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

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., MoSe, MoS, and WS). 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 MoSe 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.