Simulation of lateral manipulation with dynamic AFM: interchange of Sn and Ge adatoms on Ge(111)-c(2x8) surface

摘要

Density functional methods are used to study a recent non- contact atomic force microscopy ( NC- AFM) experiment of the lateral manipulation of Sn and Ge adatoms on a Ge( 111)- c( 2 x 8) surface, with the quest to elucidate the atomistic details of the manipulation process. Two different processes were considered, namely a tip- assisted surface diffusion, and active tip participation in the pair exchange via adatom pick- up/ deposition processes. At variance with previous results, the calculated adatom diffusion profiles indicate that the diffusion is almost isotropic in both [ 1 (1) over bar 10] and [11 (2) over bar] directions, with fairly modest energy barriers between 0.6 and 0.8 eV. These barriers can be reduced by action of the tip. Compression of a rest atom by a tip can completely delocalize the adatoms on the surface, so that they will move in a barrierless way. However, a diffusion mediated exchange mechanism appears to be precluded by a large barrier to the ( Sn, Ge) pair exchange, which in all cases studied was larger than 1 eV. This leaves the other mechanism open, whereby a tip- induced adatom pick- up/ deposition enters the exchange process.