  We demonstrate assembly of di-, tri- and tetrameric Si clusters on the
graphene surface using sub-atomically focused electron beam of a scanning
transmission electron microscope. Here, an electron beam is used to introduce
Si substitutional defects and defect clusters in graphene with spatial control
of a few nanometers, and enable controlled motion of Si atoms. The Si
substitutional defects are then further manipulated to form dimers, trimers and
more complex structures. The dynamics of a beam induced atomic scale chemical
process is captured in a time-series of images at atomic resolution. These
studies suggest that control of the e-beam induced local processes offers the
next step toward atom-by-atom nanofabrication and provides an enabling tool for
study of atomic scale chemistry in 2D materials.
