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Browsing by Subject "graphene"

Now showing 1 - 8 of 8
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    Characterization of Graphene Grown Directly on Crystalline Substrates
    (2015-09) Rothwell, Sara
    Graphene has become one of the most popular materials under research, particularly since the 2010 Nobel Prize in Physics. Many visions posit that graphene electronics will be some of the fastest and smallest circuitry physically feasible, however before this becomes reality the scientific community must gain a firm handle on the creation of semiconducting varieties of graphene. In addition, well understood epitaxial growth of graphene on insulating materials will add to the facility of fabricating all-carbon electronics. This thesis presents experimental work detailing the growth of pristine graphene grown on sapphire (GOS) through the thermal decomposition of acetylene, and the electronic characterization of graphene grown on nitrogen-seeded silicon carbide (NG), a semiconducting variety of graphene grown in collaboration with researchers at Georgia Institute of Technology and Rutgers University. GOS displays turbostratic stacking and characteristics of monolayer graphene as analyzed by Raman spectroscopy and atomic force microscopy. Scanning tunneling microscopy characterization of NG illustrates a topography of pleats from 0.5-2 nm tall, 1-4 nm thick, and 1-20 nm long, as well as atomically flat plateaus and other areas of intermixed features. Scanning tunneling spectroscopy measurements across NG features show peaks interpreted as Landau levels induced by strain. Analysis of these Landau levels in coordination with previous characterization concludes that a model employing a bandgap fits best.
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    Controlling Cocontinuous Polymer Blends with Nanofillers Jammed at the Interface
    (2017-08) Bai, Lian
    Cocontinuous polymer blends are composed of two or more immiscible or partially miscible polymers coexisted within the same volume in multiple interpenetrated networks. They can be created by either melt compounding of immiscible polymers or phase separation of partially miscible polymer pairs via spinodal decomposition. The polymer blends with cocontinuous structure have significantly improved mechanical properties and they have applications in conductive plastics, porous membranes for filtration and tissue scaffolds for drug delivery devices. As the cocontinuous morphology is in a non-equilibrium state, the thermodynamic instability causes the morphology to coarsen during post-mixing processing, which is a major drawback for applications. In order to control and optimize the phase morphology, nanofillers have been localized and jammed at the interface as an effective method to suppress the coarsening and stabilize the cocontinuous structure during annealing. However, the mechanisms involved in the morphology stabilization by interfacial nanofillers are not yet fully understood. This thesis seeks to systemically study the structure-processing-properties relationships of nanofiller stabilized cocontinuous polymer blends by providing insight to these three questions: (1) how do thermodynamic factors determine nanofiller localization and their morphology stabilization ability? (2) How do kinetic factors affect nanofiller migration during melt compounding and coarsening suppression during annealing? (3) How is the morphology dynamics of nanofiller stabilized polymer blends connected to their rheology response during annealing? Concerning the thermodynamic factors, this thesis approaches the problem via incorporating nanofillers with different surface properties into cocontinuous polymer blends. The different hydrophobicities of silica nanoparticles and the different polarities of graphene nanoplates determine the different localization of these nanofillers in the polymer blends. Nanofiller localization in one polymer phase or at the interface is explained by the system’s tendency to minimize its free energy. Wetting coefficients, which are derived from the Young’s equation and calculated based on the surface energies of nanofillers and two polymer components, have been applied to predict the nanofillers localization in the polymer blends. Concerning kinetic factors, different processing parameters during melt compounding were systemically investigated to study their effect on the migration and localization of nanofillers and their corresponding morphology stabilization ability during annealing. The proper sequence of addition of components is crucially significant to achieve the interfacial localization: nanofillers were generally premixed with the thermodynamically less favorable phase, and then melt compounded with the thermodynamically more favorable phase to enable nanofillers to migrate from the premixed phase to the interface. The effect of different melt compounding time is also systemically studied in the cocontinuous polymer blends stabilized by graphene nanoplates, and we found blends with short melt compounding time have more nanofillers jammed at the interface and more effective coarsening suppression ability during annealing. In order to correlate the morphology dynamics with rheology, we combined rheology time sweeps with morphology information from confocal microscopy, scanning electron microscopy and transmission electron microscopy. We found that morphology coarsening results in shrinkage of interfacial area and jamming of interfacial nanofillers. The nanofillers jammed at the interface contributed to stabilization of the cocontinuous morphology and formation of a 3D nanofiller network. The nanofiller network gave rise to the increase of storage modulus during annealing and the typical gel-like behavior in rheology frequency sweeps.
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    Evaluating Graphene as a Channel Material in Spintronic Logic Devices
    (2016-03) Anugrah, Yoska
    Spintronics, a class of devices that exploit the spin properties of electrons in addition to the charge properties, promises the possibility for nonvolatile logic and memory devices that operate at low power. Graphene is a material in which the spin orientation of electrons can be conserved over a long distance, which makes it an attractive channel material in spintronics devices. In this dissertation, the properties of graphene that are interesting for spintronics applications are explored. A robust fabrication process is described for graphene spin valves using Al2O3 tunnel tunnel barriers and Co ferromagnetic contacts.  Spin transport was characterized in both few-layer exfoliated and single-layer graphene, and spin diffusion lengths and spin relaxation times were extracted using the nonlocal spin valve geometry and Hanle measurements. The effect of input-output asymmetry on the spin transport was investigated.  The effect of an applied drift electric field on spin transport was investigated and the spin diffusion length was found to be tunable by a factor of ~8X (suppressed to 1.6 µm and enhanced to 13 µm from the intrinsic length of 4.6 µm using electric field of ±1800 V/cm). A mechanism to induce asymmetry without excess power dissipation is also described which utilizes a double buried-gate structure to tune the Fermi levels on the input and output sides of a graphene spin logic device independently. It was found that different spin scattering mechanisms were at play in the two halves of a small graphene strip. This suggests that the spin properties of graphene are strongly affected by its local environment, e.g. impurities, surface topography, defects. Finally, two-dimensional materials beyond graphene have been explored as spin channels.  One such material is phosphorene, which has low spin-orbit coupling and high mobility, and the interface properties of ferromagnets (cobalt and permalloy) with this material were explored.  This work could potentially enable spin injection without the need for a physical tunnel barrier to solve the conductivity mismatch problem inherent to graphene.
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    Modeling of Transport Phenomena in Two-Dimensional Semiconductors
    (2016-12) Liu, Yue
    Recently, transition metal dichalcogenides and black phosphorus (BP) emerged as new 2D semiconductors due to the advantages of moderate energy band gap, high carrier mobility, ultra thin film and high anisotropy. Together with graphene, 2D materials have been utilized in the development of biomedical devices, touch screen and display technologies, and flexible applications such as wearable electronics and IoT devices. They also open up new opportunities in research fields including spintronics, optoelectronics and next generation post-silicon transistor. In this dissertation, we present theoretical modeling for several topics related to 2D materials. Starting with the fundamental tight-binding theory of graphene, we review electronic properties for graphene including massless 2x2 Dirac Hamiltonian and pseudo-spin wave function. Followed by discussion of ballistic transport, a detailed analysis on graphene diffusive transport is provided. Ionized impurity scattering and carrier screening effect is considered in the model. The momentum relaxation time and mobility for graphene is modeled. A non-linear Thomas Fermi screening is introduced to improve the simulation accuracy. Taking the real spin into account, the new Hamiltonian is a 4x4 matrix. An external field perpendicular to the graphene breaks the reflection symmetry and introduces a Rashba spin-orbit interaction, which couples pseudo-spin and real spin. The relevant charge carrier states are no longer spin eigenstates. Rashba interaction is found to be quite small compared to Coulomb impurity scattering. To characterize the spin-polarized electrons tunneling from electrodes and transport in graphene, a spin valve device modeling and magnetoresistance calculation is developed. Black phosphorus possesses excellent properties like other 2D materials for high performance nanoelectronic applications. Moreover, there is a uniquely high in-plane anisotropy in BP due to its puckered crystal structure. To model the anisotropic transport, a model based on the BTE is developed, considering the full anisotropic electronic structure. For zero temperature calculation with ionized impurity limited scattering, anisotropy ratio 3-4 can be obtained from the model. Due to the dominating effect of screening, mobility is found to decrease weakly with increasing temperature. For , a smaller anisotropy ratio of 1.8-3.5 matching experimental measurements indicates that impurity scattering is an important mechanism for black phosphorus.
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    Temporal Control of Graphene Plasmons
    (2018-06) Wilson, Josh
    The conductivity of graphene can be controlled in real time. In this thesis we will explore the effects of modulating the conductivity on the dynamics of graphene plasmons. We will derive an equation to describe the dynamics of the plasmons and use it to investigate temporal Fresnel equations and a method for amplifying graphene plasmons.
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    Widegap semiconducting graphene from nitrogen seeded SiC
    (arXiv, 2013-06-18) Wang, F; Liu, G; Rothwell, S; Nevius, M; Tajeda, A; Taleb-Ibrahimi, A; Feldman, L C; Cohen, P I; Conrad, E H
    All carbon electronics based on graphene has been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of semiconducting graphene. While chemical functionalization was thought to be a route to semiconducting graphene, disorder in the chemical adsorbates, leading to low mobilities, have proved to be a hurdle in its production. We demonstrate a new approach to produce semiconducting graphene that uses a small concentration of covalently bonded surface nitrogen, not as a means to functionalize graphene, but instead as a way to constrain and bend graphene. We demonstrate that a submonolayer concentration of nitrogen on SiC is sufficient to pin epitaxial graphene to the SiC interface as it grows, causing the graphene to buckle. The resulting 3-dimensional modulation of the graphene opens a band-gap greater than 0.7eV in the otherwise continuous metallic graphene sheet.