Wang adds, "The hallmark of quantum mechanics is that if different paths are nondistinguishable, they must always interfere with each other. We can manipulate the interference among the quantum pathways that are responsible for Raman scattering in graphene because of graphene's peculiar electronic structure."
In Raman scattering, the quantum pathways are electronic excitations, which are optically stimulated by the incoming photons. These excitations can only happen when the initial electronic state is filled (by a charged particle such as an electron), and the final electronic state is empty.
Quantum mechanics describes electrons filling a material's available electronic states much as water fills the space in a glass: the "water surface" is called the Fermi level. All the electronic states below it are filled and all the states above it are empty. The filled states can be reduced by "doping" the material in order to shift the Fermi energy lower. As the Fermi energy is lowered, the electronic states just above it are removed, and the excitation pathways originating from these states are also removed.
"We were able to control the excitation pathways in graphene by electrostatically doping it applying voltage to drive down the Fermi energy and eliminate selected states," Wang says. "An amazing thing about graphene is that its Fermi energy can be shifted by orders of magnitude larger than conventional materials. This is ultimately due to graphene's two-dimensionality and its unusual electronic bands."
The Fermi energy of undoped graphene is located at a single point, where its electronically filled bands, graphically represented as an upward-pointing cone, meet its electronically empty bands, represented as a downward-pointing cone. To move the Fermi energy appreciably requires a strong electri
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory