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Carbon nanotube

An electronic device known as a diode can be formed by joining two nanoscale carbon tubes with different electronic properties.
An electronic device known as a diode can be formed by joining two nanoscale carbon tubes with different electronic properties.

Carbon nanotubes are tubular carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications. They exhibit unusual strength and unique electrical properties, and are extremely efficient conductors of heat.

A nanotube has a structure similar to a fullerene, but where a fullerene's carbon atoms form a sphere, a nanotube is cylindrical and each end is typically capped with half a fullerene molecule. Their name derives from their size; nanotubes are on the order of only a few nanometres wide (on the order of one ten-thousandth the width of a human hair), and their length can be millions of times greater than their width.

Nanotubes are composed entirely of spē bonds, similar to graphite. Stronger than the sp3 bonds found in diamond, this bonding structure provides them with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals force . Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking. [1]

There are two main categories of nanotubes: single-walled (SWNT) and multi-walled (MWNT). Additionally, there are a large variety of forms of each of these, idenfied by a two-digit sequence. The first digit indicates how many carbon atoms around the tube is. The second digit determines the offset of where the nanotubes wrap around to. If the second digit is a zero, the nanotubes are called "armchair". If both digits are the same, the nanotubes are called "zigzag". Otherwise, they are called "chiral".

The structure of the nanotube, as described above, strongly affects its electrical conducting properties. For example, (6,0), (6,6), (9,0), and (9,9) nanotubes are all excellent conductors. However, electron holes arise in (7,0), (8,0), (6,2), and (7,5) nanotubes, making them semiconductors. In theory, nanotubes which conduct can have an electrical current density more than 1,000 times stronger than metals such as silver and copper. All nanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube.

While it has long been known that carbon fibres can be produced with a carbon arc, and patents were issued for the process, it was not until 1991 that Sumio Iijima, a researcher with the NEC Laboratory in Tsukuba, Japan, observed that these fibres were hollow. This feature of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics. Techniques have been developed to produce nanotubes in sizeable quantities, but their cost still prohibits any large scale use of them.

Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories, and are also formed in such mundane places as candle flames. However, these naturally occurring varieties are highly irregular in size and quality, and attempting to ensure the high degree of uniformity necessary to meet the needs of research and industry is impossible in such an uncontrolled environment.

Nanotubes can be opened and filled with materials such as biological molecules , raising the possibility of applications in biotechnology. They can be used to dissipate heat from tiny computer chips.

The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. The highest tensile strength an individual SWNT has been tested to is 63 GPa [2]. In Earth's upper atmosphere, atomic oxygen errodes the carbon nanotubes, but other applications rarely need protection of the carbon nanotube surface. Though it is debatable if nanotube materials can ever be made with a tensile strength approaching that of individual tubes, composites may still yield incredible strengths potentially sufficient to allow the building of such things as space elevators, artificial muscles, ultrahigh-speed flywheels, and more. MIT is working on combat jackets utilizing carbon nanotubes for ultrastrong fibers and for monitoring its wearer's condition.

Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.

One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These diplays are known as Field Emission Display s (FEDs) A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Nanotubes have been shown to be superconducting at low temperatures.

Current progress

One application for nanotubes that is currently being researched is high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer burned out of them to make them purely nanotube or they can be left as they are.

Scientists working at the University of Texas at Dallas produced the current toughest material known in mid-2003 by spinning fibers of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibers require 600J/g to break. In comparison, the bullet-resistant fiber Kevlar is 27-33J/g.

In 2004 Alan Windle's group of scientists at the Cambridge-MIT Institute developed a way to make carbon nanotube fibre continuously at the speed of several centimetres per second just as nanotubes are produced. One thread of carbon nanotubes was more than 100 metres long. The resulting fibres are electrically conductive and as strong as ordinary textile threads. [3] [4]

High purity (80%) nanotubes with metallic properties can be extracted with electrophoretic techniques. [5]

In April of 2001, IBM announced it had developed a technique for automatically developing pure semiconductor surfaces from nanotubes.

On September 19, 2003, NEC Corporation, Japan, announced stable fabrication technology of carbon nanotube transistors.

In June 2004 scientists from China's Tsinghua University and Louisiana State University demonstrated the use of nanotubes in incandescent lamps, replacing a tungsten filament in a lightbulb with a carbon nanotube one.

Nanomechanical computer storage devices using nanotubes are currently in the prototype stages. Both high speed non-volatile memory which can be used to replace nearly all solid state memory in computers today, and high density storage that may replace hard drives, are being developed. Major limiting factors in the development of nanotubes include their cost and difficulties in orienting the nanotubes, which tend to tangle because of their length.

As of 2003, nanotubes cost upwards from 20 euro per gram to 1000 euro per gram, depending on purity, composition (single-wall, double-wall, multi-wall) and other characteristics.

Carbon nanotubes in electrical circuits

Carbon nanotubes have many properties--from their unique dimensions to an unusual current conduction mechanism--that make them ideal components of electrical circuits, and it is exciting to envision, or even to implement, novel transistors, MEMS devices, interconnect s, and other circuit elements.

The major hurdles that must be jumped for carbon nanotubes to find prominent places in circuits relate to fabrication difficulties. The carbon nanotube production processes are very different from the traditional IC fabrication process. The IC fabrication process is somewhat like sculpture--films are deposited onto a wafer and pattern-etched away. Carbon nanotubes are fundamentally different from films; they are like atomic-level spaghetti (and every bit as sticky).

Today, there is no reliable way to arrange carbon nanotubes into a circuit. Researchers sometimes resort to manipulating nanotubes one-by-one with the tip of an atomic force microscope in a painstaking, time-consuming process. Perhaps the best hope is that carbon nanotubes can be grown through a chemical vapor deposition process from patterned catalyst material on a wafer. Though such a CVD process has been shown to allow a circuit designer to locate one end of a nanotube, there is no obvious way to control where the other end goes as the nanotube grows out of the catalyst.

Even if nanotubes could be precisely positioned, there remains the problem that, to this date, engineers have been unable to control the types of nanotubes--metallic, semiconducting, single-walled, multi-walled--produced. This is a problem that chemical engineers must solve if nanotubes are to find a place in commercial circuits.

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Last updated: 11-06-2004 20:54:02