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How Do “2D” Materials Expand?

Have you ever sat around and wondered how 2D materials expand? Me neither, but if you have, then a team of researchers may have solved the answer to the question. First, 2D materials consist of just a single layer of atoms that can be densely packed together, unlike conventional materials. The applications that can benefit from 2D materials are things like transistors, solar cells, LEDs, and other devices that can run faster and perform better.

The big problem with these next-generation electronics is that they generate a lot of heat. Conventional electronics usually only reach around 80 degrees Celsius when in use, while these 2D devices will run almost twice as hot since they are packed densely in a small area. The hotter the device gets, the greater the chance of damage or failure.

One of the other issues with 2D materials is that scientists don’t have a good understanding of how 2D materials expand when temperatures go up. Basically, the materials are so thin and optically transparent that scientists have been unable to measure the thermal expansion coefficient (TEC) with standard approaches. (The thermal expansion coefficient is simply the tendency for materials to expand when temperatures are increased.)

“When people measure the thermal expansion coefficient for some bulk material, they use a scientific ruler or a microscope because, with bulk material, you have the sensitivity to measure them. The challenge with a 2D material is that we cannot really see them, so we need to turn to another type of ruler to measure the TEC,” says Yang Zhong, a graduate student in mechanical engineering.

An academic paper that illustrates such a “ruler” was co-authored by Zhong. They used laser lights to track the vibrations of the material’s atoms rather than directly detecting how the material expands. They accurately extracted the thermal expansion coefficient by measuring the same 2D material on three separate surfaces.

The results of the new study demonstrate the method’s high degree of accuracy, matching the theoretical calculations exactly. The method confirms that the TECs of 2D materials fall into a smaller range than previously believed. This knowledge might be useful to engineers creating electronics for the future.

“By confirming this narrower physical range, we give engineers a lot of material flexibility for choosing the bottom substrate when they are designing a device. They don’t need to devise a new bottom substrate just to mitigate thermal stress. We believe this has very important implications for the electronic device and packaging community,” says co-lead author and former mechanical engineering graduate student Lenan Zhang SM ’18, Ph.D. ’22, who is now a research scientist.

Co-authors of the study include senior author Evelyn N. Wang, the Ford Professor of Engineering and head of the MIT Department of Mechanical Engineering, as well as others from the Department of Electrical Engineering and Computer Science at MIT and the Department of Mechanical and Energy Engineering at Southern University of Science and Technology in Shenzhen, China. The research was published in Science Advances.

Standard techniques aren’t sensitive enough to directly detect the expansion of 2D materials because they are so small, only a few microns in size. Also, because the materials are so thin, they need to be attached to a surface like copper or silicon. Thermal stress occurs from the expansion of the 2D material and its substrate, which differ in TEC when temperatures rise.

For example, when a device is heated, the 2D material that is attached to a surface with a higher TEC will expand more than the 2D material, stretching it. As a result, it is difficult to determine a 2D material’s true TEC because the surface impacts its expansion.

These issues were solved by the researchers by concentrating on the atoms that make up the 2D substance. A substance expands when it is heated because the vibrational frequency of its atoms decreases and therefore moves farther apart. They use a method known as micro-Raman spectroscopy, which entails striking the material with a laser, to measure these vibrations. The light from the laser is scattered by the vibrating atoms, and this effect can be used to determine the vibrational frequency of the atoms.

But, the atoms of the 2D material change in vibration as the surface stretches or contracts. To focus on the material’s specific qualities, the researchers attempted to separate this surface influence. On three different substrates — copper, which has a high TEC; fused silica, which has a low TEC; and a silicon substrate with several microscopic holes — they measured the vibrational frequency of the same 2D material to achieve this. They can measure these tiny areas of freestanding material because the 2D material floats above the perforations on the latter surface.

The samples were then heated, micro-Raman spectroscopy was conducted, and each surface was placed on a thermal stage to allow for exact temperature control.

“By performing Raman measurements on the three samples, we can extract something called the temperature coefficient that is substrate-dependent. Using these three different substrates, and knowing the TECs of the fused silica and the copper, we can extract the intrinsic TEC of the 2D material,” Zhong explains.

They repeated this test on a bunch of 2D materials and found that every one of them matched the calculations. But the researchers discovered something unexpected: 2D materials were arranged in a hierarchy according to the components that make them up. For instance, the TEC of a 2D material containing molybdenum is always higher than that of a material containing tungsten.

As they probed deeper, the scientists found that electronegativity, a basic atomic characteristic, is what causes this hierarchy. When atoms bond, they have a tendency to pull or extract electrons, which is known as electronegativity. It is listed on the periodic table for each element.

They discovered that a 2D material’s thermal expansion coefficient will be lower the more the electronegativities of its constituent elements differ from one another. Instead of depending on intricate calculations that are generally performed by a supercomputer, Zhong says this approach could be used by an engineer to easily estimate the TEC for any 2D material.

“An engineer can just search the periodic table, get the electronegativities of the corresponding materials, plug them into our correlation equation and within a minute they can have a reasonably good estimation of the TEC. This is very promising for rapid materials selection for engineering applications,” Zhang says.

The researchers hope to use their approach with a lot more 2D materials in the future, maybe creating a database of TECs. They also intend to detect the TECs of heterogeneous materials, which combine various 2D materials, using micro-Raman spectroscopy. Also, they want to understand the fundamental factors that cause 2D materials’ thermal expansion to differ from bulk materials.

This work is funded, in part, by the Centers for Mechanical Engineering Research and Education at MIT and Southern University of Science and Technology, the Materials Research Science and Engineering Centers, the U.S. National Science Foundation, and the U.S. Army Research Office.

This breakthrough in understanding may lead to future developments in new 2D technology, as it seems they are on the right track to understanding how 2D materials expand.

Source and photo: MIT

 
 
 
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Written By

Tim holds an electronics engineering degree and is working toward a second degree in IT/web development. He enjoys renewable energy topics and has a passion for the environment. He is a part-time writer and web developer, full time husband and father.

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