  We demonstrate that the sub-atomically focused beam of a scanning
transmission electron microscope (STEM) can be used to controllably manipulate
individual dopant atoms in a 2D graphene lattice. We demonstrate the
manipulation of adsorbed source materials and the graphene lattice with the
electron beam such that individual vacancy defects can be controllably
passivated by Si substitutional atoms. We further demonstrate that these Si
defects may be directed through the lattice via e-beam control or modified (as
yet, uncontrollably) to form new defects which can incorporate new atoms into
the graphene lattice. These studies demonstrate the potential of STEM for
atom-by-atom nanofabrication and fundamental studies of chemical reactions in
2D materials on the atomic level.
