Field ion microscopic observations of individual Pt adatoms, dimers, and trimers which were migrating on the (001) plane showed that a positive electric field (positive tip potential) decreased, while a negative electric field increased (for monomers and dimers), the rate at which diffusion by exchange displacements occurred. The activation energy for the surface diffusion of single adatoms was found to increase from 0.44 to 0.80eV as the applied field was increased from -0.44 to 2.00V/Å. A direct comparison of self-diffusion on the (001) and (311) planes indicated that an electric field affected the rate of exchange displacements much more strongly than it affected hopping displacements. Also, the effect of a positive field upon exchange displacements was opposite to that upon hopping displacements. A sufficiently high positive electric field (about 1.5V/Å) caused the energetically favorable diffusion mode for Pt adatoms on (001) to change from exchange hopping. The observation of a similar field effect for Pt dimers and trimers on (001) confirmed theoretical predictions that they also diffused via a mechanism which involved exchange displacements.

G.L.Kellogg: Applied Surface Science, 1994, 76-77, 115-21