First-Principles Study of Water Ice Adsorption on the Methyl-Terminated Si(111) Surface

The properties of water adsorbed on the methyl-terminated Si(111) surface, CH3:Si(111), are investigated using a first principles approach to complement a recent experimental study. We confirm the existence of weak hydrogen bonds between water and the substrate; however we show that the bonding configuration is different from that assumed in the analysis of the experimental data. In fact, a water molecule prefers to interact with the substrate by adopting a "face" approach direction in which a H atom points toward the C atom of the methyl group; instead the suggested "vertex" configuration, characterized by a weak C-H center dot center dot center dot OH2 hydrogen bond, turns out to be clearly unfavored. This behavior resembles that observed in the study of hydrogen bonding in the methane-water complex. Our energetic analysis indicates that water water interactions are much stronger than those between water and the substrate, thus confirming the hydrophobic behavior of the CH3:Si(111) surface, although the contact-angle estimate (73 degrees), in good agreement with the reported experimental value, is also compatible with a partially wettable character of the substrate.