Nanometallic structures efficiently convert light to surface plasmon-polaritons (SPPs) localized to ultra-small volumes. Such structures can provide highly enhanced fields and are of interest in applications involving plasmon-enhanced nonlinear optics. In this study, the devices consist of rectangular gold nanoantennas on a graphene layer on a SiO2/Si substrate. The nanoantennas are used to exploit SPPs to enhance the interaction between graphene and light. Specifically, plasmon-enhanced Raman scattering from graphene is of interest. Here, the nanoantennas are spectrally-aligned with a Stokes wavelength of graphene. With the addition of a second laser source, stimulated Raman scattering can be achieved. The first laser source pumps the sample’s atoms and molecules into virtual excited states and the second one stimulates emission of a photon and relaxation to a higher mode of the ground state. This work involves designing and constructing a stimulated and spontaneous Raman microscope and also a reflectance measurement tool. Within the framework of this thesis, Raman scattering enhancement in graphene based on plasmonic resonant enhancement of the Stokes emission is demonstrated, providing a maximum cross-sectional gain of approximately 500 per antenna. This work also shows the normalized reflectance response of the nanoantenna structures of different length and width and how their resonant wavelengths shift.

Nanomedicine utilises biocompatible nanomaterials for therapeutic as well as imaging purposes, for the treatment of various diseases including cancer, neurological disorders and wound infections. Graphene, a material composed of a single layer of carbon atoms, has recently shown great potential to improve diagnostics and therapeutics, owing to its small size, large surface-area-to-volume ratio and unique physicochemical properties. However, the limited fabrication, in vitro and in vivo functionalities published in the literature indicate inconsistencies regarding the factors affecting metabolic fate, biodistribution as well as toxicity patterns of graphene. This thesis focuses on the biological effects of graphene-based materials, including graphene oxide (GO), reduced graphene oxide (rGO), graphene nanopores (GNPs), graphene quantum dots (GQDs) and three-dimensional graphene foam (GF). These can be used to closely mimic therapeutic functions and thereby open up new pathways to anticancer nanomedicine. In this work, a biocompatible GO-based anti-metastatic enzyme cancer therapy approach has been introduced for the first time to target the extracellular pro-metastatic and pro- tumourigenic enzymes of cathepsin D and cathepsin L, which are typically overexpressed in ovarian and breast cancers. Definitive binding and modulation of cathepsin- D and -L with GO has revealed that both of the enzymes were adsorbed onto the surface of GO through its cationic and hydrophilic residues under the biologically relevant condition of acidic pH. It has been demonstrated that low concentrations of rGO were shown to significantly produce late apoptosis and necrosis rather than early apoptotic events in lung cancer cells (A549 and SKMES-1), suggesting that it was able to disintegrate the cellular membranes in a dose-dependent manner. GNPs at lower concentrations (250μg/ml) induce upregulation of phosphatidylserine on cell surface membrane (i.e. early apoptotic event), which does not significantly disintegrate the cell membrane in the aforementioned lung cancer cells, while higher concentrations of GNPs (5 and 15 mg/kg) in rats (when intraperitoneally injected) exhibited sub-chronic toxicity in a period of 27 days. The interaction of GQDs and trypsin has revealed the strong bonding capacity of GQDs with trypsin, owing to their surface charge and surface functionalities evidencing the high bioavailability of GQDs in enzyme engineering. Finally, 3D GF was developed to probe the role of graphene-based scaffold cues in the field of regenerative medicine revealing their cell attachment to in vitro cell cultures. Furthermore, GF was shown to maintain remarkable biocompatibility with in vitro and in vivo toxicity screening models when exposed for 7 days at doses of 5, 10 and 15 mg/l. Taken together, graphene and its modified structures developed in this thesis promise to revolutionise clinical settings across the board in nanomedicine which include, but are not limited to, ultra-high sensitive enzyme adsorbents, high throughput biosensors, enzyme modulators and smart scaffolds for tissue regeneration.

Tip-enhanced Raman spectroscopy (TERS) is a scanning probe technique that utiHscs a confined, ovanescent field at the tip apex to conduct optical characterisation of a surface at length-scales below the diffraction limit. This thesis details the development of a new TER.S system based upon a shear-force scanning probe microscope (SPM) which sits atop an inverted microscope configured for bottom illumination geometry and coupled to a Raman spectrometer. The system has been optimised for use with solid silver probes and 532 nm illumination. Measurement procedures, automated scripts and data analysis software have been developed that allow reliable alignment of the tip; complex automated mapped measurements; and post processing which produces visual summary sheets to facilitate rapid review of a TERS experiment. Enhanced TERS spectra have been demonstrated on ultra-thin Rhodamine 6G films, self assembled monolayers (SAM) of thiophenole molecules, ultra-thin graphitic films and on multilayered graphene. Improvements in fabrication and alignment procedures have reduced the setup time between fabrication and approach to 20 minutes and improved the reliability of TERS tips with ~ 50% of tips demonstrating TERS activity. Using TERS, heightened defect sensitivity was observed on graphene edges, folds and overlapping regions. The TERS contrast of the defect induced D band was ~ 7.5 times the contrast of the graphene G band. Calculations show that the phonons correlating to the D and G bands interact differently with the enhanced TERS field and that the for certain defect types the D band experiences greater enhancement. Defects play an important role in tailoring the electronic and chemical properties of graphene which is key to the development of graphene based devices. The localised structural and spectral information makes TERS a highly promising tool for the characterisation of defects in graphene. This work demonstrates the potential of TERS for this exciting and important application.

The electrochemical properties of graphene were investigated using a novel and clean method to fabricate device structures with mechanically exfoliated graphene samples. Graphene is known as being particularly sensitive to both contaminating fabrication methods and the substrate it is placed on, with these effects being detrimental to accurate research into the fundamental properties and sensing applications of graphene. This thesis presents micron scale graphene electrodes that have not been subject to polymer contamination or micro-lithography methods. The effect of utilising atomically flat hexagonal boron nitride as a substrate material was investigated, believed to be the first example of this for graphene electrochemical measurements. Cyclic voltammetry demonstrated the expected steady-state behaviour for microelectrodes in the hemispherical diffusion regime. The reduction of IrCl62- in weak KCl electrolytes was studied to investigate the electron transfer characteristics of the graphene devices and the reproducibility of the measurements. Average values of the standard rate constant, k0 and the transfer coefficient, alpha were found to be 3.04 ± 0.78 ×10-3 cms-1 and 0.272 ± 0.024 respectively. These values differ significantly from previous similar studies, with the effect of reduced charge doping from the substrate and the potential dependence of the density of electronic states thought to account for the differences. Despite the clean fabrication methods, a relatively large variation between separate devices was found, highlighting an inherent variation in the properties of graphene samples.

Semiconductor manufacturing is the workhorse for a wide range of industries. It lies at the heart of consumer electronics, telecommunication equipment and medical devices. Most semiconductor electronics are made from Silicon, and are fabricated using CMOS technology. The versatility of semiconductor electronics stems from the ever-reducing cost of integrating more computing and memory functions on chip. The small cost for adding extra functions has been maintained in the past 50 years through transistor scaling. Transistor scaling focuses on shrinking the size of transistors integrated on chip. This reduction in transistor size, while keeping the overall cost of the chip fixed allowed us to reduce the cost per function with scaling, and is what is celebrated as Moore’s law. Scaling has been working gracefully up to the last decade, where the exponential rise in manufacturing cost and diminishing gains of scaling on device performance reduce its economic benefit. To revive the cost reduction trend, different techniques were proposed such as augmenting CMOS manufacturing with new materials (Beyond-CMOS), 3D integration, and integrating more non-transistor elements on-chip (More than Moore). In this work, we focus on the efficient implementation of several circuit functions using an allotropy of carbon known as graphene. Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, has unique electronic properties that has been taken the solid-state electronics community by a storm since its first experimental conception in 2004. Despite its promising electronic properties, namely the very high charge-carrier mobility and reduced scattering by impurities, graphene circuits has been held back by a plethora of nonidealities and technological roadblocks that hamper its use in traditional transistor-based circuits. In this work, we attempt to leverage the unique physical properties of graphene to implement non von-Neumann neuromorphic computing architectures, low-loss diodes and evaluate the behavior of diffusive-transport graphene couplers. We focus on the the design, fabrication and characterization of graphene devices in the presence of the current performance-limiting technological nonidealities in heterogeneous graphene-CMOS systems. We present the design, fabrication and characterization of all-graphene resistive data converters devices and diodes, discussing their performance and application as building elements of all-graphene brain-inspired computing architectures. We evaluate the performance of graphene couplers operating in the diffusive transport regime, which serve as a method to analyze the cross-coupling between adjacent graphene interconnects. We also discuss the current technological limitations hampering the performance of graphene devices, and the roles of different processing non-idealities on the characteristics of graphene devices.

No description available.

The chemical vapour deposition (CVD) of graphene is the most promising route for production of large-area graphene films. However there are still major challenges faced by the field, including control of the graphene coverage, quality, and the number of layers. These challenges can be overcome by developing a fundamental understanding of the graphene growth process. This thesis contributes to the growing body of work on graphene CVD by uniquely exploring the gas phase chemistry and fluid flow in the hot-wall graphene CVD reactor. Firstly the reported parameter space for the hot-wall CVD of graphene on copper was mapped, informing the subsequent work and providing a resource for the wider community. A CVD reactor was constructed to extend this parameter space to lower pressures using methane as a carbon source, and the films were categorised using scanning electron microscopy, Raman spectroscopy and optical dark field microscopy. The latter showed particular promise as a rapid and non-destructive characterization technique for identifying graphene films on the deposition substrate. The gas phase equilibrium compositions were calculated across the parameter space, and correlations between the stabilities of various chemical species and the types of deposition were drawn. This laid a foundation for the remainder of the experimental work, which explored the effect of diluent gases and different feedstocks on the growth to understand the importance of the identified correlations. Diluent gases (argon and nitrogen) were added to the experimental conditions and the thermodynamic model, and were found to reduce the degree of coverage of the graphene films. This result shows that the CVD of graphene is sensitive to factors other than the thermodynamic state parameters, such as the fluid flow profile in the reactor and inelastic collisions between the higher mass diluent gases and the methane/hydrogen/copper system. Using a nitrogen diluent raises the equilibrium carbon vapour pressure and seems to allow larger graphene grains to form. This suggests that thermodynamic factors can contribute to the nucleation of graphene films. Varying the hydrocarbon feedstock and the process conditions indicated that the structure of the deposited carbon is closely related to the nucleation kinetics. Three nucleation regimes are associated with different types of deposition: homogeneous nucleation with amorphous carbon or soot; uncatalysed nucleation with multilayer deposition; and nucleation processes controlled by the copper substrate withpredominantly monolayer deposition. Changing the feedstock from methane to acetylene resulted in poorer graphene coverage, showing that thermodynamic control does not apply in the portion of the parameter space at the high temperatures and lowpressures most successfully used for the deposition of continuous graphene monolayers.

The experimental work undertook in this thesis looks to integrate technologies developed by the graphene and plasmonics communities, respectively, for the purpose of producing devices of enhanced qualities to those of similar utility that have previously been produced. Furthermore, where possible, we look to offer disruptive innovation, by utilising coupled properties that may offer unique possibilities for applications. A hybrid graphene-plasmonic waveguide modulator is fabricated and shown to operate successfully at a standard telecommunications frequency. Different plasmonic-waveguide designs — the basis for the modulator — were produced to probe the coupling between graphene and the surface plasmon-polariton modes. A mode excitable at the edge of the waveguide was found to offer the best modulation, with a modulation depth of over 0.03 dB μm^−1, induced by a moderate gating voltage of about 10 V. Topologically-protected darkness (zero reflection) was produced by particular engineering of a plasmonic metamaterial. This allowed generation of a singularity in the ellipsometric phase (a particular parameter of light), allowing for measurements of mass sensitivity of ∼10 fg mm^−2, with the possibility of improving this to ∼100 ag mm^−2. Graphene was employed in a novel metrology tool to measure the sensitivity of this device. With respect to fundamental losses in plasmonics, one could find either a new plasmonic material or look to improve an existing one. Work was undertook with respect to this latter option by attempting to preserve the otherwise excellent plasmonic properties of copper and silver through a protective barrier of graphene. This was achieved and illustrated through ellipsometric measurements taken over various timescales. Fabrication of a dielectric loaded waveguide on graphene-protected copper was then carried out, with operation of the waveguide proving successful, possibly opening the field of active graphene-protected metal plasmonics.

For years, electron beam induced deposition has been used to fabricate electrical contacts for micro and nanostructures. The role of the contact resistance is key to achieve high performance and efficiency in electrical devices. The present thesis reports on the electrical, structural and chemical characterization of electron beam deposited platinum electrodes that are exposed to different steps of thermal annealing and how they are used in four-probe devices of ultrathin graphite (uG) flakes (<100nm thickness). The device integration of liquid phase exfoliated uG is demonstrated, and its performance compared to devices made with analogous mechanically exfoliated uG. For both devices, similar contact resistances of ~2kΩ were obtained. The electrical measurements confirm a 99.5% reduction in contact resistance after vacuum thermal annealing at 300 °C. Parallel to this, Raman characterization confirms the formation of a nanocrystalline carbon structure over the electrode. While this could suggest an enhancement of the electrical transport in the device, an additional thermal annealing step in air at 300 °C, promoted the oxidation and removal of the carbon shell and confirmed that the contact resistance remained the same. Overall this shows that the carbon shell along the electrode has no significant role in the contact resistance. Finally, the challenges based on topographical analysis of the deposited electrodes are discussed. Reduction of the electrode’s height down to one-third of the initial value, increased surface roughness, formation of voids along the electrodes and the onset of platinum nanoparticles near the area of deposition, represent a challenge for future work.

Graphene, a two-dimensional nanomaterial, has been given much attention since it was first isolated in 2004. Driving this intensive research effort are the unique properties of this one atom thick sheet of carbon, in particular its electrical, thermal and mechanical properties. While the technological applications proposed for graphene abound, its low-cost production in large scales is still a matter of interrogation. Simple methods to obtain few-layered graphene flakes of high structural quality are being investigated with the exfoliation of graphite taking a prominent place in this arena. From the many suggested approaches, the most promising involve the use of liquid media assisted by intercalants and shear forces acting on the basal layers of graphite. In this thesis, it is discussed how a novel method was developed to produce flakes with consistent lateral dimensions that are also few-layered and retain the expected structural and chemical characteristics of graphene. Here, the source material was a commercially available graphiteintercalated compound, also known as expandable graphite. Several exfoliation-inducing tools were investigated including the use of blenders, homogenizers, and ultrasonic processors. To aid in this process, various solvents and intercalants were explored under different reactive conditions. The more efficient approach in yielding defect-free thin flakes was the use of thermally expanded graphite in boiling dimethylformamide followed by ultrasonic processing and centrifugation. In parallel, a method to fraction the flakes as a function of their lateral size was developed. Ultimately, it was possible to obtain samples of graphene flakes with a lateral dimension of a few micrometers (<5 μm) and thickness of 1-3 nm (i.e. <10 layers).