Adsorption of oxygen atom on Zn-terminated (0001) surface of wurtzite ZnO: A density-functional theory investigation

Using first-principles density-functional theory, the structures, stabilities, and electronic structures of the clean and oxygen atom adsorbed Zn-terminated (0001) polar surface of wurtzite ZnO have been systematically investigated. The clean polar surface exhibits n-type conductivity. The calculations of the adsorption energy indicate that the hollow (H) site at the hexagon center of the ZnO surface is the most stable site for the adsorption of oxygen atom. The adsorption of oxygen atom is an exothermic process, and the adsorption is chemisorption. After the adsorption of oxygen atom, the work function of the ZnO surface increases. This indicates that the adsorbed oxygen atom is expected to withdraw electrons from the surface and there is charge transfer from the surface to the adsorbed oxygen atom, which will cause the increase of the resistance. For the ZnO surface with adsorbed oxygen atom at the H site, the work function is the lowest. Furthermore, the modifications of the electronic structures of ZnO surface by the adsorption of oxygen atom have been investigated in terms of the electron density difference and density of states. The results indicate that there are ionic bonds formed between the surface zinc and adsorbed oxygen atoms. After the adsorption of oxygen atom, the Fermi level shifts down and the n-type conduction characteristic is significantly weakened.

Summary: plot of electron density difference of ZnO (0001) surface (oxygen atom adsorbed on H site). Electron depletion and accumulation are indicated in red and blue, respectively.Figure optionsDownload as PowerPoint slideHighlights
► The clean surface exhibits n-type conductivity.
► H site of the surface is the most favorable adsorption site.
► After adsorption, work function increases, and charges transfer to the atom.
► After adsorption, Fermi level shifts down, leading to weakened n-type conductivity.