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Browsing by Author "Sambur, Justin, advisor"

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    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 member
    The 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.
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    Measuring dissolution rates and interfacial energetics of monolayer molybdenum disulfide electrodes in electrochemical systems
    (Colorado State University. Libraries, 2023) Toole, Justin R., author; Sambur, Justin, advisor; Henry, Chuck, committee member; Ackerson, Chris, committee member; Field, Stuart, committee member
    Meeting carbon zero goals within the next few decades requires advances in energy conversion efficiency, and hydrogen fuel is believed to be a key part of the solution. Photoelectrochemical (PEC) devices can contribute to a renewable-based energy portfolio by directly producing storable chemical fuels. The electrode is a key component that determines what is thermodynamically and kinetically possible for a given PEC device. Unfortunately, semiconductor electrode efficiency can come at the cost of chemical stability. Also, the energetic description of an ultra-thin semiconductor electrode at the liquid interface is unclear. Here, we studied molybdenum disulfide (MoS2), a promising two-dimensional (2D) semiconductor, to improve understanding of interfacial energetics and electron transfer. The overarching hypothesis of this work is: if we quantitatively measure band energies of this 2D material, then we improve understanding of electron transfer efficiency and rates for involved chemical reactions. Knowledge from this research informs new ways to reduce solar energy conversion losses and may improve control over chemical reactions. Our experimental approach is to make in situ optical measurements while changing two key variables: (1) the electrode applied voltage (E), and (2) the liquid redox electrolyte environment (E0'). This thesis is organized into six chapters. Chapter 1 motivates semiconductor photoelectrochemistry as a viable approach for solar energy and chemical fuel production. Following the chronology of key scientific advances over the past few decades, Chapter 2 delves deeper into the established principles of semiconductor photoelectrochemistry, the unique properties of monolayer MoS2, and the current state of the field for making in situ optical measurements in an electrochemical cell. This chapter concludes with open questions that are addressed in Chapters 3 – 5. In Chapter 3, the stability of MoS2 is tested by literally pushing the semiconductor to its anodic decomposition limit. The crucial results are identification of the MoS2 dissolution onset potential (ED) and its thickness-dependent dissolution rates. Additional insights pertain to the long-term stability differences between monolayer and multilayer material. Chapter 4 includes the most noteworthy results wherein we develop a method to quantitatively measure the electronic band gap of monolayer MoS2 using a relatively simple optical setup. For the first time, we use an all-optical approach and many-body theory to report an abrupt change in potential-dependent band gap energies of monolayer MoS2 under electrochemical conditions. Chapter 5 summarizes preliminary work investigating how redox couples in the electrolyte may tune the optical signature of a monolayer MoS2 electrode. Finally, Chapter 6 concludes the thesis with suggestions for subsequent investigations available based on the expertise and resources within the Sambur group at Colorado State University.
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    Photoelectrochemical microscopy studies of transition metal dichalcogenides nanoflakes: addressing open questions of structure-function relationships
    (Colorado State University. Libraries, 2022) Van Erdewyk, Michael, author; Sambur, Justin, advisor; Krummel, Amber, committee member; Henry, Charles, committee member; Stasevich, Tim, committee member
    Transition metal dichalcogenides (TMDs) are exciting materials for applications in solar energy conversion. However, to advance technologies that leverage these materials, a strong understanding of fundamental photoelectrochemistry and related processes is necessary. Photoelectrochemical microscopy methods are well poised in this aspect. Methods like scanning photoelectrochemical microscopy allow for the excitation of small, localized region of a material with a focused laser and the subsequent measurement of the photocurrent. The measured photocurrent can be related to the position of the laser and the physical attributes of the material surface at the location, and variations in the photocurrent across the surface can be tracked. In this way, the technique offers insight into how different surface motifs affect the photoelectrochemical behavior of the material. This method can be combined with other spectroscopies, such as photoluminescence or Raman, to can further understanding about the studied material. The following work details the use of photoelectrochemical microscopy methods to answer questions relating to both the structure and underlying properties of mechanically exfoliated TMD nanoflakes.
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    Probing buried defects in zinc oxide nanoparticles using defect-mediated energy transfer
    (Colorado State University. Libraries, 2019) Beck, Lacey, author; Sambur, Justin, advisor; Prieto, Amy, committee member; Bartels, Randy, committee member
    Semiconductor nanocrystals are actively explored as light harvesting materials for solar energy conversion and optoelectronic applications such as solar cells and light emitting diodes. The underlying processes in such systems include charge carrier generation, recombination, and transport. Defects influence these underlying processes by introducing energy levels inside the semiconductor bandgap that trap charge carriers. Despite their critical importance, the real space distribution of defect sites in semiconductor nanocrystals is often unknown. Here we demonstrate an ensemble-level energy transfer measurement approach to study the radiative defect states in a size series of ZnO nanocrystals. In this approach, ZnO defects that have energy levels inside the band gap engage in energy transfer with surface adsorbed AlexaFluor dye molecule acceptors. By quantifying the defect-mediated energy transfer efficiency as a function of nanocrystal size and reaction time, we determined that the radiative defect sites in ZnO are located between the nanocrystal core and surface (i.e., near surface sites) and the distance between the defect sites and the surface increases as the nanocrystals grow larger. The all-optical energy transfer approach represents a non-destructive characterization method to determine the spatial distribution of defects in semiconductor nanocrystals. The defect distributions can be correlated with optoelectronic or photocatalytic properties to elucidate structure/function relationships in a wide range of applications that involve light-matter interactions.
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    Single-nanoflake photoelectrochemistry of MoSe2 thin films
    (Colorado State University. Libraries, 2018) Isenberg, Allan Edward, author; Sambur, Justin, advisor; Neilson, James, committee member; de la Venta, Jose, committee member
    Transition metal dichalcogenide (TMD) thin films represent promising materials for large-area, low-cost, and high-efficiency photoelectrochemical solar energy conversion applications. The outstanding efficiency of bulk TMD crystals has been well documented, which has driven interest in large-area exfoliated TMD thin film devices in recent years. Unfortunately, the solar energy conversion efficiency of nanoflake-coated electrodes is typically much worse than bulk crystal electrodes. It is currently unclear how the high degree of variability among nanoflakes (e.g. area, thickness, types structural features, etc.) contribute to the efficiency gap between nanoflake and bulk electrodes. It is also unclear if exfoliated nanoflakes can achieve the solar conversion efficiencies demonstrated by bulk crystals. The semiconductor-electrolyte dynamics of TMD/iodide photoelectrochemical cells has also been characterized in bulk systems. Bulk TMD electrodes in an iodide electrolyte will form adsorbed oxidation products at the TMD surface, which can cause sharp drops in efficiency in these systems. A clear understanding of how this phenomenon affects the local photoelectrochemical response has not been established. Additionally, it is not clear how the surface reaction kinetics of iodide oxidation products are affected by surface structural features (e.g. basal planes, perimeter-edges, and interior step edges) on TMD nanoflakes. Here, a single-nanoflake photoelectrochemical approach is used to establish the existence of highly active champion and inactive spectator nanoflakes in mechanically exfoliated MoSe2 thin films. In the samples studied, 7% of nanoflakes are highly active champions, whose solar conversion efficiencies exceed that of the bulk crystal. Though, 68% of the deposited nanoflakes are inactive spectators, and contribute substantially to the lower photocurrent efficiencies of nanoflake-coated electrodes compared to bulk electrodes. Structural features are also shown to have a significant effect on photocurrent collection efficiencies. Photocurrent collection response is shown to increase with nanoflake area and is more negatively affected by perimeter edges than interior step edges. Moreover, local photoelectrochemical spot measurements show that while adsorbed iodide oxidation products can form at any type of surface structure, these films preferentially form at the most catalytically active and thermodynamically favorable sites for iodide oxidation. These observations reveal previously hidden performance issues associated with exfoliated TMD thin films and highlights performance aspects that can be improved upon.
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    Synthesis of monolayer MoS₂ via chemical vapor deposition
    (Colorado State University. Libraries, 2020) Varra, Travis, author; Sambur, Justin, advisor; Prieto, Amy, committee member; Yourdkhani, Mostafa, committee member
    Two-dimensional materials, specifically transition metal dichalcogenides (TMDs), have emerged as ideal candidates for lightweight and flexible optoelectronic applications. Unlike bulk solids, single layer TMDs exhibit a direct bandgap that makes next-generation device applications possible. This work describes the synthesis of single layer MoS2 via chemical vapor deposition (CVD). This method involves thermal vaporization of MoO3 and S precursors in a tube furnace. The influence of reaction conditions (e.g., temperature, pressure, reaction holding time, carrier gas flow rate, and precursor separation distance) on MoS2 sample morphology was quantified using optical microscopy. Isolated equilateral triangles with 11 μm-long edge lengths were reproducibly grown on Si/SiO2 substrates. The layer thickness was determined using Raman and photoluminescence spectroscopy.