Working at Japan’s National Institute for Materials Science, Yihua Gao and Yoshio Bando found that the height of a column of liquid gallium in a nanotube varies linearly with temperature, just as mercury does in conventional thermometers. This allows the temperature to be read in situ using a scanning electron microscope to measure the height of the gallium meniscus.

The researchers found that the gallium inside the nanotubes expands rapidly as the temperature rises – just as gallium does on a macroscopic scale – although its melting point varies greatly at different size scales. In contrast with gallium, the walls of the carbon nanotube expand very little as the temperature rises. On a microscopic scale, gallium is a liquid over a large range of temperatures, and this gives the nanothermometer a measuring range of 323 – 823 kelvin, compared with the range of 4 – 80 kelvin covered by existing nanoscale thermometers.

The nanothermometers were made using a radiofrequency furnace to react gallium oxide vapour with carbon monoxide vapour in a carbon crucible, under a flow of nitrogen gas, at a temperature of 1073 kelvin.

Carbon nanotubes are sheets of graphite molecules rolled into a cylinder and capped at both ends, formed by heating carbon. Along their length they are as strong as diamond.