Carbon Nanotubes

Carbon Nanotubes (CNTs) are one-dimensional graphitized tubular forms of carbon. They were first reported in 1991 as a concentric multi-shell form now called Multi-Walled Nanotubes (MWNTs) with diameters ranging from 5 nanometers (nm) to tens of nanometers. These original nanotubes were produced by arc-discharge of graphite. Two years later, Single-Walled Nanotubes (SWNTs) were discovered with diameters as low as about 1 nm. Carbon nanotubes can also be produced by laser vaporization (ablation) of graphite with embedded metal particles, which results in SWNTs with narrow diameters in bundles or ropes, with the nanotubes held tightly together. While these methods are effective in the production of carbon nanotubes, they have several disadvantages. The raw product is in the form of tangled ropes and bundles along with graphitic particles, metals, and other impurities. This product form requires purification steps involving chemical and thermal treatment.

Top: Schematic and SEM image of CNTFET. Nanotubes grown via LPCVD at 900°C on Si/ SiO² with 0.1 nm patterned Fe catalyst, top-contacted with semi-circular Ti/Pd electrodes. Bottom left: ID-VGS plot of two transistors. Bottom right: ID-VDS plot showing CNTFET current saturation and eventual CNT breakdown [Image credit: Albert Liao/ Eric Pop, UIUC]

The development of the Chemical Vapor Deposition (CVD) method used to produce carbon nanotubes has enabled the synthesis of high quality carbon nanotubes that grow in specified locations and with greater purity. Carbon nanotubes can be grown by CVD directly onto substrates or onto bulk-supported catalyst. The process typically consists of flowing a carbon feedstock, over a catalyst-covered substrate at elevated temperature; the feedstock molecules decompose upon the catalyst, releasing their carbon to form nanotubes. Advantages of using the CVD process for nanostructure synthesis include

• Direct synthesis onto a variety of substrates for integration of high-purity nanotubes into devices with little or no post-processing
• Control of nanotube growth location through catalyst patterning
• Potential for control of nanotube growth direction and orientation with electromagnetic fields
• Control over nanotube diameter by adjustment of the size of catalytic nanoparticles used
• Nanotube growth improvements as catalyst and substrate design develops without requiring complete retooling of the system
• Versatility of growth parameters with a wide range of gases, catalysts, substrates, temperatures, and pressures
• Scalability from chip scale to wafer scale using the same CVD process parameters

Raman spectrum of SWNT grown at 900°C under electric field alignment in a 50 mm benchtop LPCVD system [Image credit: D. Nandi, IISC Bangalore]

Carbon nanotubes have unique electrical and mechanical properties that will improve performance in many technology areas when they are successfully integrated into products. The sp2 hybridization provides for delocalized charge carriers and the conduction of current. Depending on their physical structure, the nanotubes can be metallic conductors with no band gaps or semiconducting with band gap sizes ranging from a few meV (electron volts) to approximately 1 eV. The electrical and mechanical properties of nanotubes derive from their graphene sheet origins. Nanotubes have high stiffness and resiliency; their specific strength is the highest of any known material, effectively equivalent to the in-plane strength of graphite. In addition, nanotubes have the highest thermal conductivity of any known material.