Effect of tungsten oxide concentration on structural, morphological, compositional, dispersion energy, and linear/nonlinear optical properties of niobium pentoxide thin films

Metal-oxide semiconductors with tunable optical and electrical properties are essential candidates for the next-generation development of optoelectronic devices. Interestingly, niobium pentoxide (Nb2O5) has a high refractive index and wide band-gap semiconductor. The systematically observed modification with property adjustment through controlled doping remains unexplored. In this study, the effect of tungsten oxide (WO3) incorporation on Nb2O5 was first investigated. The Nb2O5:WO3 precursors were consistently mixed at ratios of 90:10, 85:15, and 80:20 and deposited via spin coating on borosilicate glass substrates. X-ray diffraction analysis revealed the peak positions of orthorhombic Nb2O5 and monoclinic WO3 in the patterns. The structural and thermodynamic properties of Nb2O5 and WO3 were studied by DFT calculations, which revealed a strong agreement between the calculated and experimental values, with lattice parameters differing by less than 1.5 % for both materials. Field-emission scanning electron micrographs showed that the surface exhibited a very smooth, dense, and uniform morphology free of cracks and porous nature. In band gap calculation, it was found that the decrease in E-g from 3.84 +/- 0.01 to 3.40 +/- 0.02 eV with increasing WO3 concentration can be explained in terms of the occurrence of localized states created between the conduction and valence bands. Standard DFT calculations using the GGA-PBE functional yielded calculated band gaps for Nb2O5 (1.84 eV) and WO3 (1.28 eV), which are significantly lower than the experimental values. A lower transmittance value from similar to 74 to 67 % as the increasing absorption coefficient value from 0.73 x 10(3) to 7.00 x 10(3) cm(-1) with increasing WO3 concentration was obtained owing to the higher photon absorption ability in the thin film since the reflectance of the thin film was less than 10 %. The average refractive index values, in the range of 300-1000 nm, increased to 1.629, 1.630, and 1.632 with increasing WO3 concentration, implying elevated densification or packing density of the thin films with increasing E-U values from 172.58 to 182.28 meV. This imputed the increased localized defect states and demonstrated a strong correlation with the variation in dislocation density. The results from Fourier-transform infrared and micro-Raman spectroscopy revealed that the bonding of the Nb-O-Nb and W-O-W groups and a strong broadband peak corresponded to the ONbO bond. The optical, dispersion energy, optoelectrical, and nonlinear susceptibilities were calculated and investigated for different WO3 concentrations. The overall increase in average chi((1)) and chi((3)) but decrease in average n(2) values of 0.1316-0.1324, 5.636 x 10(-14) to 5.734 x 10(-14) esu., and 1.20 x 10(-12) to 1.13 x 10(-12) esu., respectively, as the energy values at a maximum point of beta(c) are decreased from 2.78 to 2.48 eV with increasing WO3 concentration rendered the expansion in the 3D network of the thin films and increase in strength of chemical bonding between the molecules of Nb2O5:WO3, which may be ascribed to a reduction in the E-g value. Photoluminescence was applied to investigate the electronic structure and electron transfer mechanism in the forbidden bandgap of a semiconductor. This study demonstrates that the synthesized thin films are promising for use in optoelectronics and other technological devices.