Ultrafast free carrier dynamics in black phosphorus-molybdenum disulfide (BP/MoS2) heterostructures

Investigation and manipulation of photocarrier dynamics in van der Waals (vdW) heterostructures with type-II band alignment have gained much interest because of their fundamental and technological significance. Despite remarkable progresses on the ultrafast dynamics of interlayer excitons in transition metal dichalcogenide (TMD) heterostructures, transient behaviors of free photocarriers in vdW heterostructures, which is a critical process governing the performance of optoelectronic devices, remain poorly understood, restricting their effective manipulation. In this study, ultrafast dynamics of photo-generated free carriers in a type-II black phosphorus-molybdenum disulfide (BP/MoS2) heterostructure was investigated. A remarkably reduced lifetime (similar to 5 ps) of interlayer electron-hole (e-h) recombination was found, compared with those of BP film (similar to 130 ps) and other exciton-rich TMD heterostructures. More interestingly, this ultrafast interlayer recombination process can well be described by the Langevin model, and this high recombination rate is fundamentally linked to the high carrier mobility in BP. In addition, broadband measurements reveal that the interlayer recombination rate is independent of the broad energy distribution of photocarriers. Our findings provide new important and complementary insights into the fundamental photo-physics of vdW heterostructures, and represent a novel proposal for designing broadband high-speed optoelectronic devices.