| Popis: |
Publisher Summary This chapter discusses the surface diffusion of metal clusters on metals. To understand the behavior of surface clusters, it is useful to first examine the properties of single metal atoms bound to a surface. The (211) plane of tungsten is particularly advantageous for this. It is made up of rows of close-packed lattice atoms; these form channels, which can be expected to restrict the motion of atoms to one dimension. This is indeed the behavior actually observed. A single rhenium atom has been deposited upon a (211) plane of tungsten formed in a highly perfect state by field evaporation at low temperatures. Observations in the field ion microscope reveal that this atom moves only along surface channels in the direction, never across them. Repeated observations do not cause any change in the location of the atom. Atom locations are never examined at room temperature, where mobility is pronounced and where the field is likely to have a significant effect. The motion evident is thus typical of diffusion in an ordinary thermal environment. It is customary to view the surface migration of an adatom as a random walk, for which the mean-square displacement in one dimension is related to the diffusion coefficient and the diffusion interval. |
