  We present scanning tunneling microscopy (STM) images of single-layer
graphene crystals examined under ultrahigh vacuum conditions. The samples, with
lateral dimensions on the micron scale, were prepared on a silicon dioxide
surface by direct exfoliation of single crystal graphite. The single-layer
films were identified using Raman spectroscopy. Topographic images of
single-layer samples display the honeycomb structure expected for the full
hexagonal symmetry of an isolated graphene monolayer. The absence of observable
defects in the STM images is indicative of the high quality of these films.
Crystals comprised of a few layers of graphene were also examined. They
exhibited dramatically different STM topography, displaying the reduced
three-fold symmetry characteristic of the surface of bulk graphite.
