Mica not only provides an atomically flat surface for 2D crystals but also holds well-ordered water layers because of its crystallinity and large hydrophilicity. Despite their low photoluminescence (PL) quantum yields, the 1L transition metal dichalcogenides (TMDs) are highly useful emitters because of the visible and NIR excitons with large binding energies. Such exploration can be best performed using a TMD/mica system. At the same time, it is also natural to expect that there can be other roles of interfacial water that have not been observed. As demonstrated by these studies, a nanoscopic amount of water can have a substantial effect on the decay of excitons in low-dimensional materials. observed the diffusion of molecular oxygen through the interfacial water layer in real time. By exploiting the PL characteristics of 1L WS 2, Kang et al. showed that sub-monolayer water accommodates an oxygen reduction reaction (ORR) that injects electrical holes in 1L WS 2 supported on SiO 2 and subsequently amplifies the excitonic PL. The direction of charge transfer responsible for the PL change was dependent on the nature of the substrates and the morphology of the water layers. observed that the photoluminescence (PL) of 1L MoS 2 on mica is modulated by the presence of interfacial water. This issue has been tackled by a few reports that used interfacial water entrapped between 2D semiconductors and solid substrates. One question of priority and yet lacking a clear understanding is how the electronic excitation in low-dimensional materials is affected by the tiny amount of water. Understanding the role of nanoscopic water in complex low-dimensional materials will eventually contribute to devising their novel functions and related devices. By gas-controlled PL imaging, we also prove that the interfacial water converted the trions into excitons by depleting native negative charges through an oxygen reduction reaction, which rendered the excited WS 2 more susceptible to nonradiative decay via exciton–exciton annihilation. Using PL spectroscopy and wide-field imaging, we show that the emission signals from A excitons and their negative trions decreased at distinctively different rates with increasing excitation power, which could be attributed to the more efficient annihilation between excitons than between trions. In this work, we report that the photoluminescence (PL) of single-layer WS 2 is substantially affected by interfacial water that is inevitably present between it and the supporting mica substrates. However, their photophysical properties are greatly affected by their surrounding environment because of their 2D nature. Because of their bandgap tunability and strong light–matter interactions, two-dimensional (2D) semiconductors are considered promising candidates for next-generation optoelectronic devices.
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