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.
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.
Levy, P., Leyva, A. G., Troiani, H. E. & Sánchez, R. D. Appl. Phys. Lett. 83, 5247–5249 (2003).
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