Browsing by Author "Krummel, Amber, advisor"
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Item Embargo Charge carrier dynamics of 2-dimensional photoelectrodes probed via ultrafast spectroelectrochemistry(Colorado State University. Libraries, 2024) Austin, Rachelle, author; Sambur, Justin, advisor; Krummel, Amber, advisor; Rappe, Anthony, committee member; Prieto, Amy, committee member; McNally, Andrew, committee member; Brewer, Samuel, committee memberThe integration of hot charge carrier-based energy conversion systems with two-dimensional (2D) semiconductors holds immense promise for enhancing the efficiency of solar energy technologies and enabling novel photochemical reactions. Current approaches, however, often rely on costly multijunction architectures. In this dissertation, I present research that combines spectroelectrochemical and in-operando transient absorption spectroscopy measurements to unveil ultrafast (<50 fs) hot exciton and free charge carrier extraction in a proof-of-concept photoelectrochemical solar cell constructed from earth-abundant monolayer (ML) MoS2. Theoretical analyses of exciton states reveal enhanced electronic coupling between hot exciton states and neighboring contacts, facilitating rapid charge transfer. Additionally, I discuss insights into the physical interpretation of transient absorption (TA) spectroscopy data in 2D semiconductors, comparing historical perspectives from physical chemistry and solid-state physics literature. My perspective encompasses various physical explanations for spectral features and experimental trends, particularly focusing on the contribution of trions to TA spectra. Furthermore, I examine how different physical interpretations and data analysis procedures can yield distinct timescales and mechanisms from the same experimental results, providing a comprehensive framework for understanding charge carrier dynamics in 2D semiconductor-based optoelectronic devices.Item Open Access Developing tools to study the interaction between the lipopeptide surfactin and phospholipid bicelles with infrared spectroscopy(Colorado State University. Libraries, 2012) Blaser, Jennifer M., author; Krummel, Amber, advisor; Levinger, Nancy, committee member; Kipper, Matthew, committee memberSurfactin has been shown to have concentration-dependent effects on lipid membranes with proposed mechanisms of action including ion chelation, ion channel formation, and a detergent-like effect. The concentration ranges for these behaviors have not been established, the structure of surfactin in a membrane has not been determined, and information regarding the dynamics of the surfactin-lipid interaction is limited at best. Therefore, a tunable phospholipid bicelle system was created to study the surfactin-lipid interaction as a function of surfactin concentration using infrared (IR) spectroscopy which can provide both structural and dynamic information. But first, the direct interaction between surfactin and bicelles was confirmed with dynamic light scattering (DLS) measurements that suggest surfactin exhibits detergent-like effects above a 2.0 mM concentration. For surfactin in Tris buffer, the IR spectra displayed a significant concentration-dependent shift in the amide-I band and a distinct change in the amide-I to amide-II band intensity ratio. These data indicate that surfactin experiences a conformational transition over the concentration range studied. The conformational transition may occur due to the formation of surfactin micelles and higher order aggregates upon increasing concentration. Surfactin was also studied in the presence of phospholipid bicelles. At low surfactin concentrations in the presence of bicelles, the amide-I band exhibits nearly identical spectral features to those found for higher concentrations of surfactin in Tris buffer, and the amide-I to amide-II band intensity ratios showed similar trends. The results of these studies indicate that the conformation of surfactin may be similar in micelles, higher order aggregates, and bicelles with the bicelles limiting the conformational distribution of the surfactin molecules. Additional studies are necessary to determine surfactin's structure in these model membranes and obtain dynamic information to better understand the mechanism of the surfactin-lipid interaction.Item Open Access Investigation of chiral porphyrin aggregates with heterodyne-detected vibrational sum frequency generation spectroscopy(Colorado State University. Libraries, 2018) Lindberg, Kathryn A., author; Krummel, Amber, advisor; Levinger, Nancy, committee member; Sambur, Justin, committee member; Gelfand, Martin, committee memberIn nature, photosynthetic organisms harvest and transport solar energy through the finely-tuned interplay between vibrational, electronic, and excitonic characteristics within photosynthetic reaction centers. These characteristics depend intimately on the precise arrangement of the reaction centers' molecular building blocks. Further knowledge of the relationship between structure and function in these natural systems is key to advancing synthetic solar technologies like dye-sensitized solar cells and artificial photosynthesis. Photosynthetic pigments, such as chlorophyll and bacteriochlorophyll, are of particular interest since their absorptive role is the first step in the solar harvesting process. Porphyrins, a group of macrocyclic organic compounds closely related to these pigments, have gained attention as simpler models for their more complicated natural counterparts. Tetra(4-sulfonatophenyl) porphyrin (TSPP), which closely resembles bacteriochlorophyll, is particularly valuable because it forms molecular aggregates analogous to the highly quantum-efficient light-harvesting "antennae" present in green sulfur bacteria chlorosomes. Imaging and spectroscopic studies indicate that the helical nanotubular TSPP aggregates are chiral and have distinct exciton contributions along different axes. However, the precise arrangement of TSPP monomers within the aggregate walls is still debated, prompting further, more detailed studies. Heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy is a phase-sensitive, second-order nonlinear technique which probes the vibrational characteristics of noncentrosymmetric molecular environments. HD-VSFG experiments can also probe excitonic and vibronic characteristics via experimental double resonance. By use of polarization conditions, theoretical modeling, and computational fitting, detailed information on the orientation of vibrational, vibronic, and excitonic transition dipoles can be extracted from HD-VSFG spectra. This work presents doubly-resonant HD-VSFG spectra of TSPP thin films drop-cast on gold, which demonstrates the technique's sensitivity to the relationship between complex phase and excitonic versus monomeric characteristics. HD-VSFG is then used to compare spectra of TSPP thin films prepared from racemic and chiral aqueous solutions. This comparison includes a polarization condition sensitive to only chiral environments, further demonstrating HD-VSFG as a valuable tool in the structural investigation of TSPP aggregates.