Nanotechnology explained: Nanowires and nanotubes

Nanowerk News) Nanowires and nanotubes, slender structures that are  only a few billionths of a meter in diameter but many thousands or millions of  times longer, have become hot materials in recent years. They exist in many  forms — made of metals, semiconductors, insulators and organic compounds — and  are being studied for use in electronics, energy conversion, optics and chemical  sensing, among other fields.

This Scanning Electron Microscope image shows an array of nanowires. (Photo:  Kristian Molhave/Opensource Handbook of Nanoscience and Nanotechnology)

The initial discovery of carbon nanotubes — tiny tubes of pure  carbon, essentially sheets of graphene rolled up unto a cylinder — is generally  credited to a paper published in 1991 by the Japanese physicist Sumio Ijima  (although some forms of carbon nanotubes had been observed earlier). Almost  immediately, there was an explosion of interest in this exotic form of a  commonplace material. Nanowires — solid crystalline fibers, rather than hollow  tubes — gained similar prominence a few years later.

Due to their extreme slenderness, both nanotubes and nanowires  are essentially one-dimensional. “They are quasi-one-dimensional materials,” says MIT associate professor of materials science and engineering Silvija  Gradecak: “Two of their dimensions are on the nanometer scale.” This  one-dimensionality confers distinctive electrical and optical properties.

For one thing, it means that the electrons and photons within  these nanowires experience “quantum confinement effects,” Gradecak says. And  yet, unlike other materials that produce such quantum effects, such as quantum  dots, nanowires’ length makes it possible for them to connect with other  macroscopic devices and the outside world.

The structure of a nanowire is so simple that there’s no room  for defects, and electrons pass through unimpeded, Gradecak explains. This  sidesteps a major problem with typical crystalline semiconductors, such as those  made from a wafer of silicon: There are always defects in those structures, and  those defects interfere with the passage of electrons.

Made of a variety of materials, nanowires can be “grown” on many  different substrates through a vapor deposition process. Tiny beads of molten  gold or other metals are deposited on a surface; the nanowire material, in  vapor, is then absorbed by the molten gold, ultimately growing from the bottom  of that bead as a skinny column of the material. By selecting the size of the  metal bead, it is possible to precisely control the size of the resulting  nanowire.

In addition, materials that don’t ordinarily mix easily can be  grown together in nanowire form. For example, layers of silicon and germanium,  two widely used semiconductors, “are very difficult to grow together in thin  films,” Gradecak says. “But in nanowires, they can be grown without any  problems.” Moreover, the equipment needed for this kind of vapor deposition is  widely used in the semiconductor industry, and can easily be adapted for the  production of nanowires.

While nanowires’ and nanotubes’ diameters are negligible, their  length can extend for hundreds of micrometers, even reaching lengths visible to  the unaided eye. No other known material can produce such extreme  length-to-diameter ratios: millions of times longer than they are wide.

Because of this, the wires have an extremely high ratio of  surface area to volume. That makes them very good as detectors, because all that  surface area can be treated to bind with specific chemical or biological  molecules. The electrical signal generated by that binding can then easily be  transmitted along the wire.

Similarly, nanowires’ shape can be used to produce narrow-beam  lasers or light-emitting diodes (LEDs), Gradecak says. These tiny light sources  might someday find applications within photonic chips, for example — chips in  which information is carried by light, instead of the electric charges that  relay information in today’s electronics.

Compared to solid nanowires, nanotubes have a more complex  structure: essentially one-atom-thick sheets of pure carbon, with the atoms  arranged in a pattern that resembles chicken wire. They behave in many ways as  one-dimensional materials, but are actually hollow tubes, like a long,  nanometer-scale drinking straw.

The properties of carbon nanotubes can vary greatly depending on  how they are rolled up, a property called chirality. (It’s similar to the  difference between forming a paper tube by rolling a sheet of paper lengthwise  versus on the diagonal: The different alignments of fibers in the paper produce  different strength in the resulting tubes.) In the case of carbon nanotubes,  chirality can determine whether the tubes behave as metals or as semiconductors.

But unlike the precise manufacturing control that is possible  with nanowires, so far methods for making nanotubes produce a random mix of  types, which must be sorted to make use of one particular kind. Besides  single-walled nanotubes, they also exist in double-walled and multi-walled  forms.

In addition to their useful electronic and optical properties,  carbon nanotubes are exceptionally strong, and are used as reinforcing fibers in  advanced composite materials. “In any application where one-dimensionality is  important, both carbon nanotubes and nanowires would provide benefits,” Gradecak  says.

Source: By David L. Chandler,  MIT

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