Researchers Stack the Odds for Novel Optoelectronic 2D Materials

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“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.”

He continued, “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 said.

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.

The team’s findings also showed fascinating effects of the vibrations between the layers. 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.

The title of the paper is “Twisted MoSe2 Bilayers with Variable Local Stacking and Interlayer Coupling Revealed by Low-Frequency Raman Spectroscopy.”

Raman spectroscopy, including aspects of theory, was conducted at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. Synthesis, including chemical vapor deposition, was supported by the DOE Office of Science. At Rensselaer Polytechnic Institute, theoretical work was supported by the National Science Foundation, and computations used resources of the Center for Computational Innovation.

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time.—by Dawn Levy

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