18 August 2008—Researchers have discovered a new way to switch current on and off in graphene, pointing the way to the possibility of molecule-size memory.
Graphene is a 1-atom-thick carbon molecule in which electrons flow 100 times as fast as they do in silicon. In theory, a graphene transistor would be 100 times as fast as the same device made of silicon. One challenge, though, is that graphene is so conductive that it’s hard to stop current from flowing, and such on-off switching is necessary for any sort of transistor.
Now, with help from the scientist in England who first isolated graphene, a group of researchers at AMO, a nanotechnology company in Germany, has come up with a novel mechanism for making a graphene switch. According to research published in the August issue of IEEE Electron Device Letters, the researchers found that by applying an electrical field to the material, they could cause a chemical change that altered the conductance of graphene. They built a transistor-like structure from graphene in which the graphene bridged two electrodes while a third electrode sat between them, separated from the graphene by a thin layer of silicon dioxide dielectric. The group applied a voltage of 5 volts to the gate electrode, causing the conductance of the graphene to decrease by more than six orders of magnitude and essentially cutting off the flow of current. When they reversed the voltage to +5 V, the conductance returned to nearly what it had been before the voltage was applied.
The group fabricated and tested tens of field-effect devices. The researchers called them devices rather than transistors to emphasize that they operate by a different mechanism than conventional transistors. In one test, they switched a device to the off state, removed the voltage, and left it that way for two days before switching it back. This experiment suggests that the system could be used to create nonvolatile memory. Although other types of memory elements are limited in how small they can get, it might be possible to shrink graphene memory down to a single molecule, says Max Lemme, one of the lead authors of the paper from AMO and now a Humboldt Research Fellow at the Center for Nanoscale Systems, at Harvard University. ”With graphene, we believe it can, in principle, be scaled down to a 1-nanometer-by-1-nanometer device,” Lemme says. The switching is not fast enough to be used in a logic circuit, he says, and researchers have not yet shown that it will work for the millions of cycles a memory device would require.