Ultrafast Carrier Dynamics in Few-Layer Colloidal Molybdenum Disulfide Probed by Broadband Transient Absorption Spectroscopy

Insights into the photophysics of molybdenum disulfide (MoS2) flakes made by exfoliation or chemical vapor deposition (CVD) have advanced the use of these materials in a broad range of applications. More recently, colloidal synthesis has been developed as an inexpensive, scalable, and highly tunable alternative to CVD for the production of MoS2 and other transition metal dichalcogenides (TMDs). Here, we present a comprehensive study on the charge-carrier relaxation in colloidal MoS2 sheets using transient absorption spectroscopy at visible and near infrared wavelengths. We show that the transient absorbance after photoexcitation originates from a reduced oscillator strength around the direct gap and a red shift of the entire absorbance and we attribute both features to state filling and band gap renormalization, respectively. In particular, the signatures of state filling exhibit a sub-picosecond decay, which reflects the trapping of hole carriers in mid-gap states. The relaxation of the band gap renormalization, on the other hand, takes several tens of picoseconds, a process that we assign to a series of charge-carrier recombination and capture events following the initial hole trapping. Since studies on CVD-grown MoS2 point toward highly similar relaxation of photogenerated charge carriers, we conclude that colloidal synthesis yields MoS2 nanosheets of comparable quality as the state-of-the-art CVD, even if both production methods involve an entirely different chemistry. This indicates that TMDs made with both approaches may benefit from similar defect passivation strategies to slow down charge-carrier trapping and enhance the exciton lifetime.