Electronic structure of self-assembled (fluoro)methylthiol monolayers on the Au(1 1 1) surface: Impact of fluorination and coverage density

We report the results of a density-functional theory study on a series of metal/organic interfaces consisting of self-assembled monolayers (SAMs) of methylthiol and mono-, di-, and tri-fluoromethylthiols, chemisorbed on the Au(1 1 1) surface. The effects of coverage density and extent of fluorination are examined. Both are found to strongly impact: (i) the component of the SAM dipole moment perpendicular to the surface; (ii) the SAM-induced work-function modification of the gold surface; and (iii) the energy-level alignments of both the highest occupied molecular levels and the molecular-signature states of the SAM with respect to the Fermi level of gold and, as a result, the ionization potential of the SAM when deposited on gold. Saturations of the effects are observed at the higher levels of fluorination and coverage.