[extropy-chat] Nature article request...

Wed Mar 3 14:23:36 UTC 2004

On Tue, Mar 02, 2004 at 06:13:06PM -0800, Robert J. Bradbury wrote:
> 
> Does anyone have a subscription to Nature who can send
> me this article?
> 
> Hueso L, Mathur N.
> Nanotechnology: dreams of a hollow future.
> Nature. 2004 Jan 22;427(6972):301-4.
> http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14737151&dopt=Abstract
> 
> I would like to see if its more of the usual dribble by
> uninformed writers/scientists.

Nature 427, 301 - 304 (22 January 2004); doi:10.1038/427301a

Nanotechnology: Dreams of a hollow future

LUIS HUESO AND NEIL MATHUR

Luis Hueso and Neil Mathur are in the Department of Materials Science and
Metallurgy, University of Cambridge, New Museums Site, Pembroke Street,
Cambridge CB2 3QZ, UK.

e-mail: ndm12 at cus.cam.ac.uk

Carbon nanotubes have become familiar components in nanotechnology. Nanotubes
made from inorganic materials are now on the rise, the latest creation being
nanoscale tubes of a complex manganese oxide.

Fabricating small structures has long been fashionable in physics. The
rationale is that reducing one or more dimensions of a system below some key
length scale can change the system's behaviour ? carbon nanotubes are a good
example. But nanotubes made from other materials are also proving useful for
technological applications. In Applied Physics Letters, Levy et al.1 add to
the catalogue with their report of the growth of nanotubes made of a
manganese oxide, namely a manganite.

Carbon nanotubes, discovered by Ijima2 in 1991, can be thought of as
rolled-up sheets of carbon atoms. The tubes have diameters as small as one
nanometre, and are typically several micrometres long. Thus they are,
effectively, one-dimensional. This reduced dimensionality creates a new
playground for physicists, where the conventional description of the
electronic structure of three-dimensional materials breaks down3. But
interesting effects are not restricted to only the smallest nanotubes. Crude
carbon-nanotube structures, consisting of imperfectly concentric cylinders
with diameters as large as a few hundred nanometres, also have technological
uses. The high aspect ratio of these structures means that electrons can be
emitted easily from their tips. If these electrons then traverse a vacuum and
excite a phosphor on a screen, this forms the basis of a display pixel.
Indeed, proof-of-principle displays using such multi-wall nanotube structures
have been fabricated and promise to be ten times more energy efficient than
competing plasma technology4.

The techniques of modern materials science also allow the fabrication of
inorganic tubular nanostructures. As with the multiwall carbon nanotubes, no
key length scales are probed, but there is, again, the promise of
technological applications. Good examples are the piezoelectric nanotubes
made from complex oxides such as barium titanate5, 6 and strontium?bismuth
tantalate7. 'Piezoelectric' means that these polycrystalline tubes can be
strained when an electrical voltage is applied, and vice versa. Each tube
could be triggered individually to release a small quantity of ink for
ink-jet printing, or to deliver drugs into a patient. Sensor, actuator and
data-storage applications are also possible.

The excitement generated by piezoelectric nanotubes has now inspired Levy et
al.1 to emulate the same growth technique using a different and resurgent
class of oxides. Manganites are complex oxides that adopt a pseudo-cubic
perovskite crystal structure. Half a century ago, it was found that an
applied magnetic field could significantly change the electrical resistance
of these materials8, but it is only in the past decade that these
'magnetoresistance' effects have been studied in detail. The catalyst for
this activity was the discovery of colossal magnetoresistance in a thin
film9, just as thin-film magnetoresistance effects were making the transition
from the laboratory to application in read heads for computer disk drives.

To fabricate their nanotubes of lanthanum?praseodymium?calcium manganite,
Levy et al. first made a porous template by chemically etching films of mylar
and polycarbonate that had been bombarded with heavy ions. They then
introduced a precursor solution into the (wetted) pores, and achieved
crystallization by heating the template. Microstructures comprising long,
thin-walled nanotubes formed spontaneously (Fig. 1). Through various
structural characterization techniques, Levy et al. confirmed that each tube
is composed of manganite nanocrystals. Moreover, rough estimates of the
magnetic properties match those expected for bulk samples of this manganite.

Figure 1 Going inorganic.   Full legend

High resolution image and legend (56k)

How might manganite nanotubes impact on technology? One possible application
is in solid-oxide fuel cells. A fuel cell differs from a battery in that
reactants may be continuously fed into it and exhausted. The microstructure
demonstrated by Levy et al. immediately suggests a means by which gases may
be efficiently distributed in such a cell. And as manganites conduct both
electrons and oxygen ions, and are resistant to high-temperature oxidizing
environments, they make good cathodes.

More speculatively, nanotubes made from metallic manganites could act as
highly localized sources of electrons possessing spins of a particular
orientation. This is possible because the spins of the conduction electrons
in manganites can be aligned perfectly, whereas in ordinary magnetic metals
such as cobalt the alignment is only partial. It is possible to imagine the
nanoscale engineering of electronic circuits in which the spin of electrons,
as well as their charge, could be manipulated with precision ? a valuable
capability for spin-sensitive scanning probe microscopy, and perhaps,
ultimately, quantum computing.

Nanotube structures may also offer a means of tuning the strong interactions
that exist between the magnetic, electronic and crystal structures of a
manganite. These interactions generate rich phase-coexistence phenomena over
a wide range of length scales, as has been revealed by imaging methods10. For
example, a ferromagnetic metallic phase may coexist with an antiferromagnetic
insulating phase. In a nanotube, the delicate balance between the diverse
phases could be tuned readily through the stresses associated with the
unconventional geometry. Exploring the parameter space of chemical
composition, grain size, tube dimensions and tube distribution should reveal
more exciting possibilities ahead. The future of nanotubes looks anything but
hollow.

References

1.
Levy, P., Leyva, A. G., Troiani, H. E. & Sánchez, R. D. Appl. Phys. Lett. 83,
5247?5249 (2003). | Article | ISI | ChemPort |

2.
Ijima, S. Nature 354, 56?58 (1991). | Article | ISI | ChemPort |

3.
Ishii, H. et al. Nature 426, 540?544
(2003). | Article | PubMed | ISI | ChemPort |

4.
Amaratunga, G. IEEE Spectrum 40, 28?32 (2003). | Article | ISI |

5.
Hernandez, B. A. et al. Chem. Mater. 14, 480?482
(2002). | Article | ISI | ChemPort |

6.
Luo, Y. et al. Appl. Phys. Lett. 83, 440?442
(2003). | Article | ISI | ChemPort |

7.
Morrison, F. D., Ramsay, L. & Scott, J. F. J. Phys. Condens. Matter 15,
L527?L532 (2003). | Article | ISI | ChemPort |

8.
Volger, J. Physica 20, 49?54 (1954). | ISI | ChemPort |

9.
Jin, S. et al. Science 264, 413?415 (1994). | ISI | ChemPort |

10.
Mathur, N. & Littlewood, P. Physics Today 56, 25?30
(2003). | ISI | ChemPort |

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