Evolution of nanostructure, phase transition and band gap tailoring in oxidized Cu thin films

Abstract

Nanostructured copper oxide films were prepared by the oxidation of copper films deposited on glass substrates by thermal evaporation of metallic copper. X-ray diffraction (XRD) studies revealed that the films oxidized at temperatures below 275 °C were of Cu 2O phase while oxidation above 325 °C produced pure CuO phase. Oxidation at temperatures between 275 and 325 °C produced films of mixed Cu 2O and CuO phases. Surface morphology of the samples using Atomic Force Microscopy revealed that the surface of the CuO phase film was closely packed when compared with that of Cu 2O phase films. Analysis of high resolution Transmission Electron Microscopy images and XRD patterns together showed that the films of Cu 2O and CuO phases have respectively (111) and (100) preferential orientations. It is evident from the Fourier Transform Infrared results that the formation of Cu 2O is privileged for lower oxidation temperatures whereas higher oxidation temperatures support the formation of CuO. The optical band gap of the film samples was determined from optical transmission spectra. The higher value of band gap for the sample composed of pure CuO phase compared to that of bulk CuO crystals indicated quantum confinement effect.