An international research team, co-led by researchers at the University of California, Riverside, which also included researchers at MIT, Nanyang Technological University, Singapore; Institute of High Performance Computing, Singapore; UC Berkeley; and National Institute for Materials Science, Japan, has found a new mechanism for highly-efficient charge and energy flow in graphene, opening the door to new types of light-harvesting devices.
The researchers made pristine graphene into different geometric shapes, connecting narrow ribbons and crosses to wide open rectangular regions. They found that when light illuminated constricted areas, such as the region where a narrow ribbon connected two wide regions, a large light-induced current, or photocurrent, was detected.
The team explains that in most solar energy harvesting devices, a photocurrent arises only in the presence of a junction between two dissimilar materials, such as p-n junctions, the boundary between two types of semiconductor materials. The electrical current is generated in the junction region and moves through the distinct regions of the two materials.
But in graphene, everything changes, said Nathaniel Gabor, an associate professor of physics at UCR, who co-led the research project. We found that photocurrents may arise in pristine graphene under a special condition in which the entire sheet of graphene is completely free of excess electronic charge. Generating the photocurrent requires no special junctions and can instead be controlled, surprisingly, by simply cutting and shaping the graphene sheet into unusual configurations, from ladder-like linear arrays of contacts, to narrowly constricted rectangles, to tapered and terraced edges.
Pristine graphene is completely charge neutral, meaning there is no excess electronic charge in the material. When wired into a device, however, an electronic charge can be introduced by applying a voltage to a nearby metal. This voltage can induce positive charge, negative charge, or perfectly balance negative and positive charges so the graphene sheet is perfectly charge neutral.
The light-harvesting device we fabricated is only as thick as a single atom, Gabor said. We could use it to engineer devices that are semi-transparent. These could be embedded in unusual environments, such as windows, or they could be combined with other more conventional light-harvesting devices to harvest excess energy that is usually not absorbed. Depending on how the edges are cut to shape, the device can give extraordinarily different signals.
The research team has found evidence that the new mechanism results in a greatly enhanced photoresponse in the infrared regime with an ultrafast operation speed.
We plan to further study this effect in a broad range of infrared and other frequencies, and measure its response speed, said first author Qiong Ma, a postdoctoral associate in physics at the Massachusetts Institute of Technology, or MIT.