A carbon nanotube optical rectenna
converts green laser light to electricity in the laboratory of Baratunde Cola
at the Georgia Institute of Technology. Credit: Rob Felt, Georgia Tech
Using nanometer-scale components, researchers have demonstrated the first
optical rectenna, a device that combines the functions of an antenna and a
rectifier diode to convert light directly into DC current.
Based on multiwall carbon nanotubes
and tiny rectifiers fabricated onto them, the optical rectennas could provide a
new technology for photodetectors that would operate without the need for
cooling, energy harvesters that would convert waste heat to electricity - and
ultimately for a new way to efficiently capture solar energy.
In the new devices, developed by
engineers at the Georgia Institute of Technology, the carbon nanotubes act as
antennas to capture light from the sun or other sources. As the waves of light
hit the nanotube antennas, they create an oscillating charge that moves through
rectifier devices attached to them. The rectifiers switch on and off at record
high petahertz speeds, creating a small direct current.
Billions of rectennas in an array
can produce significant current, though the efficiency of the devices
demonstrated so far remains below one percent. The researchers hope to boost
that output through optimization techniques, and believe that a rectenna with
commercial potential may be available within a year.
"We could ultimately make solar
cells that are twice as efficient at a cost that is ten times lower, and that
is to me an opportunity to change the world in a very big way" said
Baratunde Cola, an associate professor in the George W. Woodruff School of
Mechanical Engineering at Georgia Tech. "As a robust, high-temperature
detector, these rectennas could be a completely disruptive technology if we can
get to one percent efficiency. If we can get to higher efficiencies, we could
apply it to energy conversion technologies and solar energy capture."
The research, supported by the
Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare
(SPAWAR) Systems Center and the Army Research Office (ARO), is scheduled to be
reported September 28 in the journal Nature Nanotechnology.
Developed in the 1960s and 1970s, rectennas have operated at wavelengths as
short as ten microns, but for more than 40 years researchers have been
attempting to make devices at
optical
wavelengths. There were many challenges: making the antennas small
enough to couple optical wavelengths, and fabricating a matching rectifier
diode small enough and able to operate fast enough to capture the
electromagnetic wave oscillations. But the potential of high efficiency and low
cost kept scientists working on the technology.
"The physics and the scientific concepts have been out there," said
Cola. "Now was the perfect time to try some new things and make a device
work, thanks to advances in fabrication technology."
Using metallic multiwall carbon
nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha
Sharma, Virendra Singh and Thomas Bougher constructed devices that utilize the
wave nature of light rather than its particle nature. They also used a long series
of tests - and more than a thousand devices - to verify measurements of both
current and voltage to confirm the existence of rectenna functions that had
been predicted theoretically. The devices operated at a range of temperatures
from 5 to 77 degrees Celsius.
Fabricating the rectennas begins
with growing forests of vertically-aligned carbon nanotubes on a conductive
substrate. Using atomic layer chemical vapor deposition, the nanotubes are
coated with an aluminum oxide material to insulate them. Finally, physical
vapor deposition is used to deposit optically-transparent thin layers of
calcium then aluminum metals atop the nanotube forest. The difference of work
functions between the nanotubes and the calcium provides a potential of about
two electron volts, enough to drive electrons out of the carbon nanotube antennas
when they are excited by light.
In operation, oscillating waves of
light pass through the transparent calcium-aluminum electrode and interact with
the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve
as rectifiers switching on and off at femtosecond intervals, allowing electrons
generated by the antenna to flow one way into the top electrode. Ultra-low
capacitance, on the order of a few attofarads, enables the 10-nanometer
diameter diode to operate at these exceptional frequencies.
"A rectenna is basically an
antenna coupled to a diode, but when you move into the optical spectrum, that
usually means a nanoscale antenna coupled to a metal-insulator-metal
diode," Cola explained. "The closer you can get the antenna to the
diode, the more efficient it is. So the ideal structure uses the antenna as one
of the metals in the diode - which is the structure we made."
The rectennas fabricated by Cola's
group are grown on rigid substrates, but the goal is to grow them on a foil or
other material that would produce flexible solar cells or photodetectors.
Cola sees the rectennas built so far
as simple proof of principle. He has ideas for how to improve the efficiency by
changing the materials, opening the carbon nanotubes to allow multiple
conduction channels, and reducing resistance in the structures.
"We think we can reduce the
resistance by several orders of magnitude just by improving the fabrication of
our device structures," he said. "Based on what others have done and
what the theory is showing us, I believe that these devices could get to
greater than 40 percent efficiency."