Synchrotron X-ray Photoelectron Spectroscopy Study of Electronic Changes at the ZnO Surface Following Aryldiazonium Ion Grafting: A Metal-to-Insulator Transition C

ZnO is a member of a small class of semiconductors that includes In₂O₃, SnO₂, CdO, and InN, whose surfaces are highly unusual because their electronic bands bend downward to form a quantized potential well in which a two-dimensional electron gas is confined. At the O-polar ZnO(0001) surface, this effect arises from the adsorption of H adatoms which produces a hydroxyl-terminated surface. In this work, we investigate the effect of covalently anchored organic layers on the band bending at ZnO(0001) surfaces. We use aryldiazonium salt electrochemistry to deposit 4–5 nm thick layers of 4-nitrophenyl (NP) and 4-(trifluoromethyl)phenyl (TFMP) groups. Synchrotron X-ray photoelectron spectroscopy (XPS) showed that both NP and TFMP modifications permanently removed the downward band bending at the ZnO surface. This behavior can be explained by the direct covalent bonding of the aryl groups to the surface, also revealed by XPS analysis, and the electron-withdrawing character of both modifiers. Surprisingly, the in situ irradiation-induced reduction of the grafted NP groups to aminophenyl-like moieties resulted in a further band bending shift in the upward direction, producing a combined change of more than 1.0 eV, corresponding to strong near-surface electron depletion. This phenomenon can be explained by the participation of electrons from the ZnO surface and possibly hydrogen from subsurface donors in the reduction process. Our study shows that electrochemically grafted aryl layers can alter the fundamental nature of ZnO surfaces by producing a “metal-to-insulator transition” in its unusual surface conductivity.