Rotational and dilational reconstruction in transition metal dichalcogenide moiré bilayers

Abstract

Lattice reconstruction and corresponding strain accumulation plays a key role in defining the electronic structure of two-dimensional moiré superlattices, including those of transition metal dichalcogenides (TMDs). Imaging of TMD moirés 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 moiré layers in hBN further localizes and enhances these in-plane reconstruction pathways by 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 moiré potential.

Advance in twisted transition metal dichalcogenides: synthesis, characterization, and properties

“…Sung et al and Van Winkle et al obtained information on twist low-dimensional materials using 4D-STEM. The former observes molar lattice recombination in twist angle low-dimensional materials [92], and the latter identifies different reconstruction mechanisms in moiré homobilayers and heterobilayers [93].…”

Mechanical, electronic, optical, piezoelectric and ferroic properties of strained graphene and other strained monolayers and multilayers: an update

“…As seen in figures 36(a)-(c), different sections of the bilayer form relative conformations that explore every local unit cell configuration seen in figure 33. In principle, the energy cost varies, and certain configurations can be minimised by a process of local atomistic (strain) reconstruction [17,283,[299][300][301][302][303][304]; Cazeaux et al have deployed a process to optimise moiré bilayers that minimises a functional of the local strain [292]. It relies on an energy landscape set on a unit cell similar to the one given by equation ( 60) (but slightly simpler).…”

Influence of Atomic Relaxations on the Moiré Flat Band Wave Functions in Antiparallel Twisted Bilayer WS2

“…Due to the lack of inversion symmetry, these domains host interlayer charge transfer leading to an out-of-plane ferroelectric dipole moment. ,,, In the case of AP-aligned TMD homobilayers, no such charge transfer occurs. While AP systems do form domain wall networks at angles very close to 60°, they generally exhibit less pronounced in-plane reconstructions than P-aligned bilayers at a given twist-angle offset, leading to a more smoothly varying periodic lattice distortion over the moiré unit cell . In all cases, both out-of-plane buckling and in-plane reconstruction have profound impacts on the electronic properties of TMD homobilayers.…”

“…While AP systems do form domain wall networks at angles very close to 60°, they generally exhibit less pronounced in-plane reconstructions than P-aligned bilayers at a given twist-angle offset, leading to a more smoothly varying periodic lattice distortion over the moiréunit cell. 25 In all cases, both out-of-plane buckling and in-plane reconstruction have profound impacts on the electronic properties of TMD homobilayers. In-plane and out-of-plane deformations cause the simple band-folding description of a moirépattern band structure to break down, for example, causing a modification in the real-space position of the valence band edge wave functions.…”

“…As a result, to date, several heterobilayer TMDs such as WSe 2 /WS 2 , , MoS 2 /WS 2 , − MoS 2 /WSe 2 , , and WSe 2 /MoSe 2 have been investigated experimentally, with reports of correlated phenomena such as superconductivity, Mott insulating phases, generalized Wigner crystals, etc. ,,, Moiré patterns of TMD homobilayers are highly sensitive to the twist angle and thus expected to exhibit more twist-angle disorder. As a consequence, these systems have been studied less despite having great potential for inducing strong moiré potentials. ,,− For example, recent scanning probe microscopy experiments on a WSe 2 /WSe 2 homobilayer have demonstrated twist-angle-dependent lattice reconstructions near a 60° twist angle resulting in multiple energy-separated ultraflat valence bands for long moiré wavelengths (6–12 nm) …”