Frontiers in Energy Research: January 2013

Overcoming the First Grand Challenge

Controlling material processes at the level of electrons is not easy, but many EFRCs are up to the challenge

Scanning electron microscopy images of (a) two individual gallium nitride nanowires (the scale bar represents 10 micrometers) and (b) nanowire-pairs (the scale bar represents 5 micrometers). Researchers at SSLS successfully fabricated nanolasers that operate in a single-mode condition from these structures.

Top image was desgined by Yan Liang

Redefining the nation's energy landscape demands overcoming nature's seemingly insurmountable obstacles. Each of the Department of Energy-funded Energy Frontier Research Centers, or EFRCs, is working on at least one of five Grand Challenges to build the blueprints for truly disruptive technologies; the results are coming fast and are nothing short of remarkable. The first grand challenge engages 31 of the EFRCs to take experimental and theoretical approaches to design better materials, characterize the interactions between light and matter, develop digital memory by utilizing electron spin (electrons are negatively charged subatomic particles), create catalysts that overcome difficult fuel-producing reactions and capture and store the sun's energy as a fuel.

The barriers to accomplish these goals are daunting, but the progress thus far is impressive and a testament to the success of the centers. A deserving synopsis of each center will exceed the confines of this article; therefore, we have chosen to feature two EFRCs. However, many of the centers are featured in previous issues of this newsletter and the reader is encouraged to access our archives to learn more.

Splitting water and carbon dioxide with one catalyst. The sun represents an incredible source of energy, providing about 10,000 times the world's current daily energy use. However, solar energy is intermittent. The only way solar energy can be practical on a massive scale is to determine a way to store it.

Natural photosynthesis provides a role model in this regard, converting sunlight and carbon dioxide into carbohydrate fuels. Mimicking this process has been a subject of intense research in the emerging field of "artificial photosynthesis" with "solar fuels" as the ultimate product. The absorption of light is a necessary step, but unlike a solar cell that converts the light into electricity, artificial photosynthesis converts light directly to fuel. The target reactions are splitting water into hydrogen and oxygen and turning carbon dioxide to hydrocarbons. Both reactions are chemically challenging because of their multiple electrons and protons and the large energy barriers that must be overcome.

Researchers at the Solar Fuels Energy Frontier Research Center, or UNC EFRC, revealed that a single molecule, consisting of a ruthenium-based catalyst, could split both carbon dioxide and water. Their article in the Proceedings of the National Academy of Sciences demonstrates that the molecular catalyst reduces carbon dioxide to carbon monoxide and oxygen in the presence of an applied voltage. This discovery holds much promise for vastly simplifying the artificial photosynthetic process and is an important first step towards discovering a simple, yet highly effective method for solar fuel production.

Precise control over nanowire geometry. The advent of nanotechnology has produced semiconductor materials whose properties have given birth to the fields of nanoelectronics and nanophotonics. In particular, semiconductor nanowire lasers have been extensively studied due to their potential for low power consumption and ultra compact structure, making them invaluable in nanocircuits and devices. Nanowire lasers are slated for potential use in technologies that range from biology to information storage. For example, nanowire lasers could read more densely packed data on storage media. Until now, however, their development was hampered by poor beam and spectral properties because they could only operate with multiple frequencies (multiple modes), causing poor focusing properties and, consequently, limited spatial resolution. Ideally, a nanolaser should operate at a single frequency (single mode) with a high-quality beam.

This was recently achieved using gallium nitride nanowires by researchers at the Solid State Lighting Science Energy Frontier Research Center, or SSLS, in collaboration with the Center for High Technology Materials at the University of New Mexico. Using a top-down fabrication method, they precisely controlled the nanowire's dimensions and geometric properties, creating a structure whose dimensions enable it to operate as single-mode lasers. Additionally, for nanowires with geometries resulting in multi-mode operations, the researchers converted it to single-mode operation by taking advantage of the light interactions that occur when the nanowires are placed close to each other. With these results, the door is now open to making novel, viable technologies with this game-changing technology.

Outlook. Although seemingly unrelated, the work from the UNC EFRC and the SSLS represents the diversity of science that is being conducted by the centers and demonstrates how each contributes to solving our nation's grand challenge in energy: controlling material processes at the level of electrons. Collectively, the discoveries being made by the EFRCs are impressive (see our archives) and will ultimately overcome our grand challenges and lead to disruptive technologies that are founded on the solid understanding that can only come from basic science.

More Information: 
Chen Z, JC Javier, MK Brennaman, P Kang, MR Norris, PG Hoertz and TJ Meyer. 2012. "Splitting CO2 to CO and O2 by a Single Catalyst." Proceedings of the National Academy of Sciences 109:15606-15611. DOI: 10.1073/pnas.1203122109Li Q, JB Wright, WW Chow, TS Luk, I Brener, LF Lester and GT Wang. 2012. "Single-Mode GaN Nanowire Lasers." Optics Express 20:17873. DOI: 10.1364/OE.20.017873Xu H, JB Wright, TS Luk, JJ Figiel, K Cross, LF Lester, G Bakrishnan, GT Wang, I Brener and Q Li. 2012. "Single-mode Lasing of GaN Nanowire-pairs." Applied Physics Letters 101:113106.  DOI: 10.1063/1.4751862 
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