Mutual rotation of two monolayers of a semiconducting material creates a variety of bilayer stacking patterns, depending on the twist angle. Fast and efficient characterization of these stacking patterns may aid exploration of potential applications in electronics and optoelectronics. Credit: Image credit: Oak Ridge National Laboratory, US Dept. of Energy
Recently a team measured vibrations between 2 layers to decipher their stacking patterns with low-frequency Raman spectroscopy. This will allow for 2D materials with optical and electronic properties that strongly depend on stacking configurations eg energy-efficient transistors and solar cells.
The atoms in each layer are arranged in hexagonal arrays. When 2 layers are stacked and rotated, atoms from one layer overlap with those in the other layer and can form an infinite number of overlapping patterns. Liangbo Liang, a Wigner Fellow at ORNL said: “We are the first to show that low-frequency Raman spectra can be used as fingerprints to characterize the relative layer stacking in semiconducting 2D materials.”
In Raman scattering, an optical method for probing atomic vibrations, a material scatters monochromatic light from a laser. Whereas conventional Raman spectroscopy may probe more than approximately 3 trillion atomic vibrations per second, low-frequency Raman spectroscopy detects vibrations that are an order of magnitude slower. The low-frequency technique is sensitive to van der Waals coupling. It can provide crucial insight about layer thickness and stacking -aspects that govern fundamental properties of 2D materials.
Chemical vapor deposition, widely employed to synthesize 2D materials like graphene, was used to make perfectly triangular crystal monolayers of molybdenum diselenide 3 atoms thick. Feedstock molecules of molybdenum oxide and sulfur were reacted in a flowing gas within a high-temperature furnace to form the triangular crystals on silicon substrates.
The study revealed patterns in the stacked bilayers that strongly depend on the twist angle. Some specific twist angles, though, showed periodically repeating patches with the same stacking orientation. “These unique patterns may provide a new platform for optoelectronic applications of these materials,” Puretzky said.
Fascinating effects of the vibrations between the layers was also found. As different stacking patterns appeared when layers were displaced, variable spacings occurred between the layers at some specific twist angles. The researchers plan further measurements and modeling for different stacking configurations to better understand how these vibrational decays might alter the thermal properties of these materials-knowledge that could affect applications in heat dissipation and thermoelectric energy conversion. https://www.ornl.gov/news/ornl-researchers-stack-odds-novel-optoelectronic-2d-materials
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