Researchers have explored graphene’s extraordinary electronic properties for numerous applications over the past few years, from superfast transistors to extremely dense memory chips. Now, for the first time, IBM researchers are exploiting graphene’s unique properties for optoelectronics, using graphene sheets to make photodetectors.
Light detectors are typically made using III-V semiconductors–materials made of multiple elements such as gallium and phosphorus. When light hits these materials, each photon absorbed creates an electron-hole pair, and the electrons are then shuttled out of the material to produce an electrical current.
Graphene–a sheet of carbon atoms linked in a honeycomb structure–transports electrons tens of times faster than III-V semiconductors. That means that graphene photodetectors could work at extremely high frequencies, making them highly efficient at detecting light and transporting the resulting electrons to an external circuit. The material also absorbs wavelengths ranging from visible to infrared, whereas thin layers of III-V semiconductors don’t absorb many infrared frequencies.
Graphene has already been used to make several kinds of transistors, including ultrahigh radio-frequency devices. The highly conductive atom-thick sheets could also replace expensive and brittle indium tin oxide as the electrode material in flexible flat-panel displays and thin solar cells. People are also considering graphene for ultracapacitor electrodes and for dense and superfast computer memory.
Yet despite all these electronic applications, many experts considered graphene less than ideal for optical devices. This is because the electrons and holes generated by incoming photons normally combine in graphene within tens of picoseconds, leaving no free electrons for current. This also happens in a metal. But the speed with which the charged particles travel in graphene is key, says Phaedon Avouris, manager for nanometer scale science and technology at IBM’s T. J. Watson Research Center and the researcher who led the work, which is described in a paper published online in Nature Nanotechnology. “If we can have some kind of an electric field to separate the electron-hole pairs, we can collect them fast enough [for current].”
It is already known that when metal contacts are deposited on graphene, electric fields are generated at the interface between the two materials. So the researchers took advantage of this field. Their device is a piece of multilayered graphene with metal contacts on top. When they shine light near the contact, the field separates the electrons and holes, and a current is generated.