Graphene shows unusual thermoelectric response to light

Graphene, an exotic form of carbon consisting of sheets a single atom thick, exhibits a novel reaction to light, MIT researchers have found: sparked by light's energy, the material can produce electric current in unusual ways. The finding could lead to improvements in photodetectors and night-vision systems, and possibly to a new approach to generating electricity from sunlight.

Photo: Len Rubenstein

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 paper's lead author is postdoc Nathaniel Gabor; co-authors include four MIT students, MIT physics professor Leonid Levitov and two researchers at the National Institute for Materials Science in Tsukuba, Japan.

Instead, the MIT 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 ultralow 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.