Trapping and exciton-exciton annihilation assisted ultrafast carrier dynamics in nanosheets of 2H-MoSe2 and Cr doped 1T/2H-MoSe2

Nanosheets of transition metal dichalcogenides with prospects of photocatalysis and optoelectronics applications have significant potential in device fabrication due to their low-cost production and easily controllable morphology. Here, non-degenerate pump-probe differential transmission studies with varying pump-fluence have been carried out on single-phase 2H-MoSe2 and mixed-phase 1T/2H-MoSe2 nanosheets to characterize their excited carrier dynamics. For both the samples, the differential probe transmission data show photo-induced bleaching at earlier pump-probe delay followed by photo-induced absorption unveiling signatures of exciton-state filling, exciton trapping, defect-mediated photo-induced probe absorption and recombination of defect bound excitons. The exciton trapping and photo-induced absorption by the trapped-carriers are estimated to occur with time constant of similar to 430 to 500 fs based on multi-exponential modelling of the differential transmission till pump-probe delay of similar to 3.5 ps. Biexponential modeling of the subsequent slow-recovery of the negative differential transmission at pump-probe delay greater than or similar to 3.5 ps reveals that the exciton recombination happens via two distinct decay channels with similar to 25 to 55 ps (tau(1)) and greater than or similar to 1 ns (tau(2)) time constants. Pump-fluence dependent reduction in tau(1) and further modelling of exciton population using higher order kinetic rate equation reveals that the two-body exciton-exciton annihilation governs the exciton recombination initially with a decay rate of similar to 10-8 similar to 10-8 cm(3)s(-1). The detailed analysis suggests that the fraction of total excitons that decay via long decay channel decreases with increasing exciton density for 2H-MoSe2, in contrast to 1T/2H-MoSe2 where the fraction of excitons decaying via long decay channel remains constant.