ETDs

This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.

In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.

Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.

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Excited State Dynamics of First-Row Transition Metal Oxide Clusters

Description
Transition metal oxides are used for numerous applications, includingsemiconductors, batteries, solar cells, catalysis, magnetic devices, and are commonly observed in interstellar media. However, the atomic-scale properties which dictate the overall bulk material activity is still lacking fundamental details. Most importantly, how the electron shells of metals and O atoms mix is inherently significant…

Transition metal oxides are used for numerous applications, includingsemiconductors, batteries, solar cells, catalysis, magnetic devices, and are commonly observed in interstellar media. However, the atomic-scale properties which dictate the overall bulk material activity is still lacking fundamental details. Most importantly, how the electron shells of metals and O atoms mix is inherently significant to reactivity. This thesis compares the binding and excited state properties of highly correlated first-row transition metal oxides using four separate transition metal systems of Ti, Cr, Fe and Ni. Laser ablation coupled with femtosecond pump-probe spectroscopy is utilized to resolve the time-dependent excited state relaxation dynamics of atomically precise neutral clusters following 400 nm (3.1 eV) photoexcitation. All transition metal oxides form unique stable stoichiometries with excited state dynamics that evolve due to oxidation, size, or geometry. Theoretical calculations assist in experimental analysis, showing correlations between charge transfer characteristics, electron and hole localization, and magnetic properties to the experimentally determined excited state lifetimes. This thesis finds that neutral Ti and Cr form stable stoichiometries of MO2 (M = Ti, Cr) which easily lose up to two O atoms, while neutral Fe and Ni primarily form MO (M = Fe, Ni) geometries with suboxides also produced. TiO2 clusters possess excited state lifetimes that increase with additional cluster units to ~600 fs, owing to a larger delocalization of excited charge carriers with cluster size. CrO2 clusters show a unique inversed metallic behavior with O content, where the fast (~30 fs) metallic relaxation component associated with electron scattering increases with higher O content, connected to the percent of ligand-to-metal charge transfer (LMCT) character and higher density of states. FeO clusters show a decreased lifetime with size, reaching a plateau of ~150 fs at the size of (FeO)5 related to the density of states as clusters form 3D geometries. Finally, neutral (NiO)n clusters all have similar fast lifetimes (~110 fs), with suboxides possessing unexpected electronic transitions involving s-orbitals, increasing excited state lifetimes up to 80% and causing long-lived states lasting over 2.5 ps. Similarities are drawn between each cluster system, providing valuable information about each metal oxide species and the evolution of excited state dynamics as a result of the occupied d-shell. The work presented within this thesis will lead to novel materials of increased reactivity while facilitating a deeper fundamental understanding on the effect of electron interactions on chemical properties.
ContributorsGarcia, Jacob M. (Author) / Sayres, Scott G (Thesis advisor) / Yarger, Jeffery (Committee member) / Steimle, Timothy (Committee member) / Arizona State University (Publisher)
Created2021
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Molecular Approaches to Solar Fuel Production

Description
The conversion of water to hydrogen and of carbon dioxide to industrially relevant chemical precursors are examples of reactions that can be used to store renewable energy as fuels or chemical building blocks for creating sustainable chemical manufacturing cycles. Unfortunately, current industrial catalysts for these transformations are reliant on relatively…

The conversion of water to hydrogen and of carbon dioxide to industrially relevant chemical precursors are examples of reactions that can be used to store renewable energy as fuels or chemical building blocks for creating sustainable chemical manufacturing cycles. Unfortunately, current industrial catalysts for these transformations are reliant on relatively expensive and/or rare materials, such as platinum in the case of hydrogen generation, or lack selectivity towards producing a desired chemical product. Such drawbacks prevent global-scale applications. Although replacing such catalysts with more efficient and earth-abundant catalysts could improve this situation, the fundamental science required for this is lacking. In the first part of this dissertation, the synthesis and characterization of a novel binuclear iron fused porphyrin designed to break traditional scaling relationships in electrocatalysis is presented. Key features of the fused porphyrin include: 1) bimetallic sites, 2) a π-extended ligand that delocalizes electrons across the multimetallic scaffold, and 3) the ability to store up to six reducing equivalents. In the second part of this thesis, the electrochemical characterization of benzimidazole-phenols as “proton wires” is described. These bioinspired assemblies model the tyrosine-histidine pair of photosystem II, which serves as a redox mediator between the light-harvesting reaction center P680 and the oxygen evolution complex that enables production of molecular oxygen from water in cyanobacteria, algae, and higher plants. Results show that as the length of the hydrogen-bond network increases across a series of benzimidazole-phenols, the midpoint potential of the phenoxyl/phenol redox couple becomes less oxidizing. However, benzimidazole-phenols containing electron-withdrawing trifluoromethyl substituents enable access to potentials that are thermodynamically sufficient for oxidative processes relevant to artificial photosynthesis, including the oxidation of water, while translocating protons over ~11 Å.
ContributorsReyes Cruz, Edgar Alejandro (Author) / Moore, Gary F (Thesis advisor) / Trovitch, Ryan J (Committee member) / Sayres, Scott G (Committee member) / Arizona State University (Publisher)
Created2023
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Probing the Excited State Dynamics of Aluminum Clusters as a Means of Identifying Metallicity

Description
Pure metal clusters serve as model systems by providing an avenue for the study of fundamental phenomena, specifically the interaction between light and matter. Bulk metal materials are known to display defining characteristics, namely thermal conductivity, electrical conductivity, and luster, which provide a quantifiable measure of their metallicity. These properties…

Pure metal clusters serve as model systems by providing an avenue for the study of fundamental phenomena, specifically the interaction between light and matter. Bulk metal materials are known to display defining characteristics, namely thermal conductivity, electrical conductivity, and luster, which provide a quantifiable measure of their metallicity. These properties are all due to the electron delocalization throughout the metal. Nanoscale materials lack the ability to measure these properties, leading to the need for a manner of quantifying the metallic character at the nanoscale size regime.Excited state lifetimes vary for semiconducting and metallic systems, specifically metals relax to a ground state at a faster rate than semiconducting materials. Aluminum clusters have received decades of attention regarding their metallicity. Moreover, Al clusters have been debated to fit into the jellium model. The jellium model seeks to describe a cluster as a “superatom” where all electrons are delocalized around the positively charged metal center, like that of an atom. With three valence electrons, jellium shell closings can be met if the electrons involved in cluster bonding varies. This variance leads to a localization of electrons for instances in which all three electrons do not contribute to bonding. Localized electrons aren’t characteristic of the jellium model or metals more broadly. Tracking the excited state lifetimes of Al clusters produced through laser ablation seeks to uncover the onset of metallic character. Femtosecond pump-probe spectroscopy coupled with time-of-flight mass spectrometry has resolved the time dynamics for atomically precise Al clusters ranging in size from 1-43 atoms. At a size greater than 9 atoms, it’s identified that Al clusters show metallic character. This finding is supported by previous literature results and the fact that, above 9 atoms, Al cluster excited state lifetimes match that of the bulk scale Al excited state lifetime of ~300 fs.
ContributorsJarman, Carter Korey (Author) / Sayres, Scott G (Thesis advisor) / Birkel, Christina (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2024