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Title:Direct visualization of electron transport among nonconjugated polymeric redox-active colloids via fluorescence microscopy
Author(s):Qu, Alan (Subing)
Director of Research:Braun, Paul V.
Doctoral Committee Chair(s):Braun, Paul V.
Doctoral Committee Member(s):Chen, Qian; Rodríguez-López, Joaquín; Schroeder , Charles M.; Evans, Christopher
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Organic electronics
Colloid Physics
Fluorescence Microscopy
Abstract:Redox-based electrical conduction in nonconjugated/redox active polymers has been explored only less than a decade, yet it is already showing promise as the basis for conductivities and charge transport (CT) distances relative to well-studied conjugated polymers. My Ph.D. research has been devoted to developing a new imaging methodology based on fluorescence microscopy, to directly visualize electron transport processes among micron-sized redox active colloids (RAC) containing high densities of ethyl-viologen (EV) pendant groups. This observation was enabled by the discovery that these RAC exhibit a highly non-linear electrofluorochromism, which can be quantitatively coupled to the colloid’s state-of-charge (SOC). The mechanism of ultrasensitivity in the colloids’ quenching behavior was also discussed in the thesis. By correlating the redox state of RAC to their fluorescence intensity, I developed a working curve that is robust at varying scan rates and thus renders RAC a novel kind of colloid that can serve as a redox-meter, whether for itself or for other electrochemical systems. Subsequently, the thesis is primarily divided into two vignettes: one focused on RAC in a static scenario and the other probing into RAC’s energy-exchanging behavior in the dynamic regime. 1) Through using monolayers and sub-monolayers of RAC, electron transport between these polymeric particles and the underlying metallic electrode was visualized. And due to the percolating nature of the monolayer/submonolayer, under chronoamperometry around 10 µm intercolloid redox-based electron conduction was also successfully visualized. This observation offers experimental evidence that long-range electron hopping can occur merely through physical touching between particles at different energetic states, which was unproven prior to this work. By evaluating the quasi-Fickian nature of the charge transport kinetics, an apparent charge transport diffusion coefficient DCT was extracted in a simple manner directly from optical imaging data without resorting to electrochemical modeling or scanning electrochemical microscopy (SECM). Along with addressing more fundamental questions regarding collision and energy transport in colloidal suspensions, understanding energy transfer in redox-active polymers and particulate media has potential to impact flow batteries, organic electronics and accelerate discovery of redox-based conducting polymers with improved CT properties. 2) The working curve developed in Vignette One equipped us to further study electron/energy transfer in dilute and concentrated suspensions of RAC, where interactions and collisions are strictly pairwise. In Vignette Two, functionalizing the metallic electrode with short polymer brushes of various kinds was first tried to minimize non-specific adsorption between RAC and the metallic electrode. Moderate progress was achieved in this branch of efforts. On top of that, to bypass the issue of electrode interference entirely, I designed microfluidic devices where two streams of fully oxidized and reduced RAC were flowed in and their interactions were imaged. Owing to the small number of electrons exchanged per collision event, bright (oxidized) particles’ locations and fluorescence intensities were tracked at the single-particle level while multiple dark (reduced) particles hit them. After calibrating for photobleaching effect, we quantified the average fluorescence intensity decrease upon each collision to be 0.40%, which aligns with previously established kinetics in the viologen-based redox active polymer system. This observation indicates biomimetic behavior of this particulate materials system (akin to information and energy exchange of cells) which opens doors to the possibility of building a statistical mechanical theory for dense suspensions of these ‘energy packets’, whose interaction potential will temporarily evolve, leading, we hypothesize to unique dynamics.
Issue Date:2020-12-04
Rights Information:Copyright 2020 Alan (Subing) Qu
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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