Nov 28, 2013
FM radio transmitter breaks new size record
Researchers at Columbia University in the US have built the smallest frequency-modulated (FM) radio transmitter ever from a graphene nanomechanical system (NEMS). The device, which oscillates at a frequency of 100 megahertz, could find use in a variety of applications, including sensitive mass sensing and on-chip signal processing. It also represents an important first step towards advancing wireless signalling and designing ultrathin mobile phones, says team leader James Hone.
"Our device is much smaller than any other radio signal source ever made and, importantly, can be put on the same chip that is used for data processing," he explains.
Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Since its discovery in 2004, this "wonder material" has continued to amaze scientists with its growing list of unique electronic and mechanical properties, which include high electrical conductivity and exceptional strength. Some believe that graphene might even replace silicon as the electronic industry's material of choice in the future.
Graphene is also ideal for making nanoelectromechanical systems (NEMS) – scaled-down versions of microelectromechanical systems (MEMS) that are routinely employed in vibration sensing applications. The new device made by Hone and colleagues is a nanomechanical version of an electronic component known as a voltage controlled oscillator (VCO) and generates a frequency-modulated (FM) signal of about 100 megahertz. This frequency lies smack bang in the middle of the FM radio band (87.7 to 108 MHz) and the researchers say that they have already succeeded in using low-frequency music signals to modulate the 100 MHz carrier signal from their graphene NEMS and recover the signals again using an ordinary FM receiver.
While graphene NEMS may not replace conventional radio transmitters just yet, they will certainly be used in many other wireless signal processing applications. Although electrical circuits have been continuously shrinking over the last decades (thanks to Moore’s Law), there are still some types of devices – especially those involved in creating and processing radio-frequency (RF) signals – that are notoriously difficult to miniaturize, explains co-team leader Kenneth Shepard. These so-called off-chip components require a lot of space and electrical power and their frequency cannot be easily tuned.
Enter Graphene NEMS
Graphene NEMS come into their own here because they are very small (the active areas is only a few microns across) and they can potentially be integrated directly onto conventional CMOS chips. Most importantly, it is easy to tune their frequency over a wide range thanks to graphene’s exceptional strength.
The Columbia researchers made their devices by contacting graphene sheets to source and drain electrodes and freely suspending the sheets over metal gates. In this configuration, a DC gate voltage pulls the graphene down towards the gate and adjusts the tension and, therefore, the mechanical resonance frequency, explains Hone. A radio-frequency signal on the gate drives sheet vibrations. "Finally, we apply a DC bias across the graphene and when the graphene vibrates it acts as a transistor whose gate capacitance is constantly changing – and it is this that creates an RF source-drain current," he told nanotechweb.org.
The team studied the vibrational properties of its device at room temperature in a vacuum probe station, in an "open-loop" setup using a network analyzer. "To make an oscillator, we first adjust the signal gain to just above unity (using a variable amplifier) and the phase to zero (using a phase shifter) at the resonance frequency," said Hone. "We then connect the output to the gate. This creates a closed loop that amplifies random thermal vibrations and makes the device oscillate."
The researchers say they are now busy looking at how to put their devices directly onto integrated circuits that already contain all necessary drive and readout circuitry. They also hope to improve the performance of their oscillators and reduce device noise.
The present work is reported in Nature Nanotechnology doi:10.1038/nnano.2013.232.
About the author
Belle Dumé is contributing editor at nanotechweb.org