Novel type-II nanocrystal quantum dots and versatile oligomeric phosphine ligands

Abstract

This thesis describes the syntheses, characterizations, and applications of novel type-II quantum dots and versatile oligomeric phosphine ligands. Type-II band engineered quantum dots (CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures) are synthesized by chemical means. The optical properties of these type-II quantum dots are studied in parallel with their type-I counterparts. The spatial distributions of carriers are controlled within the type-II quantum dots, which make their properties strongly governed by the band offset of the comprising materials. This allows access to optical transition energies that are not restricted to bandgap energies. The type-II quantum dots can emit at lower energies than the bandgaps of comprising materials. The type-II emission can be tailored by the shell thickness as well as the core size. The enhanced control over the carrier distributions afforded by these type-II materials is useful for many applications such as photovoltaics, and photoconduction devices. A new family of oligomeric alkyl phosphine ligands is synthesized for quantum dots. These oligomeric phosphines show effective binding affinity to quantum dot surfaces. They form thin and secure organic shells that stabilize quantum dots in diverse environments. They maintain high photoluminescence quantum yield of the quantum dots, and enable homogeneous incorporation into various matrices. They present a chemically flexible structure that can be used for further chemistry, such as crosslinking, co-polymerization, and conjugation to biomolecules. Selection of optimal quantum dot wavelengths for biomedical assays and imaging is studied by simple mathematical modeling.

(cont.) Exploiting the design flexibilities of type-II quantum dots and the superior stabilities from oligomeric phosphine ligands, aqueous soluble near-infrared quantum dots for biomedical imaging are obtained. They are optimized for the emission wavelength, absorption cross-section, hydrodynamic size, and photoluminescence quantum yield. Using these quantum dots in small and large animal model systems, sentinel lymph node mapping, a major cancer staging procedure, is successfully performed in real-time using near-infrared light. Oliogomeric phosphine ligands with polymerizable moieties are used to incorporate quantum dots into polymer microspheres. The ligands enable co-polymerization of quantum dots with heterogeneous polymer matrices without phase separations or aggregations. Their potential applications for full color, particle-based displays, and information storage are demonstrated.