Lying-Down to Standing-Up Transitions in Self Assembly of Butanedithiol Mono layers on Gold and Substitutional Assembly by Octanethiols

Self-assembly of butanedithiol (C4DT) monolayers (SAMs) on gold and substitutional assembly by octanethiols from a lying-down doubly tethered phase of butane dithiol, were investigated in a vacuum evaporation experiment by high resolution photoemission. The intent was to address the question of transitions from a lying-down to a standing up phase in dithiol self assembly and whether a standing up phase of short chain dithiol can be formed. The lying down doubly tethered dithiol phase, with both sulfur atoms attached to gold, was formed by evaporation and then exposed to octanethiol molecules. A mixed SAM composed of standing up octanethiol and dithiol molecules is found to be formed. The degree of dithiol replacement, as evidenced by the increasing thickness of the SAM, is found to augment slowly with increasing exposure to octanethiol. A standing-up C4DT phase was found to be formed upon rapid very large exposure to C4DT vapors. Differences in valence band photoemission between the standing-up and lying-down phases were noted and accounted for theoretically. In all cases, measurements indicate more than one possible adsorption configuration as evidenced by different sulfur binding energies.