Al-doped ZnO nanocrystals: Electronic states through scanning tunneling spectroscopy

Abstract

We form a monolayer of undoped and Al-doped ZnO nanocrystals and measure tunneling current with a scanning tunneling microscope tip. From the density of states, we determine the location of conduction and valence band edges with respect to the Fermi energy. We show that with n-doping, Fermi energy of ZnO nanoparticles shifts toward the conduction band edge. The difference between the electronic states, that is, the bandgap of the nanocrystals does not change upon doping. This is in agreement with the optical absorption spectra of the nanomaterials. We also find that inhomogeneity of doping in nanoparticles is reflected in density of states. With an increase in doping concentration, the distribution of dopants among particles becomes broader. We characterize the monolayers also with Hg electrodes to comment on electrical conductivity versus doping concentration behavior.