Carbon nanotube

A carbon nanotube (CNT) is a cylindrical nanostructure composed entirely of carbon atoms arranged in a hexagonal lattice, exhibiting extraordinary mechanical, electrical, and thermal properties that have made it a subject of intense research since its discovery.

History and discovery

The existence of carbon nanotubes was first observed in 1952 by Soviet scientists L. V. Radushkevich and V. M. Lukyanovich, who published clear images of 50‑nanometer diameter tubes formed from carbon. However, this work went largely unnoticed due to the Cold War. In 1976, Morinobu Endo and colleagues reported the synthesis of nanometer‑scale carbon fibers by chemical vapor deposition, producing what are now recognized as multi‑walled carbon nanotubes. The modern era of CNT research began in 1991 when Sumio Iijima of NEC Corporation described the formation of multi‑walled carbon nanotubes using arc discharge evaporation. Shortly thereafter, single‑walled carbon nanotubes were independently synthesized by Iijima and Toshinari Ichihashi, and by Donald Bethune and colleagues at IBM in 1993.

Structure and types

Carbon nanotubes are made of sp²‑hybridized carbon atoms arranged in a cylindrical sheet of graphene. The properties of a CNT are determined by its structural parameters: diameter, chirality, and number of concentric walls.

Single‑walled carbon nanotubes (SWCNTs)

An SWCNT consists of a single graphene layer rolled into a seamless cylinder with a typical diameter of 0.7–2 nm and length up to several centimeters. The roll‑up direction, defined by the chiral vector (n,m), determines whether the tube behaves as a metal or a semiconductor. Armchair (n=m) tubes are metallic, while zigzag and chiral tubes may be metallic or semiconducting depending on the (n,m) indices.

Multi‑walled carbon nanotubes (MWCNTs)

MWCNTs comprise multiple concentric graphene cylinders nested inside one another, like a set of coaxial tubes. The interlayer spacing is approximately 0.34 nm, similar to the interplanar distance in graphite. MWCNTs are generally larger in diameter (2–100 nm) and can be grown in lengths from micrometers to millimeters. Their electrical and mechanical properties depend on the number of walls and the interlayer interactions.

Other related structures

• Double‑walled carbon nanotubes (DWCNTs) are a special class of MWCNTs with exactly two walls, offering a combination of SWCNT‑like electronic properties with improved mechanical stability.
• Carbon nanobuds are hybrid structures in which fullerene‑like protrusions are attached to the sidewall of a nanotube, altering its electronic behavior.
• Nitrogen‑ or boron‑doped CNTs can be produced to tailor their chemical reactivity or electronic band structure.

Properties

Carbon nanotubes exhibit a remarkable combination of properties that are often superior to any other known material.

Mechanical properties

CNTs are among the strongest and stiffest materials discovered. The tensile strength of an individual SWCNT can exceed 100 GPa (up to 63 times stronger than high‑strength steel), while its Young’s modulus is about 1 TPa, comparable to that of diamond. Elastic deformation up to 15–20% strain is reversible. These exceptional properties arise from the strong covalent sp² bonds in the graphene lattice.

Electrical properties

Depending on chirality, a SWCNT can be either metallic or semiconducting. Metallic CNTs can conduct electricity with minimal resistance, exhibiting ballistic transport over micrometer distances at room temperature. Semiconducting CNTs have a bandgap inversely proportional to diameter, typically ranging from 0.5 to 1.5 eV. This makes them promising for field‑effect transistors (CNTFETs) and other nanoelectronic devices.

Thermal properties

The thermal conductivity of individual CNTs is remarkably high—up to 3500 W/m·K for SWCNTs at room temperature, rivaling diamond. However, thermal transport is highly anisotropic; across the tube axis, conductivity is much lower. The high thermal conductivity is attributed to efficient phonon transport along the tube.

Optical properties

CNTs interact strongly with light. Semiconducting SWCNTs exhibit photoluminescence in the near‑infrared region, with emission wavelengths tunable by diameter. They also show strong Raman scattering, with characteristic radial breathing modes (RBM) that are used to probe their structure.

Chemical properties

The sidewalls of CNTs are relatively inert, but defects or functionalization can introduce reactive sites. Chemical or electrochemical reactions can be used to attach functional groups (e.g., –COOH, –OH) to improve solubility or compatibility with other materials.

Synthesis

Several methods exist for producing carbon nanotubes, each with distinct advantages and limitations.

Arc discharge

In this earliest method, an electric arc is struck between two graphite electrodes in an inert atmosphere, vaporizing the anode. The deposited soot contains both SWCNTs and MWCNTs. This method produces high‑quality nanotubes but in low yield and with significant impurity content.

Laser ablation

A pulsed laser vaporizes a graphite target containing metal catalysts (e.g., nickel, cobalt, iron) in a high‑temperature furnace. The resulting vapor condenses into SWCNTs. This yields high‑purity tubes with controlled diameter distribution, but the process is energy‑intensive and relatively low‑throughput.

Chemical vapor deposition (CVD)

CVD is the most widely used method for large‑scale CNT production. A carbon‑containing gas (e.g., methane, ethylene, ethanol) is decomposed over metal catalyst nanoparticles at 500–900 °C. The catalyst size and growth conditions determine the diameter, chirality, and wall number. CVD allows alignment and patterned growth on substrates, making it suitable for integration into devices. Variations such as plasma‑enhanced CVD (PECVD) or water‑assisted CVD ("super‑growth") enable rapid production of forests of long, vertically aligned CNTs.

Other methods

• Electrolysis: CNTs can be formed by electrolysis of molten carbonates or halides.
• Flame synthesis: Combustion of hydrocarbons in a controlled flame can yield nanotubes, though with high defect density.
• High‑pressure carbon monoxide (HiPco): CO disproportionation on iron catalysts gives high‑purity SWCNTs.

Applications

Carbon nanotubes have found or are being investigated for use in a vast range of fields, from electronics to medicine.

Electronics and photonics

• Transistors: Semiconducting CNTs are used in field‑effect transistors with high carrier mobility. CNTFETs are candidates for post‑silicon digital electronics.
• Interconnects: Metallic CNTs are considered for on‑chip wiring due to their high current‑carrying capacity (up to 10⁹ A/cm²) and resistance to electromigration.
• Displays and sensors: CNT‑based field‑emission displays, touch screens, and gas sensors exploit their electrical and mechanical properties.

Composite materials

CNTs are added to polymers, metals, or ceramics to enhance strength, stiffness, and electrical or thermal conductivity. For example, CNT‑reinforced epoxy composites are used in sporting goods, aerospace components, and automotive parts.

Energy storage and conversion

• Batteries and supercapacitors: CNTs provide high surface area and electrical conductivity, improving electrode performance in lithium‑ion batteries and supercapacitors.
• Fuel cells: CNT‑supported catalysts enhance the efficiency of proton‑exchange membrane fuel cells.
• Solar cells: Transparent conductive films made from CNT networks can replace indium tin oxide (ITO) in photovoltaic devices.

Biomedical applications

• Drug delivery: Functionalized CNTs can carry therapeutic molecules across cell membranes.
• Imaging: CNTs are used as contrast agents in photoacoustic and near‑infrared imaging.
• Tissue engineering: CNT‑scaffolds support cell growth and electrical stimulation for neural or cardiac tissue repair.

Environmental and other uses

• Water filtration: CNT membranes can remove bacteria, viruses, and organic contaminants by size exclusion or adsorption.
• Field emitters: CNT arrays serve as cold electron sources for X‑ray tubes, electron microscopes, and vacuum microelectronics.
• Paints and coatings: Conductive CNT‑based paints provide antistatic properties or electromagnetic interference shielding.

Safety and health concerns

The potential health risks of carbon nanotubes are under active investigation, as their needle‑like shape and biopersistence resemble those of asbestos fibers. Inhalation of airborne CNTs can cause pulmonary inflammation, granuloma formation, and fibrosis in animal studies. The degree of toxicity depends on length, diameter, surface functionalization, and aggregation. Rigorous industrial hygiene measures, including ventilation and personal protective equipment, are recommended when handling CNTs. Regulatory frameworks such as REACH in Europe and the TSCA in the United States require toxicity testing for commercial CNT products. However, many applications involve CNTs embedded in solid matrices, where release into the environment is limited.

See also

• Graphene
• Carbon nanofiber
• Fullerene
• Nanotechnology
• Molecular electronics

References

(References would be listed here in a complete encyclopedia entry, including key papers by Iijima, Bethune, and others, as well as reviews on synthesis, properties, and applications.)