Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide van der Waals Nanosheets.

Because of their earth-abundant, low-cost, and environmentally benign characteristics, two-dimensional (2D) group IV metal chalcogenides (e.g., SnSe2) with layered structures have shown great potential in optoelectronic, photovoltaic, and thermoelectric applications. However, the intrinsic motion of excited carriers and their coupling with lattice photons, which fundamentally dictates device operation and optimization, remain yet to be unraveled. Herein, we directly follow the ultrafast carrier and photon dynamics of colloidal SnSe2 nanosheets in real time using ultrafast transient absorption spectroscopy. We show ∼0.3 ps intervalley relaxation process of photoexcited energetic carriers and ∼3 ps carrier defect trapping process with a long-lived trapped carrier (∼1 ns), highlighting the importance of trapped carriers in optoelectronic devices. In addition, ultrashort laser pulse impulsively drives coherent out-of-plane lattice vibration in SnSe2, indicating strong electron-phonon coupling in SnSe2. This strong electron-phonon coupling could impose a fundamental limit on SnSe2 photovoltaic devices but benefit its thermoelectric applications.