November 25, 2008 (Tuesday)
3:00 pm in Howey N110

"Probing the Electronic Properties of Graphene at

the Atomic Scale"

Dr. Yuanbo Zhang, University of California at Berkeley

Graphene, a single atomic layer of carbon, has generated great enthusiasm recently due to its novel electronic, optical, and mechanical properties. Most strikingly, the electrons in graphene behave like two-dimensional Dirac fermions, which leads to an unusual quantum Hall as well as other quantum effects that persists even to room temperature.  Novel scattering phenomena have also been predicted to arise from graphene’s unique honey-comb lattice and its intricate interplay with impurities and defects. Scanning tunneling microscopy offers an ideal tool to study the local electronic properties of graphene at the lengthscales necessary to resolve such phenomena.  Using this technique we have observed a pronounced gap-like feature in the graphene tunneling spectrum that is pinned to the Fermi level and which arises due to unexpected phonon-assisted inelastic tunneling. We also have found that the graphene charge neutral point (called the Dirac point) manifests itself as a clear feature in our spectra that can be shifted by an applied gate voltage. By spatially mapping this feature we are able to map out electron density inhomogeneities in graphene with a spatial resolution that is two orders of magnitude higher than previous experiments.  Using this new technique we have observed charge inhomogeneities that coexist with energy-dependent electronic interference patterns, giving us new insight into the microscopic mechanisms that determine graphene electron mobility.