Tuning of the gold work function by carborane films studied using density functional theory

Using density functional theory including van der Waals interactions, we calculate the adsorption and electronic properties of dithiol-dicarba-closo-dodecaboranes chemisorbed on Au(111) surfaces. Carborane molecules consist of a cage-like structure made of boron and carbon atoms and possess a large intrinsic dipole. We consider two functionalized carborane positional isomers, with thiol linker groups attached to either carbon or boron backbone atoms, such that when adsorbed on the Au substrate, the molecular dipole points towards the metal surface or away from it. We investigate a large number of junction geometries and find that carborane adsorption can induce significant changes in the work function of the Au substrate, in the range of 1 eV. These changes depend strongly on the interface geometry at the atomistic level. From the analysis of these junction structures, we provide a picture of the driving mechanisms that determine adsorption geometries, and relate them to interface electronic structure and resulting work function modification. In particular, our results highlight the important role played in these interface quantities by distortions in the Au surface layer induced by carborane adsorption.