Control of the pH-Dependence of the Band Edges of Si(111) Surfaces Using Mixed Methyl/Allyl Monolayers

The open-circuit potentials of p-Si/((MV2+/MV+)(aq)) junctions with Si(111) surfaces functionalized with H−, CH3−, CH2CHCH2−, or mixed CH3−/CH2CHCH2− monolayers have been investigated as the solution pH was changed from 2.5 to 11. The pH sensitivity of the open-circuit potentials, and therefore the band-edge positions, was anticorrelated with the total fraction of Si atop sites that were terminated by Si−C bonds. This behavior is consistent with the hypothesis that the non Si−C terminated atop sites were initially H-terminated and were unstable to oxide growth under aqueous conditions with the oxidation-product inducing a pH-dependent dipole. Metal-semiconductor junctions between Hg and CH3-, CH2CHCH2-, or mixed CH3-/CH2CHCH2-terminated n-Si(111) surfaces formed rectifying Hg/Si Schottky junctions and exhibited mutually similar barrier-heights (∼0.9 V), suggesting similar magnitudes and direction of the surface dipoles on all of these functionalized surfaces.