  Because of their bandgap tunability and strong light-matter interactions,
two-dimensional (2D) semiconductors are considered promising candidates for
next-generation optoelectronic devices. However, their photophysical properties
are greatly affected by environments because of their 2D nature. 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
itself and supporting mica substrates. 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 can be attributed to the more efficient annihilation between
excitons than trions. By gas-controlled PL imaging, we also prove that
interfacial water converts trions into excitons by depleting native negative
charges through an oxygen reduction reaction, which renders excited WS$_2$ more
susceptible to nonradiative decay via exciton-exciton annihilation.
Understanding the roles of nanoscopic water in complex low-dimensional
materials will eventually contribute to devising their novel functions and
devices.
