Rotational and Dilational Reconstruction in Transition Metal Dichalcogenide Moir\'e Bilayers

TL;DR

This study employs advanced microscopy to quantitatively analyze how local rotations and dilations drive lattice reconstruction in transition metal dichalcogenide moiré bilayers, revealing the effects of encapsulation and heterostrain on strain distribution.

Contribution

It provides direct experimental evidence of the deformation mechanisms in TMD moiré bilayers, highlighting the roles of local rotations and dilations in lattice relaxation.

Findings

Local rotations govern relaxation in twisted homobilayers.

Local dilations are prominent in heterobilayers with lattice mismatch.

Encapsulation in hBN enhances in-plane reconstruction and suppresses out-of-plane corrugation.

Abstract

Lattice reconstruction and corresponding strain accumulation play a key role in defining the electronic structure of two-dimensional moir\'e superlattices, including those of transition metal dichalcogenides (TMDs). Imaging of TMD moir\'es has so far provided a qualitative understanding of this relaxation process in terms of interlayer stacking energy, while models of the underlying deformation mechanisms have relied on simulations. Here, we use interferometric four-dimensional scanning transmission electron microscopy to quantitatively map the mechanical deformations through which reconstruction occurs in small-angle twisted bilayer MoS2 and WSe2/MoS2 heterobilayers. We provide direct evidence that local rotations govern relaxation for twisted homobilayers, while local dilations are prominent in heterobilayers possessing a sufficiently large lattice mismatch. Encapsulation of the moir\'e layers in hBN further localizes and enhances these in-plane reconstruction pathways, suppressing out-of-plane corrugation. We also find that extrinsic uniaxial heterostrain, which introduces a lattice constant difference in twisted homobilayers, leads to accumulation and redistribution of reconstruction strain, demonstrating another route to modify the moir\'e potential.