Carbon Nanotubes Explained: Types, Properties, and Real-World Uses

A Cylinder of Carbon That Changed Materials Science

Carbon nanotubes (CNTs) are cylindrical nanostructures formed by rolling sheets of graphene into seamless tubes. First described in detail by Sumio Iijima at NEC in 1991, CNTs have been the subject of tens of thousands of research papers and are now a commercially significant material used in everything from sporting goods to aerospace components to semiconductor interconnects.

Their appeal lies in an unusual combination of properties: CNTs are among the stiffest and strongest materials ever measured, yet they are lighter than aluminium. They can be metallic or semiconducting depending on their geometry. They conduct heat as well as diamond. No other material class combines these attributes in a single structure.

Single-Walled vs Multi-Walled Carbon Nanotubes

The first distinction to understand is the number of walls:

• Single-walled carbon nanotubes (SWCNTs) — one cylindrical graphene sheet, typically 0.7–2 nm in diameter. SWCNTs can be either metallic or semiconducting depending on the chiral angle of the lattice. Semiconducting SWCNTs are the basis for experimental carbon nanotube transistors. More expensive to produce and harder to separate by electronic type.
• Multi-walled carbon nanotubes (MWCNTs) — two or more concentric graphene cylinders, outer diameters typically 5–50 nm. More practical to synthesise at scale, generally metallic in electrical character, and dominant in composite and coating applications. Lower cost per gram by an order of magnitude compared to SWCNTs.
• Double-walled carbon nanotubes (DWCNTs) — a special case of two concentric walls, combining some SWCNT electronic properties with improved chemical stability. Used in high-performance electronics research.

Key Properties of Carbon Nanotubes

• Tensile strength — theoretical ~100 GPa for individual SWCNTs; practical MWCNT bundles achieve 10–60 GPa, still far exceeding high-strength steel (~1.5 GPa)
• Young's modulus — ~1 TPa, comparable to graphene
• Electrical conductivity — metallic SWCNTs carry current densities up to 10⁹ A/cm², roughly 1,000× copper
• Thermal conductivity — axial thermal conductivity ~3,500 W/m·K for individual SWCNTs at room temperature
• Aspect ratio — length-to-diameter ratios of 1,000–10,000+ enable percolation network formation at low loading fractions in composites

Functionalization: Making CNTs Processable

Pristine CNTs are hydrophobic and prone to bundling through van der Waals forces, making dispersion in most matrices challenging. Functionalization modifies the nanotube surface to address this:

• Covalent functionalization — acid treatment introduces carboxylic and hydroxyl groups, enabling covalent coupling to polymers. Improves wettability and matrix bonding but introduces defects that reduce electrical and mechanical properties.
• Non-covalent functionalization — wrapping CNTs with surfactants, DNA, or pyrene derivatives preserves the nanotube lattice while enabling dispersion. Preferred for electronic applications.
• Amino functionalization (NH₂-CNTs) — popular for biomedical and polymer composite applications; amine groups react readily with epoxy resins and crosslinkers.
• Carboxyl functionalization (COOH-CNTs) — among the most common; good general-purpose dispersibility modifier.

Commercial Applications

Structural Composites

Adding 0.1–5 wt% MWCNTs to epoxy matrices improves tensile strength, interlaminar shear strength, and fatigue life. Aerospace, wind turbine blades, and high-performance sporting goods (tennis rackets, cycling frames, hockey sticks) are established markets. Boeing, Airbus, and Hexcel have each demonstrated or incorporated CNT-enhanced composite structures.

Electrical Conductivity Enhancement

CNT networks in polymers create conductive composites at lower loading fractions than carbon black, with less impact on mechanical properties and surface finish. Applications include electrostatic discharge (ESD) protection housings, EMI shielding, and antistatic packaging for electronics components. CNT-loaded injection-mouldable thermoplastics are available from multiple compounders.

Energy Storage

MWCNTs improve the rate capability of lithium-ion battery electrodes as conductive additives. CNT-based lithium-sulphur and sodium-ion battery architectures are active research areas promising higher energy density. SWCNT and DWCNT electrodes are used in high-performance electrochemical capacitors (supercapacitors).

Semiconductors and Interconnects

As copper interconnects approach their physical limits in advanced nodes (sub-3 nm), metallic CNT bundles are under active development as replacement interconnect materials at Intel, TSMC, and Samsung. The conductivity and electromigration resistance of CNTs offer a viable path forward. Semiconducting SWCNT arrays are the basis for CNT field-effect transistors (CNFETs) demonstrated at sub-10 nm gate lengths.

Biomedical Applications

Functionalised CNTs have been explored as drug delivery vehicles, gene transfection agents, and photothermal cancer therapy agents. CNT-based electrochemical biosensors achieve femtomolar detection limits for proteins and nucleic acids. Safety considerations around CNT biopersistence and inflammogenicity continue to be studied — always ensure you have current safety data sheets and follow institutional biosafety guidelines when working with CNTs in biological contexts.

What to Check When Sourcing CNTs

Quality variance in the CNT market is significant. Specifications to verify:

1. Purity (% carbon, % metal catalyst residues by TGA or ICP-OES)
2. Number of walls (confirmed by TEM or Raman spectroscopy)
3. Outer diameter range and distribution (TEM)
4. Length range and distribution
5. Specific surface area (BET)
6. Functional group type and density (for functionalised grades)
7. Electrical conductivity (for conductive applications)

NanoMani lists CNT products with supplier-provided specification data and verified buyer reviews, giving you the transparency needed to source with confidence.