When a material is heated, its electrical properties, such as resistance, can change. This effect it utilized in a class of devices called bolometers, which measure the change in resistance due to the heating of electrons. Bolometers are used for a variety of applications that require infrared observations, including astronomy, chemical analysis and security. Now researchers at the University of Maryland are turning to graphene to advance the technology.
Graphene is an atom-thick plane of carbon, but in this case the researchers are using bilayer graphene. Researchers have known for some time now that graphene reacts quite well to a wide range of light frequencies, stretching from terahertz radiation, through infrared, and into the optical range. This is in part because graphene does not have a band gap. Normally semiconductors are used in bolometers, because when light strikes a semiconductor, an electron is excited to a higher energy. However, semiconductors have a band gap separating the conducting and non-conducting energy levels of the electrons. This means only photons with enough energy to push an electron past the band gap are absorbed. Graphene has no band gap, so all photons can be absorbed.
There are some issues with using graphene in a bolometer though, such as heat not affecting the resistance of graphene. The researchers overcame this by placing the graphene in an electric field, which not only increased the material's resistance but also made it highly dependent on temperature. When cooled to just 5 K, the bilayer graphene bolometer was as sensitive as modern bolometers, but operated thousands of times faster, thanks to graphene's extraordinarily high electron mobility.
However there are still challenges to overcome, including the fact that the graphene is only absorbing a few percent of the light that hits it, and that this device's resistance is still higher than similar devices. The researchers are confident these can be overcome though and that we may see graphene bolometers in the future.