Matching Items (3)
Description
The performance of kilometer-scale electron accelerators, which are used for high energy physics and next-generation light sources as well as meter-scale ultra-fast electron diffraction setups is limited by the brightness of electron sources. A potential emerging candidate for such applications is the family of alkali and bi-alkali antimonides. Much of the physics of photoemission from such semiconductor photocathodes is not fully understood even today, which poses a hindrance to the complete exploration and optimization of their photoemission properties. This thesis presents the theoretical and experimental measurements which lead to advances in the understanding of the photoemission process and properties of cesium-antimonide photocathodes. First, the growth of high quantum efficiency (QE), atomically smooth and chemically homogeneous Cs$_3$Sb cathodes on lattice-matched strontium titanate substrates (STO) is demonstrated. The roughness-induced mean transverse energies (MTE) simulations indicate that the contribution to MTE from nanoscale surface roughness of Cs$_3$Sb cathodes grown on STO is inconsequential over typically used field gradients in photoinjectors. Second, the formulation of a new approach to model photoemission from cathodes with disordered surfaces is demonstrated. The model is used to explain near-threshold photoemission from thin film Cs$_3$Sb cathodes. This model suggests that the MTE values may get limited to higher values due to the defect density of states near the valence band maximum. Third, the detailed measurements of MTE and kinetic energy distribution spectra along with QE from Cs$_3$Sb cathodes using the photoemission electron microscope are presented. These measurements indicate that Cs$_3$Sb cathodes have a work function in the range of 1.5-1.6 eV. When photoemitting near this work function energy, the MTE nearly converges to the thermal limit of 26 meV. However, the QE is extremely low, of the order of 10$^{-7}$, which limits the operation of these photocathodes for high current applications. Lastly, the growth of Cs$_3$Sb cathodes using the ion beam assisted molecular beam deposition (IBA-MBE) technique is demonstrated. This technique has the potential to grow epitaxial Cs$_3$Sb cathodes in a more reproducible, easier fashion. Structural characterization of such cathodes via tools such as reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD) will be necessary to investigate the role of the IBA-MBE technique in facilitating the epitaxial, ordered growth of alkali-antimonides.
ContributorsSaha, Pallavi (Author) / Karkare, Siddharth (Thesis advisor) / Bennett, Peter (Committee member) / Nemanich, Robert (Committee member) / Kaindl, Robert (Committee member) / Arizona State University (Publisher)
Created2023
Description
The performance of accelerator applications like X-ray free electron lasers (XFELs)and ultrafast electron diffraction (UED) and microscopy (UEM) experiments is limited
by the brightness of electron beams generated by photoinjectors. In order to
maximize the brightness of an electron beam it is essential that electrons are emitted
from photocathodes with the smallest possible mean transverse energy (MTE).
Metallic photocathodes hold the record for the smallest MTE ever measured at 5
meV from a Cu(100) single crystal photocathode operated near the photoemission
threshold and cooled to 30 K. However such photocathodes have two major limitations:
poor surface stability, and a low quantum efficiency (QE) which leads to
MTE degrading non-linear photoemission effects when extracting large charge densities.
This thesis investigates the efficacy of using a graphene protective layer in order
to improve the stability of a Cu(110) single crystalline surface. The contribution to
MTE from non-linear photoemission effects is measured from a Cu(110) single crystal
photocathode at a variety of excess energies, laser fluences, and laser pulse lengths.
To conclude this thesis, the design and research capabilities of the Photocathode and
Bright Beams Lab (PBBL) are presented. Such a lab is required to develop cathode
technology to mitigate the practical limitations of metallic photocathodes.
ContributorsKnill, Christopher John (Author) / Karkare, Siddharth (Thesis advisor) / Drucker, Jeffery (Committee member) / Kaindl, Robert (Committee member) / Teitelbaum, Samuel (Committee member) / Arizona State University (Publisher)
Created2023
Description
Metalloporphyrins represent a class of molecular electrocatalysts for driving energy relevant half-reactions, including hydrogen evolution and carbon dioxide reduction. As electrocatalysts, they provide a strategy, and potential structural component, for linking renewable energy sources with the production of fuels and other value-added chemicals. In this work, porphyrins are used as structural motifs for exploring structure-function relationships in electrocatalysis and as molecular building blocks for assembling photoelectrochemical assemblies leveraging the light capture and conversion properties of a gallium phosphide (GaP) semiconductor. These concepts are further covered in Chapter 1. A direct one-step method to chemically graft metalloporphyrins to GaP surfaces is described in Chapter 2. Structural characterization of the hybrid assemblies is achieved using surface-sensitive spectroscopic methods, and functional performance for photoinduced hydrogen production is demonstrated via three-electrode electrochemical measurement combined with product analysis using gas chromatography. In Chapter 3, preparation of a novel cobalt porphyrin modified with 3-fluorophenyl groups at all four meso-positions of the porphyrin ring and a single 4-vinylphenyl surface attachment group at one of the β-positions is described. Electrochemical measurements show the 3-fluorophenyl groups perturb the reduction potentials of the complex to more positive values as compared to non-fluorinated analogs, illustrating synthetic control over the redox properties of the catalysts. The use of grazing angle attenuated total reflectance Fourier transform infrared spectroscopy to characterize chemically modified GaP surfaces containing grafted cobalt fluoro-porphyrins is presented in Chapter 4. In these hybrid constructs, porphyrin surface attachment is achieved using either a two-step method involving coordination of cobalt fluoro-porphyrin metal centers to nitrogen sites on an initially applied thin-film polypyridyl surface coating, or via a direct modification strategy using a cobalt fluoro-porphyrin precursor bearing a covalently bonded 4- vinylphenyl surface attachment group. Finally, Chapter 5 describes binuclear copper porphyrins in which two copper porphyrin macrocycles are doubly fused at the meso-β positions are shown to be active electrocatalysts for the hydrogen evolution reaction. The enhancement in catalytic performance over analogous non-fused copper porphyrins indicates extended macrocycles provide an advantageous structural motif and design element for preparing electrocatalysts that activate small molecules of consequence to renewable energy.
ContributorsKhusnutdinova, Diana (Author) / Moore, Gary F. (Thesis advisor) / Moore, Ana L. (Committee member) / Petuskey, William T. (Committee member) / Arizona State University (Publisher)
Created2019