Intrinsic photophysics of semiconductor nanocrystals in dielectric media: Formation of surface states

The photoluminescence of CdSe/ZnS nanocrystals in toluene at room temperature shows inhomogeneous emission and excitation spectra. This is accompanied by a non-exponential photoluminescence decay on time scales between 1 and 80 ns. The inhomogeneous behaviour is due to a remaining size distribution, which can be partly photoselected by a combination of excitation and emission wavelengths, which reveals the contribution of at least 4 different electronic states. The identification of these states became possible by the application of a decay time distribution analysis of the photoluminescence decay. In addition to the size-related CdSe exciton emission, two trap or surface states with decay components close to 10 and 80 ns have been identified. The 80 ns component is shifted to the red by about 100 meV from the exciton peak, whereas the 10 ns component cannot be discriminated spectrally from the exciton band.