Heat stack: Infra-red tracked in real-time

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  • Published: Dec 1, 2017
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Heat stack: Infra-red tracked in real-time

Stacked

Schematic representation of the highly efficient out-of-plane heat transfer from graphene hot electrons (yellow glow), created by optical excitation (red beam), to hyperbolic phonon-polaritons in hBN (wave lines). Credit: ICFO

Researchers at the Institute of Photonic Sciences in Barcelona, Spain, have tracked infrared energy in real-time as it flows through van der Waals stacks consisting of graphene encapsulated by the dielectric two-dimensional material hexagonal boron nitride (hBN).

ICFO scientists point out that heat flow on the nanoscale plays an important role in modern electronic and optoelectronic applications. For instance, it is critical in thermal management, photodetection, thermoelectric, and data communication. They point out that two-dimensional layered materials, such as the well-known carbon allotrope graphene and analogous boron nitride material hBN are emerging as useful substances for developing these applications. Moreover, even more promising are the so-called van der Waals heterostructures, which are hybrid substances comprising different layers of stacked two-dimensional materials. Such stacks stick together through those non-covalent van der Waals forces and can have very different physical properties to their isolated components. In addition, the real interest lies at the interfaces between the layers within the stack which can be described as "ultraclean and atomically sharp."

Heat transfer

ICFO scientists working on the European Graphene Flagship recently reported their success in tracking heat transport in such a stacked material and reported details in the journal Nature Nanotechnology. ICFO's Klaas-Jan Tielrooij, Niels Hesp, Mark Lundeberg, Mathieu Massicotte, Peter Schmidt, and Diana Davydovskaya, led by Frank Koppens have worked with colleagues in The Netherlands, Italy, Germany, and the UK. They have spotted an intriguing effect. It seems that rather than being constrained to the graphene sheet as one might expect, heat flows to the surrounding hBN sheets. This out-of-plane heat transfer process occurs on an ultrafast timescale of picoseconds and therefore predominates over competing, in-plane, heat-transfer processes, the team reports.

Heated hyperbole

The evidence suggests that this heat-transfer process occurs through hot graphene electrons, which are generated in the experiments, by incident light. These electrons couple with hyperbolic phonon-polaritons in the hBN sheets and it is these that then propagate within the hBN in a manner to visible light propagating through an optical fibre, but instead the propagation is at infrared wavelengths and on the nanoscale. The team suggests that this exotic hyperbolic mode of energy transfer is a very efficient way of carrying heat away.

It is possible that such a discovery might have far-reaching implications for various applications in which hBN-encapsulated graphene is used for heat flow control for instance.

Related Links

Nature Nanotech 2017, online: "Out-of-plane heat transfer in van der Waals stacks through electron-hyperbolic phonon coupling"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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