The structural, electronic, and superconducting properties of one dimensional materials are calculated from first principles, using the density functional theory. Nanotubes and nanowires are important building blocks in nanotechnology, in particular for nanoelectronics. In this manuscript, the growth of carbon nanotubes is studied through the interaction between carbon and the transition metal atoms used as growth catalysts. The accepted model for a new phase of nanotube-like molybdenum disulfide is critically examined using comparisons of energetic stability and types of chemical bonding in different candidate structures which have similar compositions. The epitaxial growth of diamond carbon on (100) iridium is exceptionally favorable. The differences between various substrates used experimentally are studied, and the specificity of Ir is shown. Finally, the characteristics of the electron-phonon interaction in aluminium nanowires are determined. The structural instabilities and the differences in the electron-phonon coupling are calculated for straight monoatomic wires, zigzag wires, and thicker straight wires. The constrained geometry of the wires generates a coupling which can be very strong or almost vanish, depending on the structural details, but which is concentrated in the longitudinal high-frequency phonons.

The structural, electronic, and superconducting properties of one dimensional materials are calculated from first principles, using the density functional theory. Nanotubes and nanowires are important building blocks in nanotechnology, in particular for nanoelectronics. In this manuscript, the growth of carbon nanotubes is studied through the interaction between carbon and the transition metal atoms used as growth catalysts. The accepted model for a new phase of nanotube-like molybdenum disulfide is critically examined using comparisons of energetic stability and types of chemical bonding in different candidate structures which have similar compositions. The epitaxial growth of diamond carbon on (100) iridium is exceptionally favorable. The differences between various substrates used experimentally are studied, and the specificity of Ir is shown. Finally, the characteristics of the electron-phonon interaction in aluminium nanowires are determined. The structural instabilities and the differences in the electron-phonon coupling are calculated for straight monoatomic wires, zigzag wires, and thicker straight wires. The constrained geometry of the wires generates a coupling which can be very strong or almost vanish, depending on the structural details, but which is concentrated in the longitudinal high-frequency phonons.

In this thesis I concentrate on the description of electron transport properties of microscopic objects, including molecular junctions and nano junctions, in particular, inelastic electron tunneling in surface-adsorbate systems are examined with more contemplations. Boosted by the rapid advance in experimental techniques at the microscopic scale, various electric experiments and measurements sprung up in the last decade. Electric devices, such as transistors, switches, wires, etc. are expected to be integrated into circuit and performing like traditional semiconductor integrated circuit (IC). On the other hand, detailed information about transport properties also provides new physical observable quantities to characterize the systems. For molecular electronics, which is in the state of growing up, its further applications demands more thorough understanding of the underlying mechanism, for instance, the effects of molecular configuration and conformation, inter- or intra-molecular interactions, molecular-substrate interactions, and so on. Inelastic electron tunneling spectroscopy (IETS), which reflects vibration features of the system, is also a finger print property, and can thus be employed to afford the responsibility of single molecular identification with the help of other experimental techniques and theoretical simulations.There are two parts of work presented in this thesis, the first one is devoted to the calculation of electron transport properties of molecular or nano junctions: we have designed a negative differential resistance (NDR) device based on graphene nanoribbons (GNRs), where the latter is a star material in scientific committee since its birth;The transport properties of DNA base-pair junctions are also examined by theoretical calculation, relevant experimental results on DNA sequencing have been explained and detailed issues are suggested.The second part focused on the simulation of scanning tunneling microscope mediated IETS (STM-IETS). We have implemented a numerical scheme to calculate the inelastic tunneling intensity based on Tersoff-Hamann approximation and finite difference method, benchmark results agree well with experimental and previous theoretical ones; Two applications of single molecular chemical identification are also presented following benchmarking. / QC20100630

This thesis focuses on theoretical study of charge transportproperties in molecular systems. The understanding of the transportprocess and mechanism in molecular systems is essential forthe design of new functional molecular materials and molecularelectronic devices. The molecular junctions and organic molecularcrystals have been used as the model systems to highlight the usefulnessof theoretical modelling. A molecular junction is a system that consists ofone or several molecules sandwiched between two electrodes.The charge transport in molecular junctions is a very complex processthat is affected by the interaction between molecules and electrodes,the surroundings, as well as electron-electron (e-e) andelectron-phonon (e-p) couplings. When the molecule-electrode couplingis strong, the transport process can be very quick. If the e-p couplingis weak, the inelastic tunneling has only negligible contributions to thetotal current and the elastic electron tunneling plays the dominant role.Furthermore, the hopping process becomes dominant in the case of strong e-pcoupling, for which the geometric relaxation of the molecule needsto be considered. In this thesis, we have examined these three kinds oftransport processes separately. The first studied system is a molecular junction consisting of aromaticallycoupled bimolecules. Its elastic electron tunneling property is simulatedusing Green's functional theory at density functional theory level.The dependence of the conductance of bimolecular junctions on the vertical distances,horizontal distances and the tilt angles has been systematically studied. Theinelastic electron tunneling spectra (IETS) of molecular junctions have beencalculated for several systems that were experimentally measured with conflictingresults and controversial assignments. Our calculations provide the reliableassignments for the experimental spectra and revealed unprecedented detailsabout the molecular conformations within the junctions under different conditions.It demonstrates that a combined theoretical and experimental IETS study is capableof accurately determining the structure of a single molecule inside the junction.The hopping process is a dominant charge transfer process in organic molecularcrystals. We have studied the charge transport ability of four kinds of n-typeorganic semiconductor materials to find out the related structure-to-propertyrelationship. It is done by adopting the quantum charge transfer rate equationcombined with the random walk approach. / QC 20120515

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.

The aim of this thesis was to investigate carbon based molecular materials at first principles levels. Special attention has been paid to simulations of X-ray spectroscopies, including near edge X-ray absorption fine structure (NEXAFS), X-ray photoelectron, and X-ray emission spectroscopy, which can provide detailed information about core, occupied and unoccupied molecular orbitals of the systems under investigation. Theoretical calculations have helped to assign fine spectral structures in high resolution NEXAFS spectra of five azabenzenes (pyridine, pyrazine, pyrimidine, pyridazine and s-triazine), and to identify different local chemical environments among them. With the help of NEXAFS, the characters of important chemical bonds that might be responsible for the unique magnetic properties of the tetracyanoethylene compound has been revealed. Calculations have demonstrated that X-ray spectroscopies are powerful tools for isomer identification and structure determination of fullerenes and endohedral metallofullerenes. A joint experimental and theoretical study on metallofullerene Gd@C82 has firmly determined its equilibrium structure, in which the gadolinium atom lies above the hexagon on the C2 axis. It is found that the gadolinium atom could oscillate around its equilibrium position and that its oscillation amplitude increases with increasing temperature. In this thesis, several new computational schemes for large-scale systems have been proposed. Parallel implementation of a central insertion scheme (CIS) has been realized, which allows to effectively calculate electronic structures of very large systems, up to 150,000 electrons, at hybrid density functional theory levels. In comparison with traditional computational methods, CIS provides results with the same high accuracy but requires only a fraction of computational time. One of its applications is to calculate electronic structures of nanodiamond clusters varying from 0.76 nm (29 carbons) to 7.3 nm (20,959 carbons) in diameter, which enabled to resolve the long-standing debate about the validity of the quantum confinement model for nanodiamonds. Electronic structures and X-ray spectroscopies of a series of single-walled carbon nanotubes (SWCNTs) with different diameters and lengths have been calculated, which have made it possible to interpret the existing experimental results. / QC 20100727

This thesis work was performed at AB Sandvik Coromant and aimed to enhance the knowledge about the relationships between the performance of TiN and TiAlN-coated cutting tools in metal turning and their mechanical and chemical properties. Measurements of coating material properties and turning wear tests in annealed tool steel Sverker 21, stainless steel 316L, grey cast iron V314 and nodular cast iron SS0727 were performed. The cutting temperatures were estimated from FEM-simulations. To find the dominant wear mechanism and identify the properties that are most important for the resistance against that particular wear, a correlation analysis was performed together with a wear study using LOM, SEM and EDS. The results show that relations between cutting performance and mechanical properties and/or composition of the coatings can be established. The FEM-simulations suggested that the peak tool temperature was highest, ~750°C, for turning in 316L and lowest for turning in Sverker 21, ~300°C. Turning in cast iron resulted in temperatures around 500-550°C. A mechanism for the growth of the crater on inserts tested in stainless steel 316L is proposed. Wear due to thermo-mechanical load and adhesion are believed to be the dominating wear mechanisms. The performance of the tool showed a high correlation to the composition of the coatings, with a decreased tool life for higher Al-contents. The reason for this might lie in an increased brittleness of these coatings, accelerating formation of lateral cracks above the crater. Calculated ratios of bulk and shear modulus suggests an increased brittleness for higher Al-contents. A higher tendency to stick to the work piece material might also contribute to a decrease in tool life. An Increased Al-content could also drive the formation of c-AlN to h-AlN, causing even higher wear rates. The coatings with higher substrate bias showed an enhanced performance, even though the crack pattern was worsened for these variants. The reason for the enhanced performance seen for these variants might instead originate in an enhanced adhesion to the substrate. In the flank wear resistance test in Sverker 21 the Al-content proved to be important, with an improved performance for higher Al-contents. In contrast to the test in 316L, a change in bias or hardness had no effect on the performance in this test. Scratch patterns on the flank supports that an abrasive wear mechanism is present, but no correlation between hardness and tool life could be obtained. Either some other material property than hardness is of importance for the abrasive resistance in this test, or another wear mechanism, occurring simultaneously with abrasion, is the wear rate deciding. The second part of this thesis work was to evaluate the ability of a quantum mechanical computational method, density functional theory, to predict material properties. The method predicts the lattice parameters and bulk moduli in excellent agreement with experimental values. The method also well predicts other elastic properties, with results consistent with reference values. There seems to be a constant shift of about 50-100 GPa between the calculated elastic modulus and the experimentally measured values, probably originating in contributions from grain boundaries, texture, stresses and defects present in the real coatings, and possibly also in errors in the experimental method due to an influence from the substrate. The calculated hardness values did not follow the trend of an increased hardness for TiAlN compared to TiN, which is seen in experiments.

In thiswork we present the development of a method for the prediciton of finite temperature elastic and thermodynamic properties of cubic, non-magnetic unary and binary metals from first principles calculations. Vibrational, electronic and anharmonic contributions to the free energy are accounted for while magnetic effects are neglected. The method involves the construction of a free energy surface in volume/temperature space through the use of quasi-harmonic lattice dynamics. Additional strain energy calculations are performed and fit to the derived thermal expansion to present the temperature dependence of single crystal elastic constants. The methods are developed within the framework of density functional theory, lattice dynamics, and finite elasticity. The model is first developed for FCC aluminum and BCC tungsten which demonstrate the validity of the model as well as some of the limitations arising from the approximations made such as the effects of intrinsic anharmonicity. The same procedure is then applied to the B2 systems NiAl, RuAl and IrAl which are considred for high temperature applications. Overall there is excellent correlation between the calculated properties and experimentally tabulated values. Dynamic methods for the prediction of temperature dependent properties are also introduced and a groundwork is laid for future development of a robust method.

The design and production of efficient metal-hydrides for hydrogen storage is a long standing subject. Over the years, many different types of intermetallic hydride systems were studied and some of them came out to be operable. However, none of them meet all the storage criteria perfectly. In this study, total energies, hydrogen storage capacity and stability of AB (A = Al, Be, Cu, Fe, Ni, Sb, V and B = Ti) type intermetallics were investigated with the goal of spotting a potential hydrogen storage material. The relation between thermodynamic properties and the atomic and the electronic structure of hydrides are also pointed out. For this task, first principles pseudopotential method within the generalized gradient approximation (GGA) to density functional theory (DFT) was used. Calculations correctly predict experimentally determined structures except for CuTiH. Moreover, the atomic and cell parameter were found within the allowable error interval for DFT. In CuTi intermetallic, a structure having considerably lower formation energy than experimentally found mono-hydride was determined. This contradiction may be due to metastability of the experimental phase and high activation energy for the hydrogen movement in the system. It was found that AlTi and SbTi are not suitable candidates for hydrogen storage since their hydrides are too unstable. For the other intermetallic systems, the stability of the hydrides decreases in the order of VTi, CuTi, NiTi, BeTi, FeTi. For VTi, FeTi and NiTi, a change in metallic coordination around hydrogen from octahedron to tetrahedron is predicted when tetra-hydride (MTiH4) is formed. Additionally, at this composition, FeTi and NiTi have hydride structures with positive but near-zero formation energy which may be produced with appropriate alteration in chemical makeup or storage parameters. VTi is a promising intermetallic by means of storage capacity in that even VTiH6 is found to have negative formation energy but the hydrides are too stable which can be a problem during hydrogen desorption.

A candidate hydrogen storing material should have high storage capacity and fast dehydrogenation kinetics. On this basis, magnesium hydride (MgH2) is an outstanding compound with 7.66 wt % storage capacity, despite its slow dehydriding kinetics and high desorption temperature. Therefore in this study, bulk and surface alloys of Mg with improved hydrogen desorption characteristics were investigated. In this respect, formation energies of alloyed bulk MgH2 as well as the adsorption energies on alloyed magnesium (Mg) and MgH2 surface structures were calculated by total energy pseudopotential methods. Furthermore, the effect of substitutionally placed dopants on the dissociation of hydrogen molecule (H2) at the surface of Mg was studied via Molecular Dynamics (MD). The results displayed that 31 out of 32 selected dopants contributed to the decrease in formation energy of MgH2 within a range of ~ 37 kJ/mol-H2 where only Sr did not exhibit any such effect. The most favorable elements in this respect came out to be / P, K, Tl, Si, Sn, Ag, Pb, Au, Na, v Mo, Ge and In. Afterwards, a systematical study within adsorption characteristics of hydrogen on alloyed Mg surfaces (via dynamic calculations) as well as calculations regarding adsorption energies of the impurity elements were performed. Accordingly, Mo and Ni yielded lower adsorption energies / -9.2626 and -5.2995 eV for substitutionally alloyed surfaces, respectively. MD simulations presented that Co is found to have a splitting effect on H2 in 50 fs, where the first hydrogen atom is immediately adsorbed on Mg substrate. Finally, charge density distributions were realized to verify the distinguished effects of most 3d and 4d transition metals in terms of their catalyzer effects.