World’s smallest thermometer

A new method for measuring temperature could be crucial in preventing nanotech machines from overheating.

As electronic components become smaller and the currents that drive them become more concentrated, it becomes harder to dissipate heat generated during operation, raising the risk of system failure.

The new technique, which measures the temperature of nanoscale fragments of boron nitride at a scale of around half a micron (500 nanometres), opens the way to identifying precisely if and where heat is building up.

The approach, published in the journal Physics Review Letters, was pioneered at Oak Ridge National Laboratories in the US by a team led by Juan Carlos Idrobo, using a scanning transmission electron microscope (STEM).

The team detected tiny shifts in the energy of electrons in the STEM beam caused by the thermal vibrations of the sample, a technique called energy gain and loss spectroscopy. While most of the electrons in the beam passed straight through the sample unchanged, a tiny fraction of them slowed down, adding some vibrational energy. An even smaller fraction absorbed energy and sped up.

The relative number of the two occurrences depends on the temperature – the more heat there is in the sample, the more likely it is that an electron will get sped up. The team measured the energy shifts and found they gave a good readout from room temperature up to 1600 degrees Kelvin.

“With a mercury thermometer you need to know the thermal expansion coefficient, while for an infra-red image you are limited in spatial resolution to the wavelength of the infra-red light, which is above 700 nanometre,” Idrobo explains.

“But this is a direct measure of temperature – no calibration is needed.”

The challenge is that the signal produced by the electrons that gain or lose energy is up to 100,000 times weaker than that of the ones that don’t interact. To make matters worse, the energy shift is smaller than the spread of energies in most STEM beams.

Fortunately, the team was able to use a new STEM design, made by US microscope manufacturers Nion, which emits electrons in a much narrower spread, making it possible to pick out the shifted electron energies.

As well as a narrow energy spread, the Nion STEM features a spatially narrow beam, less than 1 nanometre in diameter, which allows the new technique to probe the definition of temperature – a concept that relies on an average taken across a number of atoms in different energy levels.

“To measure the temperature of an individual atom makes no sense,” Idrobo says.

The researchers next plan to explore the localisation of temperature by applying the measuring technique to a nanowire with a temperature gradient along it.

Building a probe that could be incorporated into a micro-electronic circuit board is not straightforward – the Nion STEM is definitely not itself nano in scale, standing more than 1.5 metres tall.

“Working out how to put this onto a sample would take a couple of years, it is PhD project by itself,” Idrobo adds.