Browsing by Subject "Graphene"

Now showing 1 - 20 of 23
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    Charge Carrier Transport and Strain in Graphene Grown on Nitrogen-Seeded Silicon Carbide
    (2017-10) Torrey, Ethan R.
    The interest in graphene as a possible basis for new, faster, smaller and more flexible electronics is tempered by its lack of a band-gap. In recent years, several methods by which a gap might be created have been proposed and explored. The work presented here is a part of that exploration. In this case, the specific gap-inducing mechanism under study is a method of engineered strain. Graphene can be grown on silicon carbide. By pre-treating the silicon carbide in a process that leaves small amounts of nitrogen on its surface, the subsequently grown graphene is made to wrinkle. By controlling the wrinkling, i.e. the strain in the graphene layer, it may be possible to induce a band-gap. Indeed, Angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy results provide experimental support for this theory. At the same time, optical absorption measurements appear to contradict it. The primary focus of this dissertation is strain and transport measurements taken on devices fabricated from this type of graphene, with the expectation that these would aid in resolving the apparent contradiction in previous results.In the course of this work, a new tri-layer method of gate oxide deposition, using reactive electron beam deposition and plasma-assisted atomic layer deposition, was developed. Also, a method of enhanced Raman spectroscopy was developed for graphene-on-silicon-carbide devices. These methods were applied to a set of samples of graphene grown on nitrogen-seeded silicon carbide (NG) with the concentration of nitrogen varying between samples. In this dissertation, several transport characteristics are shown to exhibit a monotonic dependence upon the nitrogen concentration. These include changes in strain, broadening of the longitudinal resistivity peak, an offset between that peak and the zero-crossing of Hall conductivity, and a thermally activated n-doping mechanism, all measured with respect to an applied gate voltage. In addition, more complicated changes in temperature dependence and B-field dependence of the longitudinal resistivity are observed. These results, along with the surprising decrease in resistivity with the addition of nitrogen, are explained in the context of weak localization effects, increased transport by charge puddle-mediated tunneling, and edge states. While the presence of a band-gap could not be demonstrated conclusively in this, the first report of charge transport in this material, the results are in keeping with the presence of a band-gap short-circuited by edge states.
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    Development And Statistical Analysis Of Graphene-Based Gas Sensors
    (2024-03) Capman, Nyssa
    The use of graphene in gas sensors has been increasing in recent years, as graphene has many attractive properties including high carrier mobility, excellent conductivity, and high surface-area-to-volume ratio. Both individual graphene sensors and “electronic nose” (e-nose) sensor arrays have been applied to detecting many gaseous chemicals involved in indoor and outdoor air pollution, food quality, and disease detection in breath. Volatile organic compounds (VOCs) are one important category of chemicals in all of these applications. While graphene sensors have been shown to be effective at detecting and discriminating between VOCs, limitations still exist. This dissertation will describe solutions to two of these problems: Improving selectivity through functionalization and detecting target analytes in the presence of a background interferant.A graphene-based e-nose comprised of 108 sensors functionalized with 36 different chemical receptors was applied to sensing 5 VOCs at 4 concentrations each. The 5 analytes (ethanol, hexanal, methyl ethyl ketone, toluene, and octane) were chosen based on their importance as indicators of diseases such as lung cancer, since disease diagnosis in exhaled breath is one possible application of these arrays. The VOC discrimination ability of the sensor arrays was found to be near-perfect (98%) when using a Bootstrap Aggregated Random Forest classifier. Even with the addition of 1-octene, a compound highly similar to octane and therefore likely to cause high numbers of misclassifications, the sensors still achieved high classification accuracy (89%). The behavior of individual, unfunctionalized graphene varactors was also examined in the presence of VOCs mixed with oxygen. Response signal patterns unique to each VOC + oxygen mixture were revealed. As these patterns developed over the entire gas exposure period, a Long Short-Term Memory (LSTM) network was chosen to classify the gas mixtures as this algorithm utilizes the entire time series. Even in the presence of varying levels of oxygen, three VOCs (ethanol, methanol, and methyl ethyl ketone) at 5 concentrations each could be classified with 100% accuracy, and the VOC concentration could be resolved within approximately 100-200 ppm. This discrimination success was also possible despite the sensors exhibiting varied drift patterns typical of graphene sensors.
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    Electron spin-flip scattering in graphene due to substrate impurities
    (2013-01) Goswami, Aditi
    Graphene has long been known for its peculiar Dirac-like band structure which lends it many of its remarkable properties. It is a promising material for electronic and spintronic applications due to its high carrier mobility, low intrinsic spin-orbit interaction and small hyperfine coupling. However, extrinsic effects may easily dominate intrinsic mechanisms. The scattering mechanisms investigated here are those associated with non-magnetic, charged impurities in the substrate (e.g. SiO2) beneath a planar n-type graphene layer. Such impurities cause an electric field that extends through the graphene and has a non-vanishing perpendicular component. Consequently, the impurity, in addition to the conventional spin-conserving scattering can give rise to spin-flip processes. The latter are a consequence of a spatially varying Rashba spin-orbit interaction caused by the electric field of the impurity in the substrate. This work focuses on the calculation of the elastic scattering cross-sections for these mechanisms. Additionally, relaxation times are estimated for assumed impurity concentrations.
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    Functionalized Graphene Devices for Wireless Biomedical Sensing Applications
    (2018-10) Zhang, Yao
    Graphene has been attracting strong scientific and technological interest for its promising application in the development of biological and chemical sensors. The advantages of high carrier mobility, doping sensitivity and feasibility for device fabrication has made graphene an outstanding candidate material for the development of next generation biomedical sensors. Recently, a variable capacitor (varactor) based on graphene quantum capacitance effect has been studied for the capacitive sensing of target analytes, with the ultimate goal of wireless biomedical sensing. In this dissertation, the graphene varactors were investigated as a sensor platform to carry different surface functionalization for various molecules that are related to early disease diagnostics, demonstrating the potential in the research of passive wireless sensing devices. In Chapter 1, the background of graphene is introduced, including the structure, the history, the properties, and its application in the electronic device fabrication studies. The progress of graphene-based device in the field of semiconductor research is summarized with advantages and disadvantages, which leads to the motivation of this dissertation. In Chapter 2, the graphene varactor device is reviewed with the operation basis, the fabrication process, the device characteristics and the capability to carry various surface functionalization as an advanced sensing platform. In Chapter 3, a glucose sensor based on graphene varactor is developed with device characterization. The device features capacitive signal of glucose in the sensing environment with high ionic strength. In a parallel comparison of a wired measurement, the passive wireless sensing of volatile organic compounds (VOCs) using graphene varactors is demonstrated in Chapter 4. The device performance in the resonant frequency shift was analyzed in a semi-empirical model, showing the reproducible and selective response upon the exposure to different VOC conditions. Finally, the work is summarized with future developments and improvements towards optimized wireless sensing based on graphene varactors in Chapter 5. A novel sensor array with multi-VOC sensing capability is proposed for the application in real time breath monitoring.
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    Generation and Absorption of Pure Spin Currents Using Graphene Nonlocal Spin Valves
    (2018-01) Stecklein, Gordon
    This work describes the fabrication and measurement of nanoscale devices in which a spin-polarized electrical current is used to inject spins into graphene, which then diffuse. We demonstrate the electrical detection of spins in graphene devices with micron-scale spin diffusion lengths and analyze how the spin lifetime and spin diffusion lengths are affected by electrostatic gating. The spin current absorbed by an adjacent ferromagnet is calculated and demonstrated to increase as the electrical conductance of the graphene/ferromagnet interface is improved. Quantitative modeling, including a finite element model of the spatial distribution of spins and the effect of a thin metallic island, indicates that the absorbed spin current is nearing the regime necessary for future technological applications.
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    Graphene Lateral Spin Valves For Computing And Magnetic Field Sensing Applications
    (2019-01) Hu, Jiaxi
    The current complementary metal–oxide–semiconductor (CMOS) technologies are facing greater-than-ever challenges as the Moore’s law approaches to its physical limits. The search for future electronic devices began decades ago. Spintronics, which utilizes the properties of electron spins, is indeed one of the most promising solutions for the beyond-CMOS era. Over the past years, spintronics has been very successful in Hard-disc drives (HDDs) and has significantly increased the storage areal-density. Recently, because of its built-in non-volatility, spintronics has also demonstrated its potential in memory applications. On the other hand, graphene, which is a monolayer of carbon atoms arranged in hexagonal order, is very attractive as the material for spin transport. For example, graphene has the longest spin diffusion length and spin lifetime at room temperature. Therefore, as the device that combines the unique properties from both sides, the graphene lateral spin valve can be useful in many applications. This dissertation mainly explores the use of graphene lateral spin valves for future computing and magnetic field sensing applications. This thesis firstly discusses the spin-circuit model, which is capable of simulating the dc, ac and transient behavior spintronic devices. Using the spin-circuit model, the scaling and energy consumption of all-spin logic devices is quantitatively studied. As one of the original proposals for spin-based computing, ASL utilizes lateral spin valves to process information in the spin domain. By using the physics-based spin-circuit model, the simulations suggest the effect of output-input isolation may be the fundamental challenges that prevent ASL from competing with CMOS in the scheme of conventional Boolean-computing. Next, this thesis explores the application of graphene lateral spin valves in non-Boolean computing and presents an implementation of spintronic Cellular Neural Networks (CNNs). In the graphene-based spintronic CNNs, weights are programmed as spin currents. Because of the tunable spin diffusion length in graphene, the weights can be controlled as local gate voltages, which can tune the weight values over a wide range. The simulation results show that the graphene-based spintronic CNNs have significantly improved scalability, particularly as the number and accuracy of synapses increases. In the last part of this thesis, the width scaling of graphene spin channels is experimentally studied, which is crucial for both the computing and magnetic field sensing applications. By using the graphene deposited by chemical vapor deposition (CVD) and a dedicated fabrication process, a large number of graphene lateral spin valves with consistent interface properties but different channel aspect ratios are fabricated on a single chip. The experimental results show that, as the channel width is scaled from 10 µm to 0.5 µm, the change in the nonlocal spin resistance matches the theory of contact-induced spin relaxation with the interface spin polarization, P, of 3 – 5 %, and spin diffusion length, λs, of 1.5 – 2.5 µm. Meanwhile, the spin-independent baseline resistance dramatically decreases due to the reduction in charge current spreading. However, we find that a remnant baseline remains due to the thermoelectric effects of graphene. By using the gate-voltage and bias-dependent analyses, we attribute the remnant baseline signal to the Joule-heating induced Seebeck voltage. These results suggest that in lateral spin valve design, to avoid any background signals, both the charge and thermal equilibrium conditions should be satisfied.
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    Graphene Point Junctions: A Potential Platform for Achieving Valley Polarization
    (2024-08-31) Tavakley, Jack; Ren, Wei; Davydov, Konstantin; Wang, Ke
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    Graphene Quantum Capacitance Varactors
    (2015-03) Ebrish, Mona
    Graphene is an attractive material for sensing applications due to its large surface-to-volume ratio and high electrical conductivity. The concentration-dependent density of states in graphene allows the capacitance in metal-oxide-graphene structures to be tunable with carrier concentration. This feature allows graphene to act as a variable capacitor (varactor). These devices have a multitude of applications, particularly for biosensing, where the small size and wireless readout are attractive features for in vivo usage. The operation of multi-finger graphene quantum capacitance varactors fabricated using a planarized local bottom gate electrode, HfO2 gate dielectric, is described. The devices show a capacitance tuning range of 1.6:1 at room-temperature, over a voltage range of ±2 V. A characterization methodology was developed to serves as a diagnostic process to ascertain graphene varactor limitations and capabilities. Since functionalization of graphene is needed to sense variety of target analytes, the material and electrical properties of graphene functionalized with glucose oxidase (GOx) was studied. The device characteristics were explored at each step of functionalization with the end goal of realizing wireless graphene glucose sensors. Finally the effect of water vapor was explored, with a demonstration of stable and reproducible wireless humidity sensor.
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    Graphene Sensors and Perovskite Solar Cells for Water Detection
    (2020-08) Kim, Jungyoon
    Water quality test is the first step for cleaning water which is a fundamental element for human health and the environment. The objective of this research is to develop very small, cheap, fast, accurate sensors to detect pollutants including phosphate, nitrate, mercury, and chloride in waters. This is a new testing and analysis technique, which can provide accurate sensing capability to assess the cleanness of waters at a very low cost. The proposed new technology is to manufacture graphene based sensors using the micro-manufacturing. Graphene is a monolayer of carbon atoms with outstanding electrical properties well studied material for a decade by many research groups. Since graphene sensitively responds to molecules in liquids, this property will enable the tiny sensors to detect pollutants in water with very high sensitivity and super short response time to pollutants. Even though graphene responds to the surroundings, it does not have the selectivity to the specific target. In this research, the selective membranes are synthesized and applied to the graphene based sensors to detect the target ions such as nitrate, phosphate, chloride, and mercury. The selective membranes are prepared with two different key materials including molecular imprinted polymer and ionophore. The sensor is characterized by a semiconductor analyzer, and the sensors are tested with several ion solutions to verify their selectivity. The detection limits of the sensor are 0.82, 0.26, 0.87 mg/L and 1.125 µg/L for nitrate, phosphate, chloride and mercury, selectively. In addition, the detection limit of nitrate is enhanced to 0.32 mg/L using the AAO substrate. Here, this research also includes developing perovskite solar cells as the power source of the sensors. Since solar energy is clean and independent, it is one of the important renewable energy resources. Silicon solar cells have already been commercialized and used to generate electricity in various fields because solar cells can directly generate electricity from photons, and they do not cause a problem to our environment as well. Among several types of solar cells, the perovskite solar cells have been studied by many research groups owing to low-cost fabrication, low fabrication temperature and high efficiency. This research includes the preparation of the materials and fabrication of flexible perovskite solar cells. We also characterize the surface morphology of the perovskite to check the grain size by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The efficiency of solar cells is measured by the solar simulator. We study the relationship between the grain size and the CVD process time and successfully demonstrate the performance of devices. The flexible solar cells show the power conversion efficiency of 7.6 % under the AM 1.5 G. As extended research, we have tried to find the proper hole transport layer (HTL) for the device and applied two HTLs, including PEDOT:PSS and PTAA to the devices.
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    Graphene synthesis & graphene/polymer nanocomposites
    (2012-11) Liao, Ken-Hsuan
    Graphene, a two-dimensional carbon sheet with single-atom thickness, has recently attracted significant interest due to its unique mechanical and electrical properties. It has been reported that incorporation of graphene in polymers can efficiently improve the materials’ electrical and mechanical properties. For reliable integration of graphene into practical graphene/polymer nanocomposites, it is essential to have a simple, reproducible and controllable technique to produce graphene on a large scale. We successfully developed a novel, fast, hydrazine-free, high-yield method for producing single-layered graphene. Graphene sheets were formed from graphite oxide by reduction with de-ionized water at 130 ºC. Over 65% of the sheets are single graphene layers. A dehydration reaction of exfoliated graphene oxide was utilized to reduce oxygen and transform C-C bonds from sp3 to sp2. The reduction appears to occur in large uniform interconnected oxygen-free patches so that despite the presence of residual oxygen the sp2 carbon bonds formed on the sheets are sufficient to provide electronic properties comparable to reduced graphene sheets obtained using other methods. Cytotoxicity of aqueous graphene was investigated with Dr. Yu-Shen Lin by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methyl-thiazolyl-diphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. An appropriate alternate assessment, using the water soluble tetrazolium salt (WST-8) assay, reveals that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e. whether or not aggregation occurs) and mode of interaction with cells (i.e. suspension versus adherent cell types). Ultralow percolation concentration of 0.15 wt% graphene, as determined by surface resistance and modulus, was observed from in situ polymerized thermally reduced graphene (TRG)/ poly-urethane-acrylate (PUA) nanocomposite. A homogeneous dispersion of TRG in PUA was revealed by TEM images. The aspect ratio of dispersed TRG, calculated from percolation concentration and modulus, was found to be equivalent to the reported aspect ratio of single-layered free standing TRG. This indicates TRG is mono-layer-dispersed in the matrix polymer. How graphene/polymer nanocomposite glass transition temperatures (Tg) vary was investigated in this study. First we surveyed the literature. No changes in Tg were observed for graphene/polymer nanocomposites synthesized via physical blending processes such as solvent or melt blending, except aqueous blending. In contrast, chemical blending processes such as in situ polymerization or chemically modified fillers yielded significant Tg increases in graphene/polymer nanocomposites. We attribute these results to bonding interactions at the interfaces between matrix polymers and fillers. Physical blending processes cannot provide enough interaction at the interfaces, whereas chemical blending processes can yield strong interaction such as covalent bonds. Aqueous blending of graphene or graphene oxide nanocomposites with water soluble matrix polymers also cause Tg increases, even though the blending processes involve no chemical reactions. The reason for this exception is that hydrogen bonding forms between fillers (graphene oxide or reduced graphene) and water soluble matrix polymers. We then measured Tg in PMMA. We used isotactic PMMA (i-PMMA) and syndiotactic-rich atactic PMMA (a-PMMA) to make TRG/PMMA nanocomposites using solvent blending and in situ polymerization in order to investigate the stereo-regularity and processing effects on the Tg. A Tg increase was found in i-PMMA and in situ PMMA but not in a-PMMA. The results can be explained by the thin film confinement effect of polymer. We attribute the Tg increase to both a higher interaction density and a stronger hydrogen bonding at the interfaces. We have studied the elastic modulus of graphene oxide with various oxygen content. We used in situ AFM nano-indentation to measure the influence of oxygen on the elastic modulus of graphene oxide with various carbon/oxygen (C/O) ratios. The results show that chemical reduction (lower oxygen contents) decreases the elastic modulus of graphene oxide. We speculate that chemical reduction of oxygen atoms of epoxy groups on graphene oxide surface removes the bridging effect between carbon atoms, which leads to more flexible sheets.
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    Microscopic theory of supercapacitors.
    (2011-08) Skinner, Brian
    As new energy technologies are designed and implemented, there is a rising demand for improved energy storage devices. At present the most promising class of these devices is the electric double-layer capacitor (EDLC), also known as the supercapacitor. A number of recently created supercapacitors have been shown to produce remarkably large capacitance, but the microscopic mechanisms that underlie their operation remain largely mysterious. In this thesis we present an analytical, microscopic-level theory of supercapacitors, and we explain how such large capacitance can result. Specifically, we focus on four types of devices that have been shown to produce large capacitance. The first is a capacitor composed of a clean, low-temperature two-dimensional electron gas adjacent to a metal gate electrode. Recent experiments have shown that such a device can produce capacitance as much as 40% larger than that of a conventional plane capacitor. We show that this enhanced capacitance can be understood as the result of positional correlations between electrons and screening by the gate electrode in the form of image charges. Thus, the enhancement of the capacitance can be understood primarily as a classical, electrostatic phenomenon. Accounting for the quantum mechanical properties of the electron gas provides corrections to the classical theory, and these are discussed. We also present a detailed numerical calculation of the capacitance of the system based on a calculation of the system's ground state energy using the variational principle. The variational technique that we develop is broadly applicable, and we use it here to make an accurate comparison to experiment and to discuss quantitatively the behavior of the electrons' correlation function. The second device discussed in this thesis is a simple EDLC composed of an ionic liquid between two metal electrodes. We adopt a simple description of the ionic liquid and show that for realistic parameter values the capacitance can be as much as three times larger than that of a plane capacitor with thickness equal to the ion diameter. As in the previous system, this large capacitance is the result of image charge formation in the metal electrode and positional correlations between discrete ions that comprise the electric double-layer. We show that the maximum capacitance scales with the temperature to the power -1/3, and that at moderately large voltage the capacitance also decays as the inverse one third power of voltage. These results are confirmed by a Monte Carlo simulation. The third type of device we consider is that of a porous supercapacitor, where the electrode is made from a conducting material with a dense arrangement of narrow, planar pores into which ionic liquid can enter when a voltage is applied. In this case we show that when the electrode is metallic the narrow pores aggressively screen the interaction between neighboring ions in a pore, leading to an interaction energy between ions that decays exponentially. This exponential interaction between ions allows the capacitance to be nearly an order of magnitude larger than what is predicted by mean-field theories. This result is confirmed by a Monte Carlo simulation. We also present a theory for the capacitance when the electrode is not a perfect metal, but has a finite electronic screening radius. When this screening radius is larger than the distance between pores, ions begin to interact across multiple pores and the capacitance is determined by the Yukawa-like interaction of a three-dimensional, correlated arrangement of ions. Finally, we consider the case of supercapacitor electrodes made from a stack of graphene sheets with randomly-inserted "spacer" molecules. For such devices, experiments have produced very large capacitance despite the small density of states of the electrode material, which would seem to imply poor screening of the ionic charge. We show that these large capacitance values can be understood as the result of collective entrance of ions into the graphene stack (GS) and the renormalization of the ionic charge produced by nonlinear screening. The collective behavior of ions results from the strong elastic energy associated with intercalated ions deforming the GS, which creates an effective attraction between them. The result is the formation of "disks" of charge that enter the electrode collectively and have their charge renormalized by the strong, nonlinear screening of the surrounding graphene layers. This renormalization leads to a capacitance that at small voltages increases linearly with voltage and is enhanced over mean-field predictions by a large factor proportional to the number of ions within the disk to the power 9/4. At large voltages, the capacitance is dictated by the physics of graphite intercalation compounds and is proportional to the voltage raised to the power -4/5. We also examine theoretically the case where the effective fine structure constant of the GS is a small parameter, and we uncover a wealth of scaling regimes.
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    Nanoscale mechanics of helical and angular structures: exploring and expanding the capabilities of objective molecular dynamics
    (2014-06) Nikiforov, Ilia Andreyevich
    Objective molecular dynamics (OMD) is a recently developed generalization of the traditionally employed periodic boundary conditions (PBC) used in atomistic simulations. OMD allows for helical and/or rotational symmetries to be exploited in addition to translational symmetry. These symmetries are especially prevalent in nanostructures, and OMD enables or facilitates many simulations that were previously dicult or impossible to carry out. This includes simulations of pristine structures that inherently possess helical and/or angular symmetries (such as nanotubes), structures that contain defects (such as screw disclocations) or stuctures that are subjected to deformations (such as bending or torsion). OMD is already a powerful method, having been coupled with the quantum-mechanical density functional-based tight-binding (DFTB) method, as well as with classical potentials. In this work, these capabilities are used to investigate electromechanical properties of silicon nanowires, treating the mechanical simulation results in the context of continuum mechanics. The bending of graphene is studied, and the underlying molecular orbital mechanisms are investigated. The implications of the results on other simulation methods used to study bending of graphene are discussed. OMD is used in an experimental-theoretical collaboration studying the kinking of graphene and boron nitride nanoribbons. The simulations elucidate and quantify the underlying mechanism behind the kinking seen in experiments.Although theoretically, as a generalization, OMD can match or exceed the capabilities of PBC in all cases, OMD is a new method. Thus, practical implementation must be tackled to expand the capabilities of OMD to new simulation methods and simulation types. In this work, OMD is expanded to allow coupling with self-consistent charge (SCC) DFTB, by developing and implementing the required summation formulas for electrostatic and dispersion interactions. SCC-DFTB is an improved form of the standard DFTB method which includes explicit consideration of charge transfer between atoms. This allows for improved description of heteronuclear materials. To demonstrate this capability, proof-of-concept calculations are carried out on a boron nitride nanotube, a screw-dislocated zinc oxide nanowire, and a single-helix DNA molecule.Finally, preliminary development of heat current calculations under OMD is presented. Heat current calculations are used for calculating thermal conductivity of materials from equilibrium molecular dynamics. So far, heat current calculations have been implemented for the pairwise Lennard-Jones potential. The next development (not yet implemented) is the extension of the heat current calculation under OMD to the Tersoff interatomic potential. The challenges and considerations involved are discussed.
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    Plasmon Hybridization In Self-Assembled 3D Graphene-Based Metamaterials
    (2020-04) Agarwal, Kriti
    Three-dimensional (3D) photonic geometries are attractive for developing novel coupled optical modes that cannot exist in the two-dimensional (2D) nano and microfabrication world. In this thesis, the various optical properties that can be induced as a result of 3D architecture are designed, fabricated, and characterized. Even for the well-established resonance in split-ring resonator-based metamaterials, the addition of the multiple planes of symmetric coupling or decoupling induce isotropic and anisotropic resonances for applications such as ultra-sensitive molecular analysis with two-fold advantage of frequency and amplitude monitoring for small concentrations and low on-chip power inclinometers with nanodegree sensitivity, respectively. The limited spatial coverage of the plasmon-enhanced near-field in 2D graphene ribbons presents a major hurdle in practical applications. The ability to transform 2D materials into 3D structures while preserving their unique inherent properties offers enticing opportunities for the development of diverse applications for next-generation micro/nanodevices. Diverse self-assembled 3D graphene architectures are explored here that induce hybridized plasmon modes by simultaneous in-plane and out-of-plane coupling to overcome the limited coverage in 2D ribbons. While 2D graphene can only demonstrate in-plane bi-directional coupling through the edges, 3D architectures benefit from fully symmetric 360° coupling at the apex of pyramidal graphene, orthogonal four-directional coupling in cubic graphene, and uniform cross-sectional radial coupling in tubular graphene. The 3D coupled vertices, edges, surfaces, and volumes induce corresponding enhancement modes that are highly dependent on their shape and dimensions. While most of this work strives to achieve multiple coupled planes of symmetry, the same ideas are also applied to achieve multiple 3D graphene geometries that break mirror symmetry across multiple planes. The asymmetric graphene induces giant optical activity (chirality) that has remained previously unrealized due to the 2D nature of graphene. The chirality induced within the 3D graphene chiral helixes is also a strong function of the geometrical parameters that are analyzed using a machine-learning-based multivariate regression approach to determine the 3D geometry with the strongest chirality. The hybrid modes introduced through the 3D couplings amplify the limited plasmon response in 2D ribbons to deliver non-diffusion-limited sensors, high-efficiency fuel cells, and extreme propagation length optical interconnects.
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    Processing, morphology and properties of graphene reinforced polymer nanocomposites.
    (2009-09) Kim, Hyunwoo
    A unique combination of excellent electrical, thermal and mechanical properties has made graphene a multi-functional reinforcement for polymers. The goal of this research has been three-fold: exfoliation of graphite for higher surface area, development of effective strategies for processing and characterization of graphene based polymer composites and understanding their processing, structure and property relationships. Exfoliated carbon sheets can be obtained from graphite oxide (GO). Functionalized graphene sheets (FGS) are formed by rapid pyrolysis of GO. Despite size reduction and distortion in the flat graphene structure by thermal treatments, FGS have high electrical conductivity and can be melt-processed into polymers. GO can be chemically modified with isocyanate, which improves dispersability in organic solvents and polymers. Although not as thermally stable and electrical conductive as FGS, isocyanate treated GO (iGO) has a larger diameter and is advantageous for retaining high toughness of the composites. FGS and iGO were incorporated into a range of model polymers. Solvent aided blending led to better dispersion of FGS in thermoplastic polyurethane than melt processing. Via solvent mixing, polyurethane became electrically conductive at even less than 0.5 wt% of FGS. With 3 wt% iGO, tensile modulus was increased up to 10 times and nitrogen permeation was reduced by 90%, implying high aspect ratio of exfoliated sheets. Morphology of melt compounded graphite and FGS in poly(ethylene-2,6-naphthalate) was characterized with electron microscopy, X-ray scattering, melt rheology and solid property measurements. Unlike graphite, dispersion of FGS quantified from different routes spreads over a wide range due to structural irregularity and simplified model assumptions. Melt viscoelasticity and electrical properties of polycarbonate were significantly modified by graphite orientation. Flow-induced orientation reduced property gains by graphene dispersion, while quiescent-state annealing restored rigidity and electrical conductivity of the composites. Using melt-state rheological and dielectric measurements, micro-structural evolution of FGS in polystyrene was monitored through annealing. Temporal property changes were analogous to the aging response of colloidal glasses and also influenced by matrix chain relaxation dynamics. Graphene-based polymer nanocomposites can be a new versatile soft material with numerous advantages. For maximized benefits, composite morphology must be tailored appropriately with understanding of its structure-property relationships.
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    Simulation data for: "Two parameter scaling in the crossover from symmetry class BDI to AI"
    (2022-08-01) Kasturirangan, Saumitran; Kamenev, Alex; Burnell, Fiona J; kastu007@umn.edu; Kasturirangan, Saumitran
    The transport statistics at finite energies near a quantum critical point in the presence of disorder were not well understood analytically. This was approached by performing extensive simulations of transport using the package KWANT for python for disordered 1D quantum chains and metallic arm-chair graphene nanoribbons. This dataset contains the resulting data for several system sizes, strengths, and energies. This was used to establish two-parameter scaling and characterize the transport statistics.
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    Speeding-up defect analysis and modeling of graphene based tunnel field effect transistors
    (2014-05) Jaiswal, Akhilesh Ramlaut
    The hunt for post-CMOS devices has seen emergence of many new devices and materials, one among those is graphene based Tunnel Field Effect Transistor (TFET). It becomes necessary to investigate device-circuit and device-system co-design to tackle some of the challenges posed by these devices. Defect analysis and related data is necessary to study variation and effects that realistic devices would have on system level. Such defect analyses require quantum mechanical analyses and are compute and time intensive. In order to quickly gain insight and hence speed up defect analysis for graphene based TFET devices, we have developed a bandstructure based filtering mechanism which filters out severely defected devices from a pool of devices under study thus saving computation time. Effort has also been made to develop a compact model based on Landauer equation for ballistic transport and expression for quantum mechanical tunneling.
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    Strategies to Create Electrically Conductive Polymer/Graphene Composites
    (2021-08) Kou, Yangming
    Conductive polymer composites, typically constructed by melt compounding conductive fillers into a polymer matrix, enjoy specialized applications such as electrostatic discharge protection. Graphene nanoplatelets (GNPs) exhibit high inherent electrical conductivity and geometric anisotropy, thus require much lower loading (< 1 wt%) in a polymer matrix to achieve electric percolation while preserving good melt processability. However, due to their relative high cost, it is desirable to further reduce GNP loading while enhancing the polymer/GNP composite electrical conductivity. In this thesis, I demonstrate two formulation strategies to attain conductive polymer composites by controlling GNP localization in cocontinuous polymer blends using both miscible and immiscible systems. For the miscible system, poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) blends are selected. By first compounding PMMA, SAN, and GNP together at lower temperature and then inducing PMMA/SAN spinodal decomposition by heating, I create spatially regular, cocontinuous domains where GNPs preferentially localize within the thermodynamically preferred SAN-rich phase and form conductive networks. I develop a quantitative transmission electron microscopy (TEM) image analysis method to quantify both the polymer domain size and GNP localization. Dielectric measurements show that quiescent annealing improves particle connectivity of the GNP network, leading to further enhancement in electrical conductivity to ~ 10^[-8] S/cm at 1 wt% GNP concentration. For the immiscible system, polylactide/poly(ethylene-co-vinyl acetate) (PLA/EVA) blends are selected. PLA/GNP masterbatches are melt compounded with the EVA homopolymer. Since GNPs preferentially wet the EVA phase, they transfer from PLA to EVA but become kinetically trapped at the interface, as confirmed by electron microscopy. I achieve an ultra-low percolation threshold of 0.048 wt% GNPs and obtain blends with electrical conductivities of ~ 10^[-5] S/cm at 0.5 wt% GNP concentration. Rheology, in-situ dielectric measurements, and TEM imaging after nonlinear shearing and extensional deformations all show that interfacial GNP network remains at the PLA/EVA interface. Moreover, high electrical conductivity is maintained during a wide range of melt compounding times, between 2–10 minutes. In addition to cocontinuous blends, this thesis also addresses practical challenges related to homopolymer-based conductive composites. The effect of electric field-induced conductivity enhancement and dielectric breakdown due to electrical treeing formation within EVA/GNP composites is studied through in-situ measurement of the electrical conductivity. Furthermore, the relationship between shear rheology, filler dispersion, and electrical conductivity of industrially produced conductive polymer composites is studied. These analytical techniques allow for understanding of composite characteristics, enabling industrial partners to quickly determine which conductive fillers are best suited for the construction of conductive polymer composites.
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    A Study of Bianisotropy in Split Ring Structures made with Graphene and Gold
    (2016-12) Ghosh, Amartya
    An electromagnetic study of sub-wavelength structures made with graphene and gold is done with a concentration on the electro-magnetic coupling of these structures. The aim of the thesis is to analyze the reflection and transmission coefficients from the numerical simulations done with the help of COMSOL. Then homogenize these periodic array of structures for varying thicknesses so that it behaves as a continuous medium in the long wavelength limit. The next goal is to retrieve the effective electromagnetic parameters like the permittivity, permeability and refractive index from this homogenized structure. This will lead to tuning the electromagnetic properties according to the requirements-the property which is not available in naturally occurring materials, because the electrical or magnetic properties in naturally occurring materials are fixed. This new kind of material is defined as the metamaterial. The effective parameters of these materials are dependent on the properties of the basic materials with which the periodic array of structures are made - for example, it will be seen later how the effective properties is different when graphene is used instead of gold. The approach here is to use a normally incident wave on these periodic arrangement of graphene or gold structures and extract the scattering coefficients. Then invert these reflection and transmission data using basic Maxwell's equations to determine the refractive index and the impedance of the multilayered slab. From here the self consistent equations, the permittivity and permeability is determined. When the metamaterial is made with graphene it is found that the continuous slab behaves as an optical non magnetic material while with gold it behaves as a magnetic material. Some studies are also done on the dispersion of graphene nanoribbons and the electromagnetic modes associated with it.
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    Synthesis of Porous Materials and Their Applications in Electrochemistry and Additive Manufacturing
    (2020-12) Xiao, Han
    Open cellular porous materials, such as polyurethane foams, ceramic membranes, and silicon aerogels, are useful in many applications, such as gas membranes, seawater desalination, and heat insulation, because they often possess exceptionally high surface area per unit mass (>100 m2/g), high porosity (> 90%), and low mass density (< 100 mg/cm3). The simplest porous structures often consist of only a single solid material, which limits the ability to tune properties. To address this issue, fillers and other additives, such as polymers, metal nanoparticles or carbon-based substances, can be incorporated to synthesize composites with desirable properties. Polymer-carbon composites stand out from the rest, partially because the soft portions (polymers) and hard compounds (carbon) often possess distinctive yet synergistic properties. For example, incorporating a small amount (< 1 wt%) of electrically conductive graphene nanoflakes into polydimethylsiloxane (PDMS) elastomer makes the product both mechanically robust and electrically conductive, which are desirable for applications in contact sensors and flexible electronics. Pore size, morphology, isotropy, and porosity are some of the most important factors to consider when evaluating the inherent performance of porous materials. These parameters are largely determined by the processing conditions, such as temperature, concentration of porogen (a templating substance that can be easily removed during post-processing, such as water or salt, leaving behind the pores), and method of synthesis, in addition to the selection of parent solid materials. Templating is one of the many routes employed to synthesize porous structures, where a sacrificial porogen is used to first form a percolating network and is later replaced by air when removed, typically via sublimation or washing. Compared to other routes such as foaming, sol-gel transition, etching or lithography, templating enables the fabrication of complex pore shapes and geometries over large-scales with tunability in the pore size, morphology, and pore connectivity of the final product; therefore, templating is considered one of the most versatile approaches. This thesis outlines the synthesis of open cellular porous polymers and polymer composites using freezing templated methods. We first designed a carbon-polymer aerogel which is highly porous (99.6% porosity), has low density (~ 5 mg/cm3), and is electrically conductive (5.3 ± 3 × 10-2 S/cm), making it an ideal substitute for the metal current-collectors in lithium-ion batteries. Next, we explored strategies to prepare graphene oxide aerogels with aligned microstructures via bi-directional freezing. Simulations were conducted to predict the structure of the aligned aerogel, which agreed reasonably well with experimental results. Lastly, we explored camphene, a solid cyclic hydrocarbon at room temperature, as the solvent and templating agent for 3D printing porous polymers. Upon subliming camphene, the resulting porous network exhibited improved interlayer strength and reduced anisotropy, and the tensile properties were comparable to those of compression-molded samples. This new strategy to prepare porous polymer materials via direct ink writing could be further applied to other common polymers, such as polyethylene or polypropylene, two commercial-grade materials that are very challenging to print via conventional methods.
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    Toughening Thermosetting Resins with Modified Graphene Oxide
    (2018-10) He, Siyao
    In this thesis we studied the toughening effects of graphene derivatives, which have drawn much attention recently due to their high aspect ratios and outstanding mechanical properties. Graphene-based toughener can toughen resin at extremely low loading levels, which means it is economically viable for price-driven thermosetting resins market. To understand the toughening effect of graphene derivatives in resins, several GO surface modifications were developed to help disperse GO into the resins. The best performing modified GO (mGO) investigated in this work can be homogeneously dispersed into a resin with merely mechanic mixing. To simplify the materials handling and further improve the toughener dispersion, a styrene masterbatch route was developed to avoid the freeze-drying step in the mGO synthesis. The toughening effect of pristine and modified graphene oxide was tested in both unsaturated polyester and vinyl ester resins. The result indicated that GO and its derivatives can toughen UP and VE resins at a loading lower than 0.04 wt.%. Although, these tougheners are highly efficient in terms of required loading, we found that the toughness improvement obtained by adding mGO is insensitive to changes in particle-matrix interfacial strength and toughener loading. To understand this behavior, we studied the inorganic filler interference to mGO toughening, and also how the mGO toughening effect is affected by the physical dimensions of GO size and mGO aggregate size. Sophisticated data analysis involving computerized particle analysis were carried out to characterize the size differences between samples. The results show that the toughening effect of mGO is identical to that of other inorganic fillers, and this toughening effect is independent of filler mechanical properties. Finally, the toughening performance of mGO was tested in glass fiber reinforced composites, which is the target product for UP and VE resins. Both the interlaminar fracture toughness test and Izod impact test showed no improvement in composite toughness after adding mGO. A detailed fractography analysis of failed composite samples indicate that the failure happens between the resin and the glass fiber, which means increasing the fracture toughness of the resin matrix will not likely show any effect on the composite fracture toughness.