Leaky atomic traps: Upward diffusion of Au from nanoscale pits on ionic-crystal surfaces

We have investigated the formation of gold nanoclusters during submonolayer deposition on atomically flat KBr and RbI (001) ionic-crystal substrates, as well as on substrates patterned with two-dimensional (2D) nanoscale pits produced by electron stimulated desorption. In this way, it is possible to produce atomic steps free from stress and local charging, which are normally considered the reason for enhanced cluster nucleation. Easy nucleation of the Au clusters inside the pits at the lower side of the atomic step edge is not observed, while nucleation of the Au nanoclusters is found to occur preferentially at the upper step edges. Moreover, we observe that gold atoms landing inside the bottom of the pits are able to escape from them by means of thermally activated upward diffusion at the step edges. We propose that the preferential Au nucleation sites at the edges of 2D pits are activated by F centers produced in electronic processes used for nanopatterning of the ionic crystals.