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/((MV^(2+)/MV^(+))(aq)) junctions with Si(111) surfaces functionalized with H−, CH_(3)−, CH_(2)CHCH_(2)−, or mixed CH_(3)−/CH_(2)CHCH_(2)− 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 CH_(3)-, CH_(2)CHCH_(2)-, or mixed CH_(3)-/CH_(2)CHCH_(2)-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