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Researchers Find Way To Tune 2D Semiconductor Thermal Conductivity

Adding disorder to MoS2 alters thermal anisotropy ratio in a predictable way

Above: Introducing lithium ions between layers of MoS2 can tune its thermal conductivity

US researchers have found an unexpected way to control the thermal conductivity of 2D materials, which will allow electronics designers to dissipate heat in electronic devices that use these materials.

2D materials have a layered structure, with each layer having strong bonds horizontally, or "˜in plane' and weak bonds between the layers, or "˜out of plane'. These materials have unique electronic and chemical properties, and hold promise for use in creating flexible, thin, lightweight electronic devices.

For many of these potential applications, it's important to be able to dissipate heat efficiently. And this can be tricky. In 2D materials, heat is conducted differently in plane than it is out of plane.

For example, in one class of 2D materials, called transition metal dichalcogenides (TMDs), heat is conducted at 100 watts per meter per Kelvin (W/mK) in plane, but at only 2 W/mK out of plane. That gives it what is called a "˜thermal anisotropy ratio' of about 50. (Anisotropy means different properties in different directions)

To better understand the thermal conduction properties of 2D materials, a team of researchers from North Carolina State University, the University of Illinois at Urbana-Champaign (UI) and the Toyota Research Institute of North America (TRINA) began experimenting with the TMD MoS2.

The researchers found that  introducing disorder to the MoS2, by introducing lithium ions between the layers, they could significantly alter the thermal anisotropy ratio. The presence of the lithium ions does two things simultaneously: it puts the layers of the 2D material out of alignment with each other, and it forces the MoS2 to rearrange the structure of its component atoms.

When the ratio of lithium ions to MoS2 reached 0.34, the in-plane thermal conductivity was 45 W/mK, and the out-of-plane thermal conductivity dropped to 0.4 W/mK - increasing the material's thermal anisotropy ratio from 50 to more than 100. In other words, heat became more than twice as likely to travel in plane - along the layer, rather than between the layers.

And that was as good as it got. Adding fewer lithium ions made the thermal anisotropy ratio lower. Adding more ions also made it lower. But in both cases, the ratio was affected in a predictable way, meaning that the researchers could tune the material's thermal conductivity and thermal anisotropy ratio.

"This finding was very counter-intuitive," says Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State and co-corresponding author of a paper describing the work. "The conventional wisdom has been that introducing disorder to any material would decrease the thermal anisotropy ratio.

"But based on our observations, we feel that this approach to controlling thermal conductivity would apply not only to other TMDs, but to 2D materials more broadly," Liu says.

"We set out to advance our fundamental understanding of 2D materials, and we have," Liu adds. "But we also learned something that is likely to be of practical use for the development of technologies that make use of 2D materials." 'Tuning Thermal Conductivity in Molybdenum Disulfide by Electrochemical Intercalation' is published in the journal Nature Communications. Co-corresponding authors of the paper are Gaohua Zhu of TRINA and David Cahill of UI. Co-authors are Ruigang Zhang and Debasish Banerjee of TRINA, and Qiye Zheng and Dongyao Li of UI. The work was supported by TRINA.



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