Abstract
Stacked atomically thin transition metal dichalcogenides (TMDs) exhibit fundamentally new physical properties compared to those of the individual layers. The twist angle between the layers plays a crucial role in tuning these properties. Having a tool that provides high-resolution, large area mapping of the twist angle, would be of great importance in the characterization of such 2D structures. Here we use polarization-resolved second harmonic generation (P-SHG) imaging microscopy to rapidly map the twist angle in large areas of overlapping WS2 stacked layers. The robustness of our methodology lies in the combination of both intensity and polarization measurements of SHG in the overlapping region. This allows the accurate measurement and consequent pixel-by-pixel mapping of the twist angle in this area. For the specific case of 30° twist angle, P-SHG enables imaging of individual layers.
Highlights
The graphene-related atomically thin 2D transition metal dichalcogenides (TMDs) show great promise for high-tech optoelectronic applications[1,2,3,4]
In our recent work on polarization-resolved second harmonic generation (P-SHG) of TMDs, we have demonstrated that the SHG signal modulates as the angle of linear polarization of the excitation field, φ, rotates and that the modulation depends on the armchair orientation
It can be shown that upon using appropriate linear polarization for the excitation of layered WS2 with two different armchair angles overlapping in a region, we are able to decompose the interfered SHG signal originating from each individual layer
Summary
The graphene-related atomically thin 2D TMDs show great promise for high-tech optoelectronic applications[1,2,3,4]. The strain induced SHG20, the twist angle–dependent moiré-templated strain patterning[21], the interlayer valley excitons in TMD heterobilayers[22,23], the twist angle-dependent conductivities across MoS2/graphene heterojunctions[24], and the moiré excitons in heterobilayers[25,26,27,28] are just a few more studies that have been reported recently These observations indicate the strong potential to harness and tune the physical properties of layered 2D materials, via the adjustment of the twist angle of the stacked layers[29]. Phase-resolved SHG techniques have been used for the determination of the relative orientation between monolayers[30] In all these cases solely variations in SHG intensity were used to identify the armchair angle difference, a criterion that cannot unambiguously exclude other phenomena as a source of these changes, such as structural transformations and inhomogeneities. This enables the effect of optical discrimination of atomically thin layers and provides a form of axial super-resolution SHG imaging of each individual layer of stacked 2D TMDs
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