Graphene, a flat sheet of hexagonally arranged carbon atoms, can transport electrons very quickly. This has made it a promising material for high radio-frequency logic circuits, transparent electrodes for flexible flat-panel displays, and high-surface-area electrodes for ultracapacitors.
Now researchers at the National University of Singapore have made computer memory devices using graphene. This is the first step toward memory that could be much denser and faster than the magnetic memory used in today’s hard drives. The researchers have made hundreds of prototype graphene memory devices, and they work reliably, according to Barbaros Özyilmaz, the physics professor who led the work presented at a recent American Physical Society meeting in Pittsburgh. “Graphene is going to change the electronic industry,” he says. “What was missing was a way to use graphene as a memory element. So far there was almost no interest because it wasn’t [thought] doable.”
The key to making memory elements is a material that can have two different states. That is because computer memory is stored as two bits: 1 and 0. Hard drives also need to be nonvolatile, which means the material should be able to hold on to those states without requiring power. Today’s hard disks are made of magnetic cobalt alloys, and they store bits as one of two magnetic orientations of a small area on the disk.
Özyilmaz and his colleagues came up with an easy way to make graphene hold its two different levels of conductivity, or resistance. Switching between these levels requires applying and removing an electric field. The researchers deposit a thin layer of a ferroelectric material on top of the graphene. Ferroelectrics have an intrinsic electric field, and applying a voltage changes the direction of the field. The ferroelectric’s field helps graphene sustain its conductivity. And, Özyilmaz explains, “we can change the polarization of the ferroelectric, which in turn changes the conductivity of graphene.”
The new memory idea is “thrilling because it’s very simple,” says Andre Geim, professor of physics at the University of Manchester, UK, who first isolated graphene sheets from graphite. “Ferroelectrics are well known. It’s also known that an electric field changes graphene’s resistivity by a factor of typically 10. [Özyilmaz] combines those two very well-known facts.”