What Are Nanotubes?

Nanotubes are cylindrical nanostructures whose walls are one or a few atomic layers thick and whose diameters range from less than one nanometer to tens of nanometers. Carbon nanotubes are the most widely studied variety, formed by rolling a sheet of graphene into a cylinder and capping or leaving the ends open; but nanotubes of boron nitride, molybdenum disulfide, titanium dioxide, and other inorganic compounds have also been synthesized, each carrying distinct electronic, optical, and chemical properties. The field draws on condensed-matter physics, materials chemistry, and nanofabrication, and it is a foundational branch of nanotechnology as applied to both structural materials and functional devices.

The discovery of multi-walled carbon nanotubes by Sumio Iijima in 1991 and of single-walled carbon nanotubes in 1993 catalyzed a global research effort that continues today. Their combination of extremely high mechanical strength, ballistic electron transport, and chemical stability in many environments made them attractive candidates for transistors, composite reinforcement, interconnects, and sensors. The Nature publication on the atomic structure and electronic properties of single-walled carbon nanotubes established the theoretical framework that explains how the chiral geometry of the graphene roll-up determines whether a nanotube is metallic or semiconducting.

Structure and Types

Carbon nanotubes are classified by wall count and by chiral vector. Single-walled carbon nanotubes (SWCNTs) consist of a single graphene cylinder and exhibit diameter-dependent electronic character: a nanotube's chiral indices determine its band structure, so tubes of slightly different diameters or twist angles differ between metallic and semiconducting behavior. Multi-walled carbon nanotubes (MWCNTs) nest two or more concentric cylinders separated by approximately 0.34 nanometers, the interlayer spacing of graphite. The outer shells tend to be metallic and contribute primarily to current transport, while inner shells add mechanical reinforcement. Boron nitride nanotubes are structurally analogous to carbon nanotubes but are always electrically insulating with a bandgap around 5.5 electronvolts, making them useful as a chemically inert electrical barrier at the nanoscale. The NIST carbon nanotube reference materials program develops certified reference samples that support reproducible characterization across research institutions.

Mechanical and Thermal Properties

Carbon nanotubes possess some of the highest measured mechanical stiffness and tensile strength of any known material class. Theoretical and experimental estimates for the Young's modulus of SWCNTs cluster near 1 terapascal, comparable to the in-plane modulus of graphite, and their tensile strengths have been measured in the range of 11 to 63 gigapascals, with theoretical limits estimated at 100 to 200 gigapascals. Nature Communications research on the strength of carbon nanotubes and its dependence on chemical structure demonstrated that defects introduced during synthesis are the principal factor limiting measured strengths below theoretical values. Thermal conductivity along the nanotube axis exceeds 3000 watts per meter per kelvin at room temperature for clean SWCNTs, a value that exceeds diamond in the axial direction and makes nanotubes of interest for thermal management in densely packed electronics.

Synthesis Methods

Carbon nanotubes are produced by three primary methods: arc discharge between graphite electrodes in an inert atmosphere, laser ablation of graphite targets, and chemical vapor deposition (CVD) over metal catalyst particles. CVD is the most scalable and substrate-compatible method, enabling growth of aligned arrays on patterned catalyst films for device integration. Floating catalyst CVD and high-pressure CO conversion (HiPco) produce bulk SWCNT powders at gram-per-hour rates. Sorting of as-synthesized material by electronic type is accomplished by density-gradient ultracentrifugation, gel chromatography, or aqueous two-phase extraction, separating semiconducting from metallic nanotubes for transistor applications.

Applications

Nanotubes have applications across a wide range of fields, including:

• Structural composites in aerospace and sporting goods, where nanotube reinforcement increases tensile strength at low weight fractions
• Transistors and interconnects in nanoelectronics, exploiting their ballistic electron transport
• Chemical and biological sensors, where tube surface functionalization enables selective molecular recognition
• Electrodes in lithium-ion batteries and supercapacitors, benefiting from high surface area and conductivity
• Thermal interface materials for electronics packaging
• Biomedical applications including drug delivery carriers and photothermal cancer therapy