This 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. / text

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The research outlined in this dissertation focuses on understanding the role of metal-sulfur interactions as applied to bioinorganic and organometallic systems. This metal-sulfur interaction is analyzed using both gas-phase photoelectron spectroscopy (PES) and density functional theory (DFT). Gas-phase photoelectron spectroscopy is the most direct probe of electronic structure and is used in these studies to probe the molecular orbital energy levels of these model compounds, giving rise to an understanding of the metal and sulfur orbital interactions and characters (i.e. is an orbital primarily metal or sulfur based). Using density functional theory, orbital energies, overlap, and characters can be calculated and complement the PES experiments allowing for a detailed understanding of the electronic structure. The first part of my dissertation explains the design and implementation of a dual source gas-phase ultraviolet/X-ray photoelectron spectrometer (UPS/XPS). This gas-phase UPS/XPS can be used to quantify the bonding/antibonding character of frontier molecular orbitals, with specific applications to metal-sulfur interactions, allowing for a thorough analysis of the metal-sulfur interaction. The second part of the dissertation explores using model complexes, of the type Cp₂V(dithiolate) (where Cp is cyclopentadienyl and dithiolate is 1,2-ethenedithiolate or 1,2-benzenedithiolate), along with PES and DFT calculations to investigate the role of the pyranopterindithiolate cofactor and the d¹ electron configuration in modulating the redox potential and electron transfer in the active sites of molybdenum enzymes. This study shows that the d¹ electronic configuration offers a low energy electron transfer pathway for the reoxidation of the active site molybdenum center. The third part of the dissertation explores the use of model compounds that specifically focus on iron-thiolate interactions in biological systems, and the effect of electronic energy matching and sterics on the oxidation potential of this interaction. This study has shown that the metal-sulfur interaction is sensitive to the orientation of the thiolate ligand, and that during oxidation an “electronic-buffering effect” makes assigning a formal oxidation state to the metal center almost meaningless. All of these studies illustrate how the thiolate ligand can modulate the electron density and oxidation potential of the metal-sulfur interaction and the implication of this interaction to biological electron transfer.

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In this thesis first-principles calculations of point and extended defects in diamond and silicon are reported. In single crystal diamond grown by chemical vapour deposition (CVD) dislocations are observed as mixed-type 45° and edge-type dislocations lying along <100> with 1/2<110> Burgers vectors. Results are presented on the core structures, core energies and electrical properties of both types of dislocations and their interaction with nitrogen is investigated. Then the focus turns to the brown diamond problem. Despite concerted research efforts, the origin of the brown colouration of diamond is still under discussion. Recently, the attention was drawn to vacancy-related defects. Experiments on type IIa diamonds indicate that the brown colour is caused by vacancy-type extended defects, however the shape and size of these defects remained unclear. In this work, the structural, electrical and optical properties of large spherical vacancy clusters and thin vacancy disks are investigated by means of density functional theory and the calculations are compared with recent experimental measurements on brown diamond. High pressure high temperature treatment (HPHT) of brown type Ia diamonds above 2000°C results in the loss of the brown colour and the formation of nitrogen-vacancy defects. The generation of such defects requires a source of mobile vacancies during the annealing process. It is suggested that the vacancy cluster model described in this thesis can explain the observed annealing behaviour since the break-up of the clusters leads to a supersaturation of mobile vacancies which readily complex with substitutional nitrogen atoms present in the material. Therefore, the effect of HPHT treatment of brown type Ia diamond is investigated by studying the formation energies of common and rare defects and estimates of their equilibrium concentrations at different annealing stages are given. Finally, an open problem also involving nitrogen, but in a different group IV semiconductor is considered. In Czochralski-silicon, nitrogen-related shallow thermal donors are formed between 500 and 750°C. Until now the exact chemical composition and atomic structure of these defects are not well established. Here, it is shown that NO and NO_2 belong to the family of nitrogen-oxygen related shallow thermal donors. Based on the law of mass action the equilibrium defect concentrations are predicted. Finally, the theoretical results are compared to recent Fourier transform infrared (FTIR) spectroscopy measurements.

The properties of a new family of carbon structures based on stacked cyclic C6 rings and intercalated cyclic C6 structures: (C6)n and (C6)nMen-1 have been studied theoretically using ab initio DFT (Density Functional Theory). Calculations of the structural, electronic, and vibrational properties of a range of these molecules have been carried out using DFT techniques with the best correspondence to experimental results. The chemical and structural stability of structures based on stacks of cyclic C6 has also been estimated for pure carbon molecules (C6)n and for metal-organic sandwich molecules intercalated with Fe and Ru atoms. These have (C6)nFen-1 and (C6)n Run-1 compositions, respectively These structures are predicted to show a variety of new electronic, vibrational and magnetic properties. Ultra-small diameter tubular molecules are also found to have unique rotational electron states and high atomic orbital pi-sigma hybridization giving rise to a high density of electron states. All phonons in these structures have collinear wave vectors leading to an ultrahigh density of phonon states in dominant modes suggesting that some of these structures may exhibit superconductivity. These properties, as well as a predicted high electron mobility, make these structures promising as components in nanoelectronics. Experiments using femto-second laser pulses for the irradiation of organic liquids suggest that such structures may appear under certain conditions. In particular, a new type of iron carbide has been found in these experiments.

Aluminum complexes, [O3PMe]AlR(R = OtBu, OPh), containing tris-(3,5-di-tert-butyl-2-hydroxy-phenyl)phosphine ([O3P]H3) which is a novel tridentate ligand have been synthesized and characterized by NMR, X-ray diffraction and elemental analysis. Theses complexes were used as catalysts for ring-opening polymerization of £`-caprolactone. We suggested that the stereo effect of catalysts is the main factor in the ring-opening polymerization and compared the mechanism with DFT. In additional, we studied the electronic states and electronic chemistry of [O3PMe]AlR by DFT, UV and PLE. The novel ligand, bis(3,5-tert-butyl-2-phenoxy) tert-butylphosphine ([tBuOPO]H2), reacted with alkali metals such as n-BuLi, NaH and KH to form a series metal complexes, [tBuOPO]M2(Solvent)x (M = Li, Na, K). These metal complexes are all dimeric molecules characterized by X-ray diffraction, NMR and elemental analysis. Moreover, we reacted {[tBuOPO]Li2(DME)}2 with metal complexes of group 4, TiCl3 and MCl4(THF)2 (M = Ti, Zr, Hf), and we received [tBuOPO]2M and [tBuOPO]MCl2(THF) (M = Ti, Zr, Hf). We also synthesized alkoxide complexes of the series metal complexes and studied the catalytic reactivity for ring-opening polymerizations. Furthermore, tantalum complexes, [tBuOPO]2TaX (X = F, Cl) and [tBuOPO]TaCl3, have been synthesized and characterized. Especially synthesizing [tBuOPO]TaCl3 should be carefully controlled by lowering the concentration of TaCl5.

The atomic and electronic structures of Au-intercalated graphene buffer layer on SiC(0001) surface were investigated using first-principles calculations. The unique Dirac cone of the graphene near K point reappeared as the buffer layer was intercalated by Au atoms. Coherence interfaces were used to study the mismatch and strain at the interfaces. Our calculations showed that the strain at graphene/Au and Au/SiC(0001) interfaces also played a key role in the electronic structures. Futhermore, we found that at Au coverage of 3/8 ML, Au intercalation leads to strong n-type doping of graphene. At 9/8 ML, it exhibited weak p-type doping, meaning that graphene is not fully decoupled from substrate. The shift of Dirac point resulting from electronic doping is not only due to different electronegativities but also strains at the interfaces. Our calculated positions of Dirac points are consistent with those observed in the ARPES experiment [Isabella Gierz et al., Phys. Rev. B 81, 235408 (2010).].

Density functional theory (DFT) is used to study the reactivity of Pt and Pt-M (M: Pd, Co, Ni, V, and Rh) alloy catalysts towards the oxygen reduction reaction (ORR) as a function of the alloy overall composition and surface atomic distribution and compared to that on pure Pt surfaces. Reactivity is evaluated on the basis of the adsorption strength of oxygenated compounds which are intermediate species of the four-electron oxygen reduction reaction, separating the effect of the first electron-proton transfer from that of the three last electron-proton transfer steps. It is found that most homogeneous distribution PtxM catalysts thermodynamically favor the dissociation of adsorbed OOH in comparison with pure Platinum and adsorb strongly O and OH due to the strong oxyphilicity of the M elements. On the other hand, in all cases skin Platinum surfaces catalysts do not favor the dissociation of adsorbed OOH and do favor the reduction of M-O and M-OH with respect to Platinum. Considering the overall pathway of the reactions to catalyze the ORR most of the skin Platinum monolayer catalysts provide more negative free energy changes and should behave at least in a similar way than Platinum in following order: Pt3V (skin Pt) > Pt3Co (skin Pt) > Pt3Ni (skin Pt) > Pt > PtPd (skin) > Pt4Rh (skin Pt) > PtPd3 (skin ). In all cases, the reactivity is shown to be not only sensitive to the overall composition of the catalyst, but most importantly to the surface atomic distribution. Proposed electrochemical dissolution reactions of the catalyst atoms are also analyzed for the ORR catalysts, by computing the free energy changes of Platinum and bimetallic Pt-X (X: Co, Pd, Ni, and Rh) catalysts. It is found that Platinum is thermodynamically more stable than Pt-alloys in Pt3Co, Pt3Pd, Pt3Ni and Pt4Rh.