Measuring the Local Twist Angle and Layer Arrangement in Van der Waals Heterostructures

Abstract

The properties of Van der Waals (VdW) heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here, the use of spectroscopic low energy electron microscopy (LEEM) and micro low energy electron diffraction (µLEED) methods to measure these properties locally is described. The authors present results on a MoS2/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the lattice, the cleaving history of the MoS2 flake is determined, that is, which layer stems from where in the bulk.

The Transition From MoS2 Single-Layer to Bilayer Growth on the Au(111) Surface

“…This intensity difference can qualitatively be understood by realizing that in the 2H-stacked bilayer the top layer unit mesh is rotated by 60 • with respect to the first layer unit mesh, inducing differences in the local structure factor leading to changes in q -dependent I(V) measurements, with the bottom layer unit mesh being identical to the one of the whole majority-domain single-layer MoS 2 island. In fact, a similar observation for singlelayer and bilayer MoS 2 has been reported by de Jong et al [29] for exfoliated MoS 2 deposited on hexagonal boron nitride. As the key differences in the ( 10) and (01) I(V) curves in this energy range from 25 to 90 eV (in which the surface sensitivity is the highest) mostly originate from the rotation of the MoS 2 unit cell, in DF-LEEM, this reasoning would also lead to a very similar appearance of a single-layer domain and a mirror-symmetrical 2H-stacked bilayer domain.…”

“…Specifically, in the energy range from about 3 to about 9 eV, the curve of the singlelayer MoS 2 shows one characteristic dip at 7.6 eV (cf. green solid line) whereas the bilayer exhibits two characteristic dips [29] and in excellent agreement with the calculated reflectivity profiles from ab initio scattering theory (cf. dashed lines in Figure 5) as reported by the same authors, clearly justifying the assigned numbers of local MoS 2 layers.…”

Direct imaging and electronic structure modulation of moiré superlattices at the 2D/3D interface

“…Various techniques have been used to observe moiré superlattices. These include reciprocal space imaging via low energy electron diffraction 19 , 20 and convergent beam electron diffraction (CBED) 21 ; spatially resolved property measurement via scanning tunneling microscopy (STM) 22 , 23 , atomic force microscopy (AFM) modalities 2 , 24 , near-field optical microscopy 13 , and infrared nano-imaging 5 ; and imaging of transmitted intensity via high-resolution and dark field (scanning) transmission electron microscopy, (S)TEM 25 , 26 . Of these techniques, STM and (S)TEM are the only two that exhibit real-space atomic resolution.…”

Nonuniversal Transverse Electron Mean Free Path through Few-layer Graphene

“…For other, more complex materials this simple toy model does not yield valid predictions. Already for other layered crystals such as hexagonal boron nitride [7] or transition metal dichalcogenides [31,32] only the full ab initio theory can describe the reflectivity spectra correctly. This indicates that the MFPs in these materials are also strongly affected by the band structure effects discussed here.…”