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.