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, Phi, of noble-metal electrodes and, thereby, to optimize the barriers for charge-carrier injection. The achievable Phi 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.