Imaging single-molecule-resolved mass transport via electromigration at the surface of graphene field-effect transistors*

Electromigration is the process whereby applied electric fields and momentum transfer from conducting electrons induce mass transport of atoms in a material. While this causes degradation in nanoscale electronics, it also creates opportunities for controlling matter in new ways based on particle diffusion. Many studies have investigated mass transport in bulk nanostructures, but there remains a lack of understanding regarding how momentum is transferred at the single atom level. Here we use scanning tunneling microscopy to investigate current-induced molecular drift at the surface of graphene field-effect transistors (FETs). By tuning the FET source-drain current, we are able to control the direction and the magnitude of the drift velocity of molecules on the surface. Scattering theory allows us to understand how momentum is transferred from flowing electrons to surface adsorbates. Our observations are supported by theoretical non-equilibrium green's function calculations that reveal how charge redistributes when current flows and how this leads to a scattering-induced force. We observe a strong resonance effect in the magnitude of electromigration when the device Fermi level is tuned into energy alignment with molecular orbitals.