A team of researchers from the University of Pennsylvania, University of California and University of Illinois at Urbana-Champaign has demonstrated a way to change the amount of electrons that reside in a given region within a piece of graphene and have a proof-of-principle in making the fundamental building blocks of semiconductor devices using the 2D material. Their method enables this value to be tuned through the application of an electric field, which means that graphene circuits made in this way could theoretically be manipulated without physically altering the device.
Chemically doping graphene to achieve p- and n-type version of the material (similar to traditional circuits) is possible, but it comes at the price of sacrificing some of its unique electrical properties. A similar effect is possible by applying local voltage changes to the material, but manufacturing and placing the necessary electrodes is complicated. The team of researchers claims it has now come up with a non-destructive, reversible way of doping, that doesn’t involve any physical changes to the graphene.
The team’s technique involves depositing a layer of graphene so it rests on, but doesn’t bond to, a second material: lithium niobate. Lithium niobate is ferroelectric, meaning that it is polar, and its surfaces have either a positive or negative charge. Applying an electric field pulse can change the sign of the surface charges.
The researchers took advantage of the fact that a certain type of the material, periodically poled lithium niobate, is available so that it has stripes of polar regions that alternate between positive and negative. Different regions of graphene can take on different characters depending on the nature of the domain underneath. That allows for a simple means of creating a p-n junction or even an array of p-n junctions on a single flake of graphene. Such an ability should facilitate advances in graphene that might be analogous to what p-n junctions and complementary circuitry has done for current semiconductor electronics.
Also exciting is the possibility of optoelectronics using graphene and the possibility of waveguiding, lensing and periodically manipulating electrons confined in an atomically thin material. The team's experiments also involved adding a single gate to the device, which allowed for its overall carrier density to be further tuned by the application of different voltages.