Master of Science (MS), Ohio University, 2023, Physics and Astronomy (Arts and Sciences)

The pursuit of novel spintronic and quantum information devices has ignited a growing interest in rare-earth elements and their complexes, which possess exceptional magnetic, chemical, optical, and catalytic properties arising from their distinctive electronic confgurations. As a result, they exhibit immense potential as prospective candidates for the next generation of quantum information technologies. To unlock their full potential, intensive investigations have been conducted using a nascent instrumentation technique known as synchrotron X-ray scanning tunneling microscopy (SX-STM). In this particular study, we examine X-ray absorption spectra (XAS) of lanthanum and terbium complexes based on pyridine dicarboxamide that have been absorbed onto an HOPG substrate measured by using SX-STM set-up at the XTIP beamline in the Advanced Photon Source at Argonne National Laboratory. The X-ray Absorption Spectroscopy (XAS) spectra obtained exhibit distinct M4,5 absorption edges of lanthanum in the far feld regime, where only X-ray ejected electrons contribute to the observed spectra. Remarkably, the M4,5 absorption edge spectra of both lanthanum and terbium show signifcant intensity even in the tunneling regime, where X-ray excited tunneling plays a signifcant role in the spectra recorded by the SX-STM tip. Due to the extreme sensitivity of quantum tunneling to atomic positions, the observed spectra stem exclusively from X-ray excitations of a single rare-earth ion confned within a molecular complex. The validity of this claim is supported by the lack of M4,5 edge signals detected from La and Tb in the measurements, indicating that the tip is located far from the proximity of the rare-earth ions.

Committee: Hla Saw (Advisor); Justin Frantz (Committee Member); Sergio Ulloa (Committee Chair) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2022, Physics

This thesis uses scanning tunneling microscopy (STM) to study Fe dopants deposited on GaAs (110). Fe, a transition metal, can introduce magnetism into a nonmagnetic semiconductor host, GaAs, which could allow mass storage and processing of information simultaneously. An STM is a powerful tool to study individual dopants with atomic resolution. It can generate topographic images, giving structural information about the individual dopants in the semiconductor host; it can measure the electronic properties of the individual dopants, and with a spin-polarized tip, it can measure the local magnetic properties as well. The research presented here compares STM measurements taken with both a nonmagnetic and magnetic tip and shows the first magnetic contrast of individual magnetic dopants on a III-V semiconductor. We propose a preliminary model of the spin-polarized tunneling current which is affected by the tunneling rate from the sample bulk to the dopant, and the rate of tunneling from the dopant to the spin-polarized tip, as well as the exchange coupling to other spin defects.

Committee: Jay Gupta (Advisor); Andrew Heckler (Committee Member); Fengyuan Yang (Committee Member); Nandini Trivedi (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy (PhD), Ohio University, 2015, Physics and Astronomy (Arts and Sciences)

Since its inception, scanning tunneling microscopy (STM) has developed into an indispensible tool for surface science. Its sub-nanometer spatial resolution in both real space imaging and tip-sample interactions continue to demonstrate versatility in the study of novel phenomena at materials' surfaces. This dissertation explores the expansion of experimental techniques in STM through application of two uncommonly exploited interactions at the tip-sample junction: X-ray absorption and the electric field between the tip and sample. This study begins by targeting the STM tip-sample junction with X-ray photons tuned to core level electron energies of atomic species on the sample. Interactions of atomic islands with the incident light are employed to introduce elemental sensitivity in STM with a resolution of just 2 nm. Elementally sensitive images are produced simultaneously with conventional STM images, and exploited to probe X-ray cross section behavior for structures measuring just a few tens of nanometers in the lateral directions. Additionally, point spectroscopic measurements of X-ray absorption behavior vs. incident photon energy facilitate the detection of local variations in emitted electron density due to the X-ray interactions. Locally measured electron emission densities measured by specialized SXSTM smart tips demonstrate a clear dependence on incident photon energy. Next, electric field interactions between the tip and sample are used to investigate the behavior of strongly dipolar molecular rotor networks. Symmetry and structure in the molecular networks are found to inhibit rotation of molecular rotors that exhibit thermally induced switching at temperatures as low as 5 K for isolated molecules. Additionally, inelastic electron tunneling is employed to induce controlled directional rotation in a different, single molecule motor system. The directionality is explained through the structure of calculated potentials in the motor system. The mechanis (open full item for complete abstract)

Committee: Saw-Wai Hla Dr. (Advisor) Subjects: Condensed Matter Physics; Experiments; Low Temperature Physics; Molecular Physics; Molecules; Nanoscience; Nanotechnology; Physics

Doctor of Philosophy, The Ohio State University, 2003, Physics

In this work it is first shown that internal gain can be applied specifically to hot BEEM electrons without amplifying standard BEEM noise sources. It is shown that BEEM with single hot electron sensitivity (approximately a factor of 1000 improvement in the minimum detectable BEEM signal) is attainable with modified commercially existing avalanche photodiodes. This allows useful data collection at lower signal levels than previously possible. With this new low-signal capability, it was obvious that a new BEEM-like signal was being detected. We have discovered that STM tunneling generated photons that will create a false signal in most BEEM samples. Furthermore, we have characterized this effect which we call "STM-PC" and it is demonstrated with Pd/SiO2/Si and Au/SiO2/Si samples that this false signal closely mimics BEEM and is easily confused for BEEM. We discuss ways to separate real BEEM from this new effect. Separately, thermally generated kinks on steps on the Si(001) surface are counted and analyzed to determine the energy of the SB-type step. Previous work by others is extended by counting a new type of feature, the "switch" kink, to allow a more accurate determination of the energy of SB-steps in the presence of defects that will bow steps and cause non-thermal kinks. Extensive data collection along with this new extension allows a more accurate determination of the B-type kink energy than before and the first experimental evidence that this energy increases with tensile strain on the Si(001) surface. Modifications to an Omicron Variable Temperature Scanning Tunneling Microscope (VT-STM) will be presented. The VT-STM will be moved to the Electrical Engineering Department cleanroom of The Ohio State University and will allow in-situ studies of Molecular Beam Epitaxy (MBE) grown samples. Modifications, repairs, and operating procedures will be discussed for the VT-STM and supporting hardware. The bulk of the modifications to be discussed have been to allow samp (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor) Subjects:

Master of Science (MS), Ohio University, 2007, Physics (Arts and Sciences)

This thesis presents a transmission electron microscope fitted with a home designed and built nano manipulator for novel in situ experimentation. The selectivity and nano scale manipulation capabilities of the manipulator were tested on zinc oxide nanorods. Further experimentation was performed on grown gallium oxide nanobelts prepared in an argon gas flow at 950 degrees centigrade. A nanobelt was selected for deformation with the manipulator and a series of manipulations and corresponding diffraction patterns were collected. Preliminary analysis shows trends in the diffraction patterns corresponding to lengthening and contracting of lattice constants ‘a' and ‘c' by a maximum of 3 and 5 Angstroms respectively.

Committee: Martin Kordesch (Advisor) Subjects: Physics, Condensed Matter

Doctor of Philosophy, The Ohio State University, 2024, Physics

Introducing magnetism to topological insulators can produce a variety of interesting macroscopic quantum phenomena and open new paradigms for energy-efficient and high-performance computing. This dissertation presents the first scanning tunneling microscopy study of a novel heterostructure consisting of the two-dimensional itinerant ferromagnet Fe3GeTe2 (FGT) and the topological insulator Bi2Te3. The electronic and topographic structure is characterized with atomic resolution, providing insights into the interface between magnetism and topology. I show that FGT rotationally aligns to the Bi2Te3 and both materials are unstrained with an electronic density of states identical to their bulk counterparts. Bi2Te3 is confirmed to retain its topological properties via quasiparticle inference imaging of the topological surface state. FGT is shown to retain its ferromagnetic properties down to the monolayer limit via MCD measurements. In addition, this dissertation details significant development towards a nanoscale magnetism measurement technique, FMR-STM. I demonstrate a reliable and efficient procedure to measure the transfer function to a sample with a strongly nonlinear I(V) curve. The transfer function is used to apply radio frequency (RF) excitations from 1 to 20 GHz with constant amplitude at the tunnel junction. I show a measurement of the thermoelastic expansion due to heating from the RF absorption associated with cable resonances. This is an important background signal for future FMR measurements. Additionally, I report the discovery of novel RF effects on field emission resonance (FER) states and demonstrate a proof-of-principle measurement of the relative transfer function utilizing the shift in FER energies. Lastly, I present software that I developed: MacroQueue. It provides a simple GUI to allow users to automate STM measurements throughout the entire parameter space without requiring coding. Currently, MacroQueue includes functions to co (open full item for complete abstract)

Committee: Jay Gupta (Advisor); Chris Hirata (Committee Member); Brian Skinner (Committee Member); Roland Kawakami (Committee Member) Subjects: Condensed Matter Physics

Doctor of Philosophy (PhD), Wright State University, 2023, Human Factors and Industrial/Organizational Psychology PhD

Instead of characterizing transfer from short-term memory to long-term memory as the relocation of information from one structural system to another, I propose a theory that conceives of transfer as the learning processes that act on and transform the representations of the information itself. Dynamic Associative Memory posits that recently encoded memories are supported by active maintenance and the relevance of the current context. Over time, the current context becomes less relevant; therefore, the brain must learn contextually invariant associations between memories so that they may support themselves. I instantiated my theory in the ACT-R cognitive architecture and created a new module to automate and fully integrate attentional refreshing into the architecture. The DAM module extends ACT-R's spreading activation to allow activation to be shared among related items in declarative memory. It implements a novel associative learning process based on causal inference that stochastically generates new memory traces for associations between items proportionate to the causal power of one item to predict the other. I also developed another module to provide ACT-R models with a principled method for updating temporal context, and I proposed similarity functions for quantifying the contextually invariant relatedness of hierarchical relationships and the contextually mediated relatedness of features. I ran three simulation studies, systematically manipulating cognitive load, encoding instructions, and the repetition and semantic content of the to-be-remembered items, to investigate the fitness and predictions of the new model. Recall of elaborated words was better than unelaborated words, which were recalled better than non-words. Recall of lists composed of items with less semantic content benefited more from repetition. The model failed to reproduce the benchmark cognitive load effect in immediate recall, but the effect returned in delayed recall, suggesting that (open full item for complete abstract)

Committee: Ion Juvina Ph.D. (Committee Chair); Joseph Houpt Ph.D. (Committee Member); Glenn Gunzelmann Ph.D. (Committee Member); Valerie Shalin Ph.D. (Committee Member); Herbert Colle Ph.D. (Committee Member) Subjects: Cognitive Psychology

Doctor of Philosophy (PhD), Ohio University, 2024, Physics and Astronomy (Arts and Sciences)

A nascent instrument called Synchrotron X-ray Scanning Tunneling Microscope (SXSTM), which challenges the limits of conventional X-ray absorption spectroscopy (XAS) methods by accessing elemental and chemical details of matter at the ultimate atomic limit, is employed in this dissertation to observe and investigate the magnetism at the ultimate atomic limit. First, a series of X-ray Magnetic Circular Dichroism (XMCD) signals deduced from near field XAS signatures in SXSTM tip channel are used to observe surface magnetism in Ni islands grown on Cu(111) while that of the sample channel is used to identify ensemble magnetism. We observe that the magnetic moments at the surface are enhanced compared to that of the sample. A comparative study also reveals that the magnetic moments of a magnetized sample are elevated compared to that of a nonmagnetized sample. In this work we establish the first ever detection of magnetism at the ultimate atomic limit by capturing the XMCD signature of an Eu atom caged in pcam ligands which are adsorbed on top of magnetized Ni islands on Cu(111), hence displaying magnetism due to a Van-Vleck like effect while coupling ferromagnetically to the host Ni layers. Differential conductance (dI/dV) measurements backed with theory demonstrate that the Eu3+ becomes magnetic by transitioning into a non-zero total angular moment (J>0). Dimer molecules are formed with a mixture of two different precursors containing Eu and Tb in this dissertation, using Ullmann reactions on Au(111) which exhibit different chirality and various types of clustering upon dimer formation, while sequences of near field XAS point spectra provide evidence towards dimers with different as well as similar rare earth (RE) atoms caged in the same dimer. They further reveal that both Eu and Tb are preserving their original +3 oxidation state in the dimers. We also report the first-ever radiograph of a single atom by means of X-ray images carried out at M5 edges of Eu (open full item for complete abstract)

Committee: Saw Hla (Advisor); Eric Masson (Committee Member); Eric Stinaff (Committee Member); Sergio Ulloa (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Nanoscience; Physics

Doctor of Philosophy, The Ohio State University, 2023, Physics

This thesis describes the use of scanning tunneling microscopy (STM) to study oxidized Cu(100) and Cu(111) model surfaces, as well as the development of X-ray and extreme ultraviolet STM to address the lack of elemental sensitivity in conventional STM measurements. Copper based photocatalysts have shown the ability to selectively reduce CO2 into useful C1 and C2 products, but the mechanism is not well understood. Any CO2 reduction process that occurs on a catalyst surface necessarily begins with the adsorption of CO2, and STM is able to provide atomic scale information on the surface-CO2 interaction. It can probe adsorption geometry, interactions with potential active sites, such as defects or step edges, and investigate surface diffusion. The Cu(100) and Cu(111) single crystal surfaces were used as model systems in this thesis, and STM was used to investigate their oxidation. The Cu(100) surface was found to form the well-known Cu(100) missing row reconstruction, and also to form steeply sloped sections punctuated by terraces of the Cu(110) c(6×2) oxygen induced reconstruction. The Cu(111) surface was shown to form the √13R46.1° × 7R21.8° reconstruction, referred to as the 29 reconstruction, as well as a new hexagonal structure which may be the result of a three layer moire pattern. CO2 was dosed at cryogenic temperatures onto the oxidized surfaces, and was found to weakly physisorb, where individual molecules could not be stably imaged with the STM tip. A longstanding weakness of conventional STM is the lack of elemental specificity. This can be addressed by illuminating the sample with light that is energetic enough to excite core level transitions, which results in a spike in photocurrent detected by the STM tip. X-ray STM measurements were conducted at the XTIP beamline at the Advanced Photon Source at Argonne National Lab on chiral magnetic thin films of MnGe and FeGe, and the absorption edges of Mn, Ge, and Fe were detected with the STM tip both in and out (open full item for complete abstract)

Committee: Jay Gupta (Advisor); Enam Chowdhury (Committee Member); Ezekiel Johnston-Halperin (Committee Member); Ciriyam Jayaprakash (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2022, Physics

As conventional electronics approach the physical limits of size and speed, desire for more capable and energy efficient technologies has driven studies of the individual and collective behavior of electron spins within solid state materials to achieve new forms of information processing. Recent developments in this field of spintronics have led to the discovery particle-like magnetic textures called magnetic skyrmions where spins are arranged in a swirling, vortex-like pattern. To advance the understanding of materials capable of stabilizing magnetic skyrmions and the mechanisms that govern their stability and dynamical properties, we use spin-polarized scanning tunneling microscopy. The research presented here builds on the connection between the physical and magnetic structure at the surface of materials. First, we observe novel surface structures and investigate the electronic and magnetic properties of Cr(001) and Fe/Ir(111), two systems that can be used to characterize the magnetic tip of the scanning tunneling microscope. Next, we detail a technique to determine the grain structure of B20-type magnetic materials FeGe and MnGe, where the structural chirality is known to determine the chirality of the magnetic textures. The surface termination, stacking order of the layered atomic lattice, and grain chirality can all be identified by analysis of the atomic corrugation and insights from density functional theory calculations. Finally, we observe novel topological spin textures at the surface of MnGe(111). The stabilization of these is correlated to structural deformation of the film, and their three-dimensional structure is deduced based on comparison of the spin-polarized imaging with micromagnetic simulations.

Committee: Jay Gupta (Advisor); Mohit Randeria (Committee Member); Roland Kawakami (Committee Member); Enam Chowdhury (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2022, Physics and Astronomy (Arts and Sciences)

Nanoscience research is important to our life. Exotic properties of molecules are of interest to physics, chemistry, biology, materials science and even industries for their applications ranging from the nature of molecular synthesis to optical, electronic, and biological use as well as quantum information processing. Using a scanning tunneling microscope tip as a characterization and manipulation tool, individual molecules as well as some self-assembled networks of four molecular systems on metal surfaces are investigated in this dissertation. Depending on the uniqueness of the system of study, the investigations include electronic, vibronic, structural, and mechanical properties of individual molecules or self-assembled molecular networks at the atomically clean environment. As one type of blue organic light emission diode (OLED), sexiphenyl molecule offers great applications in light emission with its unique electronic structure. Molecular vibrations are important for understanding its energetic dissipation. In order to understand the molecular vibrations, first, physical properties of 1D sexiphenyl molecules are characterized. Then vibrational modes of sexiphenyl molecules adsorbed on Ag(111) are studied. Two molecular vibrational modes are found at one monolayer of sexiphenyl film. In addition, the surface phonon excitation of the Ag(111) substrate is verified to be coupled with the molecular vibrations. Metal-organic frameworks are useful in their gas separation, fuel storage and catalysis. Moreover, similar to their semiconductor counterpart, the heterostructure of MOFs also plays a key platform for band gap tuning, band alignment and charge confinement. Here, physical structures of self-assembled molecular networks of 1D terphenyl and sexiphenyl molecules are studied. The exotic patterns of zigzag and honeycomb networks are formed in specific deposition conditions and metal organic framework (MOF) models are proposed to explain the periodical unit fo (open full item for complete abstract)

Committee: Saw Wai Hla (Advisor); Jixin Chen (Committee Member); Eric Stinaff (Committee Member); Nancy Sandler (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2022, Physics and Astronomy (Arts and Sciences)

In this dissertation, studies on atomic-scale characterization and manipulation of molecular systems and heterostructures for potential applications in the emerging field of molecular nanotechnology are presented. Observing and investigating exotic properties of molecular systems often require novel techniques and state-of-the-art instrumentation; thus, we report the development and commissioning of the world's first synchrotron Xrays beamline, dubbed “XTIP”, dedicated to the synchrotron X-rays scanning tunneling microscopy (SX-STM) technique that was used on all the research projects in this dissertation. For the projects, first, we report on unexpected magnetic interface phenomena in molecular Co adsorbed on oxygenated Fe film as well as in Co/Ni/Cu(111) nanoscale heterostructure probed using the X-ray dichroism technique (XMCD). We observed that the magnetic moment of Ni islands in the heterostructure shows a considerable reduction due to partial filling of the unoccupied Ni 3d orbitals due to charge injection. In addition, using the SX-STM technique, we also report on the first-ever elemental and chemical characterization of individual Fe atoms in a molecular environment. Finally, guided by the X-rays absorption spectroscopy measurements on a rare-earth-based molecule, which shows the existence of net charges in the molecule on Au(111), we have developed a molecular motor with 100% control over its rotation direction mediated by counterions. These results open a new dimension of research where synchrotron X-rays are used to characterize materials one atom at a time.

Committee: Saw-Wai Hla (Advisor) Subjects: Condensed Matter Physics; Molecular Chemistry; Molecular Physics; Nanoscience; Nanotechnology; Physics

Master of Science, The Ohio State University, 2022, Chemical Physics

Ultra-Fast Scanning Tunneling Microscopy (UFSTM) is a novel imaging technique that uses high intensity femtosecond laser light to generate atomic defect states called "traps" and provide subsequent atomic surface imaging between pulses. Many surface engineering applications are possible through femtosecond laser induced damage (fs-LID), a process where a strong non-perturbing laser electric field generates a high density of charge carriers that eventually thermalize and impart energy into a material's lattice changing the surface morphology. The number of pulses during illumination plays a large role in the resulting morphology with current theory predicting the formation of trap states in-between pulses. The trap states are too subtle to observe using traditional methods (ie. scanning electron microscopy or optical based microscopy). This thesis presents a coordinated effort in developing a novel scanning tunneling microscope system capable of detecting atomic trap states. In particular, a discussion of the instrumentation challenges associated with UFSTM techniques are presented. This work also includes a brief summary of the X-ray STM imaging technique that can also be used for elemental resolved surface imaging. The instrumentation challenges associated with coupling x-ray light with a standard STM system is included.

Committee: Jay Gupta (Advisor); Enam Chowdhury (Committee Member) Subjects: Materials Science; Morphology; Optics; Physics

Doctor of Philosophy (PhD), Ohio University, 2021, Physics and Astronomy (Arts and Sciences)

In this dissertation, a molecule containing Rare Earth (RE) atoms was used to analyze the RE elements, La(Pcam)3, which contains a Lanthanum (La) atom. La(Pcam)3 was deposited on warm and room temperature Au(111) substrate. The La(Pcam)3 from 2, 3, 4, 6-molecule clusters on the warm Au(111) substrate. The single molecules were always accompanied by an anion attached to its ligand. The 2-molecule cluster was found with and without anion attachment. The 4-molecule cluster shows chirality, and it is argued that a 6-molecule cluster can show chirality. The dimension of single La(Pcam)3 on Au(111) is smaller than in the gas phase, and to compensate for this contraction, a model with two anions attached to single La(Pcam)3 is proposed. The electronic properties of La(Pcam)3 were also measured. The second molecule containing RE element used in this thesis is Eu(Pcam)3, containing a Europium (Eu) atom. The molecules were deposited on room temperature Au(111) substrate. The Eu(Pcam)3 also forms clusters, but not so many as La(Pcam)3. However, the single Eu(Pcam)3 was always accompanied by an anion attached to the ligand. Eu(Pcam)3 gas-phase dimension is also larger than what has been measured on Au(111) substrate. Similar to La(Pcam)3, an adsorption model with two anion attachments is proposed for Eu(Pcam)3 to explain the dimensional discrepancies. The Eu(Pcam)3 molecule can rotate at liquid Nitrogen temperature and by the tip-induced electric field at liquid Helium temperature. The threshold electric field to rotate the Eu(Pcam)3 is 2.19V/A. The dissertation also explores a ‘complex' molecule containing two functional parts, ‘rotor' and ‘base'. The rotor and the base are connected via a Ruthenium (Ru) atom. The molecule also consists of five Nickel (Ni)-porphyrin. These Ni-porphyrins are connected to the rotor. Because of the steric repulsion, the five Ni-porphyrin arms orient themselves is a specific angle relative to the surface. The molecules are d (open full item for complete abstract)

Committee: Saw-Wai Hla (Advisor); Eric Masson (Committee Member); Sergio E. Ulloa (Committee Member); Arthur R. Smith (Committee Member) Subjects: Physics

Master of Science, The Ohio State University, 2019, Chemistry

Since its discovery in 1979, the solid electrolyte interface (SEI) has drawn attention due to its importance for efficient battery performance. Despite its significance, there is still some ambiguity with regards to how certain electrolytes form a protective interface to aid Li+ intercalation whilst others irreversibly degrade the anode structure. One of the most debated examples is the “EC-PC disparity” towards the graphite anode, where ethylene carbonate (EC) results in protective SEI formation whilst propylene carbonate (PC) leads to destructive graphite exfoliation. This study focuses on the correlation between the concentration of the Li+ salt, LiPF6 dissolved in PC and the growth of SEI on graphite. In-situ and in-operando electrochemical scanning tunneling microscope (STM) observation of the basal plane of highly oriented pyrolytic graphite (HOPG) is performed in 1 M, 2.5 M and 3 M LiPF6 dissolved in propylene carbonate (PC) in sequence to cyclic voltammetry (CV) and potential hold experiments. This technique allows the study of the topographic evolution of the basal plane and edge sites on HOPG with changes in the potential, as a result of solvent co-intercalation, reduction, graphite exfoliation and SEI formation. STM images obtained whilst holding the potential at selected values, gives an insight into the nature of SEI formation, in connection to the extent of regeneration of the graphite surface. It is found that below 0.9 V vs Li, solvent decomposition, followed by extensive graphite exfoliation takes place in the 1 M and to a lesser degree in 2.5 M LiPF6 electrolyte solutions. However, in the 3 M salt solution, graphite exfoliation is scarce and SEI formation is observed at potentials as high as 1.1 V vs Li, which clearly indicates a concentration dependent SEI formation on graphite. CV experiments conducted in parallel to EC-STM, provide further confirmation. The HOPG is cycled between 3 to 0.005 V vs Li in all three salt concentrations within Swa (open full item for complete abstract)

Committee: Anne Co Dr. (Advisor); Zachary Schultz Dr. (Committee Member) Subjects: Chemistry

Doctor of Philosophy (PhD), Ohio University, 2019, Physics and Astronomy (Arts and Sciences)

This dissertation explores the manipulation of charges in a 2-D self-assembled organic molecular cluster and the manipulation of a molecular machine, a nanocar, using a low temperature ultrahigh vacuum scanning tunneling microscopy on a metal substrate. A donor-acceptor type organic charge transfer salt, α-(BEDT-TTF)2-I3 or α-ET2-I3 (BEDT-TTF=ET=Bis(ethylenedithio)tetrathiafulvalene), is thermally deposited onto Ag(111) substrate for different coverages. The scanning tunneling microscope (STM) investigation reveals different molecular assemblies in the first layer clusters, from partially ordered to ordered arrangements, at the sub-monolayer regime. At slightly higher coverages, the formation of charge density wave (CDW) is observed for the first time at both 5K and 80K substrate temperatures on 2 and 3 monolayer thick molecular islands. The nature of CDW is studied using scanning tunneling spectroscopy (STS) and density functional theory calculations. Furthermore, the observed CDW in the molecular islands is manipulated by using inelastic electron tunneling (IET) and the electric field supplied from the scanning tunneling microscope (STM) tip. The CDW patterns can be destroyed above 1.6V bias however, the same CDW pattern can be reinstated when the STM images are acquired by reducing the bias voltage. Using IET manipulation, the bi-stable deformation nature of CDW modulation is explored, and switching between two CDW states as a function of electric field and current densities are statistically analyzed. The molecular nanomachine, the nanocar, formed by using supermolecular [5]pseudorotaxane as an H-shaped frame and four Cucurbit[7]uril (CB[7]) as wheels, is thermally deposited on Au(111) substrate. The STM tip is used to extract a wheel from a nanocar to confirm its integrity upon deposition onto the surface, and to investigate the structure of the wheel. The chassis of the nanocar includes positive charges, which are used for the controlled driving of t (open full item for complete abstract)

Committee: Saw-Wai Hla (Advisor); Arthur Smith (Committee Member); Sergio Ulloa (Committee Member); Eric Stinaff (Committee Member); Hugh Richardson (Committee Member) Subjects: Condensed Matter Physics; Molecular Physics; Nanoscience; Physics; Scientific Imaging

Doctor of Philosophy (PhD), Ohio University, 2018, Physics and Astronomy (Arts and Sciences)

Most of my PhD research is focused on a two-dimensional semiconducting GaN-based manganese gallium nitride surface monolayer structure referred as MnGaN-2D and its related magnetic heterostructure systems. All of these material systems are grown by a reflection high energy electron diffraction assisted molecular beam epitaxy system. The as-grown samples are then characterized by various cutting-edge techniques including spin-polarized scanning tunneling microscopy and spectroscopy at both room temperature and low temperature, Auger electron spectroscopy, and variable temperature superconducting quantum interference magnetometry, to explore their surfaces and interfaces atomic structure, electronic structure, and spin structure. In addition, density functional theory calculations are performed for model structure, spin-polarized electronic structure, spin-orbit coupling, and lattice strain effect to help to understand and to support the experimental discoveries. The scope of this research is to investigate the magnetic properties of the MnGaN-2D monolayer and its related exchange bias systems for future practical advanced spintronic applications such as novel nonvolatile magnetic storage devices with increased integration densities and high thermal stability, and practical quantum logic computation at room temperature with low energy cost. The MnGaN-2D monolayer is promising for such applications due to its remarkable properties discovered during my research including robust intrinsic ferromagnetism at room-temperature, large spin-polarization over 95% at specific energy level and large magnetic moment 3.94 µB per Mn, perpendicular magnetic anisotropy, high thermal stability up to 700 degree Celsius, nonvolatility after direct exposure to air, lattice strain tailoring ability of its electronic and magnetic properties, directly coupled to the technological important semiconductor GaN, and etc. Chapter 1 and 2 are an overall detailed introduction of the background of m (open full item for complete abstract)

Committee: Arthur Smith (Advisor) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy (PhD), Ohio University, 2018, Physics and Astronomy (Arts and Sciences)

This work is focused on two types of nitride and spintronic material systems: a Heusler alloy on nitride semiconductor system, namely MnGa on Ga-rich GaN, and a transition metal magnetic nitride system, namely MnxNy. These systems have attracted considerable attention in spintronics research, where the spin degree of freedom plays an important role. The Heusler alloy MnGa ultra-thin films on GaN have been shown to possess giant perpendicular magnetic anisotropy, while MnxNy has diverse, phase-dependent diverse magnetic properties. The results of this study give important insights for designing quantum-sized magnetic alloy and/or magnetic nitride-based spintronic devices. The ultra-high vacuum molecular beam epitaxy approach is used for preparing thin films of the materials in this study. The crystalline growth is monitored using reflection high energy electron diffraction. Reflection high energy electron loss spectroscopy is utilized to study surface plasmon excitations of the thin films. Freshly prepared samples are transferred in situ to an adjacent ultra-high vacuum analysis chamber where surfaces are probed for their structural, electronic, and surface spin properties using scanning tunneling microscopy with both non-magnetic and magnetic-coated tips (for sensing surface spin properties) as well as with Auger electron spectroscopy for chemical analysis. For further analysis, ex situ measurements are carried out using the superconducting quantum interference device. This research begins by investigating Ga-rich GaN surfaces. The growth and crystal structure of the GaN mixed-polarity surface are studied. The N-polarity is distinguished by the formation of Ga-rich, higher-order reconstructions following growth and cooling, including the c(6×12) and the trench line structure (TLS), which can be decomposed into sub-units of the c(6×12). Whereas, the Ga-polarity is distinguished by the formation of smooth, featureless regions on the surface. A simple model is prop (open full item for complete abstract)

Committee: Arthur Smith (Advisor) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2018, Physics

This thesis addresses scanning tunneling microscopy (STM) studies of metal defects in GaAs(110). Single-atom defects are useful for their potential applications in developing nano-scale miniaturized or new types of optical, magnetic, and electronic devices, as well as expanding our understanding of atomic-scale interactions. STM is used to measure physical and electronic properties of surface and near-surface materials with atomic resolution, but affects the properties of defects being studied. Zn-doped GaAs(110) is one of the most popular commercially used semiconductor materials, and is well-understood on the macroscopic scale. By studying individual Zn defects at a wider range of doping levels than previously studied, we discovered three types of defect-based conductivity caused by dopant interactions that inform macroscopic doped semiconductor properties and atomic-scale defect properties. Additionally, we tuned these properties using electronic fields and doping level, laying the groundwork for Zn defects in solotronic or nanoscale devices. Additionally, erbium, an element with useful optical properties, has been studied on GaAs(110) for the first time, and been shown to have four types of interaction with the host surface. This improved our understanding of the significance of the interaction strength of atoms with host materials on defect charge state and electronic properties. The techniques developed in studying Zn and Er atoms have yielded preliminary results for new properties of Mn and Co atoms and the first studies of V atoms on GaAs(110).

Committee: Jay Gupta (Advisor); Amy Connolly (Committee Member); Roland Kawakami (Committee Member); Mohit Randeria (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2018, Physics

Two-dimensional (2D) materials have attracted considerable interest as the reduced dimensionality offers a playground for testing important models. Due to the extreme surface sensitivity 2D materials, it is important to characterize the interactions with metal electrodes, substrates, or adsorbates as they can have a strong effect in the inherent properties of these materials. Graphene islands grown on Cu(111) in ultra-high vacuum were characterized by scanning tunneling microscopy and spectroscopy. The changes in physical and electronic structures on the graphene regions were measured with atomic spatial resolution and compared with those of clean Cu(111) regions. On graphene, we observed two natively occurring point defects: bright and dark defects. Bright defects that occur only in graphene regions are identified as C site point defects in the graphene lattice and are most likely single C vacancies. Dark defects types are observed in both graphene and Cu regions, and are likely point defects in the Cu surface. Tunneling spectroscopy measurements indicate a decrease in surface work function and a decrease in the effective mass of the Cu(111) Shockley surface state electrons. The zero-gap of band structure of graphene is a limitation for applications in logic circuits. Hydrogen functionalization may induce a metal-insulator transition in graphene and open a band gap in its electronic spectrum. We demonstrated the crystalline hydrogenation of graphene by developing an STM tip induced field dissociation technique. With this technique, physisorbed molecular H2 on graphene islands grown on Cu(111) are dissociated with field emission local to the tip of an STM at low temperature (5 K). This process generates point defects on graphene which are attributed to chemisorbed H. At higher coverage, well ordered (√3 × √3)R30° as well as less dense 3 × 3 and 4 × 4 structures are observed on the graphene. We found that the hydrogenation process is reversible by imaging at (open full item for complete abstract)

Committee: Jay Gupta PhD (Advisor); Roland Kawakami PhD (Committee Member); Yuan-Ming Lu PhD (Committee Member); Enam Chowdhury PhD (Committee Member) Subjects: Physics