carbon nanotubes are molecular

8/9/2019 Carbon Nanotubes Are Molecular

1/4

Carbon nanotubes are molecular-scale tubes of graphitic carbon withoutstanding properties. They are among the stiffest and strongest fibres known,and have remarkable electronic properties and many other uniquecharacteristics. For these reasons they have attracted huge academic andindustrial interest, with thousands of papers on nanotubes being published every

year. Commercial applications have been rather slow to develop, however,primarily because of the high production costs of the best quality nanotubes.

History

The current huge interest in carbon nanotubes is a direct consequence of the synthesisof buckminsterfullerene, C60 , and other fullerenes, in 1985. The discovery that carboncould form stable, ordered structures other than graphite and diamond stimulatedresearchers worldwide to search for other new forms of carbon. The search was givennew impetus when it was shown in 1990 that C60 could be produced in a simple arc-evaporation apparatus readily available in all laboratories. It was using such anevaporator that the Japanese scientist Sumio Iijima discovered fullerene-related carbonnanotubes in 1991. The tubes contained at least two layers, often many more, and

ranged in outer diameter from about 3 nm to 30 nm. They were invariably closed at bothends.

A

transmission electron micrograph of some multiwalled nanotubes is shown in the figure(left). In 1993, a new class of carbon nanotube was discovered, with just a single layer.These single-walled nanotubes are generally narrower than the multiwalled tubes, withdiameters typically in the range 1-2 nm, and tend to be curved rather than straight. Theimage on the right shows some typical single-walled tubes It was soon established thatthese new fibres had a range of exceptional properties (see below), and this sparked offan explosion of research into carbon nanotubes. It is important to note, however, thatnanoscale tubes of carbon, produced catalytically, had been known for many yearsbefore Iijimas discovery. The main reason why these early tubes did not excite wide

interest is that they were structurally rather imperfect, so did not have particularlyinteresting properties. Recent research has focused on improving the quality ofcatalytically-produced nanotubes.

Structure

The bonding in carbon nanotubes is sp, with each atom joined to three neighbours, asin graphite. The tubes can therefore be considered as rolled-up graphene sheets
http://www.labs.nec.co.jp/Eng/innovative/E1/myself.htmlhttp://www.labs.nec.co.jp/Eng/innovative/E1/myself.html


2/4

(graphene is an individual graphite layer). There are three distinct ways in which agraphene sheet can be rolled into a tube, as shown in the diagram below.

The first two ofthese, known asarmchair (top left)

and zig-zag(middle left) have ahigh degree ofsymmetry. Theterms "armchair"and "zig-zag" referto the arrangementof hexagons aroundthe circumference.The third class oftube, which inpractice is the mostcommon, is knownas chiral, meaningthat it can exist intwo mirror-relatedforms. An exampleof a chiral nanotubeis shown at thebottom left.

The structure of a nanotube can be specified by a vector, (n,m), which defines how thegraphene sheet is rolled up. This can be understood with reference to figure on theright. To produce a nanotube with the indices (6,3), say, the sheet is rolled up so thatthe atom labelled (0,0) is superimposed on the one labelled (6,3). It can be seen fromthe figure that m = 0 for all zig-zag tubes, while n = m for all armchair tubes.

Synthesis


3/4

The arc-evaporation method, which produces the best quality nanotubes, involvespassing a current of about 50 amps between two graphite electrodes in an atmosphereof helium. This causes the graphite to vaporise, some of it condensing on the walls ofthe reaction vessel and some of it on the cathode. It is the deposit on the cathode whichcontains the carbon nanotubes. Single-walled nanotubes are produced when Co and Nior some other metal is added to the anode. It has been known since the 1950s, if notearlier, that carbon nanotubes can also be made by passing a carbon-containing gas,such as a hydrocarbon, over a catalyst. The catalyst consists of nano-sized particles ofmetal, usually Fe, Co or Ni. These particles catalyse the breakdown of the gaseousmolecules into carbon, and a tube then begins to grow with a metal particle at the tip. Itwas shown in 1996 that single-walled nanotubes can also be produced catalytically. Theperfection of carbon nanotubes produced in this way has generally been poorer thanthose made by arc-evaporation, but great improvements in the technique have beenmade in recent years. The big advantage of catalytic synthesis over arc-evaporation isthat it can be scaled up for volume production. The third important method for makingcarbon nanotubes involves using a powerful laser to vaporise a metal-graphite target.This can be used to produce single-walled tubes with high yield.

Properties

The strength of the sp carbon-carbon bonds gives carbon nanotubes amazingmechanical properties. The stiffness of a material is measured in terms of its Young'smodulus, the rate of change of stress with applied strain. The Young's modulus of thebest nanotubes can be as high as 1000 GPa which is approximately 5x higher thansteel. The tensile strength, or breaking strain of nanotubes can be up to 63 GPa, around50x higher than steel. These properties, coupled with the lightness of carbonnanotubes, gives them great potential in applications such as aerospace. It has evenbeen suggested that nanotubes could be used in the space elevator, an Earth-to-space cable first proposed by Arthur C. Clarke. The electronic properties of carbonnanotubes are also extraordinary. Especially notable is the fact that nanotubes can bemetallic or semiconducting depending on their structure. Thus, some nanotubes haveconductivities higher than that of copper, while others behave more like silicon. There isgreat interest in the possibility of constructing nanoscale electronic devices fromnanotubes, and some progress is being made in this area. However, in order toconstruct a useful device we would need to arrange many thousands of nanotubes in adefined pattern, and we do not yet have the degree of control necessary to achieve this.There are several areas of technology where carbon nanotubes are already being used.These include flat-panel displays, scanning probe microscopes and sensing devices.The unique properties of carbon nanotubes will undoubtedly lead to many moreapplications.

Nanohorns

Single-walled carbon cones with morphologies similar to those of nanotube caps werefirst prepared by Peter Harris, Edman Tsang and colleagues in 1994 (click here to seeour paper). They were notdiscovered by NEC scientists, as stated in a press release.They were produced by high temperature heat treatments of fullerene soot - click hereto see a typical image. Sumio Iijima's group subsequently showed that they could alsobe produced by laser ablation of graphite, and gave them the name "nanohorns". Thisgroup has demonstrated that nanohorns have remarkable adsorptive and catalytic
http://www.personal.rdg.ac.uk/~scsharip/Faraday.pdfhttp://www.nec.co.jp/press/en/0108/3001.htmlhttp://www.rdg.ac.uk/~scsharip/nanohorns.htmhttp://www.personal.rdg.ac.uk/~scsharip/Faraday.pdfhttp://www.nec.co.jp/press/en/0108/3001.htmlhttp://www.rdg.ac.uk/~scsharip/nanohorns.htm


4/4

properties, and that they can be used as components of a new generation of fuel cells.For details see the NEC press release and this news item from CNN.
http://www.nec.co.jp/press/en/0108/3001.htmlhttp://europe.cnn.com/2001/TECH/ptech/09/03/nec.pda.fuel.cell.idg/http://europe.cnn.com/2001/TECH/ptech/09/03/nec.pda.fuel.cell.idg/http://www.nec.co.jp/press/en/0108/3001.htmlhttp://europe.cnn.com/2001/TECH/ptech/09/03/nec.pda.fuel.cell.idg/

carbon nanotubes are molecular

Documents