Fluence-dependent dynamics of localized excited species in monolayer versus bulk MoS2

Transition-metal dichalcogenides are characterized by a layered lattice structure, facilitating the fabrication of two-dimensional crystals. Concerning their electronic properties, these monolayers differ fundamentally from the bulk material, exhibiting direct band gaps and significantly higher exciton binding energies. Hence, to shed light on how the crystal thickness influences the relevant excited states and in particular their dynamics, we performed time-resolved optical pump-probe spectroscopy on monolayer and bulk samples of molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}₂$) for a broad range of excitation fluences. The observed transient spectra result from photoinduced shifts in the band structure. Here, we find evidence for a localized nature of the excited species as opposed to the common model of band-gap renormalization. At high excitation densities, strong collisional broadening of the shifted absorption occurs. Within the first picosecond after excitation, the free carriers thermalize, while in the monolayer, additionally, electrons and holes pair to form excitons. On longer time scales up to several nanoseconds, the excited populations decay. In the bulk sample, the corresponding signal reduction could be described in terms of a defect-assisted Auger recombination of electrons and holes. For monolayer $\mathrm{Mo}{\mathrm{S}}₂$, in contrast, two-dimensional diffusion leads to recombination of the excitons at defect sites.