Carrier Spin Polarization in Quantum Confined System
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In this dissertation, systematic studies of magneto-photoluminescence of manganese-doped lead salt (Mn doped lead sulfide and lead selenide) quantum dots and core/shell quantum dots will be presented. It was observed that large carrier spin polarization can be obtained in manganese doped IV-VI lead salt quantum dots upon excitation by circularly polarized light; and a clear dependence of spin polarization on the sizes of quantum dots and thickness of core/shell quantum dots was observed as well. The quantum dots in this study were synthesized by solution-phase chemical method. The sizes of the quantum dots could be controlled by the growth temperature (50 to 150) and growth time (1min to 24hr). The doping concentrations in the compounds Pb 1-x Mn x S (or Pb 1-x Mn x Se) ranging from x = 0 to 8% can be adjusted by varying synthesis conditions as well. Studies on size dependence (from 3 to 10 nm), temperature dependence (from 7K to 50K), magnetic field dependence (from 0 to 7T), and laser power dependence (from 1mW to 20mW) of photoluminescence intensities and peak positions were systematically carried out. The spin-polarization, which was directly calculated from magneto-luminescence measurements, was studied as a function of quantum dot sizes, temperatures and magnetic fields. It was observed that, depending on the sizes and growth conditions (growth temperature and time), the spin polarization, as large as 40% at 7K in a 7T magnetic field, could be tuned in magnitude. Core/shell structured quantum dots with Mn 2+ ions doped in the inner core or outer shell were also studied. The spin polarization was observed in core/shell system to decrease as thickness of the shell increases. We believe the wave function overlap between carriers and dopants can be tuned by quantum confinement and therefore the magnitude of exchange interactions can be tuned via varying the sizes of quantum dots or the shell thickness of core/shell quantum dots. There are a couple of measurements that showed inversion of spin polarization but the reproducibility is low. Further studies are needed to verify the possibility of reversing the sign of spin polarization through quantum confinement, i.e., varying the sizes of quantum dots or the shell thickness of core/shell quantum dots.