Carbon nanotubes (CNTs) are cylindrical molecules made of carbon atoms arranged in a hexagonal lattice pattern — essentially a sheet of graphene rolled into a seamless tube. Discovered in 1991 by Sumio Iijima, these remarkable structures are among the strongest and most conductive materials ever measured, with properties that could transform electronics, materials science, energy storage, and medicine.
Structure and Types
Carbon nanotubes come in two main varieties. Single-walled carbon nanotubes (SWCNTs) consist of a single layer of carbon atoms rolled into a tube, typically 0.4 to 2 nanometres in diameter. Multi-walled carbon nanotubes (MWCNTs) contain multiple concentric tubes nested inside each other, with diameters ranging from 2 to 100 nanometres. The way the graphene sheet is rolled — described by a mathematical quantity called the chiral vector — determines whether a nanotube behaves as a metal or a semiconductor.
This tunability is extraordinary: by controlling the rolling angle and diameter, scientists can create materials that are either excellent electrical conductors or precise semiconductors, all from pure carbon. No other material offers such versatile electronic properties based purely on its geometry.
Extraordinary Properties
Carbon nanotubes possess a remarkable combination of physical properties. Their tensile strength exceeds 100 gigapascals — roughly 100 times stronger than steel at one-sixth the weight. They conduct electricity better than copper and heat better than diamond. Individual nanotubes can carry current densities a thousand times higher than copper wires before failing.
These properties arise from the strong covalent bonds between carbon atoms in the hexagonal lattice, combined with the seamless cylindrical geometry that eliminates edge effects and defects. The result is a material that approaches the theoretical limits of strength and conductivity for carbon-based structures.
Applications
In electronics, carbon nanotubes are being developed as replacements for silicon in transistors as conventional chip scaling approaches its physical limits. IBM, MIT, and other research groups have demonstrated nanotube-based transistors that outperform their silicon equivalents at nanometre scales. Carbon nanotube field-effect transistors could enable continued improvements in computing performance beyond the limits of Moore’s Law.
In materials science, small quantities of CNTs added to polymers, metals, or ceramics dramatically enhance strength, electrical conductivity, and thermal properties. Carbon nanotube-reinforced composites are already used in aerospace structures, sporting goods, and automotive components. Longer-term, carbon nanotube fibres — threads spun from aligned nanotubes — could enable structures previously confined to science fiction, including space elevators and ultra-lightweight aircraft.
For energy applications, carbon nanotubes improve the performance of lithium-ion batteries, supercapacitors, and fuel cells by providing high-surface-area electrodes with excellent conductivity. In solar cells, nanotube-based transparent conductors could replace expensive indium tin oxide.
In medicine, functionalised carbon nanotubes can deliver drugs directly to cancer cells, serve as scaffolds for tissue regeneration, and act as biosensors for detecting disease markers at extremely low concentrations.
Challenges and Future Outlook
Despite decades of research, large-scale manufacturing of carbon nanotubes with controlled properties remains challenging. Current production methods yield mixtures of metallic and semiconducting tubes in various diameters, and separating them is difficult and expensive. Health concerns about nanotube inhalation — some studies have compared their shape to asbestos fibres — require careful workplace safety protocols.
However, manufacturing techniques continue to improve. Recent advances in catalytic chemical vapour deposition enable growth of nanotubes with specific chiralities, and post-synthesis sorting methods are becoming more efficient. As production costs decrease and quality control improves, carbon nanotubes are steadily transitioning from laboratory curiosity to industrial material.
