  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.
