Real-time spatially resolved determination of twist angle in transition metal dichalcogenide heterobilayers

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) offer unique optoelectronic capabilities due to their direct bandgap semiconductor nature in monolayer form. Atomically thin TMDs can be assembled in vertical stacks that are held together by van der Waals forces, enabling interlayer coupling between the layers. This creates new physical properties that depend on the relative orientation (twist angle) between the TMD monolayers. Accurate and fast measurement of the twist angle is therefore of utmost importance for characterizing a 2D TMD heterostructure. Here, we present a nonlinear imaging technique based on second harmonic generation (SHG) microscopy, that enables instantaneous mapping of the twist angle between the two stacked TMD monolayers. By using a polarization beam splitter in the detection path and two detectors measuring two orthogonal SHG polarization components, we acquire with a single-shot measurement the twist angle in a WS2/MoS2 heterobilayer, in real time. Remarkably, the twist angle is measured directly in the overlapping region based on a SHG interference model. The demonstrated technique offers a powerful tool for the rapid, all-optical and spatially resolved twist angle determination in large-area 2D TMD heterostructures.

Nonlinear Optical Imaging of In‐Plane Anisotropy in Two‐Dimensional SnS

“…In order to describe the interaction of an excitation field with a 2D orthorhombic MX crystal (Figure 1a) and the subsequent production of SHG, we use the Jones formalism. [32][33][34][35][36][37][38] The two coordinate systems considered are schematically shown in Figure 1b: the laboratory frame (X, Y, Z) and the crystal coordinates (x, y, z), where z // Z. The laser beam propagates along Z axis, normally incident on the crystal, and linearly polarized along the sample plane, at an angle ϕ with respect to X laboratory axis.…”

“…Group IV monochalcogenides, also known as group IV-VI metal monochalcogenides, and denoted by MX with M = Sn, Ge and X = S, Se, are a class of layered, orthorhombic, semiconducting 2D materials attracting significant interest. [1][2][3] They are known Furthermore, it has been used to calculate and map in a pixel-bypixel manner the main crystallographic direction (armchair) of 2D TMDs, [31][32][33] and quantify their crystal quality, [32,33] determine the twist-angle in TMD homobilayers, [34,35] and heterobilayers, [36] and probe the valley population imbalance. [37] In this work, we extend the use of the P-SHG microscopy technique in order to investigate the properties of the orthorhombic 2D MXs.…”

“…and WSe 2 . [31][32][33][34][35][36][37] Specifically, it has enabled direct optical imaging of the atomic edges and boundaries of a 2D material, based on the observation of electronic structure changes at the edges of monolayer MoS 2 . [31] Two-dimensional (2D) tin(II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials with remarkable properties, such as in-plane anisotropic optical and electronic response, and multiferroic nature.…”

Second harmonic generation control in twisted bilayers of transition metal dichalcogenides

“…Polarization-resolved SHG measurements (PSHG) allows determining the crystallographic orientation of monolayers, as well as the twist angle and charge transfer dynamics in bilayer heterostructures [21][22][23][24][25][26][27]. The SHG intensity reaches a maximum value when the electric field vector of the excitation laser beam is parallel to the armchair orientation of TMD monolayers according to the non-linear susceptibility tensor χ 2 of the D 3h space group [28,29].…”

“…The twist angle between two monolayers in a heterostructure is usually measured by PSHG experiments on the constituent monolayers and not on the heterostructure region itself [10,12,26,30,31]. This approach has several drawbacks, as monolayer drift and rotation are possible during the assembly process.…”