Theory of atom transfer with a scanning tunneling microscope

Abstract

We present and discuss in detail a theory for atom transfer (or bond breaking) using the tip of a scanning tunneling microscope that was outlined by us [Solid State Commun. 84, 271 (1992)]. The theory is applied to an atomic switch [Nature 352, 600 (1991)]. In this theory the bond is broken by overcoming the associated potential barrier thanks to a gain in energy from the tunneling electrons. The barrier crossing is described by a truncated harmonic oscillator and the inelastic electron tunneling is modeled by a simple resonance model for the electronic structure. The rate of atom transfer is shown to be Arrhenius-like with a vibrational temperature set by the inelastic tunneling rate. Characteristic features of this mechanism include a crossover from current-driven to thermally activated bond breaking with decreasing applied voltage and a power-law dependence of the bond-breaking rate with the applied voltage, the latter in agreement with experimental findings. We have also identified a current-induced force in the resonance model for tunneling, which in some cases may give an important current-dependent contribution to the potential-energy surface. The general features of our theory should have relevance for many other electronically driven surface processes.