  We investigate the graphene-impurity interaction problem by combining
experimental - scanning tunneling microscopy (STM) and spectroscopy (STS) - and
theoretical - Anderson impurity model and density functional theory (DFT)
calculations - techniques. We use graphene on the SiC(000-1)(2x2)_C
reconstruction as a model system. The SiC substrate reconstruction is based on
silicon adatoms. Graphene mainly interacts with the dangling bonds of these
adatoms which act as impurities. Graphene grown on SiC(000-1)(2x2)_C shows
domains with various orientations relative to the substrate so that very
different local graphene/Si adatom stacking configurations can be probed on a
given grain. The position and width of the adatom (impurity) state can be
analyzed by STM/STS and related to its local environment owing to the high bias
electronic transparency of graphene. The experimental results are compared to
Anderson's model predictions and complemented by DFT calculations for some
specific local environments. We conclude that the adatom resonance shows a
smaller width and a larger shift toward the Dirac point for an adatom at the
center of a graphene hexagon than for an adatom just on top of a C graphene
atom.
