Nonlinear dark-field imaging of 1D defects in monolayer dichalcogenides

One-dimensional defects in 2D materials can be particularly damaging since 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 non-destructively probe grain boundaries and edges in monolayer dichalcogenides (i.e. MoSe$_2$, MoS$_2$ and WS$_2$). 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 \v{C}erenkov-type SHG emission. The frequency dependence of this emission in MoSe$_2$ monolayers is explained in terms of plasmon-enhanced SHG related to the defect's metallic character. T...