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Carbon nanotubes are hexagonal sheets of graphene, which are single layers of graphite atoms, in the form of rolled up chicken wire.
Due to their hexagonal nature, they are part of a class of molecules called fullerenes, which are closed-caged molecules containing only hexagonal and pentagonal interatomic bonding networks.

There are two types of carbon nanotubes. One type is called Single-Wall-Nanotube, or SWNT. This type consists of only one cylinder, and represents the most promising type in the use of transistors. Their diameter is usually 0.33 to 5.0 nm and their lengths are 2 to 10to nm. The other type is Multi-Walled-Nanotubes, or MWNTs , . They are a bunch of SWNT in a Russian doll type of configuration, with SWNTs placed inside larger SWNTs. A MWNT’s diameter ranges from 3 to 50 nm and their lengths are several microns.

Physical Properties

Carbon Nanotube’s hexagonal structure gives it great strength. In fact, they are 50 times stronger than steel and yet are only a quarter as dense. In addition, carbon nanotubes have very good elastic properties. No matter how much they are squeezed, the nanotube will just bend or buckle, but not break. They will always pop back into shape. Another amazing property is its heat transfer characteristics. Carbon Nanotubes are able to conduct heat so well that they are more efficient than a diamond, which is one of the best heat conductors. These three properties mean that anything made with carbon nanotubes, including transistors, will be extremely resilient and able to work even in the most inhospitable environment.

Electrical Properties

The electrical properties of carbon nanotubes (CNT) are unique and customizable. Unlike most materials, they come in both metallic and semiconducting forms. In addition, their band gaps can be set to a desired level by adjusting certain physical characteristics. Another unique property is that they are one-dimensional ballistic conductors even above the room temperature.

Multi-Walled-Nanotubes
Since MWNT’s are comprised of multiple SWNTs, there properties are complicated, since each SWNT shell can have different indices and bandgaps. However, it has been discovered that when connected to contacts, the outer shell contributes most of the electrical transport. Therefore, a simplification can be done on MWNTs for transistors, since the transistor will require the MWNT to be draped over two contacts. It should be noted that there is still some coupling between shells and ultimately these effects will have to be understood. One way to understand them is to use a procedure developed by IBM to remove parts or whole shell and study the transport of each shell.

Fabrication

There are three ways to create carbon nanotubes: arc discharge, laser ablation, and chemical vapor deposition (CVD). Each method has its advantages and disadvantages.

(1) Arc Discharge
Arc discharge is the earliest method used to create nanotubes. It involves vaporizing carbon to create the nanotube. Figure shows a schematic inside the chamber where the tubes are made.

The apparatus is kept in a helium atmosphere at 50-760 torr. A bias voltage of about 10-30V is applied. The anode, which is composed of amorphous carbon, is brought in contact with the cathode and then removed. The result is a discharge of about 50-300 amps. The discharge vaporizes part of the anode and the helium quenches the vapor to form the nanotubes. After which they collect at the end of the anode. The growth rate is directly proportional to the helium pressure.

This method will create MWNTs, but in order to create SWNTs a catalyst is required. The most common is Ni:Yi, other catalysts are Fe, Co, and Ni. The biggest requirement for a catalyst is that it be a poor carbide former. The Ni:Yi catalyst is mixed with the carbon to a 1.5% solution. Figure 10 shows the apparatus setup to create SWNTs.

The carbon vapor is absorbed into the catalyst, which is in a liquid alloy with the carbon. A layer of carbon forms around the catalyst particles. Eventually a saturation point is reached and carbon is precipitated out in the form of SWNTs. During the entire time, carbon is continually being absorbed, allowing the nanotubes to grow. This process is called vapor-liquid-solid growth (VLS growth).

This method is the cheapest, however, it results in contaminated nanotubes that require cleaning. The soot that is produced with the nanotubes needs to be removed before use and the soot can easily get in air if one is not careful. After the tubes are cleaned, they are still jumbled together. Below figure shows SEM images of both MWNTs and SWNTs created with arc discharged.
(2) Laser Ablation

The laser ablation apparatus is shown in below figure
A 10Hz pulsed laser is focused onto a graphite catalyst target in a quartz tube, which is located in a tube furnace. The laser vaporizes part of the target and produces nanotubes using VLS growth. In order for VLS growth to occur, the oven temperature must be chosen such that the catalyst carbon mixture will be in a liquid alloy state and the pure carbon can remain solid to precipitate out. To choose the correct temperature, a phase diagram is used.

The nanotubes are then swept by the flowing inert gas, argon and collected on a cold finger.

Laser ablation will only grow SWNTs and not MWNTs. The SWNT produced is a high purity nanotube. Minimal cleaning is required, but they still end up tangled up, making them difficult to use. In addition, laser ablation is very expansive, costing $2,000 per gram with catalyst, due to the laser required

(3) Chemical Vapor Deposition
Chemical vapor deposition (CVD) is the easiest method, involving the layering a catalyst and then having a hydrocarbon gas react with it to grow nanotubes.

above figure shows the apparatus used. The sample, which is a substrate layered with a catalyst particles is placed in the center and heated to around 720 degrees Celsius. The hydrocarbon gas is then flowed through the tube. The gas catalytically reacts and grows the nanotubes. The growth process is shown in below figure
The nanotube can grow in two ways. Both ways are the same except for whether the tube grows underneath the catalyst or above it. The process is similar but slower to VLS growth. The carbon form the gas is absorbed into the catalyst and the nanotubes are then precipitated out.
CVD produces the cleanest nanotubes, thus no leaning is required. Since the tubes are grown where the catalyst particles are, the nanotubes are not tangled up and are easier to use. In addition, the process is very familiar to the current IC fabrication techniques, so it would be easiest to scale up to industrial production.