Researchers from MIT and National Institute for Materials Science in Tsukuba, Japan have found that graphene can exhibit an unusual thermoelectric response to light. This current-generating effect had been observed before, but researchers had incorrectly assumed it was due to a photovoltaic effect, says Pablo Jarillo-Herrero, an assistant professor of physics at MIT and senior author of a new paper published in the journal Science.
The researchers found that shining light on a sheet of graphene, treated so that it had two regions with different electrical properties, creates a temperature difference that, in turn, generates a current. Graphene heats inconsistently when illuminated by a laser, Jarillo-Herrero and his colleagues found: The material’s electrons, which carry current, are heated by the light, but the lattice of carbon nuclei that forms graphene’s backbone remains cool. It’s this difference in temperature within the material that produces the flow of electricity. This mechanism, dubbed a “hot-carrier” response, is very unusual, Jarillo-Herrero says.
Such differential heating has been observed before, but only under very special
circumstances: either at ultra-low temperatures (measured in thousandths of a degree above absolute zero), or when materials are blasted with intense energy from a high-power laser. This response in graphene, by contrast, occurs across a broad range of temperatures all the way up to room temperature, and with light no more intense than ordinary sunlight.
The reason for this unusual thermal response, Jarillo-Herrero says, is that graphene is, pound for pound, the strongest material known. In most materials, superheated electrons would transfer energy to the lattice around them. In the case of graphene, however, that’s exceedingly hard to do, since the material’s strength means it takes very high energy to vibrate its lattice of carbon nuclei—so very little of the electrons’ heat is transferred to that lattice.
Because this phenomenon is so new, Jarillo-Herrero says it is hard to know what its ultimate applications might be. Suggestions include applications as a photodetector. Because it works very well in infrared light, which can be difficult for other detectors to handle, it could serve as an important component of devices from night-vision systems to advanced detectors for new astronomical telescopes.
The new work suggests graphene could also find uses in detection of biologically important molecules, such as toxins, disease vectors or food contaminants, many of which give off infrared light when illuminated. And graphene, made of pure and abundant carbon, could be a much cheaper detector material than presently used semiconductors that often include rare, expensive elements.
The research also suggests graphene could be a very effective material for collecting solar energy, Jarillo-Herrero says, because it responds to a broad range of wavelengths; typical photovoltaic materials are limited to specific frequencies, or colors, of light. But more research will be needed, he says, adding, “It is still unclear if it could be used for efficient energy generation. It’s too early to tell.”
The research was supported by the Air Force Office of Scientific Research, along with grants from the National Science Foundation and the Packard Foundation.
Nathaniel M. Gabor, Justin C. W. Song, Qiong Ma, Nityan L. Nair, Thiti Taychatanapat, Kenji Watanabe, Takashi Taniguchi, Leonid S. Levitov, and Pablo Jarillo-Herrero (2011) Hot Carrier–Assisted Intrinsic Photoresponse in Graphene. Science DOI: 10.1126/science.1211384