Intelligent Machines

Detecting Light with Graphene

The atom-thick carbon material could have optoelectronic applications.

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 detector: A scanning-electron-microscope image and an optical image (inset) show a high-bandwidth graphene photodetector. Metal contacts deposited on the graphene create an electric field that separates electrons, allowing the device to detect light.

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.

Light detector: A graphene photodetector takes advantage of the electric field that is created at the interface between metal contacts (gold) and graphene. When light falls on graphene, the field helps to separate electrons from holes, leading to an electric current.

A single sheet of graphene absorbs 2.3 percent of the light falling on it, a significant amount for a one-atom-thick material. “You have a photodetector that has a number of advantages: it absorbs over a wide wavelength range, it’s very fast, it has a high absorbance, it’s a single atomic layer,” Avouris says. “This combination makes it rather unique.”

Ultrafast photodetectors could find use in future optical communications networks with data rates beyond 40 gigabits per second; right now, optical networks have data rates of about 10 gigabits per second. The photodetectors could also be used in optical computers that compute with electrons but transfer data using light instead of sending it over heat-prone copper wires. Fengnian Xia, a coauthor of the paper, says that graphene would also make a better detector for terahertz radiation, which has shown promise for medical and security imaging.

“Graphene is a great material for electronics,” says Andre Geim, a professor of physics at the University of Manchester, U.K. “Very few people could think about optoelectronics being of any interest with this material. This is like fresh air.”

The researchers get current in response to light pulses at a frequency of 40 gigahertz. Frequencies higher than this are not possible with today’s electronics, says Avouris, but graphene could, in theory, enable photodetectors that work at frequencies even higher than 0.5 terahertz.

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