  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 study, we perform
density-functional-theory calculations to investigate the impact of
dithiol-terminated polyethylene glycol (PEG(thiol)) based self-assembled
monolayers (SAMs) 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,
which can be traced back to the dipole moment of the PEG(thiol) layer.
