Interfacial-Water-Modulated Photoluminescence of Single-Layer WS$_2$ on Mica

TL;DR

This study reveals how interfacial water influences the photoluminescence of single-layer WS$_2$ on mica, showing water-induced charge depletion and exciton-trion dynamics affecting emission efficiency.

Contribution

It demonstrates the significant impact of interfacial water on WS$_2$ photophysical properties and elucidates the mechanisms of exciton-trion conversion and nonradiative decay.

Findings

Interfacial water reduces PL intensity by converting trions into excitons.

Water presence causes differential decay rates of exciton and trion emissions.

Gas-controlled PL imaging confirms water-induced charge depletion and exciton-exciton annihilation.

Abstract

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.