Layer Decoupling in Twisted Bilayer WSe$_2$ Uncovered by Automated Dark-Field Tomography

TL;DR

This paper introduces an automated dark-field electron tomography technique to analyze the 3D atomic structure of twisted bilayer WSe$_2$, revealing interlayer expansion, temperature-driven decoupling, and ultrafast optically induced separation, advancing understanding of moiré materials.

Contribution

The study presents a novel 3D structural analysis method for atomically thin materials, providing new insights into interlayer spacing and dynamics in twisted bilayer WSe$_2$.

Findings

Interlayer spacing exceeds 0.1 angstrom compared to bulk

Temperature induces interlayer decoupling unique to twisted bilayers

Optical excitation causes ~0.2 angstrom separation on picosecond timescale

Abstract

Twisted bilayer systems host a wealth of emergent phenomena, such as flat-band superconductivity, ferromagnetism, and ferroelectricity, arising from moir\'e superlattices and unconventional interlayer coupling. Despite their central role, direct and quantitative access to the out-of-plane atomic structure in these systems has remained elusive due to their nanoscale dimensions. Here, we introduce an automated dark-field electron tomography technique that enables three-dimensional structural analysis of atomically thin materials with sub-angstrom precision. Applying this method to twisted bilayer WSe$_2$, we uncover a significant expansion of the interlayer spacing compared to the bulk configuration, exceeding 0.1 angstrom, along with a remarkable temperature-driven interlayer decoupling unique to the twisted bilayer. Ultrafast measurement further reveals optically induced interlayer separation of ~0.2 angstrom on the picosecond timescale, attributed to transient exciton formation. These findings not only establish a powerful approach for visualizing hidden out-of-plane structures in atomically thin micro-flake materials, but also uncover the intrinsic fragility and dynamical tunability of interlayer coupling in moir\'e-engineered 2-dimensional materials.