Light-Induced Surface Diffusion

Surface diffusion of indigenous and/or foreign atoms plays a key role in a number of physical and chemical processes. To name a few, it is important in crystal growth and epitaxy, heterogeneous catalysis, nucleation and growth of supported nanoparticles, and so on. Finding a reliable tool to control the surface diffusion processes is an attractive goal for many modern technologies. Optical photons being absorbed by the surface or by the species adsorbed onto it can alter the surface diffusion considerably. At lager intensities of illumination these alternations are mainly due to the temperature rise, while at the lower intensities non-thermal mechanisms of light-induced surface diffusion are operative. The latter are the subjects of this chapter. The electronic excitation follows after the photon absorption and changes the forces exerted by the surface onto the adsorbed atoms. After a short period of time the energy of the photon is partitioned between the surface and the adsorbed atom. The excess energy obtained by the adsorbed atom results in the increased desorption rates from as well as diffusion rates over the surface. An inhomogeneous illumination of the surface leads to the inhomogeneous steady state distribution of the adsorbed atoms over the surface. The situation is similar to the Soret effect but require a special theoretical consideration that is presented in this chapter. An unexpected result of the theoretical analysis is that the spatial distribution of the surface number density of the adsorbed atoms is non monotone. There is a pronounced maximum of the surface number density of the adsorbed atoms at the boundary between the illuminated and the dark regions. The shapes of the supported metal nanoparticles obtained via Volmer-Weber growth mode are metastable. Heating is known to speed up the equilibration of the particles shapes. In our experiments with silver and sodium nanoparticles supported on dielectric surfaces we found evidences of the reversible changes of the particle shapes. Hence, the temperature of the substrate determines the equilibrium shape of the nanoparticles. In the case of sodium, illumination speeds up the particles reshaping. This process is rationalized in terms of the light-induced diffusion of the indigenous atoms over the metal nanoparticle surface, while the main step of the process is identified as the photo-induced detachment of an atom from the terraces. The latter is found to be the rate limiting step in the nanoparticle reshaping process.