Spin Polarization and Dynamics of DMS Colloidal Quantum Dots
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CdSe colloidal quantum dots (QD) is probably one of the most studied QDs due to their tunable bandgap around visible range by quantum confinement. This thesis is composed of three parts that are all based on the investigation of interactions between the carriers and Mn 2+ ions incorporated into colloidal CdSe QDs. In the first part, we studied photoluminescence (PL) of Mn 2+ doped CdSe QDs at different sizes. We demonstrated excitonic energy of the QDs can be adjusted to lie below or above the Mn 2+ characteristic emission by adjusting the size of the QDs and hence quantum confinement. In each case, we investigated PL of Mn 2+ doped QDs. We demonstrated that for highly confined QDs PL is dominated by Mn 2+ internal transition emission and PL lifetime of excitonic emission is greatly reduced by rapid energy transfer from the conduction band to Mn 2+ excited states. In contrast, for bigger QDs, we showed that PL is dominated by excitonic emission and PL lifetime of the excitonic state is three orders of magnitude longer than the PL lifetime of smaller QDs. In the second part, we investigated dynamics of formation of magnetic polarons and carrier spin polarization in Mn 2+ doped CdSe QDs using continuous wave magneto-PL and time resolved magneto-PL spectrospies. In Mn 2+ doped CdSe QDs, we observed large Zeeman splitting and Landau g factor of two orders of magnitude larger than the g factor for CdSe. Formation of magnetic polarons and carrier spin polarization were measured simultaneously through time resolved magneto-PL spectrospy. The observed characteristic time scale of magnetic polaron formation of ∼700 ps is surprisingly one order of magnitude longer than the carrier spin relaxation time of ∼ 60ps. We attribute this difference to the relatively slow alignment of the correlated Mn 2+ ion spins within the exchange field of the carriers. In addition, the increase in the degree of Mn 2+ spin alignment during magnetic polaron formation leads to an increase in the carrier spin polarization. In the last part, we investigated the exchange interactions in the CdSe-CdMnSe, core-shell QDs and demonstrated sign reversal of carrier spin polarization through magneto-PL spectroscopy. The sign reversal in the carrier spin polarization is explained by sign reversal of exchange interactions between conduction band electrons and Mn 2+ ions due to quantum confinement, together with a different degree of spatial overlap between the electrons and holes and Mn 2+ ions in the CdSe-CdMnSe QDs through band engineering.