Doctor of Philosophy, University of Toledo, 2023, Physics

Low-cost thin film absorber layer materials with high absorption coefficients (> 105 cm-1 in visible spectral range) and bandgap close to the ideal value for efficient photovoltaic conversion efficiency are leading candidates for thin film photovoltaic (PV) applications. This dissertation discusses the fabrication and optical and microstructural properties of magnetron-sputtered glancing angle deposited CdTe thin film absorber layer material and its application as an interlayer in CdS/CdTe solar cells. In addition, optoelectronic properties of non-toxic and earth-abundant absorber layer material, antimony selenide (Sb2Se3), and optimization of polycrystalline VO2 fabrication from amorphous vanadium oxide (VOx) film along with its optical properties have been discussed. Sb2Se3 is a promising candidate as an absorber layer material in PV applications. I have performed optical property characterization of thin film Sb2Se3 and identified electronic losses when used in a PV device. The indirect bandgap, direct bandgap, and Urbach energy have been determined to be 1.12 eV, 1.17 eV, and 21.1 meV, respectively using photothermal deflection spectroscopy. Optical properties of Sb2Se3 in the form of complex dielectric function (ε = ε1 + iε2) spectra in 0.75 to 4 eV spectral range is determined using spectroscopic ellipsometry. The line shape of ε is obtained using a parametric model which incorporates an Urbach tail, a band edge function, and five critical point oscillators. The optical property spectra in ε and structural parameters in terms of the thickness of solar cell layer components are used as input parameters for external quantum efficiency (EQE) simulation to investigate the electronic and optical losses in Sb2Se3-based solar cells. A carrier collection length of ~ 400 nm and a ~97 % carrier collection probability near the heterojunction in the Sb2Se3 solar cell are identified by comparing experimental and simulated EQE. Next, I describe deposition and characterizati (open full item for complete abstract)

Committee: Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Yanfa Yan (Committee Member); Song Cheng (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

Doctor of Philosophy (Ph.D.), University of Dayton, 2021, Electro-Optics

Reconfigurable photonics has been at the forefront of modern optics research, especially as optics and electronics merge into proper single-device integration. Within that umbrella, we can identify a series of critical devices commonly used in free-space applications adaptive optical elements, such as liquid crystal on silicon spatial light modulators (SLMs) and micromechanical adaptive beam steering mirrors. Light modulating devices play an essential role in spatio-temporal beam shaping, image processing, and display technologies for their role in converting intensity patterns into phase or amplitude light modulation. At the core, the physical idea remains the same: locally controlling the refractive index of the constituent material to affect the amplitude, phase, and polarization of incident light. Critical issues common to many reconfigurable devices are bulkiness, low speeds, and large voltage requirements or power consumption. However, phase change materials (PCMs) such as Ge2Sb2Te5 (GST) offer an alternative path for high-speed light modulation circumventing each of these issues. In general, PCMs alter their atomic structure via a thermal stimulus which yields large electrical, thermal, and optical contrast. Moreover, the nanosecond transition between the amorphous and metastable rock salt phase state leads to a substantial difference in the complex refractive index. In this work, an investigation is conducted on GST as a solid-state material for light modulators in the visible and infrared regimes. A holistic approach is taken to investigate the design, fabrication, and performance of each device. This work addresses critical issues in each design such as mitigating the electrical contact resistance, maximizing amplitude modulation, and improving phase transition speeds. Additionally, this work investigates the optical properties of nanopatterned GST using both top-down and bottom-up fabrication approaches that incorporate the necessary thermal mana (open full item for complete abstract)

Committee: Imad Agha (Advisor); Thomas Searles (Committee Member); Jay Mathews (Committee Member); Andrew Sarangan (Committee Member) Subjects: Optics; Physics

Doctor of Philosophy, University of Toledo, 2019, Physics

Thin film solar cells are promising candidates for generation of low cost and pollution-free energy. The materials used in these devices, mainly the active absorber layer, can be deposited in a variety of industry-friendly ways, so that the cost associated with manufacturing is generally lower than for competing technologies such as crystalline silicon. This dissertation will focus on the fabrication and characterization of nanocrystalline hydrogenated silicon (nc-Si:H) and polycrystalline cadmium telluride (CdTe) thin films by industrially scalable, non-toxic, and comparatively simple magnetron sputtering. The performance of the solar cells incorporating these films as an active absorber layers are discussed. In this work, spectroscopic ellipsometry is used as the primary tool for the characterization of optical and structural properties of thin films and bulk material. As a first case study, the anisotropic optical properties of single crystal strontium lanthanum aluminum oxide (SrLaAlO4) in the form of birefringence and dichroism is obtained from Mueller matrix ellipsometry. SrLaAlO4 exhibit uniaxial anisotropic optical properties and the indirect optical band gap of 2.74 eV. A parametric model consisting of parabolic band critical points (CPs) for electronic transitions and a gap function is used to describe the complex dielectric function spectra in both the ordinary and extra-ordinary directions. The modeling in this case study has applications to both nc-Si:H, an indirect band gap semiconductor, and CdTe which may exhibit microstructural anisotropy depending upon the deposition method. Fabrication and characterization of hydrogenated silicon (Si:H) thin films produced by reactive magnetron sputtering is the second case in this study. RTSE and a virtual interface analysis (VIA) are used to track the growth evolution of sputtered Si:H. From these studies, growth evolution diagrams depicting the nucleation of nanocrystallites from the amorphous phase and (open full item for complete abstract)

Committee: Nikolas Podraza (Committee Chair); Robert Collins (Committee Member); Yanfa Yan (Committee Member); Michael Cushing (Committee Member); Sylvain Marsillac (Committee Member) Subjects: Energy; Materials Science; Optics; Physics

Master of Science (M.S.), University of Dayton, 2017, Bioengineering

In this study, nanostructured, functionalized microcantilevers have been designed, fabricated, and characterized for the sensing of volatile organic compounds. Sensing devices were fabricated with either four or eight hammerhead-shaped cantilevers. These cantilevers vibrate laterally in-plane making them highly suitable for sensing in both air and liquid. Silicon oxide nanostructure was deposited on the cantilevers to increase the surface area and sensitivity of the devices. Molecular recognition peptides were chemically tethered to the surfaces to create a selective response for the analytes of interest. When the analytes have bound to the surface of the cantilever, a shift in resonance frequency is produced and detected by piezoresistive sensors. This frequency shift can be used to determine the mass of analyte bound to the surface. The cantilever sensors are expected to provide fast and highly sensitive detection, and can be fabricated in an array format for sensing multiple compounds in complex samples. The nanostructured cantilever sensors show strong potential for applications in medical, environmental, food safety, and hazardous gas monitoring applications.

Committee: Karolyn Hansen Ph.D. (Advisor); Kristen Comfort Ph.D. (Committee Member); Matthew Lopper Ph.D. (Committee Member) Subjects: Biochemistry; Biomedical Engineering; Chemical Engineering; Electrical Engineering; Mechanical Engineering

Doctor of Philosophy, University of Toledo, 2014, Physics

In this thesis, the nucleation, growth, defect structure, and dynamical behavior of a variety of different nanoscale systems and processes, ranging from nanoparti- cle self-assembly to multilayer metal thin-film growth to nanocolumns, are studied. In our simulations, a variety of different methods have been used including rate- equations, molecular dynamics, and analytical methods. In addition, a new compu- tational method to use graphical processing units (GPUs) to improve the efficiency of accelerated dynamics calculations is described. In the first project, which was motivated by experiments on colloidal nanoparticle (NP) island growth, the development of a self-consistent rate-equation (RE) approach to irreversible island growth and nucleation which takes into account cluster mobility and coalescence is presented. As a first application, we consider the irreversible growth of compact submonolayer islands on a two-dimensional (2D) substrate in the presence of monomer deposition as well as monomer and island diffusion. Our results are compared with kinetic Monte Carlo simulations for different values of the exponent µ describing the dependence of the island diffusion constant on island size. We find excellent agreement between our self-consistent RE results and simulation results for the island and monomer densities, up to and somewhat beyond the coverage corresponding to the peak island density. We also find good agreement between our self-consistent RE and simulation results for the portion of the island size distribution (ISD) corresponding to island sizes less than the average island size S. Our self- consistent RE approach also demonstrates that geometric effects play a crucial role in determining the power-law behavior of the ISD for µ = 1. We then present simulation results for the critical island size, stability, and mor- phology of 2D colloidal Au nanoparticle islands formed during drop-drying, which were carried out in order to explain recent experimen (open full item for complete abstract)

Committee: Jacques Amar (Advisor); Sanjay Khare (Committee Member); Bo Gao (Committee Member); Randall Ellingson (Committee Member); Terry Bigioni (Committee Member) Subjects: Condensed Matter Physics; Physics