The effective synthesis of two-dimensional transition metal dichalcogenides alloy is essential for successful application in electronic and optical devices based on a tunable band gap. Here we show a synthesis process for Mo_1−xW_xS₂ alloy using sulfurization of super-cycle atomic layer deposition Mo_1−xW_xO_y. Various spectroscopic and microscopic results indicate that the synthesized Mo_1−xW_xS₂ alloys have complete mixing of Mo and W atoms and tunable band gap by systematically controlled composition and layer number. Based on this, we synthesize a vertically composition-controlled (VCC) Mo_1−xW_xS₂ multilayer using five continuous super-cycles with different cycle ratios for each super-cycle. Angle-resolved X-ray photoemission spectroscopy, Raman and ultraviolet–visible spectrophotometer results reveal that a VCC Mo_1−xW_xS₂ multilayer has different vertical composition and broadband light absorption with strong interlayer coupling within a VCC Mo_1−xW_xS₂ multilayer. Further, we demonstrate that a VCC Mo_1−xW_xS₂ multilayer photodetector generates three to four times greater photocurrent than MoS₂- and WS₂-based devices, owing to the broadband light absorption.

We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS${}_2$. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons ($A$ and $B$) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the $B$-exciton linewidth in turn lowers the peak spectral amplitude of the $A$ exciton. Resonant excitation at the $B$-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.

The efficient synthesis of two-dimensional molybdenum disulfide (2D MoS₂) at low temperatures is essential for use in flexible devices.