  Single atoms and few-atom nanoclusters are of high interest in catalysis and
plasmonics, but pathways for their fabrication and stable placement remain
scarce. We report here the self-assembly of room-temperature-stable single
indium (In) atoms and few-atom In clusters (2-6 atoms) that are anchored to
substitutional silicon (Si) impurity atoms in suspended monolayer graphene
membranes. Using atomically resolved scanning transmission electron microscopy
(STEM), we find that the exact atomic arrangements of the In atoms depend
strongly on the original coordination of the Si anchors in the graphene
lattice: Single In atoms and In clusters with 3-fold symmetry readily form on
3-fold coordinated Si atoms, whereas 4-fold symmetric clusters are found
attached to 4-fold coordinated Si atoms. All structures are produced by our
fabrication route without the requirement for electron-beam induced materials
modification. In turn, when activated by electron beam irradiation in the STEM,
we observe in situ the formation, restructuring and translation dynamics of the
Si-anchored In structures: Hexagon-centered 4-fold symmetric In clusters can
(reversibly) transform into In chains or In dimers, whereas C-centered 3-fold
symmetric In clusters can move along the zig-zag direction of the graphene
lattice due to the migration of Si atoms during electron-beam irradiation, or
transform to Si-anchored single In atoms. Our results provide a novel framework
for the controlled self-assembly and heteroatomic anchoring of single atoms and
few-atom clusters on graphene.
