  Extended defects with one dimensionality smaller than that of the host, such
as 2D grain boundaries in 3D materials or 1D grain boundaries 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. Unfortunately, a technique to rapidly and
non-destructively image 1D defects in 2D materials is lacking. Scanning
transmission electron microscopy (STEM), Raman, photoluminescence and nonlinear
optical spectroscopies, are all extremely valuable, but current implementations
suffer from low throughput and a destructive nature (STEM) 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. 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 Cerenkov-type
SHG emission. The frequency dependence of this emission in MoSe2 monolayers is
explained in terms of plasmon-enhanced SHG related to the defects 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 towards both applications and
fundamental materials discoveries.
