Influence of temperature-induced nanocrystallization on the optical properties of Ni+2-containing glassy tellurite thin films

Thin films of tellurite glasses are prepared using thermal evaporation method from the two bulk systems of glass 75TeO2-5 WO3-15Nb2O5–5NiO (TWNN5) and 78TeO2-5WO3-15Nb2O5-2NiO (TWNN2). The effect of annealing in temperature ranging from 200 to 400 °C on the optical and structural properties of the glassy systems in the form of thin films have been investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV–Vis-NIR Spectrometry techniques. The XRD pattern showed that at 400 °C, the annealed TWNN5 film were polycrystalline with four phases while the sample TWNN2 has only one crystalline phase (orthorhombic γ-TeO2). The SEM and AFM images show the presence of different particle size of nanocrystals at 400 °C which confirmed via XRD pattern for annealed films. The EDX spectrum confirms the presence of Te, W, Nb, Ni and O elements in the prepared films. The transmittance, absorbance and reflectance of as-prepared and annealed thin films were measured spectrophotometry and from which optical, dielectric and dispersion energy parameters have been extracted. The direct energy band gap Eg of the film sample TWNN2 is increased from 2.92 to 3.92 eV with the annealing temperature. While Eg decreases from 2.92 to 2.21 eV with increasing NiO content from 2% to 5%. Tuning of structural and optical properties for TWNN thin films open the avenue towards to be a potential for optoelectronic applications.