Structure and dynamics of amorphous silica surfaces

We use molecular dynamics computer simulations to study the equilibrium pro perties of the surface of amorphous silica. Two types of geometries are inv estigated: (i) clusters with different diameters (13.5, 19, and 26.5 Angstr om) and (ii) a thin film with thickness 29 Angstrom. We find that the shape of the clusters is independent of temperature and that it becomes more sph erical with increasing size. The surface energy is in qualitative agreement with the experimental value for the surface tension. The density distribut ion function shows a small peak just below the surface, the origin of which is traced back to a local chemical ordering at the surface. Close to the s urface the partial radial distribution functions as well as the distributio ns of the bond-bond angles show features which are not observed in the inte rior of the systems. By calculating the distribution of the length of the S i-O rings we can show that these additional features are related to the pre sence of two-membered rings at the surface. The surface density of these st ructures is around 0.6/nm(2), in good agreement with experimental estimates . From the behavior of the mean-squared displacement at low temperatures, w e conclude that at the surface the cage of the particles is larger than the one in the bulk. Close to the surface the diffusion constant is somewhat l arger than the one in the bulk and with decreasing temperature the relative difference grows. The total vibrational density of states at the surface i s similar to the one in the bulk. However, if only the one for the silicon atoms is considered, significant differences are found. (C) 2001 American I nstitute of Physics.