Diamond is an important material in many industrial applications (e.g., machining of hard materials, bio-electronics, optics, electronics, etc.) because of its exceptional properties such as hardness, tolerance to aggressive environments, compatibility with human tissues, and high carrier mobility. However, a highly controlled method for growing artificial high-purity diamond on a range of different substrates is needed to exploit these exceptional properties. The Chemical Vapour Deposition (CVD) method is a useful tool for this purpose, but the process still needs to be developed further to achieve better control of growth. In this context, the introduction of dopant species into the gas phase has been shown to strongly influence growth rate and surface morphology. Density Functional Theory (DFT) methods are used to deepen our atomic-level understanding of the effect of dopants on the mechanism for CVD growth on diamond. More specifically, the effect of four dopants (N, P, B and S) has been studied on the important reaction steps in the growth mechanism of diamond. Substitution of N into the diamond lattice has generally been found to disfavour critical reaction steps in the growth of the 100-face in diamond. This negative effect has been related to electron transfer from the N dopant into an empty surface state, e.g., a surface carbon radical. In addition, strong surface stabilization is observed for N substitution in certain sites via a beta-scission reconstruction, with the formation of sp2 carbon. These observations correlate well with observed surface degradation and decrease in growth rate when a high concentration of nitrogen gas is introduced into the CVD growth process. The effect of co-adsorbed P, S and B onto the diamond surface has also been investigated for two reaction steps: CH3 adsorption and H abstraction. While P and B are observed to influence these reaction steps, the effect of S is rather limited.

Nanowire heterostructures play a crucial role in nanoscale electronics, i.e., one-dimensional electronics derives benefits from the growth of heterostructures along the nanowire axis. We use first-principles plane-wave calculations within density functional theory with the localized density approximation (LDA) to get information about the structural and electronic properties of bare and hydrogen passivated GaAs/AlAs nanowire heterostructures. We also take into account the reconstruction of the nanowire surfaces. Modeled nanowire heterostructures are constructed using bulk atomic positions along [001] and [111] direction of zinc-blende structures and cutting out wires from this GaAs/AlAs heterostructure crystal with a diameter of 1 nm. We study for the effects of the surface passivation on the band gap and the band offsets for the planar GaAs/AlAs bulk heterostructure system and GaAs/AlAs nanowire heterostructure system. It is possible to control the potential that carriers feel in semiconductor heterostructures. For the planar lattice-matched heterostructures, the macroscopic average of potential of the two materials is constant far from the interface and there is a discontinuity at the interface depending on the composition of the heterostructure. In order to obtain the valence band offset in the heterostructure system, the shift in the macroscopic potential at the interface and the difference between the valence band maximum values of the two constituents must be added. In nanoscale heterostructures, the potential profile presents a more complex picture. The results indicate that while the discontinuity remains close to the planar limit right at the interface, there are fluctuations on the average potential profile beyond the interface developed by the inhomogeneous surface termination, that is, there are variations of the band edges beyond the interface. We report a first-principles study of the electronic properties of surface dangling-bond (SDB) states in hydrogen passivated GaAs/AlAs nanowire heterostructures with a diameter of 1 nm, where the SDB is defined as the defect due to an incomplete passivation of a surface atom. The charge transition levels of SDB states serve as a common energy reference level, such that charge transition level value for group III and V atoms is a constant value and a periodic table atomic property. We have carried out first-principles electronic structure and total energy calculations of aluminum nanowires for a series of different diameters ranging from 3 Angtrom-10 Angstrom, which is cut out from a slab of ideal bulk structure along the [001] direction. First-principles calculations of aluminum nanowires have been carried out within the density-functional theory. We use the norm-conserving pseudopotentials that are shown to yield successful results for ultrathin nanowire regime. Our results show that the number of bands crossing the Fermi level decreases with decreasing wire diameter and all wires studied are metallic.

Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

This thesis concerns methods for electronic structure calculations and some applications of the methods. The augmented planewave (APW) basis set and it’s relatives LAPW (linearised APW) and APW+lo (local orbitals) have been widely used for electronic structure calculations. Here a modification of the APW basis set based on a transformation of the basis functions from a curvilinear coordinate system. Applications to a few test systems show that the modified basis set may speed up electronic structure calculations of sparse systems. The local density approximation (LDA) is used in density functional theory. Although it is the simplest possible approximation possible for the unknown exchange-correlation energy functional, it has proven to give quite accurate results for a wide range of systems. LDA fails in systems where the non-local effects are important. By including non-local effects by adding an orbital dependent term to the energy functional, through for example the LDA+U method, the calculated properties of many materials are closer to experimental observations. In the thesis the most general formulation of the LDA+U method is presented and a new way of interpreting the results of a calculations by formulating the orbital dependent part of the energy functional in terms of multipole momentum tensors. Applications to some early actinide systems leads to a reformulations of Hund’s rules for polarisations associated with the spin and orbital magnetic moment and a suggestion for similar rules, Katt’s rules, valid in the strong spin orbit coupling regime.

Density functional theory has been used to investigate cathode materials for rechargeable batteries, carbon nanotube interactions with catalyst particles and transition metal catalyzed hydrogen release in magnesium hydride nanoclusters. An effort has been made to the understand structural and electrochemical properties of lithium iron silicate (Li2FeSiO4) and its manganese-doped analogue. Starting from the X-ray measurements, the crystal structure of Li2FeSiO4 was refined, and several metastable phases of partially delithiated Li2FeSiO4 were identified. There are signs that manganese doping leads to structural instability and that lithium extraction beyond 50% capacity only occurs at impractically high potentials in the new material. The chemical interaction energies of single-walled carbon nanotubes and nanoclusters were calculated. It is found that the interaction needs to be strong enough to compete with the energy gained by detaching the nanotubes and forming closed ends with carbon caps. This represents a new criterion for determining catalyst metal suitability. The stability of isolated carbon nanotube fragments were also studied, and it is argued that chirality selection during growth is best achieved by exploiting the much wider energy span of open-ended carbon nanotube fragments. Magnesium hydride nanoclusters were doped with transition metals Ti, V, Fe, and Ni. The resulting changes in hydrogen desorption energies from the surface were calculated, and the associated changes in the cluster structures reveal that the transition metals not only lower the desorption energy of hydrogen, but also seem to work as proposed in the gateway hypothesis of transition metal catalysis.

The purpose of this thesis has been to investigate the elastic properties of an fcc FeMg alloy with 10 at.% magnesium under high pressure. Recent research has shown that magnesium can be a possible candidate for light element impurities in the Earth’s inner core, something that was previously not considered possible because of the low miscibility of magnesium in iron at ambient pressure. Gaining knowledge about the composition of the Earth’s core can help us better understand such phenomena as seismic activity and the fluctuations of the Earth’s magnetic field. The elastic constants of the FeMg alloy was calculated using ab-initio methods based on Density Functional Theory. The Exact Muffin-Tin Orbitals method was used in conjunction with the Coherent Potential Approximation. The FeMg alloy was found to be overall considerably softer than pure iron, and the softening effect on the elastic constants was also found to increase with pressure. The results also showed that 10% Mg alloying increased the anisotropy with about 40% compared to pure iron.

The main purpose of this thesis is to implement the Gauss--Legendre quadrature for the dispersion coefficient. This has been done and can be now be made with different number of points. The calculations with this implementation has shown that the relativistic impact on helium, neon, argon and krypton is largest for krypton, that has the highest charge of its nucleus. It was also seen that the polarizability of neon as a function of the imaginary angular frequency decreases monotonically from a static value.

Phytochromes constitute one of the six well-characterized families of photosensory proteins in Nature. From the viewpoint of computational modeling, however, phytochromes have been the subject of much fewer studies than most other families of photosensory proteins, which is likely a consequence of relevant high-resolution structural data becoming available only in recent years. In this work, hybrid quantum mechanics/molecular mechanics (QM/MM) methods are used to calculate UV-vis absorption spectra of Deinococcus radiodurans bacteriophytochrome. We investigate how the choice of QM/MM methodology affects the resulting spectra and demonstrate that QM/MM methods can reproduce the experimental absorption maxima of both the Q and Soret bands with an accuracy of about 0.15 eV. Furthermore, we assess how the protein environment influences the intrinsic absorption of the bilin chromophore, with particular focus on the Q band underlying the primary photochemistry of phytochromes.

The reactions between aqueous radiolysis products and oxide surfaces are important in nuclear technology in many ways. In solid-liquid systems, they affect (and at the same time are dependent on) both the solution chemistry and the stability of materials under the influence of ionizing radiation. The stability of surface oxides is a factor that determines the longevity of the materials where such oxides are formed. Additionally, the aqueous radiolysis products are responsible for corrosion and erosion of the materials.   In this study, the reactions between radiolysis products of water – mainly H2O2 and HO radicals – with metal, lanthanide and actinide oxides are investigated. For this, experimental and computational chemistry methods are employed. For the experimental study of these systems it was necessary to implement new methodologies especially for the study of the reactive species – the HO radicals. Similarly, the computational study also required the development of models and benchmarking of methods. The experiments combined with the computational chemistry studies produced valuable kinetic, energetic and mechanistic data.   It is demonstrated here that the HO radicals are a primary product of the decomposition of H2O2. For all the materials, the catalytic decomposition of H2O2 consists first of molecular adsorption onto the surfaces of the oxides. This step is followed by the cleavage of the O-O bond in H2O2 to form HO radicals. The HO radicals are able to react further with the hydroxylated surfaces of the oxides to form water and a surface bound HO• center. The dynamics of formation of HO• vary widely for the different materials studied. These differences are also observed in the activation energies and kinetics for decomposition of H2O2. It is found further that the removal of HO• from the system where H2O2 undergoes decomposition, by means of a scavenger, leads to the spontaneous formation of H2.   The combined theoretical-experimental methodology led to mechanistic understanding of the reactivity of the oxide materials towards H2O2 and HO radicals. This reactivity can be expressed in terms of fundamental properties of the cations present in the oxides. Correlations were found between several properties of the metal cations present in the oxides and adsorption energies of H2O, adsorption energies of HO radicals and energy barriers for H2O2 decomposition. This knowledge can aid in improving materials and processes important for nuclear technological systems, catalysis, and energy storage, and also help to better understand geochemical processes. / <p>QC 20130322</p>

c-BN has been in focus for several years due to its interesting properties. The possibility for large area CVD is a requirement for the realization of these different properties in various applications. Unfortunately, there are at present severe problems in the CVD growth of c-BN. The purpose with this research project has been to theoretically investigate, using DFT calculations, the possibility for a layer-by-layer CVD growth of c-BN. It could be established that, PEALD, using a BF3-H2-NH3-F2 pulse cycle and a diamond substrate, is a promising method for deposition of c-BN films. The gaseous species will decompose in the plasma and form BFx, H, NHx, and F species (x = 0, 1, 2, 3). The H and F radicals will uphold the cubic structure by completely hydrogenate, or fluorinate, the growing surface. However, surface radical sites will appear during the growth process as a result of atomic H, or F, abstraction reactions. The addition of NHx growth species (x = 0, 1, 2) to B radical sites, and BFx growth species (x = 0, 1, 2) to N radical sites, will then result in a continuous growth of c-BN.