Oxygen vacancy diffusion in bare ZnO nanowires

Oxygen vacancies (V_O) are known to be common native defects in zinc oxide (ZnO) and to play important roles in many applications. Based on density functional theory, we present a study for the migration of oxygen vacancies in ultra-thin ZnO nanowires (NWs). We find that under equilibrium growth conditions V_O has a higher formation energy (E_f) inside the wire than that at shallow sites and surface sites, with different geometric relaxations and structural reconstructions. The migration of V_O has lower barriers in the NW than in the bulk and is found to be energetically favorable in the direction from the bulk to the surface. These results imply a higher concentration of V_O at surface sites and also a relative ease of diffusion in the NW structure. Our results support the previous experimental observations and are important for the development of ZnO-based devices in photocatalysis and optoelectronics.