Identifying the Intermediate Free-Carrier Dynamics Across the Charge Separation in Monolayer MoS2/ReSe2 Heterostructures

Van der Waals heterostructures composed of different two-dimensional films offer a unique platform for engineering and promoting photoelectric performances, which highly demands the understanding of photocarrier dynamics. Herein, large-scale vertically stacked heterostructures with MoS₂ and ReSe₂ monolayers are fabricated. Correspondingly, the carrier dynamics have been thoroughly investigated using different ultrafast spectroscopies, including Terahertz (THz) emission spectroscopy, time-resolved THz spectroscopy (TRTS), and near-infrared optical pump-probe spectroscopy (OPPS), providing complementary dynamic information for the out-of-plane charge separation and in-plane charge transport at different stages. The initial charge transfer (CT) within the first 170 fs, generating a transient directional current, is directly demonstrated by the THz emissions. Furthermore, the TRTS explicitly unveils an intermediate free-carrier relaxation pathway, featuring a pronounced augmentation of THz photoconductivity compared to the isolated ReSe₂ layer, which likely contains the evolution from immigrant hot charged free carriers to bounded interlayer excitons (∼0.7 ps) and the surface defect trapping (∼13 ps). In addition, the OPPS reveals a distinct enhancement in the saturable absorption along with long-lived dynamics (∼365 ps), which originated from the CT and interlayer exciton recombination. Our work provides comprehensive insight into the photocarrier dynamics across the charge separation and will help with the development of optoelectronic devices based on ReSe₂-MoS₂ heterostructures.