An Insightful Picture of Multi-Particle Recombination in Few-Layer MoS2 Nanosheets

The electronic and optical properties of transition metal dichalcogenides are well understood. However, much less are known about the role of defects and free carriers in exciton recombination, which is of fundamental importance for optoelectronic applications. Here, we investigate the photoexcited carrier recombination mechanism in few-layer (4–6 L) MoS2 nanosheets by employing pump energy and fluence-dependent femtosecond transient absorption spectroscopy. We demonstrate that the multi-particle (excitons and free carriers) generated by 3.1 eV excitation well above the electronic bandgap exhibit distinct recombination times. For instance, free carriers slow down the recombination by orders of magnitude relative to excitons. In contrast, the recombination time of excitons generated upon near quasi-particle excitation (1.94–2.22 eV) drops to ∼3 ps, which is associated with fast exciton capture to defects. To understand the nature of defects, we have investigated different configurations of sulfur vacancies from density functional theory (DFT) and time-dependent DFT.