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12/23/11

Nanowires

Nanowires
A nanowire is a nanostructure and a solid, cylindrical wire with a diameter usually less than 100 nm or structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length where quantum mechanical effects are important.
Types
Many different types of nanowires exist, including metallic (e.g., Ni, Pt, Au), semi conducting (e.g., Si, InP, GaN, etc.), and insulating (e.g., SiO2, TiO2). Molecular nanowires are composed of repeating molecular units either organic (e.g. DNA) or inorganic (e.g. Mo6S9-xIx). The nanowires could be used to link tiny components into extremely small circuits.
Classification
Generally, nanowires are classified according to their structures: a) crystalline, those with structured alignments of polymer chains, b) polycrystalline, those with repeating chemical units for molecules, and c) nearly amorphous, those with random alignment of polymer chains. The varying shape is because the nanowire is only periodic along its axis. In all other dimensions, the nanowire will assume shapes are energetically favorable.
Fabrication
Nanowires are not naturally found but they can be fabricated from a wide variety of materials including germanium, metals, oxides, gallium nitrate, and silicon. There is no single standardized method for the fabrication of nanowires but can be done by nanolithography via an election beam and a relatively recent expensive method known as Molecular Beam Epitaxy (MBE) and by direct chemical synthesis by Vapor-Liquid-Solid (VLS) synthesis method.
High volume fabrication
Scientists at Harvard University in collaboration with researchers from the German universities of Jena, Gottingen, and Bremen, have developed a new technique for fabricating nanowire photonic and electronic integrated circuits that may one day be suitable for high-volume commercial production. By incorporating spin-on glass technology, used in Silicon integrated circuits manufacturing, and photolithography, transferring a circuit pattern onto a substrate with light, the team demonstrated a reproducible, high-volume, and low-cost fabrication method for integrating nanowire devices directly onto silicon. The structure of the team's nanowire devices is based on sandwich geometry: a nanowire is placed between the highly conductive substrate, which functions as a common bottom contact, and a top metallic contact, using spin-on glass as a spacer layer to prevent the metal contact from shorting to the substrate. As a result current can be uniformly injected along the length of the nanowires. These devices can then function as light-emitting diodes, with the color of light determined by the type of semiconductor nanowire used.
Semiconductor nanowires
Semiconductor nanowires are used in the development of cheaper and more efficient solar cells, as well as batteries with higher storage capacity and mainly in nanoelectronics. But, manufacturing semiconductor nanowires on an industrial scale is very expensive because of high temperatures at which they are produced using expensive catalysts, such as gold. But scientists at the Max Planck Institute have produced crystalline semiconductor nanowires at a much lower temperature using inexpensive catalysts, such as aluminium.
Nanowires for solar cell
University of California, San Diego electrical engineers have created experimental solar cells spiked with nanowires that could lead to highly efficient thin-film solar cells of the future. Indium phosphide (InP) nanowires can serve as electron superhighways that carry electrons kicked loose by photons of light directly to the device’s electron-attracting electrode and this scenario could boost thin-film solar cell efficiency.
Nanowire laser
Researchers have grown long, thin crystals of antimony-doped zinc oxide on a thin film of pure zinc oxide as nanowires. These nanowires have diameters of about 200 nm and are about 3 ┬Ám long and the ends of the nanowires were fused into a single crystal with the thin film underneath to work as an ultraviolet laser, producing light with a variety of wavelengths closely spaced around 385 nm.
Future developments
Already large-area lasing devices have been demonstrated, but the true potential of nanowires has now been realized. But the challenge is to fabricate single-nanowire laser diodes with ease. Researchers feel that the real significance of their research lies in developing single-nanowires lasers that could be used to study living cells so that they can insert this tiny laser into the cell or even smaller tissue inside the cell as it would be a powerful tool for doing basic biological and biomedical research into the single cell and perhaps even for killing viruses.


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