"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.
The current can also be varied by changing the voltage between tip and surface, which gives information about the electronic structure of the material in particular the local density of states (LDOS, an energy-dependent electron density) below the tip. Combining STM microscopy and spectroscopy allows a researcher to construct an image of the spatial distribution of the electronic states.
The Crommie group's experiments used exfoliated graphene, individual flakes made by mechanically cleaving a sheet of atoms from a larger chunk of carbon. The group attached electrodes to both the graphene flake and an underlying substrate consisting of a conducting layer of silicon, which was separated from the flake by an insulating layer of silicon dioxide. The experimental setup was thus able to uniquely incorporate two distinct voltage differences, that between the tip of the STM and the surface (the "bias" voltage) and that between the graphene flake and the underlying substrate (the "gate" voltage).
"The purpose of controlling the
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory