Nanoscale fine-structure evaluation of RF magnetron sputtered anatase films using HRTEM, AFM, micro-Raman spectroscopy and fractal analysis

Nanocrystalline titanium dioxide (TiO2) thin film is one of the most investigated oxide material owing to its important applications in areas like energy conversion and photocatalysis. In the present study nanostructured TiO2 film were deposited by radio frequency magnetron sputtering. The as-deposited films were found to be amorphous from grazing incidence X-ray diffraction analysis. Annealing at different temperatures (300 to 600 °C) transformed the amorphous films into crystalline films of the anatase phase. The annealed films had a high degree of orientation along the (004) direction. The grain size and crystalline perfection of the films were analyzed using high resolution transmission electron microscopy. Atomic force microscopy images of the samples were analyzed to investigate surface morphological features. The samples heat treated at higher temperatures exhibited rough surface texture with high mountains and deep valleys generated by the fusion together of particles at the inter particle contacts. Analysis of the characteristics of nanostructure of the films was attempted in making use of the theory of fractals. Raman spectra of the film samples had shown that annealing the samples at high temperatures resulted in an improvement of crystalline perfection. Analysis of UV–Visible transmittance spectra showed the optical band gap of the films. Thermal treatment of the film samples lead to an increase in the band gap values due to a reduction in the extent of the band tailing. GIXRD results along with AFM and HRTEM analyses showed that RF magnetron sputtered, annealed, anatase films had suitable nanostructure for practical applications such as dye sensitized solar cells.