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

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The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been investigated by density functional ab-initio calculations. An on top H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be energetically favorable. The H covered surface is metallic and spin-polarized, with a noticeable magnetic moment at the surface region. Lower hydrogen coverages lead to strengthening of the H-Zn bonds, corrugation of the surface layer and to an insulating surface. Our results explain experimental observations of hydrogen adsorption on this surface, and not only predict a metal-insulator transition, but primarily provide a method to reversible switch surface magnetism by varying the hydrogen density on the surface.