Consisting of only a single layer of atoms, two-dimensional materials can be packed more tightly than conventional materials, so they can be used to make transistors, solar cells, LEDs and other devices that work faster and perform better.
One problem holding these next-generation electronics back is the heat they generate when in use. Conventional electronics typically reach about 80 degrees Celsius, but the materials in 2D devices are so tightly packed in such a small area that the devices can get twice as hot. This one temperature increase may damage the device.
This problem is exacerbated by the fact that scientists don’t have a good understanding of how 2D materials expand as the temperature rises. Because the materials are so thin and optically transparent, their coefficient of thermal expansion (TEC)—the material’s tendency to expand as temperature rises—is nearly impossible to measure with standard approaches.
“If people measure the coefficient of thermal expansion for some bulk, 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 can’t really see them, so we have to turn to a different type of ruler to measure the TEC,” said Yang Zhong, a graduate student in mechanical engineering.
Zhong is co-lead author of a research paper demonstrating such a “ruler”. Instead of directly measuring how the material expands, they use laser light to track vibrations of the atoms that make up the material. By taking measurements of one 2D material on three different surfaces or substrates, they can accurately extract the coefficient of thermal expansion.
The new study shows that this method is very accurate and produces results consistent with theoretical calculations. The approach confirms that the TECs of 2D materials fall into a much narrower range than previously thought. This information can help engineers design next generation electronics.
“By confirming this narrower physical range, we give engineers a lot of material flexibility when choosing the soil substrate when they design a device. They don’t have to invent a new soil substrate to reduce thermal stress. We believe this has very important implications for the electronic device and packaging community,” said co-lead author and former mechanical engineering graduate student Lenan Zhang SM ’18, Ph.D. ’22, who is now a research scientist.
Co-authors 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 from Southern University of Science and Technology in Shenzhen, China. The research has been published in Scientific progress.
Because 2D materials are so small, perhaps just a few microns in size, standard tools are not sensitive enough to measure their expansion directly. In addition, the materials are so thin that they must be bonded to a substrate such as silicon or copper. If the 2D material and substrate have different TECs, they will expand differently as the temperature rises, causing thermal stress.
For example, if a 2D material is bonded to a substrate with a higher TEC, the substrate will expand more than the 2D material when heated, causing it to stretch. This makes it difficult to measure the actual TEC of a 2D material, as the substrate influences the expansion.
The researchers overcame these problems by focusing on the atoms that make up the 2D material. When a material is heated, the atoms vibrate at a lower frequency and move farther apart, causing the material to expand. They measure these vibrations using a technique called micro-Raman spectroscopy, where they hit the material with a laser. The vibrating atoms scatter the laser’s light, and this interaction can be used to detect their vibrational frequency.
But as the substrate expands or compresses, it affects how the atoms of the 2D material vibrate. The researchers had to decouple this substrate effect in order to map the intrinsic properties of the material. They did this by measuring the vibrational frequency of the same 2D material on three different substrates: copper, which has a high TEC; fused silica, which has a low TEC; and a silicon substrate riddled with tiny holes. Because the 2D material floats above the holes on the latter substrate, they can take measurements on these small pieces of free-standing material.
The researchers then placed each substrate on a thermal stage to precisely control temperature, heated each sample and performed micro-Raman spectroscopy.
“By doing Raman measurements on the three samples, we can extract something called the temperature coefficient that depends on the substrate. By using these three different substrates and knowing the TECs of the fused silica and the copper, can we extract the intrinsic TEC of the 2D material,” Zhong explains.
A curious result
They performed this analysis on several 2D materials and found that they all matched theoretical calculations. But the researchers saw something they didn’t expect: 2D materials fell into a hierarchy based on the elements that make them up. For example, a 2D material containing molybdenum will always have a larger TEC than a material containing tungsten.
The researchers dug deeper and found that this hierarchy is caused by a fundamental atomic property known as electronegativity. Electronegativity describes the tendency of atoms to attract or withdraw electrons when they bond. It is listed on the periodic table for each part.
They found that the greater the difference between the electronegativities of elements that make up a 2D material, the lower the material’s coefficient of thermal expansion will be. An engineer could use this method to quickly estimate the TEC for any 2D material, instead of relying on complex calculations that would normally have to be made by a supercomputer, Zhong says.
“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 pretty good estimate of the TEC. This holds promise for rapid material selection for engineering applications,” says Zhang.
In the future, the researchers want to apply their methodology to many more 2D materials, perhaps building a database of TECs. They also want to use micro-Raman spectroscopy to measure TECs of heterogeneous materials, which combine multiple 2D materials. And they hope to learn the underlying reasons why thermal expansion of 2D materials is different from that of bulk materials.
Yang Zhong et al, A unified approach and descriptor for the thermal expansion of two-dimensional monolayers of transition metal dichalcogenide, Scientific progress (2022). DOI: 10.1126/sciadv.abo3783. www.science.org/doi/10.1126/sciadv.abo3783
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Quote: New technique accurately measures how 2D materials expand when heated (2022, November 18) Retrieved November 18, 2022 from https://phys.org/news/2022-11-technique-accurately-2d-materials.html
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