  Atomic resolution high angle annular dark field imaging of suspended,
single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al and Au atoms
had been deposited was carried out in an aberration corrected scanning
transmission electron microscope. In combination with electron energy loss
spectroscopy, employed to identify individual impurity atoms, it was shown that
nano-scale holes were etched into graphene, initiated at sites where single
atoms of all the metal species except for gold come into close contact with the
graphene. The e-beam scanning process is instrumental in promoting metal atoms
from clusters formed during the original metal deposition process onto the
clean graphene surface, where they initiate the hole-forming process. Our
observations are discussed in the light of calculations in the literature,
predicting a much lowered vacancy formation in graphene when metal ad-atoms are
present. The requirement and importance of oxygen atoms in this process,
although not predicted by such previous calculations, is also discussed,
following our observations of hole formation in pristine graphene in the
presence of Si-impurity atoms, supported by new calculations which predict a
dramatic decrease of the vacancy formation energy, when SiOx molecules are
present.
