GaN One Better with Single-Crystal Nanotubes

GaN One Better with Single-Crystal Nanotubes

by John Emsley

WHAT'S HOT IN CHEMISTRY
Rank	Paper	Citations This Period (Nov-Dec 04)	Rank Last Period (Sep-Oct 04)
1	A.L. Spek, *"Single-crystal structure validation with the program PLATON,"* J. Appl. Cryst., 36: 7-13, February 2003. [Utrecht U., Netherlands] *636LK	69	1
2	N.L. Rosi, et al., "Hydrogen storage in microporous metal-organic frameworks," Science, 300(5622): 1127-9, 16 May 2003. [5 U.S. institutions] *678VC	16	6
3	P. Wang, et al., "Gelation of ionic liquid-based electrolytes with silica nanoparticles for quasi-solid-state dye-sensitized solar cells," J. Amer. Chem. Soc., 125(5): 1166-7, 5 February 2003. [Swiss Fed. Inst. Tech., Lausanne; N’Tera Batteries, Lausanne, Switzerland] *640XB	11	†
4	S.P Brown, et al., "The direct and enantioselective organocatalytic ALPHA-oxidation of aldehydes," J. Amer. Chem. Soc., 125(36): 10808-9, 10 September 2003. [Caltech, Pasadena] *718NY	10	†
5	Z. Tang, et al., "Novel small organic molecules for a highly enantioselective direct aldol reaction," J. Amer. Chem. Soc., 125(18): 5262-3, 7 May 2003. [Chengdu Inst. Organic Chem., China; Sichuan U., Chengdu, China; Peking U., Beijing] *675FL	10	†
6	D. Gelman, S.L. Buchwald, "Efficient palladium-catalyzed coupling of aryl chlorides and toxylates with terminal alkynes: Use of a copper cocatalyst inhibits the reaction," Angew. Chem. Int. Ed., 42(48): 5993-6, 15 December 2003. [MIT, Cambridge] *757PJ	10	†
7	A. Babel, S.A. Jenekhe, et al., "High electron mobility in ladder polymer field-effect transistors," J. Amer. Chem. Soc., 125(45): 13656-7, 12 November 2003. [U. Washington, Seattle] *740WG	10	†
8	X.F. Duan, et al., "Single-nanowire electrically driven lasers," Nature, 421(6920): 241-5, 16 January 2003. [Harvard U., Cambridge, MA] *635KG	9	4
9	J. Goldberger, et al., "Single-crystal gallium nitride nanotubes," Nature, 422(6932): 599-602, 10 April 2003. [U. Calif., Berkeley; Lawrence Berkeley Natl. Lab., CA] *665GN	9	†
10	D.R. Larson, et al., "Water soluble quantum dots for multiphoton fluorescence imaging in vivo," Science, 300(5624): 1434-6, 30 May 2003. [Cornell U., Ithaca, NY; Quantum Dot Corp., Hayward, CA] *683ZW	9	†
SOURCE: ISI’s Hot Papers Database. the Legend.

allium nitride (GaN) is the semiconductor that promises to outshine silicon and even eclipse gallium arsenide (GaAs). Not only is it more robust but it can deliver 50 times the power performance of GaAs. What is rather strange is that GaN works well even though it is riddled with dislocations that would normally be fatal to a microelectronic material.

Sales of GaN devices already exceed $1 billion per year and are rising fast. What makes GaN popular for lighting devices is that it emits in the short-wavelength part of the visible spectrum, and there are now so-called "Blu-ray" DVD players using GaN lasers operating at 405 nm. This means that the information-storing pits on a DVD’s surface can be made much smaller, so that in theory a single disc could hold 50 gigabytes. The large breakdown voltages inherent in GaN technology makes it well suited to high-power microwave and high-voltage wireless applications. With GaN, the base stations for mobile phones could be ten times further apart than at present.

And just when you thought things could not get better for GaN, a whole new field of research has suddenly opened up in the form of GaN nanotubes. These might one day be used in nanoscale electronics, optoelectronics, and biochemical-sensing applications. The synthesis of single-crystal nanotubes was reported in paper #9, which comes from the research group of Peidong Yang of the Lawrence Berkeley National Laboratory, Berkeley, California. Yang featured in the November/December 2002 issue of Science Watch (13[6]: 7, 2002), when his method of making zinc oxide (ZnO) nanowires was being heavily cited. Now he has gone one stage further and used those same wires as templates on which to grow GaN nanotubes.

The Berkeley researchers placed an array of ZnO nanowires, grown on sapphire wafers, inside a reaction tube in which chemical vapor deposition was undertaken using trimethylgallium and ammonia. These were fed into the chamber on a stream of argon and heated at 600-700° C, whereupon they decomposed to form a layer of GaN on the ZnO fibers. The inner oxide core was subsequently removed by heating with hydrogen gas at 600° C to leave hollow tubes of GaN. (Alternatively the ZnO can be etched away simply by heating with ammonia.)

This method of synthesis is likened by Yang to the age-old "lost wax" technique for making hollow sculptures. "It is simple and easy to implement, and very versatile," he says, and this explains the growing number of citations for paper #9. "Other research groups have been extending the concept to make different single crystalline semiconductor nanotubes, such as silicon and indium phosphide."

The GaN nanotubes were checked with X-ray diffraction, which revealed that almost all of the ZnO had been removed, although a few atoms of zinc had been incorporated into the GaN tube wall. The nanotubes were studied using transmission electron microscopy (TEM), which showed them to have wall thicknesses of 5-50 nm and inner diameters of 30-200 nm. Electron diffraction measurements showed them to be single crystals, and in this respect they are very different from other reported inorganic nanotubes. Most of the GaN nanotubes had only one open end but some tubes had both ends open, probably due to mechanical breakage during TEM sample preparation. Otherwise the GaN nanotubes are mechanically robust and are both electrically and optically active.

More recently Yang has published in the general area of one-dimensional nanostructures (see T. Kuykendall, et al., Nature Materials, 3[8]: 524-8, 2004), and he is currently studying ionic transport within hollow single-crystal nanotubes. He claims that this work is revealing a completely new kind of chemical and physical behavior and he promises that it will soon be published. "The introduction of these hollow nanotubes opens up an exciting field of nanotube nanofluidics, which should have significant implication in areas such as chemical and biological sensing, DNA sequencing, as well as in proteomics."

Yang’s work has already featured twice in Science Watch, a rare event in itself, but it may well be that a third call to his labs may soon be on the cards. Watch this space!

Dr. John Emsley is based at the Department of Chemistry, University of Cambridge, U.K.


	View the top 10 scientists and/or top 3 Hot Papers in Chemistry; for the period of January 1, 1994-December 31, 2004.

Science Watch^®, May/June 2005, Vol. 16, No. 3
Citing URL: http://www.sciencewatch.com/may-june2005/sw_may-june2005_page7.htm