Microscale spectroscopic mapping of defect evolution and filling in large-area growth of monolayer MoS2

Defects in monolayer transition metal dichalcogenides are known to be problematic in that they usually dete-riorate optical and electronic properties, thus acting as a constraint in nanodevice applications. In this regard, understanding defect-related strain and charge distributions is intriguing to identify the inhomogeneous energy landscape of two-dimensional materials. Herein, simultaneous Raman and photoluminescence results unveil how strain and charge carriers evolve over the large-area growth of monolayer MoS2. Unexpectedly, spatially -resolved correlation analysis between the in-plane and out-of-plane phonon frequencies of MoS2 reveals that both compressive strain and charge carriers increase as the morphology evolves from a triangular flake to a partially coalesced film and, finally, to a fully covered film, indicating that the number of sulfur vacancies in-creases with surface coverage. Theoretically calculated formation energies of sulfur vacancies support the evo-lution scenarios with respect to morphological variations. Photoluminescence mappings of excitons and trions provide further quantitative information on electron concentrations that are consistent with Raman correlation data. Notably, chemical treatment of defective MoS2 with a p-type dopant leads to a relaxation of compressive strain and a decrease in excess electrons, where in particular, the p-doping effects are more prominent in the fully covered film region than in the partially coalesced region.

Automated summary

In this study, the evolution of strain and charge carriers in monolayer MoS2 during large-area growth was investigated using Raman and photoluminescence techniques. The results showed that both compressive strain and charge carriers increased as the morphology changed from a triangular flake to a partially coalesced film and finally to a fully covered film, indicating an increase in sulfur vacancies with surface coverage. Theoretical calculations supported these evolution scenarios. Photoluminescence and Raman correlation data provided quantitative information about electron concentrations. Additionally, chemical treatment with a p-type dopant relaxed compressive strain and decreased excess electrons, with a more significant effect in the fully covered film region.