ZnO is a prime candidate for future use in transparent electronics; however, development of practical materials requires attention to factors including control of its unusual surface band bending and surface reactivity. In this work, we have modified the O-polar (0001̅), Zn-polar (0001), and m-plane (101̅0) surfaces of ZnO with phosphonic acid (PA) derivatives and measured the effect on the surface band bending and surface sensitivity to atmospheric oxygen. Core level and valence band synchrotron X-ray photoemission spectroscopy was used to measure the surface band bending introduced by PA modifiers with substituents of opposite polarity dipole moment: octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (FOPA). Both PAs act as surface electron donors, increasing the downward band bending and the strength of the two-dimensional surface electron accumulation layer on all of the ZnO surfaces investigated. On the O-polar (0001̅) and m-plane (101̅0) surfaces, the ODPA modifier produced the largest increase in downward band bending relative to the hydroxyl-terminated unmodified surface of 0.55 and 0.35 eV, respectively. On the Zn-polar (0001) face, the FOPA modifier gave the largest increase (by 0.50 eV) producing a total downward band bending of 1.00 eV, representing ∼30% of the ZnO band gap. Ultraviolet (UV) photoinduced surface wettability and photoconductivity measurements demonstrated that the PA modifiers are effective at decreasing the sensitivity of the surface toward atmospheric oxygen. Modification with PA derivatives produced a large increase in the persistence of UV-induced photoconductivity and a large reduction in UV-induced changes in surface wettability.

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(0001̅) 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(0001̅) 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.

A downward to upward surface band bending change can be induced by grafted 4-(trifluoromethyl)phenyl groups on SnO2.