Imaging Graphene
Research into graphene, a two dimensional crystal consisting of a single atomic plane of carbon atoms, has gone from strength to strength since its discovery by Geim's group in the Physics Department at Manchester University.
Interest in graphene has originally been driven by the extraordinary properties of its charge carriers, mimicking relativistic elementary particles and exhibiting ballistic electron transport on the submicron scale, even at room temperature. Hence graphene bears great promise for manufacture of ultra fast devices, e.g., transistors. Graphene is also being investigated for its mechanical, physio- and chemi-sorption properties which make it a potential candidate for a gas sensor.
In all this it has been of prime importance to give direct evidence that graphene, in particular free-standing graphene, does indeed exist. This could only be achieved by atomic resolution electron microscopy. Several laboratories around the world have attempted this by employing high resolution phase contrast imaging Images obtained by this method, however, are subject to interpretation. Besides, many of these images have been of multi layer graphene.
Uschi Bangert from the School of Materials and her colleagues have, for the first time, obtained atomic scale images of free-standing single layer graphene by using the high angle annular dark field imaging technique, carried out in an aberration corrected scanning transmission electron microscope (the Daresbury SuperSTEM). The atomic lattice seen here is a direct depiction of the ball-and-stick model, where bright contrast corresponds to atoms and dark contrast to the gaps in between (above). This means also that defects, such as missing atoms, are immediately visible (see black region on left hand side of the image).
The ability of free-standing graphene to exist is the subject of a long-standing theoretical debate, and it has previously been suggested that 2D films could achieve stability in 3D space by out-of-plane undulations. By using specific spatial frequency filters, ripples in free-standing graphene can be easily revealed, as can be witnessed in the image below. The black centres of the atomic hexagonal rings are visible throughout the image, but out-of-plane bending of atoms becomes apparent by the yellow regions in the image. These ripples are less then 0.5 nm in height.
Image of Graphene Ripples
Further Reading
- Novoselov K S, Geim A K, Morozov S V, Jiang D, Katsnelson M I, Grigorieva I V, Dubonos S V and Firsov A A, Nature 438, 197-200 (2005).
- Zhang Y, Tan Y W, Stormer H L and Kim P, Nature 438, 201-204 (2005).
- Meyer J C, Geim A K, Katsnelson M I, Novoselov K S, Booth T J and Roth S, Nature 446, 60-63 (2007)
- Meyer J.C., Kisielowski C., Erni R., Rossell M.D., Crommie M.F., Zettl A., Nano Lett., DOI: 10.1021/nl801386m (2008)
- Gass M H, Bangert U, Bleloch A L, Wang P, Nair R R, Geim A K, Nature Nanotech. doi: 10.1038/nnano.2008.280. (2008)
- A. Fasolino, J. H. Los and M. I. Katsnelson, Nature Materials 6, 858 (Nov 2007)