What Are Carbon Nanotubes?

Carbon nanotubes (CNTs) are cylindrical nanostructures formed entirely from carbon atoms arranged in a hexagonal lattice, rolled into a seamless tube with diameters typically between 0.4 and 40 nanometers and lengths that can extend to several centimeters. They were first characterized systematically by Sumio Iijima in 1991 using high-resolution transmission electron microscopy. CNTs exist in two principal forms: single-walled nanotubes (SWCNTs), consisting of a single rolled graphene sheet, and multi-walled nanotubes (MWCNTs), which contain multiple concentric cylindrical shells separated by approximately 0.34 nanometers.

The electrical, mechanical, and thermal properties of CNTs are determined largely by the way the graphene lattice is rolled, described mathematically by a chiral vector. Depending on this chirality, a SWCNT can behave as a metallic conductor or as a semiconductor, which makes the same material family relevant to both interconnects and transistor channels. The field draws on condensed matter physics, materials chemistry, and electrical engineering.

Structural Properties and Chirality

The chiral vector (n, m) specifies the orientation of the carbon hexagons relative to the tube axis and controls whether a nanotube is metallic or semiconducting. When n minus m is divisible by three, the nanotube is metallic; otherwise it is semiconducting with a bandgap inversely proportional to the tube diameter. Metallic SWCNTs can carry current densities exceeding 10^9 A/cm^2, far above the limits of copper interconnects. MWCNTs contribute additional mechanical stiffness through the van der Waals coupling between shells, producing Young's moduli on the order of one terapascal.

Carbon Nanotube Field-Effect Transistors

Carbon nanotube field-effect transistors (CNTFETs) use semiconducting SWCNTs as the channel material between source and drain contacts. Carrier transport in a short-channel CNTFET approaches the ballistic limit, meaning electrons traverse the channel with minimal scattering. This produces high carrier mobility and low power dissipation relative to silicon transistors of similar gate length. The Proceedings of the IEEE review on carbon nanotube electronics by Avouris and colleagues established the foundational framework for understanding CNTFET operation. A key challenge in CNTFET fabrication is controlled placement and chirality sorting, since metallic tubes in an array short-circuit the gate. Research from Stanford's Pop Lab and others has developed compact models that guide circuit design at sub-10 nm gate lengths.

Synthesis and Fabrication Challenges

CNTs are synthesized primarily by arc discharge, laser ablation, or chemical vapor deposition (CVD). CVD is the most scalable method and allows site-directed growth on patterned substrates, but it produces a mixture of chiralities. Post-synthesis separation techniques, including density-gradient ultracentrifugation and gel chromatography, can enrich samples toward semiconducting tubes, though achieving the purity required for large-scale integrated circuits remains an active research area. MWCNTs are generally easier to produce in bulk but are less suited to transistor channels because the inner shells contribute leakage paths.

Applications

Carbon nanotubes have applications in a wide range of fields, including:

• Transistors and logic circuits in post-silicon nanoelectronics
• High-current interconnects in advanced integrated circuit packaging
• Structural reinforcement in composite materials for aerospace and sporting goods
• Chemical and biological sensors exploiting surface conductance changes
• Field-emission cathodes in flat-panel displays and electron sources
• Electrode materials in supercapacitors and lithium-ion batteries

Research on high-performance CNT electronic devices continues to address the remaining barriers between laboratory demonstrations and manufacturable products.