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

Sotiris Psilodimitrakopoulos; Leonidas Mouchliadis; Ioannis Paradisanos; Andreas Lemonis; George Kioseoglou; Emmanuel Stratakis

doi:10.1038/lsa.2018.5

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

Light Sci Appl. 2018 May 4:7:18005. doi: 10.1038/lsa.2018.5. eCollection 2018.

Authors

Sotiris Psilodimitrakopoulos¹, Leonidas Mouchliadis¹, Ioannis Paradisanos^{1

2}, Andreas Lemonis¹, George Kioseoglou^{1

3}, Emmanuel Stratakis^{1

3}

Affiliations

¹ Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, Heraklion Crete 71110, Greece.
² Department of Physics, University of Crete, Heraklion Crete 71003, Greece.
³ Department of Materials Science and Technology, University of Crete, Heraklion Crete 71003, Greece.

Abstract

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 nm² 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.

Keywords: armchair orientation mapping; atomically thin transition metal dichalcogenides; crystal quality marker; graphene-related materials; polarization-resolved second-harmonic generation.