Controlling heat flow is a major challenge for many technologies. In electronic and photonic devices, for example, heat dissipation can limit the performance and efficiency, as well as their potential for further miniaturisation. At the same time, two-dimensional (2D) materials, which are made of layers just a few atoms thick, have emerged as a promising platform in these fields. For example, 2D semiconductors are expected to be used in conduction channels of future transistors. However, their thermal behaviour remains difficult to predict and control.
Now, an international team of researchers led by ICN2, UAB, TU/e, and McGill has discovered a new regime of heat transport in ultrathin materials. The study shows that in 2D semiconductors, in particular molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂), heat can behave in a completely new way, known as hydro-thermoelastic transport, where thermal diffusion is highly impeded. These findings, published in Nature Physics, could have a significant impact on the development of new strategies for thermal management in devices.
A combination of unexpected phenomena
Under normal conditions, heat spreads gradually from hot regions to cold ones. However, in these ultrathin materials, more complex effects occur. As Dr Sebin Varghese, first author of the paper, remarks: “Our results challenge the conventional picture of diffusive heat transport and reveal a richer, more complex transport mechanism in ultrathin semiconductors.” One of the effects that occur is phonon hydrodynamics, whereby heat is carried collectively and behaves like a viscous fluid. At the same time, heating induces mechanical deformations in the material, which also affect how heat moves. Although these types of effects were already known, they had never been observed in this type of materials.
The interplay of these phenomena results in unexpected behaviour: heat propagates much more slowly than predicted, with the thermal diffusivity reduced by up to an order of magnitude. To reach these conclusions, the researchers used an advanced optothermal technique that enabled them to track heat flow in real time with nanometre resolution. Prof. F. Xavier Alvarez from the Department of Physics at the UAB, who led the theoretical part of the work, notes that “for the first time, we observe how mechanical stress can redirect — and even obstruct — the flow of heat in a material.”
Can heat flow “the opposite way”?
The experiments show that, in these ultrathin materials, heat tends to remain concentrated around the heated region for longer than expected. This happens because heating causes the material deformations that alter how heat moves through the material, even pushing the heat flow in unexpected directions.
As Prof. Klaas-Jan Tielrooij (ICN2 and TU/e), who led the study, explains: “What surprised us most is that heat can, under certain conditions, resist leaving the hot region, which is due to contributions to the heat flux that point from cold to hot regions, rather than the conventional flux that points from hot to cold regions. This opens up a completely new way to control heat flow intrinsically, without the need to modify the material’s structure.”
This discovery provides new fundamental insight into how heat is transported at the nanoscale and could pave the way for designing electronic, photonic, and thermal devices with new functionalities. The ability to control rather than simply dissipate heat could be pivotal for future technologies, from improving the thermal management of chips to making thermoelectric systems more efficient.
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