Two-dimensional materials made of just a single layer of atoms can be packed more densely than conventional materials, so they could be used to make transistors, solar cells, LEDs and other devices that run faster and perform better.
One issue holding back these next-generation electronics is the heat they generate when in use. Conventional electronics typically reach temperatures of around 80 degrees Celsius, but the materials in a two-dimensional device are packed so densely in such a small area that the temperature of the device could be double that. This temperature increase can damage the device.
The problem is compounded by the fact that scientists don’t have a good understanding of how 2D materials expand when heated. Because these materials are so thin and optically transparent, their thermal expansion coefficient (TEC) (the tendency of a material to expand as the temperature increases) is almost impossible to measure using standard methods.
“When people measure the coefficient of thermal expansion of certain bulk materials, they use a scientific ruler or a microscope because with bulk materials you have the sensitivity to measure them. The challenge with 2D materials is that we can’t really see them, so we need to turn to Another type of ruler to measure TEC,” said Yang Zhong, a graduate student in mechanical engineering.
Chung is co-lead author of a research paper demonstrating such a “ruler.” Instead of directly measuring how the material expands, they used laser light to track the vibrations of the atoms that make up the material. Measuring a 2D material on three different surfaces or substrates allowed them to accurately extract its coefficient of thermal expansion.
The new study shows that this method is very accurate, achieving results that match theoretical calculations. This method confirms that the TEC of 2D materials falls within a narrower range than previously thought. This information can help engineers design the next generation of electronic products.
“By identifying this narrower physical range, we are giving engineers a lot of material flexibility in choosing the bottom substrate when designing a device. They don’t need to design a new bottom substrate to relieve thermal stress. We believe this is beneficial for electronic devices and The packaging world is very important,” 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, Ford Professor of Engineering and Chair of MIT’s Department of Mechanical Engineering, and others from MIT’s Department of Electrical Engineering and Computer Science and MIT’s Department of Mechanical and Energy Engineering at Southern Tech The university is located in Shenzhen, China.The study was published today in scientific progress.
Because 2D materials are so small—perhaps just a few micrometers—standard tools are not sensitive enough to directly measure their expansion. In addition, these materials are very thin and must be bonded to substrates such as silicon or copper. If the 2D material and its substrate have different TECs, they will expand differently when the temperature increases, leading to thermal stress.
For example, if a 2D material is bonded to a substrate with a higher TEC, when the device is heated, the substrate will expand more than the 2D material, stretching it. This makes it difficult to measure the actual TEC of 2D materials, since the substrate affects their expansion.
The researchers overcame these problems by focusing on the atoms that make up 2D materials. When a material is heated, its atoms vibrate at a lower frequency and move farther, which causes the material to expand. They measured these vibrations using a technique called micro-Raman spectroscopy, which involves hitting the material with a laser. The vibrating atoms scatter the laser light, and this interaction can be used to detect their vibrational frequency.
But if the substrate expands or compresses, it affects how the atoms in the 2D material vibrate. Researchers need to decouple this substrate effect to zero on the intrinsic properties of the material. They did this by measuring the vibration frequencies 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 dotted with small holes. Since the 2D material hovers over the holes in the latter substrate, they can perform measurements on tiny regions of these free-standing materials.
The researchers then placed each substrate on a hot stage to precisely control the temperature, heated each sample, and performed microscopic Raman spectroscopy.
“By performing Raman measurements on three samples, we can extract something called the temperature coefficient, which is substrate dependent. Using these three different substrates, and knowing the TEC of fused silica and copper, we can extract the Intrinsic TEC,” explained Zhong.
a strange result
They performed this analysis on several 2D materials and found that they all matched their theoretical calculations. But the researchers saw something they didn’t expect: 2D materials organized into a hierarchy according to the elements that make them up. For example, the TEC of 2D materials containing molybdenum is consistently higher than that of 2D materials containing tungsten.
The researchers dug deeper and learned that this level is caused by a fundamental atomic property called electronegativity. Electronegativity describes the tendency of atoms to pull or extract electrons when bonding. It is listed in the periodic table of every element.
They found that the greater the difference in electronegativity of the elements that make up the 2D material, the lower the material’s coefficient of thermal expansion. Engineers can use this method to quickly estimate the TEC of any two-dimensional material, Zhong said, rather than relying on complex calculations that typically have to be handled by supercomputers.
“Engineers can simply search the periodic table, get the electronegativity of the corresponding materials, plug them into our correlation equations, and within a minute they can make a pretty good estimate of the TEC. This is very promising for fast material selection for engineering applications ,” Zhang said.
Going forward, the researchers hope to apply their method to more 2D materials, perhaps building a TEC database. They also want to use microscopic Raman spectroscopy to measure the TEC of heterogeneous materials that combine multiple 2D materials. They wanted to understand the fundamental reason why 2D materials thermally expand differently than bulk materials.
This work was funded in part by the MIT and SUSTech Mechanical Engineering Research and Education Centers, the Materials Research Science and Engineering Center, the National Science Foundation, and the U.S. Army Office of Research.