Browsing by Subject "Charge Transfer"
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Item Continuous and Reversible Modulation of Interfacial Electrochemical Phenomena on Back-Gated Two Dimensional Semiconductors(2017-08) Kim, Chang-HyunThe continuous increases in global population and energy consumption have raised major concerns about the security of our energy future. Limited amount of fossil fuels and increasing concerns over their combustion product, carbon dioxide, on climate change make renewable energy sources, such as sun light and wind, attractive options. Accordingly, energy conversion and storage devices based on (photo)electrochemical processes (e.g., batteries, fuel cells, water splitting, solar-to-fuel conversion, etc.) have received great attention as promising solutions to overcome the challenges in the intermittent renewable energy sources. To develop highly efficient and economically viable energy devices, fundamental understanding of electrochemical phenomena occurring at the electrode/electrolyte interfaces is essential. In this dissertation, we introduce a new approach, inspired by the operating mechanism of field-effect transistors (FETs), to modify and study such electrochemical interfaces. The devices studied in this dissertation project have a “gate-insulator-electrode” stack structure, which is essentially similar to that of a FET but the (typically) thick semiconductor layer in regular FETs is replaced with an ultrathin or two-dimensional (2D) active electrode for electrochemical processes in our devices. In such a device, due to the extreme thinness of the active electrode, electronic properties at the electrode surface can be dramatically altered by extra charge carriers induced with a voltage bias to the gate. This, in turn, makes thermodynamics and kinetics of electrochemical processes at the electrode/electrolyte interface be determined by the gate bias as well as by the electrode potential (with respect to the solution or reference electrode potential). In this project, three interfacial electrochemical phenomena are of our main interest: (1) electric double layer charging, (2) electron transfer across interface, and (3) surface binding of reaction species on electrode surface. First, responses of electric double layer structure to the gate bias is investigated using graphene devices, and a method to experimentally separate the band filling potential and the double layer charging potential has been developed. Second, continuous and reversible modulation of outer-sphere electron transfer kinetics by a gate bias was demonstrated on ZnO devices for the first time using cyclic voltammetry. Third, quantitative analysis of the electron transfer kinetics has been conducted using microchannel flow cells, in which continuous supply of fresh electrolyte through the microfluidic channel generates time-invariant diffusion layers near the active electrode surface, allowing electrochemical measurements in steady states. To collectively explain our observations, a simple but very useful physical model is proposed; the model indicates that the observed changes in the interfacial electrochemical phenomena essentially result from the gate-induced band alignment shift at the electrode/electrolyte interfaces. Lastly, based upon the results and the insight gained from the previous experiments, possibilities and challenges in field-effect control of surface binding energies in electrocatalytic systems are explored, and rational strategies to overcome the difficulties are proposed.Item Investigating Charge Transfer Dynamics in Organic Crystals and Photocatalytic Solutions with Femtosecond Stimulated Raman Spectroscopy(2019-12) Cassabaum, AlyssaFemtosecond stimulated Raman spectroscopy (FSRS) is a non-destructive vibrational technique that is used to track evolving structural dynamics over the course of a photoinduced reaction. Due to its high spectral (cm-1) and temporal (fs) resolution, FSRS is able to follow spectral signatures as a function of time relative to photoexcitation, discerning ultrafast reaction dynamics and providing chemically specific structural information on relevant timescales of chemical reactivity, thus enabling the identification of structure-function relationships. Historically, FSRS has been used to determine polymorph identity via phonon mode assignments, map exciton transport, and isolate essential molecular structures in charge transfer processes. This thesis discusses the improvements made to the FSRS technique, enhancing its ability to probe solid state materials, and how FSRS can be used to follow charge transfer processes in organic single crystals and homogeneous photocatalytic systems. Initial studies utilize FSRS to probe the effects of crystal orientation and laser polarization on the charge transfer process in betaine-30. This work demonstrated that changing the crystal orientation does not always have the same effect as changing the laser polarization and further examined differences in ultrafast chemical dynamics at various polarization and orientation combinations. Next, I explore intermolecular charge transfer dynamics in an organic co-crystal of perylene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Using FSRS, I track the photoexcitation of perylene and subsequent charge transfer to TCNQ within a single crystal and examine the kinetics of the molecular dynamics tracked throughout the charge transfer process. Finally, I investigate the photoinduced reduction of carbon dioxide in a homogeneous photocatalytic system. In this chapter, I demonstrate my ability to spectroscopically detect methanol, the desired reduction product, and discuss my efforts to investigate the ultrafast dynamics and elucidate the mechanism of carbon dioxide in this specific system. Herein, I demonstrate the ability of FSRS to examine charge transfer processes in a variety of material states. This thesis focuses on the improvement to FSRS to better probe charge transfer events in solid state materials, discusses my results in both solid-state and liquid samples, and provides insight into future avenues for promising research studies.