Work function measurements on nano-crystalline zinc oxide surfaces

The work function of nano-crystalline zinc oxide (ZnO) thin films was examined using photoemission spectroscopy (PES). Colloidally dispersed ZnO nano-particles were electrospray-deposited in vacuum to form nano-crystalline thin films. The samples showed an immediate work function reduction by 0.35 eV during ultraviolet photoemission spectroscopy (UPS) measurements. This artifact was detected and quantified through low intensity x-ray photoemission spectroscopy (LIXPS) measurements, which use a very low photon flux. This prevented significant photochemical changes on the measured surface, i.e. the true work function unaffected by the UPS artifact can be measured. Annealing of an identical sample removed all ambient contamination from the ZnO surface with the effect to prevent the work function lowering artifact. This allowed the conclusion that ambient contamination is essential for the artifact to occur, similar to what was observed earlier on indium tin oxide and TiO2 surfaces. In an additional experiment, exposure of the annealed sample to the ambient resulted in a sample that again showed the artifact further demonstrating the necessity of water to be present. This experiment also demonstrated that the solubility enhancing surfactant shell of the nano-crystals does not play a significant role in the artifact, since it was removed during the annealing process.