Preparation of nanocrystalline ZnO/CoxOy and CNT/CoxOy bilayers for photoabsorption potential: XPS and some surface structural characterization

Zinc oxide and carbon nanotubes (CNT) have been separately grown atop cobalt oxide (CoxOy) thin film to form bilayer structures. The structures were characterized by some surface probing techniques. Surface morphology shows that particles of the underlying CoxOy film are evenly distributed across substrate, its average size increases between 60 and 73 nm with deposition voltage. A transparent layer of ZnO completely laminated the CoxOy film in the first set of bilayer sample. Thickness of overgrown CNT on CoxOy is about 16 nm. Dispersion of CoxOy crystallites along the path of the adsorbed CNT was also observed. Optical studies indicated that both the CoxOy film and the ZnO/CoxOy bilayers are fairly transparent to visible light. Values of their energy band gap were estimated in the range between 2.0 and 2.14 eV. Photoemission and crystallographic studies revealed possible interaction between particles of CoxOy and ZnO films but not with the substrate. The study suggests that both the laminated II-VI bilayer and the CNT enhanced structures could be a recipe for effective charge harvesting and separation in nanostructured photovoltaic devices.