Obviously, the precise characterization of the twist-angle in moiré superlattices is essential for a global understanding and quality control in such 2D material systems, as well as precise tuning of the respective vdW devices’ performance.Īlthough TEM is the most commonly used technique to atomically reconstruct twisted TMD bilayers 7, it requires tedious sample transfer on TEM grids, which is incompatible with most 2D materials fabrication and characterization techniques. Consequently by tuning the twist-angle in real-space, the change in the moiré pattern and consequently to the moiré periodic potential, one can control the interlayer coupling in order to obtain the desired superlattice properties. In this context, the twist-angle is regarded as a new degree of freedom, enabling tuning of the physical properties of the TMD superlattices. The moiré pattern in the crystal symmetry of a twisted bilayer can be controlled through the rotation of the adjacent layers. More recent studies revealed the presence of moiré excitons in twisted TMD homo- and hetero-bilayers 4, 5, 6. Recently, it was also discovered that the moiré periodic potential in twisted MoS 2 bilayer can modify the properties of phonons in the respective ML constituents to generate Raman modes related to moiré phonons 3. Photoluminescence (PL) studies on MoS 2 TMD bilayers with different twist-angles reveal an interlayer electronic coupling, which corresponds to an indirect bandgap recombination which varies with twist-angle 2. The relative direction between the two MLs is called the twist-angle. In such TMD bilayers, the two constituent MLs may possess different crystal directions creating in their overlapping region a moiré superlattice. Besides this, vertical stacks of two TMD monolayers (ML) forming a bilayer demonstrate exciting optoelectronic properties, not present in individual MLs 1. We envisage that the optical P-SHG imaging could become the gold standard for the quality examination of TMD superlattice-based devices.įollowing 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 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. It is found that the twist-angle imaging of WS 2 bilayers, using the P-SHG technique is in excellent agreement with that obtained using electron diffraction. 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). 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. Atomically thin two-dimensional (2D) materials can be vertically stacked with van der Waals bonds, which enable interlayer coupling.
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