Nonlinear Dark-Field Imaging of One-Dimensional Defects in Monolayer Dichalcogenides

Abstract

One-dimensional defects in two-dimensional (2D) materials can be particularly damaging because they directly impede the transport of charge, spin, or heat and can introduce a metallic character into otherwise semiconducting systems. Current characterization techniques suffer from low throughput and a destructive nature or limitations in their unambiguous sensitivity at the nanoscale. Here we demonstrate that dark-field second harmonic generation (SHG) microscopy can rapidly, efficiently, and nondestructively probe grain boundaries and edges in monolayer dichalcogenides (i.e., MoSe2, MoS2, and WS2). Dark-field SHG efficiently separates the spatial components of the emitted light and exploits interference effects from crystal domains of different orientations to localize grain boundaries and edges as very bright 1D patterns through a Čerenkov-type SHG emission. The frequency dependence of this emission in MoSe2 monolayers is explained in terms of plasmon-enhanced SHG related to the defect’s metallic character. This new technique for nanometer-scale imaging of the grain structure, domain orientation and localized 1D plasmons in 2D different semiconductors, thus enables more rapid progress toward both applications and fundamental materials discoveries.

Nonlinear Optical Characterization of 2D Materials

“…Different techniques have been used to characterize various types of defects in 2D materials to assess crystal quality. For example, TEM can directly solve the atomic details of defects, but TEM is time-consuming, and the size of the TEM characterization area is limited to nanoscale [ 78 ]. Photoluminescence (PL) spectroscopy imaging can also be used to visualize grain boundaries (GBs) in materials.…”

“…One-dimensional (1D) defects in 2D materials are sometimes extremely destructive because they directly hinder the transfer of charge, spin, or heat [ 78 ]. The grain boundary is a common 1D defect.…”

“…The grain boundary is a common 1D defect. The appearance of grain boundaries has been observed in many 2D materials, such as MoSe 2 , MoS 2 , WS 2 , and BP [ 53 , 78 , 79 , 87 , 88 ], and these 2D materials are synthesized by different methods. Figure 9 a shows MoS 2 grown by a solid-phase sulfurization strategy, and the area indicated by the yellow arrow is the grain boundary.…”

“…A 1D defect contrast factor is defined to evaluate the bright field (BF) SHG and dark field (DF) SHG microscopy, and it is found that the contrast factor of DF-SHG is three times that of BF-SHG when characterizing the grain boundary. Obvious difference regarding contrast can be clearly seen through the white line of Figure 9 d. The advantages of DF conditions are also demonstrated by the imaging of MoSe 2 , MoS 2 , and WS 2 samples under DF conditions [ 78 ]. SHG microscopy can only be used to characterize the grain boundaries of non-centrosymmetric materials.…”

“…The insets depict the cross-section (dashed white line), which is used to compare the contrast of the BF-SHG and DF-SHG response on the same crystal. Reproduced from [ 78 ], with permission from American Chemical Society, 2020. ( e ) Upper panel: Experimental SHG image.…”