First-principles study of lateral atom manipulation assisted by structural relaxation of a scanning tip apex

Abstract

We report first-principles density-functional calculations that elucidate mechanisms of atom manipulation with precise placement of the tip of a noncontact atomic force microscope (AFM). We focus on the vacancy-mediated lateral manipulation of the Si adatom on the Si(111)-($7\ifmmode\times\else\texttimes\fi{}7$) surface where intriguing adatom movement approaching (approach move) or following (follow move) the tip has been observed experimentally. We identify the diffusion pathways of the adatom from a stable position to another with and without the AFM tip and obtain the corresponding diffusion barriers. We find that a diffusion barrier as high as 1 eV without the AFM tip is reduced by a half eV with the assistance of the tip, indicating the increased feasibility of the adatom motion with the assistance of the AFM tip. More importantly, we find that the energy landscape of the adatom motion is modified drastically with the presence of the AFM tip: The most stable positions without the tip become metastable with the tip, whereas the metastable position becomes the most stable; the metastable position becomes unstable in some cases. We find that all these modifications of the energy landscapes depend on the atom-scale positioning of the AFM tip, inducing the spontaneous adatom move, i.e., the atom manipulation via the AFM tip. The obtained theoretical findings unveil the reason for the approach move and follow move. The underlying physics and chemistry of this atom manipulation are found to be the structural relaxation of the tip apex atom and the subsequent bond formation with the diffusing adatom.