Energy Level Pinning in Self-Assembled Alkanethiol Monolayers

Abstract

Potassium deposition in ultrahigh vacuum on a self-assembled monolayer (SAM) of 1-dodecanethiol on Au(111) results in penetration of a large fraction of the metal to the gold−SAM interface, as determined by angle-resolved X-ray photoelectron spectroscopy (XPS) and high-resolution XPS. This results in a change in the work function (WF) of the surface, due to a change in the WF of the gold substrate beneath the SAM. The measured binding energy of the C1s photoemission peak related to the aliphatic chain also shifts and correlates directly with the change in WF. In contrast, the binding energy of the S2p peak from the thiolate headgroup is independent of the substrate WF. These findings indicate that photoemission from the core levels of the aliphatic chain and headgroup atom in an alkanethiolate SAM should be described in different frameworks, assuming, respectively, either Fermi or vacuum level alignment between the respective parts of the SAM and substrate. The results may be explained by slow dynamics of charge transfer along the aliphatic chain.