Spectroscopy and kinetic studies of electron-hole recombination in CdSe nanoparticles

Abstract

This thesis presents an experimental study of the energy and time-resolved optical response of chemically prepared CdSe nanoparticles with different sizes, shapes (dots, rods, and tetrapods), and lattice structures (wurtzite and zinc blende). The first part of the thesis concerns a model system: spherical CdSe quantum dots with a wurtzite lattice structure. We have investigated their fluorescence spectra and decay kinetics as a function of size, laser excitation power, detection energy, and temperature. The experiments were performed using a nanosecond time-correlated single photon counting technique and have revealed kinetics from different short and long-lived states. In contrast to comparable literature studies, we covered a wide temporal window (up to 1 µs). The study shows at least four order of magnitude of signal to background ratio. Generally, at low excitation power, we always find four independent decay components covering a range of lifetimes between one nanosecond up to hundreds of nanoseconds. The first component is < 2 ns and probably corresponds to fast relaxation and trapping processes. A second component of 3-8 ns can be attributed to the lifetime of charged excitons, while the third one is in the range of 20-30 ns is due to the radiative electron-hole recombination. Finally, a longer decay time of 80-100 ns with low amplitude (< 10 %) appears for all sizes and experimental condition. This component is related to trap states. At high power (corresponding to more than one exciton per particle), an additional fast component, in the same range as the first one (< 2 ns), appears and is due to the multiexciton effect. The second part of the thesis concerns CdSe semiconductor nanocrystals from a slightly different point of view focusing on the crystal structure, by investigating quantum dots with a cubic lattice structure, synthesized in our group. We performed low- and high-resolution luminescence and excitation spectroscopy, and time-resolved spectroscopy as a function of size and temperature from room temperature down to 4K. These measurements reveal the optical properties and relaxation processes, and finally infer the amount of the structure-dependent field effect on the band edge exciton structure. Overall, we observe no difference in all these properties compared to wurtzite dots. We conclude that the crystal field is much less important than shape asymmetries and exchange interaction. The appearance of a permanent dipole moment, in a cubic lattice structure of spherical quantum dots, can explain the similar response of the two different lattice structure dots in our experiments. The third part of this work concerns the investigation of the shape effect on the spectroscopic and kinetic properties of CdSe nanocrystals. New fabrication methods have enabled the synthesis of high-quality CdSe nanorods and tetrapods. We investigated the temperature dependence of the absorption and fluorescence spectra of the different shapes of CdSe nanocrystals in the range of 4 to 300 K. We find that, while the shift of the fluorescence maximum indicates a little dependence on the shape, the broadening of the emission spectrum behaves very differently for dots and rods, indicating major differences in the broadening mechanisms for different shapes. Tetrapods behave more similar to dots, which suggest that the lowest exciton state is centered at the core. This is confirmed by the decay kinetics, which is again identical between dot and tetrapod nanocrystals, while an opposite temperature dependence decay was recorded for the kinetics of nanorods. We attribute this behaviour in tetrapods to the dot like centre. Nanorods show very different kinetics, because the lowest exciton state becomes allowed.