Graphene has many talents. And now it can count radio transmission among them.
A team led by James Hone and Kenneth Shepard at Columbia University in New York has demonstrated a device built from a strip of graphene that can transmit FM radio signals. The device, the team says, is the smallest FM transmitter yet made.
Many research groups have built graphene transistors that could be used in future RF circuits such as signal processors. Hone and his colleagues decided to test a different radio application for graphene, by building a moving, vibrating, electromechanical device. The team reckons that such graphene-based nanoelectromechanical systems (NEMS) could be more compact and easier to integrate onto chips than silicon MEMS and quartz devices, which are used today to pick up and filter RF signals in smartphones and other gadgets.
To build a graphene transmitter, the team suspended a 2-4 micrometer-long strip of graphene above a metal electrode. By applying a voltage to the electrode, they could draw the strip of graphene down. The resulting strain altered the strip's resonant frequency, tuning it up much as you might tighten a guitar string. By altering the voltage on the gate, the team found they could use the graphene device to generate a frequency-modulated electromagnetic signal. In a paper published this week in Nature Nanotechnology, they report the device could transmit radio signals at 100 MHz, right in the center of the FM band.
For an aural demonstration, the team queued up the now classic K-pop song "Gangnam Style" on an iPhone and fed it into one of their graphene devices. They picked up the result on a regular FM radio tuner that Hone had brought in from home. Here’s what it sounds like:
You’ll notice a fair amount of static in the audio clip. That’s partly because the graphene oscillator is quite sensitive to electrical noise: a small voltage on the gate electrode can dramatically shift the frequency, Hone says. The team didn't add insulation in order to optimize their set-up for this demonstration. “We were also trying to operate at 100 MHz, right smack in the middle of the FM spectrum, where you can pick up a lot of FM signals,” he adds.
But the transmitter was just a proof of principle demonstration. “I think the big kinds of applications here are in filters and signal processing," Hone says. The team next hopes to show that they can integrate graphene NEMS devices onto silicon chips.
Rachel Courtland, an unabashed astronomy aficionado, is a former senior associate editor at Spectrum. She now works in the editorial department at Nature. At Spectrum, she wrote about a variety of engineering efforts, including the quest for energy-producing fusion at the National Ignition Facility and the hunt for dark matter using an ultraquiet radio receiver. In 2014, she received a Neal Award for her feature on shrinking transistors and how the semiconductor industry talks about the challenge.