Carboxyl- and amine-functionalized carboranethiol SAMs on Au(111): A dispersion-corrected density functional theory study

The morphological and electronic properties of isolated and monolayer phases of carboxyl- and amine-functionalized carboranethiols on unreconstructed Au(111) were determined using density functional theory calculations including van der Waals interactions. The groups are effective in the assembly of pristine adlayers on gold and also offer functionality when exposed at the SAM-environment interface. Anisotropy brought by both functional groups increases tilting of carboranethiols relative to the surface normal and absolute values of the dissociative chemisorption energies. Positional isomerization and the functional groups modify the molecular dipole moments which influence the adsorption characteristics. Even though carboxylic acid and amine groups have different chemical properties, they have similar effects on the adsorption characteristics of carboranethiols. Dense packing favors intermolecular interactions which gives a stronger binding relative to isolated adsorption. The carboranethiols with the functional groups can be arranged in the same lateral direction or in a dimer conformation with molecules facing each other. Carboxyl and amine groups offer functionalization of carboranethiol SAMs and in conjunction with positional isomerization shift tunability ranges of the work function of gold to higher energies.