Nanowires and nanotubes

Nanowires and nanotubes, slender structures that are only a few billionths of a meter in diameter but many thousands or millions of times longer. The intense research started at the beginning of the 90s, precisely 1991 after Japanese physicist Sumio Ijima discovered carbon nanotubes. These carbon tubes are tiny tubes made of pure carbon which essentially makes a sheet of grapheme rolled up into a cylinder. After that discovery it was automatically generated a huge interest in this exotic form of a commonplace material. Nanowire is a nanostructure which has a diameter of the order of a nanometer (10^-9 meters). At these scales quantum mechanical effects are important hence the name quantum wires. 

Due to their slenderness we can treat nanowires and nanotubes as one-dimensional which means that we can neglect second dimension because of small value. We can call them “quasi-one-dimensional materials”. According to associate professor of materials science and engineering Silvija Gradečak two of their dimensions are on the nanometer scale and this one-dimensionality confers distinctive electrical and optical properties. It means that electrons and photons within these nanowires experience so called quantum confinement effects. 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.

According to S. Gradečak the structure of nanowires are so simple that there is no room for defects, and electrons pass through nanowire unimpeded. This sidesteps a major problem with typical crystalline semiconductors, such as those made from water or silicon. So far when they made these semiconductors there were always some defects and those defects always interfere with the passage of electrons.

This nanowire can be made of different materials through process called vapor deposition process. In this process the tiny beds of molten gold or other metals are deposited on a surface; then the nanowire material in vapor is then absorbed by the molten gold, ultimately grown from the bottom of that bead as a skinny column of the material. In this process by selecting the size of the metal bead, it is possible to precisely control the size of the resulting nanowire. The scientists have discovered that the materials that don’t ordinarily mix together can be very easily mixed together in nanowire form. For example, layers of silicon and germanium, the two widely used materials in semiconductor technology are very difficult to grown together in thin films. But in nanowires they can be grown without any problems. This procedure of joining silicon and germanium together in nanowire is very simple and for the tool which is necessary for these procedure is the same that they use in semiconductor industry.

While the nanowires and nanotubes diameter are negligible their length can be about hundreds of micrometers even reaching lengths visible to the naked eye. Because of this, the wires have an extremely high ratio of surface area to volume and make them very good detectors, because all that surface area can be treated to bind with specific chemical or biological molecules.

In addition to their useful electronic and optical properties, carbon nanotubes are exceptionally strong and are used for reinforcing fiber in advanced composite materials.