Origin of nonlocal interactions in adsorption of polar molecules on Si(001) -2X1

Abstract

Using density functional theory slab calculations, we have investigated sid the origin of nonlocal interactions occurring in the adsorption of small polar molecules sH2O,NH3,CH3OH,CH3NH2d on the clean Sis001d-231 surface and siid the nonlocal effects on two-dimensional arrangement of adsorbates. Our results show the adsorption properties are significantly altered in the presence of adsorbates on an adjacent dimer along a row. We have identified that the coverage dependent behavior arises from a combination of sid surface polarization change, siid adsorbate-induced charge delocalization, siiid adsorbate-adsorbate repulsion, and sivd hydrogen bonding. The nucleophilic-electrophilic molecular adsorption involves charge delocalization to neighboring dimers along a row, which in turn undermines molecular adsorption on the neighboring dimers. Nonlocal effects associated with polar interactions with neighboring dimers and adsorbates vary with adsorption system. While such polar interactions are unimportant in CH3OH adsorption, hydrogen bonding and adsorbate-adsorbate repulsion play an important role in determining the adsorption structures of H2O and NH3/CH3NH2, respectively. In addition, the lectrostatic attraction with the buckled-up Si atoms of adjacent dimers contributes to stabilization of H2O, NH3, and CH3NH2 adsorption.We also discuss kinetic effects on two-dimensional ordering of adsorbates, in conjunction with surface phase transition and adsorption-dissociation rates.