March 19, 2001
Volume 79, Number 12
CENEAR 79 12 pp.10
ISSN 0009-2347
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New method to grow, characterize BN materials with intriguing properties


Researchers at Northwestern University 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 [Phys. Rev. Lett., 86, 2385 (2001)].

SYNTHESIZER Marks stands near microscope and surface science equipment used to prepare and analyze BN nanotubes. PHOTO BY MITCH JACOBY
SHAPES OF THINGS Occasional four- and eight-membered rings cause boron nitride nanotubes to bend and adopt a variety of shapes (red = boron; blue = nitrogen).
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," asserts Laurence D. Marks, a professor of materials science and engineering. Marks, who conducted the investigation with graduate student Erman Bengu, 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.

"This is clearly an advance in noncarbon nanotube technology," comments Pulickel M. Ajayan, associate professor of materials science and engineering at Rensselaer Polytechnic Institute. "The microscopy and analysis have been done superbly."

Ajayan explains that the Northwestern study "stands out from other reports on the subject" in that the BN nanotubes were grown in a pristine high-vacuum environment and then imaged without exposing the specimens to air. By confining the samples to a clean atmosphere with less than one-billionth the number of molecules typically present in air, Marks and Bengu avoid the question that has bedeviled earlier studies: Did reactions with water, oxygen, or other contaminants introduce structural artifacts

Unlike arc-discharge methods or other techniques for making BN nanotubes in the gas phase, the Northwestern scientists 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.

Marks and Bengu 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.

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