The effect of covalently bonded aryl layers on the band bending and electron density of SnO2 surfaces probed by synchrotron X-ray photoelectron spectroscopy

Abstract

Tin(IV) dioxide (SnO₂) is a technologically important transparent conducting oxide with high chemical stability. In air, the SnO₂ surface is terminated with hydroxyl groups which cause the electronic bands to bend downward at the surface capturing a two-dimensional surface electron accumulation layer (SEAL). The SEAL promotes adsorption at the surface, giving environmentally-sensitive electronic properties; this sensitivity is a barrier to some potential applications of the material. This work investigates surface modification of SnO₂via reaction with an aryldiazonium salt as a route to controlling the surface band bending. We compare the surface layers formed by reaction at open-circuit potential and under potential control of 4-(trifluoromethyl)benzene diazonium ions with moderately- and highly-doped (101) SnO₂ thin films grown by plasma-assisted molecular beam epitaxy. Atomic force microscopy and synchrotron X-ray photoelectron spectroscopy (XPS) measurements demonstrate that both reaction conditions lead to covalently-attached 4-(trifluoromethyl)phenyl groups, with grafting at open-circuit potential giving thinner layers (<2 nm) and fewer direct bonds to the surface than electrografting (layer thickness >3 nm). Valence band investigations show that for all samples the 4-(trifluoromethyl)phenyl layers decrease the surface downward band bending with the greatest effect observed for the electrografted sample. In the latter case, a +0.29 eV shift in band bending relative to that of the unmodified material indicates the success in turning the surface electron accumulation layer into a depletion layer.