of worldwide attention from government, academia and industry. Because of its ultra-light weight and superior material properties, CNTs are considered to be one of the most promising novel materials suitable for a wide range of applications in nanotechnolgy from nanoelectronics and bioengineering to biotechnology. 

Two of the most common forms of carbon nanotubes are single- and multi-wall. A single-walled nanotube (SWNT) consists of carbon atoms that resemble rolled up sheets of graphite, ~ 1.5 nm in diameter. Laboratory tests show that a single-walled carbon nanotube has a mean tensile strength as high as 30

Manufacturing Techniques - Several techniques based upon similar principles of vaporization and condensation of carbon are used in CNT manufacturing. The least complicated technique is the Carbon Arc or Arc Discharge. In this technique, two graphite electrode rods containing transition metal catalysts (Co or Ni) are placed in a low-pressure chamber filled with an inert gas (helium or argon). An electrical current (50A - 100A) between anode and cathode graphite rods causes a high temperature discharge and vaporization of the anode graphite rod. Vaporized carbon is condensed on the cool chamber sidewall and forms nanotubes and soot. 

In laboratories such as the NASA Ames Research Center [3], the purest form of SWNTs can be produced at the rate of 0.3 to 0.4 grams per hour using the pulsed laser vaporization (PLV) technique. In this technique, a carbon graphite target containing ~ 0.5 atomic percent of Co and/or Ni as a catalyst is used. The target is placed in a pressurized tube at elevated temperatures, ~ 1200ºC and then ablated or evaporated using high-power Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet) lasers. Carbon vapor is condensed on the cool wall collector in the form of SWNTs.

Large-scale production of SWNTs can be done using either a chemical vapor deposition (CVD) [4] or "High-Pressure CO conversion" (HiPCO) [5] techniques. In a chemical vapor deposition (CVD) technique, hydrocarbon gas flows through a transition metal catalyst substrate in a reactor tube operated at temperatures between 600 and 1200ºC. CNTs are formed directly on the substrate as a result of the dissolution of hydrocarbon gas molecules catalyzed by the transition metal at high temperatures. This technique is highly suitable for nanoelectronics since selective SWNTs can be grown on substrate using  standard microlithography processes. 

In the HiPCO technique, SWNTs are produced by flowing a gas mix of CO with a small amount of iron pentacabonyl Fe(CO)₅ catalytic particles, through high-pressured heated carbon monoxide (CO) gas in a quartz tube reactor, operated at ~ 1200ºC. Because CO gas is used as the primary gas source, CNTs produced by the HiPCO technique contain no hydrocarbon and are considerably cleaner than CNTs produced by the CVD technique. 

Applications - Although small companies such as Chapel Hill, N.C.-based Xintek, have developed and commercialized carbon nanotube materials, carbon nanotubes for cold cathodes, Atomic Force Microscope (AFM) tips and x-ray tubes since 2000, most products are still in the laboratory development stage. In Japan, carbon nanotube-based cold cathodes, for example, are being developed as field-emission displays (FEDs) for future ultrahigh-definition televisions.

Research scientists at Munich, Germany-based Infineon Technologies AG (NYSE ADR: IFX) announced that they have already successfully grown carbon nanotubes on a 150-mm silicon wafer substrate using a selective CVD technique [6]. The company's first plan is to use the carbon nanotubes to replace the conventional vias, the contact bridges between two metal layers in the chip, and later all of the metal layers with the CNT material.

Purdue University researchers have shown that carbon nanotubes could be used to create brain probes and implants to study and treat neurological damage and disorders [7]. Probes made of silicon that are currently used, cause the body to produce scar tissue that eventually accumulates and prevents the devices from making good electrical contact with brain cells called neurons.

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