Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item EXPLORING AN ALTERNATIVE TECHNOLOGY FOR MANUFACTURING ELECTRONICS FOR EXTREME TEMPERATURES(2023) Patel, Mital; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Within our increasingly digital world, there is a demand to integrate electronics into every industry to take advantage of applications in communication, optimization, and artificial intelligence. Relatively untapped areas for electronics implementation are the extreme environments where high temperatures (>300°C) are present. These environments are common within energy, automotive, and aerospace industry es. Current high temperature technologies limit reliable use of electronics to ~200°C. Emerging technologies, such as transient liquid phase (TLP) bonding, copper sintering, and thick films, have not yet demonstrated resilient operation above 300°C. Possessing various remarkable properties, diamond is a promising material that can be used in manufacturing electronic devices operable well above 500°C. Graphene and graphite additionally can serve as conductive material for circuitry or other electronic elements. The compatibility and versatility of these three materials demonstrate the potential for robust, all-carbon electronics for high temperature applications. Chemical vapor deposition (CVD), the predominant method of synthesizing diamond for electronics, involves very costly, long processes at extreme temperatures. A relatively underdeveloped, alternative method utilizes the pyrolysis of polymer precursors into diamond. This study aims to further explore this method using Poly(naphthalene-co-hydridocarbyne) (PNHC). The polymer synthesis, processing, and pyrolysis have been performed here, and the process parameters and outcomes at each step have been documented. Native graphite and graphene growth on diamond surfaces allows for the integration of conductive material on insulating diamond. Four known methods of diamond graphitization, assisted with the metal catalysts nickel, copper, and iron, have also been applied to support the fabrication of carbon-based electronics. Ultimately in this study, the synthesis of diamond has been unsuccessful, but multi-layer graphene has been grown on polycrystalline diamond with high sheet carrier concentration and mobility values of 1.0*1015 cm-2 and 629.1 cm2 Vs-1, respectively.Item WATER, ION, AND GRAPHENE: AN ODYSSEY THROUGH THE MOLECULAR SIMULATIONS(2019) Wang, Yanbin; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Water is known as the most common and complicated liquid on earth. Meanwhile, graphene, defined as single/few layer graphite, is the first member in the 2-dimensional materials family and has emerged as a magic material. Interactions between water and graphene generate many interesting phenomena and applications. This thesis focuses on applying molecular dynamics (MD), a powerful computational tool, for investigating the graphene-water interactions associated with various energetic and environmental applications, ranging from the wettability modification, species adsorption, and nanofluidic transport to seawater desalination. A key component of one domain of applications involves a third component, namely salt ions. This thesis attempts that and discovers a fundamentally new way in which the behavior of ions with the air-water interfaces should be probed. In Chapter 1, we introduce the motivation and methods and the overall structure of this thesis. Chapter 2 focuses on how MD simulations connect the statistical mechanics theory with the experimental observations. Chapter 3 discusses the simulation results revealing that the spreading of a droplet on a nanopillared graphene surface is driven by a pinned contact line and bending liquid-surface dynamics. Chapter 4 probes the interactions between a water drop and a holey graphene membrane, which is prepared by removing carbon atoms in a circular shape and which can serve as catalyst carriers. Accordingly, chapter 5 studies the effects of various terminations on water-holey graphene interactions, showing that water flows faster and more thoroughly through the membrane with hydrophobic terminations, compared to that with hydrophilic terminations. In chapter 6, simulations describe the generation of enhanced water-graphene surface area during the water-holey-graphene interactions in presence of an applied time-varying force on the water drop. In chapter 7, we focus on the ion-water interaction at the water-air interface to fully understand the fluidic dynamics during any seawater desalination. Our research revisits the energetic change while ion approaches water-air interface and shows that the presence of ion at the interface enhances capillary-wave fluctuation. Finally, in chapter 8 we summarize the main findings of the thesis and provide the scope of future research.Item INTERLAYER INTERACTION PHENOMENA IN NOVEL MATERIALS(2014) Pershoguba, Sergii; Yakovenko, Victor M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recently, there has been a considerable interest in various novel two-dimensional (2D) materials, such as graphene, topological insulators, etc. These materials host a plethora of exotic phenomena owing to their unconventional electronic structure. Physics of these 2D materials is understood fairly well, so a natural generalization is to assemble these materials into three-dimensional (3D) stacks. In this thesis, we study a number of multilayer systems, where the interlayer interaction plays a salient role. We commence with studying graphene multilayers coupled via interlayer tunneling amplitude. We calculate the energy spectrum of the system in magnetic field B parallel to the layers. The parallel magnetic field leads to a relative gauge shift of the momentum spaces of the individual 2D layers. When the interlayer tunneling is introduced, we find the Landau levels. We observe two qualitatively distinct domains in the Landau spectrum and analyze them using semiclassical arguments. Then, we include electric field E perpendicular to the layers, and analyze the spectrum in the crossed-field geometry. If the fields are in resonance E = vB , where v is the velocity of carriers in graphene, the wave-functions delocalize in the direction along the field E. We compare this prediction to a tunneling spectroscopy study of a graphite mesa in the parallel magnetic field. Indeed, the tunneling spectrum displays a peak, which grows linearly with the applied magnetic field B, and is, thus, consistent with our theoretical analysis. Then, we move on to a discussion of Z2 topological insulators within the Shock-ley model. We generalize the one dimensional (1D) Shockley model by replacing atomic sites of the original model by the 2D Rashba spin-orbit layers. We analyze surface states of a topological insulator using a construction of vortex lines in the 3D momentum space. We also study a topological insulator in a thin film geometry, where the opposite surface states are coupled by the tunneling amplitude. We cal-culate the tunneling current between the opposite surfaces and a spin polarization of the current as a function of the in-plane magnetic field. We conclude with studying a novel chiral order in cuprates. We construct a helical interlayer pattern of loop-currents. The interlayer magnetic coupling and magnetoelectric effect lead to optical gyrotropy.Item Organic Molecular Thin Films on Device-Relevant Substrates(2013) Groce, Michelle Anne; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Organic thin films are central to many cutting-edge electronic devices. Improving our understanding of the characteristics of thin films is important not only to the development of condensed matter physics but also to our ability to engineer specialized devices that we demand be ever smaller, less expensive, and more efficient. This thesis applies the experimental techniques of scanning tunneling microscopy and spectroscopy to the task of characterizing submonolayer thin films of two types: the organic semiconductor C60 on silicon oxide, and self-assembling porous networks of trimesic acid on graphite. Capture zone analysis of the initial nucleation regime for C60 on ultrathin silicon oxide is reported. The critical nucleus size, reflecting the largest unstable cluster of particles on a surface, is found to have a parabolic dependence on temperature rather than a monotonically increasing one. Between stages of stable monomers (i=0$) at < 300 K and > 480 K, a peak corresponding to i=1 is found at 386±3 K. This unique temperature dependence is attributed to defect-like variation in the silicon oxide surface. The first successful room-temperature UHV STM of trimesic acid on graphite is also presented here. These exploratory studies indicate the potential for a variety of porous hexagonal networks of trimesic acid to exist on a graphitic surface at room temperature. Significant electronic effects on graphite from trimesic acid lattices are shown via scanning tunneling spectroscopy, including an electronic state at -0.14 V that appears in networks whose pores are filled with excess TMA guest molecules. Ultimately, if the growth of TMA films could be extended to graphene, then the periodicity of electronegative oxygen atoms in molecules physisorbed on the graphene surface is predicted to provide a slight energy shift between the degenerate sublattices, opening a band gap. Promising directions for future research in these areas are also discussed.Item Theory of graphene transport properties(2013) Li, Qiuzi; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene is of great fundamental interest and has potential applications in disruptive novel technologies. In order to study the novel phenomena in graphene, it is essential to understand its electron transport properties and in particular the main factors limiting its transport mobility. In this dissertation, we study the transport properties of graphene in the presence of electron-hole puddles induced by charged impurities which are invariably present in the graphene environment. We calculate the graphene conductivity by taking into account the non-mean-field two-component nature of transport in the highly inhomogeneous density and potential landscape, where activated transport across the potential fluctuations in the puddle regimes coexists with regular metallic diffusive transport. Our theoretical calculation explains the non-monotonic feature of the temperature dependent transport, which is experimentally generically observed in low mobility graphene samples. Our theory also predicts the existence of an intriguing "disorder by order" phenomenon in graphene transport where higher-quality (and thus more ordered) samples, while having higher mobility at high carrier density, will manifest more strongly insulating (and thus effectively more disordered) behavior as the carrier density is lowered compared with lower quality samples (with higher disorder), which exhibit an approximate resistivity saturation phenomenon at low carrier density near the Dirac point. This predicted behavior, simulating a metal-insulator transition, arises from the suppression of Coulomb disorder induced inhomogeneous puddles near the charge neutrality point in high quality graphene samples. We then study carrier transport through graphene on SrTiO3 substrates by considering the relative contributions of Coulomb and resonant impurity scattering to graphene resistivity. We establish that the nonuniversal high-density behavior of &sigma(n) in different graphene samples on various substrates arises from the competition among different scattering mechanisms, and it is entirely possible for graphene transport to be dominated by qualitatively different scattering mechanisms at high and low carrier densities. Finally, we calculate the graphene conductivity as a function of carrier density, taking into account possible correlations in the spatial distribution of the Coulomb impurity disorder in the environment. We find that the conductivity could increase with increasing impurity density if there is sufficient inter-impurity correlation present in the system.Item Electronic Transport in Dirac Materials:Graphene and a Topological Insulator(Bi2Se3)(2011) Cho, Sungjae; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Materials with Dirac electronic spectra ("Dirac materials") have attracted much interest since the first successful electronic transport measurement in graphene in 2004. Dirac quasiparticles have novel physical properties such as absence of backscattering and Klein tunneling. Topological insulators are a more recently discovered class of materials that have a bulk band gap and gapless edge/surface states. The surface state in 3D topological insulators has a Dirac electronic spectrum like graphene, but is singly spin-degenerate, with spin-momentum locking. This thesis will describe electronic transport experiments in graphene and in Bi2Se3 ultrathin films, which are predicted to be either 2D topological insulators or conventional insulators. The basic quantum physics of a particle confined in a box is demonstrated using electrons in single and bilayer graphene as examples of massless and massive 2D Fermions, respectively. Ballistic metal-graphene-metal devices act as Fabry- Pérot cavities for electrons, and resonant states of the Fabry-Pérot cavity observed in electronic transport are used to measure the density of states as a function of particle number for massless and massive 2D Fermions. Nonlocal spin-valve experiments are demonstrated up to room temperature in mesoscopic graphene contacted by ferromagnetic electrodes. At low temperature the spin-valve signal shows changes in magnitude and sign with back-gate voltage, which may also result from quantum-coherent transport through Fabry Pérot cavities. The temperature- and magnetic-field-dependent longitudinal (ρxx) and Hall(ρxy) components of the resistivity of graphene were measured. Near the minimum conductivity point ρxx(H) is strongly enhanced and ρxy(H) is suppressed, indicating nearly equal electron and hole contributions to the current. The data are inconsistent with the standard two-fluid model, but consistent with the prediction for inhomogeneously distributed electron and hole regions of equal mobility. Ultrathin three quintuple layer (3QL) Bi2Se3 field effect transistors (FETs) were fabricated by mechanical exfoliation on 300 nm SiO2/Si susbtrates. Temperature and gate-voltage-dependent conductance measurements show a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV, implying that 3QL-Bi2Se3 films are conventional insulators rather than 2D topological insulators, likely due to coupling of the topological surface states through the thin bulk.Item Nanoelectronic Materials(2010) Moore, Tracy; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores fabrication methods and characterization of novel materials used in field effect transistors, including metallic nanowires, carbon nanotubes, and graphene. Networks of conductive nanotubes are promising candidates for thin film electrode alternatives due to their desirable transparency, flexibility, and potential for large-scale processing. Silver nanowire and carbon nanotube networks are evaluated for their use as thin film electrode alternatives. Growth of silver nanowires in porous alumina membranes, dispersion onto a variety of substrates, and patterning is described. Metallic carbon nanotubes are suspended in aqueous solutions, airbrushed onto substrates, and patterned. The conductivity and transparency of both networks is evaluated against industry standards. Graphene is a two dimensional gapless semimetal that demonstrates outstanding room temperature mobilities, optical transparency, mechanical strength, and sustains large current densities, all desirable properties for semiconductors used in field effect transistors. Graphene's low on/off ratio and low throughput fabrication techniques have yet to be overcome before it becomes commercially viable. Silicon oxide substrates are common dielectrics in field effect transistors and instrumental in locating mechanically exfoliated graphene. The morphology of two different silicon oxides have been studied statistically with atomic force microscopy and scaling analysis. Tailoring the physical properties of these substrates may provide a control of graphene's electrical properties. A silicon oxide substrate may also be chemically altered to control the properties of graphene. I have modified silicon oxide with self-assembled monolayers with various terminal groups to control the field near the graphene. I characterize the monolayers with atomic force microscopy, x-ray photospectroscopy, and contact angles. I characterize graphene on these substrates using Raman microscopy and transport measurements. Finally, I examine low frequency noise in graphene field effect transistors on conventional silicon oxide substrates. As devices become smaller, the signal to noise ratio of these devices becomes important. Low frequency noise occurs on long time scales and must be controlled for device stability. I measure novel behavior of low frequency noise in multiple graphene devices. The noise may be described electron-hole puddles in the graphene that are caused by trapped charges near the surface of silicon oxide.Item Materials for large-area electronics: characterization of pentacene and graphene thin films by ac transport, Raman spectroscopy, and optics(2010) Lenski, Daniel Roy; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation explores techniques for fabricating and characterizing two classes of novel materials which may be useful for large-area electronics applications: organic semiconductors and graphene. Organic semiconductors show promise for large-area electronics because of their low cost, compatibility with a variety of substrates, and relative ease of fabricating and patterning thin-film transistors (TFTs). Nearly all published work has focused on the dc electronic transport properties of these materials, rather than their ac behavior, which could be affected by their polycrystalline, granular structure. To address this, I have constructed a model of organic TFTs based on lossy transmission lines, and determined the relationship between the film conductivity and the overall device behavior for a bottom-contacted TFT. I apply this transmission-line framework to interpret my experiments on pentacene TFTs designed in a special long-channel geometry to hasten the onset of high-frequency effects. The experiments reveal an intrinsic frequency-dependent conductivity of polycrystalline pentacene, which can be understood within the context of the universal dielectric response model of ac conduction in disordered solids. The results are important for establishing practical limits on pentacene's ac performance. Graphene is a two-dimensional crystalline form of carbon, with a remarkably simple structure. It is a gapless semiconductor with an extremely high mobility and very high optical transparency, attracting great interest both for its possible uses as a replacement for silicon and as a transparent conducting material. I have synthesized large-area films of graphene via atmospheric-pressure chemical vapor deposition (CVD) on copper substrates, adapting a low-pressure CVD method previously reported to produce exclusively monolayer graphene films. I have transferred the graphene films to insulating SiO2, and characterized them using optical transparency, Raman spectroscopy, and atomic-force microscopy, observing significant differences from the measured properties of widely studied mechanically-exfoliated graphene. I analyze the strengths and weaknesses of these three techniques for distinguishing films of different layer number, and relate them to uncertainties in the known properties of one- and few-layer graphene. I conclude that atmospheric-pressure CVD of graphene on copper produces significant areas of multilayer, rotationally-misoriented graphene, in a significant departure from results on low-pressure CVD of graphene on copper.Item Diffusive Charge Transport in Graphene(2009) Chen, Jianhao; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The physical mechanisms limiting the mobility of graphene on SiO2 are studied and printed graphene devices on a flexible substrate are realized. Intentional addition of charged scattering impurities is used to study the effects of charged impurities. Atomic-scale defects are created by noble-gas ions irradiation to study the effect of unitary scatterers. The results show that charged impurities and atomic-scale defects both lead to conductivity linear in density in graphene, with a scattering magnitude that agrees quantitatively with theoretical estimates. While charged impurities cause intravalley scattering and induce a small change in the minimum conductivity, defects in graphene scatter electrons between the valleys and suppress the minimum conductivity below the metallic limit. Temperature-dependent measurements show that longitudinal acoustic phonons in graphene produce a small resistivity which is linear in temperature and independent of carrier density; at higher temperatures, polar optical phonons of the SiO2 substrate give rise to an activated, carrier density-dependent resistivity. Graphene is also made into high mobility transparent and flexible field effect device via the transfer-printing method. Together the results paint a complete picture of charge carrier transport in graphene on SiO2 in the diffusive regime, and show the promise of graphene as a novel electronic material that have potential applications not only on conventional inorganic substrates, but also on flexible substrates.