June 26, 2002 They're cute little shavers, and they could play a key role in the "small" revolution about us.
They're boron nanowhiskers, the world's first such crystalline nanowires, made by chemists at Washington University in St. Louis.
Reporting in the May 1 issue of the Journal of the American Chemical Society (JACS), graduate student Carolyn Jones Otten, her advisor William. E. Buhro, Ph.D., Washington University professor of chemistry, and their collaborators report that they have made boron nanowhiskers by chemical vapor deposition. The particles have diameters in the range of 20 to 200 nanometers and the whiskers (also called nanowires) are semiconducting and show properties of elemental boron.
To get an idea of scale, one nanometer is one one-thousandth of a micrometer; in comparison, a strand of human hair is typically 50 to 100 micrometers thick.
In the nano-world, the carbon nanotube is king, considered the particle most likely to make new materials, and increasingly valued as potential metallic conductors in the burgeoning experimental world of molecular electronics. However, carbon has its limitations: its cell wall structure and variable conductivity make it unreliable as a conductor--only one-third of those grown have metallic characteristics; the others are semiconductors. And one specific type can't predictably be grown; instead, a mix of types is grown together.
The Buhro group at Washington University in St. Louis turned to boron, one spot to the left of carbon in the periodic table, to see if it would be a good candidate. If nanotubes could be made of boron and produced in large quantities, they should have the advantage of having consistent properties despite individual variation in diameter and wall structure. The discovery that the nanowhiskers are semiconducting make them promising candidates for nanoscale electronic wires.
"The theoretical papers predicted that boron nanotubes may exist and if they do, should have consistent electrical properties regardless of their helicity. This would be a distinct advantage over carbon nanotubes," said Otten. "So, we set out to make these. We had already done some work on boron nitride nanotubes, which are similar in structure to carbon nanotubes but they are electrically insulating. So, we used a similar method to try to make boron nanotubes. We grew things that looked very promising--long thin wire-like structures. At first we thought they were hollow, but after closer examination, we determined that they were dense whiskers, not hollow nanotubes. "
The notion of boron nanotubes creates more excitement in nanotechnology than nanowhiskers because of their unique structure, which could be likened to a distinct form of an element. Carbon, for instance, is present as graphite and diamond, and, recently discovered, in "buckyball" and nanotube conformations. Also, boron nanotubes are predicted by theory to have very high conductivity, something groups like Buhro's are eager to measure .
The nanowhiskers made by Buhro's group were electrically characterized to see if they were good conductors despite being whiskers rather than tubes. They were found to exhibit semiconducting behavior. However, bulk boron can be "doped" with other atoms to increase its conductivity. Otten, Buhro and their collaborators are now working on trying to do the same thing with boron nanowhiskers to increase their conductivity. Carbon nanotubes have been doped, as have various other kinds of nanowires, and assembled in combinations of conducting and semiconducting ones to make for several different microscale electronic components such as rectifiers, field-effect transistors and diodes
"Now we're trying to dope our boron nano-whiskers to see if we can increase their conductivity," Otten said. "We would still be interested in discovering boron nanotubes, but we're just not quite sure how to make them."
Since the early 90s Buhro and his group have been making many kinds of nanowires and nanotubes that might ultimately be incorporated into nanoelectronic devices. Nanowires and nanotubes are receiving much current attention as potential transistors, wires, and switches for ultra-small circuits and devices to be built from them on almost a molecular scale.
"If you want to make electronics smaller and smaller, you have to make the component devices and the wires that interconnect them smaller and smaller," Buhro said. "We are trying to build the scientific infrastructure for electronic nanotechnology, and to understand the basic principles involved. We have to find out how these nano-wires work and how to connect them into circuits and functional devices. Even when we have that, nobody yet knows how a computer chip will be made that uses these things. That is a wide-open, unsolved problem. But the fundamental science to be done is potentially important and is going to be very fun."
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