Physics
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Item Cyclotron resonance gain in the presence of collisions(2017) Cole, Nightvid; Antonsen, Thomas M; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The conditions needed for the amplication of radiation by an ensemble of magnetized, relativistic electrons that are collisionally slowing down are investigated. The current study is aimed at extending the work of other researchers in developing solid-state sources of Terahertz radiation. The source type considered here is based on gyrotron-like dynamics of graphene electrons, or it can alternately be viewed as a solid state laser source that uses Landau levels as its band structure and is thus similar to a quantum cascade laser. Such sources are appealing because they oer the potential for a compact, tunable source of Terahertz radiation that could have commercial applications in scanning, communication, or energy transfer. An exploration is undertaken, using linear and nonlinear theories, of the conditions under which such sources might be viable, assuming realistic parameters. Classical physics is used, and the model involves electrons in monolayer graphene assumed to be pumped by a laser, follow classical laws of motion with the dissipation represented by a damping force term, and lose energy to the electromagnetic eld as well. The graphene is assumed to be in a homogeneous magnetic eld, and is sandwiched between two partially-transmissive mirrors so that the device acts as an oscillator. This thesis incorporates the results of two approaches to the study of the problem. In the rst approach, a linear model is derived semi-analytically, which is relevant to the conditions under which there is gain in the device and thus stable operation is possible, versus the regime in which there is no net gain. In the second approach, a numerical simulation is employed to explore the nonlinear regime and saturation behavior of the oscillator. The simulation and the linear model both assume the same original equations of motion for the eld and particles that interact self-consistently. The model used here is very simplied, but the aim here is to elucidate the basic principles and scaling behavior of such devices, not necessarily to calculate what the exact dynamics, outputs, and parameters of a fully commercially realized device will be.Item Optical and magnetic measurements of a levitated, gyroscopically stabilized graphene nanoplatelet(2017) Coppock, Joyce Elizabeth; Wellstood, Frederick; Kane, Bruce; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I discuss the design and operation of a system for levitating a charged, $\mu$m-scale, multilayer graphene nanoplatelet in a quadrupole electric field trap in high vacuum. Levitation decouples the platelet from its environment and enables sensitive mechanical and magnetic measurements. First, I describe a method of generating and trapping the nanoplatelets. The platelets are generated via liquid exfoliation of graphite pellets and charged via electrospray ionization. Individual platelets are trapped at a pressure of several hundred mTorr and transferred to a trap in a second chamber, which is pumped to UHV pressures for further study. All measurements of the trapped platelet's motion are performed via optical scattering. Second, I present a method of gyroscopically stabilizing the levitated platelet. The rotation frequency of the platelet is locked to an applied radio frequency (rf) electric field $\bm{E}_{\mathrm{rf}}$. Over time, frequency-locking stabilizes the platelet so that its axis of rotation is normal to the platelet and perpendicular to $\bm{E}_{\mathrm{rf}}$. Finally, I present optical data on the interaction of a multilayer graphene platelet with an applied magnetic field. The stabilized nanoplatelet is extremely sensitive to external torques, and its low-frequency dynamics are determined by an applied magnetic field. Two mechanisms of interaction are observed: a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation. A model is constructed to describe this data, and experimental values are compared to theory.Item Surface Studies of Graphene and Graphene Substrates(2013) Burson, Kristen M.; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene has attracted a great deal of attention for its exceptional electronic and mechanical properties. As graphene, a two-dimensional lattice of carbon atoms, is an `all surface' material, its interactions with the underlying substrate play a crucial role in determining graphene device behavior. In order to tailor graphene device properties, the interaction between graphene and the underlying substrate must be clearly understood. This thesis addresses the question of the relationship between graphene and graphene substrates by considering both the substrate topography and the impact of charged impurities in the substrate. Utilizing scanning tunneling microscopy and high-resolution atomic force microscopy, we measure the topography of silicon dioxide (SiO2) supported graphene and the underlying SiO2(300nm)/Si substrates. We conclude that the graphene adheres conformally to the substrate with 99% fidelity and resolve finer substrate features by atomic force microscopy than previously reported. To quantify the density of charged impurities, simultaneous atomic force microscopy (AFM) and Kelvin probe microscopy are used to measure the potential and topographic landscape of graphene substrates, SiO2 and hexagonal boron nitride (h-BN). We find that the surface potential of SiO2 is well described by a random two-dimensional surface charge distribution with charge densities of ~1011 cm-2, while BN exhibits charge fluctuations that are an order of magnitude lower than this. Charged impurities in the substrate present a scattering source for transport through graphene transistors, and the difference in magnitude in measured substrate charged impurities densities for SiO2 and BN is consistent with the observed improvement in charged carrier mobility in graphene devices on h-BN over graphene devices on SiO2. Finally, this thesis presents a theoretical model elucidating the challenges of imaging corrugated substrates by non-contact AFM and an experimental work using Kelvin probe microscopy to characterize the electrostatic potential steps at interfaces of small-molecule organic heterojunctions.Item TWO-DIMENSIONAL CRYSTALS ON SUBSTRATES: MORPHOLOGY AND CHEMICAL REACTIVITY(2013) Yamamoto, Mahito; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-dimensional crystals such as graphene and transition metal dichalcogenides have emerged as a new class of materials. They serve as rich playgrounds for two-dimensional physics but also have great potential for a wide range of applications due to their exceptional tunability via external influences such as electric fields, light, chemical adsorbates, defects, and stress. This dissertation aims to understand, as a fundamental step toward their application, the response of two-dimensional crystals to such external perturbations imposed by supporting substrates. First, the mechanical response of graphene supported on corrugated substrates is studied. I find that the structural evolution of graphene depends on the roughness of the substrate and the graphene thickness. On SiO2 substrates decorated with a low-density of SiO2 nanoparticles, adhesion dominates graphene elasticity and, hence, graphene conforms to the substrate. With increasing nanoparticle density, however, the elastic stretching energy is reduced by the formation of wrinkles. As the graphene membrane is made thicker, graphene becomes stiffer and delaminates from the substrate. Second, the effect of substrates on chemical reactivity of graphene is probed. Single-layer graphene on low charge-trap density boron nitride is not etched and shows little doping after oxygen treatment, in sharp contrast with oxidation under similar conditions of graphene on high charge-trap density SiO2 and mica. Furthermore, bilayer graphene shows reduced reactivity compared to single-layer graphene regardless of its substrate-induced roughness. Together the observations indicate that graphene's reactivity is predominantly controlled by charge- inhomogeneity-induced potential fluctuations rather than by surface roughness. Lastly, the oxidative reactivity of atomically thin molybdenum disulfide (MoS2) on SiO2 is studied. MoS2 is etched by oxygen treatment. However, unlike graphene on SiO2, the density of etch pits barely depends on MoS2 thickness, oxidation time, oxidation temperature, but varies significantly from sample to sample. The observations suggest that the oxidative etching of atomically thin MoS2 is initiated at native defect sites on the basal-plane surface rather than activated by substrate effects such as charged impurities and surface roughness. The findings provide insight into the mechanical and chemical properties of two-dimensional crystals and may have important implications for their applications.Item CHARGE TRANSPORT IN GRAPHENE WITH ADATOM OVER-LAYERS ; CHARGED IMPURITY SCATTERING, DIELECTRIC SCREENING, AND LOCALIZATION.(2011) Jang, Chaun; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene, a single atom thick plane of graphite, is a novel two-dimensional electron system in which the low-energy electrons behave as massless chiral Dirac fermions. This thesis explores the effects of disorder in graphene through controlled surface modification in ultra-high vacuum (UHV), coupled with in situ electronic transport experiments. Three different roles of adatom overlayers on graphene are investigated. First, the effects of charged impurity scattering are studied by introducing potassium ions on the graphene at low temperature in UHV. The theoretically expected magnitude and linear density-dependence of the conductivity due to long range Coulomb scattering is verified. Second, the effective dielectric constant of graphene is modified by adding ice overlayers at low temperature in UHV. The opposing effects of screening on scattering by long range (charged impurity) and short range impurities are observed as variations in conductivity, and the changes are in agreement with Boltzmann theory for graphene transport within the random phase approximation. The minimum conductivity of graphene is roughly independent of charged impurity density and dielectric constant, in agreement with the self-consistent theory of screened carrier density inhomogeneity (electron and hole puddles). Taken together, the experimental results on charged impurity scattering and dielectric screening strongly support that long range Coulomb scattering is the dominant scattering mechanism in as-fabricated graphene on SiO2. In addition to the semi-classical transport properties, quantum transport is also studied with cobalt decorated graphene. Strong localization is achieved in the disordered graphene through deposition of cobalt nanoclusters. In finite magnetic field a phase transition occurs from the localized state to the quantum Hall state. Scaling analysis confirms that the transition is a quantum phase transition which is similar to the localization - delocalization transitions in other two dimensional electron systems.Item Kelvin Probe Microscopy Studies of Epitaxial Graphene on SiC(0001)(2011) Curtin, Alexandra E.; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Epitaxial graphene on SiC(0001) presents a promising platform for device applications and fundamental investigations. Graphene growth on SiC(0001) can produce consistent monolayer thickness on terraces and good electronic properties. In exfoliated graphene on SiO2, random charged impurities in the SiO2 surface are thought to be the dominant scatterers, explaining the observed transport properties as well as the spatial charge inhomogeneity seen in scanned-probe experiments. In contrast, the scattering mechanisms and charge distribution in epitaxial graphene remain relatively unexplored. Here I use Kelvin probe microscopy (KPM) in ambient and UHV conditions to directly measure the surface potential of epitaxial graphene on SiC(0001). Ambient-environment KPM on graphene/SiC(0001) shows surface potential variations of only 12 meV. Taken together with transport measurements, the data suggest that the graphene samples in ambient are in the low-doped regime, near the minimum conductivity of roughly 4e2/h. I am also able to use UHV KPM of graphene/ SiC(0001) to identify the discrete surface potentials of monolayer and bilayer graphene as well as the insulating interfacial carbon layer and bare SiC, correlated with scanning electron micrographs of the same location. The surface potential differences between monolayer and bilayer graphene and between IFL and monolayer graphene are both suggestive of low doping (≤1012 cm-2). The surface potentials of monolayer and bilayer graphene are relatively smooth, while the IFL and bare SiC, in contrast, showed larger variations in surface potential suggesting the presence of unscreened charged impurities present on the IFL that are later screened by the overgrown graphene. I model the potential variations for unscreened and graphene-screened charged impurities using the self-consistent theory of graphene developed by Adam et al. The results show that although surface potential variations are, as expected, larger in the IFL than in graphene, both surfaces display surface potential variations 10-40 times smaller than predicted by theory. While ambient electronic transport data and surface potential steps suggest our samples are only lightly doped (≤1012 cm-2), in a regime dominated by electron-hole puddles, we do not observe these puddles in UHV. The absence of puddles in UHV leaves the source of doping in these samples an open question.Item Many-body Effects in Graphene(2008-05-14) Tse, Wang-Kong; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene is a novel material that features a quasi-relativistic linear energy dispersion with the quantum mechanical motion of electrons obeying the massless Dirac equation. In this dissertation, we study the many-body effects in graphene due to Coulomb interaction and electron-phonon interaction. Interaction effects can appear in both transport and electronic properties. For many-body effects in transport, we formulate the theory for Coulomb drag in double-layer graphene. We calculate the drag resistivity and study its dependence on temperature, density and interlayer spacing, finding zero drag if one of the graphene layers is intrinsic (i.e., undoped) and a non-zero drag exhibiting a similar behavior to regular bilayer drag if both graphene layers are extrinsic (i.e., doped). For many-body effects in electronic properties, we formulate the theory for quasiparticle and phonon renormalization due to Coulomb and electron-phonon interaction. We first study renormalization of electron properties due to Coulomb interaction by calculating the renormalized quasiparticle parameters from the electron self-energy, showing that intrinsic graphene behaves as a marginal Fermi liquid and extrinsic graphene behaves as a regular Fermi liquid. We then study renormalization of electron properties due to electron-phonon interaction. We calculate the electron self-energy and the renormalized quasiparticle velocity, finding that the renormalized band structure exhibits a kink at the phonon energy in agreement with angle-resolved photoemission spectroscopy (ARPES) experiment. We finally study renormalization of phonon energy due to electron-phonon interaction. We calculate the phonon self-energy and the renormalized phonon energy dispersion, showing that multiple Kohn anomalies arise which are completely different from the Kohn anomaly in usual metals.