Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface

Total energy calculations for the adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface predict that a metal-insulator transition and the reversible switch of surface magnetism can be achieved by varying the hydrogen density on the surface. An on top H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be energetically favorable. The H(1x1) covered surface is metallic and spin polarized. Lower hydrogen coverages lead to a nonmagnetic insulating surface, with strengthened H-Zn bonds and corrugation of the topmost layers. Our results explain the experimental observation of formation of an ordered H(1x1) overlayer on the ZnO(0001) surface and its unexpected evolution toward a disordered layer. Furthermore, we identify a mechanism which can contribute to the room-temperature ferromagnetism measured in ZnO thin films and nanoparticles.