Graphene is a single layer two-dimensional honeycomb lattice of carbon atoms. It is one of the toughest, lightest, and most conductive materials known. Graphene was first isolated using adhesive tape in 2004, and awarded the Physics Nobel Prize in 2010. Here we focus on the mechanical properties of graphene. First we present an analytical study, together with numerical simulations, of ripples in graphene. We show that ripples observed in free-standing graphene sheets can be a consequence of adsorbed OH molecules sitting on random sites. The adsorbates cause the bonds between carbon atoms to lengthen slightly. In the second part of this work we study the fracture mechanics of graphene. Experiments on free-standing graphene can expose the graphene sheets to out-of-plane forces. Here we show that out-of-plane forces can cause free-standing graphene to fracture. This fracture mode is known as the tearing mode and is common in materials such as paper. We present a numerical study of the propagation of cracks in clamped, free-standing graphene as a function of the out-of-plane force. We also obtain an analytical expression for the minimum force required to tear a two dimensional sheet, which is our model of graphene, in terms of the initial crack length. / text

Graphene bilayers in Bernal stacking exhibit a transverse electric (E) field dependent band gap, thanks to the on-site electron energy asymmetry between the two layers, which can be used to increase the channel resistivity, and enable higher on/off ratio devices. Using dual-gated device structure, we investigate the transport characteristics of exfoliated graphene bilayers as a function of carrier density and E-field at temperature from 295 K down to 0.3 K. At high E-field, strong conduction suppression near the charge neutrality point is observed, a primary characteristic introduced by band gap opening. The conductivity suppression persists up to the finite threshold voltages, which increase with increasing the E-field, similar to a gapped semiconductor. We extract the transport gap as a function of E-field from the threshold measurement, and further discuss the impact of disorder. At gate bias higher than the threshold, conductivity increases linearly as carrier density increases, which contrasts to the sub-linear dependence in graphene monolayer. Mobility shows decreasing tendency with the increasing E-field, which changes little as temperature changes. Besides, we probe the electrical characteristics of quasi-free-standing graphene bilayers grown on SiC at temperature down to 0.3 K, based on the study on the exfoliated graphene bilayers. The epitaxial graphene bilayer on SiC is prepared by atmospheric pressure graphitization in Ar, followed by H₂ intercalation, which renders the material quasi-free-standing. At the charge neutrality point, the longitudinal resistance shows an insulating behavior, and follows a temperature dependence consistent with variable range hopping transport in a gapped state. Besides, clear linear dependence of the conductivity on the carrier density is observed, which is distinguishable from the sub-linear dependence in graphene monolayer. These properties show that the epitaxial graphene bilayer grown on the SiC exhibits band-gap opening and Bernal stacked arrangement. / text

Electrochemical double layer capacitors (EDLCs, also called supercapacitors or ultracapacitors) are high power density energy storage devices that operate through the separation of charge at the electrochemical interface between an electrode and a supporting electrolyte. Numerous types of carbon materials with high surface area and internal porosity, such as activated carbon, carbon fabrics, nanotubes, and reduced graphene oxide have been studied as electrode materials. Electrolytes such as aqueous alkaline and acid solutions usually give high capacitance, while organic and ionic liquids provide a wider operation voltage. Graphene, due to its high theoretical surface area of 2630 m2/g, good electrical conductivity, and relatively low density, is being studied as an electrode material in EDLCs. The objective of this dissertation is thus to study effective methods for synthesis of graphene-based materials, and to investigate their behavior in EDLCs. This work explored microwave assisted synthesis of graphite oxide (‘MEGO’, prepared in less than one minute by irradiation of graphite oxide by microwave). This material was further chemically activated to obtain a unique carbon material, activated microwave exfoliated graphite oxide (‘a-MEGO’) with specific surface areas up to 3100 m2/g. Gas adsorption measurements were used to study the specific surface area and porosity of a set of a-MEGO samples, which were also studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for their structure, and by combustion analysis (i.e., elemental analysis) and X-ray photoelectron spectroscopy (XPS) to understand their elemental composition. Cyclic voltammetry (CV), galvanostatic charge/discharge, and frequency response, tests were done in order to study the performance of these new carbon materials as electrodes in both aqueous and organic electrolytes in a two electrode cell set up. / text

Increasing energy demand makes the development of higher energy storage batteries, imperative. However, one of the major advantages of fossil fuels as an energy source is they can provide variably large quantities of power when desired. This is where electrochemical capacitors can continue to carve out a niche market supplying moderate energy storage, but with high specific power output. However, current issues with carbon precursors necessitate further development. Further, production requires high temperature, energy intensive carbonization to create the active pore sites and develop the pores. Double-layer capacitive materials researched to replace active carbons generally require properties that include: very high surface area, high pore accessibility and wettability, strong electrical conductivity, structural stability, and optionally reversible functional groups that lend to energy storage through pseudocapacitive mechanisms. In recent years, nanostructured carbon materials which could in future be tailored through bottom up processing have the potential to exhibit favourable properties have also contributed to the growth in this field. This thesis presents research on graphene, an emerging 2-dimensional carbon material. So far, production of graphene in bulk exhibits issues including restacking, structural damage and poor exfoliation. However, the high chemical stability, moderate conductivity and high electroactive behaviour even with moderate exposed surface area makes them an excellent standalone material or a potential support material. Two projects presented focus on enhancing the capacitance through functionality and controlling graphene formation to enhance performance. The first study addresses graphene enhancement possible with heteroatom functionality, produced by a single step low temperature hydrothermal reduction process. The dopant methodology was successful in adding nitrogen functionality to the reduced graphene oxide basal and the effect of nitrogen type was considered. The second study addresses the need for greater control of the rGO structure on the macro-scale. By harnessing the change in interactions between the GO intermediate and final rGO sheets we were able to successfully control the assembly of graphene, creating micro and macro-pore order and high capacitive performance. Further, self assembly directly onto the current collector eliminates process steps involved in the production of EDLC electrodes.

Graphene oxide (GO) is a material of both fundamental and applied interest. Elucidating this complex material is crucial to both control its physical chemical properties and enable its applications in technology. Graphene oxide films synthesized from epitaxial graphene on silicon carbide constitute a particular -- simplified -- form of GO, suitable for fundamental physical chemistry studies of oxidized sp2 carbon materials. In this thesis work, I used density functional theory calculations and I developed a lattice-model Monte Carlo scheme to elucidate puzzling experimental observations and to gain molecular insight into the chemical composition, thermochemical and structural properties of this type of ultrathin GO films on silicon carbide substrates.

<p>We report on graphene-based Superconductor-Normal metal-Superconductor Joseph- son junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to &#1113094; 2 K. We are able to detect a small, but non-zero, resistance despite the Josephson junctions being in the superconducting state. We attribute this resistance to the phase diffusion regime, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further charac- terize the nature of electromagnetic environment and dissipation in our samples. In addition we modulate the critical current through grapehene by an external magnetic field; the resulting Fraunhofer interference pattern shows several periods of oscilla- tions. However, deviations from the perfect Fraunhofer pattern are observed, and their cause is explained by a simulation that takes into account the sample design.</p> / Dissertation

This thesis concerns development and characterization of the FEBID technique to improve interfacial properties at MWCNT/graphene-metal junctions by forming graphitic nanojoints using hydrocarbon precursors. A fabrication protocol for ultralow-resistant, Ohmic contacts at MWCNT-metal junctions with FEBID graphitic nanojoints was developed, based on an in-depth topological/ compositional/electrical material characterization, yielding high performance “end” contacts to multiple conducting channels of MWCNT interconnect. Using the FEBID technique as a contact fabrication tool, three fabrication strategies of electrical contacts between the mechanically exfoliated multilayer graphene and a metal interconnect using graphitic nanojoints were proposed and demonstrated experimentally, suggesting one of them, the post-deposited FEBID graphitic interlayer formation, as the most efficient strategy. A patterned CVD grown monolayer graphene, which is a promising material for large area graphene device fabrication, was contacted to metal electrodes through the FEBID graphitic interlayer, whose formation and chemical coupling to graphene and metal were theoretically and experimentally explored. The effects of FEBID process on the graphitic interlayer formation and graphene electronic devices were demonstrated through electrical measurements, including the transmission line method (TLM) measurements for separate evaluation of sheet and contact resistances. Modifications of the graphene channel as well as interfacial properties of the graphene-metal junctions were achieved, highlighting a unique promise of the FEBID technique as a tool for enhancing chemical, thermo-mechanical, and electrical properties of graphene-metal interfaces along with controllable tuning of doping states of the graphene channel.

In 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.

Motivated by the few work has done on the performance measurement of graphene passive devices compared to graphene active devices, several different types of passive devices are fabricated and measured. In general, the fabricated devices are divided into two parts: the DC devices and the RF devices, which based on the different electrical properties we measure in Chapter 7. For the DC devices, attention has been given to the resistance of CVD graphene that we later use in all the RF devices. The Dirac point seems only appears in the exfoliated graphene measurement, which is caused by the doping concentration difference between the exfoliated and CVD graphene. Meanwhile, the sheet resistance of graphene is calculated based on the four-point measurement. The sheet resistance of CVD graphene is around 291 Ω/sqFor the RF devices, the measurement is conducted on the two types of graphene passive devices from 0-110 GHz. The first type of graphene devices is the graphene CPW resonator. We measure the input impedances of the graphene resonators on different substrates (Si/SiO2 and GaAs) and with different graphene lengths (440 micro metre, 500 micro metre and 1415 micro metre). For the graphene resonators on Si/SiO2 substrate, the input impedance does show the resonance shift compared to the graphene-removed structure. The frequency position of the resonance that appears is consistent with the theoretical calculation result. Besides, the influence of the external conditions such as temperature on the performance of graphene resonators has been investigated. The input impedance resonance shows the shift when the external temperature varying from 40o C（313K） to 160o C （433K）. This measurement is undertaken with the graphene resonator on GaAs substrate. The second type of graphene devices is the graphene CPW transmission line on Si/SiO2 substrate. The S-parameters measured from VNA reveal that graphene within the transmission line acts as the transmission channel, which is a little lossy at the microwave frequency range. The poor transmission is also partially caused by the mismatching of the parasitic impedance, as well as the substrate loss, which is verified by the comparison result between the graphene transmission line and the graphene-removed transmission line. Similarly, the concern on the signal line coupling is also eliminated by using the graphene-removed structure.

Terahertz (THz) radiation has become increasingly important in many scientific and commercial fields in recent years. It possesses many remarkable features resulting in an increased use of THz radiation for various applications, like biomedical imaging, security screening, and industrial quality control. The capability of these applications depends directly on the availability of powerful THz sources and high-responsivity, fast THz detectors. Current commercial products used to detect THz radiation, like Golay cells and pyroelectric detectors, have only slow detection rates and poor sensitivities. Other commercial THz detectors, like bolometers, are more sensitive but require liquid helium cooling. In this thesis, two types of room-temperature high-responsivity graphene-based THz detectors are presented, relying on the unique properties of graphene and the function of plasmonic antenna arrays which boost the interaction between THz waves and graphene. Graphene has been demonstrated as a promising material for THz detection. However, the challenge is its insufficient light absorption that largely limits the responsivity. The first design is based on an array of planar antennas arranged in series and shorted by graphene squares. Highly efficient photodetection can be achieved by using the metallic antenna to simultaneously improve both light absorption, as resonant elements, and photocarrier collection, as electrodes. The device has been characterized with quantum cascade lasers, yielding a maximum responsivity of ~2 mA/W at 2 THz. The second detector is based on an array of interdigitated bow-tie antennas connected in parallel and shunted by graphene squares. The arms of the bow-tie antennas were made of two metals with different work functions to create a built-in electric field and improve the responsivity. The device has been characterized and yields a maximum responsivity of ∼34 μA/W at 2 THz. Efficient THz imaging is presented by integrating the detector in a QCL-based THz imaging system.