Title

Ultrafast Relaxation Dynamics in Graphene Oxide-Dye and Perovskites Nanocomposites

Date of Award

12-2019

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dr. Ramakrishna Guda

Second Advisor

Dr. Ekk Sinn

Third Advisor

Dr. Charles Ide

Fourth Advisor

Dr. Sherine Obare

Keywords

Ultrafast relaxation dynamics, graphene oxide dye, perovskite nanocomposites

Abstract

Novel materials such as graphene oxide (GO), reduced graphene oxide (RGO), and Perovskites nanocomposites nanosheets have shown interesting electrical and optical properties. These materials have shown prominence in research regarding optical sensing applications. The interaction of different fluorescent molecules like dye molecules with GO, and RGO have been studied recently to develop novel optical sensors, photo-catalysts, and light-harvesting agents. In this study, we have monitored the excited state interactions of dyes covalently attached to GO and RGO nanosheets. Three amine derivatives of anthracene, pyrene, and coumarin were covalently bound to different systems via amide bonds and diazotization. Characterization of different systems via TEM, infrared spectroscopy, and optical absorption measurements have shown that the dyes changed upon binding. The fluorescence of the dyes was quenched upon binding to GO and RGO; suggesting efficient excited-state interactions. Time-resolved fluorescence and ultrafast transient absorption measurements suggested charge-transfer interactions for the dyes when they are bound the systems. Ultrafast transient absorption decay traces supported by transient absorption anisotropy data shed light on charge transfer and energy transfer interactions. The results show that there is an efficient electron transfer from Anthracene to RGO with time constants of 230 fs and 2.1 ps. However, the charge recombination was found to be equally faster with time constants of 23 ps and 1 ns. Contrarily, electron transfer from RGO to pyrene was observed with a time constant of 3.5 ps.

Regarding cesium lead halide perovskites (CsPbX3), we have synthesized and characterized CsPbX3 perovskites that can form nanosheets at room temperature synthesis conditions and nanocrystals at high temperatures. The synthesis has yielded highly luminescent perovskite with their absorption and luminescence being adjustable in the entire visible region by changing the halide composition from chloride to Iodide. The photoluminescence (PL) was narrow for crystals while it is broader for nanosheets suggesting the presence of more surface states in nanosheets. Ultrafast luminescence upconversion luminescence measurements carried out for sheets, and crystals have shown that the luminescence decay is faster for nanosheets compared to nanocrystals, suggesting efficient leakage of charge carriers to surface states. On the other hand, femtosecond transient absorption measurements have shown similar transient features for both sheet and crystals forms with dominant excited state absorption and stimulated emission. We have probed the two-photon absorption (2PA) cross-sections of CsPb-halide perovskites using a two-photon excited fluorescence technique. The 2PA cross-sections were found to be in the order of 106 GM for perovskites, while normal organic dye molecules have cross-sections reaching only 100 GM. The fact that these nanoperovskites can be easily made into thin films and increase in photostability makes these perovskites nanoarchitectures ideal for broadband optical sensor protection systems.

Our results have shown interesting large cross-sections for both sheets and crystals. The larger optical nonlinearities can be ascribed to greater number of freely moving excitons in perovskites. Synthesis of cesium lead halide perovskite nanocrystals at different reaction conditions using different ligands to improve the water solubility of such systems for biophoton imaging and luminescent biosensors application. To investigate block copolymers as ligands and shown increase the water solubility for some provskites.

Access Setting

Dissertation-Campus Only

Restricted to Campus until

12-2020

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