increased absorption and causes a shift in optical absorption edge towards longer wavelengths, and then the bandgap decreases [50]. In Figure 6c, the energy gap decreased linearly with the increase in CuSO .5H O concentration due to the increased grain size and low sulfur 4 2 ions [24]. The obtained energy gap was slightly larger than the previous report by Ismail R.A. et al., which was prepared by a different precursor (CuCl ) [24]. The energy gap of thin films prepared at different pH values decreased as the pH value of bath solution increased due to the increase in thickness, as shown in Table 2. The obtained value is almost the same as the value obtained by Ahmed H.S. et al. [51], which was prepared by Cu S thin films of a different precursor (CuCl .2H O). The effect of thickness on the energy gap could increase 2 2 due to the height of crystalline films and large density of dislocation [21]. The energy gap increased as the SC (NH ) concentration increased due to the higher concentration 2 2 of S ions, which leads to a decrease in the thickness of thin films. The obtained values are slightly different from the obtained values by Muhammed A.M. et al. [52], which was prepared by Cu S thin films of a different precursor (CuCl ). 2 2 A (a .u ) A (a .u ) A (a .u ) A (a .u ) A (a .u ) Photonics 2022, 9, 161 10 of 13 Photonics 2022, 9, x FOR PEER REVIEW 10 of 13 13 14 8.0x10 1.0x10 14 o 1.8x10 (b) (a) t = 40 C t =6 min. 14 t = 60 C 1.6x10 6.0x10 o t = 70 C 1.4x10 8.0x10 1.2x10 6.0x10 13 4.0x10 1.0x10 8.0x10 2.0x10 6.0x10 13 6.0x10 13 4.0x10 4.0x10 0.0 2.0x10 1.0 1.5 2.0 2.5 3.0 3.5 4.0x10 0.0 hν (eV) 1.0 1.5 2.0 2.5 3.0 3.5 t =2 min. hν(nm) t =4 min. 13 o 2.0x10 t = 50 C t =8 min. 2.0x10 t = 80 C t =10 min. o t = 90 C 0.0 0.0 1.0 1.5 2.0 2.5 3.0 3.5 1.0 1.5 2.0 2.5 3.0 3.5 hν (eV) hν (eV) 8.0x10 (c) CuSO .5H O=0.1 M 8.00E+13 4 2 pH=11 (d) CuSO .5H O= 0.3 M 8.0×10 4 2 CuSO .5H O=0.5 M 6.00E+13 4 2 6.0x10 4.00E+13 6.0×10 2.00E+13 4.0x10 0.00E+00 13 1.0 1.5 2.0 2.5 3.0 3.5 4.0×10 hυ(eV) pH=8 2.0x10 pH=9 2.0×10 pH= 10 0.0 0.0 1.0 1.5 2.0 2.5 3.0 3.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 hυ(eV) hν(eV) (e) Tu=0.8M 1.6×10 6.00E+13 4.00E+13 1.2×10 2.00E+13 8.0×10 0.00E+00 1.0 1.5 2.0 2.5 3.0 3.5 hν (nm) Tu=0.4 M 4.0×10 Tu=0.6 M Tu=0.1 M Tu=1.2 M 0.0 1.01.5 2.02.5 3.03.5 hν (nm) Figure 6. Variation of (αhʋ) with (hʋ) with deposition parameters): (a) Deposition time, (b) bath Figure 6. Variation of (ahu) with (hu) with deposition parameters): (a) Deposition time, (b) bath temperature, (c) CuSO4.5H2O concentration, (d) pH value, and (e) SC (NH2)2 concentration. temperature, (c) CuSO .5H O concentration, (d) pH value, and (e) SC (NH ) concentration. 4 2 2 2 Table 2. Bandgap variation with different deposition parameters. Bath No. 1 2 3 4 5 66 Deposition time (min) 2 4 6 8 10 - Eg (eV) 3.01 2.92 2.9 2.86 2.95 - Bath temprature (°C) 40 50 60 70 80 90 Eg (eV) 3.04 2.92 2.9 2.86 2.72 2.63 2 -1 2 2 -1 2 (αhν) (eVm ) ( α h ν ) ( e V m ) 2 -1 2 ( α h ν ) ( e V m ) 2 -1 2 (αhν) (eVm ) 2 -1 2 (αhν) (eVm ) 2 -1 2 2 -1 2 (α h ν ) (e V m ) (αhυ) (eVm ) 2 -1 2 (αhυ) (eVm ) 2 -1 2 (α h ν ) (e V m ) Photonics 2022, 9, 161 11 of 13 Table 2. Bandgap variation with different deposition parameters. Bath No. 1 2 3 4 5 6 Deposition time (min) 2 4 6 8 10 - Eg (eV) 3.01 2.92 2.9 2.86 2.95 - Bath temprature ( C) 40 50 60 70 80 90 Eg (eV) 3.04 2.92 2.9 2.86 2.72 2.63 CuSO .5H O concentration (M) 0.1 0.3 0.5 - - - 4 2 Eg (eV) 3.1 2.86 2.65 pH Value 8 9 10 11 - - Eg (eV) 3.14 2.9 2.86 2.75 SC (NH ) concentration (M) 0.4 0.6 0.8 1 1.2 - 2 2 Eg (eV) 2.79 2.81 2.83 2.86 3.09 - 4. Conclusions Copper sulfide thin films were successfully deposited by the chemical bath deposition technique. The obtained results indicate that the thickness of deposited thin films strongly depends on the deposition parameters. It increased from 185 to 281 nm as deposition time increased, and from 183 to 291 nm as bath temperature increased. In addition, the thickness increased from 257 to 303 nm as the precursor concentration increased and from 185 to 297 nm as the pH value increased. However, the thickness decreased from 299 to 234 nm with the increasing thiourea concentration. The saturation was found only in the deposition time and bath temperature. The morphology of deposited thin films noticeably changed with the deposition times. The grains’ number and size increased with the deposition time. Moreover, the quality improved with the increasing deposition time and decreased at 10 min of deposition time. The deposited Cu S thin films at various CuSO .5H O molar 2 4 2 concentrations were compact, uniform, and homogeneous with different particle sizes and clusters, and the agglomerated nanoparticles increased as the CuSO .5H O molar concen- 4 2 tration increased. Lower concentrations of thiourea provided compact and homogeneous thin films. The average particle sizes increased as the thiourea concentration increased. The morphology of deposited thin films improved and agglomerated nanoparticles increased as bath temperature increased. The morphology of deposited thin films remarkably changed with the pH value. Deposited thin films at higher pH values were covered almost com- pletely with deposited nanoparticles. The optical measurements demonstrate that almost all of the deposited thin films have high transmission and low absorption in the visible region, while transmittance decreased and absorbance increased in the UV and NIR regions. The energy bandgap of thin films decreased with the increasing deposition time (3.01–2.95 eV), bath temperature (3.04–2.63 eV), CuSO .5H O concentration (3.1–2.6 eV), and pH value 4 2 (3.14–2.75 eV), except for thiourea concentration, where it decreased with the increase in thiourea concentration (2.79–3.09 eV). Furthermore, all of the deposited thin films have high energy bandgaps and are slightly larger than those obtained by other works. Author Contributions: Conceptualization, R.Y.M.; methodology, R.Y.M. and H.S.A.; software, H.S.A.; validation, R.Y.M. and H.S.A.; formal analysis, R.Y.M. and H.S.A.; investigation, R.Y.M. and H.S.A.; resources, R.Y.M.; data curation, R.Y.M. and H.S.A.; writing—original draft preparation, H.S.A.; writing—review and editing, R.Y.M.; visualization, R.Y.M. and H.S.A.; supervision, R.Y.M.; project administration, R.Y.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors are thankful to the University of Duhok for their full support. Photonics 2022, 9, 161 12 of 13 Conflicts of Interest: The authors declare no conflict of interest. References 1. Cruz, J.S.; Hernández, S.M.; Coronel, J. 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The Effect of Deposition Parameters on Morphological and Optical Properties of Cu2S Thin Films Grown by Chemical Bath Deposition Technique

Photonics , Volume 9 (3) – Mar 6, 2022

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hv photonics Article The Effect of Deposition Parameters on Morphological and Optical Properties of Cu S Thin Films Grown by Chemical Bath Deposition Technique Honar S. Ahmed and Raghad Y. Mohammed * Department of Physics, College of Science, University of Duhok, Duhok 42001, Kurdistan Region, Iraq; honar.salah@uod.ac * Correspondence: ssraghad@uod.ac Abstract: The chemical bath deposition technique has been used for the deposition of Cu S thin films on glass substrates. The thickness of deposited thin films strongly depends on the deposition parameters. The present study revealed that the thickness increased from 185 to 281 nm as deposition time increased and from 183 to 291 nm as bath temperature increased. In addition, the thickness increased from 257 to 303 nm with the increment of precursors concentration and from 185 to 297 nm as the pH value increased. However, the thickness decreased from 299 to 234 nm with the increment of precursors concentration. The morphology of Cu S thin films remarkably changed as the deposition parameters varied. The increase in deposition time, bath temperature, and CuSO .5H O concentration 4 2 leads to the increase in particle sizes, homogeneity, compactness of the thin films, and the number of clusters, and agglomeration, while the increase in thiourea concentration leads to the decrease in particle sizes and quality of films. Optical results demonstrated that the transmission of thin films rapidly increased in the UV–VIS region at ( = 350–500 nm) until it reached its maximum peak at ( = 600–650 nm) in the visible region, then it decreased in the NIR region. The high absorption was obtained in the UV–VIS region at ( = 350–500 nm) before it decreased to its minimum value in the visible region, and then increased in the NIR region. The energy bandgap of thin films Citation: Ahmed, H.S.; Mohammed, effectively depends on the deposition parameters. It decreased with the increasing deposition time R.Y. The Effect of Deposition (3.01–2.95 eV), bath temperature (3.04–2.63 eV), CuSO .5H O concentration (3.1–2.6 eV), and pH 4 2 Parameters on Morphological and value (3.14–2.75 eV), except for thiourea concentration, while it decreased with the increasing thiourea Optical Properties of Cu S Thin Films concentration (2.79–3.09 eV). Grown by Chemical Bath Deposition Technique. Photonics 2022, 9, 161. https://doi.org/10.3390/ Keywords: Cu S thin film; chemical bath deposition; energy bandgap; thin films; optical properties; photonics9030161 thickness measurements; deposition parameters Received: 17 January 2022 Accepted: 4 March 2022 Published: 6 March 2022 1. Introduction Publisher’s Note: MDPI stays neutral Transition metal chalcogenides have been attracting great interest due to their physical with regard to jurisdictional claims in and chemical properties. These properties are beneficial in various applications, such published maps and institutional affil- as optical sensors, solar energy conversion, sensors for low temperatures, catalysts, and iations. microelectronic devices [1–5]. The copper sulfide (Cu S) system has five different phases, including chalcocite, djurleite, digenite, anilite, and covellite. Additionally, it depends on the value of X = 2, 1.95, 1.8, 1.75, and 1, respectively [6–9]. Bulk Cu S solids are classified as Copyright: © 2022 by the authors. phase (stable above 425 C), phase (high chalcocite; stable between 105 and 425 C), and Licensee MDPI, Basel, Switzerland. phase (low chalcocite; the first solid–liquid hybrid phase and stable below 105 C) [10,11]. This article is an open access article Cu S has crucial properties, such as non-toxicity, low cost [12], and an ideal bandgap. distributed under the terms and In addition, it plays an important role in various applications, including solar energy ab- conditions of the Creative Commons sorbers [13], electroconductive coatings [14], tabular solar collectors [15], ion batteries and Attribution (CC BY) license (https:// superconductors [16,17], heterojunction photodetectors, such as Cu S/CdS, ZnO/Cu S, 2 2 creativecommons.org/licenses/by/ Cu S/ZnS, and Cu S/n–Si [18–20], and sensors [3]. Various deposition techniques are 2 2 4.0/). Photonics 2022, 9, 161. https://doi.org/10.3390/photonics9030161 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 161 2 of 13 utilized for the synthetization of Cu S thin films, such as vacuum evaporation [21], photo- chemical deposition [22], chemical bath deposition [15,23,24], sputtering [25], continuous flow microreactor [26,27], spray pyrolysis [28,29], and successive ionic layer adsorption and reaction technique (SILAR) [30,31]. The chemical bath deposition technique is one of the most popular techniques for the deposition of thin films due to its advantage over other deposition techniques, such as low cost, low pressure, ability to deposit large areas, and low-temperature requirement [15,32,33]. The properties of chemically deposited thin films strongly depend on the deposition parameters, such as deposition time [23], pH value [34,35], bath temperature [36], and substrate nature [13,33,37]. In the present study, the chemical bath deposition technique has been utilized for the synthetization of Cu S thin films. The growth, morphological, structural, and optical properties of deposited thin films have been studied as a function of deposition parameters (deposition time (t ), bath temperature (T ), pH value, precursors concentration (copper d b sulfate pentahydrate (CuSO .5H O), and thiourea concentration (SC (NH ) ). 4 2 2 2 2. Experiment 2.1. Cleaning Process Glass substrates with dimensions of 25  75  1 mm have been immersed in chromic acid for 24 h. Then, the substrates were ultrasonically cleaned with distilled water and rinsed in ethanol. Thereafter, they were maintained inside a desiccator to deposit Cu S thin films. 2.2. Deposition of Cu S Thin Films Initially, the CuSO .5H O precursor was used as a source of Cu ions by diluting the 4 2 solution in 50 mL of distilled water. Under continuous stirring, 2 mL of triethanolamine (TEA; C H NO ) acted as a complex agent. Then, 15% of ammonia solution was consec- 6 15 3 utively added to achieve the desired pH value. Subsequently, the color of the solution changed to dark blue, which indicates the formation of copper ions. Then, 50 mL of thiourea (SC (NH ) ) solution as a source of S was added to the solution. The color of the solution 2 2 changed from dark blue to dark brown during the deposition process, which indicates Photonics 2022, 9, x FOR PEER REVIEW 3 of 13 the steps of the formation of Cu S thin films, as shown in Figure 1. The reactions inside the bath can be expressed as shown in Equations (1) and (2) [36]. Thereafter, the substrate was immersed vertically in the solution. The deposition process was carried out at various deposition parameters and presented in Table 1. Δ𝑥 𝜆 (3) 𝑑= 𝑥 2 2+ CuSO .5H O + n(TEA) ! [Cu(TEA) ] +SO +5H O (1) 4 2 n 4 2 where d is the thin film thickness, He–Ne is the laser wavelength (λ = 632.8 nm), ∆x is the 2+ [Cu(TEA) ] +SC (NH ) +2OH ! Cu S + [NC_NH ] +2H O (2) distance between two f n ringes, and x2 is 2 the width of the f 2 ringe. 2 2 Figure 1. The stages of chemical bath deposition of Cu S: (a) 0 min, (b) after 2 min, (c) 3 min, (d) 5 min, Figure 1. The stages of chemical bath deposition of Cu 2 2S: (a) 0 min, (b) after 2 min, (c) 3 min, (d) 5 and (e) 6.5 min with CuSO . H O = 3 M, SC (NH ) = 1 M, bath temperature = 70 C, and pH = 10. min, and (e) 6.5 min with CuSO4.5H2O = 3 M, SC (NH2)2 = 1 M, bath temperature = 70 °C, and pH = 4 5 2 2 2 2.3. Characterization of Cu2S Thin Films The morphological properties of Cu2S thin films has been studied by the MIRA 3 TESCAN scanning electron microscopy (SEM). The optical transmission measurements of Cu2S thin films were obtained using an UV–Vis spectrophotometer (JANEWAY 6850) in the range of 350–1100 nm. The direct energy bandgap of Cu2S thin films was calculated using Tauc’s equation: (4) (𝛼ℎ𝜈) =𝐵( ℎ𝜈− 𝑔𝐸) where the bandgap energy (Eg), the transmittance (T), the absorption (A), and the absorp- tion coefficient (α) are given by (=2.303 log (T/d)). In addition, (hʋ) is the incident photon energy, n depends on the transition type (which is equal to 1/2 for the allowed direct tran- sition and 2 for the indirect one), and (d) is the film thickness [41,42]. 3. Results and Discussion 3.1. Deposition Parameters Figure 2 presents the thickness and growth rate variation of Cu2S thin films as a func- tion of deposition parameters (deposition time, bath temperature, CuSO4.5H2O concen- tration, pH value, and SC (NH2)2 concentration). Photonics 2022, 9, 161 3 of 13 Table 1. Chemical bath solutions. CuSO .5H O SC (NH ) 4 2 2 2 Bath No. t (min) T ( C) pH Value TEA (mL) (M) (M) 1 2–10 step 2 0.3 1 70 10 2 2 8 0.1, 0.5 1 70 10 2 0.4–1.2 3 8 0.3 70 10 2 step 0.2 4 8 0.3 1 70 8, 9, and 11 2 40, 50, 60, 5 8 0.3 1 10 2 80, and 90 The thickness of Cu S was measured using an optical interferometer technique [38–40]: Dx l d = (3) x 2 where d is the thin film thickness, He–Ne is the laser wavelength (l = 632.8 nm), Dx is the distance between two fringes, and x is the width of the fringe. 2.3. Characterization of Cu S Thin Films The morphological properties of Cu S thin films has been studied by the MIRA 3 TESCAN scanning electron microscopy (SEM). The optical transmission measurements of Cu S thin films were obtained using an UV–Vis spectrophotometer (JANEWAY 6850) in the range of 350–1100 nm. The direct energy bandgap of Cu S thin films was calculated using Tauc’s equation: 2 n (ahn) = B(hn Eg) (4) where the bandgap energy (Eg), the transmittance (T), the absorption (A), and the absorption coefficient ( ) are given by (=2.303 log (T/d)). In addition, (hu) is the incident photon energy, n depends on the transition type (which is equal to 1/2 for the allowed direct transition and 2 for the indirect one), and (d) is the film thickness [41,42]. 3. Results and Discussion 3.1. Deposition Parameters Figure 2 presents the thickness and growth rate variation of Cu S thin films as a function of deposition parameters (deposition time, bath temperature, CuSO .5H O con- 4 2 centration, pH value, and SC (NH ) concentration). 2 2 In Figure 2a, the thickness of deposited thin films increased almost linearly as deposi- tion time increased from 2 to 6 min. Thereafter, the thickness started to decrease when the time reached 8 min, with a significant decrease at deposition time of 10 min. This decrement can be attributed to the porous formation of an outer layer and peeling off from the glass substrate [43]. The growth rate of deposited films (Figure 2a) gradually decreased with the deposition time, which can be attributed to the precursor consumption over time [42]. The thickness increased as bath temperature increased, until it reached its saturation point at a bath temperature of 70 C. Thereafter, when the temperature reached 80 C, the thickness started to decrease due to the desorption and/or dissolution of preformed Cu S [44], as shown in Figure 2b. While the growth rate of deposited films gradually increased with the bath temperature due to additional generations of colloidal ions [42], as shown in Figure 2b. Photonics 2022, 9, x FOR PEER REVIEW 4 of 13 Photonics 2022, 9, 161 4 of 13 100 300 40 Thickness Thickness Growth rate 38 b growth rate 40 26 24 68 10 40 50 60 70 80 90 t (min) d T ( C) 310 40 40 Thickness Thickness 300 Growth rate 39 38 Growth rate c d 38 36 37 34 280 35 240 30 34 28 33 26 32 24 31 22 250 30 20 0.1 0.2 0.3 0.4 0.5 8 9 10 11 12 CuSO .H O concentration (M) pH 4 2 Thickness e Growth rate 38 0.4 0.6 0.8 1.0 1.2 SC (NH ) concentration (M) 2 2 Figure 2. Figure 2. The t Thehthickness ickness and growth rate variation of and growth rate variation of Cu Cu S 2thin S thin films films as aas a function function of of deposition deposition parameters: ( parameters:a) ( Deposition a) Deposition time, ( time, (b b)) ba bath th te temperatur mperature, ( e, (c) c CuSO ) CuS.5H O4.5H O 2 concentration, O concentration, ( (d) pH d) pH value, value, 4 2 (e) and SC (e) and SC (NH (NH 2)2 concentration. ) concentration. 2 2 In Figure 2a, the thickness of deposited thin films increased almost linearly as depo- sition time increased from 2 to 6 min. Thereafter, the thickness started to decrease when the time reached 8 min, with a significant decrease at deposition time of 10 min. This dec- rement can be attributed to the porous formation of an outer layer and peeling off from the glass substrate [43]. The growth rate of deposited films (Figure 2a) gradually d (nm) d (nm) d (nm) Growth rate (nm/min Growth rate (nm/min) d (nm) d (nm) Growth rate (nm/min) Growth rate (nm/min) Growth rate (nm/min) Photonics 2022, 9, x FOR PEER REVIEW 5 of 13 Photonics 2022, 9, 161 5 of 13 decreased with the deposition time, which can be attributed to the precursor consumption over time [42]. The thickness increased as bath temperature increased, until it reached its saturation In Figure 2c, the thickness and growth rate increased as CuSO .5H O concentration 4 2 point at a bath temperature of 70 °C. Thereafter, when the temperature reached 80 °C, the increased. This is attributed to the competition of heterogeneous nucleation on the substrate thickness started to decrease due to the desorption and/or dissolution of preformed Cu2S and homogeneous nucleation in the solution, which could modify the growth of thin films [44], as shown in Figure 2b. While the growth rate of deposited films gradually increased and increase the thickness [45]. with the bath temperature due to additional generations of colloidal ions [42], as shown The thickness of Cu S thin films almost linearly increased from 185 to 297 nm as the in Figure 2b. pH value of bath solution increased from 8 to 11, as shown in Figure 2d. In Figure 2c, the thickness This increase and g in thickness rowth ra iste incr attributed eased to as C the incr uSO ement 4.5H2of O concentration OH ions concentration in the solution that pushes the reaction of thiourea hydrolysis forward, causing the high increased. This is attributed to the competition of heterogeneous nucleation on the sub- generation of sulfide ions [46], as shown in Figure 2d. In addition, the growth rate increased strate and homogeneous nucleation in the solution, which could modify the growth of as the pH value increased (Figure 2d). thin films and increase the thickness [45]. Figure 2e shows the effect of SC (NH ) concentration on the thickness of deposited 2 2 The thickness of Cu2S thin films almost linearly increased from 185 to 297 nm as the thin films. pH value of bath solution increased from 8 to 11, as shown in Figure 2d. Notably, the thickness of deposited thin films decreased as the SC (NH ) concentration 2 2 2+ - 2 This increase in thickness is attributed to the increment of OH ions concentration in increased due to the limited concentration of Cu and S ions that was released in the solution [39]. The growth rate of Cu S thin films can be seen in Figure 2e. the solution that pushes the reaction of thiourea hydr 2 olysis forward, causing the high gen- eration of sulfide ions [46], as shown in Figure 2d. In addition, the growth rate increased 3.2. Physical Properties as the pH value increased (Figure 2d). 3.2.1. Morphological Properties Figure 2e shows the effect of SC (NH2)2 concentration on the thickness of deposited Figure 3 demonstrated the SEM images of Cu S thin films at various deposition thin films. parameters (deposition time, bath temperature, pH value, CuSO .5H O concentration, and 4 2 Notably, the thickness of deposited thin films decreased as the SC (NH2)2 concentra- SC (NH ) concentration). 2 2 2+ 2− tion increased due to the li Figuremi 3ate shows d conc that entr the atio FESEM n of Cu images andof Sthin ions t films hat at w deposition as released time in of 6 min were uniform, well covered, and homogenous without cracks. In addition, the increase in the solution [39]. The growth rate of Cu2S thin films can be seen in Figure 2e. deposition time leads to the increase in aggregations. Moreover, few cracks were found in both deposited thin films at 8 and 10 min. The number and size of the grain increased 3.2. Physical Properties from 37.25 to 99.57 nm as deposition time increased from 6 to 10 min. Figure 3a indicates 3.2.1. Morphological Properties that additional nucleation sites have formed [47]. The increase in bath temperature leads to the increase in compactness, homogeneity, and uniformity of films, as shown in Figure 3b. Figure 3 demonstrated the SEM images of Cu2S thin films at various deposition pa- The surface of films deposited at 40 C was not completely covered and had few cracks. rameters (deposition time, bath temperature, pH value, CuSO4.5H2O concentration, and However, the surface of films deposited at 70 and 90 C was homogeneous, compact, and SC (NH2)2 concentration). relatively completely covered and uniform. Figure 3. Cont. Photonics 2022, 9, x FOR PEER REVIEW 6 of 13 Photonics 2022, 9, 161 6 of 13 Figure 3. Cont. Photonics 2022, 9, x FOR PEER REVIEW 7 of 13 Photonics 2022, 9, 161 7 of 13 Figure 3. FESEM images of Cu S thin films at different deposition parameters: (a) Deposition time Figure 3. FESEM images of Cu2S thin films at different deposition parameters: (a) Deposition time (t ), (b) bath temperature (T ), (c) SC (NH ) concentration, (d) CuSO .5H O concentration, and d b 2 2 4 2 (td), (b) bath temperature (Tb), (c) SC (NH2)2 concentration, (d) CuSO4.5H2O concentration, and (e) (e) pH value. pH value. The deposited films at 0.6 and 1 M of SC (NH ) concentration are well covered, 2 2 uniform, and homogeneous with few small cracks on the surface deposited at 1 M of SC Figure 3a shows that the FESEM images of thin films at deposition time of 6 min were (NH ) concentration. The quality of thin films decreased as the SC (NH ) concentration 2 2 2 2 uniform, well covered, and homogenous without cracks. In addition, the increase in dep- increased to 1.2 M. Agglomerations and cracks existed on the film’s surface, as shown in osition time leads to the increase in aggregations. Moreover, few cracks were found in Figure 3c. Deposited thin films at various CuSO .5H O molar concentrations were well 4 2 both deposited thin films at 8 and 10 min. The number and size of the grain increased covered, uniform, homogeneous, and few small cracks existed on the surface of the films from 37.25 to 99.57 nm as deposition time increased from 6 to 10 min. Figure 3a indicates deposited at 0.3 and 0.5 M, as shown in Figure 3d. However, the average particle sizes were that additional nucleation sites have formed [47]. The increase in bath temperature leads 36.87, 55.03, and 43.98 nm for the deposited thin films at 0.1, 0.3, and 0.5 M of CuSO .5H O 4 2 molar concentration, respectively. The increase in CuSO .5H O molar concentration leads to the increase in compactness, homogeneity, and uniformity of films, as shown in Figure 4 2 to the increase in the number of clusters and agglomerated nanoparticles. The FESEM 3b. The surface of films deposited at 40 °C was not completely covered and had few cracks. images of deposited films at different pH values are shown in Figure 3e. The films were However, the surface of films deposited at 70 and 90 °C was homogeneous, compact, and homogeneous and well covered. However, the agglomerations and clusters increased as relatively completely covered and uniform. the pH value increased. The average particle sizes decreased from 65.72 to 33.28 nm as The deposited films at 0.6 and 1 M of SC (NH2)2 concentration are well covered, uni- the pH value increased from 8 to 11. The increase in the pH value leads to noticeable form, and homogeneous with few small cracks on the surface deposited at 1 M of SC changes in the morphology of deposited thin films due to the increment of OH ions. This (NH2)2 concentrat leads ion. The the reaction quality of forwar th d, inincr films dec ease there attractive ased asfor thce e S and C (NH decrease 2)2 con repculsive entration force, which allow for the growth of oriented attachment (OA). When the particles are perfectly aligned, increased to 1.2 M. Agglomerations and cracks existed on the film’s surface, as shown in the common boundary is eliminated, resulting in the formation of larger well-defined Figure 3c. Deposited thin films at various CuSO4.5H2O molar concentrations were well morphology particles [48,49]. covered, uniform, homogeneous, and few small cracks existed on the surface of the films deposited at 0.3 and 0.5 M, as shown in Figure 3d. However, the average particle sizes 3.2.2. Optical Properties were 36.87, 55.03, and 43.98 nm for the deposited thin films at 0.1, 0.3, and 0.5 M of Figure 4 shows the transmission of Cu S thin films deposited at different deposition CuSO4.5H2O molar concentration, respectively. The increase in CuSO4.5H2O molar con- parameters (deposition time, bath temperature, CuSO .5H O concentration, SC (NH ) 4 2 2 2 centration leads tconcentration, o the increaseand in th pH e nu value). mber of Notably clust , the ers transmittance and agglomerat decr ed eased nano aspart these icle parameters s. increased. However, the transmission increased as the SC (NH ) concentration increased. 2 2 The FESEM images of deposited films at different pH values are shown in Figure 3e. The In addition, the transmittance of deposited thin films in the UV–Vis region rapidly increased films were homogeneous and well covered. However, the agglomerations and clusters until it reached its maximum peak at ( = 600–650 nm). Thereafter, a slight decrease was increased as the pH value increased. The average particle sizes decreased from 65.72 to found in the visible part before the rapid decrease in the NIR region, except for thin films 33.28 nm as the pH value increased from 8 to 11. The increase in the pH value leads to deposited at (TB = 40 C ) and (pH = 8), which have high transmittance of 94% and 86.2% noticeable changes in the morphology of deposited thin films due to the increment of OH at the NIR region. This decrease in transmission is due to the increase in the thickness of ions. This leads the reaction forward, increase the attractive force and decrease repulsive deposited Cu S thin films [23]. force, which allow for the growth of oriented attachment (OA). When the particles are perfectly aligned, the common boundary is eliminated, resulting in the formation of larger well-defined morphology particles [48,49]. 3.2.2. Optical Properties Figure 4 shows the transmission of Cu2S thin films deposited at different deposition parameters (deposition time, bath temperature, CuSO4.5H2O concentration, SC (NH2)2 concentration, and pH value). Notably, the transmittance decreased as these parameters Photonics 2022, 9, x FOR PEER REVIEW 8 of 13 increased. However, the transmission increased as the SC (NH2)2 concentration increased. In addition, the transmittance of deposited thin films in the UV–Vis region rapidly in- creased until it reached its maximum peak at (𝜆 = 600–650 nm). Thereafter, a slight de- crease was found in the visible part before the rapid decrease in the NIR region, except for thin films deposited at (TB = 40 °C°) and (pH = 8), which have high transmittance of 94% and 86.2% at the NIR region. This decrease in transmission is due to the increase in Photonics 2022, 9, 161 8 of 13 the thickness of deposited Cu2S thin films [23]. 100 85 2 min 0.1 M (a) (b) 90 (c) 4 min 0.3 M 6 min 0.5 M 8 min 40 C 10 min 50 C 60 C 70 C 70 80 C 90 C 55 50 50 45 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) λ (nm) Tu= 0.4 M (d) 85 (e) 90 Tu= 0.6 M Tu= 0.8 M Tu= 1 M 75 Tu= 1.2 M pH=8 pH=9 pH=10 pH=11 65 40 30 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) Figure 4. Transmission spectrums of Cu S thin films as a function of different deposition parameters: Figure 4. Transmission spectrums of Cu2S thin films as a function of different deposition parame- (ters: ( a) Deposition a) Deposition time, ( time, (b) bath b) b temperatur ath temperature, ( e, (c) CuSO c) CuSO .5H4.5 OH concentration, 2O concentratio (d n, ( ) pH d) pH value, value and , and ( (e) SC e) 4 2 SC (NH2)2 concentration. (NH ) concentration. 2 2 Figure 5 shows the absorbance spectra of Cu S thin films deposited at different deposi- Figure 5 shows the absorbance spectra of Cu2S thin films deposited at different dep- tion parameters. The absorbance of deposited thin films increased as deposition time, bath osition parameters. The absorbance of deposited thin films increased as deposition time, temperature, CuSO .5H O concentration, and pH value increased. In contrast, it decreased bath temperature, CuSO 4 2 4.5H2O concentration, and pH value increased. In contrast, it de- when the SC (NH ) concentration increased. In general, all of the deposited thin films creased when the SC (NH2)2 concentration increased. In general, all of the deposited thin 2 2 have high absorbance at UV and NIR regions, except for thin films deposited at (T = 40 C) films have high absorbance at UV and NIR regions, except for thin films deposited at (Tb and (pH = 8), which had low absorbance at the NIR region. In addition, thin films had low = 40 °C) and (pH = 8), which had low absorbance at the NIR region. In addition, thin films absorbance at the VIS region. Low transparency in UV region provides thin films with a had low absorbance at the VIS region. Low transparency in UV region provides thin films coating quality for use in eyeglasses to protect the eye from UV radiation [23]. with a coating quality for use in eyeglasses to protect the eye from UV radiation [23]. The high absorption coefficient indicates that deposited Cu S thin films with various The high absorption coefficient indicates that deposited Cu 2 2S thin films with various deposition parameters have a direct energy bandgap [19]. This was calculated from the deposition parameters have a direct energy bandgap [19]. This was calculated from the extrapolation straight-line portion of the curve ( h) versus photon energy, as shown in Figure 6. T ( % ) T (% ) T % T (% ) T ( % ) Photonics 2022, 9, x FOR PEER REVIEW 9 of 13 extrapolation straight-line portion of the curve (αhυ) versus photon energy, as shown in Figure 6. In Figure 6a and Table 2, the energy gap of deposited thin films decreased as deposi- tion time increased due to the increase in thin films’ thickness [19] and crystallinity in the quantum size effect [43]. The decrease in energy gap with the increment of bath tempera- ture is due to the increase in crystallite size, as shown in Figure 6b. This leads to the in- creased absorption and causes a shift in optical absorption edge towards longer wave- lengths, and then the bandgap decreases [50]. In Figure 6c, the energy gap decreased lin- early with the increase in CuSO4.5H2O concentration due to the increased grain size and low sulfur ions [24]. The obtained energy gap was slightly larger than the previous report by Ismail R.A. et al., which was prepared by a different precursor (CuCl2) [24]. The energy gap of thin films prepared at different pH values decreased as the pH value of bath solu- tion increased due to the increase in thickness, as shown in Table 2. The obtained value is almost the same as the value obtained by Ahmed H.S. et al. [51], which was prepared by Cu2S thin films of a different precursor (CuCl2.2H2O). The effect of thickness on the energy gap could increase due to the height of crystalline films and large density of dislocation [21]. The energy gap increased as the SC (NH2)2 concentration increased due to the higher concentration of S ions, which leads to a decrease in the thickness of thin films. The ob- Photonics 2022, 9, 161 9 of 13 tained values are slightly different from the obtained values by Muhammed A.M. et al. [52], which was prepared by Cu2S thin films of a different precursor (CuCl2). 0.30 (c) (a) 0.30 (b) 0.32 0.25 0.25 0.28 0.20 0.20 0.24 40 C 0.15 0.20 0.15 50 C 60 C 0.16 0.10 70 C 0.10 2 min 80 C 4 min 0.12 90 C 0.05 6 min 0.1 M 8 min 0.3 M 0.05 10 min 0.08 0.5 M 0.00 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) λ (nm) 0.35 0.50 (d) Tu= 0.4 M (e) 0.45 Tu= 0.6 M 0.30 Tu= 0.8 M 0.40 Tu= 1 M Tu= 1.2 M 0.25 0.35 0.30 0.20 0.25 0.15 pH=8 0.20 pH=9 pH=10 0.15 0.10 pH=11 0.10 0.05 0.05 0.00 400 500 600 700 800 900 1000 1100 1200 400 500 600 700 800 900 1000 1100 1200 λ (nm) λ (nm) Figure 5. Absorbance of Cu2S thin films as a function of different deposition parameters: (a) Depo- Figure 5. Absorbance of Cu S thin films as a function of different deposition parameters: (a) Depo- sition time, (b) bath temperature, (c) CuSO4.5H2O concentration, (d) pH value, and (e) SC (NH2)2 sition time, (b) bath temperature, (c) CuSO .5H O concentration, (d) pH value, and (e) SC (NH ) 4 2 2 2 concentration. concentration. In Figure 6a and Table 2, the energy gap of deposited thin films decreased as deposition time increased due to the increase in thin films’ thickness [19] and crystallinity in the quantum size effect [43]. The decrease in energy gap with the increment of bath temperature is due to the increase in crystallite size, as shown in Figure 6b. This leads to the increased absorption and causes a shift in optical absorption edge towards longer wavelengths, and then the bandgap decreases [50]. In Figure 6c, the energy gap decreased linearly with the increase in CuSO .5H O concentration due to the increased grain size and low sulfur 4 2 ions [24]. The obtained energy gap was slightly larger than the previous report by Ismail R.A. et al., which was prepared by a different precursor (CuCl ) [24]. The energy gap of thin films prepared at different pH values decreased as the pH value of bath solution increased due to the increase in thickness, as shown in Table 2. The obtained value is almost the same as the value obtained by Ahmed H.S. et al. [51], which was prepared by Cu S thin films of a different precursor (CuCl .2H O). The effect of thickness on the energy gap could increase 2 2 due to the height of crystalline films and large density of dislocation [21]. The energy gap increased as the SC (NH ) concentration increased due to the higher concentration 2 2 of S ions, which leads to a decrease in the thickness of thin films. The obtained values are slightly different from the obtained values by Muhammed A.M. et al. [52], which was prepared by Cu S thin films of a different precursor (CuCl ). 2 2 A (a .u ) A (a .u ) A (a .u ) A (a .u ) A (a .u ) Photonics 2022, 9, 161 10 of 13 Photonics 2022, 9, x FOR PEER REVIEW 10 of 13 13 14 8.0x10 1.0x10 14 o 1.8x10 (b) (a) t = 40 C t =6 min. 14 t = 60 C 1.6x10 6.0x10 o t = 70 C 1.4x10 8.0x10 1.2x10 6.0x10 13 4.0x10 1.0x10 8.0x10 2.0x10 6.0x10 13 6.0x10 13 4.0x10 4.0x10 0.0 2.0x10 1.0 1.5 2.0 2.5 3.0 3.5 4.0x10 0.0 hν (eV) 1.0 1.5 2.0 2.5 3.0 3.5 t =2 min. hν(nm) t =4 min. 13 o 2.0x10 t = 50 C t =8 min. 2.0x10 t = 80 C t =10 min. o t = 90 C 0.0 0.0 1.0 1.5 2.0 2.5 3.0 3.5 1.0 1.5 2.0 2.5 3.0 3.5 hν (eV) hν (eV) 8.0x10 (c) CuSO .5H O=0.1 M 8.00E+13 4 2 pH=11 (d) CuSO .5H O= 0.3 M 8.0×10 4 2 CuSO .5H O=0.5 M 6.00E+13 4 2 6.0x10 4.00E+13 6.0×10 2.00E+13 4.0x10 0.00E+00 13 1.0 1.5 2.0 2.5 3.0 3.5 4.0×10 hυ(eV) pH=8 2.0x10 pH=9 2.0×10 pH= 10 0.0 0.0 1.0 1.5 2.0 2.5 3.0 3.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 hυ(eV) hν(eV) (e) Tu=0.8M 1.6×10 6.00E+13 4.00E+13 1.2×10 2.00E+13 8.0×10 0.00E+00 1.0 1.5 2.0 2.5 3.0 3.5 hν (nm) Tu=0.4 M 4.0×10 Tu=0.6 M Tu=0.1 M Tu=1.2 M 0.0 1.01.5 2.02.5 3.03.5 hν (nm) Figure 6. Variation of (αhʋ) with (hʋ) with deposition parameters): (a) Deposition time, (b) bath Figure 6. Variation of (ahu) with (hu) with deposition parameters): (a) Deposition time, (b) bath temperature, (c) CuSO4.5H2O concentration, (d) pH value, and (e) SC (NH2)2 concentration. temperature, (c) CuSO .5H O concentration, (d) pH value, and (e) SC (NH ) concentration. 4 2 2 2 Table 2. Bandgap variation with different deposition parameters. Bath No. 1 2 3 4 5 66 Deposition time (min) 2 4 6 8 10 - Eg (eV) 3.01 2.92 2.9 2.86 2.95 - Bath temprature (°C) 40 50 60 70 80 90 Eg (eV) 3.04 2.92 2.9 2.86 2.72 2.63 2 -1 2 2 -1 2 (αhν) (eVm ) ( α h ν ) ( e V m ) 2 -1 2 ( α h ν ) ( e V m ) 2 -1 2 (αhν) (eVm ) 2 -1 2 (αhν) (eVm ) 2 -1 2 2 -1 2 (α h ν ) (e V m ) (αhυ) (eVm ) 2 -1 2 (αhυ) (eVm ) 2 -1 2 (α h ν ) (e V m ) Photonics 2022, 9, 161 11 of 13 Table 2. Bandgap variation with different deposition parameters. Bath No. 1 2 3 4 5 6 Deposition time (min) 2 4 6 8 10 - Eg (eV) 3.01 2.92 2.9 2.86 2.95 - Bath temprature ( C) 40 50 60 70 80 90 Eg (eV) 3.04 2.92 2.9 2.86 2.72 2.63 CuSO .5H O concentration (M) 0.1 0.3 0.5 - - - 4 2 Eg (eV) 3.1 2.86 2.65 pH Value 8 9 10 11 - - Eg (eV) 3.14 2.9 2.86 2.75 SC (NH ) concentration (M) 0.4 0.6 0.8 1 1.2 - 2 2 Eg (eV) 2.79 2.81 2.83 2.86 3.09 - 4. Conclusions Copper sulfide thin films were successfully deposited by the chemical bath deposition technique. The obtained results indicate that the thickness of deposited thin films strongly depends on the deposition parameters. It increased from 185 to 281 nm as deposition time increased, and from 183 to 291 nm as bath temperature increased. In addition, the thickness increased from 257 to 303 nm as the precursor concentration increased and from 185 to 297 nm as the pH value increased. However, the thickness decreased from 299 to 234 nm with the increasing thiourea concentration. The saturation was found only in the deposition time and bath temperature. The morphology of deposited thin films noticeably changed with the deposition times. The grains’ number and size increased with the deposition time. Moreover, the quality improved with the increasing deposition time and decreased at 10 min of deposition time. The deposited Cu S thin films at various CuSO .5H O molar 2 4 2 concentrations were compact, uniform, and homogeneous with different particle sizes and clusters, and the agglomerated nanoparticles increased as the CuSO .5H O molar concen- 4 2 tration increased. Lower concentrations of thiourea provided compact and homogeneous thin films. The average particle sizes increased as the thiourea concentration increased. The morphology of deposited thin films improved and agglomerated nanoparticles increased as bath temperature increased. The morphology of deposited thin films remarkably changed with the pH value. Deposited thin films at higher pH values were covered almost com- pletely with deposited nanoparticles. The optical measurements demonstrate that almost all of the deposited thin films have high transmission and low absorption in the visible region, while transmittance decreased and absorbance increased in the UV and NIR regions. The energy bandgap of thin films decreased with the increasing deposition time (3.01–2.95 eV), bath temperature (3.04–2.63 eV), CuSO .5H O concentration (3.1–2.6 eV), and pH value 4 2 (3.14–2.75 eV), except for thiourea concentration, where it decreased with the increase in thiourea concentration (2.79–3.09 eV). Furthermore, all of the deposited thin films have high energy bandgaps and are slightly larger than those obtained by other works. Author Contributions: Conceptualization, R.Y.M.; methodology, R.Y.M. and H.S.A.; software, H.S.A.; validation, R.Y.M. and H.S.A.; formal analysis, R.Y.M. and H.S.A.; investigation, R.Y.M. and H.S.A.; resources, R.Y.M.; data curation, R.Y.M. and H.S.A.; writing—original draft preparation, H.S.A.; writing—review and editing, R.Y.M.; visualization, R.Y.M. and H.S.A.; supervision, R.Y.M.; project administration, R.Y.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors are thankful to the University of Duhok for their full support. Photonics 2022, 9, 161 12 of 13 Conflicts of Interest: The authors declare no conflict of interest. References 1. Cruz, J.S.; Hernández, S.M.; Coronel, J. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Mar 6, 2022

Keywords: Cu2S thin film; chemical bath deposition; energy bandgap; thin films; optical properties; thickness measurements; deposition parameters

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