Comparative study on surface states and CO gas sensing characteristics of CuO thin films synthesised by vacuum evaporation and sputtering processes

CuO thin films have been synthesised by thermal oxidation of Cu films deposited using vacuum evaporation and sputtering processes. Single-phase polycrystalline CuO thin films with monoclinic crystalline structure and porous granular surface morphology are obtained by thermal oxidation of Cu films at 400 °C for 5 h, as characterized by x-ray diffraction and scanning electron microscopy. X-ray photoelectron spectroscopy revealed a relatively larger presence of Cu2+ ions on the surface of sputtered CuO film compared to the evaporated film. The CuO film synthesised by sputtering process exhibits 15~20 % higher sensing response towards 91 and 915 ppm concentrations of CO gas than that prepared with vacuum evaporation process. The better CO sensing characteristics of sputtered CuO film are understood based on the combined effects of smaller crystallite size, finer granular surface morphology and higher surface presence of Cu2+ ions as compared to CuO film synthesised by vacuum evaporation process.