Ultrafast Recombination\nDynamics under Lateral Confinement\nand Cryogenic Temperatures in Colloidal MoS<sub>2</sub>

Colloidal synthesis opened up a versatile way to tailor\ntwo-dimensional\ntransition metal dichalcogenides. In particular, the huge oscillator\nstrength of excitons in colloidal MoS<sub>2</sub> makes the material\nappealing for excitonic devices. In this context, we studied ultrafast\nexcitons with transient absorption (TA) spectroscopy in MoS<sub>2</sub> nanosheets (NSs, 22 ± 9 nm) and nanoplatelets (NPLs, 8 ±\n4 nm) with NPLs providing additional lateral confinement of excitons\nin the structures. Carrier relaxation mechanisms are identified by\nextending the TA setup with a custom micro cryostat and by extracting\ntime-dependent line shapes. The exciton lifetime in MoS<sub>2</sub> NSs almost doubles from 12 to 22 ps by cooling the samples from\n295 to 9 K, which is attributed to phonon scattering. Temperature-dependent\nrecombination dynamics are found for the sparsely studied C and D\nexcitons, thus covering the full visible spectrum by time-resolved\ncharacterization. For smaller MoS<sub>2</sub> NPLs, the A exciton\nsignature reappears at cryogenic temperatures, allowing laterally\nconfined excitons in MoS<sub>2</sub> to be extensively studied for\nthe first time.