Synthesis, Crystallography, Microstructure, Crystal Defects, Optical and Optoelectronic Properties of ZnO:CeO2 Mixed Oxide Thin Films

We report the synthesis and characterization of pure ZnO, pure CeO2, and ZnO:CeO2 mixed oxide thin films dip-coated on glass substrates using a sol-gel technique. The structural properties of as-prepared thin film are investigated using the XRD technique. In particular, pure ZnO thin film is found to exhibit a hexagonal structure, while pure CeO2 thin film is found to exhibit a fluorite cubic structure. The diffraction patterns also show the formation of mixed oxide materials containing well-dispersed phases of semi-crystalline nature from both constituent oxides. Furthermore, optical properties of thin films are investigated by performing UV–Vis spectrophotometer measurements. In the visible region, transmittance of all investigated thin films attains values as high as 85%. Moreover, refractive index of pure ZnO film was found to exhibit values ranging between 1.57 and 1.85 while for CeO2 thin film, it exhibits values ranging between 1.73 and 2.25 as the wavelength of incident light decreases from 700 nm to 400 nm. Remarkably, refractive index of ZnO:CeO2 mixed oxide-thin films are tuned by controlling the concentration of CeO2 properly. Mixed oxide-thin films of controllable refractive indices constitute an important class of smart functional materials. We have also investigated the optoelectronic and dispersion properties of ZnO:CeO2 mixed oxide-thin films by employing well-established classical models. The melodramatic boost of optical and optoelectronic properties of ZnO:CeO2 mixed oxide thin films establish a strong ground to modify these properties in a skillful manner enabling their use as key potential candidates for the fabrication of scaled optoelectronic devices and thin film transistors.


Introduction
Zinc oxide (ZnO) is an essential and promising material for several modern technological applications owing to its high transmittance of visible light, wide direct band gap of 3.37 eV,

Preparation Pure ZnO and Pure CeO 2 Solution
The pure ZnO/pure CeO 2 solution was prepared by dissolving 4.38 g of zinc acetate dehydrated (Zn (CH 3 CO 2 ) 2 2H 2 O) and 2.21 g of Ce (III) nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O) separately in 50 mL 2-methoxyethanol. ZnO solution was then mixed thoroughly on a magnetic stirrer for 50 min at room temperature, while CeO 2 solution was then mixed thoroughly on a magnetic stirrer for 60 min at 65 • C. Then, the ethanolamine of 1.7 mL and acetylacetone of 1.5 mL as a stabilizer was added to the ZnO and CeO 2 solutions, respectively, in terms of drop by drop while stirring the solutions until they become transparent. The resulting mixtures was stirred for 40 min to get homogeneous solutions. The solutions were filtered by paper-filter with 0.45 µm in dimension [1,2,42].

Deposition of ZnO:CeO 2 Mixed Oxides Thin Films
To prepare the desired mixed-oxides thin films, pure ZnO, pure CeO 2 and mixed ZnO:CeO 2 are deposited for 2 h on pre-cleaned glass substrates by dip coating technique. The deposited thin films are then air dried at 110 • C in an oven at atmospheric pressure for 15 min in order to evaporate the solvents and organic residues. In the final step of the synthesis, we anneal the deposited dried pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films in air at 500 • C for 2 h. The procedure followed ensures that the obtained thin films are structurally optimized [1,2].

Characterizations of ZnO:CeO 2 Mixed Oxides Thin Films
Crystallinity of ZnO:CeO 2 mixed oxides thin films is investigated by Powder XRD (Malvern Panalytical Ltd., Malvern, UK) (220-230 VAC 50/60 Hz 40 A) using Cu Kα 1 ray with a wavelength of 0.1540598 nm at room temperature from angles of incident photon on the surface ranges from 10 • to 70 • with a step of 0.02 • with an energy resolution of 20%. Angular resolution: 0.026 • FWHM on LaB 6, and Angular reproducibility < 0.0002 • . Maximum angular speed of 15 deg./s. The optical transmittance and reflectance spectra are measured using UV-Vis spectrophotometer (U-3900H) with the total internal integrating sphere at room temperature. Figure 1 shows the XRD patterns of pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxide thin films annealed at 500 • C for 2 h. Pure ZnO thin films exhibit peaks at Bragg's angles of 32.13 • , 34.75 • and 36.58 • corresponding to (100), (002) and (101) crystallographic planes, respectively. In addition, XRD patterns confirm that ZnO thin film is polycrystalline and exhibits wurtzite hexagonal structure [3,43]. In addition, the main peaks of pure CeO 2 thin film are observed at Bragg's angles of 28.89 • , 33.54 • and 47.88 • corresponding to CeO 2 crystallographic planes indexed by Miller indices of (111), (200) and (220), respectively. Comparison of observed and standard 'd' (hkl) planes as matched with JCPDS cards no. 034-394 indicates that CeO 2 thin film is polycrystalline and exhibit cubic phase with fluorite structure [44]. Furthermore, XRD patterns indicate that pure CeO 2 and ZnO thin films have a higher degree of crystallinity than that of ZnO:CeO 2 mixed thin films. Careful examination of XRD patterns reveal that ZnO:CeO 2 mixed thin films encompass distinct crystalline phases of ZnO and CeO 2 components [45]. XRD patterns of ZnO:CeO 2 mixed thin films containing 25%-75% of CeO 2 NPs reveal that the intensity of ZnO reflections at (100), (002) and (101) decrease quite rapidly while that of the CeO 2 (111) peak increases. This could be understood in terms of the amount of scattered X-ray from Ce 4+ and Zn 2+ ions. The diffraction factor f of Ce 4+ ions is higher than that of Zn 2+ ions. Since f of an atom depends mainly on sin θ/λ where θ is the scattering angle and λ is the wavelength of X-ray and atomic number Z, it is challenging to identify lighter atoms in presence of heavier atoms. The diffraction pattern of ZnO is much weaker than that of CeO 2 [44]. Obviously, the individual peak positions of CeO 2 and ZnO remain unchanged in the XRD pattern of ZnO:CeO 2 composite oxides. This provides substantial evidence that XRD pattern of ZnO:CeO 2 consists mainly of distinct phases of ceria and ZnO. The existence of weaker peaks indicates that dissolving of small amount of ZnO in ceria and vice versa is possible for a certain extent. Mixing ZnO hexagonal lattice with CeO 2 fluorite leads to a drastic change of diffraction pattern. Thus, lattice constant of the composite changes significantly. The lattice constant 'a' for the cubic phase structure and 'a & c' for the hexagonal phase structure are evaluated as:

X-Ray Diffraction (XRD) Analysis
1 where d is the inter-planar spacing and can be determined using Bragg's law (λ = 2d hkl sin θ hkl ). The lattice constants of pure ZnO and CeO 2 thin films are presented in Table 1. The lattice constants (a and c) of pure wurtzite ZnO thin film are calculated to be 3.216 Å and 5.161 Å, respectively, in a good agreement with those reported in [46]. The lattice constant a of pure CeO 2 thin film is found to be 5.350 Å is in a good agreement with that reported in [44].
Photonics 2020, 7, x FOR PEER REVIEW 4 of 20 of the CeO2 (111) peak increases. This could be understood in terms of the amount of scattered X-ray from Ce 4+ and Zn 2+ ions. The diffraction factor f of Ce 4+ ions is higher than that of Zn 2+ ions. Since f of an atom depends mainly on sin / where is the scattering angle and is the wavelength of X-ray and atomic number Z, it is challenging to identify lighter atoms in presence of heavier atoms. The diffraction pattern of ZnO is much weaker than that of CeO2 [44]. Obviously, the individual peak positions of CeO2 and ZnO remain unchanged in the XRD pattern of ZnO:CeO2 composite oxides. This provides substantial evidence that XRD pattern of ZnO:CeO2 consists mainly of distinct phases of ceria and ZnO. The existence of weaker peaks indicates that dissolving of small amount of ZnO in ceria and vice versa is possible for a certain extent. Mixing ZnO hexagonal lattice with CeO2 fluorite leads to a drastic change of diffraction pattern. Thus, lattice constant of the composite changes significantly. The lattice constant ' ' for the cubic phase structure and ' & ' for the hexagonal phase structure are evaluated as: where is the inter-planar spacing and can be determined using Bragg's law ( = 2 sin ). The lattice constants of pure ZnO and CeO2 thin films are presented in Table 1. The lattice constants ( and ) of pure wurtzite ZnO thin film are calculated to be 3.216 Å and 5.161 Å, respectively, in a good agreement with those reported in [46]. The lattice constant of pure CeO2 thin film is found to be 5.350 Å is in a good agreement with that reported in [44].   To elucidate the elastic properties of synthesized mixed oxide thin films, crystallite size (D) and microstrain (ε) originated from the lattice misfitting and mismatching are deduced from the peak width and the shift of the 2θ peak position [47]. Fundamentally, average D can be computed from the Debye Scherrer's formula, D = kλ/β cos θ [48,49] where λ is the wavelength of the X-ray (λ = 0.154184 nm), β (in radians) is the full width at half maximum (FWHM), θ is Bragg's angle and k = 0.94 is the Scherrer constant [50,51]. The average microstrain ε = β cot θ/4 [48,52]. The estimated values of D and ε of ZnO:CeO 2 mixed oxide thin films are plotted as functions of CeO 2 concentrations as shown in Figure 2. The value of D of pure ZnO and CeO 2 thin films was found to be 18.93 nm and 9.26 nm, respectively. However, D of ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 were found to be 12.09 nm, 11.58 nm and 11.50 nm, respectively. Obviously, Figure 2 shows an inverse relationship between ε and D which could be attributed to the decrease in the volume occupied by constituents inside the combined crystalline structure. In addition, as the volume of the unit cells increases, the total surface area is increased causing a shift in planes positions [53]. To elucidate the elastic properties of synthesized mixed oxide thin films, crystallite size ( ) and microstrain ( ) originated from the lattice misfitting and mismatching are deduced from the peak width and the shift of the 2θ peak position [47]. Fundamentally, average can be computed from the Debye Scherrer's formula, = cos ⁄ [48,49] where is the wavelength of the X-ray ( = 0.154184 nm), (in radians) is the full width at half maximum (FWHM), is Bragg's angle and = 0.94 is the Scherrer constant [50,51]. The average microstrain = cot 4 ⁄ [48,52]. The estimated values of and 〈 〉 of ZnO:CeO2 mixed oxide thin films are plotted as functions of CeO2 concentrations as shown in Figure 2. The value of D of pure ZnO and CeO2 thin films was found to be 18.93 nm and 9.26 nm, respectively. However, D of ZnO:CeO2 mixed oxide thin films containing 25%, 50% and 75% of CeO2 were found to be 12.09 nm, 11.58 nm and 11.50 nm, respectively. Obviously, Figure 2 shows an inverse relationship between and D which could be attributed to the decrease in the volume occupied by constituents inside the combined crystalline structure. In addition, as the volume of the unit cells increases, the total surface area is increased causing a shift in planes positions [53]. Scherrer formula considers only the effect of crystallite size on the XRD peak broadening. However, it does not provide details of the intrinsic strain generated in the nanocomposites. It is induced by the point defect, grain boundary, triple junction and stacking faults [54,55]. There are many methods such as Williamsons Hall method (WH), Warren-Averbach method etc., which considers the effect of the strain induced by XRD peak broadening and can be used for the calculation of the intrinsic strain along with the particle size. Among these methods, Williamson-Hall (W-H) method which is considered to be easy, elegant and simple [56]. According to WH method, the line broadening of X-ray diffraction peak occurs due to the size and microstrain of the nanocrystals and the total broadening can be written as: Scherrer formula considers only the effect of crystallite size on the XRD peak broadening. However, it does not provide details of the intrinsic strain generated in the nanocomposites. It is induced by the point defect, grain boundary, triple junction and stacking faults [54,55]. There are many methods such as Williamsons Hall method (WH), Warren-Averbach method etc., which considers the effect of the strain induced by XRD peak broadening and can be used for the calculation of the intrinsic strain along with the particle size. Among these methods, Williamson-Hall (W-H) method which is considered to be easy, elegant and simple [56]. According to WH method, the line broadening of X-ray diffraction peak occurs due to the size and microstrain of the nanocrystals and the total broadening can be written as: In the present work, average particle size and microstrain have been calculated using modified WH equation implementing Uniform deformation model (UDM). The method deliberately uses uniform strain all the way through the crystallographic direction that is induced in the nanocrystals due to crystal imperfections. In other words, UDM considers strain, which is isotropic in nature [57]. This intrinsic strain essentially affects the physical broadening of the XRD profile inducing peak broadening: Photonics 2020, 7, 112 6 of 19 So, the total broadening due to strain and size in a particular peak indexed by hkl Miller indices, can be expressed as: where, β hkl is the full width at half of the maximum intensity for different crystallographic diffraction planes. Consequently, WH equation is modified by UDM can be given by: By plotting β hkl cos θ along Y-axis as a function of 4 sin θ on the X-axis corresponding to each diffraction peak, we can calculate the values of crystallite size (D) and microstrain (ε). The estimated values of D and ε of ZnO:CeO 2 mixed oxide thin films are plotted as functions of CeO 2 concentrations as shown in Figure 3. The peaks of ZnO; (100), (002), (101) and the peaks of CeO 2 ; (111), (200), (220) and (311) are used in equation 6 to calculate D and ε. The value of D of pure ZnO and CeO 2 thin films is estimated to be 28.05 nm and 8.10 nm, respectively. However, D of ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 were found to be 25.89 nm, 19.85 nm and 12.12 nm, respectively. It can be seen that the calculated crystallite size of the ZnO:CeO 2 mixed oxide thin films does not vary analytically with the Ce concentration. This could be attributed to the lattice disorder, produced in the powder samples at higher Ce concentrations due to the difference in the ionic radii of Zn 2+ and Ce 4+ species. Obviously, Figure 2 shows an inverse relationship between ε and D which could be attributed to the decrease in the volume occupied by the constituents inside the combined crystalline structure. In addition, as the volume of the unit cells increases, the total surface area is increased causing a shift in planes positions [53].
WH equation implementing Uniform deformation model (UDM). The method deliberately uses uniform strain all the way through the crystallographic direction that is induced in the nanocrystals due to crystal imperfections. In other words, UDM considers strain, which is isotropic in nature [57]. This intrinsic strain essentially affects the physical broadening of the XRD profile inducing peak broadening: So, the total broadening due to strain and size in a particular peak indexed by hkl Miller indices, can be expressed as: where, is the full width at half of the maximum intensity for different crystallographic diffraction planes. Consequently, WH equation is modified by UDM can be given by: By plotting cos along Y-axis as a function of 4 sin on the X-axis corresponding to each diffraction peak, we can calculate the values of crystallite size (D) and microstrain (ε). The estimated values of and 〈 〉 of ZnO:CeO2 mixed oxide thin films are plotted as functions of CeO2 concentrations as shown in Figure 3. The peaks of ZnO; (100), (002), (101) and the peaks of CeO2; (111), (200), (220) and (311) are used in equation 6 to calculate D and ε. The value of D of pure ZnO and CeO2 thin films is estimated to be 28.05 nm and 8.10 nm, respectively. However, D of ZnO:CeO2 mixed oxide thin films containing 25%, 50% and 75% of CeO2 were found to be 25.89 nm, 19.85 nm and 12.12 nm, respectively. It can be seen that the calculated crystallite size of the ZnO:CeO2 mixed oxide thin films does not vary analytically with the Ce concentration. This could be attributed to the lattice disorder, produced in the powder samples at higher Ce concentrations due to the difference in the ionic radii of Zn 2+ and Ce 4+ species. Obviously, Figure 2 shows an inverse relationship between and D which could be attributed to the decrease in the volume occupied by the constituents inside the combined crystalline structure. In addition, as the volume of the unit cells increases, the total surface area is increased causing a shift in planes positions [53].  Our results indicate that the values of the crystallite size using Scherrer method exhibit lower values than those obtained using WH method. However, the average microstrain, an inverse trend is obtained. The two methods differ from each other in the sense that WH method considers the effects of the broadening arise from the crystallite size and the microstrain. Whereas Scherrer method takes into account the broadening of the X-ray diffraction lines originates from the crystallite size only. Thus, Scherrer method considers the films as if they are a perfect system. Another major difference between the two methods lies in the fact that, WH method can be applied to analyze XRD patterns composed of of several diffraction lines, while in case of applying Scherrer method; it does not require more than one diffraction line. We conclude that, WH method is a general method and yields more accurate results than Scherrer method does.
To obtain a deeper insight into crystal defects, we investigate the dislocations that may be caused by internal stresses in the sub-lattices. Dislocations are formed at the boundaries between the glided and the un-slipped regions of the strained structure. The number per unit volume and the strength of dislocations determine the strength and ductility of materials. [58]. We calculate the density of dislocations (δ) by using the line profile analysis of XRD (LPA-XRD) patterns and using simple Williamson-Smallman formula given by δ = 1/D 2 , where D is the crystallite size calculated using Williamsons Hall equation [48,[58][59][60][61]. Figure 4a shows the dislocation density of the ZnO:CeO 2 mixed oxides thin films as a function of CeO 2 concentration. The value of δ of pure ZnO thin film is found to be 0.12 × 10 12 lines/cm 2 and that of pure CeO 2 thin film is 1.17 × 10 12 lines/cm 2 . The values of δ of ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 are calculated to be 0.15 × 10 12 , 0.25 × 10 12 and 0.68 × 10 12 lines/cm 2 , respectively. The value of (δ) measures the agglomerations of crystalline clusters. The increasing (δ) with increasing the concentration of CeO 2 in the mixed-oxide composite implies a decreasing of the crystallization and significant increasing of vacancies [58,60]. The increasing in dislocation density can be attributed to the internal strain during film formation in addition to the partial interstitial substitution of the Ce 4+ ion instead of Zn 2+ .

UV-Vis Spectroscopy Measurements of Optical Properties
Transmittance %( ) spectra of pure ZnO, pure CeO2 and ZnO:CeO2 mixed oxides thin films are displayed in Figure 5. Transmittance spectra of pure ZnO thin film show steep rising from about 0% up to 88% as wavelength increases from 300-350 nm. It maintains almost constant value for the 350-700 nm spectral range. Moreover, the transmittance spectra of pure CeO2 thin film demonstrate fast increasing trend about 0% up to 86% for the 300-420 nm spectral range. Similarly, it attains a constant value for 420-700 nm range. As can be clearly seen from Figure 5, transmittance of ZnO:CeO2 mixed oxides thin films have the same behavior of pure CeO2 thin film. However, it exhibits different transmittance values in 420-700 nm range. Interestingly, absorption edge is shifted towards the red region (lower energy) as CeO2 concentration in ZnO:CeO2 mixed oxides thin films is increased. Consequently, a considerable decrease in band gap energy is observed. To elucidate the significant The crystallites density (N) of thin films can be calculated from the estimated values of D and given by N = t/D 3 [62,63] where t is the film thickness found to be 500 nm estimated using SEM micrograph. Figure 4b shows N of ZnO:CeO 2 mixed oxides thin films as a function of CeO 2 concentration. The value of N of pure ZnO thin film is found to be 0.02 × 10 12 cryst./cm 2 while for pure CeO 2 thin film it exhibits a value of 0.63 × 10 12 cryst./cm 2 . We found that N exhibits values of 0.03 × 10 12 , 0.06 × 10 12 and 0.28 × 10 12 cryst./cm 2 for ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 , respectively. Temperature-induced lattice dynamical vibrations and mechanical-induced phase transitions and deformations are measured by the internal lattice stress. The value of the total internal stress (σ) can be obtained from the linear region of stress-strain graph, σ = E * ε where E is the Young's Modulus (E). The value of σ of pure ZnO thin film is found to be 0.72 × 10 12 N/m 2 and that of pure CeO 2 thin film is 1.73 × 10 12 N/m 2 . For ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 , σ takes on values of 0.82 ×10 12 , 2.41 × 10 12 and 2.61 × 10 12 cryst./cm 2 , respectively as displayed in Figure 4c. The strain energy density within thin films (E d ) [J/m 3 ] is closely related to Young's modulus of the film, the volume of the unit cell (V) and the internal micro strain ε . It can be expressed as [59,64]. The calculated value of E d of pure ZnO thin film is 2.35 × 10 12 J/m 3 and that of pure CeO 2 thin film is 13.54 × 10 12 J/m 3 . For ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 , σ is computed to be 3.05 × 10 12 , 26.38 × 10 12 and 30.84 × 10 12 N/m 3 , respectively, as shown in Figure 4d.

UV-Vis Spectroscopy Measurements of Optical Properties
Transmittance T%(λ) spectra of pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films are displayed in Figure 5. Transmittance spectra of pure ZnO thin film show steep rising from about 0% up to 88% as wavelength increases from 300-350 nm. It maintains almost constant value for the 350-700 nm spectral range. Moreover, the transmittance spectra of pure CeO 2 thin film demonstrate fast increasing trend about 0% up to 86% for the 300-420 nm spectral range. Similarly, it attains a constant value for 420-700 nm range. As can be clearly seen from Figure 5, transmittance of ZnO:CeO 2 mixed oxides thin films have the same behavior of pure CeO 2 thin film. However, it exhibits different transmittance values in 420-700 nm range. Interestingly, absorption edge is shifted towards the red region (lower energy) as CeO 2 concentration in ZnO:CeO 2 mixed oxides thin films is increased. Consequently, a considerable decrease in band gap energy is observed. To elucidate the significant reduction in the optical and gap energy, several factors are identified to contribute to this reduction. Among these is the fact that CeO 2 thin films have lower band gap energy than ZnO thin films. In addition, enhanced fs-p interactions between the band electrons of ZnO:CeO 2 mixed oxides and the localized fs electrons of cerium and the strong mismatch between the electronegativities of Ce and Zn in the ZnO: CeO 2 mixed oxide system contribute significantly [65][66][67][68]. Small shoulders in transmittance spectra CeO 2 and ZnO:CeO 2 mixed oxides thin films appear at a wavelength of~350 confirming the excitonic absorption characteristics of those thin films [69]. Reflectance spectra are shown in Figure 6. The reflectance values of pure ZnO and pure CeO2 thin film are found to be ranging from 5%-10% and 7%-15% as the wavelength decreases from 700 nm to 400 nm, respectively. Reflectance spectra of ZnO:CeO2 mixed oxides thin films demonstrate a tuning of %( ) as the concentration of CeO2 in ZnO:CeO2 mixed oxides is increased. This can be attributed to the changes in crystalline size and microstrain of ZnO:CeO2 mixed oxides crystals. In addition, ZnO:CeO2 mixed oxides thin films exhibit absorb photons of wavelength ranging between Reflectance spectra are shown in Figure 6. The reflectance values of pure ZnO and pure CeO 2 thin film are found to be ranging from 5%-10% and 7%-15% as the wavelength decreases from 700 nm to Photonics 2020, 7, 112 9 of 19 400 nm, respectively. Reflectance spectra of ZnO:CeO 2 mixed oxides thin films demonstrate a tuning of R%(λ) as the concentration of CeO 2 in ZnO:CeO 2 mixed oxides is increased. This can be attributed to the changes in crystalline size and microstrain of ZnO:CeO 2 mixed oxides crystals. In addition, ZnO:CeO 2 mixed oxides thin films exhibit absorb photons of wavelength ranging between 280 nm to 330 nm efficiently. Consequently, incorporation of ZnO with CeO 2 enhance UV-filter efficiency of thin films especially for UVB and UVC irradiation. Overall, ZnO:CeO 2 mixed oxides thin films could be potential candidates for high transparency and UV-filter applications. Reflectance spectra are shown in Figure 6. The reflectance values of pure ZnO and pure CeO2 thin film are found to be ranging from 5%-10% and 7%-15% as the wavelength decreases from 700 nm to 400 nm, respectively. Reflectance spectra of ZnO:CeO2 mixed oxides thin films demonstrate a tuning of %( ) as the concentration of CeO2 in ZnO:CeO2 mixed oxides is increased. This can be attributed to the changes in crystalline size and microstrain of ZnO:CeO2 mixed oxides crystals. In addition, ZnO:CeO2 mixed oxides thin films exhibit absorb photons of wavelength ranging between 280 nm to 330 nm efficiently. Consequently, incorporation of ZnO with CeO2 enhance UV-filter efficiency of thin films especially for UVB and UVC irradiation. Overall, ZnO:CeO2 mixed oxides thin films could be potential candidates for high transparency and UV-filter applications.

Analysis of Transmittance Spectra
Absorption coefficient ( ) describes the intensity attenuation of the electromagnetic waves passing through a thin film [70]. The parameter can be expressed using the corrected transmission ( ), as = (1⁄ )ln(1⁄ ) [2,71,72], where is the thickness of thin films measured using SEM micrograph and estimated to be 500 nm. Figure 7 shows the variation of of pure ZnO, pure CeO2 and ZnO:CeO2 mixed oxides thin films with the incident photon wavelength. Obviously, exhibits

Analysis of Transmittance Spectra
Absorption coefficient (α) describes the intensity attenuation of the electromagnetic waves passing through a thin film [70]. The α parameter can be expressed using the corrected transmission (T), as α = (1/d) ln(1/T) [2,71,72], where d is the thickness of thin films measured using SEM micrograph and estimated to be 500 nm. Figure 7 shows the variation of α of pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films with the incident photon wavelength. Obviously, α exhibits higher values just before the absorption edge compared with those after the absorption edge. This can be attributed to the resonance of the frequency of incident photons with that of electronic polarizability. The dramatic reduction of α at the absorption edge could be explained in terms of the coupling occur between electrons of the constituents [73][74][75][76]. Furthermore, the red shift of absorption edge upon the introduction of CeO 2 into ZnO:CeO 2 mixed oxides thin films is due to the change in the width of localized states and consequent changes in the energy gap.
Photonics 2020, 7, x FOR PEER REVIEW 10 of 20 higher values just before the absorption edge compared with those after the absorption edge. This can be attributed to the resonance of the frequency of incident photons with that of electronic polarizability. The dramatic reduction of at the absorption edge could be explained in terms of the coupling occur between electrons of the constituents [73][74][75][76]. Furthermore, the red shift of absorption edge upon the introduction of CeO2 into ZnO:CeO2 mixed oxides thin films is due to the change in the width of localized states and consequent changes in the energy gap. To elucidate the attenuation of electromagnetic waves by the proposed thin films in this work, we calculate the extinction coefficient . It is related to the absorption coefficient,  To elucidate the attenuation of electromagnetic waves by the proposed thin films in this work, we calculate the extinction coefficient k. It is related to the absorption coefficient, k = αλ/4π [2]. Figure 8 shows k of pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films as a function of incident photon wavelength. A drastic decrease of k is noticed prior to the absorption edge indicating that thin films allow electromagnetic waves to pass through without any decay or damping for λ ≥ 350 nm. Moreover, k exhibits an increasing trend as the concentration of CeO 2 introduced in ZnO:CeO 2 mixed oxides thin films is increased. Therefore, ZnO:CeO 2 mixed oxides thin films have a larger ability to attenuate incident light by scattering and absorption. To elucidate the attenuation of electromagnetic waves by the proposed thin films in this work, we calculate the extinction coefficient . It is related to the absorption coefficient, = 4 ⁄ [2]. Figure 8 shows of pure ZnO, pure CeO2 and ZnO:CeO2 mixed oxides thin films as a function of incident photon wavelength. A drastic decrease of k is noticed prior to the absorption edge indicating that thin films allow electromagnetic waves to pass through without any decay or damping for 350 . Moreover, k exhibits an increasing trend as the concentration of CeO2 introduced in ZnO:CeO2 mixed oxides thin films is increased. Therefore, ZnO:CeO2 mixed oxides thin films have a larger ability to attenuate incident light by scattering and absorption.  The specific landscapes of the band structure and optical absorption edge of thin films can be simply interpreted by Tauc model [2]. The functional dependence of (αhv) 2 of pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films versus photon energy is shown in Figure 9a. The optical band gaps obtained from Tauc plots are 3.296 eV for ZnO thin films and 3.092 eV for CeO 2 thin films. The band gap energy decreases as 25% of CeO 2 is introduced in ZnO:CeO 2 mixed oxides thin films. A slight increase of the band gap is noticed as the concentration of CeO 2 is further increased as shown in Figure 9b. We analyze the decrease in band gap and find that three factors contribute unequally to this decrease. Namely, CeO 2 thin films exhibit lower band gap energy than ZnO thin films. Consequently, the increase of CeO 2 content in the mixed oxide composites decreases the optical band gap. Furthermore, the strong ds-p interactions in ZnO:CeO 2 mixed oxides and the large difference between the electronegativities of Ce and Zn in the ZnO:CeO 2 mixed oxide system contribute to the optical band gap reduction [65][66][67][68]. Urbach energy, E U parameter can be used effectively to measure the degree of disorder in the thin films [75,77]. Near the optical band edge, absorption coefficient, α = α 0 exp(hv/E U ), where α 0 is a constant, hv is the incident photon energy and E U is the Urbach energy. Urbach energy of ZnO thin film is found to be 221 meV and that of CeO 2 thin film is 531 meV. For ZnO:CeO 2 mixed oxides thin films, E U exhibits values intermediate between those of ZnO and CeO 2 thin films. Figure 9c shows the variation of E U as a function of CeO 2 concentration in ZnO:CeO 2 mixed oxides thin films. We find that E U increases as concentration CeO 2 is increased. For high CeO 2 concentrations, E U attains nearly constant value. Furthermore, E U exhibits the lowest value for pure ZnO thin film indicating minimum disorder before introducing CeO 2 consistent with the value of the microstrain calculated previously in XRD section. Interestingly, a direct relationship between E U and ε is observed as the concentration of CeO 2 is increased. Linear fitting of the plots shows a direct relation between E U and ε as illustrated in Figure 9d. E U and ε follows the empirical relationship: ZnO thin film indicating minimum disorder before introducing CeO2 consistent with the value of the microstrain calculated previously in XRD section. Interestingly, a direct relationship between and ε is observed as the concentration of CeO2 is increased. Linear fitting of the plots shows a direct relation between and ε as illustrated in Figure 9d. and ε follows the empirical relationship: = 31982 * ε + 1.6888 (7)

Refractive Index Analysis
Refractive index, n, of a thin film is closely related to electronic polarization of ions and the local electric and lattice fields [78,79]. The n parameter can be expressed as n = (1 + R/1 − R) + 4R/(1 − R) 2 − k 2 [2]. Figure 10 shows n of pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films as a function of the wavelength of incident light. Obviously, spectral behavior of n can be divided into two regions. For (λ < 400 nm) called anomalous dispersion region, the frequency of incident photon resonates with plasma frequency leading to strong electron coupling to the oscillating electric field [80]. For λ > 400 nm (normal dispersion region), n decreases continuously with increasing wavelength and shows significant normal dispersion. We found that n of ZnO and CeO 2 thin films exhibits values ranging between 1.57 and 1.85 and 1.73 and 2.25 as the wavelength decreases from 700 nm to 400 nm, respectively. We found that increasing the concentration of CeO 2 in ZnO:CeO 2 mixed oxides thin films leads to considerable change of n values. This can be attributed to the changes induced in crystalline size and microstrain of ZnO:CeO 2 mixed oxides crystals that leads to change in surface morphology and vacancies inside the crystal structure. High refractive index materials can be used in many optoelectronic applications such as enhancing the visual properties of electronic displays, which includes LCDs, OLEDs, and quantum dot (QDLED) televisions [81,82].
in ZnO:CeO2 mixed oxides thin films leads to considerable change of n values. This can be attributed to the changes induced in crystalline size and microstrain of ZnO:CeO2 mixed oxides crystals that leads to change in surface morphology and vacancies inside the crystal structure. High refractive index materials can be used in many optoelectronic applications such as enhancing the visual properties of electronic displays, which includes LCDs, OLEDs, and quantum dot (QDLED) televisions [81,82]. The dispersion of refractive index has been investigated intensively [83,84]. Wemple-DiDomenico (WDD) model is used to relate n with other key optical parameters such as effective single oscillator energy ( ) and dispersion energy ( ). The model has been successful in estimating zero-frequency refractive index and zero-frequency dielectric constant ( ) [12,85,86]. WDD model provides physical interpretation of the related optical parameters: Therefore, plotting (ℎ ) versus ( − 1) yields the values of the dispersion parameters via fitting the linear relation. Figure 11a shows (ℎ ) versus ( − 1) for pure ZnO, pure CeO2 and  The dispersion of refractive index has been investigated intensively [83,84]. Wemple-DiDomenico (WDD) model is used to relate n with other key optical parameters such as effective single oscillator energy (E 0 ) and dispersion energy (E d ). The model has been successful in estimating zero-frequency refractive index and zero-frequency dielectric constant (ε 0 ) [12,85,86]. WDD model provides physical interpretation of the related optical parameters: Therefore, plotting (hv) 2 versus n 2 − 1  Table 2. The value of E 0 of pure ZnO and pure CeO 2 thin films are found to be 3.959 eV and 3.603, respectively. The values of E 0 for ZnO:CeO 2 mixed oxides thin films are found to be intermediate between those of pure ZnO and pure CeO 2 thin films. Introducing CeO 2 in ZnO:CeO 2 mixed oxides thin films leads to a decrease in the energy of bonding and a change in the iconicity of these bonds in the nanocomposite. Consequently a decrease in the value of E 0 [76]. Moreover, the dispersion energy E d increases upon introducing CeO 2 in ZnO:CeO 2 mixed oxides. This increase could be attributed to the fact that cubic structure has higher dispersion energy than hexagonal structure. The values of E 0 and E d are used to estimate the zero-frequency dielectric constant ε 0 and the zero-frequency refractive index n 0 by rewriting Equation (8) and setting hv = 0 leading to    The calculated values of ε 0 are presented in Table 2. It is worth mentioning that the obtained values of n 0 are in good agreement with theoretical and the experimental values of the normal refractive index.
Sellmeier model is used to calculate the average oscillator wavelength (λ 0 ) and the oscillator length strength (S 0 ) parameters [87]. According to this model, Plotting n 2 − 1 −1 against λ −2 gives the oscillator parameters by fitting the linear relationship as demonstrated in Figure 11b. The values of S 0 and λ 0 are presented in Table 2. Our calculations indicate that λ 0 increases as CeO 2 introduces in ZnO:CeO 2 mixed oxides. However, S 0 -value decreases as 25% and 50% of CeO 2 are introduced in ZnO:CeO 2 mixed oxides. Moreover, inserting 75% of CeO 2 increases S 0 -value. This is due to the fact that λ 0 is inversely proportional to E 0 whereas S 0 is directly proportional to E d . Obviously, introducing CeO 2 into ZnO:CeO 2 mixed oxides leads to an increase of λ 0 and a decrease of S 0 , as a result of the decline of the surface morphology.
To elucidate the dielectric properties of mixed oxides thin film, the square of n is correlated with the density of states (N/m * ) and the high-frequency dielectric constant ε ∞ as formulated by Spitzer-Fan [88,89]: where e is the electronic charge, c is the speed of light in vacuum, N c is the charge carrier density and m * is the effective mass of the carrier. Plotting n 2 versus λ 2 is used to determine N/m * and ε ∞ . Figure 11c shows the dispersion nature of the index of refraction of our thin films. The estimated values of both ε ∞ and N c /m * are listed in Table 2. The values of ε ∞ of pure ZnO and pure CeO 2 thin films were found to be 2.868 and 3.801, respectively. The values of ε ∞ of ZnO:CeO 2 mixed oxides thin films are found to increase upon increasing the concentration of CeO 2 in ZnO:CeO 2 mixed oxides thin films consistent with that of n [90,91]. We found that charge carrier density boosts upon increasing CeO 2 content in ZnO:CeO 2 mixed oxides thin films. The imaginary part of the dielectric function (ε ) in relation to the wavelength of the incident photon could be analyzed to determine the relaxation time (τ) defined by Drude free electron model [88]: Figure 11d shows the variation of ε with λ 3 for pure ZnO, pure CeO 2 and ZnO:CeO 2 mixed oxides thin films. The relaxation time τ could be determined from the slope of the plot between ε and λ 3 , and from the value of N c /m * calculated from Equation (7) taking m * = 0.44m e [43].

Summary and Conclusions
In summary, synthesis, crystallography, microstructure, crystal defects, optical and optoelectronic characterization of ZnO:CeO 2 mixed oxide-thin films dip-coated on glass substrates using a sol-gel technique are investigated and interpreted. Pure ZnO thin film is found to exhibit a hexagonal structure whereas that of pure CeO 2 thin film is a fluorite cubic. XRD patterns demonstrate the formation of mixed oxide materials containing semi-crystalline nature from the constituent oxides. ZnO-CeO 2 films were obtained with a hexagonal structure and extremely favored orientation with the c-axis perpendicular to glass substrates. The as-prepared mixed-oxide thin films exhibit several crystallographic orientations determined by the lattice mismatch between ZnO and CeO 2 thin films and the concentration of added CeO 2 thin films.
To elucidate deeper insight into crystal defects, we estimate the average microstrain and the crystallite size of pure ZnO and CeO 2 thin films found to be 18.93 nm and 9.26 nm, respectively. In addition, we found that ZnO:CeO 2 mixed oxide thin films containing 25%, 50% and 75% of CeO 2 exhibit compositional-averaged crystallite sizes of 12.09 nm, 11.58 nm and 11.50 nm, respectively. Transmittance of all investigated thin films is found to attain high values of 85% in the visible region. Moreover, the refractive index of pure ZnO film was found to exhibit values ranging between 1.57 and 1.85, while that of CeO 2 thin film take on values ranging between 1.73 and 2.25 as the wavelength decreases from 700 nm to 400 nm. Interestingly, ZnO:CeO 2 mixed oxides thin films exhibit refractive indices that are strongly dependent on the content level of CeO 2 . We found a significant decrease of optical band gap as the concentration of CeO 2 is increased in the mixed oxide thin films. Therefore, band gap engineering is achieved efficiently in this mixed oxide system.
The strong correlation among crystallinity, surface morphology and optical properties of the as-synthesized mixed oxide thin films is investigated and interpreted using XRD and UV-Vis spectroscopy. Different classical models such as WDD model, Sellmeier model, Spitzer-Fan model and Drude free electron model are employed to calculate and interpret the optoelectronic and dispersion properties of ZnO:CeO 2 mixed oxides thin films. Obtaining mixed oxide thin films with controllable refractive index and tunable optical and optoelectronic properties provides a pathway to design smart multi-functional materials. Such materials may act as potential candidates for the fabrication of modern optoelectronic devices and thin film transistors.