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Surface waves can help nanostructured devices keep their cool

Surface waves can help nanostructured devices keep their cool
A research team led by the Institute of Industrial Science, the University of Tokyo finds that hybrid surface waves called surface phonon-polaritons can conduct heat away from nanoscale material structures Credit: Institute of Industrial Science, the University of Tokyo

Due to the continuing progress in miniaturization of silicon microelectronic and photonic devices, the cooling of device structures is increasingly challenging. Conventional heat transport in bulk materials is dominated by acoustic phonons, which are quasiparticles that represent the material’s lattice vibrations, similar to the way that photons represent light waves. Unfortunately, this type of cooling is reaching its limits in these tiny structures.


However, surface effects become dominant as the materials in nanostructured devices become thinner, which means that surface waves may provide the thermal transport solution required. Surface phonon-polaritons (SPhPs) – hybrid waves composed of surface electromagnetic waves and optical phonons that propagate along the surfaces of dielectric membranes—have shown particular promise, and a team led by researchers from the Institute of Industrial Science, the University of Tokyo has now demonstrated and verified the thermal conductivity enhancements provided by these waves.

“We generated SPhPs on silicon nitride membranes with various thicknesses and measured the thermal conductivities of these membranes over wide temperature ranges,” says lead author of the study Yunhui Wu. “This allowed us to establish the specific contributions of the SPhPs to the improved thermal conductivity observed in the thinner membranes.”

The team observed that the thermal conductivity of membranes with thicknesses of 50 nm or less actually doubled when the temperature increased from 300 K to 800 K (approximately 27°C to 527°C). In contrast, the conductivity of a 200-nm-thick membrane decreased over the same temperature range because the acoustic phonons still dominated at that thickness.

“Measurements showed that the dielectric function of silicon nitride did not change greatly over the experimental temperature range, which meant that the observed thermal enhancements could be attributed to the action of the SPhPs,” explains the Institute of Industrial Science’s Masahiro Nomura, senior author of the study. “The SPhP propagation length along the membrane interface increases when the membrane thickness decreases, which allows SPhPs to conduct much more thermal energy than acoustic phonons when using these very thin membranes.”

The new cooling channel provided by the SPhPs can thus compensate for the reduced phonon thermal conductivity that occurs in nanostructured materials. SPhPs are thus expected to find applications in thermal management of silicon-based microelectronic and photonic devices.


Minimizing thermal conductivity of crystalline material with optimal nanostructure


More information:
Y. Wu et al, Enhanced thermal conduction by surface phonon-polaritons, Science Advances (2020). DOI: 10.1126/sciadv.abb4461
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University of Tokyo

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Surface waves can help nanostructured devices keep their cool (2020, October 13)
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Graphite sheets to help next-gen smartphones to keep their cool

Next-gen smartphones to keep their cool
Model for NGF growth with respect to the Ni surface topography. The variable number of graphene layers correlates with the orientation, size and boundaries of the Ni grains at the surface of the polycrystalline metal foil. Credit: KAUST; Xavier Pita

It can be a significant challenge to cool the powerful electronics packed inside the latest smartphones. KAUST researchers have developed a fast and efficient way to make a carbon material that could be ideally suited to dissipating heat in electronic devices. This versatile material could also have additional uses ranging from gas sensors to solar cells.


Many electronic devices use graphite films to draw away and dissipate the heat generated by their electronic components. Although graphite is a naturally occurring form of carbon, heat management of electronics is a demanding application and usually relies on use of high-quality micrometer-thick manufactured graphite films. “However, the method used to make these graphite films, using polymer as a source material, is complex and very energy intensive,” says Geetanjali Deokar, a postdoc in Pedro Costa’s lab, who led the work. The films are made in a multistep process that requires temperatures of up to 3200 degrees Celsius and which cannot produce films any thinner than a few micrometers.

Deokar, Costa and their colleagues have developed a quick, energy-efficient way to make graphite sheets that are approximately 100 nanometers thick. The team grew nanometer-thick graphite films (NGF) on nickel foils using a technique called chemical vapor deposition (CVD) in which the nickel catalytically converts hot methane gas into graphite on its surface. “We achieved NGFs with a CVD growth step of just five minutes at a reaction temperature of 900 degrees Celsius,” Deokar says.

Next-gen smartphones to keep their cool
Polymer-free wet chemical transfer process for NGFs grown on Ni foil. Credit: KAUST; Xavier Pita

The NGFs, which could be grown in sheets of up to 55 square centimeters, grew on both sides of the foil. It could be extracted and transferred to other surfaces without the need of a polymer supporting layer, which is a common requirement when handling single-layer graphene films.

Working with electron microscopy specialist Alessandro Genovese, the team captured cross-sectional transmission electron microscopy (TEM) images of the NGF on nickel. “Observing the interface of the graphite films to the nickel foil was an unprecedented achievement that will shed additional light on the growth mechanisms of these films,” Costa says.

In terms of thickness, NGF sits between commercially available micrometer-thick graphite films and single-layer graphene. “NGFs complement graphene and industrial graphite sheets, adding to the toolbox of layered carbon films,” Costa says. Due to its flexibility, for example, NGF could lend itself to heat management in flexible phones now starting to appear on the market. “NGF integration would be cheaper and more robust than what could be obtained with a graphene film,” he adds.

However, NGFs could find many applications in addition to heat dissipation. One intriguing feature, highlighted in the TEM images, was that some sections of the NGF were just a few carbon sheets thick. “Remarkably,