Ultrafast Recombination Dynamics under Lateral Confinement and Cryogenic Temperatures in Colloidal MoS₂

Colloidal synthesis opened up a versatile way to tailor two-dimensional transition metal dichalcogenides. In particular, the huge oscillator strength of excitons in colloidal MoS₂ makes the material appealing for excitonic devices. In this context, we studied ultrafast excitons with transient absorption (TA) spectroscopy in MoS₂ nanosheets (NSs, 22 ± 9 nm) and nanoplatelets (NPLs, 8 ± 4 nm) with NPLs providing additional lateral confinement of excitons in the structures. Carrier relaxation mechanisms are identified by extending the TA setup with a custom micro cryostat and by extracting time-dependent line shapes. The exciton lifetime in MoS₂ NSs almost doubles from 12 to 22 ps by cooling the samples from 295 to 9 K, which is attributed to phonon scattering. Temperature-dependent recombination dynamics are found for the sparsely studied C and D excitons, thus covering the full visible spectrum by time-resolved characterization. For smaller MoS₂ NPLs, the A exciton signature reappears at cryogenic temperatures, allowing laterally confined excitons in MoS₂ to be extensively studied for the first time.