Interfacial Charge Transfer and Recombination Dynamics in van der Waals Heterojunctions of 2D Semiconductors*

Heterojunctions of transition metal dichalcogenides (TMDC) are being explored for optoelectronics, photovoltaics and spin-valleytronics at the 2D limit. Using time-resolved microscopic transient reflectance spectroscopy, we measured the interfacial charge transfer and recombination dynamics in two dimensional MoS$_{2}$/WSe$_{2}$ heterojunctions as a function of interlayer momentum mismatch. The observed ultrafast ($<$ 40 fs) formation of interlayer exciton is independent of the relative orientation between the two layers, indicating a hot-carrier mediated charge transfer mechanism. The lifetime of charge transfer excitons span two orders of magnitude (0.04 - 3 ns) with no clear dependence on momentum mismatch, in accordance with the defect-mediated non-radiative recombination mechanism. Our results suggest the importance of defect reduction in revealing the intrinsic properties of charge transfer excitons in two dimensional van der Waals heterojunctions.