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In this thesis, we model theoretically spectra measured for bilayer graphene obtained using the angle-resolved photoemission spectroscopy, magneto-optical absorption spectroscopy and electronic Raman spectroscopy The theories are based on the tight-binding description of the pi bands in the material. In particular, we concentrate on the comparison of the four-band model and its effective low-energy approximation neglecting the split high-energy bands, in the description of specific spectra. We demonstrate that both for monolayer and bilayer graphene, the observed anisotropy of angle-resolved photoelectron spectroscopy spectra reflects the electronic chirality in the system. However, for bilayer graphene, the influence of the nonchiral dimer states not captured within the effective approximation is significant and should not be neglected. We also show that the anisotropy of the constant-energy maps may be used to extract information about the magnitude and sign of interlayer coupling parameters and about symmetry breaking inflicted on a bilayer by the underlying substrate. We then determine selection rules and optical strengths of the inter-Landau-level excitations among any of the pi bands and including the physically most relevant symmetry-breaking parameters. We then present a self-consistent calculation of the interlayer asymmetry caused by an applied electric field in magnetic fields. We show how this asymmetry influences the Landau level spectrum in bilayer graphene and the observable inter-Landau level transitions when they are studied as a function of high magnetic field at fixed filling factor as measured experimentally. We also analyse the magneto-optical spectra of bilayer flakes in the photon-energy range corresponding to transitions between degenerate and split bands of bilayers. Finally, we investigate the contribution of the low-energy electronic excitations toward the Raman spectrum of bilayer graphene for the incoming photon energy Ω " 1eV. Using the four-band model, we de rive an effective scattering amplitude that can be incorporated into the two-band approximation and show that this amplitude is different from the contact interaction amplitude obtained within the two-band model alone. We then calculate the spectral density of the inelastic light scattering accompanied by the excitation of electron-hole pairs in bilayer graphene. In the absence of a magnetic field this contribution is constant and in doped structures has a threshold at twice the Fermi energy. In an external magnetic field, the dominant Raman-active modes are the n- --- > n+ inter-Landau-level transitions with crossed polarization of in/out photons. We estimate the quantum efficiency of a single n- --- > n+ transition in the magnetic field of 10 T as In- - > n+ ~ 10. 12.

This work focuses on investigating the structural and electronic properties of ReS2, a layered semiconducting member of the transition metal dichalcogenide (TMD) family, which is distinguished by its high in-plane anisotropy. The structural and electronic properties of layered materials ReS2 have been probed at the nanometre scale by employing low temperature ultra high vacuum (LT-UHV) scanning probe techniques including: Non contact atomic force microscopy (NC-AFM), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). By performing both STM and NC-AFM, we were able to distinguish the non-equivalent chalcogen atoms as well as identify the b-axis (or Re-chain direction); and it was possible to assess the structural and/or electronic contribution of each of the S and Re sublattices to the features in images recorded using both techniques as they result from different phenomena. Analysis shows that, as opposed to STM, the NC-AFM images show a clear transition with the imaging parameters that allows the Re-sublattice contribution to be measured or "picked up" more readily. We believe that this is the first NC-AFM study performed on ReS2. The ability of the NC-AFM technique to reveal information from subsurface atomic layers can be exploited further, in the imaging of subsurface defects. The STM/STS measurements reveal the effect of defects in ReS2 on the electronic structure of the material. Localized defects, associated with S monovacancies, as well as local density of states (LDOS) disturbances extending over several nm in lateral size and associated with subsurface, intercalated dopants, have been identified, as well as their signature density of states spectra, leading to localized, in-gap states. These in-gap states significantly reduce the energy gap in the vicinity of these defects, leading to electronic perturbations across the ReS2 surface. Finally, a separate study was done to probe the anisotropic transport in few-layer ReS2 by localised current injection and conductance measurements using mobile scanning probe microscopy (SPM) probes. The angle-dependent conductance measurements on a 4-layer mechanically exfoliated ReS2 flake show strong angle-dependence over a narrow angle range around the b-axis of the material: this is consistent with the existence of localised structural anisotropy. To reach this conclusion, finite element analysis of transport through ReS2 films of different geometries was performed; and a detailed understanding of the current-voltage characteristics acquired via two-local probe measurements and the nature of the contact junctions achieved with this method was necessary. This study proposes methods to improve the large in-built transport anisotropy in multi-layer ReS2 films by introducing structural inhomogeneity into the system.

We study the semiconducting rhenium dichalcogenides ReX₂, where X is sulfur (S) or selenium (Se), a class of transition metal dichalcogenides (TMDs) with a distorted lattice structure. These materials have a single symmetry operation: inversion symmetry, which leads to a lack of spin-orbit splitting. We use density functional theory (DFT) to study the electronic dispersion of bulk ReSe₂ and ReS₂, which have complicated band structures with many close-lying band gaps. We show the importance of studying the full Brillouin zone rather than only the high symmetry points when trying to determine the nature of the band gap in these materials. Thereafter, we calculate the band structure of monolayer ReSe₂ and identify which orbitals contribute to the low energy behaviour at the valence band edge. We compare our DFT calculations to experimental angle-resolved photoemission spectroscopy (ARPES) measurements. Furthermore, we compare DFT data to ARPES data which uses different polarisations, exploiting the symmetry of the underlying atomic orbitals. We find that the out-of-plane orbitals do not contribute significantly to the top of the valence band, unlike in many other TMDs. These materials are thus shown to display quasi-one-dimensional behaviour, with anisotropy along the Re chains. We then calculate band structures of Janus ReSSe, where the chalcogen layers on either side of Re are composed of S on one side and Se on the other. The distorted structures of rhenium TMDs motivates the study of Janus-like rhenium dichalcogenides with varying numbers of substituted chalcogens in pure ReSe₂ or ReS₂. We calculate formation energies and predict the substitutions most likely to take place, before showing the effect of these substitutions on the valence and conduction bands. The broken inversion symmetry in these Janus and Janus-like materials leads to significant spinorbit splitting, which could have uses in spintronics applications. Hence, we have studied the band structures of rhenium dichalcogenides and shown that these materials have complicated band structures and strong anisotropy which lead to interesting effects not seen in other TMDs, making them promising candidates for applications in optoelectronic devices.

In this thesis, we present the tight binding method to describe electronic properties of graphene. We begin with a theoretical description of electronic Raman scattering (ERS) in graphene, under the framework of time-dependent perturbation theory. Our original work is presented in chapters 4, 5 and 6. In each chapter, we model the Raman spectra of graphene-based systems of special interest in carbon research: superconducting graphene, graphite and twisted bilayer graphene. Our findings underpin that purely electronic excitations in Raman spectra generate distinctive features that allows us to characterise and study these materials. In particular, Raman spectra give us insights into the position of van Hove singularities, the size of gaps or the flatness of bands. At the end of each chapter, we provide additional information about the current experimental status.

The Food and Drug Administration requires that all sterile medical devices be tested and documented for a labeled shelflife. This thesis analyzed the effects of 12 months of accelerated aging testing, obtained at two equivalent aging programs, on a medical-grade chevron pouch constructed from a proprietary BOPET/PE film sealed to 1073B uncoated Tyvek®. The equivalent of 12 of months accelerated aging was obtained at 55°C and 70°C, and samples were also evaluated pre-aging to serve as a baseline for comparison. The aging model was based upon ASTM F-1980, "Standard Guide For Accelerated Aging of Sterile Barrier Systems for Medical Devices" and the Arrhenius equation. The peel strength of the manufacturer's seal was evaluated per ASTM F-88, "Standard Test Method for Seal Strength of Flexible Barrier Materials." This thesis demonstrated that there does exist an effect on seal strength due to accelerated aging of this pouch, and that effect is equivalent between 55°C and 70°C.

In this thesis, we model the electronic and optical properties of stacked structures of two-dimensional materials involving graphene. In particular, we discuss how the interplay between the type and alignment of these stacked two-dimensional crystals affects the observed physical properties. Tuning the misalignment between two graphene layers leads to stark modification to the resulting electronic band structure. By employing electronic Raman scattering, we show that it is theoretically possible to accurately determine the twist angle in twisted bilayer graphene samples close to the magic-angle. We propose a new parameterisation for the interlayer coupling between twisted graphene layers, and demonstrate that twisted trilayer graphene is the simplest system in which the coupling in aligned and misaligned graphene bilayer can be probed. By comparison to experimental photoemission data, we demonstrate the validity and self-consistency of our model. We discuss the tunnelling current across two van der Waals tunnelling transistors. In the first, one or both of the electrodes are made of two crystals forminga moiré superlattice at their interface. We investigate structures containing either aligned graphene/hexagonal boron nitride heterostructure, twisted bilayer graphene or aligned graphene on α-In2Te2 and show negative differential resistance is possible in such transistors. In the second case we propose a tunnelling junction in an external magnetic field perpendicular to the layers, where the source and drain electrode are comprised of bilayer and monolayer graphene respectively. We show that, due to the effective difference in tunnelling barrier width for electrons on the two layers of bilayer graphene and the valley-dependent wave function distribution between these layers, the valley polarisation of the current can be electrically tuned. We demonstrate that strong valley polarisation can be obtained in the clean limit, where strong-momentum conserving tunnelling dominates, and in lower quality devices where this constraint is lifted.

Following the successful isolation of a continuously growing number of layeredmaterials in monolayer forms, these can now be assembled into stacks, referred toas van der Waals heterostructures. In this thesis, we investigate theoretically theelectronic properties of heterostructures based on bilayer graphene, two coupledlayers of carbon. We first study the minibands of bilayer graphene placed on asemiconducting substrate with a unit cell about, but not exactly, three times largerthan that of graphene. While the former introduces asymmetry in the distributionof the electronic wave function between the layers and opens a band gap in theelectronic spectrum, the latter generates a long wavelength moire perturbationthat couples states in inequivalent graphene Brillouin zone corners. We show that,depending on the details of the moire perturbation, the miniband structure canbe tuned to a situation where a single narrow miniband is separated from therest of the spectrum by small gaps. We then discuss electron tunneling betweenbilayer and monolayer graphene across a hexagonal boron nitride barrier in thepresence of a strong perpendicular magnetic field. We demonstrate that in sucha device, valley-polarization close to unity can be produced in fields of order 1 T.Finally, we discuss electron transport in a van der Waals tunnelling transistor inwhich the electronic density of states in one of the electrodes has been modulatedby a superlattice perturbation. Using the example of twisted bilayer grapheneand a similar system of monolayer graphene on hexagonal boron nitride, we showthat negative differential resistance is possible in such transistors as a consequenceof the moire-induced changes in the density of states and without the need ofmomentum conserving tunnelling.