"The problem with transport measurements is that they are too sensitive to defects," says Wang. "A tiny amount of impurity or defect doping can create a big change in the resistance of the graphene and mask the intrinsic behavior of the material. That's why we decided to go with optical measurements at the Advanced Light Source."
Using infrared beamline 1.4 at the ALS, under the direction of ALS physicist Michael Martin and Zhao Hao of the Earth Sciences Division, Wang and his colleagues were able to send a tight beam of synchrotron light, focused on the graphene layers, right through the device. As the researchers tuned the electrical fields by precisely varying the voltage of the gate electrodes, they were able to measure variations in the light absorbed by the gated graphene layers. The absorption peak in each spectrum provided a direct measurement of the bandgap at each gate voltage.
"In principle we could have used a tunable laser to measure the optical transmission, but the 1.4 beamline is very bright and can be focused down to the diffraction limit an important consideration when the graphene-flake target is so small," Wang says. "Also, compared to a laser, the beamline provides a wider range of frequencies all at once, so we don't have to painstakingly tune to each absorption frequency we're trying to measure."
The malleable electronic structure of bilayer graphene
The results from the ALS measurements were obtained with relative ease and efficiency, and showed that by independently manipulating the voltage of the two gates, the researchers could control two important parameters, the size of the bandgap and the degree of doping of the graphene bilayer. In essence, they created a virtual semiconductor from a material that is not inherently a semiconductor at all.
In ordinary semiconductors, the gap between the conduction band (unoccupied by electrons) the valence band (occupied by
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DOE/Lawrence Berkeley National Laboratory