Collective surface diffusion: An experimentalist's view

Surface diffusion (SD) is a really ubiquitous phenomenon playing a highly important part in a wealth of natural and technological processes. The effect of SD is to move surface atoms, molecules and clusters and allow them to assemble into some desirable configurations or, vise versa, to destroy the configurations that have been purposely created. Growth of crystals and thin films; catalysis; sintering and powder metallurgy; capillary phenomena; corrosion; nanotechnologies of all kinds; strengthening of materials; soldering; a multitude of processes that occur on various biological interfaces, etc.- this is an incomplete list of scientific and practical areas where SD can be a rate-controlling stage. SD has much in common with volume diffusion from the point of view of the mathematical description, but at the same time some important differences between them are connected with the lower dimensionality of SD and the peculiarities of surface interactions. This review presents an introduction to experimental techniques used in SD investigations and to some theoretical challenges which arise in the interpretation of experimental results on SD. It is stressed that SD is basically a many-body (collective) phenomenon. Even in the case when one observes random walks of an individual atom, their kinetics depends on the interaction of the adatom with substrate atoms, concerted motions of the atoms, lattice dynamics, and energy exchange and dissipation. At low adatom concentrations, the lateral (interadatom) interactions can result in formation of clusters, which demonstrate an amazing diversity of SD mechanisms and substantially affect the kinetics. At dense (approaching a monolayer) coverages, the lateral interactions give rise to formation of various two-dimensional adatom structures in the SD zone. Actually, there occurs a self-organization of this zone: it represents a (nonequilibrium) phase portrait of the adsorbed layer. The adsorbate phases that provide fast SD occupy major areas in the SD zone. Surface diffusion is of the utmost importance in nano-objects and nanostructures, where its role may be either useful or adverse. An intriguing challenge are nonlinear processes in SD. The mechanisms of SD of large organic molecules in chemistry and biology also call for much more attention both from theoreticians and experimentalists.