| STUDY |
Researchers have developed a technique to prepare boron nitride nanotubes and other shaped structures and have uncovered new details of the materials' atomic structure and growth mechanisms. Carbon nanotubes have been the focus of intense research interest since their discovery a decade ago because of their unique physical and electronic properties. However, even though nanosized structures of boron and nitrogen possess certain properties that could make them superior to carbon nanotubes, they have received relatively little attention. The new study may change that. "Boron nitride has important advantages over carbon. Authors points out that boron nitride is "far more resistant to oxidation than carbon and therefore suited for high-temperature applications in which carbon nanostructures would burn." In addition, BN nanotubes are expected to be semiconducting, with predictable electronic properties that are independent of tube diameter and number of layers, unlike tubes made of carbon. Unlike arc-discharge methods or other techniques for making BN nanotubes in the gas phase, the authors prepare nanotubes by depositing boron and nitrogen ions on a hot, electrically biased tungsten surface. Then the researchers shuttle freshly prepared specimens into a transmission electron microscope that is connected to the deposition chamber. From atomic-resolution images, the team can see that boron and nitrogen combine on the tungsten surface to form "a woolen yarn" that contains a variety of structures, including single-wall nanotubes, BN fullerenes, cones, and buckyonions. The microscopists note that the wool grows from the ends of the fibers that dangle in the vacuum--not the ends attached to the surface. Authors use structural models and computed images to aid in interpreting the recorded microscopy data. They find that the tiny structures are composed of rolled-up sheets that consist mainly of hexagons of boron and nitrogen with occasional four- and eight-membered rings that allow the tubes to bend and endow them with characteristic shapes. By contrast, carbon nanotubes consist of hexagons, with five- and seven-membered rings here and there. By further developing an understanding of the growth process, the researchers aim to optimize and manipulate synthesis conditions in order to prepare tailored nano-structures for technological applications. |
| UPDATE | 19.03.2001 |
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