Second‐Harmonic Spectroscopy for Defects Engineering Monitoring in Transition Metal Dichalcogenides

Abstract

Reports on micromachining, [11,12] atomic healing, [13] nanoparticles decoration, [14,15] lateral heterostructures, [16,17] and heterocrystals [18] have demonstrated the feasibility of defects manipulation and postprocessing techniques. All those results have been summarized in important recent reviews on graphene [19] and TMDs. [20,21] One very important aspect is how the defect-engineered and postprocessed materials are characterized. The most widely used characterization techniques are scanning transmission electron microscopy (STEM) [2] for atomically resolved analysis of crystalline structure and different material phases, and PL emission [3,5] for micrometric-resolved linear optical characterization. However, to the best of our knowledge, no report has yet explored the potential of second-harmonic generation (SHG) as a characterization tool for point defects in TMDs. SHG has been extensively applied in studies about nonlinear optical properties of 2D materials, [22][23][24][25][26] mostly related to crystal symmetry investigation, grain boundaries characterization, [27] and in van der Waals heterostructures properties. [28] Particularly for monolayer TMDs, which lack inversion symmetry, changes in SHG signal can be used to track the density of defects in the material as compared to the pristine sample, with spatial resolutions typically at the order of sub-micrometer size for diffraction-limited laser beams.Lateral homojunctions in as-grown chemical transport deposition monolayer tungsten disulfide (WS 2 ) have been recently reported by Liu et al. [29] These homojunctions, as observed by fluorescence microscopy, may give rise to PL concentric patterns of alternating bright and dark regions. Further energydispersive X-ray spectroscopy (EDX) analysis and density functional theory (DFT) calculation revealed that this phenomenon is due to chemical heterogeneity: bright PL areas have less sulfur (S) vacancies, and since it is dominated by excitons, the PL emission is strong; dark PL areas have more S vacancies, increasing the density of mid-gap states, changing the bandgap from direct to indirect, and making PL emission weaker.The modulation in the density of defects should also impact the nonlinear optical properties of monolayer WS 2 crystal, since the second-order susceptibility of the material may experience an improvement in the infrared due to the presence of midgap states related to defects. In this work, we report SHG as Defects engineering in transition metal dichalcogenides is a topic of intense research recently, since crystal properties can be controlled and tailored during and after fabrication. In this context, defects characterization is key to understand the material structure and enable specific applications. In this work, second-harmonic generation (SHG) spectroscopy is used to map concentric triangular defective regions in as-grown monolayer tungsten disulfide, demonstrating that SHG can be used for defects observation and characterization in layered noncentrosymmetric nanomaterials. In monola...

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

“…The S vacancies in 2D materials create midgap states which reduce the bandgap of MoS 2 . These states arise due to the unsaturated Mo atoms near the vacant S sites [ 284 ]. When NO 2 molecules are adsorbed to the MoS 2 surface, the NO 2 molecule dissociates at the S vacancy sites into NO and O.…”

Twist Angle mapping in layered WS2 by Polarization-Resolved Second Harmonic Generation

“…(2)) originating from layered regions depends on both the number of SLs and their relative armchair orientation, i.e., the twist angle δ. Consequently, a change in the SHG amplitude in a SL is an indication of either the presence of a second TMD SL, or a change in the stacking order of the same SL (e.g. from 2 H to 3 R stacking 34,35 ).…”

“…The WS 2 samples were grown by the low-pressure chemical vapor deposition method (LP-CVD) on a c -cut (0001) sapphire substrate (2D Semiconductors). Note that in the case of CVD-grown samples the stacking of layers is not artificial like in 12 but occurs naturally during the growth 35 . Nevertheless, we expect a similar behavior like in 35 from CVD-grown layered samples.…”

“…Note that in the case of CVD-grown samples the stacking of layers is not artificial like in 12 but occurs naturally during the growth 35 . Nevertheless, we expect a similar behavior like in 35 from CVD-grown layered samples. This effect has been observed previously and is attributed to the nucleation that occurs during the CVD-growth and commences from the center 36 .…”

Nonlinear Optical Characterization of 2D Materials

“…From Figure 8 a–c, it can be seen that as the N vacancies increase, that is, defects gradually increase, the TPEF signal weakens and the CARS signal increases. Due to the larger defect, the surface of defective g-C 3 N 4 is of a larger roughness, so the SHG signals greatly increase [ 82 ]. For example, during the synthetic process of TMDs, for the high sulfur vacancy defect region of the monolayer WS 2 grown by chemical vapor deposition (CVD), the SHG signals experience a magnitude enhancement of two orders due to the existence of mid-gap states (as shown in Figure 8 d) [ 83 ].…”