PSHG vs 4D STEM imaging of twist angle in 2D transition metal dichalcogenide bilayers

Atomically thin two-dimensional materials can be vertically stacked with van der Waals bonds that enable interlayer coupling. In the particular case of transition metal dichalcogenide (TMD) bilayers, the relative orientation between the two monolayers, i.e., the twist-angle, modifies the crystal symmetry and creates a superlattice with exciting properties. In this work, we demonstrate an all-optical, high resolution method for pixel-by-pixel mapping of the twist-angle, via polarization-resolved second harmonic generation (PSHG) microscopy and compare it with four-dimensional scanning transmission electron microscopy (4D STEM). It is found that the twist-angle imaging of WS2 bilayers, using the PSHG technique is in excellent agreement with that obtained using electron diffraction. The main advantages of the nonlinear optical method are that the characterization is performed on the same substrate that the bilayer is created on and that it is three orders of magnitude faster than the 4D STEM.