Optical studies of chemically-synthesized and MBE-grown semiconductor quantum nanostructures

In recent years, there are growing interests around semiconductor quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs), which are important quantum systems in the exciting research fields of nanoscience and nanotechnology. In this dissertation, the optical properties of both chemically-synthesized and MBE-grown semiconductor quantum nanostructures were systematically investigated by photoluminescence (PL) and PL decay measurements. The results obtained from these studies are very useful for both fundamental studies and device applications of these low-dimensional semiconductor quantum nanostructures. The topics covered in this dissertation can be described as follows: (i) The spontaneous emission (SpE) lifetime of colloidal QDs can be greatly modified in the presence of a semiconductor surface and inside the photonic crystals, which is a direct result of the modified photonic mode densities at the QD locations. (ii) The electronic structure transformations among zero-dimensional (OD), one-dimensional (1D), and two-dimensional (2D) quantum nanostructures were studied. The transformation from OD QDs to 1D QWRs was observed in both colloidal CdSe quantum rods and MBE-grown InGaAs/GaAs QD chains. We also observed the electronic structure transformation from 1D QWRs to 2D QWs in MBE-grown InGaAs/GaAs QWRs fabricated on (331)B GaAs templates with nanoscale fluctuations. (iii) The surface states in colloidal QDs were studied by both time-resolved and PL up-conversion spectroscopies. We observed lattice contraction from highly-luminescent colloidal CdSe QDs with the corresponding surface-related emissions, which can be explained by the existence of surface states. We also used up-converted PL (UCPL) to probe the electron and hole surface states within the energy bandgap of colloidal CdTe QDs. (iv) The electronic coupling or energy transfer processes between both MBE-grown and colloidal QDs were studied in the forms of carrier transfer and dipole coupling, respectively. A more complete picture of the energy and carrier transfer processes in multi-modal and high-density MBE-grown InAs QD systems was uncovered by the intensity and temperature dependent optical studies. It was also shown that electrons yielded by photo-irradiation can assist the oxidation process of colloidal CdSe QDs, which can significantly improve PL intensity because the coupling strength is significantly enhanced.