Figure 6.

Schematic representation of the different mechanisms proposed for the exciton radiative recombination in CuInS₂ NCs and the expected optical spectra and PL decay for each scenario (see text for details). (A) Donor–acceptor pair recombination. (B) Localized electron recombines with delocalized valence band hole. (C) Delocalized conduction band electron recombines with localized hole. (D) Exciton fine-structure: absorption transition occurs to a higher-energy hole state while emission takes place from a lower-energy hole state. (E–H) Absorption and PL spectra expected for the scenarios represented in panels A–D. All models would result in broad bands and large Stokes shift, except the fine-structure model that predicts narrow PL bands. (I–L) PL decay curves showing long radiative decay times due to limited electron–hole wave function overlap (scenario’s A–C) or the forbidden nature of the transition (scenario D). In scenarios A–C, also multiexponential radiative decay is expected because of a distribution of localized states. However, single-exponential radiative decay is expected in scenario D. (M–P) Transient absorption spectra expected for the different models. In the donor–acceptor pair recombination model both charge carriers localize on subnanosecond time scales, resulting in a very weak or absent negative bleach signal that decays fast. The other three models could result in both a negative bleach signal and a positive photo-induced absorption signal. The dynamics (here not schematically drawn) would however be different (see text for a detailed discussion). The bleach magnitude in panels N, O, and P is not necessarily the same but depends on the degeneracy of the electron and hole band edge levels.