In this work the electronic and structural properties of anatase TiO2 (100) surface and gold adsorption have been investigated by using the first-principles calculations based on density functional theory (DFT). TiO2 is a wide band-gap material and to this effects it finds numerous applications in technology such as, cleaning of water, self-cleaning, coating, solar cells and so on. Primarily, the relation between the surface energy of the anatase (100)-1x1 phase and the TiO2-layers is examined. After an appropriate atomic layer has been chosen according to the stationary state of the TiO2 slab, the adsorption behavior of the Au atom and in the different combinations are searched for both the surface and the surface which is supported by a single Au atom/atoms. It has been observed that a single Au atom tends to adsorb to the surface which has an impurity of Au atom or atoms. Although, the high metal concentration on the surface have increased the strength of the adsorption, it is indicated that the system gains a metallic property which is believed to cause problems in the applications. In addition, the gold clusters of the dimer (Au2) and the trimer (Au3) have been adsorbed on the surface and their behavior on the surface is investigate. It is observed that the interaction between Au atoms in the atomic cluster each other is stronger than that of gold clusters and the surface.

The correlation between NMR chemical shifts of interstitial atoms and electronic structures of boron- and carbon-centered hexazirconium halide clusters was investigated by density functional theory (DFT) calculation. The influences of bridging halide and terminal ligand variations on electronic structure were examined respectively. Inverse proportionality was found between the chemical shifts and the calculated energy gaps between two Kohn-Sham orbitals of t1u symmetry, which arose from the bonding and antibonding interaction between the zirconium cage bonding orbitals and the interstitial 2p orbitals. Chemical shielding properties of the interstitial atoms were calculated with Gauge Including Atomic Orbital (GIAO) method. Stepwise ligand substitution of terminal chlorides on [(Zr6CCl12)Cl6]4-cluster by tri(n-butyl)-phosphine oxide (Bu3PO) was conducted with the aid of TlPF6. Composition of the reaction mixtures was analyzed by use of both 13C and 31P NMR. A preliminary scheme for synthesis and separation of [(Zr6CCl12)Cl6-x(Bu3PO)x]x-4 (x = 3 ?? 5) mixture based on solubility difference was reevaluated. Three 1,10-phenanthroline based bidentate ligands, namely, 2,9-Bis(diphenyl-phosphinyl)-1,10-phenanthroline, 2,9-Bis(diethoxyphosphoryl)-1,10-phenanthroline, and 2,9-Bis(di-n-butoxyphosphoryl)-1,10-phenantholine, were synthesized for bridge-chelating the hexazirconium clusters. Coordination chemistry of these ligands with the [Zr6BCl12] and [Zr6CCl12] clusters was subject to preliminary investigation.

Metal hydrides are promising candidates for H2 storage, but high stability and poor kinetics are the important challenges which have to be solved for vehicular applications. Most of recent experimental reports for improving thermodynamics of metal hydrides have been focused on lowering reaction enthalpies of a metal hydride by mixing other compounds. However, finding out metal hydride mixtures satisfying favorable thermodynamics among a large number of possible metal hydride mixtures is inefficient and thus a systematic approach is required for an efficient and rigorous solution. Our approaches introduced in this thesis allow a systematic screening of promising metal hydrides or their mixtures from all possible metal hydrides and their mixtures. Our approaches basically suggest two directions for improving metal hydride thermodynamics. First, our calculations for examining the relation between the particle size of simple metal hydrides and thermodynamics of their decomposition reactions provide that the relation would depend on the total surface energy difference between a metal and its hydride form. It ultimately suggests that we will be able to screen metal hydride nanoparticles having favorable thermodynamics from all possible metal hydrides by examining the total surface differences. Second, more importantly, we suggest that our thermodynamic calculations combined with the grand canonical linear programming method and updated database efficiently and rigorously screen potential promising bulk metal hydrides and their mixtures from a large collection of possible combinations. The screened promising metal hydrides and their mixtures can release H2 via single step or multi step. Our additional free energy calculations for a few selected promising single step reactions and their metastable paths show that we can identify the most stable free energy paths for any selected reactant mixtures. In this thesis, we also demonstrate that a total free energy minimization method can predict the possible evolution of impurity other than H2 for several specified mixtures. However, it is not ready to predict reaction thermodynamics from a large number of compounds.

Theoretical strength of solids is defined as the ultimate strength beyond which plastic deformation, fracture, or decohesion would occur. Understanding the microscopic origin from quantum mechanics and thermoelastic formulation is of great importance to mechanical properties and engineering design of various solids. While quite a few theory models have been made in the past century by several generations of scientists, including Frankel and Born, a general and convincing framework has not been fully established. We study this issue from three respects: (1) Unify various elastic stability criteria for solids that determine an upper bound of theoretical strength; (2) with ab initio method, we test the elastic stability conditions of crystal Au. The phenomenon of bifurcation is observed: under hydrostatic expansion, the rhombohedral modulus reaches zero first of all; while under uniaxial tensile stress, the tetragonal shear modulus first reaches zero; (3) propose a nonlinear theoretical formulation of stability criterion. As an analytic method, this scheme is quite simple, in the mean time, it saves computation resource.

Plastic deformation in crystalline materials largely depends on the properties of dislocations, in particular their mobility. While continuum description of deformation of a crystalline metal can be made reasonably well by considering the elastic properties of dislocations and neglecting the core, crystallographic aspects of dislocation motion require precise understanding of the core effects. The concept of the generalized stacking fault (GSF) energy was proposed as means to describe this. GSF energy, a fundamental property of a given material, can be determined using first principles total energy calculations. In this thesis, we use GSF to understand some of the intriguing mechanical responses recently observed in some metallic systems. First, we examine the structures and stacking fault energies in Mg-Zn-Y alloy system. This system is unique in the sense that trace additions of Zn and or Y result in long period stacking sequences such as 6l and 14l, as reported in recent literature. Further, these alloys exhibit extraordinary mechanical properties. We attempt to rationalize these experimental observations through first principles calculations of energies of periodic structures with different stacking sequences and stacking faults. For pure Mg, we find that the 6-layer structure with the ABACAB stacking is most stable after the lowest energy hcp structure with ABAB stacking. Charge density analysis shows that the 2l and 6l structures are electronically similar, which might be a cause for better stability of 6l structure over a 4l sequence or other periodic structures. Addition of 2 atomic% Y leads to stabilization of the structure to 6l sequence whereas the addition of 2 atomic% Zn makes the 6l energetically comparable to that of the hcp. Stacking fault (SF) on the basal plane of 6l structure is higher in energy than that of the hcp 2l Mg, which further increases upon Y doping and decreases significantly with Zn doping. SF energy surface for the prismatic slip indicates dissociation of dislocations in alloys with a 6l structure. Thus, in an Mg-Zn-Y alloy, Y stabilizes the long periodicity, while Zn doping provides a synergistic effect in improving the mechanical properties alongwith strengthening due to long periodic phases. Our investigation of surface properties and magnetism in Ni revealed that, the universal binding energy relation (UBER) derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleavage cracked surfaces are allowed to relax through atomic displacements. We find that two characteristic length-scales are involved in the cleavage of a crystal accompanied by structural relaxation at the cleaved surface. Based on that, we suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of relaxed surfaces upon cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in fcc, bcc, and hcp metals, as well as covalently bonded materials. We also found that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occuring at higher displacements. We have attempted understanding these ideas through images obtained from electronic densities and eigen states. Our work should be useful in providing inputs to multi-scale simulations of fracture in materials. The third phase of the work reports the stacking fault energy and twinning in Ni with a particular emphasis on the size effect. Experimental and computational research on Nan crystalline metals (mostly on Ni) indicates unique facets of dislocation activity (extended partial dislocations) and modes of deformation (twinning). In order to capture the intrinsic scaling eject in the nano-regime, it is imperative to account for the complex electronic structure of the metal in question. The stacking fault (SF) and twinning fault (TF) energies in nano-thin elm of Ni with 7, 13, 19, and 25 layers of (111) planes were determined using rest-principles density functional theory (DFT) total energy calculations. Generalized planar fault (GPF) energy curves of the nano-thin alms show higher extreme vis-a-vis the bulk, indicating that creation of SFs in nano-Ni is relatively difficult. In contrast, the ratios of energy barriers relevant to nucleation of dislocations and twinning support the observed enhanced tendency for extended partial dislocation formation and twinning in the nano-thin films in comparison with bulk. Our results should be useful in understanding deformation behavior of nano-structured Ni-based alloys used as advanced structural materials.

The description of the magnetic properties of interacting many-particle systems has been one of the most important goals of physics. The problem is to derive the magnetic properties of such systems from quantum mechanical principles. It is well understood that the magnetization in an atom described by quantum numbers, spin (S), orbital (L), and total angular momentum (J) of its electrons. A set of guidelines, known as Hund's rules, discovered by Friedrich Hermann Hunds help us to determine the quantum numbers for the ground states of free atoms. The question ``to which extent are Hund's rules applicable on different systems such as molecules and solids?'' is still on the agenda. The main problem is that of finding the ground state of the considered system. Density functional theory (DFT) methods apparently are the most widely spread self-consistent methods to investigate the ground state properties. This is due to their high computational efficiency and very good accuracy. In the framework of DFT, usually the total energy is decomposed into kinetic energy, Coulomb energy, and a term called the exchange-correlation energy. Taking into account the relativistic kinetic energy leads to direct and indirect relativistic effects on the electronic structure of a solid. The most pronounced direct effect (although not the biggest in magnitude) is the spin-orbit splitting of band states. A well-known indirect relativistic effect is the change of screening of valence electrons from the nuclear charge by inner-shell electrons. One can ask that how relativistic effects come into play in ordinary density functional theory. Of course ordinary density functional theory does not include those effect. Four-current density functional theory (CDFT), the quantum electrodynamic version of the Hohenberg-Kohn theory is a powerful tool to treat relativistic effects. Although it is principally designed for systems in strong magnetic fields, CDFT can also be applied in situations where currents are present without external magnetic fields. As already pointed out by Rajagopal and Callaway (1973), the most natural way to incorporate magnetism into DFT is the generalization to CDFT. These authors, however, treated its most simple approximation, the spin density functional theory (SDFT), which keeps the spin current only and neglects completely correlation effects of orbital currents. By using the Kohn-Sham-Dirac (KSD) equation, spin-orbit coupling is introduced kinematically. The part of the orbital magnetism that is a consequence of Hund's second rule coupling is absent in this theory and there is not any more a one-to-one mapping of spin densities onto external fields. In solids, in particular in metals, the importance of Hund's second rule coupling (orbital polarization) and Hund's third rule (spin-orbit coupling) is usually interchanged in comparison to atoms. Thus, in applications of the relativistic CDFT to solids, the usual way has been to keep the spin-orbit coupling in the KSD equation (an extension to ordinary Kohn-Sham (KS) equation) and to neglect the orbital contribution to the total current density and approximate exchange-correlation energy functional with spin density only. This scheme includes a spontaneous exchange and correlation spin polarization. Orbital polarization, on the other hand, comes into play not as a correlation effect but also as an effect due to the interplay of spin polarization and spin-orbit coupling: In the presence of both couplings, time reversal symmetry is broken and a non-zero orbital current density may occur. Application of this scheme to 3d and 4f magnets yields orbital moments that are smaller than related experimental values by typically a factor of two. Orbital magnetism in a solid is strongly influenced by the ligand field, originating from the structural environment and geometry of the solid. The orbital moments in a solid with cubic symmetry are expected to be quenched if spin-orbit coupling is neglected. However, spin-orbit coupling induces orbital moments, accordingly. The relativistic nature of the spin-orbit coupling requires orbital magnetism to be treated within QED, and the treatment of QED in solids is possible in the frame of current density functional theory. The kinematic spin-orbit coupling is accounted for in many DFT calculations of magnetic systems within the LSDA. However, a strong deviation of the LSDA orbital moments from experiment is found in such approaches. To avoid such deviations, orbital polarization corrections would be desirable. In this Thesis, those corrections have been investigated in the framework of CDFT. After a short review for CDFT in Chapter 2, in Chapter 3, an "ad hoc" OP correction term (OPB) suggested by Brooks and Eriksson is given. This correction in some cases gives quite reasonable corrections to orbital moments of magnetic materials. Another OP correction (OPE), which has been introduced recently, was derived from the CDFT in the non-relativistic limit. Unfortunately, the program can only incompletely be carried through, as there are reasonable but uncontrolled approximations to be made in two steps of the derivation. Nevertheless, the result is quite close to the "ad hoc"ansatz. The calculated OPE energies for 3d and 4f free ions are in qualitative agreement with OPB energies. In Chapter 4, both corrections are implemented in the FPLO scheme to calculate orbital moments in solids. We found that both OPB and OPE corrections implemented in FPLO method, yield reasonably well the orbital magnetic moments of bcc Fe, hcp Co and fcc Ni compared with experiment. In Chapter 5, the effect of spin-orbit coupling and orbital polarization corrections on the spin and orbital magnetism of full-Heusler alloys is investigated by means of local spin density calculations. It is demonstrated, that OP corrections are needed to explain the experimental orbital moments. Model calculations employing one ligand field parameter yield the correct order of magnitude of the orbital moments, but do not account for its quantitative composition dependence. The spin-orbit coupling reduces the degree of spin polarization of the density of states at Fermi level by a few percent. We have shown that the orbital polarization corrections do not change significantly the spin polarization degree at the Fermi level. We also provide arguments that Co2FeSi might not be a half-metal as suggested by recent experiments. In Chapter 6, to understand recent XMCD data for Co impurities in gold, the electronic structure of Co impurities inside gold has been calculated in the framework of local spin density approximation. The orbital and spin magnetic moment have been evaluated. In agreement with experimental findings, the orbital moment is enhanced with respect to Co metal. On the other hand, internal relaxations are found to reduce the orbital moment considerably, whereas the spin moment is less affected. Both OPB and OPE yield a large orbital moment for Co impurities. However, those calculated orbital moments are almost by a factor of two larger than the experimental values. We also found that the orbital magnetic moment of Co may strongly depend on pressure.

Computational approaches play an important role in today's materials science owing to the remarkable advances in modern supercomputing architecture and algorithms. Ab initio simulations solely based on a quantum description of matter are now very able to tackle materials problems in which the system contains up to a few thousands atoms. This dissertation aims to address the modern electronic structure calculation methods applied to a range of various materials such as liquid and amorphous phase materials, nanostructures, and small organic molecules. Our simulations were performed within the density functional theory framework, emphasizing the use of real-space ab initio pseudopotentials. On the first part of our study, we performed liquid and amorphous phase simulations by employing a molecular dynamics technique accelerated by a Chebyshev-subspace filtering algorithm. We applied this technique to find l- and a- SiO₂ structural properties that were in a good agreement with experiments. On the second part, we studied nanostructured semiconducting oxide materials, i.e., SnO₂ and TiO₂, focusing on the electronic structures and optical properties. Lastly, we developed an efficient simulation method for non-contact atomic force microscopy. This fast and simple method was found to be a very powerful tool for predicting AFM images for many surface and molecular systems. / text

This thesis presents the basic concepts of electronic structure theory and the chemical properties of mercury. The theoretical foundation of DFT and the consequences of relativity are also introduced. The electronic structure of Hg(II) ions, [Hg(L)n(H2O)m]q (L = HO-, Cl-, HS-, S2-) has been studied. We show, in this thesis, that the charge transfer (that is calculated from the hard-soft-acid-base principle (Pearson’s principle)), the total NBO charge and the interaction energies are strongly correlated. Our studies indicate the effect of the solvent on the global electrophilicity, the charge transfer and consequently the interaction strength between Hg(II) and ligand L. The formation constants, logK, of Hg2+−complexes are calculated. The procedure that we follow in this thesis to calculate the formation constants, logK’s, are in good agreement with the extrapolated experimental values. We introduce and explain why it is important adding water molecules explicitly during the calculations of the logK. The recommended logK value of HgS is 27.2. We examined two different types of organic compounds as sensors for heavy metal ions: lumazine (Lm) and 6-thienyllumazine (TLm). We found that the simple calculation of pKa values using DFT methods and implicit solvent models failed to reproduce the experimental values. However, calculated orbital energies and gas phase acidities both indicate that the compound TLm is inherently more acidic than the parent species Lm. We demonstrate that: (1) we need to take in our consideration the population of each tautomer and conformer during the calculations of the pKa values, and (2) thienyl group has indirect effect on the acidity of the proton on N1 in the uracil ring. Last but not least, the fluorescence spectrum of the sensors (L) and their [(L)nM(H2O)m]2+ complexes (L = Lumazine (Lm) and 6-thienyllumazine (TLm) and M = Cd2+and Hg2+) are calculated using time dependent DFT (TDDFT). The results show that TDDFT is in good agreement with experimental results. This chapter provides a new concept in the design of fluorescence turn-on/off sensors that has wider applicability for other systems. Finally, we provide a summary of the works compiled in this thesis and an outlook on potential future work. / October 2015

In the first part of the dissertation, we develop a theoretical approach to analyze nonlinear spectroscopy experiments based on the formalism of quantum state (QST) and process tomography (QPT). In it, a quantum system is regarded as a black box which can be systematically tested in its performance, very much like an electric circuit is tested by sending a series of inputs and measuring the corresponding outputs, but in the quantum sense. We show how to collect a series of pump-probe or photon-echo experiments, and by varying polarizations and frequency components of the perturbations, reconstruct the quantum state (density matrix) of the probed system for a set of different initial conditions, hence simultaneously achieving QST and QPT. Furthermore, we establish the conditions under which a set of two-dimensional optical spectra also yield the desired results. Simulations of noisy experiments with inhomogeneous broadening show the feasibility of the protocol. A spin-off of this work is our suggestion of a witness that distinguishes between spectroscopic time-oscillations corresponding to vibronic only coherences against their electronic counterparts. We conclude by noting that the QST/QPT approach to nonlinear spectroscopy sheds light on the amount of quantum information contained in the output of an experiment, and hence, is a convenient theoretical and experimental paradigm even when the goal is not to perform a full QPT. In the second part of the thesis, we discuss a methodology to study the electronic dynamics of complex molecular systems, such as photosynthetic units, in the framework of time-dependent density functional theory (TD-DFT). By treating the electronic degrees of freedom as the system and the nuclear ones as the bath, we develop an open quantum systems (OQS) approach to TD-DFT. We formally extend the theoretical backbone of TD-DFT to OQS, and suggest a Markovian bath functional which can be readily included in electronic structure codes.

This thesis explores multiscale approaches to describe structural and chemical defects in metals. Particular emphasis is placed on investigating processes involving grain boundaries (GBs) in combination with impurity and vacancy defects. The defects and their interactions are calculated to very high accuracy using density functional theory (DFT) and connected to the macroscopic behavior within the two multiscale formalisms presented here. We begin with a sequential approach to address chemical embrittlement of nickel by sulfur impurities. Effects at both a \(\Sigma 5 (012)\) symmetric tilt GB and in the bulk are studied by considering competing mechanisms for ductile and brittle behavior. For the bulk, this takes the form of Rice’s theory, where the ratio of the surface and unstable stacking energy is used as a measure of ductility. This is generalized to the GB by considering GB sliding (GBS) and intergranular decohesion. Clear evidence that chemical embrittlement of nickel by sulfur is a GB driven effect is found. Next, a concurrent multiscale approach is described. A small region, containing the defects, is treated with Kohn-Sham DFT and coupled to the bulk, described with the embedded atom method. We apply this novel method to elucidate the chemical embrittlement of a copper \(\Sigma 5 (012)\) symmetric tilt GB. Intergranular decohesion for three substitutional impurities, bismuth, lead and silver, is investigated by considering the work of separation \((W_s)\) and the tensile strength \((\sigma_t)\). Bismuth and lead show a significant decrease in \(W_s\) and \(\sigma_t\), consistent with embrittlement, whilst silver has only a minor effect. Then, the concurrent multiscale method is applied to the process of GBS in copper. It is found that the resistance against sliding increases significantly for bismuth, lead and silver impurities. The underlying mechanisms for this increase are found to be dominated by size effects for bismuth and lead. For silver, chemical effects are of greater importance. Similar results are found for the underlying mechanisms of intergranular decohesion. The effect of a mono-vacancy on GBS is studied for copper. The multiscale approach enables improved decoupling of the mono-vacancy. It is found that the monovacancy enhances GBS by 22%. / Engineering and Applied Sciences