Doctor of Philosophy, University of Toledo, 2022, Physics

This dissertation represents a collection of related studies that employ the various capabilities of spectroscopic ellipsometry (SE) to characterize and gain insights into the properties of the materials of relevance to advanced cadmium telluride (CdTe) photovoltaic technology. The structural, optical, and electrical properties of the component layers of the CdTe solar cell have been investigated using different SE data collection modes and analysis techniques. The component layers of the CdTe solar cell are deposited on soda lime and TEC-15 glass substrates in the superstrate configuration, i.e., with the solar irradiance entering through the glass substrate. The key layers include a transparent conducting oxide front contact, typically pyrolytic fluorine-doped tin oxide (SnO2:F); a high resistivity transparent layer (HRT) of either pyrolytic SnO2, sputtered MgxZn1 xO (MZO), or both; semiconductor layers of either n-type cadmium sulfide (CdS), cadmium selenide (CdSe), or both; p-type CdTe; a p+ back contact interlayer typically copper based; and a metallic conducting back contact layer, such as gold. In this research, SE-deduced component layer properties have been correlated with the CdTe device performance parameters. Applying various SE capabilities not only identifies the correlations between the material properties and solar cell performance but also enables optimization of the preparation conditions (e.g., substrate temperature) and resulting properties (e.g., thickness) of the CdTe device components. First, we have employed UV-VIS SE and ex-situ mapping SE results to correlate between the CdSe optical and structural properties with the CdTe solar cell performance. The effects of varying CdSe layer thickness on the CdTe solar cell performance have been studied, focusing on the TEC-15/HRT/CdSe/CdTe/Cu/Au cell structure. A set of four 6.5 cm x 6.5 cm TEC-15/HRT structures were coated with different nominal thicknesses of CdSe for incorporation within the (open full item for complete abstract)

Committee: Robert Collins Dr. (Advisor); Jone Bjorkman Dr. (Committee Member); Stephen O'Leary Dr. (Committee Member); Randall Ellingson Dr. (Committee Member); Nikolas Podraza Dr. (Committee Member) Subjects: Meteorology; Optics; Physics

Doctor of Philosophy, Case Western Reserve University, 2016, Physics

In this thesis we studied the signature of quantum confined electronic structure of CdSe nanocrystal quantum dots (NCQDs) in the electron spin behavior via time resolved Faraday rotation (TRFR) measurement at room temperature. We found that there is a distinction between the optical spin pumping efficiency(SPE) spectrum and the optical absorbance. Although the first peak in the SPE coincides with the band edge absorption peak, the rest of the two spectra show significant distinctions. There is a second peak in the signal from the larger NCQDs 200~meV higher in energy. The SPE diminishes afterwards. In smaller NCQDs there is only one peak in the signal. We model this behavior following a 6-band effective mass approximation spin dependent absorbance calculation. We also investigated the dynamics of the spin signal and the presence of two precessing components in the TRFR signal. We find that the inhomogeneous dephasing is not sufficient to explain the dynamics of both components. We associate one of the components to the spin signal from neutral excitons and the other one to positively charged trions. We describe the data using a magnetic field dependent spin decoherence mechanism that occurs via fluctuation of the exciton energy levels in between the available fine structure states. This model captures the TRFR experiment data on both short and long timescales. Additionally, we developed a NCQD-polymer composite optical waveguide and investigated the optical spin measurement on these structures.

Committee: Berezovsky Jesse (Advisor) Subjects: Optics; Physics

Doctor of Philosophy, The Ohio State University, 2014, Chemistry

We show that commercially available TiO2, SiO2, and ZnO nanoparticles are all internalized by C2BBe1 intestinal epithelial cells, but do not appear to be toxic, even after long term repeat-exposures. When particles were exposed to a simulated digestion protocol mimicking the stomach and intestinal environment, TiO2 particles did show mild toxicity by MTT assay, indicating a decrease in metabolic activity. IR spectra of these particles indicate presence of material from the digestion media, and these absorbed species may be responsible for the effects noted. Though the three particles were not significantly toxic, we note internalization by the intestinal epithelial cells, opening a possibility for absorption into circulation where they may localize in organs throughout the body. This will be observed by functionalizing the particles with fluorophores, after which they can be measured via fluorescence. To optimize the quantum yield efficiency, and thus the brightness, of one such fluorophore, we seek to improve a microwave synthesis of CdSe/CdS/ZnS quantum dots our lab has previously reported. By coupling the microwave reactor to a fluorescence spectrometer via fiber optic cables, we were able to monitor the development of the particles throughout the microwave heating. Time-dependent fluorescence shows the development of an early fluorescence peak at 502 nm attributed to CdSe cores. We then note two isosbestic points which we attribute to the development of CdS layer around CdSe cores, and eventually the formation of outer ZnS shell. We utilize this in situ monitoring along with a study of various nucleation temperatures ranging from 0 to 100 ºC, and pre-and-post microwave heating UV exposure treatments to obtain optimized CdSe/CdS/ZnS particles with a QY of 40%. This is an improvement over our previous particles' 13% QY, and the highest yet reported for an aqueous synthesis of CdSe/ZnS type particles. Finally, we incorporate these QDs as well as two organic flu (open full item for complete abstract)

Committee: Prabir Dutta (Advisor); James Waldman (Committee Member); Heather Allen (Committee Member); Prosper Boyaka (Committee Member) Subjects: Chemistry

PhD, University of Cincinnati, 2001, Arts and Sciences : Physics

We use photoluminescence spectroscopy to investigate the optical properties of two different II-VI semiconductor nanostructure systems: ZnMnSe/ZnSe multiple quantum wells and self-assembled CdSe quantum dots. The behavior of excitons in ZnMnSe quantum wells is examined using polarized magneto-photoluminescence, while (temperature-dependent) time-resolved photoluminescence is used to study the dynamics of excitons confined to quantum dot structures. When Zn0.86Mn0.14Se/ZnSe multiple quantum wells are placed in an external magnetic fields, the spin-down holes become confined to the ZnMnSe “barriers,” while the electrons remain in the ZnSe “wells.” This spatial separation of electrons and holes results in weak electron-hole overlap, which should result in a large increase of the radiative lifetime of the spin-down exciton. As a result, we see the formation of exciton magnetic polarons (EMP) which lower their energy by spontaneously aligning the magnetic impurities in the exciton Bohr radius. We find that the EMP polarization approaches 100% in fields as small as 200 mT in these spatially indirect EMP, which is consistent with their extremely long recombination lifetime (~10 ns). This contrasts with previous measurements on other magnetic quantum well systems for which the polarization of short-lived spatially direct EMP never saturates. Time-resolved photoluminescence measurements of excitons confined to CdSe self-assembled quantum dots (SAQDs) reveal the existence of two different exciton decay times: a short 450 ps lifetime and a much longer ( > 4 ns) lifetime. While the emission resulting from the short lifetime excitons persists to room temperatures, the longlifetime component decreases in intensity with increasing temperature and is nearly completely gone by 60 K. Time-resolved spectra further reveal that the long lifetime component arises from spectrally sharp features (~200 µeV), while the rapid decay results from an underlying broad emission (~60 meV). Further (open full item for complete abstract)

Committee: Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

We study the dynamics of excitons in bimodal CdSe quantum dots. The effect of exciton localization is investigated by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe an exciton emission from low energy (QDs1) and weaker emission from high energy (QDs2) at low excitation levels at 10 K. Temperature-dependent photoluminescence (PL) studies reveal a thermally activated exciton transfer from QDs1 to QDs2. Time-resolved PL estimate the characteristic radiative and nonradiative decay rates as well as the trapping rate from the QD-precursor layer. The observed PL is reasonably reproduced using a coupled rate equation model. We investigate 10 nm Zn0.94Mg0.06Se/ZnSe quantum well (QW) with two-beam four-wave mixing (FWM) using 90 fs pulses. At high intensity the signal is dominated by χ(3) FWM processes and at low intensity it reveals an exciton resonant phase coherent photorefractive (PCP) effect that is attributed to the formation of an electron grating within the QW by the interference of coherent QW excitons. The observed traces and spectra are reproduced by the model based on a 15-level system and a phenomenological PCP model. The dynamical properties of the electron grating responsible for the PCP is further studied reducing the pulse repetition at 45, 55 and 65 K. The PCP diffraction reveals a nearly constant efficiency up to 1 μs that implies a constant average equilibrium electron density. With increasing temperature, the efficiency decreases due to QW electron escape back to the substrate reducing the grating lifetime. The observed PCP efficiency is studied with a model that considers the equilibrium density dynamics in the QW. We further report on PCP effect in Zn0.9Mg0.1Se/ZnSe QW by performing intensity dependent and polarization dependent two-beam FWM experiments using 30 fs pulses at 2.79 and 2.84 eV. The PCP effect is attributed to the formation of an electron grating within the QW by the interference of exc (open full item for complete abstract)

Committee: Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter

Master of Science, The Ohio State University, 2011, Chemistry

The synthesis, characteristics, and macrophage uptake of CdSe/ZnS core/shell quantum dots were studied. Insights into the mechanism of nucleation and growth of the quantum dots were gained by performing in-situ fluorescence spectroscopy during a microwave synthesis. The size and surface charge of quantum dots capped by 3-mercaptopropionic acid (3-MPA, negatively charged) and thiocholine (positively charged) were characterized by dynamic light scattering (DLS) and electrophoretic light scattering (ELS). Finally, macrophage uptake studies were performed by Amber Nagy via flow cytometry to determine the level of quantum dot association with murine alveolar macrophages, and to determine a possible uptake pathway into the cells. The mechanism for the CdSe/ZnS synthesis was determined by an in-situ fluorescence experiment. A fast nucleation step occurred, resulting in small CdSe seed nanoparticles which were protected from aggregation by the 3-MPA. Upon microwave heating, these caps were removed from the surface and began to deteriorate. The CdSe cores underwent Ostwald ripening in which smaller particles dissolved and provided free ions to increase the size of the larger particles. After this period, free zinc in the solution reacted with sulfur, freed from MPA decomposition, to form a ZnS shell around the CdSe core. The ZnS shell passivated the surface of the CdSe core, resulting in increased quantum yield. A maximum quantum yield of 22% was attained after 80 minutes of microwave heating, after which it began to decrease. Both MPA and thiocholine coated quantum dots were stable in water and in Roswell Park Memorial Institute (RPMI) media. MPA coated quantum dots aggregated in fetal bovine serum and serum free media, but remained stable. In both cases the surface charge was reduced, indicating the association of a species in the media with the quantum dots. We propose this was due to bovine serum albumin. Thiocholine coated quantum dots aggregated significantly in dilu (open full item for complete abstract)

Committee: Dr. Prabir Dutta (Advisor); Dr. Susan Olesik (Committee Member) Subjects: Chemistry

Doctor of Philosophy (PhD), Ohio University, 2008, Chemistry and Biochemistry (Arts and Sciences)

The reduction in the threshold of biexciton formation in CdTe semiconductor nanocrystals (NCs) was studied, as an approach towards reducing the optical-gain threshold and improving the lasing abilities of semiconductor nanocrystals.CdTe quantum dots (QDs) were assembled into an antenna-like architecture. In this nanocomposite, an energy-acceptor CdTe QD was at the center and surrounded by several energy-donor CdTe QDs. The electronic structures of the involved QDs were tailored so that the donor QDs have larger band gap than the band gap of the acceptor QD, which directs the energy irreversibly from the donors to the acceptor through electronic energy transfer. Focusing the excitation energy at the center of this nano-antenna simply reduces the pump intensity required to excite the acceptor QD. In order to realize the required band-gap gradient, small CdTe QDs (2.9 nm) were used as donors, and large CdTe QDs (3.9 nm) were used as acceptors. Deriving the self-assembly of CdTe QDs to form the desired nanocomposite was managed by electrostatic interaction between the ligands attached to the surface of the QDs. Donor QDs were synthesized with thioglycolic acid on their surface (TGA), and acceptor QDs were synthesized with 2-mercaptoethylamine (MA) on their surface. The electrostatic interaction between the positive charge of MA and negative charge of TGA in the neutral medium leads to the desired self-assembly of the donors and acceptors. Occurrence of electronic energy transfer from the donors to the acceptors was investigated by analyzing the steady state photoluminescence (PL) spectra and the PL decay dynamics of the free and assembled donors and acceptors. Analyzing the PL decay dynamics of the free acceptors and comparing them with the dynamics of the acceptors in the nano-antenna shows reduction in the threshold of the biexciton formation by factor 3.6±2 times. This result is promising in the future reduction of the optical-gain threshold and so improving the lasi (open full item for complete abstract)

Committee: P. Gregory Van Patten Ph.D. (Advisor); Liwei Chen Ph.D. (Committee Member); Glen Jackson Ph.D. (Committee Member); Alexander Govorov Ph.D. (Committee Member) Subjects: Chemistry

Doctor of Philosophy, Case Western Reserve University, 2005, Chemistry

Since the discovery of the photocatalytic splitting of water on TiO2 electrode by Fujishima and Honda in 1972, enormous effort has been spent on the study of TiO2 under light illumination, due to its various potential applications, such as photovoltaics and photocatalysis. The optical properties, in particular the absorption, of TiO2 are essential to its photon-driven applications. Typically, TiO2 absorbs in the UV regime, which is only a small fraction of the sun's energy (< 10%). The performance of TiO2 can be enhanced by shifting the onset of its absorption from the UV to the visible region. Metals have been employed to tune the electronic structure of TiO2-based material. The photocatalytic reactivity of metal-doped TiO2 depends on many factors, and metal doping can result in thermal instability and increased carrier trapping. The desired visible-light absorption of TiO2 can be also achieved by using main group dopants. In this dissertation, different non-metal elements, C, N and S, are incorporated to the lattice of TiO2 to induce the absorption in the visible-light regime. Both bottom-up and top-down methods are used to synthesize these doped TiO2 nanoparticles. The optical, physical, electronic and photocatalytic properties of these doped TiO2 nanoparticles are explored with different techniques. The relationship between the optical, electronic and photocatalytic properties are elucidated. The photocatalytic performance of the doped TiO2 nanoparticles is applied not only to the model photodegradation of methylene blue, but also on other industrial dyes under natural sun-light illumination. The non-metal-doped TiO2 nanoparticles demonstrated improved photocatalytic performance over the non-doped TiO2 nanoparticles, i.e. in the visible-light regime. On the other hand, as the size of nanoparticles decreases, the surface-to-volume ratio increases dramatically (~ 1/r), so does the surface area (1/r2). The high surface area brought by the small size of nanopartic (open full item for complete abstract)

Committee: Clemens Burda (Advisor) Subjects:

Master of Science (MS), Bowling Green State University, 2012, Physics

Colloidal semiconductor nanocrystals (NCs) and composite metal-semiconductor (M-S) structures are emerging as a promising class of nanomaterials for the development of solid state optical applications. Some of these applications require colloidal nanocrystals in form of thin films (matrices), possessing both thermal and chemical stability, and demonstrating high energy transfer efficiency. In this study, we addressed the issues of low emission yield and instability of NC matrices through the use of ligand-free encapsulation of colloidal nanocrystals within all-inorganic crystalline solids. The present methodology relies on solution-processing of CdSe nanocrystals into a crystalline matrix ofa wide bandgap semiconductor (CdS, ZnS), which replaces the original molecular ligands on nanocrystal surfaces with an inorganic medium. Such matrices efficiently protect nanoparticles from the surrounding environment and preserve the quantum confinement of electrical charges in embedded NCs. Assembly of thin film matrices from NC “inks” is cost-effective approach for the development of next-generation materials with high efficiency photoluminescence. Specifically, the main accomplishments of this work included: (i) hot-injection synthesis of CdSe/CdS core/shell semiconductor NCs; (ii) processing of NCs into light emitting solid films through spin-coating with consecutive ligand exchange; (iii) optimization of film morphology towards improving the emission yield and stability; (iv) further enhancement of developed morphology via introducing the M-S composites: a. development of chemical strategies for the fabrication of M-S nanocomposites that demonstrate an efficient and potentially tunable energy exchange between excitons in semiconductor and plasmons in metal nanoparticles; b. investigation of the nature of underlying exciton-plasmon interactions and the ultrafast electron processes in the fabricated M-S heteronanocrystals, using modern spectroscopy methods. A general st (open full item for complete abstract)

Committee: Mikhail Zamkov PhD (Advisor); Haowen Xi PhD (Committee Member) Subjects: Chemistry; Nanoscience; Nanotechnology; Physics

Doctor of Philosophy, University of Toledo, 2012, Physics

CdTe has recently become the most commercially successful polycrystalline thin film solar module material. Its low cost, large-area solar module is reshaping the silicondominated solar panel market; however, CdTe has much room to improve and thus more fundamental understanding is needed. Current thin film solar cell research is focused on two areas: One is identifying loss mechanisms and understanding the polycrystalline nature of single junction device to improve device performance. Another is searching for new materials and fabricating tandem solar cells. In this study, along with other people's work to improve the efficiency of CdTe solar module, I studied loss mechanism and growth mode of CdTe solar cells to have fundamental understanding of polycrystalline films. In addition to that, in an effort to make tandem solar cells, I fabricated and characterized CdSe solar cells, which is considered as an ideal candidate for the top cell with its band gap of 1.7 eV. This dissertation is designed to show similarities and differences between CdTe and CdSe solar cells, side by side. After the introduction (Chapter1), I will review the physical properties of CdTe and CdSe solar cells (Chapter 2). Two primary tools to study defects and surface morphology were photoluminescence (PL) and atomic force microscopy (AFM). PL showed information on the crystallinity and defects of CdTe and CdSe films before and after annealing.(Chapter 3). AFM measurements and their analysis using scaling theory revealed information on the growth modes of CdTe and CdSe films.(Chapter 4). With the goal of exploring suitability for tandem structures with ~1.7 eV top cell and ~1.1 eV bottom cell, I fabricated and characterized single-junction CdSe devices. (Chapter 5) In addition, for the bottom cell I fabricated HgCdTe cells with Eg~1.1 eV. Single junction HgCdTe and two-terminal CdTe/HgCdTe tandem solar cells were fabricated and characterized. (Chapter 5)

Committee: Alvin Compaan (Committee Chair); Jacques Amar (Committee Member); Victor Karpov (Committee Member); Thomas Kvale (Committee Member); Dean Giolando (Committee Member) Subjects: Physics

Master of Science, University of Toledo, 2012, Chemistry

The high affinity choline uptake transmembrane protein (CHT) is an important part of the cholinergic transport cycle and yet the structure of this transmembrane protein has still not been determined. The concentration and functionality of choline has been observed to be depleted in neurodegenerative diseases, so obtaining as much data about the cycle is an important part of diagnosis and possible treatment for these diseases, such as Alzheimer's Disease (AD). Previous research has focused on developing extremely sensitive methods of monitoring acetylcholine and choline throughout the uptake process using capillary electrophoresis with electrochemical detection (CEEC). These quantitative methods have provided nonradiochemical strategies for the measurement of choline transport including measurements under conditions of selective cholinergic inhibition. A cholinomimetic quantum dot (QD) was designed and synthesized to provide qualitative imaging opportunities for monitoring and observing CHT. These QDs were synthesized using a CdSe/ZnSe/ZnS inorganic core in a colloidal organic matrix with trioctylphosphine, trioctylphosphine oxide and hexadecylamine as the coordinating ligands and passivating agents to control growth. The QDs were ~6.2 nm in diameter and exhibited strong and narrow fluorescence intensity and broad UV-absorption properties. The organic ligands were exchanged for water-soluble mercaptosuccinic acid ligands which functionalized the surface with a useful carboxylic acid. An aqueous coupling reaction with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) was utilized to create three distinct functionalized quantum dots; one quantum dot with the –OH moiety on the surface, the second with the quaternary ammonium group on the surface and the third with both functional groups on the surface to work together as a cholinomimetic probe, as the quaternary ammonium and the alcohol groups are the two important groups on choline and the mode of interaction with (open full item for complete abstract)

Committee: Jon Kirchhoff PhD (Committee Chair); L. Viranga Tillekeratne PhD (Committee Member); Jared Anderson PhD (Committee Member); Mark Mason PhD (Committee Member) Subjects: Chemistry