Photochemical Sciences Ph.D. Dissertations

Energy Transport in Colloidal Inorganic Nanocrystals

Date of Award

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Mikhail Zamkov (Advisor)

Second Advisor

Jenny Toonstra (Other)

Third Advisor

Liangfeng Sun (Committee Member)

Fourth Advisor

Alexey Zayak (Committee Member)

Abstract

Excitonic energy transfer (ET) represents the primary step of energy conversion during photosynthesis and is the key mechanism of the energy flow in excitonic solids and organic crystals. Unlike the charge-mediated energy transfer in bulk semiconductors, energy transfer in most nanoscale systems proceeds through the electrically excitons transport. The donor and acceptor can be any optical materials such like quantums, metal ions and organic crystals. Here, we demonstrate a strategy that can be utilized as a simple post synthetic procedure for controlling the surface chemistry, adjusting the average particle size and reducing the particle size dispersion of semiconductor colloids. The low dispersion of nanocrystal shapes can facilitate the energy transfer efficiency between donor and acceptor nanoscale materials. After making particles uniform, we show the energy transport in both metal and semiconductor systems. Typically, the interaction of molecular fluorophores with surface plasmons in metals result in either quenching or enhancement of the dye excitation energy, we demonstrate that fluorescent molecules can also engage in a reversible energy transfer with proximal metal surface, the quenching of the dye emission via the energy transfer to localized surface plasmons can trigger delayed ET from metal back to the fluorescent molecule. The resulting two-step process leads to the sustained delayed photoluminescence (PL) in metal-conjugated fluorophores. In the meantime, artificial solids of CsPbBr3 perovskite nanocrystals are well known for their promising charge transport characteristics, we show that the same set of electronic properties allows CsPbBr3 NC solids to act as superior energy transport materials, which support a long-range diffusion of electrically neutral excitons. By performing time-resolved bulk quenching measurements on halide-treated CsPbBr3 NC films, we observed average exciton diffusion lengths of 52 and 71 nm for I−- and Cl− -treated solids, respectively. Steady-state fluorescence quenching studies have been employed to explain such a large diffusion length as due to a high defect tolerance and a low disorder of exciton energies in CsPbBr3 NC solids. The long-range exciton transport ability of halide treated CsPbBr3 NC solids could be beneficial for applications in light energy concentration, as was demonstrated in this work through energy transfer measurements in assemblies of perovskite NC donors and CdSe quantum dot acceptors.

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