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

We calculated the electrostatic field (the gradient of the molecular electr ostatic potential) near steps and corners on the Si(111)-(2 x 1) surface us ing the semiempirical AM1 molecular orbital method. Calculations on appropr iate models, derived from the experimental surface structure, indicate that the magnitude of the electrostatic field increases considerably in the vic inity 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 s teps and corners. Semiempirical AM1 molecular orbital calculations on model s with ammonia and methane molecules adsorbed near surface defects indicate that NH and CH bond lengths increase, the corresponding bond orders decrea se, compared with the gas phase, This means that the enhanced electrostatic field weakens covalent bonds, promoting dissociation. On this basis we sug gest that the electrostatic field is a determining factor of the enhanced c atalytic activity near surface discontinuities. (C) 1998 Elsevier Science B .V. All rights reserved.