Real-Time Monitoring of Chemical Treatment on TMDs

Chemical treatment is one of the major route for tuning the properties of two-dimensional transition metal dichalcogenides (TMDs), a class of ultrathin semiconductors. Firstly, solution-based chemical approaches using TFSI-based ionic salts (e.g. using Li-TFSI) enhance semiconducting properties by passivating surface defects and unwanted doping, notably enhancing photoluminescence (PL) yield. Secondly, chemical lithiation using organolithiation agents (e.g. n-butyllithium) changes the crystallographic phase of TMDs from the natural trigonal prismatic (2H) to octahedral (1T). Here, employing various home-built microscopy techniques, we monitored the time- and spatially-resolved PL enhancement and phase transition of mono-, and few-layered MoS₂ during solution processes. For Li-TFSI treatment, we focused on the evolution and homogeneity during the chemical treatments. We defined the treatment time, and figured out the inhomogeneity remains during the treatment, which presented non-varying intrinsic defect density over time. In organolithiation-based phase engineering of MoS₂, we discovered that this process is a charge-limited, surface-driven intercalation that can be tuned by illumination energy.