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  • 1. Smith, Rebekah Scanning Tunneling Microscopy Studies of Fe Dopants on GaAs (110)

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

  • 2. Galiano, Kevin Scanning Probe Microscopy Measurements and Simulations of Traps and Schottky Barrier Heights of Gallium Nitride and Gallium Oxide

    Doctor of Philosophy, The Ohio State University, 2020, Physics

    Gallium Nitride (GaN) and Gallium Oxide (Ga2O3) are two semiconductors of significant interest for high power and high frequency electronics. However, the performance of these electronics can be inhibited by the presence of defects which can produce "trap" states in the "forbidden" bandgap of semiconductors. These traps can then degrade the output current of transistors and cause undesireable time-dependent phenomenon. This work investigates the physical origin of the most common trap, EC - 0.57 eV, in Gallium Nitride (which happens to be detrimental for certain transistors) by using Scanning Probe - Deep Level Transient Spectroscopy (SP-DLTS) to probe its spatial distribution. For the first time, this trap species is mapped with high spatial resolution and it is found to exhibit strong spatial localization in the form of "trap clusters". Through a correlative study with Electron Channeling Contrast Imaging (ECCI), this trap is found to be located at pure edge dislocations. In another study, the impact of iron on the spatial distribution of this trap is investigated, and it is found that the iron causes a more spatially-uniform trap distribution. One possible explanation is that the EC - 0.57 eV traps are directly related to iron atoms that are gettered by edge dislocations in Gallium Nitride. To better understand how the SP-DLTS maps relate to the trap concentration, simulations are performed. A comparison between the measurement and simulation shows reasonable agreement for the two GaN samples studied here. In collaboration with fellow graduate student Darryl Gleason, a study is conducted on a different device geometry (AlGaN/GaN heterostructures with semi-insulating GaN layers). This study allows for the characterization of two trap species in the GaN layer (one of which is the EC - 0.57 eV trap), and good agreement is found between macroscopic DLTS and SP-DLTS for both trap species. Finally, the first Ballistic Electron Emission Microscopy (BEEM) measurements on (open full item for complete abstract)
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    Committee: Jonathan Pelz (Advisor); Steven Ringel (Advisor); Nandini Trivedi (Committee Member); Yuri Kovchegov (Committee Member) Subjects: Physics

  • 3. Foley, Andrew Magnetic and Interfacial Properties of the Metal-Rich Phases and Reconstructions of MnxNy and GaN Thin Films

    Doctor of Philosophy (PhD), Ohio University, 2017, Individual Interdisciplinary Program

    The interfacial and magnetic properties of the metal-rich phases and reconstructions of MnxNy and GaN are investigated. Thin films of the two most metal-rich phases of MnxNy (ε and ζ) are grown on MgO(001) using a custom-built ultra high vacuum molecular beam epitaxy growth system. By the same means, thin films of GaN with the two most metal-rich reconstructions [c(6×12) and psuedo-1×1+1⁄12] are grown on GaN(0001) and Al2O3(0001). The interfacial properties of these materials such as surface structure and local density of states are investigated in situ with low temperature scanning tunneling microscopy and reflection high energy electron diff raction. The bulk structure of these thin films are investigated using x-ray diff raction. Measurements of the morphology and magnetism of these thin films are made ex situ using atomic/magnetic force microscopy, spin-polarized scanning tunneling microscopy, scanning electron microscopy, vibrating sample magnetometry, and superconducting quantum interference device magnetometry techniques. Measurements of the chemical composition of the samples are made using back scattered electron scanning electron microscopy, energy dispersive x-ray spectroscopy, and Rutherford backscattering spectrometry. These techniques reveal that growth temperature heavily influences the quality of the ε-Mn4N grown. A nucleation temperature below 480 °C is observed to result in the growth of substantial antiferromagnetic η-Mn3N2 grains alongside ferrimagnetic ε-Mn4N grains. The most significant component of perpendicular magnetic anisotropy in ε-Mn4N thin films (9 to 300 nm thick) grown on MgO(001) is attributed to the shape induced partial overlap of Ising domains out-of-plane. Spin-polarized scanning tunneling microscopy and magnetic force microscopy measurements are consistent with the interpretation that these ε-Mn4N thin films form single material out-of-plane spin valves due to this out-of-plane overlapping of Ising domains. The ζ-Mn1 (open full item for complete abstract)
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    Committee: Arthur Smith (Advisor); Wojciech Jadwisienczak (Committee Member); Savas Kaya (Committee Member); David Ingram (Committee Member) Subjects: Electrical Engineering; Physics

  • 4. Heller, Eric Ultra low signals in ballistic electron emission microscopy

    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)
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    Committee: Jonathan Pelz (Advisor) Subjects:

  • 5. Shields, Seth Scanning Tunneling Microscopy Studies of Oxidized Copper Surfaces and the Development of X-ray and Extreme Ultraviolet Scanning Tunneling Microscopy

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

  • 6. Walko, Robert An Investigation of Materials at the Intersection of Topology and Magnetism Using Scanning Tunneling Microscopy

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

    Material systems that combine magnetism and topology have garnered intense interest recently due to predictions of a variety of phenomena such as the quantum anomalous Hall effect, chiral topological edge states, magnetoelectric effects, and Weyl semimetal and axion insulator phases. In this dissertation scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are used to investigate the structural and electronic properties of several materials that exist at the intersection of topology and magnetism such as topological insulators, van der Waals layered materials, two-dimensional magnets, and intrinsic magnetic topological insulators. Specifically, STM was used to confirm the growth quality and properties of thin films grown by molecular beam epitaxy including Bi2Se3, SnSe2, MnSe, and Fe3GeTe2. In the van der Waals heterostructure SnSe2/Bi2Se3 a moire pattern was observed which was found to be correlated to a set of localized electronic states in STS measurements. STM was also used to provide feedback for the growth of the heterostructure MnSe/Bi2Se3, which led to successful growth of partial monolayers of MnSe. In Fe3GeTe2, a magnetic field dependent Kondo lattice behavior was observed as well as a ferromagnetic hysteresis loop using spin-polarized STM. In addition, this work reports the first STM study of the recently experimentally verified intrinsic antiferromagnetic topological insulator (AFM TI) MnBi2Se4. Its atomic and layered properties are found to reasonably match theoretical predictions. Two different terminations of its layered structure are observed on the surface with distinct electronic properties. In-gap states are observed near some step edges which could be related to predicted topological edge states. Another AFM TI, the related material MnBi2Te4, was also studied in which nanoscale surface manipulation using an STM tip was demonstrated.
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    Committee: Jay Gupta (Advisor); Dan Gauthier (Committee Member); Nandini Trivedi (Committee Member); Ezekiel Johnston-Halperin (Committee Member) Subjects: Condensed Matter Physics; Physics

  • 7. Ajayi, Tolulope Characterizations of Complex Molecular Systems and Nanoscale Heterostructures Using Synchrotron X-rays at the Ultimate Atomic Scale

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

  • 8. Rodriguez, Ryan Development of Optically-coupled Scanning Tunneling Microscope for Investigation of Multi-pulse Laser Induced Defect States and Time Resolved Dynamics

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

  • 9. Dehiwala Liyanage, Chamathka In-situ scanning tunneling microscopy studies of the SEI formation on graphite anodes in propylene carbonate

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

  • 10. Tumbleson, Ryan Investigation of a Robust Chiral Molecular Propeller Using Scanning Tunneling Microscopy.

    Bachelor of Science (BS), Ohio University, 2019, Engineering Physics

    Molecular machines are ubiquitous in nature and are essential for many biological functions but only exist in select environments. Here, I present a synthetic molecular machine designed to operate on a material surface that can convert energy into mechanical motion. The molecular machine presented is a multi-component propeller molecule composed of a ratchet-shaped gear stator, three propeller blades acting as a rotator and a single Ruthenium atom axle functioning as an atomic ball bearing. Once adsorbed onto a gold crystal surface, the symmetry of the stator breaks and induces chirality in the molecule. The ratchet-shaped gear allows for unidirectional and step-wise rotations of the propeller. This rotation can be induced and directly visualized by means of scanning tunneling microscopy, confirming a unidirectional rotation of both left and right handed molecular propellers into clockwise and anticlockwise directions, respectively.
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    Committee: Saw Wai Hla (Advisor) Subjects: Nanotechnology; Physics; Quantum Physics

  • 11. Chang, Hao Synchrotron X-ray Scanning Tunneling Microscopy Investigation of Interfacial Properties of Nanoscale Materials

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

    Nanoscale materials have demonstrated unique properties in condensed matter studies and played a crucial rule in modern devices. Fundamental research on nanoscale systems provides deep understanding into science and enables advancing and reshaping of nanotechnology. Scanning tunneling microscopy is a comprehensive tool which is capable of characterizing topographic and electronic properties of nanoscale materials in real space down to atomic scale. To date many emerging techniques have been combined with scanning tunneling microscopy to discover and explore new phenomena. This dissertation explicitly demonstrates novel application of synchrotron X-ray scanning tunneling microscopy which combines synchrotron X-ray and scanning tunneling microscopy to study magnetic, electronic, and structural properties of materials interfaces. A scanning tunneling microscope tip is used to capture X-ray magnetic circular dichroism and near edge X-ray absorption fine structure signals, which explain charge transfer and magnetic properties of oxide materials interfaces with chemical and elemental sensitivities. Using X-ray absorption spectroscopy and spectroscopic imaging with a scanning tunneling microscope tip, the effect of charge transfer at the interfaces formed by transition metals of cobalt and nickel in nanoscale clusters and islands on a Cu (111) surface has been explored. Finally, X-ray standing wave formed by the interference of the incident and diffracted X-ray beams is used to characterize the structural properties of a cobalt thin film grown on a Au (111) surface. These results open novel research directions where material characterizations will be able to perform simultaneous chemical, structural and magnetic contrast potentially down to atomic scale.
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    Committee: Saw-wai Hla (Committee Chair); Nancy Sandler (Committee Member); Gang Chen (Committee Member); Hugh Richardson (Committee Member); Volker Rose (Committee Member) Subjects: Materials Science; Nanoscience; Physics

  • 12. Benjamin, Anne Scanning Tunneling Microscopy Studies of Defects in Semiconductors: Inter-Defect and Host Interactions of Zn, Er, Mn, V, and Co Single-Atom Defects in GaAs(110)

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

  • 13. Alam, Khan Growth, Structural, Electronic, and Magnetic Characterization of GaN, CrN, Fe Islands on CrN, and Fe/CrN Bilayer Thin Films

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

    As a part of my Ph.D research, initially I was involved in construction and calibration of an ultra-high vacuum thin film facility, and later on I studied structural, electronic, and magnetic properties of GaN, CrN, Fe/CrN bilayers, and Fe islands on CrN thin films. All of these films were grown by molecular beam epitaxy and characterized with a variety of state-of-the-art techniques including variable temperature reflection high energy electron diffraction, low temperature scanning tunneling microscopy and spectroscopy, variable temperature vibrating sample magnetometry, variable temperature neutron diffraction and reflectometry, variable temperature x-ray diffraction, x-ray reflectometry, Rutherford backscattering, Auger electron spectroscopy, and cross-sectional tunneling electron microscopy. The experimental results are furthermore understood by comparing with numerical calculations using generalized gradient approximation, local density approximation with Hubbard correction, Refl1D, and data analysis and visual environment program. In my first research project, I studied Ga gas adatoms on GaN surfaces. We discovered frozen-out gallium gas adatoms on atomically smooth c(6×12) GaN(000¯1) surface using low temperature scanning tunneling microscopy. We identified adsorption sites of the Ga adatoms on c(6×12) reconstructed surface. Their bonding is determined by measuring low unoccupied molecular orbital level. Absorption sites of the Ga gas adatoms on centered 6$\times$12 are identified, and their asymmetric absorption on the chiral domains is investigated. In second project, I investigated magneto-structural phase transition in chromium nitride (CrN) thin films. The CrN thin films are grown by molecular beam epitaxy. Structural and magnetic transition are studied using variable temperature reflection high energy electron diffraction and variable temperature neutron diffraction. We observed a structural phase transition at the surface at 277±2 K, and a sharp (open full item for complete abstract)
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    Committee: Arthur Smith (Advisor); Sergio Ulloa (Committee Member); Tatiana Savin (Committee Chair); Eric Stinaff (Committee Member) Subjects: Condensed Matter Physics; Experiments; Low Temperature Physics; Nanoscience; Physical Chemistry; Physics

  • 14. Mandru, Andrada Oana Ferromagnetic Thin and Ultra-Thin Film Alloys of Manganese and Iron with Gallium and Their Structural, Electronic, and Magnetic Properties

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

    The behavior of ferromagnetic alloys of manganese and iron with gallium when coupled with different magnetic and/or non-magnetic systems is investigated. The studies explore how the structural and electronic/magnetic properties vary with thickness and composition, probing systems in the sub-monolayer, ultra-thin, and thin film regimes. Molecular beam epitaxy is used to prepare clean sample surfaces that are subsequently investigated in-situ down to atomic level using scanning tunneling microscopy and Auger electron spectroscopy. A variety of ex-situ methods are also utilized to obtain information about the overall system properties, with additional theoretical calculations accompanying the experimental findings for two of the investigated systems. The first study refers to L10-structured ferromagnetic MnGa(111) ultra-thin films grown on semiconducting GaN(0001) substrates under lightly Mn-rich conditions. Room temperature scanning tunneling microscopy investigations reveal smooth and reconstructed terraces, with the surface structure consisting primarily of a hexagonal-like 2 x 2 reconstruction. Theoretical calculations are carried out using density functional theory, revealing that a Mn-rich 2 x 2 surface structure gives the best agreement with the observed experimental images and Auger electron spectroscopy surface composition investigations. It is found that under such growth conditions, the Mn atoms incorporate at di fferent rates: surfaces become highly Mn-rich, while the bulk remains stoichiometric, making the MnGa system very sensitive to the ratio of elements in its structure. Such behavior reveals a potential recipe for tuning, for example, magnetic properties by carefully controlling the surface reconstruction during growth. With the aim of understanding how the properties change as the growth conditions are varied, we also investigate the structure, surface, and magnetism of ferromagnetic Ga-rich MnGa thin and ultra-thin films grown again on GaN(00 (open full item for complete abstract)
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    Committee: Arthur Smith (Advisor) Subjects: Condensed Matter Physics; Physics

  • 15. Kersell, Heath Alternative Excitation Methods in Scanning Tunneling Microscopy

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

  • 16. Alhashem, Zakia Growth and Scanning Tunneling Microscopy Studies of Novel Trench-Like Formation and Relation to Manganese Induced Structures on w-GaN (000-1)

    Master of Science (MS), Ohio University, 2015, Physics and Astronomy (Arts and Sciences)

    A trench-like structure has been observed after sub-monolayer Mn is deposited onto N-polar gallium nitride (GaN) surfaces. It is reminiscent of the well-documented formation of the vacancy line structures on Si(001) surfaces. However, little is known about what really causes this trench-like structure to form. Although it was previously hypothesized that it may be due to the Mn atoms, it may in fact be present before Mn deposition, and possibly caused by annealing or some other variations in the growth conditions. This study investigates the causes of the appearance of this trench-like structure on N-polar w-GaN surfaces by optimizing the growth recipe to obtain reproducible trench structures. Additionally, surface structure after Mn deposition onto w-GaN (0001 ¯) was observed and the role of the Mn was analyzed. In the current study, GaN samples were grown on sapphire (0001) substrates using ultra high vacuum molecular beam epitaxy (UHV-MBE). The growth was monitored using a reflection high energy electron diffraction (RHEED) system. For room temperature scanning tunneling microscopy (RT-STM) studies, the samples were transferred in-situ to the analysis chamber, where STM images were taken using a tungsten tip under a constant current condition. In the first attempt, the RHEED patterns of the GaN growth showed the well-known 3×3 reconstruction, which was further confirmed by atomic resolution STM. The sample was annealed at a high temperature range of ~ 600°C to 700°C but no trench-like structure was observed. Another GaN sample was grown under highly Ga-rich conditions and then annealed at a higher temperature, ~ 800°C, for ~ 6 minutes. These growth conditions resulted in the trench-like formation often appearing nearby the c(6×12) reconstruction. The trench-like structure is composed of two sub-units of the c(6×12) reconstruction because of the observable similarities between the trench-like formation and the c(6×12) reconstruction. After depositing ~ 0.15 (open full item for complete abstract)
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    Committee: Arthur Smith (Advisor) Subjects: Physics

  • 17. Nielsen, Jon Energetically and Kinetically Driven Step Formation and Evolution on Silicon Surfaces

    Doctor of Philosophy, The Ohio State University, 2001, Physics

    Energetically and kinetically driven step formation and evolution on Si(001) and Si(111) surfaces has been investigated experimentally using scanning tunneling microscopy (STM), atomic force microscopy (AFM), optical microscopy, and low-energy electron microscopy (LEEM). Four systems are investigated: (1) Detailed STM measurements of boron-doped Si(001) surfaces is presented, along with large-scale AFM and LEEM observations of the well-known boron-induced ‘striped' phase at elevated temperatures. Boron is shown to induce a variety of related atomic-scale structures, some of which tend to decorate surface step-edges. This, in turn, could provide an explanation for the observed boron-induced reduction in step formation energy. However, the observed boron-accumulation at step-edges does not appear to vary systematically with annealing temperature, leaving the well-known temperature dependence of the striped phase unresolved. Real-time LEEM observations of striped step formation on Si(001) during diborane (B2H6) exposure at elevated temperatures are used to demonstrate the controlled formation of large (>5 mm) surface regions with highly uniform striped step structures. (2) Large-scale step rearrangements have been investigated on Si(001) and Si(111) surfaces heated to sublimation temperatures (>900 °C) using a direct current. These surfaces undergo dramatic morphological changes, which are believed to arise from a directional drift of diffusing surface atoms in the presence of an applied electric field. Such ‘electromigration' phenomena include step ‘bunching' and step ‘wandering', as well as a predicted step ‘bending' instability. Using AFM and optical microscopy, we argue that the direction of surface atom electromigration on Si(001) can be parallel, anti-parallel, or even sideways to the applied electric field, depending on the direction of the applied field with the high-symmetry <110> crystal directions. In addition, the first experimental evidence for the predi (open full item for complete abstract)
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    Committee: Jon Pelz (Advisor) Subjects: Physics, Condensed Matter

  • 18. Gohlke, David Tuning the Properties and Interactions of Manganese Acceptors in Gallium Arsenide with STM

    Doctor of Philosophy, The Ohio State University, 2012, Physics

    Dilute magnetic semiconductors have the electrical properties of semiconductors while allowing for long-range alignment of spin. In particular, manganese-doped gallium arsenide (Mn-GaAs) is a ferromagnetic semiconductor with measured Curie temperatures as high as 200 K. These manganese dopants act as electron acceptors while introducing a magnetic moment, facilitating bulk ferromagnetism. Low-temperature (5 K) scanning tunneling microscopy (STM) allows for atomic-scale measurements and manipulation, enabling the study of individual dopants. STM can be used to modify surfaces with sub-nanometer precision, thereby providing the capability to tune the interaction between individual atoms. The research presented here shows that the binding energy of a single Mn acceptor embedded into the GaAs(110) substrate can be tuned by the presence of charged atoms adsorbed on the surface, allowing direct measurement of the charge of these adatoms. We observe a decrease of the acceptor binding energy due to Coulomb repulsion with charged defects located further than 2 nm away, while closer defects induce sufficient band bending to forbid tunneling through this acceptor state, resulting in a turnaround behavior in the measured resonance energy. Additionally, the anisotropy of the zincblende crystal lattice of GaAs establishes the manner of magnetic coupling between Mn dopants, which can be ferromagnetic or antiferromagnetic depending on their respective orientation. Here the controlled movement of charged adatoms is shown to tune the coupling strength between pairs of surface-layer Mn atoms. This control of the magnetic interactions between dopants will lead to deeper understanding of these dilute magnetic semiconductors and, hopefully, to designs for spintronic materials that can function at room temperature.
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    Committee: Jay Gupta (Advisor); Louis F. DiMauro (Committee Member); Ratnasingham Sooryakumar (Committee Member); Nandini Trivedi (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Physics; Solid State Physics

  • 19. Choi, Taeyoung STM studies of charge transfer and transport through metal-molecule complexes on ultrathin insulating films

    Doctor of Philosophy, The Ohio State University, 2011, Physics

    Traditional electronic and memory devices face a fundamental limit as their size shrinks to an order of one nanometer. An understanding of the electronic and magnetic properties of single atoms and molecules is needed for their incorporation into passive or active components of such devices. Ultrathin insulating films comprising only a few atomic layers are useful materials for building nanostructures from atoms and molecules and for controlling their electronic and magnetic coupling at the nanoscale. Scanning Tunneling Microscopy (STM) is used to study the geometric and electronic structure of an ultrathin insulating film, Cu2N, grown on a Cu(100) surface. We find that the Cu2N lattice is incommensurate with the Cu(100) lattice, suggesting that the resulting strain contributes to the self-assembly of Cu2N into square shaped, ~25nm2 islands. Atomically resolved STM images enable us to register Cu and N atoms within the islands and reveal a contrast reversal with voltage that is similar to that for the GaAs(110) surface. Scanning Tunneling Spectroscopy (STS) indicates that the Cu2N islands suppress the local density of state (LDOS) of Cu(100) within a range from +2eV to -4eV and thus act as an insulator. We study single magnetic atoms (Fe and Co) and clusters on the Cu2N islands grown on Cu(100). Our tunneling spectra show step-like features which are due to the opening of inelastic tunneling channels. The inelastic conductance originates from excitations among spin states (spin-IETS). We observe the evolution of magnetic moments as a function of the number of atoms in clusters or chains. Transition metal ion (M=V, Mn, Co, and Fe)-tetracyanoethylene (TCNE, C6N4) compounds (e.g. V(TCNE)2) are well known organic semiconductors exhibiting ferromagnetism above room temperature. As a bottom-up approach, we studied single TCNE molecules on Cu(111). The single molecules display five distinct configurations which are reversibly switchable by voltage pulses from the STM (open full item for complete abstract)
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    Committee: Jay Gupta PhD (Advisor); Arthur Epstein PhD (Committee Member); Louis DiMauro PhD (Committee Member); David Stroud PhD (Committee Member); Lafont Jean-Francois PhD (Committee Member) Subjects: Physics

  • 20. Daughton, David Electronic structure dependence on molecular orientation: a Scanning Tunneling Microscopy study of C60 on Cu(100)

    Doctor of Philosophy, The Ohio State University, 2010, Physics

    As with traditional semiconductor electronic and photoelectronic devices, successful implementation of organometallic device architectures requires understanding and engineering of molecule-electrode interfaces. The intramolecular and electronic structure of C60 stabilized in monolayer films on a Cu(100) surface has been studied with low temperature scanning tunneling microscopy (STM) and spectroscopy. In contrast to other single crystal metal surfaces, C60 adopts four unique orientations on the Cu(100) surface due to a reconstruction of the underlying copper atoms. Spectroscopy and spatial imaging of molecular orbital resonances from LUMO to LUMO+3 for the four distinct adsorption geometries are presented. Shifts in the MO resonances indicate different degrees of electronic molecule-surface hybridization for the four geometries. Additionally, nanoscale modulation of the relative work function is observed across the film which correlates with the proposed atomic-scale electrode reconstruction.
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    Committee: Jay Gupta PhD (Advisor); Louis DiMauro PhD (Committee Member); Ratnasingham Sooryakumar PhD (Committee Member); Stroud David PhD (Committee Member) Subjects: Physics