A material made by stacking 2D sheets of molybdenum disulfude is a strong thermal conductor in two dimensions but blocks heat transfer from layer to layer. The material conducts heat 900 times as well in-plane as it does perpendicularly, setting a record for directional heat control (Nature 2021, DOI: 10.1038/s41586-021-03867-8). This proportion is threefold higher than the previous record-holder, single crystalline graphite.
Materials engineered to carefully control the flow of heat could prevent computer chips from overheating, and keep electronic devices cool to the touch. But when using conventional materials, making heat move only in a particular direction is challenging, says Jiwoong Park, a materials chemist at the University of Chicago. Park and his collaborators, David Cahill at the University of Illinois and Paul Erhart of the Chalmers University of Technology, were curious about how well sheets of randomly stacked, atomically thin crystals would do.
Other researchers have tried to create such directional insulation before by stacking 2D materials. But these materials had slower heat flow in the plane than the new material, thanks to its high quality crystalline structure. Park and colleagues “made things pretty crystalline in the plane, and that helped them get really high in-plane thermal conductivity,” says David Johnson, a materials chemist at the University of Oregon.
To make the stacks of the new material, the researchers used methods Park’s team previously developed (Nature 2015, DOI: 10.1038/nature14417) to grow highly crystalline MoS2 on a silicon dioxide wafer. “After it’s grown, it’s not bonded to the surface, but hovering on it,” says Park. The researchers can peel off the material like a sticky note and stack layers of it on another surface. The team tested stacks of up to 22 sheets of MoS2.
In crystalline materials, heat is carried by the synchronized oscillation of atoms in the crystal lattice. Because each layer of MoS2 is a high-quality crystal, heat flows rapidly through it. However, the crystalline sheets are out of alignment with each other in the vertical direction. Park says this stacking creates “intentional chaos” preventing heat from transferring between the layers.
“This is a beautiful piece of basic science,” Johnson says. Park hopes it will be more than that. He says if the team can scale up production of these materials, they could be used to coat computer chips and consumer devices to keep them cool.
Eric Pop, a materials scientist at Stanford University who has developed stacked 2D thermal materials, says he wonders about the electrical conductivity of these materials. Reached over email, he said engineering both thermal and electrical properties is key to applications such as thermoelectric devices, which convert heat into electricity and vice versa, and for emerging computer memory technologies.