Two-dimensional Electron Gas at Thiol/ZnO Interface

Electronic modification of O-terminated ZnO(000 (1) over bar) and Zn-terminated ZnO(0001) by ethanethiol (C2H5SH) adsorption was investigated by angle-resolved photoelectron spectroscopy (ARPES) and X-ray photoelectron spectroscopy (XPS). Ethanethiol dissociates on both surfaces at room temperature to form C2H5S and H. The saturation coverage of ethanethiol is nearly three times larger on ZnO(0001) than on ZnO(000 (1) over bar). This suggests that Zn atoms exposed to the surface should act as adsorption and dissociation sites. Adsorption on the (000 (1) over bar) surface induces large downward band bending and accompanies a Zn 4s-derived metallic state at the center of the surface Brillouin zone. This proves that ethanethiol is a good electron donor on ZnO(000 (1) over bar). Contrastingly, ethanethiol on ZnO(0001) hardly affects the energetic position of the ZnO band so that surface metallization is not brought about.