Photo- and Thermally-Induced Huge Layer Decoupling in Twisted Bilayer WSe₂.

Abstract

Twisted bilayer systems host a wealth of emergent phenomena, such as flat-band superconductivity, ferromagnetism, and ferroelectricity, arising from moiré superlattices and unconventional interlayer coupling. Despite their central role, direct and quantitative access to the 3D atomic arrangement in these systems has remained elusive due to their nanoscale dimensions. Here, we introduce an automated dark-field electron tomography technique that enables quantitative 3D structural analysis of atomically thin materials with sub-ångström precision. By applying this method to twisted bilayer WSe₂, we precisely visualize the twist-angle-dependent structural relaxation appearing as the AB/BA stacking domains separated by 10-20 nm domain walls. In the marginally twisted region (θ ≤ 0.1°), we uncover a significant expansion of the interlayer spacing compared to the bulk configuration, exceeding 0.1 Å, along with a remarkable temperature-driven interlayer decoupling. Ultrafast measurements further reveal optically induced interlayer separation of ∼0.2 Å 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 moiré-engineered 2D materials.