Mechanistic Insight to the Chemical Treatments of Monolayer Transition Metal Disulfides for Photoluminescence Enhancement

There is a growing interest in obtaining high quality monolayer transition metal disulfides (TMDSs) for optoelectronic device applications. Surface chemical treatments using a range of chemicals on monolayer TMDSs have proven effective to improve their photoluminescence (PL) yield. However, the underlying mechanism for PL enhancement by these treatments is not clear, which prevents a rational design of passivation strategies. In this work, a simple and effective approach to significantly enhance PL of TMDSs is demonstrated by using a family of cation donors, which we show to be much more effective than commonly used p-dopants which achieve PL enhancement through electron transfer. We develop a detailed mechanistic picture for the action of these cation donors and demonstrate that one of them, Li-TFSI (bistriflimide), enhances the PL of both MoS2 and WS2 to a level double that compared to the widely discussed and currently best performing super acid H-TFSI treatment. In addition, the ionic salts used in chemical treatments are compatible with a range of greener solvents and are easier to handle than super-acids, which provides the possibility of directly treating TMDSs during device fabrication. This work sets up rational selection rules for ionic chemicals to passivate TMDSs and increases the potential of TMDSs in practical optoelectronic applications.