First-principles Study of Adsorption Energetics of Alkanethiols on GaAs(001)

Adsorption of alkanethiols on GaAs (001) surface under low coverage conditions was studied using density functional calculations in a periodic supercell approach. The study of physisorbed precursor and transition to chemisorption revealed that hydrogen atoms stay on the surface upon S-H bond cleavage and significantly affect desorption products and energies, in agreement with available experimental data. Binding of thiols to GaAs is found to be comparable or stronger than that of thiols to noble metals surfaces. Calculated thiolate-surface binding energies are found to be higher for Ga-rich than for As-rich surfaces, and are strongly dependent on surface reconstruction, adsorption site and coverage. This dependence is explained by the violation or fulfillment of electron counting rule, rehybridization of surface atom orbitals and strain relaxation upon addition of extra electrons brought by adsorption