Elucidation of the highest valence band and lowest conduction band shifts using XPS for ZnO and Zn0.99Cu0.01O band gap changes

ZnO and Zn0.99Cu0.01O nanostructures were prepared by a simple sol-gel method. The band gaps of the materials were systematically studied based on the dependence of the dimensions of the nanostructures as well as the presence of a dopant material, Cu. ZnO and Zn0.99Cu0.01O nanostructures were found to exhibit band gap widening whilst substitution of Cu in the lattice of ZnO caused its band gap to narrow with respect to the pure ZnO materials. In order to understand the phenomenon of band gap change, structural, spectroscopic, particle size and morphological studies were done. The band gap change occurring when the materials were in the nanostructured phase was proven to be mainly due to the downward shift of the valence band. Interestingly, when the band gaps of the pure ZnO and Cu doped ZnO were compared, the band gap changes were due to different shifts of the valence bands.