  In the last decade, the atomically focused beam of a scanning transmission
electron microscope (STEM) was shown to induce a broad set of transformations
of material structure, open pathways for probing atomic-scale reactions and
atom-by-atom matter assembly. However, the mechanisms of beam-induced
transformations remain largely unknown, due to an extreme mismatch between the
energy and time scales of electron passage through solids and atomic and
molecular motion. Here, we demonstrate that a single dopant Si atom in the
graphene lattice can be used as an atomic scale force sensor, providing
information on the random force exerted by the beam on chemically-relevant time
scales. Using stochastic reconstruction of molecular dynamic simulations, we
recover the potential energy landscape of the atom and use it to determine the
beam-induced effects in the thermal (i.e. white noise) approximation. We
further demonstrate that the moving atom under beam excitation can be used to
map potential energy along step edges, providing information about atomic-scale
potentials in solids. These studies open the pathway for quantitative studies
of beam-induced atomic dynamics, elementary mechanisms of solid-state
transformations, and predictive atom-by-atom fabrication.
