BN nanostructures stronger, lighter than steel

BN nanostructures stronger, lighter than steel
EVANSTON, Ill. — Northwestern University research has delivered the world's first nanotubes, -cones and -spheres constructed from boron nitride rather than the usual carbon. Single-walled boron-nitride (BN) nanostructures are hypothetically stronger and lighter than steel, but were only recently demonstrated here by professor Laurence Marks. Once BN nanostructures are embedded into polymers, they could serve to ruggedize the surface of metal parts, as well as form the basis for oxidation-proof coating.

"These are basic scientific results, not applications," said Marks, who is director of the new Center for Transportation Nanotechnology at Northwestern. "But BN nanostructures could lead to incredibly durable materials that have better electronic qualities and are much easier to work with than carbon nanostructures."

Carbon nanostructures, such as the 60-molecule spheres known as buckyballs, have found wide technological uses, but undesirable electronic and mechanical properties have limited their applications. BN nanostructures, however, are semiconducting with a gap of roughly 5.5 electron-volts, independent of tube diameter, number of layers and chirality. They also grow from their exposed ends, rather than from the substrate side, making them easier to form into oxidation-proof coatings that are lighter but stronger than steel.

BN nanostructures have been hypothesized since carbon buckyballs were first built in the 1980s. But previous evidence of BN nanostructures was limited to multiwalled nanotubes, concentric fullerenes and nano-arches, the images for which were obtained only after the samples had been exposed to air, contaminating them with artifacts.

Working with research engineer Erman Bengu at Intel Corp. in San Francisco, Marks utilized a low-energy electron-cyclotron resonance (ECR) plasma to deposit not multiple, but single atomic layers of BN nanostructures onto polished substrates of polycrystalline tungsten.

The substrates were jet-polished, then perfected down to the single crystal by repeated cycles of sputtering and annealing until the surfaces could be verified as free of impurities by X-ray photoelectron spectroscopy. Single-atomic layers of BN were then deposited. For boron, a conventional electron-beam evaporation source was used. Nitrogen ions were provided using a Compact ECR source from Astex.

After forming BN nanostructures, high-resolution electron microscopy was then performed in situ, without exposing the sample to air, using an ultrahigh-vacuum surface-analysis system.

In general, high temperatures (600°C) and bias (--400 V) resulted in a surface similar to woolen yarn, with many nanotubes a single atom thick growing on the substrate out into the vacuum like loose threads. Large variations in the diameter of these single-walled nanotubes — from .5 to 3 nanometers — was observed over relatively short distances, making it possible to create cones and other tapered shapes.

By modeling the observed features of the BN nanostructures — in particular the alternating acute and obtuse angles that dominated the observed curvatures — Marks was able to explain the results using atomic models of specific BN molecules of about 400 atoms each.

Marks hypothesizes that hexagons of boron and nitrogen are broken by the occasional fourfold and eightfold rings that enable them to bend into distinct shapes. Carbon nanotubes, on the other hand, are primarily hexagons with occasional fivefold and sevenfold rings. That makes BN the much more stable material, since with even-numbered rings there is no need for B-to-B or N-to-N bonds.

Four-member rings corresponded to the observed acute angles, whereas eight-member rings corresponded to the observed obtuse angles. The quick tapering between the different sizes corresponded to cone-shaped nanostructures resembling mountains with incomplete fullerenes on top, like the hollow top of a capped volcano.

Detailed observations of the BN nanostructures' growth process — including tubes, buckyballs and cones more than 1,000 times smaller than a human hair — revealed how temperature and substrate bias could be used to control their shapes and sizes. Overall, growth patterns were observed to occur on the surface of the film independent of its underlying thickness, with new atoms rapidly diffusing until they found an open site on the surface.

To encourage single-atomic thicknesses, a low-energy ionic species from the ECR plasma source was able to stabilize single-walled structures.

For the future, the researchers hope to extend their technique to another world first: direct deposition of carbon nitride nanostructures, which are hypothesized to have extensive electronic applications.