Electronic Properties of Biphenylthiolates on Au(111): The Impact of Coverage Revisited

We study the impact of coverage on the electronic structure of substituted biphenylthiolate-based self-assembled monolayers (SAMs) on Au(111) surfaces with a particular focus on SAM-induced work-function changes, Delta Phi. This is done using density functional theory accounting also for van der Waals interactions. We find that the tilt angle of the molecules increases significantly when reducing the coverage, which results in a marked decrease of the perpendicular component of the molecular dipole moment. However, Delta Phi does not follow the trend that one would expect on purely geometrical grounds. While for donor-substituted SAMs, Delta Phi decreases much more slowly than anticipated, for acceptor-substituted SAMs the coverage-induced reduction of Delta Phi is clearly more pronounced than expected. In fact, in that case Delta Phi already vanishes around half coverage. This is in part associated with the (coverage-dependent) bond dipole originating from the Au-S-C bond. Especially for low coverages, however, the relevance of the Au-S-C dipole diminishes, and we observe a significant contribution of Pauli-Pushback (also known as cushion effect) to the interfacial charge rearrangements, an effect that hitherto received only minor attention in the discussion of covalently bonded SAMs.