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Overview on the discovery, structure, properties and production of Carbon Nanotubes

What are Carbon Nanotubes?

Carbon is an amazingly versatile element in its ability to bond in various ways to form materials which have very different properties. The most abundant form of pure carbon on earth is graphite, which is composed of sheets of trigonally bonded carbon atoms arranged in hexagonal sheets (called graphene layers) as shown in Figure 1(a). Graphite is a soft, grey solid with high electrical conductivity along the direction of its graphene layers. Under conditions of extreme temperature and/or extreme pressure, carbon forms diamond, which is composed of tetrahedrally bonded carbon atoms as shown Figure 1(b). Diamond is a precious stone which is transparent, insulating and the hardest material known on earth. Then there also exists a whole range of closed-caged carbon structures called fullerenes, the most famous of which is the C₆₀ molecule (Figure 1(c)) or the Buckminster fullerene which was discovered by Kroto in 1985. In 1991, Iijima, whilst studying the carbonaceous deposit from an arc discharge between graphite electrodes, found highly crystallized carbon filaments which were merely a few nanometers in diameter and few microns long. These high aspect ratio structures had a unique form – they contained carbon atoms arranged in graphene sheets which were rolled together to form a seamless cylindrical tube (Figure 1(d)), and each filament contained a ‘Russian doll’ arrangement of coaxial tubes – hence, the term ‘nanotube’ was born to describe these structures. Nanotubes can be single walled (ie. one tube) or multiwalled (ie. multiple concentric tubes).

Why are Carbon Nanotubes technologically important?

With graphene tubes parallel to the filament axis, nanotubes would inherit several important properties of ‘intra-plane’ graphite. This imparts a very unique combination of properties on this material, namely:

High aspect ratio structures with diameters in nanometers, lengths in microns
High mechanical strength (tensile strength 60GPa) and modulus (Young’s modulus 1TPa)
High electrical conductivity (10^-6 ohm m typically), and for well crystallised nanotubes ballistic transport is observed
High thermal conductivity (1750-5800 W/mK)
Being covalently bonded, as electrical conductors they do not suffer from electromigration or atomic diffusion and thus can carry high current densities (10⁷ -10⁹ A/cm² )
Single wall nanotubes can be metallic or semi-conducting
Chemically inert, not attacked by strong acids or alkali
Collectively, nanotubes can exhibit extremely high surface area

How are Carbon Nanotubes produced?

Today, nanotubes are produced by 3 main techniques, namely electric arc discharge, laser ablation and chemical vapour deposition. The arc discharge technique (Figure 2(a)) involves the generation of an electric arc between two graphite electrodes, one of which is usually filled with a catalyst metal powder (eg. iron, nickel, cobalt), in a Helium atmosphere. The laser ablation method (Figure 2(b)) uses a laser to evaporate a graphite target which is usually filled with a catalyst metal powder too. The arc discharge and laser ablation techniques tend to produce an ensemble of carbonaceous material which contain nanotubes (30-70%), amorphous carbon and carbon particles (usually closed-caged ones). The nanotubes must then be extracted by some form of purification process before being manipulated into place for specific applications. The chemical vapour deposition process (Figure 2(c)) utilises nanoparticles of metal catalyst to react with a hydrocarbon gas at temperatures of 500-900°C. A variant of this is plasma enhanced chemical vapour deposition (Figure 2(d)) in which vertically aligned carbon nanotubes can easily be grown. In these chemical vapour deposition processes, the catalyst decomposes the hydrocarbon gas to produce carbon and hydrogen. The carbon dissolves into the particle and precipitates out from its circumference as the carbon nanotube. Thus, the catalyst acts as a ‘template’ from which the carbon nanotube is formed, and by controlling the catalyst size and reaction time, one can easily tailor the nanotube diameter and length respectively to suit. Carbon tubes, in contrast to a solid carbon filament, will tend to form when the catalyst particle is ~50nm or less because if a filament of graphitic sheets were to form, it would contain an enormous percentage of ‘edge’ atoms in the structure. These edge atoms have dangling bonds which makes the structure energetically unfavourable. The closed structure of tubular graphene shells is a stable, dangling-bond free solution to this problem, and hence the carbon nanotube is the energetically favourable and stable structural form of carbon at these tiny dimensions.