Nanotube Research Really Starts Rolling

Nanotube Research Really Starts Rolling

by John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Sep-Oct 98	Rank Last Period Jul-Aug 98
1	A.P. Scott, L. Radom, "Harmonic vibrational frequencies: an evaluation of Hartree-Fock, Moller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors," J. Phys. Chem., 100(41):16502-13, 10 October 1996. [Australian. Natl. U., Canberra] *VL579	29	2
2	S.J. Tans, et al., "Individual single-wall carbon nanotubes as quantum wires," Nature, 386(6624):474-7, 3 April 1997. [Delft U. Technol., Netherlands; Rice U., Houston, TX] *WR256	25	4
3	J.W.G. Wildoer, et al., "Electronic structure of atomically resolved carbon nanotubes," Nature, 391(6662):59-62, 1 January 1998. [Delft U. Technol., Netherlands; Rice U., Houston, TX] *YP888	21	†
4	T.W. Odom, et al., "Atomic structure and electronic properties of single-walled carbon nanotubes," Nature, 391(6662):62-4, 1 January 1998. [Harvard U., Cambridge, MA] *YP888	20	†
5	G.N. Murshudov, A.A. Vagin, E.J. Dodson, "Refinement of macromolecular structures by the maximum-likelihood method," Acta Cryst., D53:240-55, May 1997. [U. York, England; Free U. Brussels, Belgium] *XB256	17	3
6	K. Matyjaszewski, T.E. Patten, J. Xia, "Controlled/"living" radical polymerization. Kinetics of the homogeneous atom transfer radical polymerization of styrene," J. Amer. Chem. Soc., 119(4):674-80, 29 January 1997. [Carnegie Mellon U., Pittsburgh, PA] *WE826	15	†
7	C. Granel, et al., "Controlled radical polymerization of methacrylic monomers in the presence of a Bis(ortho-chelated) arylnickel (II) complex and different activated alklyl halides," Macromolecules, 29(27):8576-82, 30 December 1996. [U. Liege, Belgium] *WB741	14	9
8	L.A. Curtiss, et al., "Assessment of Gaussian-2 and density functional theories for the computation of enthalpies of formation," J. Chem. Phys., 106(3):1063-79, 15 January 1997. [Argonne Natl. Lab., IL; Bell Labs, Lucent Technol., Murray Hill, NJ; Northwestern U., Evanston, IL] *WD070	14	†
9	H. Dai, et al., "Nanotubes as nanoprobes in scanning probe microscopy," Nature, 384(6605):147-50, 14 November 1996. [Rice U., Houston, TX] *VT336	14	†
10	A.M. Rao, et al., "Diameter-selective raman scattering from vibrational modes in carbon nanotubes," Science, 275(5297):187-91, 10 January 1997. [5 U.S. institutions] *WC022	14	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

Five of the papers in the current list are about carbon nanotubes: #2, #3, #4, #9 and #10. All but one of these (#4) bear the name of Nobel laureate Richard E. Smalley, and all but one (#2) are new to the list. However, two in particular stand out as quite remarkable (#3 and #4), and together they report some definitive work on the structure and conductivity of single wall nanotubes (SWNT).

Graphite consists of layers of carbon atoms, called graphene sheets, in which carbon atoms are arranged in a honeycomb pattern with a film of delocalized electrons covering the whole layer. It is this which makes non-metallic graphite able to conduct electricity. But if a single graphene sheet is rolled into a seamless tube, a SWNT, will it still be as good at conducting electricity?

The answer is that it all depends on its structure, because such a tube can be rolled straight or at an angle. If it is straight, with a diameter of ten carbon atoms, then it is called a (10,0) tube. If it is rolled an angle and links up with a carbon that is seven atoms further along the sheet then it is a helical tube coded (10,7). Only one of these arrangements will be conducting.

Nanotubes were discovered in 1991, and predictions about their electrical behavior suggested that some SWNTs would conduct while others would be semiconductors. Proving this posed seemingly insurmountable problems. Laser vaporized graphite condenses into a tangle of nanotubes of all shapes and sizes, including multi-layered tubes. To pick out just the right SWNTs and measure their electronic properties looked an impossible task, but in the January 1st, 1998 edition of Nature there appeared two remarkable papers that reported success. These are papers #3 and #4 in the current Hot Ten.

The first comes from the laboratory of the world’s leading authority on complex carbon structures, Richard E. Smalley at Rice University, Houston, Texas, and the work was done in conjunction with Jeroen Wildoer’s groups at Delft University of Technology in The Netherlands. While #4 comes from Charles Lieber and colleagues based at Harvard University. Both papers report essentially the same information, albeit with slightly different emphasis. Both groups have resolved the structure of SWNT tube walls with an accuracy hitherto unachievable, and sufficient for them to determine not only the diameter of the tube, but their exact helicity. Lieber found that samples of SWNT exhibited many different structures, with no one species dominating thereby refuting earlier suggestions that one particular type of tube—the (10,10) tube—would dominate.

Paper #3 reports the electronic properties of 27 SWNTs of various sizes and wrapping angles. These spanned a range of structural types, and for 18 of them their electronic features were measured: 12 were found to be semiconductors, 6 were conducting. The former had energy band gaps of around 0.5 - 0.6 electronvolts, whereas the latter had Egap in the range 1.7-2.0 eV showing they were metallic.

Together papers #3 and #4 confirm the theory that (n,0) tubes will be metallic when n/3 is an integer, and the same will be true of (n,m) tubes if 2n+m is exactly divisible by three. Thus (10,0) won’t conduct but (10,7) will.

"These structure-dependent variations in electronic properties are unique to SWNT materials and offer great promise for the emerging field on nano molecular electronics," says Lieber, but he acknowledges that there is still a long way to go before this happens. "Until methods are developed to either separate or synthesize tubes with specific diameters and wrapping angles, then structure-dependent variations in electronic properties will inhibit the use of SWNTs in electronic applications."

However, Lieber points out that important applications are already emerging, such as in molecularly sharp probes with chemical and biological functions; he cites more recent work in this area that the Harvard group have been doing (Nature, 398:52, 1998). A recent issue of the Journal of Materials Research, which was devoted entirely to nanotubes, also carried a paper from his group (13:2380, 1998), and Lieber hints at more exciting revelations in the coming months. Watch this space!

Dr. John Emsley is Science Writer in Residence
at the Department of Chemistry, University of Cambridge, U.K.