Electronic structure and optical properties of quantum dots

Because of complete (three dimensional) confinement, quantum dots have a most dramatic quantum size effect. Due to the finite size of the dot, the conduction and valence bands of semiconductor dots are quantized and quantum dot spectra exhibit a series of discrete electronic transitions and depend strongly on the size of the nanocrystal.; In this thesis we study the electronic structure and the optical properties of semiconductor quantum dots and semiconductor quantum dot systems. Different properties and different dots and dot systems are described.; The first topic is the electric polarization in a semiconductor dot (II-VI compound). A simple theoretical model for the origin of spontaneous polarization in single nanocrystals is developed, based on the proposal that the origin of the spontaneous polarization is in the strained layer between “cap” and the nanocrystal. The internal electric field in the dot is due to the piezoelectric effect caused by the strain existing in the interface region of material with different lattice constants. Based on spherical rotation symmetry without inversion SO(3), the model employs a distribution of polarization with symmetry which is a subgroup of SO(3), consistent with the hexagonal structure of wurtzite structure.; The second topic we study is a distribution of many quantum dots, which are arranged together in an array. We present a new model to implement organic exciton-inorganic (semiconductor) exciton hybridization. We consider embedding a quantum dot array into an organic medium. A Wannier-Mott transfer exciton is formed when the exciton in each semiconductor dot interacts via multipole-multipole coupling with other excitons in the different semiconductor dots of the array. A new hybrid exciton appears in the system owing to strong dipole-dipole interaction of the Frenkel exciton of organic molecules with the Wannier Mott transfer exciton of the quantum dot array. This hybrid exciton has both a large oscillator strength (Frenkel-like) and a large exciton Bohr radius (Wannier-like). At resonance between these two types of excitons, the optical non-linearity is very high and can be controlled by changing parameters of the system such as dot radius and dot-dot spacing.; As the third topic, which differs from the pure nanocrystal in the above study, we also present our study on Mn-doped semiconductor nanocrystals such as the ZnS:Mn quantum dot. The effect of an extra electron “injected” into the doped quantum dot with a substitutional Mn 2+ at the center is considered. The electron confined in the dot will be strongly coupled by exchange interaction with the Mn ion, and will split and mix Mn crystal-field energy levels. As a result, this will strongly break the previous selection rules. The optical transition of interest is the 4T₁ − 6 A₁ transition. Using this model we evaluate the energy structure, wavefunctions, luminescent efficiency and transition life time of a Mn doped quantum dot and compare our results with experimental data.