Second harmonic generation in two-dimensional transition metal dichalcogenides with growth and post-synthesis defects

Abstract

Strong second harmonic generation (SHG) in monolayer transition metal dichalcogenides demonstrates great promise for nonlinear photonic applications. However, to utilize these materials in photonic applications, it is imperative to investigate the robustness of the nonlinear optical properties against various defects introduced during or post synthesis. Here we study the SHG in hexagonal monolayer WS2 with synthesis defects, in triangular monolayer WS2 with focused ion beam induced structural defects, and in WS2 under ambient and aqueous environmental conditions. We find that WS2 hexagonal monolayers exhibit significantly more uniform SHG intensity across the sample with a coefficient of variation, c(v) = 4.9% than the varying photoluminescence (PL) intensity (c(v) = 32.83%). We also show that introducing structural defects in WS2 monolayers leads to diminishing, but not vanishing SHG. To further explore the robustness of SHG in comparison with PL, we show that aged WS2 monolayers have greatly varying PL across different samples (c(v) = 35.6%), while exhibiting much more uniform SHG (c(v) = 8.31%). Lastly, we find that WS2 monolayers submerged in deionized water shows similar intensity in SHG signal. To understand these experimental results, density functional theory calculations have been performed. Our analysis suggests that potential defects and water introduced to the WS2 monolayers produce relatively little change in the second-order nonlinear susceptibility, whereas nonradiative recombination caused by these defects can significantly affect the PL. These results show a good qualitative agreement with the experimental results.