Thesis defense of Gang Qiang

Understanding of the spin and exciton properties in colloidal nanostructures paves the way for their applications in a variety of fields, such as spintronics and quantum science and technology. Using magneto-optical experimental techniques, the fundamental properties such as polarization, g-factor, spin dynamics in three kind of semiconductor nanocrystals (NCs), i.e. CdSe, CuCl and all-inorganic perovskite (CsPbBr3, CsPbI3) are studied, which help to understand the exciton fine structures and related interactions in each material. By combining both the experimental and theoretical efforts, the puzzling behavior of the polarized emission of dark excitons in the ensemble of CdSe NCs is explained by considering the nanocrystal size dispersion and phonon effect. The spin coherent dynamics in the pump-probe measurements show two components, i.e. one oscillating and one nonoscillating. The Larmor frequency from the oscillating component is unambiguously assigned to the electron based on the theoretical analysis. While the nonoscillating component is clarified to be contributed by the frozen exciton spin polarization created by the pump pulse in NCs with heavy-light hole splitting determined by the crystal field. The polarization properties and g-factors in CuCl NCs are investigated comprehensively, where the degree of circular polarization of the Z3 exciton emission is found to increase linearly with the magnetic field at low temperatures up to 8 T. In the spin-flip Raman scattering measurements, a g-factor about 2 is resolved which can be assigned to the electron. The magnetic field and temperature dependent recombination dynamics reveal a dark ground state in both CsPbBr3 and CsPbI3 NCs with corresponding bright-dark exciton splitting being 4.2 meV and 6.5 meV, respectively. In CsPbI3 NCs, very interesting anomalous polarization and spin dynamics are observed, which is related to the interaction between the dark exciton state and the bright exciton fine structure. The spin-flip Raman scattering measurements on CsPbI3 NCs also reveal two g-factors, i.e. one is about 2.5 and another one about 1.5.

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