Molecular physisorption on atom-adsorbed unreconstructed (111) silicon surfaces

Abstract

The potential energy surfaces of ammonia and water molecules physisorbed on (111) silicon surface are determined, using semi-empirical potentials. The nature and the geometry of the adsorption sites is studied for three cases: unreconstructed (111) Si surface. H-terminated (111) Si surface and ideally O-adsorbed Si(111)−(1 × 1) surface. Results show that the adsorption energy increases respectively for the three cases, being larger for the O-terminated surface. On top positions are not stable sites for the molecules; ammonia stands upright on the surfaces with its dipole moment pointing up, whereas water is either flat on the H-terminated surface or in a dipod orientation on the O-adsorhed one.