Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
The optical properties of elongated semiconductor dot-rods (DRs) comprising a CdSe core (dot) surrounded by an elongated shell of CdS (rod) have been investigated for several years by now. In general it is believed that the photo generated holes are strongly localized in the CdSe core, while the photo generated electrons have a large degree of freedom in the length direction of the rod, due to very small band offsets of the respective conduction bands and low effective masses of the electrons. This leads to interesting optical properties such as large absorption thresholds and 1D transition dipole moments. However, the detailed optical properties strongly depend on the degree of charge carrier localization, which is strongly dependent on length and diameter of the DRs, as well as on fluctuating excess charges in and around the DRs. Here we present a detailed optical and theoretical study, where we compare the optical and electronic properties of red (λPL > 600 nm) and green (λPL < 550 nm) emitting CdSe/CdS DRs. For example we investigate the fluorescence blinking and polarization anisotropy of single DRs in dependence of the DR length and diameter. Moreover upon recording the fluorescence spectra of individual DRs at low temperature (T < 5K) for time periods of several hours we observed temporary spectral jumps of up to 30 meV and changes in the corresponding fluorescence lifetime. To explain the observed results we developed an effective mass model based on discretized potentials and wave functions. In general this model allows for the calculation of transition energies and wave functions of the corresponding electrons and holes in arbitrary potentials, including potential modifications through a mutual interaction of charge carriers. Here we used this model to calculate the degree of electron delocalization for green and red DRs with different radial confinement, assuming a variety of band offsets as discussed in the literature. Also we could show that the spectral jumps are most likely caused by excess charge carriers localized at different positions on the DR-surface. For example we calculated how surface charges would modify the valence- and conduction band potentials, and affect the transition energies and fluorescence lifetimes for the photo excited electron hole pairs.
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