Electronic structure and optical properties of lead sulphide and lead selenide nanocrystal quantum dots.

摘要

The electronic structure and optical properties of PbS and PbSe nanocrystal quantum dots are the main topics of this dissertation. In chapter 1, recent advances in the study of II-VI quantum dots are briefly summarized and the investigation of PbS and PbSe quantum dots is motivated. Chapter 2 presents a detailed description of the electronic and linear optical properties of PbS and PbSe quantum dots based on a four-band envelope-function formalism. This is the first self-consistent multi-band calculation of the electronic structure of these materials. The electronic structure of PbS and PbSe quantum dots is successfully described using a four-band {dollar}kcdot p{dollar} Hamiltonian which accounts for the band anisotropy and interband interaction of PbS and PbSe. The effects of many-body interactions and the intervalley interaction are also given. Chapter 3 describes a novel experimental technique, spectrally-resolved two-beam coupling, for measuring very small optical nonlinearity. The technique is based on spectrally resolving a weak probe beam which is modulated by a strong pump beam in a nonlinear medium. It is pertinent for measuring the nonlinear response of dilute quantum dot samples since the nonlinear response of individual nanocrystals can be obscured by the response of the host material. In Chapter 3, the results of two-photon absorption spectroscopy of PbS quantum dots are presented. For systems with inversion symmetry, two-photon spectroscopy probes optical transitions not allowed by the one-photon selection rules, and thus gives complementary information of the electronic structure. Unlike for II-VI quantum dots, two-photon spectroscopy of PbS quantum dots should yield unambiguous information on the symmetry of the electronic states due to the centrosymmetric lattice and simple electronic structure. The measurement reveals that the electronic states of PbS quantum dots are not parity eigenstates. The implications of this measurement for other quantum dots are discussed. In chapter 5, some thoughts on future directions in the research of PbS and PbSe quantum dots are given. Finally description of the numerical routines that are needed to calculate the electronic structure of PbS and PbSe quantum dots is given in appendix.