Copper indium sulfide (CIS) nanocrystals (NCs) are attracting increasing attention due to their intrinsic low toxicity and size-dependent optical spectra, which are tunable in the visible to near-IR spectral window, making CIS NCs promising materials for applications in thin-film photovoltaics, lighting and biolabeling. These NCs are often overcoated with CdS or ZnS shells to increase their photoluminescence (PL) quantum yields and stability.  

The PL of CIS and CIS-based core/shell NCs is characterized by broad bandwidths (FWHM 300 meV), large global Stokes shifts (~600 meV), and multi-exponential PL decays with long lifetimes (~200 ns). In recent years, these spectral characteristics have been tentatively explained by using models based on the electronic structure of CdSe NCs in combination with knowledge of emission of bulk CIS, in which intrinsic crystal defects are involved. Mechanisms proposed include trapping of charge carriers and donor-acceptor recombination. Nevertheless, the exact electronic structure of CIS NCs and the origin of their luminescence properties remains unclear. To gain insight into this fundamental problem we carried out (time-resolved) PL measurements in combination with absorption and transient absorption (TA) spectroscopy on fs-ns timescales. 

Our study shows a striking difference between the optical properties of bare CIS NCs and CIS/CdS core/shell NCs. For the core/shell system, the PL decay traces can be fitted well with a bi-exponential function, while those of CIS NCs are strongly multiexponential. The PL lifetimes of CIS/CdS NCs are longer than those of CIS NCs, and in TA measurements there is no recovery of the ground state bleach up to 2 ns. These results show that the type-II band alignment between bulk CIS and CdS still holds on the NC-scale, leading to a (partial) spatial separation of the charge carriers. Moreover, the TA results imply that the hole cannot be trapped when the electron is delocalized, since that would result in a recovery of the ground state bleach. Furthermore, we studied the dynamics of the PL energy in the 1-3000 ns time window. The red-shift of the core/shell system is much larger and faster, while the peak width narrows in the first 200 ns and then broadens again, which is not observed for bare CIS NCs. This observation in combination with the clear 2-exponential PL decay, indicates the existence of two distinct recombination pathways with two well distinguishable rates for CIS/CdS core/shell NCs, while for bare CIS NCs there is a much more complicated interplay of recombination pathways.