Graphene and monoatomic boron nitride as members of the new class of two dimensional materials are discussed in this thesis. Since the discovery of graphene in 2004, various aspects of this one atom thick material have been studied with previously unexpected results. Out of many outstanding amazing properties of graphene, its elastic properties are remarkable as graphene can bear strain up to 20% of its actual size without breaking. This is the record value amongst all known materials. In this work experiments were conducted to study the mechanical behaviour of graphene under compression and tension. For this purpose graphene monolayers were prepared on top of polymer (PMMA) substrates. They were then successfully subjected to uniaxial deformation (tension- compression) using a micromechanical technique known as cantilever beam analysis. The mechanical response of graphene was monitored by Raman spectroscopy. A nonlinear behaviour of the graphene G and 2D Raman bands was observed under uniaxial deformation of the graphene monolayers. Furthermore the buckling strength of graphene monolayers embedded in the Polymer was determined. The critical buckling strain as the moment of the final failure of the graphene was found to be dependent on the size and the geometry of the graphene monolayer flakes. Classical Euler analysis show that graphene monolayers embedded in the polymer provide higher values of the critical buckling strain as compared to the suspended graphene monolayers. From these studies we find that the lateral support provided by the polymer substrate enhances the buckling strain more than 6 orders of magnitude as compared to the suspended graphene. This property of bearing stress more than any other material can be utilized in different applications including graphene polymer nanocomposites and strain engineering on graphene based devices. The second part of the thesis focuses on a two dimensional insulator, single layer boron nitride. These novel two dimensional crystals have been successfully isolated and thoroughly characterized. Large area boron nitride layers were prepared by mechanical exfoliation from bulk boron nitride onto an oxidized silicon wafer. For their detection, it is described that how varying the thickness of SiO2 and using optical filters improves the low optical contrast of ultrathin boron nitride layers. Raman spectroscopy studies are presented showing how this technique allows to identify the number of boron nitride layers. The Raman frequency shift and intensity of the characteristic Raman peak of boron nitride layers of different thickness was analyzed for this purpose. Monolayer boron nitride shows an upward shift as compared to the other thicknesses up to bulk boron nitride. The Raman intensity decreases as the number of boron nitride layers decreases. Complementary studies have been carried out using atomic force microscopy. With the achieved results it is now possible to successfully employ ultrathin boron nitride crystals for precise fabrication of artificial heterostrutures such as graphene-boron nitride heterostrutures.

The possibility of creating superconductivity in Highly Oriented Pyrolytic Graphite (HOPG) by means of doping was investigated. Bulk HOPG samples were doped with phosphorous using either ion-implantation or by Chemical Vapor Deposition growth with phosphine in the gas mixture. The methods for testing the graphene samples, once doped, were done by performing R vs. T measurements, and determining via observation suppressed superconductive characteristics signaling the presence of the Meissner Effect in a strong applied magnetic field. Before doping, the resistance vs. temperature (R vs. T) characteristic of the HOPG was measured. The R vs. T characteristic was again measured after doping, and for surface multilayers of graphene exfoliated from the post doped bulk sample. A 100 to 350 mT magnetic field was supplied, and the R vs. T characteristic was re-measured on a number of samples. Phosphorous-implanted HOPG samples exhibit deviations from the expected rise in resistance as the temperature is reduced to some point above 100 K. The application of a modest magnetic field reverses this trend. A step in resistance at a temperature of approximately 50-60 K in all of the samples is clearly observed, as well as a second step at 100-120 K, a third at a temperature range of 150-180 K and a fourth from about 200-240 K. A response consistent with the presence of magnetic field flux pancake vortices in phosphorous implanted HOPG and in phosphorous-doped exfoliated multilayer graphene has been observed. The lack of zero resistance at low temperatures is also consistent with pancake vortex behaviour in the flux-flow regime. The presence of magnetic vortices requires, and is direct evidence of superconductivity.

Nanoelectromechanical systems (NEMS) have drawn considerable attention towards several sensing applications such as force, spin, charge and mass. These devices due to their smaller size, operate at very high frequencies (MHz - GHz) and have very high quality factors (102 -105). However, the early onset of nonlinearity limits the linear dynamic range of these devices. In this work we investigate the nonlinearities and their effect on the performance of graphene based NEMS. Electromechanical devices based on 2D materials are extremely sensitive to strain. We studied the effect of strain on the performance of single layer Graphene NEMS and show how the strain in Graphene NEMS can be tuned to increase the range of linear operation. Electromechanical properties of the doubly clamped graphene resonators deviates from the flat rectangular plate as the former possesses geometrical imperfections which are sometimes orders of magnitude larger than the thickness of the resonator. Due to these imperfections we report an initial softening behavior, turning to strong hardening nonlinearity for larger vibration amplitude in the back-bone curve. We have also studied the frequency stability of graphene resonators. Frequency stability analysis indicates departure from the nominal frequency of the resonator with time. We have used Allan Variance as a tool to characterize the frequency stability of the device. Frequency stability of graphene resonator is studied in an open loop configuration as a function of temperature and bias voltage. The thesis concludes with a remark on the future work that can be carried out based on the present studies.

Polymer nanocomposites are prepared by appropriately dispersing nanoscale fillers into polymer matrices. Graphene, a two-dimensional nano-carbon material with outstanding physical properties, has been widely studied as a conductive filler for nanocomposites. In this work, a gum Arabic aqueous solution was proposed as a new media to exfoliate graphite into few-layer graphene by liquid-phase sonication. Successful exfoliation was confirmed by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Four types of graphene nanoplatelets were used to study the effects of the filler's aspect ratio. The one with the largest aspect ratio showed the best performance, where the conductivity of neat epoxy was increased by five orders of magnitude at 10 wt.%. Using a hot sonication technique and adding a small amount of second fillers further improved the electrical conductivities. The highest conductivity obtained in this study was 0.025 S/cm, which met the requirements of electromagnetic shielding material.

No description available.

This thesis reports on two graphene-based structures that have been proposed and fabricated as possible prototypes for high-spatial-resolution Hall sensors with potential application in research on high-density magnetic recording technology such as bit patterned magnetic recording (BPMR) and other areas where the measurement of highly inhomogeneous fields is required. There is a direct graphene-metal contact in the first structure, which is named as TYPE I in this thesis, so that the anomalous Hall effect (AHE) in the ferromagnetic islands deposited on the graphene could be detected. Meanwhile, the graphene and the metal are isolated by an h-BN layer in the second structure which is named as TYPE II, so that only the stray field from the islands can be detected using the ordinary Hall effect (OHE).The transport measurements performed on TYPE I devices revealed there is no AHE or stray field signal detectable, and their Hall resistance relations are non-linear and do not pass through the origin point. A finite element simulation comparing the resistance of the empty graphene cross and the island-occupied cross indicates that the current in the graphene may not redistribute through the metallic islands due to interface current blocking, resulting in the non-appearance of the expected AHE signal. Moreover, an analysis on the data of the longitudinal magnetoresistance (MR) reveals that a two-fluid model and effective medium theory (EMT) model might be the major graphene MR mechanisms in the regime away from and near to the charge neutrality point (CNP) respectively. As a combined result of the above findings, a joint MR-Hall effect model under the condition of the presence of a pre-existing transverse offset current, is proposed to explain the unusual behaviour of the Hall measurement data of the TYPE I devices. The model gives qualitatively correct fitting for all longitudinal and transverse transport data of TYPE I devices. In addition, the nature of the graphene/metal contact is considered as the reason responsible for the non-appearance of the expected AHE and stray field signal, although further experimental work is needed, and suggested in the thesis, to clarify this issue. On the other hand, the TYPE II devices have shown their potential to be developed as a Hall sensor being able to detect a sub-micron magnetic island in the future, but there is still a large space for the performance of the devices to be improved. At the end of the thesis, future experimental work, which could lead to the eventual development of a high-sensitivity high-spatial-resolution Hall sensor on the basis of TYPE I and TYPE II structures, are suggested and described.

Intercalation of graphite with alkali metals has previously been shown to, in some cases, produce superconducting compounds from the two non-superconducting components. The use of graphene as a basis to continue this research offers new possibilities as confinement of intercalant species is reduced from bulk graphite. Papers comprised of exfoliated graphene flakes were doped with Li, Cs, K and Ca atoms via vapour transport methods in order to investigate superconducting properties. While Li, Cs and K-doping showed no signs of a superconducting transition as low as 1.8 K, observed through magnetic measurements, Ca-doped graphene became superconducting below 6.4 K – a lower transition temperature than Ca-doped graphite, TC = 11.5 K. The carrier concentration could also be changed using composite papers made from graphene and various proportions of insulating boron nitride flakes, allowing TC to be varied. Optical reflectivity spectra were used to determine the level of doping present in each compound, directly calculated from their estimated plasmon energy. Ca-doped graphene paper exhibited a 20% lower carrier concentration than Ca-intercalated graphite, offering an explanation for the lower value of TC. To allow insight into the partial doping of graphene papers, samples were exposed to air and monitored via dynamic x-ray diffraction techniques and optical analysis during degradation. With prolonged reaction in air, the carrier concentration was found to drop monotonically, while the interlayer separations contracted as intercalant species vacated the structure, leaving an arrangement of flakes similar to that of the initial, un-doped, graphene paper. The range of carrier concentrations observed suggests that doping of graphene flakes is non-discrete, thus implying tunable TC.

Graphene – a monolayer of carbon atoms densely packed in a honeycomb lattice – was isolated for the first time in 2004 and, since then, has established itself as one of the most remarkable materials available to condensed matter scientists today. Theory predicts a whole spectrum of magnetic phenomena in graphene, including several mechanisms for intrinsic ferromagnetism and spin-ordering effects that arise due to its low-dimensionality and highly unusual electronic properties (e.g. Dirac-like spectrum). In this experimental work, SQUID (Superconducting Quantum Interference Device) magnetic measurements have been carried out in graphene laminates with masses up to ≈ mg obtained by ultrasonic exfoliation of highly oriented pyrolytic graphite (HOPG) in N-methyl-pyrrolidone (NMP). Scanning electron microscopy (SEM) and X-ray diffraction experiments revealed that the laminates are made of decoupled graphene crystallites with typical flake size below 50 nm. Atomic force microscopy (AFM) measurements carried out for graphene suspensions dispersed onto a SiO2 substrate allowed the recognition of thin crystallites associable to single and double-layer graphene. X-ray dispersive fluorescence (XRF) and electron dispersive diffraction (EDX) confirmed the laminates chemical purity with absence of metals and/or magnetic inclusions. Pristine laminates exhibit Curie paramagnetism noticeable below ≈ 50 K, which contributes to about one moment per crystallite at 2 K. The laminates are strongly diamagnetic, although a decrease of the diamagnetic susceptibility by about three times with respect to graphite was observed for fields applied perpendicularly to the ab plane. The same graphene laminates were employed as a reference system to study magnetism of point defects, such as fluorine adatoms and vacancies generated through ion irradiation. The unambiguous spin value J=1/2 found for both species of defects confirms theoretical expectations. In the case of fluorine atoms a magnetic moment of 1 µB per ≈ 1000 adatoms was obtained, associated to the tendency of fluorine to cluster in graphene. Vacancies produced a value of the magnetic moment much closer to the expected 1 µB for point defects. No sign of defect related ferromagnetism was observed. On the other hand, our study performed on NT-MTD HOPG crystals (the same adopted for the fabrication of our graphene laminates), revealed ferromagnetic signals up to 3∙10-3 emu/g. Backscattering electron microscopy (BSE), performed alongside EDX chemical analysis, confirmed that the observed magnetic behaviour is due to ferromagnetic inclusions, such as magnetite and titano-magnetite. Therefore, weak and poorly reproducible ferromagnetic-like signals in graphene laminates were attributed to the same contaminations present in the original material (i.e. HOPG).

Since 2004, graphene attracts intensive attention from scientists and engineers all over the world. During the last decades, the research relates to graphene and other 2 dimensional (2D) materials are rapidly increasing. Approximately, ten thousand journal papers have been published after the discovery of graphene in relative topics widely spread. On the other hand, the simple graphene properties research is nearly completed. Researchers turn their attention to other 2D materials or Van der Waals heterostructures. By increasing the liberty and knowledge of 2D materials, the Van der Waals heterostructures can start to build something on this 2D wander land. In this thesis the Van der Waals heterostructures is based on graphene and some other well known 2D materials such as hexagonal boron nitride (hBN) to study fundamental physics and possible applications in near future. In this thesis, three published papers which are related to Van der Waals heterostructures have been included. The electronic properties of encapsulated graphene on different 2D crystals have been investigated by the capacitance spectroscopy. Several 2D crystals have been tested as a substrate such as MoS2, WS2, mica, LiNbO3…etc. The quality of encapsulated device is correlates the interface self-cleaning. Follow with the fundamental physics study employed by a simple Van der Waals heterostructure. Graphene and hBN is lattice aligned within 2 degrees in difference and creates a new superlattice structure which just like moire pattern happens while two similar patterns overlapped. The basic electronic properties do not vary at near Dirac point. Away from the first generation Dirac point, the superlattice structure affects the band structure in higher carrier concentration. In this paper, aligned graphene-hBN capacitors have been demonstrated to discover more fine details of these many-body interactions in this superlattice structure. The final part is related to twist controlled graphene-graphene resonance tunneling transistors. A Van der Waals heterostructure is constructed by two aligned graphene stripes with a thin layer of hBN as a spacer. The electrons are tunneled from one stripe to another graphene stripe while a bias voltage applied. The resonance tunneling is occurred when two graphene flakes are aligned at certain bias voltage. In this paper, we contribute the resonance tunneling to momentum conservation of tunnelling electrons. Theory simulation is highly agreed with our experiment results.

Graphene has attracted significant research attention for next generation of semiconductor devices due to its high electron mobility and compatibility with planar semiconductor processing. In this dissertation, the influences of Ohmic metals and high dielectric (high-k) constant aluminum oxide (Al2O3) deposition on the structural and electrical properties of multi-layer epitaxial graphene (MLG) grown by graphitization of silicon carbide (SiC) substrates have been investigated. Uniform MLG was successfully grown by sublimation of silicon from epitaxy-ready, Si and C terminated, 6H-SiC wafers in high-vacuum and argon atmosphere. The graphene formation was accompanied by a significant enhancement of Ohmic behavior, and, was found to be sensitive to the temperature ramp-up rate and annealing time. High-resolution transmission electron microscopy (HRTEM) showed that the interface between the metal and SiC remained sharp and free of macroscopic defects even after 30 min, 1430 °C anneals. The impact of high dielectric constant Al2O3 and its deposition by radio frequency (RF) magnetron sputtering on the structural and electrical properties of MLG is discussed. HRTEM analysis confirms that the Al2O3/MLG interface is relatively sharp and that thickness approximation of the MLG using angle resolved X-ray photoelectron spectroscopy (ARXPS) as well as variable-angle spectroscopic ellipsometry (VASE) is accurate. The totality of results indicate that ARXPS can be used as a nondestructive tool to measure the thickness of MLG, and that RF sputtered Al2O3 can be used as a (high-k) constant gate oxide in multilayer grapheme based transistor applications.