Nonlinear imaging of grain boundaries in monolayer semiconducting transition metal dichalcogenides

Locating and imaging defects in two-dimensional (2D) materials is a key challenge to probe the material quality for different target applications. Recently, second harmonic generation (SHG) has been used to characterise different 2D materials and defective regions like grain boundaries [1]. However it has been shown that these grain boundaries were revelled by interference between the two neighbouring domains, leading to a dark region (no SHG emission) between then [1]. Here we implemented a nonlinear dark-field SHG microscopy setup to accurately probe grain boundaries and edges from semiconducting transition metal dichalcogenide monolayers. The dark-field SHG efficiently separates the spatial components of the emitted light and accurately locates grain boundaries and edges as bright patterns. The far field SHG pattern is calculated considering the interference between two second harmonic dipoles, which is in agreement with our experimental finding. Therefore the dark field second harmonic imaging open new opportunities to characterize defects in 2D materials [2].
[1] X.Yin et. al, Science 344, 488, 2014.
[2] Bruno Carvalho et al., submitted (2018).