Determination of Electronic Energy Levels in Type-II CdTe-Core/CdSe-Shell and CdSe-Core/CdTe-Shell Nanocrystals by Cyclic Voltammetry and Optical Spectroscopy

Two different sets of type-H core/shell semiconductor nanocrystals (NCs) are prepared and studied by optical absorption, time-resolved and time-integrated photoluminescence (PL) spectroscopy, as well as cyclic voltammetry (CV). In particular, NCs with a CdTe core and a CdSe shell of variable thickness, where the holes are localized in the core and the electrons are mainly in the shell, are compared with their inverse system of a CdSe core and a CdTe shell. All measurements are correlated to model calculations based on the effective mass approximation (EMA). The comprehensive study reveals a good congruence between optical and electrochemical measurements and theoretical modeling. In particular, we find a good coincidence between the shell-thickness dependence of the band gap as measured in PL experiments, as determined from CV data, and as calculated. Interestingly, the cyclic voltammograms, which also allow for the determination of absolute electronic energy levels, are rich in features: for various shell thicknesses, several reduction and oxidation features are observed. The comparison of the energy positions and intensities of the CV signals with calculated energy levels and probability density functions from the EMA for different shell thicknesses reveals that several CV signals can be attributed to reduction or oxidation via quantized electronic ground or excited states of the type-II core/shell NCs, while others are assigned to surface states.