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

TL;DR

This paper introduces an all-optical polarization-resolved second harmonic generation microscopy method for mapping twist-angles in 2D TMD bilayers, offering high resolution and rapid analysis compared to electron diffraction.

Contribution

It presents a novel optical technique for twist-angle imaging in TMD bilayers that is faster and compatible with device substrates, matching electron diffraction results.

Findings

Optical P-SHG accurately maps twist-angles in WS2 bilayers.

P-SHG is three orders of magnitude faster than 4D-STEM.

The method is non-destructive and compatible with device fabrication processes.

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 resolution of 0.23 degrees, 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.