The Role of Free Carriers and Defects in Exciton Recombination in Few-layer MoS₂ Nanosheets

Understanding the carrier recombination of transition metal dichalcogenides (TMDCs) is a fundamental importance in many-body physics and various optoelectronic applications. Although the electronic and optical properties are well understood in TMDCs, much less is known about the role of defects and free carriers in exciton recombination. To this aim 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 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 (1.94-2.2 eV) drops to ~3 ps, which is associated with fast exciton capture to defects. To understand the nature of defects in MoS₂, we have investigated different defect configurations with various possible densities of sulfur vacancies from density functional theory (DFT) and time-dependent DFT. Our systematic studies in few-layer MoS₂ nanosheets reveal crucial information on the exciton recombination in TMDCs for several optoelectronic applications.