Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation

We use laser-scanning nonlinear imaging microscopy in atomically thin transition metal dichalcogenides(TMDs)to reveal information on the crystalline orientation distribution,within the 2D lattice.In particular,we perform polarization-resolved second-harmonic generation(PSHG)imaging in a stationary,raster-scanned chemical vapor deposition(CVD)-grown WS2 flake,in order to obtain with high precision a spatially resolved map of the orientation of its main crystallographic axis(armchair orientation).By fitting the experimental PSHG images of sub-micron resolution into a generalized nonlinear model,we are able to determine the armchair orientation for every pixel of the image of the 2D material,with further improved resolution.This pixel-wise mapping of the armchair orientation of 2D WS2 allows us to distinguish between different domains,reveal fine structure,and estimate the crystal orientation variability,which can be used as a unique crystal quality marker over large areas.The necessity and superiority of a polarization-resolved analysis over intensity-only measurements is experimentally demonstrated,while the advantages of PSHG over other techniques are analysed and discussed.

Ultrahigh-resolution nonlinear optical imaging of the armchair orientation in 2D transition metal dichalcogenides

We used nonlinear laser scanning optical microscopy to study atomically thin transition metal dichalcogenides(TMDs)and revealed,with unprecedented resolution,the orientational distribution of armchair directions and their degree of organization in the two-dimensional(2D)crystal lattice.In particular,we carried out polarization-resolved second-harmonic generation(PSHG)imaging for monolayer WS2 and obtained,with high-precision,the orientation of the main crystallographic axis(armchair orientation)for each individual 120×120 nm^(2) pixel of the 2D crystal area.Such nanoscale resolution was realized by fitting the experimental PSHG images,obtained with sub-micron precision,to a new generalized theoretical model that accounts for the nonlinear optical properties of TMDs.This enabled us to distinguish between different crystallographic domains,locate boundaries and reveal fine structure.As a consequence,we can calculate the mean orientational average of armchair angle distributions in specific regions of interest and define the corresponding standard deviation as a figure-of-merit for the 2D crystal quality.