Chemisorption and physisorption of alkanethiols on Cu(1 1 1). A quantum mechanical investigation

The adsorption of n-alkanethiols on the 1 1 1 surface of copper was studied by means of quantum mechanical calculations. Charge transfer processes between the adsorbates and the metal were investigated in order to elucidate the nature of the adsorbed species. The electronic structure of the adsorbates in the proximity of the Fermi level was also investigated to obtain a better understanding of the electronic states which contribute to scanning tunneling microscopy (STM) images. We found that the bonding of the n-alkanethiols is independent of the chain length when the adsorbates are perpendicular to the surface. This bonding mainly involved the sulfur p-orbitals and, to a lesser extent, the orbitals of the first methyl group. From the analysis of dipole moment curves and Mulliken populations, a charge transfer of 0.6 electrons from the metal towards the adsorbate was obtained, thus confirming the anionic nature of the adsorbate. A depletion of electron density was observed in the metal atoms of the primary chemisorption site as well as a net accumulation of charge around the sulfur atom. Constant current STM images of thiols adsorbed flat on the surface showed a pronounced protrusion on the position of sulfur due to the accumulation of electron density around this atom in the proximity of the Fermi level. However, for thiols adsorbed perpendicular to the surface, the STM images were found to be sensitive to the position of the terminal methyl group. A corrugation of 1.5 إ was obtained for a methyl radical adsorbed perpendicular to the surface.