Exciton recombination in few-layer MoS2 nanosheets: Role of free carriers and defects

A current obstacle to the development of extremely flexible and high-performance optoelectronic devices using transition metal dichalcogenide nanosheets is the incomplete understanding of the exciton and free carrier recombination in the presence of defects. Here, taking liquid-phase exfoliated few-layer MoS2 nanosheets as a model system, we demonstrate the exciton and free carrier recombination mechanism by employing pump energy and fluence-dependent ultrafast transient absorption spectroscopy. We demonstrate that 3.10 eV pump excitation, much above the lowest energy A exciton of four to six layers of MoS2 nanosheets (similar to 1.84 eV), generates excitons and free carriers. The excitons decay quickly within similar to 3 ps due to the defect capture via the Shockley-Read-Hall mechanism. In contrast, free carriers show slower recombination (similar to 1000 ps), which is an order of magnitude larger than the exciton recombination time. We verified this idea by exciting the sample with 1.94-2.22 eV pump excitations that predominantly generate excitons decaying within similar to 3 ps. Our systematic studies in few-layer MoS2 nanosheets reveal crucial information on the unexplored domain of excitons and free carriers recombination in the presence of defects for several optoelectronic applications.