Master of Science (MS), Bowling Green State University, 2009, Physics

In this thesis I report on an attempted colloidal synthesis of heterostructured quantum solids comprising of a staggered heterojunction of nearly lattice matched cadmium sulfide and zinc selenide semiconductor quantum dots. I present compelling evidence of photoinduced charge separation between zinc selenide and cadmium sulfide domains, via absorption and photoluminescence spectra, but can not provide conclusive evidence via transmission electron microscopy of the merging of the quantum dots. Also in this thesis I report on a colloidal synthesis of lead selenide, titanium dioxide heterostructures, comprising of small diameter lead selenide nanocrystals, grown onto the surface of titanium dioxide nanorods. The deposition of lead sulfide on titanium dioxide proceeds via formation of sub-2 nm lead selenide islands that can be controllably grown to 5 nm by introducing secondary precursor injections. Evidence of the formation of lead selenide nanocrystal islands on the titanium dioxide rods was determined via the acquisition of transmission electron microscopy images that confirm the statistically distributed formation of lead selenide islands.

Committee: Mikhail Zamkov PhD (Advisor); Robert Boughton PhD (Committee Member); Eric Mandell PhD (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2012, Physics and Astronomy (Arts and Sciences)

Device miniaturization with advanced fabrication techniques has revolutionized the semiconductor industry along with innovative concepts of carrier spin, potentially important at the fundamental physical limits of scalability. For spin-based information processing, semiconductor coupled quantum dots (CQDs) provide excellent control in spin dynamics due to 3-D confinement, discrete energy levels, and optical orientation and coupling. The research presented in this dissertation investigates spin interactions and exciton relaxation channels in semiconductor CQDs measured through optical control and time-resolved experimental techniques. Our experiments involving photoluminescence (PL) and photoluminescence excitation (PLE) methods revealed effects arising from the structural properties of semiconductor nanostructures, including quantum rings and CQDs. High resolution PL measurements on positively charged exciton states demonstrated experimental evidence of isotropic exchange interaction. Controlling exchange interaction in different spin configurations is fundamental to quantum logic operations. Hence, polarization dependent PL experiments were executed and electric field tunable exchange interaction effects were reported on the neutral exciton states. Next, time-resolved measurements were performed while pumping above the InAs wetting layer (WL) energy and probing below the WL to determine the dynamics of the optically generated electric field in CQDs. The observed, rapid onset of the optically generated electric field may provide the use of CQDs for optical switching applications. Finally, carrier relaxations in the CQDs were identified through the dynamics of the spatially indirect exciton state using a mode-locked laser excitation source and standard time-resolved single photon counting technique. Wave function distribution, carrier tunneling, and phonon scattering led to the observed lifetime and intensity modulations. With time-resolved PLE, bi-exponential lifet (open full item for complete abstract)

Committee: Dr. Eric A. Stinaff (Advisor); Dr. Alexander Govorov (Committee Member); Dr. Jeffrey Rack (Committee Member); Dr. Saw-Wai Hla (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Molecular Chemistry; Molecular Physics; Nanoscience; Nanotechnology; Optics; Physics; Quantum Physics; Solid State Physics

Master of Science (MS), Bowling Green State University, 2023, Physics

Colloidal semiconductor nanocrystals (NCs) have been utilized to great effect in optoelectronic applications in the last half-century due to their favorable optical and electronic properties. However, these materials suffer significant performance decline as optical or electrical excitation power increases. The culprit of this decline is the Auger recombination of multiple excitons that produces excess heat from the NC energy. This process poses a significant obstacle to NC performance in photodetectors, X-ray scintillators, lasers, and high-brightness LEDs. A new NC structure, semiconductor quantum shells (QSs) are an emerging NC morphology that solve this problem by providing one of the slowest rates of Auger decay for colloidal NCs yet. These NCs are synthesized in a series of time-dependent injections that layer the desired materials in successive shells. Early iterations of these NCs were still limited by a high rate of surface-trap recombination. Alloying the outer CdS layer with ZnS to produce a CdS-CdSe-CdS-ZnS QS has proven to reduce surface carrier decay. The resulting QSs have photoluminescence quantum yields as high as 90%, and biexciton emission quantum yields as high as 79%. These characteristics make QSs favorable for high-excitation applications when compared to other low-dimensional semiconductors.

Committee: Mikhail Zamkov Ph.D. (Committee Chair); Marco Nardone Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Materials Science; Physical Chemistry; Physics

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

We have used electrical transport measurements, photocurrent spectroscopy (PC), photoluminescence (PL) and photoluminescence excitation spectroscopies (PLE) to investigate electronic band structure and quantum confined states in single (MOCVD) grown semiconductor nanowires. The nanowires used in this study include Zn2As3, GaAs, GaAsSb, GaAs/AlGaAs core-shell and GaAs/AlGaAs core-multishell (Quantum well tube (QWT)) nanowires. Single nanowire devices were fabricated using photolithography to deposit metal contacts on either end of the nanowire to allow optoelectronic measurements. Electrical transport and photocurrent spectroscopy were used to characterize a novel II-V semiconductor nanowire device, Zn3As2, at room and low (10 K) temperature. Employing metal-semiconductor-metal modeling and self-consistent fitting, we have extracted relevant intrinsic semiconductor parameters from room temperature current-voltage characteristics. The extracted acceptor doping densities for nanowire and nanoplatelate devices are of 1.67×1018cm-3 and 7.41×1018cm-3 respectively. These values agree well with doping densities determined using transient Rayleigh scattering. The photocurrent spectra measurements reported here allow an estimation of the band gap of this material to be 1.13 eV which is in the near-infrared at 10 K. The electronic band structure of single GaAs nanowires was also characterized using photocurrent spectroscopy at room and low (10 K) temperature. In bare 100 nm diameter GaAs nanowire devices we observed the non-linear dark current arising from back-to-back Schottky behavior. We observed saturation of photocurrent as we increased the bias for fixed photon energy above the band gap at room temperature. We attributed this saturation to the diffusion length of minority carriers resulting in the complete extraction of photogenerated carriers at high biases. Photocurrent spectra at 10 K exhibit a peak near the band edge of GaAs ~1.5eV, in both bare GaAs core and GaAs/ (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Philip Argyres Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member); Frank Pinski Ph.D. (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

In this dissertation, a wide variety of optical spectroscopies including photoluminescence (PL), photoluminescence excitation (PLE), photoluminescence imaging, time-resolved photoluminescence (TRPL), as well as photocurrent (PC) spectroscopy have been used to explore the optical and electronic structures of Zn-doped GaAs twinning superlattices (TSL) and GaAs/AlGaAs quantum well tube (QWT) nanowires. Low Temperature PL spectra of a single Zn-doped GaAs TSL nanowire reveal a broad band low energy emission centerd at 1.47 eV in addition to the free exciton emission of zincblende GaAs. We do not observe any changes in the relative ratio of these emissions as a function of excitation power, suggesting that the low energy emission is not related to defects or carbon impurities, this is further confirmed by PC measurement. A simple theoretical model with holes confined in the wurtzite segment is developed in order to understand the origin of the emissions. In GaAs/AlGaAs QWT nanowires, low temperature PL spectra of 8 and 4 nm QWTs show that the exciton confinement energy have been shifted up by 57 meV and 185 meV with respect to the GaAs core. A first excited state transition has been observed in PLE spectra of only the 8 nm QWT. A simple theoretical modeling using a cylindrical quantum well shows the calculations are in good agreement with both direct and indirect transitions observed in PL and PLE measurements. A more detailed model including the hexagonal symmetric facets shows that the electron and holes ground states are strongly localized to the corners, while the excited states are confined to the long facets of the QWT. In order to gain more information about quantum confined energy in these radial heterostructures, we then studied a series of QWTs with thicknesses ranging from 1.5 to 8 nm. High resolution spatially-resolved PL images reveal several ultranarrow emission lines (localized states) distributed at different spatial positions along the long axis of (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Howard Everett Jackson Ph.D. (Committee Member); Randy Johnson Ph.D. (Committee Member); Nayana Shah Ph.D. (Committee Member) Subjects: Condensation

Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)

Semiconductor nanostructures such as quantum dots, quantum wires and quantum wells have gained significant attention in the scientific community due to their peculiar properties, which arise from the quantum confinement of charge carriers. In such systems, confinement plays key role and governs the emission spectra. With the advancements in growth techniques, which enable the fabrication of these nanostructured devices with great precision down to the atomic scale, it is intriguing to study and observe quantum mechanical effects through light-matter interactions and new physics governed by the confinement, size, shape and alloy composition. The goal is to reduce the size of semiconductor bulk material to few nanometers, which in turn localizes the charge carriers inside these structures such that the spin associated with them is used to carry and process information within ultra-short time scales. The main focus of this dissertation is the optical studies of quantum dot molecule (QDM) systems. A system where the electrons can tunnel between the two dots leading to observable tunneling effects. The emission spectra of such system has been demonstrated to have both intradot transitions (electron-hole pair residing in the same dot) and interdot transitions (electron-hole pair participating in the recombination origin from different dots). In such a system, it is possible to apply electric field such that the wavefunction associated with the charge carriers can be tuned to an extent of delocalizing between the two dots. This forms the first project of this dissertation, which addresses the origin of the fine structure splitting in the exciton-biexciton cascade. Moreover, we also show how this fine structure can be tuned in the quantum dot molecule system with the application of electric field along the growth direction. This is demonstrated through high resolution polarization dependent photoluminescence spectroscopy on a single QDM, which was described in great det (open full item for complete abstract)

Committee: Eric A. Stinaff Prof. (Advisor); Sergio E. Ulloa Prof. (Committee Member); Arthur R. Smith Prof. (Committee Member); Wojciech M. Jadwisienczak Prof. (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Nanoscience; Nanotechnology; Optics; Physics; Quantum Physics; Solid State Physics

Master of Science (MS), Bowling Green State University, 2014, Physics

Colloidal semiconductor nanocrystals are becoming widely used materials in developing high performing light emitting devices in the Infrared region. The ability of tuning their properties at the colloidal stage and easy-low-cost processing of these Quantum Dot solutions in to nanocrystal solid devices makes them a perfect candidate in the device engineering process. One of the main challenges that present methods of making Infrared emitting thin film devices face is that both quantum yield efficiency and stability is compromised when processing them from colloidal stage to the solid state. The proposed method provides a better solution to this problem allowing a better assembly of Infrared emitting PbS nanocrystals encapsulated into an all inorganic matrix of wide band gap CdS. The newly proposed Semiconductor Matrix Encapsulation Nanocrystal Array (SMENA) method provides a better passivation in the PbS surfaces which can be optimized to reduce the non-radiative exciton decaying processes preserving the emission characteristics of the film. Due to the strong localization of the electrical charges, the films fabricated using modified SMENA method shows a bright emission yield compared to the current reported techniques. In addition to a high emission quantum yield, fabricated films exhibit excellent thermal and chemical stability, which avails their integration into solid state IR emitting technologies.

Committee: Mikhail Zamkov (Advisor); Haowen Xi (Committee Member); Liangfeng Sun (Committee Member) Subjects: Chemical Engineering; Chemistry; Condensed Matter Physics; Engineering; Experiments; Materials Science; Nanoscience; Nanotechnology; Physics

Master of Science (MS), Bowling Green State University, 2013, Physics

I report the design, fabrication, and characterization of colloidal PbS nanocrystals (NC's semiconductors for there use as photo voltaic structures and thin films. I also explain the design and method for film deposition of the NC's for use of the Hall effect experiment. Using a layer by layer deposition technique via dip coating a controlled layer of PbS NC'S can be grown on top of an amorphous substrate. These films were electrically conductive with thickness from .365 (μ m) to .571(μ m) having a roughness on the average of Ra = 0.017(μ m). Conductivity of these films vaired by the size of the quantum dots and the ligands used to spatially separate adjacent quantum dots. PbS QD films were fabricated using 1,2-ethanedithiol (EDT), 3-mercatopropionic acid (MPA), 6-mercaptohexanoic acid (MHA) and 8-mercaptaoctanaic acid (MOA) to spatially separate quantum dots from one another. The EDT separated 7.2 nm QD film .365 (μ m) had a conductivity of (1.85 X 10-4Ω-1cm-1) and the .571 μm had a conductivity of (2.61 X 10-4Ω-1cm-1 ). The EDT separated 5.1 nm QD film .311 μm had a conductivity of (1.98 X 10-3Ω-1cm-1). Films using EDT ligands had the highest conductivity due to the main means of charge transport being tunneling through hopping conduction. These films using (MOA, MHA) produced photo-luminescent films with higher intensity and more distinguished absorption peaks. Hall mobilities couldn't be accurately determined due to low carrier concentration limiting the magnitude of the Hall voltage.

Committee: LiangFeng Sun (Advisor); Haowen Xi (Committee Member); Mikhail Zamkov (Committee Member) Subjects: Nanotechnology; Optics; Physics

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

Doctor of Philosophy (PhD), Ohio University, 2012, Physics and Astronomy (Arts and Sciences)

The study and control of individual carrier spins in semiconductor heterostructures can be effected by interaction with light. Owing to their high degree of tunability, the spectroscopic properties of vertically coupled InAs quantum dots grown on GaAs substrate are great candidates for study as they have not been fully characterized. The central theme of this dissertation is therefore to collect spectroscopic information, with the ultimate aim of identification and characterization of the spin and charge properties of quantum dots to help lay the groundwork for future uses in optical devices and design of efficient and reliable qubit states for quantum computers. The main results of this work include : (i) a detailed large sample size study of the electron and hole positions via dipole Stark shifts in various barrier sizes, leading to a model explaining how these positions are affected by quantum mechanical tunneling. This model successfully predicts the Stark shifts of previously unidentified exciton states; and (ii) the measurement of circular polarization memory properties of the various excitonic charge states that make up this system, which, in addition to aiding in the identification of the charge states in quantum dot molecules, reveal information about methods of mitigating the effects of the anisotropic exchange interaction, which is a potential spin decoherence mechanism. Finally, this work discusses in detail (iii) the design of a polarimetry setup to measure with a high degree of precision the Stokes parameters of the coupled quantum dot system, which elucidate the complete polarization state of the various charge states, with the ultimate goal of using this information to select the states that are best suited for quantum computation applications.

Committee: Eric Stinaff PhD (Advisor); Martin Kordesch PhD (Committee Member); Hugh Richardson PhD (Committee Member); Sergio Ulloa PhD (Committee Member) Subjects: Condensed Matter Physics

Doctor of Philosophy (PhD), Ohio University, 2011, Physics and Astronomy (Arts and Sciences)

We investigate different aspects of the coherent dynamics of excitons in quantum dot molecules. A theoretical model is developed in order to extract the Forster energy transfer signatures in tunnel coupled quantum dot molecules in the presence of strong interdot tunneling. It is found that Forster coupling can induce spectral doublets in the excitonic dressed spectrum, which is suitable for detection in level anticrossing spectroscopy. The coherent exciton dynamics is investigated both in the closed and open quantum system approach by means of the Lindblad master equation. An adiabatic elimination procedure using the projection operator formalism allows us to extract effective Hamiltonians to describe analytically all relevant anticrossing gaps of the dressed spectrum. It is found that a pair of two indirect excitons can be used as the computational basis of a qubit. An adiabatic control pulse is constructed in order to manipulate the indirect exciton qubit and characterize its coherent dynamics, as well as its decoherence due to spontaneous recombination. On the other hand, recent experiments have shown that indirect excitons in hole tunnel coupled quantum dot molecules exhibit indirect exciton oscillatory relaxation rates, as function of an applied electric field. To this end we developed a model for the experimental results, in which we incorporate relaxation due to exciton-acoustic phonon coupling. We characterize the scattering structure factor and found that it contains an electrically tunable phase relationship between the phonon wave and the hole wave function, which leads to interference effects and oscillatory relaxation rates.

Committee: Sergio Ulloa Dr (Advisor); Alexander Govorov Dr. (Committee Member); Eric Stinaff Dr. (Committee Member); P. Gregory Van Patten Dr. (Committee Member) Subjects: Materials Science; Molecular Chemistry; Molecular Physics; Nanoscience; Optics; Physics; Quantum Physics; Solid State Physics; Theoretical Physics

Doctor of Philosophy (PhD), Ohio University, 2011, Physics and Astronomy (Arts and Sciences)

In this thesis, we study the electronic transport properties through nanostructures in which the effects of spin-orbit interaction play non-trivial roles. Firstly, we investigate the spin-dependent transport properties of two-dimensional electron-gas systems formed in diluted magnetic semiconductors and in the presence of Rashba spin-orbit interaction in the framework of the scattering matrix approach. We focus on nanostructures consisting of realistic magnetic barriers produced by the deposition of ferromagnetic strips on heterostructures. Secondly, we study ballistic transport through semiconductor quantum point contact systems under different confinement geometries and applied fields. In particular, we investigate how the lateral spin-orbit coupling, introduced by asymmetric lateral confinement potentials, affects the spin polarization of the current. Finally, we present studies of the Coulomb blockade and Kondo regimes of transport of a quantum dot connected to current leads through spin-polarizing quantum point contacts (QPCs). We address the spin-dependent transport properties of the QD structures where electron-electron correlations dominate the electronic dynamics. We show that QPCs can generate finite spin-polarized currents due to the combination of lateral and perpendicular spin-orbit interactions. Our theoretical investigations provide a new approach for exploring spintronic devices, spin polarized sources and spin filters.

Committee: Sergio Ulloa E (Advisor); Art Smith (Committee Member); Martin Kordesch (Committee Member); Savas Kaya (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2010, Physics and Astronomy (Arts and Sciences)

Coupled quantum dots present an active field of study, both at the fundamental and applied level, due to their atomic and molecular-like energy structure and the ability to design and tune their parameters. Being single-photon emitters, they are systems that behave fully according to the laws of quantum mechanics. The work presented here involved the experimental study of the electro-optical properties of Indium Arsenide, coupled quantum dots. Initial experiments involved the use of spectroscopic methods such as photoluminescence and photoluminescence excitation (PLE). Through such techniques, the top dot's hole energy level structure was mapped and different types of resonant absorption were identified. The characterization of these excited states and the knowledge of how to resonantly excite into them is an integral part of the development of certain controlled spin gates in quantum computation. Additionally, a shift of the spectra in the electric field was observed with varying excitation wavelength through and above the wetting layer, which allowed for direct measurement of the optically-created electric field within the device. This extends the quantum dots' capabilities to using them as electric-field nano-probes and opens up the possibility of an all-optical, fast switching mechanism. In the course of these studies, a novel data visualization method for PLE in this type of system was developed. Finally, to study correlated photon effects, a Hanbury Brown - Twiss experiment was built which revealed bunching and antibunching signals typical of quantum statistics in biexciton cascade emissions. This is an important step towards the experimental investigation of entangled states in coupled quantum dots.

Committee: Eric A. Stinaff PhD (Advisor) Subjects: Experiments; Optics; Physics; Technology

Doctor of Philosophy (PhD), Ohio University, 2004, Physics (Arts and Sciences)

Physical nanoscale systems have been analyzed both from an electrostatic point of view and quantum mechanically with respect to quantum computation. We introduce an elaborate code for the efficient numerical simulation of nanoscale electrostatics via a higher – order relaxation algorithm with a large variety of boundary conditions which then is applied to a set of physically relevant problems. Great emphasis is put on screening effects as well as capacitive coupling between spatially separated conducting regions. Specifically, we analyze the depletion of a two – dimensional electron gas using different methods. The effect of surface charges due to the pinning of the Fermi level at a semiconductor surface is shown to play an important role in that it can shift the whole system characteristics, underlining the importance of chemical potentials and work functions. The capacitive coupling is further used to model the interactions in an interacting network of quantum dots, and the use of the capacitance formalism in the quantum mechanical context is explicitly justified. Quantum dot arrays are then analyzed on a general footing with respect to quantum computation and charge qubits based on an extended Hubbard Hamiltonian model. For systems with at most two operative electrons, general restrictions apply, introducing certain constraints on what realizations of this type of charge qubit may eventually look like. Furthermore, the interaction of the macroscopic world with the quantum dot network via quantum gates is discussed. Again, general arguments allow us to rule out certain scenarios of quantum gates. For example it turns out that capacitive coupling alone is not sufficient for full single qubit operation. Alternative ways are discussed, and finally, by using an external magnetic field and its resulting Aharonov – Bohm phases on the array, full single qubit operation based on charge is demonstrated.

Committee: Sergio Ulloa (Advisor) Subjects: Physics, Condensed Matter

Master of Science (MS), Ohio University, 2013, Physics and Astronomy (Arts and Sciences)

The use of metal droplet epitaxy may provide a novel method of growing laterally coupled nanostructures. We will present optical studies of InAs/GaAs nanostructures which result in twin quantum dots (QD) formed on a single quantum ring (QR). Previous studies have investigated the coupling between vertically grown quantum dot pairs. In this thesis, we have used photoluminescence (PL) and photoluminescence excitation (PLE) to examine the possibility of energy transfer and coupling between quantum dot pairs in a single InGaAs quantum ring grown by droplet epitaxy. Power dependent photoluminescence spectra reveal a few peaks at low power, which are identified with emission from the ground state of the individual dots. As the power is increased we observe multi-exciton and excited state emission. We then perform PLE, tuning the excitation laser energy continuously from the high energy ring emission down to the individual dot states. We have observed resonant PLE emission in the QD/QR structures both at high laser energy and when resonant with the identified states of one of the QDs which may indicate energy transfer and/or coupling between the dots.

Committee: Eric Stinaff Professor (Advisor); David Drabold Professor (Committee Chair); Alexander Govorov Professor (Committee Member) Subjects: Condensed Matter Physics

PhD, University of Cincinnati, 2023, Engineering and Applied Science: Chemical Engineering

The research presented in this dissertation focuses on the remarkable photothermal properties and application potential of carbon quantum dots (CQDs), specifically red graphene quantum dots (RGQDs). These nanomaterials exhibit promising photothermal performance across various applications, with a meta-analysis revealing the impact of synthesis protocols, precursor types, and numerous factors on their performance. Despite significant progress, challenges persist in achieving the desired functionality compared to metallic competitors. Strategies and future research directions are outlined, emphasizing the need for mechanistic understanding, a model for parameter optimization, and the exploration of new applications. Therefore, our work delves into the synthesis and characterization of Cyan, Green-Yellow, and Red Color Emissive Carbon Dots (CDs). The study uncovers the intricate relationship between synthesis conditions and the resulting chemical and optical properties. The role of heteroatoms (O and N) in fluorescence emission characteristics is elucidated, offering opportunities for tailored optical properties. In the context of photothermal heating, a comprehensive analysis involving these carbon dots is presented, showcasing the impressive photothermal conversion efficiency (PCE) of RGQDs. The research investigates factors affecting the PCE of CDs and provides a mechanistic understating of their photothermal conversion as promised. The compatibility of RGQDs in Polymerase Chain Reaction (PCR) is explored, revealing their impact on DNA condensation which opens the path for potential applications in molecular biology and nanotechnology research. Innovative approaches are proposed such as the RGQDs/Hydrogel setup that offers the utilization of RGQDs as photothermal heating agents to develop a cost-effective, portable, and efficient solution for in Nucleic Acid Amplifications (NAAs). Customer interviews and a (open full item for complete abstract)

Committee: Aashish Priye Ph.D. (Committee Chair); Benjamin Yavitt Ph.D. (Committee Member); Vesselin Shanov Ph.D. (Committee Member); Jason Heikenfeld Ph.D. (Committee Member) Subjects: Chemical Engineering

Master of Science (MS), Bowling Green State University, 2023, Physics

Due to their tunable energy gap and high quantum efficiency, lead sulfide (PbS) nanosheets exhibit high potential in photovoltaics and optoelectronics applications. Exciton dynamics play a vital role in the performance of the devices based on this material. We measured the time-resolved photoluminescence spectra of PbS nanosheets to study the dynamics of the excitons in PbS nanosheets. The photoluminescence peak of the nanosheets shifts to longer wavelengths in a few hundred nanoseconds time window after excitation by a picosecond pulsed laser. The Stark effect leads to the redshift of the photoluminescence spectrum of the PbS nanosheets. The dangling bonds of the surface sulfur atoms lead to a static electric field which reduces the energy gap of the nanosheet – so called Stark effect. Under pulsed laser excitation, the photoexcited charge carriers (excitons) screen the static electric field, reducing the Stark effect. Consequently, a blue-shifted photoluminescence peak, corresponding to a larger band gap, is observed. The carrier recombination resumes the Stark effect, leading to a red shift of the photoluminescence peak. Since the surface states, either charged or polarized, attract the excitons to them before they undergo recombination, the time-dependent photoluminescence spectrum reveals the exciton diffusion process in the 2D nanosheets. The control experiment using quantum dots, however, does not show such time-dependent red shift of the photoluminescence spectrum. The absence of the red shift is due to the small size of the quantum dots where no excess excitons exist to reduce the Stark effect.

Committee: Liangfeng Sun Ph.D. (Committee Chair); Marco Nardone Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Materials Science; Nanotechnology; Physics

PhD, University of Cincinnati, 2022, Engineering and Applied Science: Chemical Engineering

The concentration of carbon dioxide (CO2) in the atmosphere reaches a record high level of 412 parts per million. This high CO2 concentration has caused a series of undesirable climate effects, like erratic weather. There is a pressing need to develop a sustainable strategy to achieve negative CO2 emissions. Electrochemical CO2 reduction at ambient conditions employing renewable energy is recognized as a viable technology for the distributed generation of chemicals such as methane (CH4) and ethylene (C2H4) using CO2 recovered from industrial exhaust streams. This dissertation optimizes the CO2 reduction performance from the aspects of intrinsic catalyst design and extrinsic micro-environment regulation. Chapter 2 rationally designs metal-free graphene quantum dots (GQDs) catalysts for CO2 to CH4 conversion by regulating functional groups. The CH4 Faradaic efficiency reaches 70% at 200 mA cm-2 partial current density. Electron-donating functional groups facilitate the yield of CH4 while electron-withdrawing groups suppress CO2 reduction. Chapters 3, 4, and 5 study the role of the micro-environment in promoting the active and durable CO2 to C2H4 conversion. Chapters 3 and 4 design tandem gas diffusion electrodes (GDE), which integrate a CO-selective catalyst for complementary CO supply and a C2H4-selective catalyst for further CO reduction, to implement the cascade CO2?CO?C2H4 conversion. The C2H4 productivity was determined to be positively correlated with CO concentration (reaction order > 0). Therefore, the tandem GDEs were designed following the principle of plug flow reactor (PFR) in which CO intermediate conversion maximizes compared to the counterpart of continuous stirred tank reactor (CSTR). Chapter 3 investigates the PFR-analogous layered tandem GDEs with a CO-selective catalyst layer (CL) on the top and a C2H4-selective CL underneath. The CO concentration tops at the electrode/electrolyte interface and is consumed gradually along the electrode (open full item for complete abstract)

Committee: Jingjie Wu Ph.D. (Committee Member); Joo-Youp Lee Ph.D. (Committee Member); Maobing Tu Ph.D. (Committee Member); Yujie Sun Ph.D. (Committee Member); Peter Panagiotis Smirniotis Ph.D. (Committee Member) Subjects: Chemical Engineering

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2022, Photochemical Sciences

The following work introduces a novel approach for shape and size control of semiconductor nanocrystals within colloidal solution by thermodynamically-driven aggregative growth. The presented technique is based on the employment of coordinating ligands that reduce surface energy, resulting in crystal melting. Compared to traditional growth approaches with precursors, here, nanoparticles act as building blocks offering more predictive control over the course of coalescence. We demonstrate that an innovative approach to thermodynamically-driven aggregative growth of colloidal semiconductor nanocrystals yields unique and diverse geometries (cubes, spheres, nanorods, nanorings) with narrow size dispersion by the use of X-, L-, and Z-type coordinating solvent, making it a valuable technique for the development of new optoelectronic materials. We also investigated the effect of external electric field on products of thermodynamically-driven aggregative growth. It was shown that solution-proceeded semiconductor nanocrystals undergo photoinduced rotation driven by excited-state dipole moment and counterbalanced by the viscosity of a solvent. Compared to solid assemblies, where dipoles are randomly positioned, solution-proceeded nanocrystals align along electric field and cause prominent optical changes due to quantum confined Stark effect. We demonstrate that organized alignment can be preserved by slow crystallization from a solid environment. This unique approach could aid the development of new electro-optical and voltage-sensitive devices for exotic applications.

Committee: Mikhail Zamkov Ph.D. (Advisor); Mihai Staic Ph.D. (Other); Liangfeng Sun Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Chemistry; Nanoscience; Physics

Master of Science in Materials Science and Engineering (MSMSE), Wright State University, 2021, Materials Science and Engineering

Semiconducting nanoparticles have received significant attention due to their unique optoelectronic properties. Quantum dots (QDs), a class of spherical nanoparticles, possess a size-dependent bandgap and photoluminescence at visible wavelengths. QDs have many applications including biological labelling, solar cells, chemical impurity detection, and optical glasses. Doping QDs into optical glasses is highly desirable. High-quality QDs can be synthesized via liquid solution methods. However, solution-synthesized QDs often degrade over time and they cannot survive incorporation into a glass melt without protection. In this work, the aqueous synthesis of ZnSe QDs and coating with nanometer silica and alumina protective shells are investigated. The effects of synthesis conditions on the structure and coating quality of ZnSe nanocrystals are systematically assessed via X-Ray Diffraction, Electron Microscopy, and UV-Vis Spectroscopy. Results indicate the successful fabrication of ZnSe nanocrystals and deposition of both silica and alumina shells.

Committee: Hong Huang Ph.D. (Advisor); Raghavan Srinivasan Ph.D., P.E. (Committee Member); Steven Fairchild Ph.D. (Committee Member) Subjects: Materials Science; Nanoscience; Optics