Electronic and relating behavior of Mn-doped ZnO nanostructures: An x-ray absorption spectroscopy study

Abstract

Controlled synthesis of Mn-doped ZnO nanostructures with Mn concentrations of 1%, 3%, and 10% at. has been carried out using sol–gel methods and temperature treatments at 400, 600, and 800 °C. It is found that Mn is successfully introduced into the hcp oxide lattice of ZnO nanoparticles of a range of sizes from a few nm to 102 nm, depending on temperature conditions. It is also found that a secondary phase appears as the Mn concentration and processing temperature increase, most probably in the form appropriately described as MnOx clusters on the surface, although the dominant component remains hcp ZnO. The x-ray absorption near edge structure at all edges of interest reveals that the Mn2+ ion substitutes Zn2+ at the tetrahedral site and that the secondary phase exhibits a clear signature of the octahedral local environment at the Mn L3,2 and O K-edge. X-ray excited optical luminescence excited at 1085 eV (just above the Zn L3,2 edge) shows that the characteristic bandgap emission is slightly blue shifted and the luminescence from both the bandgap and defect emission is quenched somewhat with the latter significantly shifted to longer wavelengths in the region observed for surface and near surface defects. The Mn-doped samples processed at low temperature are poor light emitters due to the high degree of disorder and improve markedly with annealing at higher temperature. The magnetic properties of these systems were also investigated. The results suggest that Mn doping impedes radiative recombination, which is in favor of improved photocatalytic behavior. The implication of these findings is discussed.