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Maragkakis G M, Psilodimitrakopoulos S, Mouchliadis L, Paradisanos I, Lemonis A et al. Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation. Opto-Electron Adv 2, 190026 (2019). doi: 10.29026/oea.2019.190026
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Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation
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Abstract
We use laser-scanning nonlinear imaging microscopy in atomically thin transition metal dichalcogenides (TMDs) to reveal information on the crystalline orientation distribution, within the 2D lattice. In particular, we perform polarization-resolved second-harmonic generation (PSHG) imaging in a stationary, raster-scanned chemical vapor deposition (CVD)-grown WS2 flake, in order to obtain with high precision a spatially resolved map of the orientation of its main crystallographic axis (armchair orientation). By fitting the experimental PSHG images of sub-micron resolution into a generalized nonlinear model, we are able to determine the armchair orientation for every pixel of the image of the 2D material, with further improved resolution. This pixel-wise mapping of the armchair orientation of 2D WS2 allows us to distinguish between different domains, reveal fine structure, and estimate the crystal orientation variability, which can be used as a unique crystal quality marker over large areas. The necessity and superiority of a polarization-resolved analysis over intensity-only measurements is experimentally demonstrated, while the advantages of PSHG over other techniques are analysed and discussed. -
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References
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Supplementary Information
Supplementary information for Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation 
oea-2019-0026 PSHG modulation.avi
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Maragkakis G M, Psilodimitrakopoulos S, Mouchliadis L, Paradisanos I, Lemonis A et al. Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation. Opto-Electron Adv 2, 190026 (2019). doi: 10.29026/oea.2019.190026
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Figure 1.
Schematic representation of the experimental setup, also adopted in ref.23, allowing high-resolution PSHG measurements in stationary, raster-scanned samples.
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Figure 3.
The experimental configuration, showing the laboratory X-Y-Z, and the crystalline x-y-z coordinate systems.
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Figure 2.
Schematic representation of the structure of 2D TMDs, containing three sublattices, with a plane of metal atoms being hexagonally packed between two planes of chalgogen atoms.
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Figure 4.
Simulated PSHG modulation presented in polar diagrams, as function of the linear polarization orientation φ, with φ ϵ [1°, 360°], for fixed polarizer at angle (a) ζ=0° and (b) ζ=90°.
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Figure 5.
Snapshots of experimental PSHG images of a WS2 flake, CVD-grown on a sapphire substrate.
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Figure 6.
(a) Integration of the experimentally detected PSHG intensity from the WS2 island, for φ ϵ[0°–90°] with step 1°, presented upon marking three POIs and two LOIs for further analysis. The POIs are actually single pixels of the 1200×1200 original image, magnified here, for illustration purposes. (b) Intensity profile of the experimental PSHG modulation presented in (a), along the LOIs shown there. As may be seen for LOI i, the intensity in the central, brighter area is magnified by a factor of ~4, which suggests the presence of second layer29. (c) Polar diagrams of the experimental PSHG modulation for φ ϵ[0°–360°] with step 1°, for the POIs illustrated in (a). We show with red color the raw data, and with blue the fitting using Eq. (3). We also present the retrieved values of the armchair orientation θ and the quality of fitting R2.
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Figure 7.
Mapping in (a) 2D diagram and (b) histogram of the armchair orientation distribution of the WS2 flake (with < θ > being the mean vale), based on the pixel-by- pixel fitting (R2≥0.88) of Eq. (3) on the experimental PSHG modulation for φ ϵ[0°–360°] with step 1°.
