Electrostatics and reactivity of surface defects on Si(111)-(2X1)

We calculated the electrostatic field (the gradient of the molecular electrostatic potential) near steps and corners on the Si(111)-(2 x 1) surface using the semiempirical AM1 molecular orbital method. Calculations on appropriate models, derived from the experimental surface structure, indicate that the magnitude of the electrostatic field increases considerably in the vicinity of discontinuities. While the field near the unreconstructed perfect surface is less than 2 V/nm, it becomes larger by a factor of 2 to 4 near steps and corners. Semiempirical AM1 molecular orbital calculations on models with ammonia and methane molecules adsorbed near surface defects indicate that NH and CH bond lengths increase, the corresponding bond orders decrease, compared with the gas phase, This means that the enhanced electrostatic field weakens covalent bonds, promoting dissociation. On this basis we suggest that the electrostatic field is a determining factor of the enhanced catalytic activity near surface discontinuities. (C) 1998 Elsevier Science B.V. All rights reserved.