Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
In this presentation we investigate the temporal evolution of the fluorescence emitted by individual semiconductor CdSe-CdS dot-rod nanocrystals at cryogenic temperatures. Typically, for colloidal semiconductor nanocrystals the emission wavelength spontaneously shifts on different timescales ranging from below milliseconds up to hours, commonly attributed to spectral diffusion caused by migrating surface charge. We demonstrate, that simultaneously recording of the emission wavelength and the fluorescence decay time can give insight into the localization of the electron- and the hole-wavefunctions inside the nanocrystal. We present experimental results on CdSe/CdS dotrods with type I band alignment leading to a localization of electron and hole inside the CdSe core [1] as well as first results on ZnSe/CdS dotrods with type II band alignment where the hole localizes inside the ZnSe core while the electron can spread over the CdS shell. We find a nearly linear correlation of the emission energy shift and the relative recombination rate evaluated at different times of the recorded data.
The experimental results are compared to model calculations where we simulated the influence of one or more point charges located on the surface of the dotrod on the electron and the hole inside the dotrod. In the calculations, we numerically solve the single particle Schrödinger equations (SDE) of electron and hole using effective mass approximations. Also the coulomb interaction of electron and hole is taken into account by iteratively solving the SDE of the electron (hole) affected by the coulomb potential of the hole (electron) until convergence is achieved. We clearly observe a correlation between the emission energy shift and the relative squared wavefunction overlap (following Fermis golden rule) and the spectral shift from the experimental data.
By comparing the experimental and theoretical results of the two systems with type I and type II band alignment we discuss how the spectral diffusion can monitor the wavefunction localization of electron and hole. While in the case of CdSe/CdS dotrods with type I band alignment the influence of the surface charges on electron and hole wavefunctions is only moderate strong energy shifts and changes of the recombination rates are found in the case of ZnSe/CdS dotrods with type II band alignment.
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