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Researchers at Rensselaer Polytechnic Institute, New York, are hoping to spread the word far and wide of a new analytical technique that can help scientists and technologists working with nanomaterials. They say that their discovery could help accelerate the development of materials for the next generation of solar energy conversion and computer data storage. Gwo-Ching Wang, Toh-Ming Lu, and Fu Tang explain that their new approach is based on the relatively well-known technique of reflection high-energy electron diffraction, or RHEED. In a traditional RHEED system, an interference pattern of the surface of the nanomaterials is obtained and provides a mere snapshot of the growing surface, the team says. In order to observe growth, the researchers modified the traditional RHEED technique by simply rotating the substrate on which they are growing a particular nanomaterial. This gives them a diffraction pattern containing the complete information on the crystal orientation distribution of the growing surface. The new technique is different from conventional diffraction techniques because it provides a way to monitor the surface structure of a material as it grows. X-ray and other technologies measure the entire material, from the tip of the new growth straight through the substrate that the material is growing on. The new RHEED technique shows the growth of only a few nanometres of a material at a time, the researchers explain. "Since we discovered this technique, we have been trying to get the word out to the nanoscience and nanotechnology research community,? enthuses Wang, who helped discover the technique. "It is inexpensive because it uses existing technology and vastly increases the rate of discovery by giving researchers a very clear picture of how to perfect and duplicate the growth of a new nanomaterial without spending months characterizing its structures after the growth." Critical to understanding the potential of the technique is that being able to obtain a surface crystal orientation diagram is particularly important for revealing the nature of the growth of nanostructures such as nanodots, nanorods, and nanoblades. The problem with conventional analytical approaches to their study is that their orientation can change dramatically over time as they grow, affecting detrimentally their properties. This unpredictability means that the commercialisation of novel nanomaterials would be limited because of a lack of consistency from one manufactured device to the next. Wang's work could change that for the next generation of devices based on nanomaterials. He is particularly focused on materials for solar energy conversion. Indeed, he views these materials as one of the most important applications for the new technique. For instance, the work could change the way photovoltaics are made. Currently, the most efficient solar panels on the market are comprised of a single crystal material, a semiconductor. To work properly, this material has to have no grain boundaries. Grain boundaries in a nanomaterial cause huge decreases in energy-conversion capabilities. But, single crystal solar cells are so costly that they could never be widely used in the consumer market, Wang believes. As such, many researchers and solar cell companies are investigating polycrystalline materials that grow in such a way that they can still convert light into electricity efficiently despite containing grain boundaries. Such materials would be much cheaper than single crystal devices. "The problem with creating high-quality polycrystalline materials is that you need a powerful technique to monitor them at nanoscale dimensions as they grow so you can quickly work on recreating the material to maximize its efficiency," Wang explains. "The new RHEED technique really allows researchers to create a material, see how it formed, and then turn around and recreate the most ideal version of that material without extensive experimentation." The team most recently evangelized on their technique at the American Vacuum Society 55th International Symposium and Exhibition and to representatives from the US Department of Energy in October. Reference:
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Wang, extolling the virtues of rotating RHEED |
