A carbon nanotube is a tiny hollow tube of carbon atoms, linked in a pattern of hexagons like the sheets that make up graphite. A single-walled nanotube, whose wall is one atom thick, is typically about 1 nanometer across. A nanometer is a billionth of a meter. Nanotubes are far longer than they are wide, with lengths measured in micrometers, or millionths of a meter.

Size

The explorer's 1 nanometer is the diameter of a typical single-walled nanotube, measured across the tube. Widths vary from tube to tube. One study of 34 single-walled nanotubes measured diameters between 0.7 and 1.2 nanometers.

Multi-walled nanotubes are several tubes nested one inside another, so they are wider. The ones Sumio Iijima described in 1991 had from 2 to about 50 walls, and were a few to a few tens of nanometers across.

Lengths vary too. The study of 34 tubes measured only tubes longer than 3 micrometers, so each was at least 2,500 times as long as it was wide.

Size comparisons

• About 80,000–100,000 single-walled nanotubes side by side would span a human hair, which is 0.08–0.1 millimeters wide.
• A buckyball, a closed ball of 60 carbon atoms, is about 1 nanometer across, about as wide as a typical single-walled nanotube.
• If a nanotube 1 nanometer wide and 3 micrometers long were enlarged until it was as wide as a human hair, 0.1 millimeters, it would be about 30 centimeters (12 inches) long, the length of a ruler.

How nanotubes were found

Iijima reported carbon nanotubes in the journal Nature on November 7, 1991. He made them with an electric arc, a method like the one used to make buckyballs. Needle-like tubes grew on the arc's negative electrode. Under an electron microscope, each needle turned out to be a set of 2 to about 50 tubes of carbon sheets, nested one inside another.

The hexagons of carbon atoms in each tube wind around it in a spiral. Iijima noted that making these tubes suggested carbon structures could be built at scales much larger than buckyballs.

In 1993, two teams reported single-walled nanotubes. Iijima and Toshinari Ichihashi found tubes about 1 nanometer across. D. S. Bethune and colleagues, who added cobalt to the carbon, found tubes about 1.2 nanometers across.

Watching a single nanotube

A single nanotube is far too thin to see, but some kinds give off a near-infrared glow. In a 2009 study, researchers used that glow to film single nanotubes in water with a fluorescence microscope. The tubes bent back and forth as the liquid around them jostled them at random, a motion called Brownian motion.

From that bending, the researchers found that a nanotube's stiffness grows with the cube of its diameter. A tube twice as wide is eight times as hard to bend. Their great length for their width, their strength and their electrical properties make nanotubes well suited to uses in engineering materials and in medicine.

Sources

• Diameter-dependent bending dynamics of single-walled carbon nanotubes in liquids. Fakhri et al., Proceedings of the National Academy of Sciences, 2009.
• Helical microtubules of graphitic carbon. Sumio Iijima, Nature, 1991.
• Single-shell carbon nanotubes of 1-nm diameter. Iijima and Ichihashi, Nature, 1993.
• Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls. Bethune et al., Nature, 1993.
• Thermally Activated Sliding of C60 on Gold. Pierno et al., ACS Omega, 2025.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.