Sub-Bandgap Emission and Intraband Defect-Related Excited-State Dynamics in Colloidal CuInS₂/ZnS Quantum Dots Revealed by Femtosecond Pump–Dump–Probe Spectroscopy

We consider the highly radiative, long-lived photoluminescence (PL) component observed in colloidal CuInS2/ZnS core/shell quantum dots (CIS/ZnS QDs) and provide evidence of the involvement of intragap defect states in the emission, settling a long ongoing discussion in the literature. Femtosecond transient absorption (fs-TA) spectroscopy was used to investigate subpicosecond dynamics in these technologically important QDs. Spectral and kinetic analysis of the fs-TA data, in combination with femtosecond pump-dump-probe experiments, revealed a stimulated emission component in CIS/ZnS QDs for the first time. PDP experiments showed that the excited-state absorption signal, originating from the conduction band (CB), was immune to the depopulation of the emitting state by a third, "dump" laser centered close to the luminescence maximum. We conclude that the optical transition responsible for the observed room-temperature PL in CIS/ZnS QDs does not originate from the CB as postulated in the literature but rather from high-lying intraband donor states most likely associated with indium-copper antisite defects. Filling of the emitting sub-bandgap state was assigned with a time constant of 0.5 ps, and de-excitation via remaining surface states was associated with a 1.8 ps time constant. A third longer decay constant (27 ps) was attributed to Auger recombination.