Work-function modification of PEG(thiol) adsorbed on the Au(111) surface: A first-principles study

The possibility of modifying the work function of electrodes is important for optimizing the energy barriers for charge injection (extraction) at the interface to an organic material. In this paper, we perform density-functional theory calculations to investigate the impact of dithiol-terminated polyethylene glycol [PEG(thiol)] based self-assembled monolayers with different numbers of PEG repeat units on the work function of the Au(111) surface. We find that a monolayer of PEG(thiol) decreases the work function of the Au(111) surface, where the magnitude of this reduction strongly depends on the length of the PEG backbone. The main contribution arises from the dipole due to the adsorption-induced charge rearrangement at the interface. Our work reveals a pronounced odd-even effect that can be traced back to the dipole moment of the PEG(thiol) layer.