Browsing by Subject "graphene"
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Item Computational Study of the Development of Graphene Based Devices(2012-02-14) Bellido Sosa, EdsonGraphene is a promising material for many technological applications. To realize these applications, new fabrication techniques that allow precise control of the physical properties, as well as large scale integration between single devices are needed. In this work, a series of studies are performed in order to develop graphene based devices. First, using MD simulations we study the effects of irradiating graphene with a carbon ion atom at several positions and energies from 0.1 eV to 100 keV. The simulations show four types of processes adsorption, reflection, transmission, and vacancy formation. At energies below 10 eV the dominant process is reflection, between 10 and 100 eV is adsorption, and between 100 eV and 100 keV the dominant process is transmission. Vacancy formation is a low rate process that takes place at energies above 30 eV. Three types of defects were found: adatom, single vacancy, and 5-8-5 defect formed from a double vacancy defect. Also a bottom-up fabrication method is studied, in this method, the controlled folding of graphene structures, driven by molecular interactions with water nanodroplets, is analyzed considering the interactions with substrates such as SiO2, HMDS and IPA on SiO2. When the graphene is supported on SiO2, the attraction between graphene and the substrate prevents graphene from folding but if the substrate has HMDS or IPA, the interaction between graphene and the substrate is weak, and depending on the geometry of the graphene structure, folding is possible. Finally, to evaluate the characteristics of graphene based devices, we model the vibrational bending modes of graphene ribbons with different dimensions. The resonant frequencies of the ribbons and relations between the size of the ribbon and their resonant frequencies are calculated. The interaction of a graphene vibronic device with water and IPA molecules are simulated and demonstrate that this device can be used as a sensitive vibronic molecular sensor that is able to distinguish the chemical nature of the detected molecule. Also, the electrical properties of the graphene vibronic with armchair and zigzag border are calculated; the latter has the potential to generate THz electrical signals as demonstrated in this work.Item Density functional study of graphene on insulating substrates(2009-08) Jadaun, Priyamvada; Banerjee, Sanjay; Sahu, Bhagawan R.; Fiete, GregoryThis is a study of the structural and electronic behavior and properties of graphene on α-quartz and α-sapphire using Density Functional Theory. We construct initial structures using the above 2 substrates, place a layer of graphene on them and subsequently allow the atoms to relax. After relaxation we study any structural changes, band structures, density of states, charge density to determine the electronic properties of the entire structure. Eventually this study will help in the search for good substrates for graphene based transistors.Item Electron Irradiation Induced Changes of the Electrical Transport Properties of Graphene(2014-08-06) Woo, Sung OhThis research investigates the effect of electron irradiation on transport properties in graphene Field Effect Transistor (FET) devices. Upon irradiation, graphene is doped with electrons and adsorbs molecules by transfer of accumulated electrons in graphene to environmental molecules, resulting in the deterioration of transport properties. Molecules adsorbed after electron irradiation in ambient condition were observed by Atomic Force Microscopy (AFM). In-situ transport properties were measured in a vacuum after electron irradiation. In addition, hysteresis in the transport properties appeared as a result of electron irradiation. The origin of the hysteresis was investigated by exposing the electron beam irradiated graphene to ambient condition. As environmental molecules are adsorbed on graphene, the hysteresis disappears. In addition, annealing in a vacuum also removes the hysteresis. We conclude that the hysteresis is the result of polar adsorbates on graphene induced by electron irradiation. In addition, the effect of electron irradiation on a PMMA (Poly Methyl Methacrylate)/Graphene bilayer was studied. We observed a deterioration of the electrical transport properties of a graphene FET. Prior to electron irradiation, we observed that the PMMA layer on graphene did not degrade the carrier transport of graphene but improved its electrical properties instead. As a result of the electron irradiation on the bilayer, defects could be observed by Raman spectroscopy. We suggest that the degradation of the transport behavior originates from the binding of atoms or molecules generated by the PMMA backbone secession process.Item Linear and Nonlinear Optics in a System of Massless Dirac Fermions(2014-08-10) Yao, XianghanGraphene electrons possess linear energy dispersion relation, and thus behave as two-dimensional (2D) Dirac fermions. Consequently, compared with the conventional 2D electron gas systems (2DEG) found in MOSFETs and quantum wells, graphene exhibits a variety of electronic and optoelectronic properties that are characteristic of 2D Dirac fermions. Similar 2D Dirac fermions are found at the surface layer of 3D topological insulator, and they are topological protected from backscattering due to spin-orbital coupling and time reversal symmetry. We here calculate the linear and nonlinear optical response of graphene in strong magnetic and optical fields, using a quantum-mechanical density-matrix formalism. We show that graphene in a magnetic field possesses a giant mid- or far-infrared optical nonlinearity, perhaps the highest among known materials. The high nonlinearity originates from the unique electronic properties and selection rules near the Dirac point. As a result, even one monolayer of graphene gives rise to an appreciable nonlinear frequency conversion efficiency for incident infrared radiation. Inspired by the highly efficient four-wave mixing process in the 2D Dirac fermion systems, we further propose a new mechanism of generating polarization-entangled photons based on the parametric generation process in the third section of this dissertation. Unique properties of quantized electron states in a magnetized graphene and optical selection rules near the Dirac point give rise to a giant optical nonlinearity and a high rate of photon production in the mid- or far-infrared range. A similar mechanism of photon entanglement may exist in topological insulators where the surface states have a Dirac-cone dispersion and demonstrate similar properties of magneto-optical absorption. In the absence of a magnetic field, the surface plasmon resonance provides an alternative method to enhance nonlinear frequency conversion efficiency. In the forth section of this dissertation, a graphene-based difference frequency generation (DFG) of terahertz plasmons is proposed as an example to study nonlinear photonplasmon interaction in 2D Dirac fermion systems. Our results demonstrate strong enhancement of the DFG efficiency near the plasmon resonance and the feasibility of phase-matched nonlinear generation of plasmons over a broad range of frequencies. Considering graphene plasmonics' superiorities in wave confinement, dissipation and tunability, a graphene-based nonlinear terahertz plasmon generation process promises applications in terahertz sources and sensors, as well as integrated photonic circuits.Item Mechanochemical Fabrication and Characterization of Novel Low-dimensional Materials(2012-10-19) Huitink, David RyanIn this research, for the first time, a novel nanofabrication process is developed to produce graphene-based nanoparticles using mechanochemical principles. Utilizing strain energy at the interface of Si and graphite via the use of a tribometer, a reaction between nanometer sized graphite particles with a reducing agent (hydrazine) was initiated. This simple method demonstrated the synthesis of lamellar platelets (lamellae of ~2nm) with diameters greater than 100 micrometers and thicknesses less than 30 nm directly on the surface of a substrate under rubbing conditions. Spectroscopic evaluation of the particles verified them to be graphene-based platelets, with functionalized molecules including C-N and C-Si bonding. Furthermore, the size of the particles was shown to be highly correlated to the applied pressure at the point of contact, such that three-body friction (with intermediate particles) was shown to enhance the size effect, though with greater variation in size among a single test sample. A chemical rate equation model was developed to help explain the formation of the chemically modified graphene platelets, wherein the pressure applied at the surface can be used to explain the net energy supplied in terms of local flash temperature and strain energy. The activation energy calculated as a result of this method (~42kJ/mol) was found to be extraordinarily close to the difference in bond enthalpies for C-O and the C-N, and C-Si bonds, indicating the input energy required to form the platelets is equivalent to the energy required to replace one chemical bond with another, which follows nicely with the laws of thermodynamics. The ability to produce graphene-based materials using a tribochemical approach is a simple, one-step process that does not necessarily require specialized equipment. This development could potentially be translated into a direct-write nanopatterning procedure for graphene-based technologies, which promise to make electronics faster, cheaper and more reliable. The tribochemical model proposed provides insight into nanomanufacturing using a tribochemical approach, and suggests that further progress can be accomplished through the reduction of the activation energy required for graphene formation.Item Strong Suppression of Electronic Coherence Time by Flexural Phonons in Graphene --- Example of a New Dephasing Mechanism(2014-07-08) Zhao, WeiWe investigate decoherence of an electron in graphene caused by electron-flexural phonon interaction.We find out that the flexural phonons can produce dephasing rate comparable to the electron-electron one. The problem appears to be quite special because there is a large interval of temperatures where dephasing rate cannot be obtained using the golden rule. We evaluate this rate for a wide range of n and T and determine several asymptotic regions with temperature dependence crossing over from ?_?^(-1)?T^2 to? ??_?^(-1)?T when temperature increases. We also find ?_?^(-1) to be a non-monotonous function of n. These distinctive features of the new contribution can provide an effective way to identify flexural phonons in graphene through the electronic transport by measuring the weak localization corrections in magnetoresistance.