Atom and molecule manipulation by low-temperature STM

Surface imaging by STM is usually performed at constant tunneling current and at a tip-to-surface distance of several Ångstroms. Reducing the tip-to-surface distance and scanning the tip across the surface enables to move single atoms and molecules along the surface by short-range tip-adsorbate interactions. On metal surfaces, we normally take advantage of these short-range forces to move adsorbates and to assemble artificial structures (note that STM does not permit to directly measure these forces). Generally, different processes such as force interactions, inelastic excitations by the tunneling electrons, and the tip-induced electric field may allow for a controlled repositioning of adsorbates. On semiconductor surfaces, for example, we find that the latter two processes are important for the vertical manipulation of adsorbates (that is, the reversible transfer of adsorbates between STM tip and surface). Cryogenic temperatures are an essential requirement for atom manipulation in order to (i) freeze out surface diffusion and to (ii) achieve sufficient drift stability for manipulation with atomic-scale precision.

        Lateral manipulation:   Moving atoms/molecules along the surface

          Vertical manipulation: Transfer of atoms/molecules between surface and STM tip