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

Abstract

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.

Dimensional Roadmap for Maximizing the Piezoelectrical Response of ZnO Nanowire-Based Transducers: Impact of Growth Method

“…Eventually, the engineering of surface trap density in ZnO NWs appears as an alternative way to tune the radius to a relevant range that is compatible with the different growth methods. It has been shown that the use of chemical adsorbates on the polar and non-polar planes of ZnO has a great effect on the nature and magnitude of the band bending [ 86 , 87 ]. The intricate nature of all these approaches represents the major difficulty to get ZnO NWs with the optimal dedicated properties.…”

Electroreduction of Aryldiazonium Ion at the Polar and Non‐Polar Faces of ZnO: Characterisation of the Grafted Films and Their Influence on Near‐Surface Band Bending

“…The procedure most commonly generates loosely‐packed multilayer films covalently linked to the substrate [7] . For the O‐polar MBE ZnO used in our earlier work, X‐ray photoelectron spectroscopy (XPS) analysis, obtained with synchrotron radiation, strongly supported the presence of Zn−O−C bonds arising from covalent anchoring of aryl groups to the substrate [6] . The same bonding configuration has been demonstrated, or assumed, at other hydroxylated surfaces [8] .…”

“…The focus was to examine the mode of interaction of the film with the surface and to monitor the effect on the band bending at each crystal face. NBD was selected for the study because grafted nitrophenyl (NP) films were found to decrease the downward band bending at O‐polar MBE ZnO [6] . Our second aim was to gain insight into a very intriguing result obtained in our earlier study: during synchrotron XPS measurements, the grafted NP groups were substantially, and rapidly, converted to aminophenyl‐like groups, and concurrently, there was a dramatic change in surface band bending from downward to upward [6] .…”

“…In earlier work using Sb‐doped O‐polar ZnO thin films grown by molecular beam epitaxy (MBE), we demonstrated that surface modification by electroreduction of aryldiazonium ions leads to a strong decrease in downward band banding [6] . Reduction of aryldiazonium ions is a simple and popular route to deposition of low‐nanoscale organic layers on many different substrates.…”

Heterogeneous-phase Sonogashira cross-coupling reaction on COC surface for the grafting of biomolecules – Application to isatin

“…Recently, we have successfully functionalized COC surfaces in absence of any plasma activation by reducing grafting of aryldiazonium salts via transient formation of the corresponding aryl radicals [16]. Widely used to modify metallic surfaces for their electro-or chemical grafting ability [17][18][19][20], these species also proved to be competent for the functionalization of polymeric surface [21]. From their reduced form, radicals covalently bind to the surface allowing various chemical functions (acids, nitro or bromo groups) to be introduced on the COC surface [16].…”