  We demonstrate that 60 keV electron irradiation drives the diffusion of
threefold coordinated Si dopants in graphene by one lattice site at a time.
First principles simulations reveal that each step is caused by an electron
impact on a C atom next to the dopant. Although the atomic motion happens below
our experimental time resolution, stochastic analysis of 38 such lattice jumps
reveals a probability for their occurrence in a good agreement with the
simulations. Conversions from three- to fourfold coordinated dopant structures
and the subsequent reverse process are significantly less likely than the
direct bond inversion. Our results thus provide a model of non-destructive and
atomically precise structural modification and detection for two-dimensional
materials.
