Doing the twist with stacked 2D materials

Stacking layers of nanometer-thin semiconducting materials at different angles offers a new approach for designing the next generation of energy-efficient transistors and solar cells. The atoms in each layer are arranged in hexagonal arrays; when two layers are stacked and rotated, atoms in one layer overlap with those in the other layer and can form an infinite number of overlapping patterns. This is like the Moiré patterns that result when two screens are overlaid and one is rotated on top of the other. Theoretical calculations predict excellent electronic and optical properties for certain stacking patterns, but how can these stacking patterns be created and characterized experimentally?

Now, a team led by researchers from the Department of Energy's Oak Ridge National Laboratory has come up with a novel method for characterizing these various stacking patterns, by studying the vibrations between two layers as they are twisted in different directions. The team employed a method called low-frequency Raman spectroscopy to measure how the layers vibrate with respect to each other and compared the frequencies of the measured vibrations with their theoretically-predicted values.

This study provides a platform for engineering two-dimensional (2D) materials with optical and electronic properties that strongly depend on stacking configurations. The findings are published in ACS Nano.

"Low-frequency Raman spectroscopy, in combination with first-principles modeling, offers a quick and easy approach to reveal complex stacking configurations in the twisted bilayers of a promising semiconductor, without relying on other expensive and time-consuming experimental techniques," said co-lead author Liangbo Liang at ORNL. "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."

Raman scattering is an optical method for probing atomic vibrations in a material based on the scattering of monochromatic light from a laser. Whereas conventional Raman spectroscopy can probe more than approximately three 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 weak attractive forces between layers, called van der Waals coupling, and can provide crucial insight about layer thickness and stacking – aspects that govern fundamental properties of 2D materials.

"This work combines state-of-the art synthesis and processing of 2D materials, their unique spectroscopic characterization, and data interpretation using first-principles theory," said co-lead author Alex Puretzky. "High-resolution Raman spectroscopy that can probe low-frequency modes requires specialized instrumentation, and only a few places around the world have such a capability together with advanced synthesis and characterization tools, and theory and computational modeling expertise. The Center for Nanophase Materials Sciences at ORNL is among them."

Chemical vapor deposition, which is widely employed to synthesize 2D materials like graphene, was used to make perfectly triangular crystal monolayers of molybdenum diselenide just three atoms thick. This involved reacting feedstock molecules of molybdenum oxide and selenium in a flowing gas within a high-temperature furnace to form the triangular crystals on silicon substrates.

"Numerous parameters need to be properly adjusted to synthesize large, triangular 2D crystals successfully," Puretzky said. "Then, carefully removing the crystals and stacking them precisely in different orientations is a big challenge. The precise, equilateral triangular shape of the synthesized and transferred crystals allowed us to measure the twist angles with a high precision using standard optical and atomic force microscopy images, which was a key factor in our experiments."

Theoretical and computational aspects were challenging too. "Raman spectroscopy is heavily based on theory for interpretation and assignment of the observed Raman spectra, especially for new materials that have never before been measured," Puretzky explained.

The study revealed patterns in the stacked bilayers that strongly depended on the twist angle. Some specific twist angles, though, produced periodically repeating patches with the same stacking orientation. "These unique patterns may provide a new platform for optoelectronic applications of these materials," Puretzky said.

The team also showed that the vibrations between the layers produced some interesting effects. As different stacking patterns appeared when layers were displaced, variable spacings occurred between the layers at certain 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.

This story is adapted from material from Oak Ridge National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.