Technological development is all about finding engineering solutions to scientific theories, and this is especially true in the field of nanotechnology. The work of Gerd Binnig and Heinrich Rohrer in inventing the Scanning Tunneling Microscope comes to mind as an example of trying to engineer something of substance out of little more than an idea based on some sound physics.
Now Brian Willis, associate professor of chemical, materials, and biomolecular engineering at the University of Connecticut, is applying his atomic layer deposition (ALD) fabrication process, which he developed in 2011 while at the University of Delaware, to create nano-antenna arrays for highly efficient solar power devices.
The theory is straightforward: If you could build nano-antenna arrays so that the core electrodes were no more than 1 or 2 nanometers apart, they would serve to both absorb and rectify solar energy—thus the name “rectennas.” These rectennas should be able to collect as much as 70 percent of the sun’s electromagnetic radiation and simultaneously convert that light into direct current electrical power.
With those kinds of potential yields (no pun intended), research into nano-antenna arrays has been growing of late, with some of the more recent research out of MIT looking at ways of using them for holographic TVs. However, for the specific use in photovoltaics the problem has been getting the core electrodes close enough. The best that could be previously achieved, using electron guns, was somewhere in the neighborhood of a 10 to 20 nanometers gap between the core electrodes.
This is where Willis comes in. After the core electrodes have been cut with an electron gun so that one of the pair of electrodes has been shaped into a sharp tip, Willis is able to coat the surfaces of both electrodes with copper atoms using his ALD process, reducing the gap to 1.5 nanometers.
At this close proximity, a tunnel junction is created that allows the electrons to pass quickly between the electrodes. The combination of the sharp tip and the small gap means that the electrons can tunnel to the opposite electrode before their electrical current reverses and they try to go back.
“Until the advent of selective atomic layer deposition (ALD), it has not been possible to fabricate practical and reproducible rectenna arrays that can harness solar energy from the infrared through the visible,” says Darin Zimmerman, a physics professor at Penn State Altoona in a press release. “ALD is a vitally important processing step, making the creation of these devices possible. Ultimately, the fabrication, characterization, and modeling of the proposed rectenna arrays will lead to increased understanding of the physical processes underlying these devices, with the promise of greatly increasing the efficiency of solar power conversion technology.”
ALD looks to be a critical step towards realizing the potential of rectenna arrays for solar power devices, but what they’ve built so far is just proof that they can get the electrodes close enough for one to work. Now they have to orient the electrodes so that the electrons actually achieve their desired effect.
“We’ve already made a first version of the device,” says Willis in the release. “Now we’re looking for ways to modify the rectenna so it tunes into frequencies better. I compare it to the days when televisions relied on rabbit ear antennas for reception. Everything was a static blur until you moved the antenna around and saw the ghost of an image. Then you kept moving it around until the image was clearer. That’s what we’re looking for, that ghost of an image. Once we have that, we can work on making it more robust and repeatable.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.