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Scientists measure radiative heat exchange at the nanoscale


In the near future thermal radiation might assist nano-lithography for computer technology. Photovoltaics could also take advantage of easier thermal transfer at nanoscale. Scientists of CEA, CNRS and ESRF have recently followed up and measured the law that governs heat transfers between two surfaces kept at very small distances, confirming previous unverified theoretical speculations. They publish their results in the September issue of Nature Photonics.

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The team compared the theory with the experimental results they got on the thermal heat exchange between surfaces kept at different temperature and distant only in few tens of nanometres. The experiments were conducted in the Surface Science Laboratory of the ESRF.

The results showed a considerable distance from the classical Stefan-Boltzmann prediction of the invariance of thermal flow over the change of distance between the two surfaces.

A complete theory on the thermal exchange at very small distances had been developed by Polder and van Hove later, in 1971, but despite the numerous attempts it had never been verified. According to this theory, the growth of thermal flux at nanoscale is linked to the increasing role in this regime of the so-called surface evanescent waves. The results of the team are in agreement with this theory, showing an effective increase of the radiation transfer as the two surfaces get closer and closer.

The scientists measured the transfer through a gap going from 2.5 microns down to 30 nanometres between a heated, flat surface and a micrometre-sized sphere that was attached to the cantilever of an atomic force microscope.

The next step for the team is to see how the heat transfer between two surfaces at the nanoscale can be modulated by selected activation of electronic properties of the two surfaces.

Rousseau, E. et al, Nature Photonics, 3, 514-517 (2009).

Top image: The experimental set-up used by the team to make their highly sensitive measurements of heat flow. Credits: Nature Photonics.