Tiny tool measures heat at the nanoscale

February 27, 2014 by Anne Ju in Physics / General Physics
Tiny tool measures heat at the nanoscale
Design of the spectrometer to probe phonon transmission through silicon nanosheet arrays. Credit: Robinson group


Cornell researchers have developed a new way to precisely measure the extremely subtle movement of in nanostructures. Recently published online in Nano Letters and highlighted in Physics Today, the study features the researchers' phonon spectrometer, whose measurements are 10 times sharper than standard methods. This boosted sensitivity has uncovered never-before-seen effects of phonon transport.



"If waters are calm you see a reflection, but in choppy waters you see diffuse scattering," said Jared Hertzberg, the paper's first author, a former postdoctoral associate. "This diffuse scattering slows down the transmission of phonons. This decrease in phonon transport becomes particularly important in nanoscale materials where surfaces play a larger role in the ."


"The fundamental science of heat flow is not as well understood in nanostructures as it is in bulk materials," Robinson said. "If we can precisely understand how this process works, then we can begin to engineer heat flow at the nanoscale, which can lead to more efficient alternate energy applications, such as thermoelectrics, or advanced phononic heat-logic circuits. We've just scratched the surface, so to speak, of how heat behaves at the nanoscale. There's so much more to learn, and so much more that can be done with these phonons now that we know how to spectroscopically measure them."

The researchers fabricated silicon nanosheets and measured phonon transmission rates with their spectrometer, and gauged the nanosheets' surface roughness using atomic force microscopy. By comparing transmission rates with those predicted by theory, they could assess the validity of a 50-year-old theory called the Casimir-Ziman theory, which determines the probability of phonon scattering based on and the phonon's wavelength. While a perfectly smooth surface will reflect phonons perfectly, and a perfectly rough surface randomly scatters phonons in all directions, real surfaces fall somewhere in between.


Since diffusive scattering effectively lowers phonon transmission, high scattering rates have implications for thermal conductivity in nanostructures: The actual thermal conductance will be much lower than predicted using the standard Casimir-Ziman theory.

More information: Nano Lett., 2014, 14 (2), pp 403–415 DOI: 10.1021/nl402701a

Provided by Cornell University

"Tiny tool measures heat at the nanoscale" February 27, 2014 http://phys.org/news/2014-02-tiny-tool-nanoscale.html