Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But there’s a catch: graphene has no bandgap.
“Having no bandgap greatly limits graphene’s uses in electronics,” says Feng Wang of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, where he is a member of the Materials Sciences Division. “For one thing, you can build field-effect transistors with graphene, but if there’s no bandgap you can’t turn them off! If you could achieve a graphene bandgap, however, you should be able to make very good transistors.”
Wang, who is also an assistant professor in the Department of Physics at the University of California at Berkeley, has achieved just that. He and his colleagues have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts (250 meV, or .25 eV).
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.”
What these researchers basically did was to create a material that could replace semiconductors one day with a cheap and simple structure, allowing multicolour LEDs to be fabricated. They could be printed on virtually anything, and unleash a whole new set of displaying possibilities.
