To design an efficient solar energy conversion device, theoretical input is extremely important to provide the basic guideline for experimental scientists, to fabricate the most efficient, cheap, and stable device with less efforts. This desire can be made possible if computational scientist use a proper theoretical protocol, design an energy material, then the experimentalist will only invest weeks or months on the synthetic effort. This thesis highlights my recent efforts in this direction. Monoclinic BiVO4 is has been using as a photocatalyst due to its stability, cheap, easily synthesizable, narrow band gap and ideal VB (-6.80 eV vs vacuum) but inappropriate CB (-4.56 eV vs vacuum) edge position, responsible for its low efficiency. We have carried out a comprehensive experimental and periodic density functional theory (DFT) simulations of the pristine, Oxygen defective (Ov), Se doped monoclinic BiVO4 and heterojunction with Selenium (Se-BiVO4), to improve not only its CB edge position but photocatalytic and charge carrier properties. It is found that Ov (1% Oxygen vacancy) and mild doped BiVO4 (1 to 2% Se) are thermodynamically stable, have ideal band edges ~ -4.30 eV), band gaps (~1.96 eV), and small effective masses of electrons and holes. We have also investigated the contribution of Se to higher performance by effecting morphology, light absorption and charge transfer properties in heterojunction. Finally, it is found that Se makes a direct Z-scheme (band alignments) with BiVO4 where the photoexcited electron of BiVO4 recombine with the VB of Se, consequences electron-hole separation at Se and BiVO4, respectively, as a result, enhanced photocurrent is obtained. Theoretical study of β-TaON in the form of primitive unit cell, supercell and its N, Ta, and O terminated surfaces are carried out with the help of periodic DFT. Optical and electronic properties of all these different species are simulated, which predict TaON as the best candidate for photocatalytic water splitting contrast to their Ta2O5 and Ta3N5 counterparts. The calculated bandgap, valence band, and conduction band edge positions predict that β-TaON should be an efficient photoanodic material. The valence band is made up of N 2p orbitals with a minor contribution from O 2p, while the conduction band is made up of Ta 5d. Turning to thin films, the valence band maximum; VBM (−6.4 eV vs. vacuum) and the conduction band minimum; CBM (−3.3 eV vs. vacuum) of (010)-O terminated surface are respectively well below and above the redox potentials of water as required for photocatalysis. Charge carriers have smaller effective masses than in the (001)-N terminated film (VBM −5.8 and CBM −3.7 eV vs. vacuum). However, due to wide band gap (3.0 eV) of (010)-O terminated surface, it cannot absorb visible wavelengths. On the other hand, the (001)-N terminated TaON thin film has a smaller band gap in the visible region (2.1 eV) but the bands are not aligned to the redox potential of water. Possibly a mixed phase material would produce an efficient photoanode for solar water splitting, where one phase performs the oxidation and the other reduction. Computational study of an optically transparent, near-infrared-absorbing low energy gap conjugated polymer, donor−acceptor−donor (D-A-D) with promising attributes for photovoltaic application is reported herein. The D and A moiety on the polymeric backbone have been found to be responsible for tuning the band gap, optical gap, open circuit (Voc) and short-circuit current density (Jsc) in the polymers solar cells (PSC). Reduction in the band gap, high charge transformation, and enhanced visible light absorption in the D-A-D system is because of strong overlapping of molecular orbitals of D and A. In addition, the enhanced planarity and weak steric hindrance between adjacent units of D-A-D, resulted in red-shifting of its onset of absorption. Finally, PSC properties of the designed D-A-D was modeled in the bulk heterojunction solar cell, which gives theoretical Voc of about 1.02 eV. DFT study has been carried out to design a new All-Solid-State dye-sensitized solar cell (SDSC), by applying a donor-acceptor conjugated polymer instead of liquid electrolyte. The typical redox mediator (I1−/I3−) is replaced with a narrow band gap, hole transporting material (HTM). A unique “upstairs” like band energy diagram is created by packing N3 between HTM and TiO2. Our theoretical simulations prove that the proposed configuration will be highly efficient as the HOMO level of HTM is 1.19 eV above the HOMO of sanitizer (dye); providing an efficient pathway for charge transfer. High short-circuit current density and power conversion efficiency is promised from the strong overlapping of molecular orbitals of HTM and sensitizer. A low reorganization energy of 0.21 eV and exciton binding energy of 0.55 eV, confirm the high efficiency of HTM. Theoretical and experimental studies of a series of four porphyrin-furan dyads were designed and synthesized, having anchoring groups, either at meso-phenyl or pyrrole-β position of a zinc porphyrin based on donor–π–acceptor (D–π–A) approach. The porphyrin macrocycle acts as donor, furan hetero cycle acts as π-spacer and either cyanoacetic acid or malonic acid group acts as acceptor. Optical bandgap, natural bonding, and molecular bonding orbital (HOMO–LUMO) analysis confirm the high efficiency pyrrole-β substituted zinc porphyrins contrast to meso-phenyl dyads. DFT study of polypyrrole-TiO2 composites has been carried out to explore their optical, electronic and charge transfer properties for the development of an efficient photocatalyst. Titanium dioxide (Ti16O32) was interacted with a range of pyrrole (Py) oligomers to predict the optimum composition of nPy-TiO2 composite with suitable band structure for efficient photocatalytic properties. The study has revealed that Py-Ti16O32 composites have narrow band gap and better visible light absorption capability compared to individual constituents. A red-shifting in λmax, narrowing band gap, and strong intermolecular interaction energy (-41 to −72 kcal/mol) of nPy-Ti16O32 composites confirm the existence of strong covalent type interactions. Electron−hole transferring phenomena are simulated with natural bonding orbital analysis where Py oligomers found as donor and Ti16O32 as an acceptor in nPy-Ti16O32 composites. Sensitivity and selectivity of polypyrrole (PPy) towards NH3, CO2 and CO have been studied at DFT. PPy oligomers are used both, in the doped (PPy+) and neutral (PPy) form, for their sensing abilities to realize the best state for gas sensing. Interaction energies and amount of charges (NBO and Mulliken charge analysis) are simulated which reveal the sensing ability of PPy towards these gases. PPy, both in doped and neutral state, is more sensitive to NH3 compared to CO2 and CO. More interestingly, NH3 causes doping of PPy and de-doping of PPy+, providing evidence that PPy/PPy+ is an excellent sensor for NH3 gas. UV-vis and UV-vis-near-IR spectra of nPy, nPy+, and nPy/nPy+-X complexes demonstrate strong interaction of PPy/PPy+ with these atmospheric gases. The applications of graphene (GR) and its derivatives in the field of composite materials for solar energy conversion, energy storage, environment purification and biosensor applications have been reviewed. The vast coverage of advancements in environmental applications of GR-based materials for photocatalytic degradation of organic pollutants, gas sensing and removal of heavy metal ions is presented. Additionally, the presences of graphene composites in the bio-sensing field have been also discussed in this review.

The spinel structure (general formula AB2O4) is widely occurring in natural and synthetic materials, and has a marked technological and scientific significance due to its magnetic, electric and multiferroic behaviours. The presence of transition metal cations with multiple oxidation state and the resulting charge, orbital and spin degrees of freedom of the partially occupied d-orbitals lead to uniquely ordered ground states. The coupling of all the three degrees of freedom can result in a structurally distorted ground state where the direct metal-metal interaction forms atomic clusters, or 'orbital molecules'. The Verwey phase of magnetite (Fe3O4), occurring below TV ~ 125 K, is driven by a cooperative bond distortion that forms linear Fe3+-Fe2+-Fe3+ arrangement (trimeron). The effect of non-stoichiometry and chemical modification on this complex structure has been investigated with a variety of samples through microcrystal synchrotron XRD. A mineral sample (Al, Si, Mg and Mn impurities, TV = 119 K) confirms the Verwey phase as the most complex long-range electronic order known to occur naturally; its relevance in space sciences is discussed. Moreover, the structural analysis of two synthetic magnetites (Fe3(1-δ)O4 with 3δ = 0.012 and TV = 102 K, Fe3-xZnxO4 with x = 0.03 and TV = 90 K) univocally confirmed the persistence of the transition, and its first order, at doping level > 1 %, contrary to previous reports. Moreover, the temperature evolution of the trimerons and their persistence above TV was probed through X-ray Pair Distribution Function analysis on pure Fe3O4: the data analysis between 90 K < T < 923 K show that the Verwey phase goes from long-range ordered (T < 125 K) to short-range ordered (T > 850 K). Magnetite can thus only be considered to have a regular cubic spinel structure above the Curie temperature (TC = 858 K). The pyrochlore lattice of B cations in a spinel gives the structure the potential for frustration upon antiferromagnetic ordering. Fe2GeO4 and γ-Fe2SiO4 were synthesised through conventional solid state routes, with the use of high-pressure synthesis for the latter. Magnetometry and heat capacity measurements highlighted two transitions (Tm1 = 8.6 K and Tm2 = 7.2 K, and Tm1 = 11.2 K and Tm2 = 7.5 K respectively). Powder neutron diffraction data between 2 K < T < 25 K showed that both materials stay undistorted below TN. Magnetic Rietveld refinement led to two highly unconventional magnetic structures, with incommensurate propagation vectors and modulation of the moment magnitude. γ-Fe2SiO4 also shows a spin-ice order below Tm2. The results are unique and unusual for transition metal oxides; the models are systematised by proposing a 'frustration wave' model, in which the degree of frustration is a spatial quantity that can be distributed through the structure in order to stabilise the ground state.

Layered lithium transition-metal oxides (LMOs) are used as the positive electrode material in rechargeable lithium-ion batteries. Because transition metals undergo redox reactions when lithium ions intercalate in and disintercalate from the lattice, the selection and composition of transition metals largely influence the electrochemical performance of LMOs. Recently, a Ni-rich compound, LiNi0.8Co0.1Mn0.1O2 (NCM811), has drawn much attention. It is expected to replace its state-of-the-art cousins, LiCoO2 (LCO) and LiNi1/3Co1/3Mn1/3O2 (NCM111), because of its higher capacity, lower cost, and reduced toxicity. However, the excess Ni, as a transition-metal element in NCM811, can cause structural and cycling instability. Starting from NCM811, I modified the composition of transition metals by two approaches: 1) introducing cobalt deficiency and 2) substituting Ni, Co, and Mn with Zr. Their influences on the phase, structure, cycling performance, rate capability, and ionic transport were investigated by a variety of characterization techniques. I found that cobalt non-stoichiometry can suppress Ni2+/Li+ cation mixing, but simultaneously promotes the formation of oxygen vacancies, leading to rapid capacity fade and inferior rate capability compared to pristine NCM811. On the other hand, Zr can reside on and expand the lattice of NCM811, and form Li-rich lithium zirconates on their surfaces. In particular, 1% Zr substitution can increase the stability of NCM811 and facilitate Li-ion transport, resulting in enhanced cycling durability and high-rate performance. My studies help improve the understanding of the effects of transition metals on the degradation of the Ni-rich layered positive electrode material and provide modification strategies to enhance its performance and durability for Li-ion battery applications.

<p>The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials.</p><p>Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics.</p><p>The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface.</p><p>Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO₂, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.</p>

Two framework oxide materials of the MO��� network type have been synthesized and structurally characterized by synchrotron and X-ray powder diffraction and the Rietveld method in the temperature range 25~500 K. The results show one of them to be a low thermal expansion material. Theoretical studies of negative thermal expansion (NTE) in framework oxides were conducted with two methods, geometrical modeling by Rigid Unit Mode (RUM) method and lattice dynamic calculations by free energy minimization (FEM) method, the results are compared with each other as well as with experimental observations. RUM analysis of all five types of framework oxide structures negates any simple and direct correlation between presence or absence of RUMs in a structure and the sign of its thermal expansion. Instead, results suggest that NTE of a crystalline solid can not be explained by pure geometrical considerations over its structure alone, and for a better understanding of structure-relationship in negative thermal expansion structures, specific interatomic interactions present in each one must be brought in explicitly. FEM calculation of two negative thermal expansion structures indicates on a structure by structure basis NTE could be predicted and understood within the Gruneisen model, which attributes NTE of a structure to special vibration modes in a structure that softens when the lattice shrinks. The soft NTE modes are, however, not necessarily RUM or RUM like vibration motions. / Graduation date: 2003

Oxide materials exhibit a wide variety of structures and properties. In particular, transition metal oxides tend to be highly stable while exhibiting a wide range of properties that can be used for numerous applications. This work focuses on investigating how the structures��� of 4d and 5d transition metal oxides influences their properties. Specifically oxides of Ru, Rh and Ir were investigated. A complete solid solution was found between isostructural Pb���Mn���O������ and Pb���Rh���O������. Pb���Rh���O������ shows a Verwey-type transition at 185 K. This transition remains with a 3 % substitution of Mn for Rh but disappears with a 4 % substitution of Mn for Rh. The structure was found to expand in the direction perpendicular to the layers of the structure, which is the c-axis, despite a contracting unit cell. Bi for Pb substitution in Pb���Mn���O������ was found to be limited as compared to in Pb���Rh���O������. Alkali metal substitution on the A-site of the orthorhombic perovskite SrRuO��� showed only low substitution levels were possible. Nonetheless, the substituted phases showed decreased ferromagnetic Curie temperatures, increased electrical resisitivity and relatively unchanged Seebeck coefficients. Thermoelectric studies of Sr[subscript 2-x]La[subscript x]CoRuO��� perovskite phases showed Sr���.���La���.���CoRuO��� with the best thermoelectric performance. This system showed possible correlations between cation ordering on the B-site and the charge carrier transport. A similar thermoelectric study of (RhV)[subscript 1+x]Ti[subscript 1-2x]O��� phases crystallizing in a disordered trirutile structure was done. Electron carriers were found to be dominant and dependent on Ti content. The electron carriers appear to become diminished at higher temperatures. Sr���IrO��� crystallizes in a K���NiF���-type structure. Effects of Ti, Fe and Co substitution for Ir were investigated. A complete Sr���Ir[subscript 1-x]Ti[subscript x]O��� solid solution was synthesized and characterized while limited solubility was found for Fe and Co substitutions. All substitutions showed a decrease in the c-cell parameter coupled with a decrease in octahedral tilting. All substitutions also showed a decrease in magnetic susceptibility and an increase in the paramagnetic effective moment was observed for Co and Fe doped samples. An incomplete solid solution was formed for Sr���Ti[subscript 1-x]Rh[subscript x]O��� phases; however effects of increased octahedral tilting with higher Rh content were observed. / Graduation date: 2013

The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials. Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics. The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface. Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO2, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.

Organic molecules adsorbed on inorganic substrates are the topics of interest in this thesis. Interfaces of this kind are found in dye sensitized solar cells that convert solar energy to electricity, a promising environmentally friendly energy source which might provide a route to replace fossil fuels. Another field where these interfaces play a role is in molecular electronics, an approach to solve the down scaling in the ever increasing hunt for miniaturized electronic devices. The motivation for this work lies among other in these applications and surface science is a suitable approach to investigate the electronic and morphologic properties of the interfaces as it provides detailed knowledge on an atomic level. Phthalocyanines are the organic molecules investigated and the inorganic substrates range from wide band gap via narrow band gap semiconductors to metals. Photoelectron and X-ray spectroscopy experiments are performed to shed light on the electronic properties of the adsorbed molecules and the substrate, as well as the chemical interaction between adsorbate and substrate at the interface. The ordering of the adsorbate at the interface is important as ordered molecular thin films may have other properties than amorphous films due to the anisotropic electronic properties of the organic molecules; this is investigated using scanning tunneling microscopy. We find that the phthalocyanines are affected by adsorption when the substrate is TiO2 or Ag, where charge transfer from the molecule occurs or an interface state is formed respectively. The molecules are adsorbed flat on these surfaces giving a large contact area and a relatively strong bond. On Ag, ordered structures appear with different symmetry depending on initial coverage. The reactivity of the TiO2 surface is not ideal in the solar cell application and by modifying the surface with a thin organic layer, the negative influence on the adsorbed phthalocyanine is reduced. ZnO is not as reactive as TiO2, thanks maybe to the upright adsorption mode of the phthalocyanines. The semiconductor InSb is less reactive leading to self-assembled molecular structures on the (001) surface, either homogenously distributed in a one monolayer thick film or in strands along the reconstruction rows. InAs on the other hand has a larger influence on the adsorbed molecules resulting in a metallic film upon thermal treatment. / QC 20100812

This work presents a study of the electronic structure of four transition metal oxides (TMOs) using spectroscopic data and a variety of theoretical models. TMOs are a class of materials made from d-block metals in the periodic table, and one or more oxygen atoms. The nature of d-electrons is examined and theoretical models used to treat d-electron systems are tested against experimental data. Background theory of condensed matter physics is outlined. An overview of density functional theory (DFT) as a theoretical model for calculating the electronic structure of materials is presented. A variety of exchange-correlation (XC) functionals used within the DFT framework are outlined and tested for their applicability to the TMO systems in question. X-ray spectroscopy is briefly outlined and used to test the validity of the different XC functionals. All four compounds, AgO, Ag2O, CuO, and Cu2O require a Hubbard U term in the XC functional to most accurately reproduce experimental results. The effects of varying the value of U is examined in depth. The oxygen K-edge X-ray emission spectra (XES) exhibits a“two peak” structure for all compounds; the effect of varying the U value is to change the intensity ratio of the two peaks. The ratio of the two peaks as a function of U shows a linear trend in all compounds. A simple line is fit to the peak ratio vs. U curve. A common line between all compounds would provide an important metric with which to predict the appropriate U value needed in similar materials based on simple experimental data. However, the parameters of the fitted line were not common between the four compounds and any metric derived from this method would be system-dependent and not widely applicable to other systems. There are, however, interesting trends in the data when the U value is varied that provide subjects for future research. A number of fundamental quantities are determined both from experiment and theoretical calculations. Calculated bandgap values are shown to be lower than the experimental values for most functionals tested. This is not unexpected as DFT methods are known to predict much smaller bandgaps than expected. The Heyd-Scuseria-Ernzerhof (HSE) functional used for Ag2O and Cu2O does predict the bandgaps very accurately. The core-hole effect is estimated and proven to be negligible in these systems. Charge transfer and on-site Coulomb repulsion energies, important quantities in the electronic behaviour of TMOs, are determined and compared to previously reported values.

There is no apparent, dominant interaction in heavy transition metal oxides (TMO), especially in 5d-TMO, where all relevant interactions are of comparable energy scales, and therefore strongly compete. In particular, the spin-orbit interaction (SOI) strongly competes with the electron-lattice and on-site Coulomb interaction (U). Therefore, any tool that allows one to tune the relative strengths of SOI and U is expected to offer an opportunity for the discovery and study of novel materials. BaIrO3 is a magnetic insulator driven by SOI whereas the isostructural BaRuO3 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of SOI and U via a systematic chemical substitution of the Ru4+(4d4) ions for Ir4+(5d5) ions in BaIrO3, i.e., in high quality single crystals of BaIr1-xRuxO3(0.0 < x < 1.0) . Our investigation of structural, magnetic, transport and thermal properties reveals that Ru substitution directly rebalances the competing energies so profoundly that it generates a rich phase diagram for BaIr1-xRuxO3 featuring two major effects: (1) Light Ru doping (0 < x < 0.15) prompts a simultaneous and precipitous drop in both the magnetic ordering temperature TC and the electrical resistivity, which exhibits metal-insulator transition at around TC. (2) Heavier Ru doping (0.41 < x < 0.82) induces a robust metallic and spin frustration state. For comparison and contrast, we also substituted Rh4+(4d5) ions for Ir4+(5d5) ions in BaIrO3, i.e. BaIr1-xRhxO3(0.0 < x < 0.10), where Rh only reduces the SOI, but without altering the band filling. Hence, this system remains tuned at the Mott instability and is very susceptible to disorder scattering which gives rise to Anderson localization.