  The recent discovery of graphene has sparked significant interest, which has
so far been focused on the peculiar electronic structure of this material, in
which charge carriers mimic massless relativistic particle. However, the
structure of graphene - a single layer of carbon atoms densely packed in a
honeycomb crystal lattice - is also puzzling. On the one hand, graphene appears
to be a strictly two-dimensional (2D) material and exhibits such a high crystal
quality that electrons can travel submicron distances without scattering. On
the other hand, perfect 2D crystals cannot exist in the free state, according
to both theory and experiment. This is often reconciled by the fact that all
graphene structures studied so far were an integral part of larger 3D
structures, either supported by a bulk substrate or embedded in a 3D matrix.
Here we report individual graphene sheets freely suspended on a microfabricated
scaffold in vacuum or air. These membranes are only one atom thick and still
display a long-range crystalline order. However, our studies by transmission
electron microscopy (TEM) have revealed that suspended graphene sheets are not
perfectly flat but exhibit intrinsic microscopic roughening such that the
surface normal varies by several degrees and out-of-plane deformations reach 1
nm. The atomically-thin single-crystal membranes offer an ample scope for
fundamental research and new technologies whereas the observed corrugations in
the third dimension may shed light on subtle reasons behind the stability of 2D
crystals.
