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John O | January 2018

European scientists detect fast-flowing heat in layered material heterostructures

scientists from the graphene flagship, a european research consortium of more than 150 academic and industrial groups that launched in 2013, have detected and followed in real-time the out-of-plane heat transfer of graphene in van der waals heterostructures, according to a report on its website.


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). (icfo)


the research was led by a team from the institute of photonic sciences (icfo) in barcelona, spain.


“heterostructures consisting of different layered materials stacked one on top of the other open an even wider range of opportunities,” the article explained. “these stacks can consist of materials with different physical properties, while the interfaces between them are ultraclean and atomically sharp.”


graphene’s in-plane thermal conductivity has made it an interesting topic of research for thermal management experts and this new research on out-of-plane heat transfer can advance graphene’s use in electronic devices.


researchers studied heat transport in stacks composed of graphene surrounded by a dielectric layer of hexagonal boron nitride (hbn). the researchers saw that heat flowed from the graphene into the layer of hbn and does so at a scale of picoseconds (one millionth of a millionth of a second).


“this was found to rely on two interesting phenomena: hot electrons in graphene and hyperbolic phonons in hbn,” the article said. “graphene makes hot electrons by transferring incident light into electrical heat and it is these hot electrons that interact with the adjacent hbn layer and enable the out-of-plane heat transfer.”


it continued, “the process for this involves the coupling of the hot electrons to hyperbolic phonon-polaritons in the hbn sheets which then propagate within the hbn as light does in an optical fiber, but in this case for infrared wavelengths and at the nanometer scale. it turns out that these hyperbolic modes are very efficient at carrying heat away.”


according to researchers, the heat transfer was dependent on graphene carrier density, substrate temperature, hot electron temperature and hbn sheet thickness.


the research was recently published in nature nanotechnology. the abstract stated:


“van der waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities and superior electrical and optoelectronic properties. applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting require a thorough understanding of (nanoscale) heat flow.


“here, using time-resolved photocurrent measurements, we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons in the encapsulating layered material.


“this hyperbolic cooling is particularly efficient, giving picosecond cooling times for hexagonal bn, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. we study this heat transfer mechanism using distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters.


“these insights may lead to the ability to control heat flow in van der waals heterostructures.”

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