Is there a Au–S bond dipole in self-assembled monolayers on gold?

Self-assembled monolayers (SAMs) of functionalized thiols are widely used in organic (opto)electronic devices to tune the work function, Φ, of noble-metal electrodes and, thereby, to optimize the barriers for charge-carrier injection. The achievable Φ values not only depend on the intrinsic molecular dipole moment of the thiols but, importantly, also on the bond dipole at the Au–S interface. Here, on the basis of extensive density-functional theory calculations, we clarify the ongoing controversy regarding the existence, the magnitude, and the nature of that bond dipole.