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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 electrons) is finite, and fixed by the crystalline structure of the material. In bilayer graphene, however, as Wang's team demonstrated, the bandgap is variable and can be controlled by an electrical field. Although a pristine graphene bilayer has zero bandgap and conducts like a metal, a gated bilayer can have a bandgap as big as 250 milli-electron volts and behave like a semiconductor.
With precision control of its bandgap over a wide range, plus independent manipulation of its electronic states through electrical doping, dual-gated bilayer graphene becomes a remarkably flexible tool for nanoscale electronic devices.
Wang emphasizes that these first experiments are only the beginning. "The electrical performance of our demonstration device is still limited, and there are many routes to improvement, for example through extra measures to purify the substrate."
Nevertheless, he says, "We've demonstrated that we can arbitrarily change the bandgap in bilayer graphene from zero to 250 milli-electron volts at room temperature, which is remarkable in itself and shows the potential of bilayer graphene for nanoelectronics. This is a narrower bandgap than common semiconductors like silicon or gallium arsenide, and it could enable new kinds of optoelectronic devices for generating, amplifying, and detecting infrared light."
"Direct observation of a widely tunable bandgap in bilayer graphene," by Yuanbo Zhang, Tsung-Ta Tang, Caglar Girit, Zhao Hao, Michael C. Martin, Alex Zettl, Michael F. Crommie, Y. Ron Shen, and Feng Wang, appears in the June 11, 2009 issue of Nature. Zhang, Tang, and Girit are members of UC Berkeley's Department of Physics, in the groups of Professors Crommie, Shen, and Zettl respectively; Zettl, Crommie, and Shen are also members of Berkeley Lab's Materials Sciences Division.
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