CARBON NANOTUBES
 

        Carbon nanotubes, also known as tubular fullerenes, are cylindrical graphene sheets of sp2-bonded carbon atoms.31 Previous STM and AFM studies of these nanotubes have measured various properties of carbon nanotubes. These nanotubes are single molecules measuring a few nanometers in diameter and several microns in length (Figure 2333).

Sumio Iijima discovered carbon nanotubes in 1991. He was making C60 molecules with the carbon arc process. In the same soot as the C60 molecules, he found carbon nanotubes. Since then, fabrication methods have been optimized to produce nanotubes in yields higher than 70 percent.32
        Carbon nanotubes come in a variety of diameters and lengths. Depending on the growth process, the length of the tubes can be from approximately 100 nanometers to several microns. Diameters vary from 1 to 20 nanometers. Another parameter describing carbon nanotubes is their chiral angle. This angle is specified by how the graphene sheet is rolled into a cylinder (Figure 24). By convention, a nanotube with its axis
 

collinear with the horizontal (q = 0) line in Figure 24 is called a "zigzag" nanotube. This name derives from the appearance of the half fullerene molecule that can cap the end of the tube.
        Nanotubes can form with axes collinear to several lines forming chiral angles from 0 to 30 degrees. The number of unique end caps for each possible chiral angle depends on the diameter of the nanotube.
 

Properties
        Being one giant molecule, carbon nanotubes have unusual mechanical and electrical properties. The conductivity of single wall carbon nanotubes can vary from semi-conductive to metallic depending on the chiral angle of the tube and its diameter. The mechanical properties of nanotubes are also unusual. Numerical simulations predict the Young?s modulus of single wall nanotubes to be in excess of a terapascal. Transmission electron microscope (TEM) photographs have shown individual nanotubes bent into a radius of curvature of less than 20 nanometers.
 
Fabrication Methods
        Various techniques are capable of synthesizing carbon nanotubes. The carbon arc method, used initially for producing C60 fullerenes, is the most common and perhaps easiest way to produce carbon nanotubes. Chemical vapor deposition in an apparatus used for creating vapor grown carbon fibers has also produced carbon nanotubes. Finally, laser vaporization of a carbon block has produced the most uniform single wall carbon nanotubes. The carbon arc method was employed in the procedure described here and thus discussion will focus on this method specifically.34
        Initially, the carbon arc method was developed to produce C60 fullerenes. This method creates nanotubes through the arc-vaporization of two carbon rods placed end to end separated by approximately 1mm. A direct current of 50 to 100 A driven by approximately 20 V creates a high temperature discharge between the two electrodes. The discharge vaporizes one of the carbon rods and forms a small rod shaped deposit on the other rod. Producing nanotubes in high yield depends on the uniformity of the plasma arc and the temperature of the deposit form on the carbon electrode.
        The laser vaporization method produces single wall carbon nanotubes in high yields. A graphite target is heated to 1200 °C in a quartz tube. A Nd-YAG laser ablates carbon off of this graphite target. At the end of the furnace a water cooled brass cone collects the soot from the ablation. This soot contains a high percentage of single wall carbon nanotubes.
        Two theories concerning the growth mechanism for tubular fullerenes are currently under debate. The first assumes that the nanotubes are always capped and C2 molecules are absorbed at the pentagonal defects at the caps. The second method assumes that the nanotubes are open during the growth process and carbon molecules are added to the open ends of the nanotubes.
        The growth mechanism in the arc discharge method is believed to be the open tube method. Absorption of a single C2 dimer at an active dangling bond adds one hexagon to the open end. The sequential addition of C2 dimers results in continuous growth of the nanotube. What keeps the nanotube open ended is unknown, creating a serious problem for this model. One suggestion is the high electric field in the region of tube growth. In the carbon arc method the voltage between the electrodes is generally on the of 20 volts. A 1 mm separation distance creates an electric field of 20000 V/m. But the temperatures involved ionize the carbon atoms, thus creating some shielding from the electrodes. This may reduce the effective separation distance to the order of a few microns subsequently increasing the field strength. A field this high could prevent the closure of the nanotube ends.