Chemical synthetic strategy for single-layer transition-metal chalcogenides

A solution-phase synthetic protocol to form two-dimensional (2D) single-layer transition-metal chalcogenides (TMCs) has long been sought; however, such efforts have been plagued with the spontaneous formation of multilayer sheets. In this study, we discovered a solution-phase synthetic protocol, called "diluted chalcogen continuous influx (DCCI)", where controlling the chalcogen source influx (e.g., H₂S) during its reaction with the transition-metal halide precursor is the critical parameter for the formation of single-layer sheets as examined for the cases of group IV TMCs. The continuous influx of dilute H₂S throughout the entire growth period is necessary for large sheet formation through the exclusive a- and b-axial growth processes. By contrast, the burst influx of highly concentrated H₂S in the early stages of the growth process forms multilayer TMC nanodiscs. Our DCCI protocol is a new synthetic concept for single-layer TMCs and, in principle, can be operative for wide range of TMC nanosheets.