Power/Performance Bits: Sept. 29

Optical rectenna

Engineers at the Georgia Institute of Technology 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, 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.

“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,” said Baratunde Cola, an associate professor of Mechanical Engineering at Georgia Tech. “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.”

This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology. (Source: Thomas Bougher, Georgia Tech)

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 Cola, who 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.

Faster screens with zinc-based transistors

Researchers at Korea University and the Samsung Advanced Institute of Technology developed a new type of thin film transistor that’s significantly faster than its predecessors, which could help speed up image display on devices like TVs and smartphone screens. The scientists made the transistor from zinc oxynitride, or ZnON, which they then plasma treated with argon gas.

Much of the thin-film research involving zinc oxide materials has focused on the addition of metal cations, such as indium with gallium, hafnium, zirconium and lanthanide. When used in semiconductors, these exhibit mobility values — the speed at which electrons subjected to an electric field travel through a material — from 5 to 20 cm2/voltseconds. While extreme electron mobilities have been recorded under ideal lab conditions — graphene’s vaunted mobility is around 200,000 cm2/voltseconds — thin-film transistors speeds in commercial electronics largely fall within the single to double digit range.

Active oxide semiconductors, such as ZnON, provide a variety of benefits, such as a low cost of production and a relatively low temperature of fabrication — below 300° C — which makes them suitable for display applications and makes it easy to integrate them with a variety of other inorganic and organic materials.

As a thin film, zinc oxynitride — a glassy composite of ZnO, ZnO_xN_x and Zn₃N₂ — exhibits extremely fast mobility rates due to its ability to deactivate oxygen vacancies, which are defects in transition metal oxide surfaces that occur due to inevitable bonding flaws in creating the compound. When the oxygen anion in ZnO is substituted with a nitrogen anion, the valence band edge, or fully occupied energy band edge, of ZnON is positioned above the previous location of the neutral oxygen vacancy in ZnO, effectively burying the vacancy below the valence band edge and preventing gaps in conductance.

Cross-sectional high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image and nanobeam diffraction pattern of Ar plasma treated ZnON. (Source: E. Lee & S. Jeon/Samsung Advanced Institute of Technology & Korea University)

When subjected to conductance tests, the channel mobility, or semiconductor speed, of the nanocrystalline thin film transistor was found to be 138 cm2/vs — an order of magnitude higher than that of the group’s previous indium-gallium-zinc-oxide film.

“We believe that zinc oxynitride, tailored by reactive sputtering and plasma processes, will constitute another significant breakthrough in the field of thin film electronics,” said Sanghun Jeon, an associate professor at Korea University.