Unraveling the tunable transient optical response of colloidal 2D Mo_1-xW_xS₂ nanosheets

Understanding the excited state dynamics of colloidal transition metal dichalcogenides (TMDCs) is vital for advancing their tunable optical properties toward innovative photonic and electro-optic applications. Upon photoexcitation, alloyed TMDCs exhibit strong exciton-phonon coupling and an associated band gap renormalization (BGR), which is characterized by a pronounced red-shift of the transient absorption (TA) of the materials and which is e.g. highly interesting for signal modulation. Here, we characterize a series of colloidal semiconducting alloyed Mo_1-xW_xS₂ (x = 0, 0.5, 1) nanosheets (NSs) with mono- to few-layer thickness and ∼14 nm lateral size by femtosecond broadband TA spectroscopy. Our analysis allows for the probing of composition-dependent optical changes within Mo_1-xW_xS₂ alloys directly in solution. By changing the molybdenum to tungsten ratio in the synthesis, it is possible to tune the absorption of the A exciton between 614 and 655 nm. We treat the complicated combination of spectrally overlapping sub-picosecond up to microsecond persisting transients in TA spectra by targeted global analysis for the in depth interpretation of charge carrier recombination processes in alloyed Mo_1-xW_xS₂ NSs. We find our model to be particularly well suited for describing the probed decays over a wide temporal range and extract constants spanning from hot charge carrier interactions (0.6-0.8 ps) over exciton-phonon coupling and BGR (29-61 ps) to band gap recombination (2-6 ns) up to trapping of charge carriers (0.6-1.6 µs) respectively. These values indicate that colloidal Mo_1-xW_xS₂ NS alloys are indeed promising for an optical signal modulation by BGR, which is tunable in wavelength (614-655 nm), transient red shift (18-27 eV) and timescale, while solution-processability provides maximum flexibility.