In this thesis I have studied some problems of condensed matter at high pressures and temperatures by means of numerical simulations based on Density Functional Theory (DFT). The stability of MgCO3 and CaCO3 carbonates at the Earth's mantle conditions may play an important role in the global carbon cycle through the subduction of the oceanic crust. By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transformation is characterized by the change of the sp-hybridization state of carbon atom and indicates a change to a new crystal-chemical regime. By performing ab initio Molecular Dynamics simulations we show the new phase to be stable at 250 GPa and 1000K. Thus, the formation of sp3 hybridized bonds in carbonates can explain the stability of MaCO3 and CaCO3 at pressures corresponding to the Earth's lower mantle conditions. We have also calculated phase transition sequence in CaCO3, SrCO3 and BaCO3, and have found that, despite the fact that these carbonates are isostructural and undergo the same type of aragonite to post-aragonite transition, their phase transformation sequences are different at high pressures. The continuous improvement of the high-pressure technique led to the discovery of new composite structures at high pressures and complex phases of many elements in the periodic table have been determined as composite host-guest incommensurate structures. We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells and exploiting the fact that the total energy of the system is a function of structural parameters. By applying this method to the Sc-II phase, we have determined the incommensurate ratio, lattice parameters and Wyckoff positions of Sc-II in excellent agreement with the available experimental data. Moreover, we predict the occurrence of an incommensurate high-pressure phase in Ca from first-principle calculations within this approach. The implementation of DFT in modern electronic structure calculation methods proved to be very successful in predicting the physical properties of a solid at low temperature. One can rigorously describe the thermodynamics of a crystal via the collective excitation of the ionic lattice, and the ab initio calculations give an accurate phonon spectra in the quasi-harmonic approximation. Recently an elegant method to calculate phonon spectra at finite temperature in a self-consistent way by using first principles methods has been developed. Within the framework of self-consistent ab initio lattice dynamics approach (SCAILD) it is possible to reproduce the observed stable phonon spectra of high-temperature bcc phase of Ti, Zr and Hf with a good accuracy. We show that this method gives also a good description of the thermodynamics of hcp and bcc phases of Ti, Zr and Hf at high temperatures, and we provide a procedure for the correct estimation of the hcp to bcc phase transition temperature.

The analysis of nature of bonding in non-classical structures is always an intriguing area of research. Typical examples of such systems are polyhedral boranes that exhibit fascinating cluster bonding where the traditional 2-center-2-electron (2c-2e) bond model fails. This thesis involves the investigation of such polyhedral borane structures and their reactivity by employing both qualitative and quantitative tools of electronic structure theory. There is an intense current interest in the macropolyhedral boranes for their applications pharmaceuticals and materials chemistry. The mno rule had been formulated to account for the electronic requirements for the macropolyhedral structures. Though useful in explaining and designing structures, electron counting rules provide a yes or no answer; not all the molelcules having stipulated number of electrons are equally stable. We have used the concept of orbital compatibility to explain the relative energies of different macropolyhedral structural patterns such as closo-closo, closo-nido and nido-nido. One of the major problems in polyhedral boron cage chemistry has been the lack of general synthetic routes for the construction of large cage systems . With this view, we explored the mechanism of the reaction of macropolyhedron B20H16 with MeCN and similar ligands, which provide an understanding of the skeletal rearrangement that occur in macropolyhedral boranes. This can help in the design and synthesis of new macropolyhedral boranes. The early examples of metallaboranes were found to adopt structures which are analogous to that of boranes and carboranes. Hypercloso metallaboranes have closo structure with less number of electrons than required by Wades rule. We have carried out a detailed DFT analysis to explore the structure and electronic relationship of 9-12 vertex closo and hypercloso structures of both borane and metallaboranes. Calculations show that in vertex hypercloso metallaborane needs only n skeleton electron pairs rather than n+1 as suggested by Wade’s rules. Stabilization of supraicosahedral boranes with more than 12 vertices by substituting BH groups by transition metal fragments is also explored with DFT calculations. Calculations show that as the number and the size of the metal atom increases the stability of supraicosahedral and condensed supraicosahedral borane structures also increases. These studies will open up new possibilities for the development of polyhedral clusters of extraordinary size.

In this thesis point defects in semiconductors are studied by electronic structure calculations. Results are presented for the stability and equilibrium concentrations of native defects in GaP, InP, InAs, and InSb, for the entire range of doping conditions and stoichiometry. The native defects are also studied on the (110) surfaces of InP, InAs, and InSb. Comparing the relative stability at the surface and in the bulk, it is concluded that the defects have a tendency to migrate to the surface. It is found that the cation vacancy is not stable, but decomposes into an anion antisite-anion vacancy complex. The surface charge accumulation in InAs is explained by complementary intrinsic doping by native defects and extrinsic doping by residual hydrogen. A technical investigation of the supercell treatment of defects is performed, testing existing correction schemes and suggesting a more reliable alternative. It is shown that the defect level of [2VCu-IIICu] in the solarcell-material CuIn1-xGaxSe2 leads to a smaller band gap of the ordered defect γ-phase, which possibly explains why the maximal efficiency for CuIn1-xGaxSe2 has been found for x=0.3 and not for x=0.6, as expected from the band gap of the α-phase. It is found that Zn diffuses via the kick-out mechanism in InP and GaP with activation energies of 1.60 eV and 2.49 eV, respectively. Explanations are found for the tendency of Zn to accumulate at pn-junctions in InP and to why a relatively low fraction of Zn is found on substitutional sites in InP. Finally, it is shown that the equilibrium solubility of dopants in semiconductors can be increased significantly by strategic alloying. This is shown to be due to the local stress in the material, and the solubility in an alloy can in fact be much higher than in either of the constituting elements. The equilibrium solubility of Zn in Ga0.9In0.1P is for example five orders of magnitude larger than in GaP or InP.

The experimental work presented in this thesis is based on a wide assortment of very advanced and highly sophisticated photoelectron spectroscopy (PES) techniques. The objective of the present study has been to reveal and understand the electronic structure and electron dynamics in a broad spectrum of materials, ranging from wide band gap oxides, via semiconductors along with metals, and finally high-temperature superconductors. The first part of the thesis concerns laser-based pump-and-probe PES. This unique experimental technique has permitted a study of the excited electronic structure and the electron dynamics of several semiconductor surfaces. An insight into details of the adatom to restatom charge-transfer of the Ge(111)c(2x8) surface is presented, as well as an estimate for the timescale in which the dynamic adatoms of the Ge(111):Sn(sqrt3xsqrt3)R30deg surface operate. Further results comprise a novel unoccupied surface state at the GaSb(001) surface as well as a time-resolved study of the charge accumulation layer at the InAs(111)A/B surfaces. In the second part, high-resolution synchrotron based angle-resolved PES (ARPES) data from the cuprate high-temperature superconductor La(2-x)Sr(x)CuO(4) (LSCO) is presented. This extensive study, reveals detailed information about how the Fermi surface and electronic excitations evolve with doping in the superconducting state. The results comprise support for a connection between high- and low-energy electronic responses, the characteristics of the superconducting gap, and indication of a quantum phase transition between two different superconducting phases. In the third group of experiments we move away from the two-dimensional systems and concentrate on fully three-dimensional compounds. By the use of soft x-ray ARPES it is possible to extract the three-dimensional electronic structure in a straightforward manner with increased k(perpendicular)-resolution. As a result the first high-quality ARPES data from Cu2O is presented, as well as a novel method for extracting the (real space) electron density by ARPES. These experiments clearly display the advantages of using soft x-ray ARPES. If the material and type of experiment is chosen wisely, the benefit of the increased k||-window and the free electron final state, surpass the drawbacks of decreased count-rate and inferior energy resolution. Finally we return to the high-temperature superconductors (NCCO & Nd-LSCO) and make use of the increased bulk-sensitivity. From an evident change in the shape of the Fermi surface when moving from low to high photon energies, the durface to bulk difference in electronic structure is highlighted. / QC 20100810

This thesis is based on experimental studies in chemical physics. Titanium dioxide (TiO2) and phthalocyanine’s (Pc’s), interesting in many future perspectives, have been deposited as thin films and studied as follows. Information has been obtained on e.g. molecular orientation, crystal structure, depth profile of the chemical composition, electrochemical properties and electronic structure. This has been achieved by means of a combination of techniques: X-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), density functional theory calculations (DFT), UV-visible absorption spectroscopy (UVVIS) and cyclic voltammetry (CV). Metal-free phthalcyanine (H2Pc) has been shown to form films with different crystal structure and molecular orientation depending on deposition method, evaporation/sublimation or powder deposition, on commercial conducting glass (fluorine doped tin oxide, FTO), which is used e.g. in solar cells and organic light emitting devices (OLEDs). The unoccupied molecular orbitals are divided in x, y and z space coordinates of the molecule and also divided in inequivalent nitrogen components. The electronic structure is also studied for a sublimated titanyl phthalocyanine (TiOPc) film and related to the metal-free phthalocyanine. The ligand field around the titanium atom in TiOPc is compared with that of TiO2 to delineate the unoccupied levels recorded by means of x-ray absorption spectroscopy. Nanostructured TiO2 films were manufactured by screen printing/doctor blading on FTO. Such films were additionally covered with lutetium diphthalocyanine (LuPc2) by means of surface assembly from solution. LuPc2-, LuPc2+ and LuPc2H were identified and the stability of the electrochromic reactions in this system was monitored. Chemical vapor deposition (CVD) has been used to grow nanometer sized anatase TiO2 crystals on pre-oxidized Si (111) without formation of interfacial carbon and with an interface layer of the size of 15- 25Å. The interface layer was found to be amorphous TiSixOy with graded stoichiometry.

Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis. The compounds TlCo2S2, TlCo2Se2 and their solid solution TlCo2Se2-xSx, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo2S2, the net result is ferromagnetism, while TlCo2Se2 exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo2Se2 (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0). The systems TlCo2-xMexSe2 show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu2Se2, but now with the moments perpendicular to the metal sheets. TlCrTe2 forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr3+.

A systematic study of trends in band gap and lattice energies for bare zinc oxide nanoparticles were performed by means of quantum chemical density functional theory (DFT) calculations and density of states (DOS) calculations. The geometry of the optimized structures and the appearance of their frontier orbitals were also studied. The particles studied varied in sizes from (ZnO)6 up to (ZnO)192.The functionalization of bare and hydroxylated ZnO surfaces with MPTMS was studied with emphasis on the adsorption energies for adsorption to different surfaces and the effects on the band gap for such adsorptions.

Noncovalent interactions are of pivotal importance in many areas of chemistry, biology, and materials science, and the intermolecular interactions involving aromatic rings in particular, are fundamental to molecular organization and recognition processes. The work detailed in this thesis involves the application of state-of-the-art ab initio electronic structure theory methods to elucidate the nature of pi-pi interactions. The binding energies, and geometrical and orientational preferences of the simplest prototype of aromatic pi-pi interactions, the benzene dimer, are explored. We obtain the first converged values of the binding energies using highly accurate methods and large basis sets. Results from this study predict the T-shaped and parallel-displaced configurations of benzene dimer to be nearly isoenergetic. The role of substituents in tuning pi-pi interaction is investigated. By studying dimers of benzene with various monosubstituted benzenes (in the sandwich and two T-shaped configurations), we surprisingly find that all of the substituted sandwich dimers considered bind more strongly than benzene dimer. We also find that these interactions can be tuned by a modest degree of substitution. Energy decomposition analysis using symmetry-adapted perturbation theory (SAPT) reveals that models based solely on electrostatic effects will have difficulty in reliably predicting substituent effects in pi-pi interactions.

The accuracy of a quantum chemical calculation inherently depends on the ability to account for the completeness of the one- and n-particle spaces. The size of the basis set used can be systematically increased until it reaches the complete one-particle basis set limit (CBS) while the n-particle space approaches its exact full configuration interaction (FCI) limit by following a hierarchy of electron correlation methods developed over the last seventy years. If extremely high accuracy is desired, properly correcting for very small effects such as those resulting the Born-Oppenheimer approximation and the neglect of relativistic effects becomes indispensable. For a series of chemically interesting and challenging systems, we identify the limits of conventional approaches and use state-of-the-art quantum chemical methods along with large basis sets to get the “right answer for the right reasons.” First, we quantify the importance of small effects that are ignored in conventional quantum chemical calculations and manage to achieve spectroscopic accuracy (agreement of 1 cm−1 or less with experimental harmonic vibrational frequencies) for BH, CH+ and NH. We then definitively resolve the global minimum structure for Li₆ , Li₆⁺ , and Li₆- using high accuracy calculations of the binding energies, ionization potentials, electron affinities and vertical excitation spectra for the competing isomers. The same rigorous approach is used to study a series of hydrogen transfer reactions and validate the necessary parameters for the hydrogen abstraction and donation steps in the mechanosynthesis of diamondoids. Finally, in an effort to overcome the steep computational scaling of most high-level methods, a new hybrid methodology which scales as O(N⁵) but performs comparably to O(N⁶) methods is benchmarked for its performance in the equilibrium and dissociation regimes.

Conjugated polymers and oligomers have been widely studied based on their wide range of useful properties and applications. Given the importance of self-assembly and charge transfer in the development of conjugated materials for use in electronic applications, it is crucial to: (i) prepare functional materials by molecular design, (ii) evaluate the structure-property relationships of new materials, and (iii) develop fundamental understanding of electronic structure and charge transport behavior. The use of conjugated polymeric materials in electronic applications relies on control of the assembly and orientation of the polymer chains in the solid state. Conjugated polymers with liquid crystalline behavior could be used to implement an additional level of control over orientation and resultant properties. Substitution of the conjugated polythiophene backbone with semifluoroalkyl side chains (i.e., the diblock -(CH2)m(CF2)nF) has afforded materials with unusual properties. The mutual immiscibility of the aromatic backbone, the alkyl side-chain segments, and the fluoroalkyl side-chain termini provides control over supramolecular packing. A series of eight polymers has been synthesized, in which the lengths of the alkyl (m) and fluoroalkyl (n) segments are varied. One regiorandom analogue and two poly(3-alkylthiophene)s were also synthesized for comparative purposes. The structure, molecular weight, and regioregularity of the polymers were evaluated using a variety of techniques. The semifluoroalkyl-substituted polymers have been systematically studied to determine the effect of side chain length and m:n block ratios on their solution state, liquid crystalline, and solid state properties. The effect of side chains on conjugation was determined, where solubility allowed, by solution-state UV-visible and fluorescence spectroscopy. The thermal and liquid crystalline properties of the homopolymers were evaluated by DSC, variable-temperature X-ray diffraction, and polarized optical microscopy. Several semifluoroalkyl-substituted polythiophene homologues show liquid crystalline behavior. Molecular packing and charge transport are key factors governing the use of conjugated materials in electronic applications. A wide variety of oligomers have been studied as models for charge migration in conjugated polymers. One-dimensional models do not adequately represent two-dimensional charge transport; thus, a variety of two-dimensional, covalently-linked models have been developed. Previous work by our group, and others, led to the proposal of bis(oligothienyl) compounds as models to study the interaction of the ð-conjugated chains. Previous reports by other researchers described the synthesis and characterization of hydrogen-terminated analogues of 1,8-bis(oligothienyl)naphthalenes. However, these materials proved to be unsuitable for use as charge transport models, as they were subject to irreversible polymerization upon oxidation. Installation of methyl groups at the terminal a-positions of 1,8-bis(oligothienyl)naphthalenes allowed us to create a series of models in which conjugated chains are held in close proximity. This provides access to multiple redox states, and future systems based on these molecules may be used as models for charge transport or as functional materials for incorporation into devices.