Moiré Potential, Lattice Relaxation, and Layer Polarization in Marginally Twisted MoS₂ Bilayers

Abstract

Artificially twisted heterostructures of semiconducting transition-metal dichalcogenides (TMDs) offer unprecedented control over their electronic and optical properties via the spatial modulation of interlayer interactions and structural reconstruction.Here we study twisted MoS 2 bilayers in a wide range of twist angles near 0°using scanning tunneling microscopy/spectroscopy. We investigate the twist angle dependence of the moirépattern, which is dominated by lattice reconstruction for small angles (<2°), leading to large triangular domains with rhombohedral stacking. Local spectroscopy measurements reveal a large moire-potential strength of 100−200 meV for angles <3°. In reconstructed regions, we see a bias-dependent asymmetry between neighboring triangular domains, which we relate to the vertical polarization that is intrinsic to rhombohedral stacked TMDs. This viewpoint is further supported by spectroscopy maps and ambient piezoresponse measurements. Our results provide a microscopic perspective of this new class of interfacial ferroelectrics and can offer clues for designing novel heterostructures that harness this effect.

Nonlinear Optical Response Modulated by Lattice Reconstruction in Small‐Angle Twisted Bilayer MoS2

“…[59] Therefore, for distinct highly symmetric structures, the Г point VBM at the AA site undergoes a notably downward energy shift in contrast to the MX and XM sites. [24,60] Although the conduction band edge also exhibited a downward and d) SHG intensity versus the excitation power on the monolayer MoS 2 . e) Variation of SHG intensity with wavelength and intensity ratio between the 3°twisted MoS 2 bilayer and the monolayer sample.…”

“…This suggests that, at 𝜃 ≤ 2°, the sample undergoes significant atomic-level reconstruction, resulting in the discrete corresponding domains being separated by 1D DWs, in contrast to the smoothly varying rigid lattice Moiré pattern. [24,26] When the two atomic layers are nearly aligned, the interlayer stacking energy and intralayer strain energy compete. After taking the lattice deformation analysis by measuring the atomicresolved STM images of the DW and MX stacking regions, we acquire that there are 1.34% tensile strain and 0.20% compressive strain in the DW region of the reconstructed moiré superlattice (see Figure S6, Supporting Information).…”

“…For the second point, it is well established that the valence band maximum (VBM) at the Г point exhibits an acute sensitivity to interlayer interactions, whereas the VBM and conductivity band minimum (CBM) at the K point are not sensitive. [24,58] This is because the susceptibility to interlayer interactions can be explained by the compositional wave functions, whereby the VBM wave function at the Г point manifests lobed protrusions emanating from sulfur atoms, in contrast to the confinement of the VBM and CBM wave functions at the K-point within the transition-metal sublattice. In particular, the S-p z feature of the Г point VBM renders it more receptive to interlayer spacing modifications in comparison to the K point VBM or CBM.…”

“…In addition, scanning tunneling microscopy (STM), which is characterized by high-resolution, real-time imaging, has emerged as an indispensable instrument for delving into and navigating nanowires. Some studies have investigated the microscopic properties of twisted bilayer MoS 2 using STM [24] and pulse force microscopy, revealing that structural remodeling occurs at small angles and that structural reconstruction can have a great impact on crystal properties, such as ferroelectric properties. [25] Furthermore, Quan et al [26] unearthed a nuanced relationship between phonon spectroscopy and Moiré lattice reconstructions.…”

Strain Solitons in an Epitaxially Strained van der Waals-like Material

“…Novel electronic properties such as local band gap modification, charge carrier confinement, flatbands, − and topological edge modes , have been both predicted and observed to occur within the strain solitons. Strain solitons have been experimentally observed in mechanically strained bulk crystals, exfoliated graphene layers, ,,− hBN multilayers, and twisted transition metal dichalcogenides. , However, a wafer scale approach for the generation of strain solitons has yet to be demonstrated. The lack of a wafer scale method to generate strain solitons poses a significant challenge for the utilization of the novel properties of strain solitons in industrial devices.…”