First STM spectroscopy of graphene flakes yields new surprises
Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley have performed the first scanning tunneling spectroscopy of graphene flakes equipped with a “gate” electrode.
The result is the latest in a series of surprising insights into the electronic behavior of this unique, two-dimensional crystal form of carbon: an unexpected gap-like feature in the energy spectrum of electrons tunneling into graphene’s single layer of atoms.
Michael Crommie, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor in the Department of Physics at UC Berkeley, explains that this peculiar feature of the electronic structure of graphene arises from the interaction of the tunneling electrons with phonons, the quantized vibrations of the 2-D graphene crystal, and may lead to novel applications for future graphene nanodevices.
A team led by Yuanbo Zhang, a postdoctoral fellow in Crommie’s research group, discovered graphene’s mysterious energy gap; the research appears in advanced online publication on the Nature Physics website at http://dx.doi.org/10.1038/nphys1022.
“Monolayer graphene – flakes a single atom thick — were first isolated by Andrew Geim’s group at the University of Manchester, England, in 2005, and have been intensely studied since then,” says Crommie. “Graphene’s interesting electronic effects opens a new realm of basic science. It’s an entirely new material, with new physics that could lead to new practical devices and applications. In that respect it’s as promising as carbon nanotubes — but graphene’s planar geometry is potentially even more versatile.”
Crommie says, “Because graphene is two-dimensional, it can be carved up and cut into tailored shapes, like cutting a sheet of paper.” The shape might include features like narrow sections to control the flow of electrons, edges with unique magnetic properties, and dopant atoms implanted at precise locations in the 2-D matrix.
“Two-dimensionality confers an amazing degree of flexibility,” he says, “but to take full advantage of this new material, we will need to understand what is happening at atomic length scales. That’s where the STM — the scanning tunneling microscope — comes in.”
Studying gated graphene with the STM
The business end of the STM is the tip, a fine metal wire placed in close proximity to a conducting surface — in this case a flake of graphene contacted by thin metal electrodes. An applied voltage between the tip and sample causes electrons to tunnel between them — a “tunnel current.” At constant voltage the tunnel current depends on the position of the tip with respect to the surface, so by scanning the tip across the flake the surface topography can be mapped.
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