Steady state and time resolved photoluminescence properties of CuInS2/ZnS quantum dots in solutions and in solid films

The photoluminescence properties of CuInS2/ZnS quantum dots (QDs) dispersed in solutions of different concentrations and solvent polarity and deposited as solid films on quartz substrates by drop-casting and spin-coating are studied. Both steady state and time-resolved photoluminescence spectroscopy have been used. The CuInS2/ZnS QDs in solutions exhibit a red-shift of their absorption and photoluminescence spectra by increasing concentration and solvent polarity. In addition, they exhibit a three-exponential decay with time constants 1-3, 20-40 and 200-300 ns, depending on solvent, concentration and detection wavelength. In films, a red-shifted photoluminescence spectrum is observed for films made by drop-casting compared to those prepared by spin-coating. The time-resolved photoluminescence decays in films, apart from the three mechanisms observed in solutions, also exhibit a fast decay component of <1 ns, which is more pronounced in the spin coated films and especially at long emission wavelengths. The time resolved photoluminescence spectra in the drop-casted films experience a larger transient red-shift than in the spin-coated ones, indicative of a possible energy transfer among adjacent QDs. In general, it is shown that the chemical environment and the presence of defects play a central role in the recombination processes.