Quantum Dot-Organic Dye Conjugates: Synthesis and Characterization for Energy Transfer Systems

Colloidal semiconductor nanocrystals, also known as quantum dots (QDs), have been employed in diverse applications including biological imaging and sensing as well as in photovoltaic and energy harvesting systems. QDs add value due to their exceptional photostability and size-dependent optoelectronic properties. They are also good substrates for the study of energy transfer mechanisms. This dissertation includes research on the synthesis and characterization of QDs to create a greater understanding of energy transfer processes at the nanoscale, which will impact the development of biological sensing and photovoltaic applications. Indium phosphide quantum dots are a less toxic alternative for cadmium-based materials that are near ubiquitous. Currently, the synthesis of InP QDs is problematic due to the limited availability of competent phosphorus reagents, many of which are overly reactive. To address this challenge, sterically hindered tris(triethylsilyl) phosphine and tris(tributylsilyl) phosphine were synthesized and applied to prepare InP QDs and other materials. Our hypothesis was that the steric encumbrance would minimize the reactivity of the reagents and lead to enhanced monodispersity. And while it was found that the quality of the InP QDs were improved in terms of the quantum yields and emission color saturation, these enhancements were relatively modest. Nevertheless, these reagents add value to the field of materials synthesis due to their reduced pyrophoricity and the fact that they can be applied to create several types of phosphide semiconductors and organophosphorus compounds. Concerning energy harvesting and biological imaging, coupled quantum dot–organic dye chromophores have gained considerable attention as they enable new functionalities that add value to these applications. To develop a microscopic picture of the energy transfer between a QD donor and a dye acceptor, it is necessary to investigate the FRET pairs at the single construct level. This is due to the dynamics of fluorescence intermittency (“blinking”), which is the switching between “On” and “Off” states that we hypothesize influences FRET energy transfer. Several QD-dye coupled chromophores were prepared, and the fluorescence intensity of the dye acceptors were characterized using TIRF microscopy. Statistical characterizations of the blinking trajectories revealed that the average on/off durations of dye acceptors within QD–dye chromophore pairs are significantly reduced by the blinking behavior of the QD donors. In biological applications, this effect is advantageous as it minimizes the photobleaching of the acceptor dye. Concerning alternative energy, however, the consequences are less favorable as the capacity of a device that incorporates QD-dye chromophores will have a 95% reduction in the energy throughput. During this study it was also demonstrated that the paradigm of power-law statistical behavior of QDs is not fully correct. Rather, QDs may display log-normal behavior under several conditions, which is consistent with a model for blinking whereby electrons become localized in trap states by crossing over a fluctuating kinetic barrier. These observations not only challenge a 20 year paradigm of the optical properties of nanomaterials, it also simplifies our basic understanding of quantum dot photochemistry.