Optical versus electron diffraction imaging of Twist-angle in 2D transition metal dichalcogenide bilayers

S Psilodimitrakopoulos,L Mouchliadis,G Kourmoulakis,J Verbeeck,G M Maragkakis,N Gauquelin,D Jannis,G Kioseoglou,A Orekhov,E Stratakis

doi:10.1038/s41699-021-00258-5

Abstract

Atomically thin two-dimensional (2D) materials can be vertically stacked with van der Waals bonds, which enable interlayer coupling. In the particular case of transition metal dichalcogenide (TMD) bilayers, the relative direction between the two monolayers, coined as twist-angle, modifies the crystal symmetry and creates a superlattice with exciting properties. Here, we demonstrate an all-optical method for pixel-by-pixel mapping of the twist-angle with a resolution of 0.55(°), via polarization-resolved second harmonic generation (P-SHG) microscopy and we compare it with four-dimensional scanning transmission electron microscopy (4D STEM). It is found that the twist-angle imaging of WS2 bilayers, using the P-SHG technique is in excellent agreement with that obtained using electron diffraction. The main advantages of the optical approach are that the characterization is performed on the same substrate that the device is created on and that it is three orders of magnitude faster than the 4D STEM. We envisage that the optical P-SHG imaging could become the gold standard for the quality examination of TMD superlattice-based devices.

Highlights

Following the discovery of graphene, the appearance of 2D transition metal dichalcogenides (TMD) significantly broadened the knowledge in the field of 2D materials, as well as opening potential optoelectronic applications
Having a tool that spatially resolves, with high-resolution and minimally invasively, the twistangle in large area vdW heterostructures, would be of great importance in the quality characterization of such structures. We demonstrate such a technique based on the areal imaging of polarization-resolved SHG (P-SHG) signals from TMD superlattices complemented with theoretical modeling that predicts the SHG signals interference from the respective overlapping areas of twisted-bilayers
The excitation source used for the SHG experiments is an fs oscillator, at 1030 nm and repetition rate in the order of MHz, which is adequate to excite non-linear signals like SHG

Summary

Introduction

Following the discovery of graphene, the appearance of 2D transition metal dichalcogenides (TMD) significantly broadened the knowledge in the field of 2D materials, as well as opening potential optoelectronic applications. The graphical representation of Eq (1) and the corresponding visualization in a polar diagram (presented in Fig. 2) demonstrates a fourfold symmetry of the P-SHG intensity modulation that rotates for different armchair directions θ.

Results

Conclusion