The Other Nanotubes
Carbon nanotubes get all the attention. Hundreds of scientists study these tubes of pure carbon, which are just nanometers in diameter, and which have high strength and novel electronic properties. But many proposed applications for carbon nanotubes might be better accomplished with boron nitride (BN) nanotubes, which are less chemically fragile. BN tubes are harder to make and analyze, but the 12 March PRL describes some significant progress. By fabricating BN structures on a surface (rather than the usual gas phase) and imaging them without exposure to air, the researchers have taken the most detailed look at BN tubes and nanostructures to date. Their electron micrographs showed many new details of the nanotube growth and defect formation process.
BN tubes are similar in structure to carbon tubes–both consist of rolled up sheets of hexagonal arrays of atoms. “Carbon’s great stuff,” says Laurence Marks of Northwestern University in Evanston, IL, “but boron nitride can be just as great, if not greater.” That’s because boron nitride nanotubes have higher heat tolerance and are less likely to oxidize than carbon. They may also be good for electronic devices because they are semiconducting and more predictable in their electronic properties.
To optimize the synthesis of BN nanotubes, researchers would like to know more about how they grow atom-by-atom and how defects form. When each formation step is well understood at the atomic scale, they should be able to control and guide the growth process with higher precision. One central question concerns the rings that form on the closed ends of nanotubes: Are they 5- and 7-atom rings, like carbon nanotubes? Or does BN make rings with four and eight atoms apiece?
One reason for the difficulty in characterizing the structures is that experimenters have to expose their samples to air before imaging, leading some to question whether the proposed structures result from unintended chemical reactions. Marks and graduate student Erman Bengu have now developed the first system that can image BN nanotubes without first exposing them to air. Part of their method is a new way to synthesize the nanotubes on a surface. They spray boron and energetic nitrogen atoms onto a clean, heated tungsten surface, held at 250 to 500 volts, in an ultrahigh vacuum. The nanotubes and other BN structures grow as a sort of fine, tangled “hair” on the tungsten, lengthening by perhaps one angstrom per second.
By comparing their electron micrographs with computer simulated structures, Marks and Bengu showed that the rings at the tube end “caps” have 4- and 8-fold symmetry. They also found that the tubes grow from the ends that dangle in the vacuum, rather than from the ends attached to the tungsten surface. This conclusion, Marks explains, comes from the team’s observation that these free ends remained unchanged with time: If growth came from the surface-attached ends, the free ends would have gradually become more tangled.
“The microscopy and analysis have been done superbly,” says Pulickel Ajayan of Rensselaer Polytechnic Institute in Troy, NY, “and some real clues are obtained on the growth mechanisms of nanotubes.” Making and imaging the samples in the same ultrahigh vacuum is “a feat that could be achieved only in a very few places in the world,” Ajayan says. Another “exciting and unique” aspect, he adds, is that a scaled-up version of the synthesis technique might make nanotube coatings possible in industry. Marks thinks a coating of BN nanotube “hair” might reduce the friction on bearings in heavy machinery.