Elementary Processes at Surfaces II. Surface Diffusion

Surface diffusion is the motion of adparticles, such as atoms or molecules, over the surface of a solid substrate. The diffusing particles might be the same chemical species as the substrate (the case referred to as a self-diffusion) or another one (the case of heterodiffusion). In most cases, the adparticle becomes mobile due to thermal activation and its motion is described as a random walk. In the presence of a concentration gradient (in the more general case, of the gradient of the chemical potential), the random walk motion of many particles results in their net diffusion motion in the direction opposite to the direction of the gradient. The diffusion process is affected by many factors, such as interaction between diffusing adspecies, formation of surface phases, presence of defects, etc.