Intensive and broad bound exciton emission at cryogenic temperature in suspended monolayer transition metal dichalcogenides

Two-dimensional (2D) transition metal chalcogenides (TMDCs) are expected to play an important role in next-generation optoelectronic devices. However, the properties of 2D TMDCs can be largely affected by defects, and thus, many efforts have been made to characterize and eliminate the defects. Here, we investigate defect-related trap states, particularly in large-area suspended ${\mathrm{WS}}_{2}$ monolayers at cryogenic temperatures, using steady-state and time-resolved photoluminescence (TRPL) spectroscopy. We observed an intensive and broad (full width half maximum $\ensuremath{\sim}170\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) photoluminescence (PL) emission centered at 1.83 eV, 0.3 eV lower than the free exciton transition energy. From temperature-dependent PL spectra, the thermal activation energy of excitons escaping from trap states is determined to be 30 meV. The TRPL spectrum shows an ultralong lifetime at 13 K. The long-lived emission with the broad PL linewidth is attributed to excitons bound to defects of possibly chalcogenide-site substitution and sulfur vacancies. Compared with the supported monolayer, the defect-related trap states are found to be deeper in the suspended layer and can trap excitons more efficiently due to less dielectric screening. In this letter, we provide further understanding of trap states and decay channels in monolayer TMDCs and show that suspended 2D TMDCs can provide a pure platform for studying defect-related properties.