Light-Harvesting Studies in Electrostatically Bonded All-Quantum Dot Assemblies

Abstract

Colloidal semiconductor quantum dots (QDs) have attracted substantial interest in the direction of modern energy research owing to their unique size- and shape-dependent optoelectronic properties. This includes sensors, photovoltaic diodes, display devices, photocatalysis and so on. Besides the size-dependent quantum confinement effect, QD also possesses several other advantages, such as large absorption cross-section, narrow emission bandwidth, superior photostability, and high quantum yield, which makes them an ideal alternative to existing luminescent materials such as organic dyes, inorganic-organic complexes, and so on. This drives the development of donor-acceptor assemblies solely based on QDs for energy harvesting and solid-state lighting applications, a move towards mimicking of natural light-harvesting system. To date, the majority of the developed all-QD based nanohybrid systems are composed of Cd- and Pb-based toxic metal ions, which restricts their use in commercial and industrial applications. In this direction, our main objective was to introduce InP based all-QD assemblies as an environmentally friendly alternative to state-of-the-art Cd-based QDs. However, this requires addressing one of the core issues that the InP family was confronting, which was the lack of complete colour gamut in the visible region. Here, the piece of the jigsaw was to generate pure-blue emitting InP QDs, and that too in aqueous medium. Thus, the proposed thesis aims to develop strategies for the preparation of pure-blue emitting InP QDs, and study the exciton dynamics in fundamental photophysical processes. Light induced energy and electron transfer processes are chosen as two key photophysical processes for this investigation. In our first attempt, strategies were developed to produce InP/ZnS QDs emitting in the pure-blue region, in water. The developed blue-emitting InP/ZnS QDs exhibited excellent cell viability and multicolour bio-imaging ability inside the HeLa cells. Moreover, the ability of InP based pure-blue emitters to participate in an efficient Förster resonance energy transfer (FRET) process was demonstrated in a QD-Dye model donor-acceptor system. Next, in the subsequent work, blue-emitting InP/ZnS QDs were subjected to an efficient electron transfer process in a model QD-dye nanohybrid system composed of blue-emitting InP/ZnS QDs donor and methylene blue acceptor dye. Steady-state and time-resolved spectroscopy studies confirms a photoinduced electron transfer from blue-emitting InP/ZnS QDs to MB dye. After the successful demonstration of energy and electron transfer processes with blue-emitting InP/ZnS QDs, in the final Chapter, a series of all-QD based energy transfer systems were developed at the dyad level, based on blue-, green-, & red-emitting InP/ZnS QD and CuInS2/ZnS QDs (blue-green, green-red, and blue-red all-QD assemblies). Detailed steady-state and time-resolved spectroscopic experiments were performed to conclude the process of energy transfer in these all-QD based donor-acceptor systems. Now, all the primary colors are available, and groundwork is done for the future realization of InP-QD based multicomponent system, at triad and other higher levels. In summary, the demonstration of resonance energy transfer process in all-QD based donor–acceptor systems are fundamentally intriguing and can have far reaching applications in the areas of biophysics as well as other light harvesting applications including photovoltaics and photocatalysis.