A green laser shines onto an atomically thin material

How do 2D materials expand when heated?

Two-dimensional (2D) materials composed of a single atomic layer are commonly used in modern miniaturized devices. However, the operation of the device can cause a significant increase in temperature and thermal stress, which will lead to device failure.

One such problem arises due to a poor understanding of how 2D materials expand with increasing temperatures. These materials are thin and optically transparent, making their coefficient of thermal expansion (TEC) nearly impossible to measure using standard approaches. To overcome such thermal challenges, it is important to have a good understanding of the coefficient of thermal expansion (TEC).

A new MIT study highlights a new technique that can accurately measure how atomic-thin materials expand when heated. Instead of directly measuring how the material stretches, they used laser light to track the vibrations of the material’s atoms. They accurately measured the coefficient of thermal expansion by measuring the same 2D material on three different surfaces or substrates.

This method is very accurate and gives results that correspond to the theoretical calculations. The approach confirms that the TECs of 2D materials fall in a much narrower range than previously thought. This information could help engineers design next-generation electronics.

Co-lead author and former mechanical engineering student Lenan Zhang SM ’18, Ph.D. ’22, who is now a research scientist, said: “By confirming this narrower physical range, we give engineers a lot of material flexibility in choosing the bottom substrate when designing a device. You don’t need to use a new floor substrate to mitigate thermal stress. We believe this has important implications for the electronic device and packaging community.”

Scientists solved the problem by focusing on the atoms that make up the 2D material. As the temperature increases, its atoms vibrate at a lower frequency and move farther apart. This causes the material to expand.

A technique called micro-Raman spectroscopy was used to measure these vibrations. With this method, the material is hit with a laser. The vibrating atoms scatter the light from the laser, and this interaction can be used to detect their vibrational frequency.

However, the atoms of the 2D material change their vibration when the substrate stretches or contracts. In order to focus on the intrinsic properties of the material, scientists need to decouple this substrate influence. On three different substrates – copper, which has a high TEC, fused silica, which has a low TEC; and a silicon substrate with multiple microscopic holes – they measured the vibration frequency of the same 2D material. You can measure these tiny areas of freestanding material as the 2D material levitates over the holes on the latter substrate.

Later, scientists placed each substrate on a hot stage to precisely control the temperature, heated each sample, and performed micro-Raman spectroscopy.

The results also showed something unexpected: 2D materials fell into a hierarchy based on the elements that made them up. For example, a 2D material containing molybdenum will always have a larger TEC than one containing tungsten.

When scientists dig deeper, they find that this hierarchy results from a fundamental atomic property known as electronegativity.

Yang Zhong, a graduate student in mechanical engineering, said: “They found that the larger the difference between the electronegativity of elements that make up a 2D material, the lower the material’s coefficient of thermal expansion. An engineer could use this method to quickly estimate the TEC for any 2D material, rather than relying on complex calculations that typically have to be handled by a supercomputer.”

zhang said “An engineer can just search the periodic table, get the electronegativity of the relevant materials, plug them into our correlation equation and within a minute they can have a reasonably good estimate of the TEC. This is very promising for rapid material selection for engineering applications.”

Scientists now plan to apply their technique to many more 2D materials. You now want to create a database of TECs.

Magazine reference:

  1. Yang Zhong, Lenan Zhang et al. A unified approach and descriptor for the thermal expansion of two-dimensional transition-metal dichalcogenide monolayers. scientific advances. DOI: 10.1126/sciadv.abo3783

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