Nano-optical investigation of grain boundaries, strain and edges in CVD grown MoS$_{2}$ monolayers

TL;DR

This study employs advanced nano-optical techniques to reveal nanoscale heterogeneities, strain, and defect effects in CVD-grown MoS₂ monolayers, providing insights into their local optical properties and defect-related phenomena.

Contribution

It demonstrates the use of tip-enhanced photoluminescence and Raman spectroscopies to probe nanoscale defect features in MoS₂ monolayers, overcoming spatial resolution limits of conventional methods.

Findings

Enhanced exciton peaks at grain boundaries indicate doping effects.

Localized strain causes non-uniform photoluminescence and Raman shifts.

Different edges show distinct optical responses related to strain and passivation.

Abstract

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a wide researched topic over the past decades. Optical characterization such as photoluminescence and Raman spectroscopies are important tools to probe their physical properties and the impact of defects. However, conventional optical techniques present a spatial resolution limitation lying in a $\mu$m-scale, which can be overcomed by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil nanoscale optical heterogeneities at grain boundaries, local strain fields and edges in grown MoS$_{2}$ monolayers. A noticeable enhancement of the exciton peak intensity corresponding to a trion emission quenching is observed at narrow regions down to 47 nm of width at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, distinct samples present different nano-optical responses at their edges due to strain and passivation defects. The passivated defective edges show a photoluminescence intensity enhancement and energy blueshift as well as a frequency blueshift of the 2LA Raman mode. On the other hand, the strained edges display a photoluminescence energy redshift and frequency redshifts for E$_{2g}$ and 2LA Raman modes. Our work shows that different defect features can be only probed by using optical spectroscopies with a nanometric resolution, thus revealing hindered local impact of different nanoscale defects in two-dimensional materials.