When current is applied, explains Geim, an electron jumps from the source to the quantum dot, and then to the drain. Quantum dots can only accommodate the single electron, therefore “electrons flow one at a time, or not at all,” says Geim. “By changing the charge of the central dot, you can control the flow of electrons through it.”
The problem with most single-electron transistors, says Geim, is that materials used for the quantum dots aren’t stable enough at room temperature. When most metals are shrunk to the size of quantum dots–about two to three nanometers wide–they become fragile and move around, “like a droplet of liquid on a hot plate,” he says. Graphene quantum dots, however, are stable at room temperature and can even withstand high electrical current. “The most important part of our work,” Geim says, “is to demonstrate that graphene can be confined to a few nanometers in size.”
Still, manufacturing these transistors will be tough. In fact, there is no fabrication technique that can reliably produce three-nanometer-size quantum dots. Geim’s devices were made using electron-beam lithography, but most of the transistors didn’t have usable quantum dots. The researchers “had to rely on luck,” he says, to get the right-size dots–and thus usable transistors. However, Geim notes, as the semiconductor industry improves its fabrication techniques, graphene will benefit.Geim’s work was published in the current issue of Nature Materials as part of a peer-reviewed article summarizing the state of graphene research. He does not intend to fully report his results for another six months. In the meantime, other groups are working on similar designs. Researchers at Georgia Institute of Technology, in Atlanta, led by Walter de Heer, a professor of physics, have been working to develop graphene transistors since 2001. The work is supported by Intel, and patents have already been issued for graphene-transistor designs.
Right now, graphene research is competitive, and it’s still not clear which approach will prevail. As graphene gains traction, engineers are enthusiastic about the possibilities of the material. “The recently developed graphene devices have created enormous excitement in the field,” says Ali Javey, professor of electrical engineering and computer sciences at the University of California, Berkeley. And he believes that after silicon, graphene will play an important role in electronics.