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New Way to Control Heat in 2D Materials

Slipping a few lithium ions between layers of 2D materials changes how they conduct heat

2 min read
Introducing lithium ions between layers of molybdenum sulfide can tune the thermal conductivity of the material.
Image: Jun Liu/North Carolina State University

Two-dimensional (2D) materials such as those made from transition metal dichalcogenides (TMDs) are  layered one on top of another to create devices that could potentially be used for electronics. In fact, how these materials are layered determines to a large extent the electronic properties of the final device.

One property that engineers have kept in mind when plotting the layering of TMDs and other 2D materials is how they dissipate heat. Researchers at North Carolina State University, the University of Illinois at Urbana-Champaign (UI) and the Toyota Research Institute of North America (TRINA) have discovered an almost counterintuitive phenomenon in the layering of 2D materials that should help them to dissipate heat when they are fabricated into electronic devices.

In research described in the journal Nature Communications, the researchers wanted to address the issue of the difference in how heat is conducted in the strong, horizontal “in plane” bonds of the layers versus the weak, “out of plane” bonds between the layers. What they discovered was that in TMDs, heat was conducted at 100 watts per meter per Kelvin (W/mK) in plane, but at only 2 W/mK out of plane. This gives the TMDs a “thermal anisotropy ratio” of about 50. Anisotropy is when a material has a physical property that is different in one direction versus another. For instance, wood is stronger along its grain rather than against it.

The researchers wanted to see if they could increase these 2D materials’ heat conduction ratio to an even higher number. That would almost guarantee that the heat would travel in plane—along the layer—rather than between the layers.

To accomplish this, they added a few lithium ions between layers of molybdenum disulfide (a TMD). Two key changes were observed: First, it put the two layers out of alignment; and, second, it forced the atoms of molybdenum disulfide to rearrange. Those molecular adjustments were sufficient to yield an in-plane thermal conductivity of 45 W/mK and an out-of-plane thermal conductivity of only 0.4 W/mK. In other words, the thermal anisotropy ratio rose from 50 to more than 100.

Curiously, the number of lithium ions they struck upon was just right; in iterations of the experiment when they added more or fewer lithium ions, the result was actually a dip in the material’s thermal anisotropy ratio. But this taught them that the change in heat conductivity was tunable.

“This finding was very counterintuitive,” said Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State and co-corresponding author of a paper, in press release. “The conventional wisdom has been that introducing disorder to any material would decrease the thermal anisotropy ratio.

Liu added:

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. We set out to advance our fundamental understanding of 2D materials, and we have. But we also learned something that is likely to be of practical use for the development of technologies that make use of 2D materials.

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3 Ways 3D Chip Tech Is Upending Computing

AMD, Graphcore, and Intel show why the industry’s leading edge is going vertical

8 min read
A stack of 3 images.  One of a chip, another is a group of chips and a single grey chip.
Intel; Graphcore; AMD

A crop of high-performance processors is showing that the new direction for continuing Moore’s Law is all about up. Each generation of processor needs to perform better than the last, and, at its most basic, that means integrating more logic onto the silicon. But there are two problems: One is that our ability to shrink transistors and the logic and memory blocks they make up is slowing down. The other is that chips have reached their size limits. Photolithography tools can pattern only an area of about 850 square millimeters, which is about the size of a top-of-the-line Nvidia GPU.

For a few years now, developers of systems-on-chips have begun to break up their ever-larger designs into smaller chiplets and link them together inside the same package to effectively increase the silicon area, among other advantages. In CPUs, these links have mostly been so-called 2.5D, where the chiplets are set beside each other and connected using short, dense interconnects. Momentum for this type of integration will likely only grow now that most of the major manufacturers have agreed on a 2.5D chiplet-to-chiplet communications standard.

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