Carbon Nanotube
A graphene sheet rolled into a seamless cylinder one atom thick — stronger than steel at a fraction of the weight, and electrically a near-perfect wire.
Carbon nanotubes carry mechanical loads and electrical current in extreme conditions. As a structural reinforcement they stiffen polymer composites at very low loading fractions. As conductors, metallic single-walled tubes carry current densities a thousand times higher than copper before failing.
A single-walled CNT (SWCNT) is a graphene sheet — a one-atom-thick lattice of carbon hexagons — rolled into a seamless cylinder 0.7–2 nm in diameter. The direction of rolling (the 'chirality', described by two integers called the chiral indices) determines whether the tube is metallic or semiconducting. In a metallic tube, electrons travel as ballistic carriers: the tube's one-dimensional band structure lets them pass from end to end with almost no collisions up to mean free paths of micrometres at room temperature. Multi-walled CNTs (MWCNTs) nest multiple coaxial tubes and show the same mechanical extremes, but electronic conduction is dominated by the outermost shell.
In plain terms
Imagine taking a sheet of chicken wire and rolling it into a tube the width of a single atom. That tube is so perfectly made, with every atom exactly in place, that it's stronger than almost anything you can squeeze, and electricity races through it without bumping into anything.
Also called: CNT, SWCNT, MWCNT, Buckytube.
Why it matters
- Theoretical tensile strength for a defect-free SWCNT exceeds 100 GPa — roughly 50 times stronger than structural steel at one-sixth the density.
- Ballistic electron transport means near-zero resistive heating at the nanoscale, relevant to nanoscale interconnects.
- Semiconducting CNTs have a diameter-tunable bandgap, making them candidates for sub-5 nm transistor channels.
- As a filler in composites, sub-1 wt% loading measurably increases stiffness and electrical conductivity simultaneously.
Where it lives
- Carbon nanotube transistors (IBM, 2017 — 1.8 nm gate-length CNT FET)
- CNT-reinforced carbon fibre composite structural panels
- Field-emission electron sources in X-ray tubes and flat-panel displays
- Conductive CNT inks for printed electronics
Key numbers
- Wall thickness (SWCNT): ~0.34 nm (One carbon atom thick; the interlayer spacing of graphite)
- Diameter (SWCNT): 0.7–2 nm (Typical arc-discharge or CVD product)
- Tensile strength (theoretical, defect-free SWCNT): >100 GPa (Measured SWCNT: 25–66 GPa (Takakura et al., Nat. Commun. 2019); measured MWCNT outermost shell: 11–63 GPa (Yu et al., Science 2000))
- Young's modulus: ~1 TPa (Terapascal range confirmed by thermal-vibration TEM method, Treacy et al., Nature 1996)
- Ballistic mean free path: ~1 µm (Room temperature, metallic SWCNT; PRL 2007)
- Current density (metallic SWCNT): ~10⁹ A/cm² (Roughly 1,000× that of copper before electromigration failure)
Surprising facts
- A defect-free single-walled CNT has a theoretical tensile strength above 100 GPa — you could suspend a car from a CNT rope thinner than a human hair, if you could make one long enough without defects.
- Electrons in a metallic single-walled nanotube travel without scattering for micrometres at room temperature — a mean free path roughly 1,000 times longer than in bulk copper.
- The wall of a single-walled CNT is exactly one carbon atom thick — about 0.34 nm — yet the tube is seamless enough to hold a pressure difference of tens of atmospheres.
- The chiral angle of a CNT — essentially the direction the graphene was 'rolled' — determines whether it conducts like a metal or a semiconductor, even though both are made of identical carbon atoms.
A short history
Sumio Iijima at NEC's Tsukuba laboratory described multi-walled carbon nanotubes in November 1991 in Nature (354, 56–58), observing coaxial graphitic cylinders 2 to ~50 walls wide grown at the cathode of a carbon arc-discharge apparatus. Single-walled nanotubes were reported simultaneously by Iijima & Ichihashi and by Bethune et al. in 1993. The first direct mechanical measurements came from Treacy, Ebbesen, and Gibson in 1996 (Nature 381, 678–680), who used thermal vibration amplitudes in a TEM to obtain Young's moduli in the terapascal range. In 2000 Yu, Lourie, Dyer et al. (Science 287, 637) loaded individual MWCNTs in tension inside a scanning electron microscope, measuring tensile strengths of 11–63 GPa in the outermost shell. Theoretical predictions for defect-free SWCNTs consistently exceed 100 GPa; the first structure-resolved measurements by Takakura et al. in 2019 (Nature Communications) measured 25–66 GPa and confirmed a chiral-structure dependence.
Sources
- Iijima, S. — Helical microtubules of graphitic carbon, Nature 354:56–58 (1991) (https://www.nature.com/articles/354056a0)
- Yu et al. — Strength and Breaking Mechanism of MWCNTs Under Tensile Load, Science 287:637 (2000) (https://www.science.org/doi/10.1126/science.287.5453.637)
- Treacy, Ebbesen & Gibson — Exceptionally high Young's modulus of individual CNTs, Nature 381:678 (1996) (https://www.nature.com/articles/381678a0)
- Takakura et al. — Strength of CNTs depends on their chemical structures, Nat. Commun. (2019) (https://www.nature.com/articles/s41467-019-10959-7)
- Zhong et al. — Scaling of resistance and electron mean free path of SWCNTs, PRL 98:186808 (2007) (https://link.aps.org/doi/10.1103/PhysRevLett.98.186808)