Carbon nanotube

Carbon nanotubes (CNTs) are cylindrical nanostructures composed entirely of carbon atoms arranged in a graphitic lattice. With diameters in the nanometre scale and lengths that may extend to several micrometres or more, they represent a remarkable class of carbon allotropes noted for their exceptional mechanical, thermal and electrical properties. Their discovery and development have significantly influenced materials science, nanotechnology and applied physics.

Types and Fundamental Characteristics

Two broad categories of carbon nanotubes are recognised:

• Single-walled carbon nanotubes (SWCNTs): These consist of a single graphene sheet rolled seamlessly into a hollow cylinder. Their diameters range from about 0.5 to 2.0 nanometres, making them roughly 100,000 times thinner than a human hair. Depending on their atomic arrangement, SWCNTs may be metallic or semiconducting, a feature that has major implications for nanoelectronics.
• Multi-walled carbon nanotubes (MWCNTs): These comprise several concentric cylindrical layers of graphene, arranged in a tube-in-tube structure. Double-walled and triple-walled nanotubes are intermediate forms. MWCNTs typically exhibit greater robustness but less tunable electronic characteristics than SWCNTs.

Carbon nanotubes owe their extraordinary properties to the strength of the carbon–carbon bonds within the hexagonal lattice. Many CNTs exhibit exceptional tensile strength, high thermal conductivity and notable electrical conductivity, enabling their use in composite materials, conductive films and nanoelectronic devices. Their surfaces may also be chemically modified, enhancing their compatibility with polymers, biological molecules and metallic systems.

Historical Development

Although the modern scientific interest in carbon nanotubes began in the early 1990s, earlier observations predate this surge in activity. A detailed reassessment of the historical record has shown that hollow graphitic tubes were imaged decades before they gained widespread recognition.

• 1952: L. V. Radushkevich and V. M. Lukyanovich published electron micrographs of carbon tubes with diameters of approximately 50 nm. These early observations appeared in a Russian journal with limited international circulation.
• 1976: Morinobu Endo observed hollow graphitic tubules during studies of vapour-grown carbon fibres. This work is now recognised as an early identification of structures resembling both SWCNTs and MWCNTs.
• 1979: John Abrahamson presented evidence of nanotube-like carbon fibres produced by arc discharge during a conference at Pennsylvania State University.
• 1981: Soviet researchers described multilayer tubular crystals produced via thermocatalytic disproportionation of carbon monoxide, proposing models for their rolled graphene structures including armchair and chiral geometries.
• 1987: H. G. Tennent was granted a patent for producing cylindrical carbon fibrils with nanoscale diameters, closely related to modern MWCNTs.

The breakthrough that catalysed worldwide interest came in 1991, when Sumio Iijima reported multi-walled carbon nanotubes formed in arc-burned graphite residue. His publication drew global attention and inspired extensive research into carbon nanostructures.
Further advances occurred in 1993, when Iijima and Ichihashi at NEC, and independently Bethune and colleagues at IBM, demonstrated that transition metals such as iron or cobalt could catalyse the specific formation of SWCNTs. Subsequent work by Thess and co-workers refined catalytic vapour processes to greatly improve SWCNT yield and purity.
Archaeological findings in 2020 revealed that coatings containing carbon nanotubes had inadvertently formed on 2600-year-old pottery excavated at Keezhadi, India, suggesting that CNT-like structures may have occurred historically under specific pyrolytic conditions.

Structure of Single-Walled Nanotubes

The idealised structure of an infinitely long SWCNT is equivalent to a graphene lattice rolled into a cylinder. The arrangement of carbon atoms is determined by how the graphene sheet is “wrapped”, described mathematically by a chiral vector defined on the hexagonal lattice.
Three principal configurations are recognised:

• Armchair nanotubes: characterised by a wrapping that produces parallel carbon rows resembling the armrests of a chair. These structures are metallic.
• Zigzag nanotubes: formed when the wrapping direction follows alternating turns of 60 degrees. These can be metallic or semiconducting depending on their indices.
• Chiral nanotubes: general structures where neither armchair nor zigzag symmetry applies. These exhibit a helical arrangement and may have semiconducting or metallic properties.

The atomic positions on the nanotube surface can be constructed by conceptually slicing the tube along its axis, unrolling it into a strip aligned with an infinite graphene sheet and identifying the edges that would reconnect to form a cylinder. Constraints on the vectors that connect carbon atoms ensure that the nanotube’s circumference aligns with permissible lattice translations.

Properties and Applications

Carbon nanotubes possess a suite of remarkable properties:

• Mechanical: CNTs exhibit extremely high tensile strength and resilience due to the robust sp² carbon lattice. Their stiffness exceeds that of steel at a fraction of the weight.
• Electrical: Depending on their chirality, SWCNTs may behave as metals or semiconductors, enabling applications in nanoscale transistors, sensors and conductive networks.
• Thermal: CNTs show exceptional thermal conductivity along their lengths, making them valuable for heat dissipation.
• Chemical: Their surfaces can be modified through covalent or non-covalent functionalisation, allowing incorporation into polymers, biological systems and catalytic assemblies.

These features make CNTs suitable for use in:

• high-performance composite materials,
• nanoelectronic and optoelectronic devices,
• energy storage and conversion systems,
• nanomedicine and targeted delivery methods,
• reinforcement of carbon fibre technologies,
• advanced coatings and sensors.