Search

This research was for the fundamental understanding and theoretical design of novel nanostructured semiconductor materials with a potential impact in the areas of thermoelectrics, solar energy, and the role of impurities and embedded nanoparticles. The most challenging phase of the theoretical research was attacked so as to help understand the underlying fundamental electronic structures, find the key mechanism, and finally suggest design principles for desirable thermoelectric and solar energy materials. This involves developing new insights via theoretical calculations for interfaces, nanostructures, and defect structures including important refinements for electronic structures such as screened exchange (sX)-LDA.

The functionality of NiO interfacial layers in enhancing bulk heterojunction (BHJ) organic photovoltaic (OPV) cell performance is investigated by integrated characterization of the electrical properties, microstructure, electronic structure, and optical properties of thin NiO films grown on glass/ITO electrodes. These NiO layers are found to be advantageous in BHJ OPV applications due to favorable energy band levels, interface passivation, p-type character, crystallinity smooth surfaces, and optical transparency. The NiO overlayers are fabricated via pulsed-laser deposition and found to have a work function of 5.3 eV. They are investigated by both topographic and conductive atomic force microscopy and shown to passivate interfacial charge traps. The films also have an average optical transparency of 80% in the visible range, crucial for efficient OPV function, and have a near-stoichiometric Ni:O surface composition. By grazing-incidence X-ray diffraction, the NiO thin films are shown to grow preferentially in the (111) direction and to have the fcc NaCl crystal structure. Diodes of p-n structure and first-principles electronic structure calculations indicate that the NiO interlayer is preferentially conductive to holes, with a lower hole charge carrier effective mass versus that of electrons. Finally, the implications of these attributes in advancing efficiencies for state-of-the-art OPV systems--in particular, improving the open circuit voltage (VOC)--are discussed., Published in Chemistry Materials v23, p2218 2226, 2011. Supported in part by NSF-ECS-0609064, DE-SC0001059.

To fulfill the great potential of intermetallic alloys for high temperature structural applications, it is essential to understand the mechanisms controlling their mechanical behavior on the microscopic level. We focus on the mechanical behavior of homogeneous intermetallics with LI 2 and B2 structures, two-phase `ly' alloys, and bcc transition metals and their alloys. Based on highly-accurate total energy calculations and large-scale Peierls-Nabarro and atomistic modeling, we investigated the relation between electronic structure, dislocation properties, and brittle-ductile behavior in these metals and alloys. We demonstrated that Ir- and Rh-based alloys are intrinsically brittle and established the dislocation structure in Ir and Rh-based LI2 alloys. We determined the temperature dependence of the lattice misfit parameter in y/y' superalloys, and established its simple relation with the alloy phase diagram. We studied the structure of dislocations in bcc Nb, Ta, Mo and W alloys, analyzed the effect of alloying with 3d and Sd transition metal elements on dislocation energetics, and established the electronic origins of the solid solution softening/hardening phenomena. We investigated the effect of topologically close packed phase formation on the solubility of interstitial impurities in Cr- and Mo-based alloys and demonstrated their effect in preventing the formation of embrittling phases. We showed that Co- and Y-based B2 intermetallics are intrinsically ductile, established the dislocation structure in Co-based alloys, analyzed the origins of their unusual mechanical behavior, and identified the microscopic origins of the yield stress anomaly.

To fulfill the great potential of intermetallic alloys for use in high temperature structural applications, it is necessary to understand on the microscopic level the mechanisms controlling their mechanical behavior, including such key phenomena as dislocation structure and mobility, crack blunting and propagation, the role and the effect of alloying additions: while they have been characterized by mesoscopic length and energy scales, they are determined on the microscopic level by the electronic structure.

The electronic band structure of artificially ordered superlattice alloys (SLA) of Bi and Sb, prepared by alternately depositing thin layers of Bi and Sb as a superlattice, could be modified from a semimetal to a semiconductor by changing the period of the superlattice. This new SLA opens a possibility to engineer an alloy material to get more desirable thermoelectric properties, because a thermal conductivity reduction, due to an increase in phonon scattering from the superlattice interfaces, is expected; certain electronic properties are altered at the same time. To check the performance of an in-plane Bi/BiSb cooling device, anisotropy in the films was investigated. Bi showed a strongly anisotropic and field-direction-dependent magneto-Seebeck effect. The magneto-transport properties of Bi(1-x)Sb(sub x) films and Bi/CdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis and multicarrier fitting to the magnetic-field-dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. Using the local density relativistic full-potential linear muffin-tin orbital method, we found that the internal displacement changes the Bi electronic structure from a metal to a semimetal and that an increase of the trigonal shear angle can lead to a semimetal-semiconductor transition in Bi. Post annealing of Bi films near its melting point improved the magnetoresistance ratios drastically, which can be ascribed to the electron mobility enhancement. The calculation of the electronic structure of Bi2Te3 suggested that the quasi-two dimensional crystal structure of Bi2Te3 gives a large, but finite, anisotropy in the effective mass which enhances the thermoelectric figure of merit., Prepared in collaboration with the Naval Research Laboratory, Washington, DC.

We have fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using molecular-beam epitaxy. We observe that by changing the superlattice period thickness, the electronic structure can be “tuned” from a semimetal, through zero gap, to a narrow-gap semiconductor. These unique properties, which are distinct from those in random alloys, are believed to be a consequence of ordered atomic configurations.

In high-temperature intermetallics, dislocations play an especially important role for understanding mechanisms of their deformation and fracture behavior and anomalous mechanical response which are still far from being well understood and remain a challenge to theoretical explanation. Progress in this complex area requires understanding such key phenomena as dislocation structure and mobility, and crack blunting and propagation: while they have been characterized by mesoscopic length and energy scales, they are determined on the microscopic level by the electronic structure which has, in many cases to be carefully taken into account using abinitio techniques. The most important and challenging component of our research objectives is to bridge the gap between a microscopic quantum-mechanical description of the chemical bonding and the mesoscopic phenomena which govern the mechanical response. It is highly desirable that theory describes dislocation core structure in a compact and physically transparent form which would provide a natural link with larger length theories of dislocation motion processes (kink formation, propagation and cross-slip) as well as to study how dislocations, as typical extended defects, influence functional properties.

The objective of this research was to investigate fundamental aspects of the fracture and deformation behavior of ordered intermetallic aerospace alloys on the basis of the ab-initio determination of the parameters needed for further (1) model theoretical, (2) band structure and (3) chemical bonding analyses. The research was targeted at investigating the microscopic mechanisms governing the deformation and fracture behavior of intermetallic alloys in order to contribute to the development of a fundamental basis for computer-aided alloys design. Progress in this complex area required understanding such key phenomena as dislocation structure and mobility, and crack blunting and propagation: while they have been characterized by mesoscopic length and energy scales, they are determined on the microscopic level by the electronic structure. Thus, this most important and challenging component of our research was to bridge the gap between a microscopic quantum-mechanical description of the chemical bonding and the mesoscopic phenomena which govern the mechanical response of intermetallics. Especially for intermetallic compounds and metals of interest. we concentrated on (1) fundamental aspects of dislocation structure and mobility; (2) solid-solution hardening problems in the context of the fundamental aspects of impurity dislocation interaction theory; (3) the electronic structure of dislocations and related properties of real crystals. These important. complex problems required the use of a hierarchy of methods. We focussed on the application and further development of both state-of-the-art band structure and real-space large scale cluster electronic structure methods which provide parameters needed for atomistic simulations and model mesoscopic simulations. In this way, we greatly amplified the impact of our research based on our quantum-mechanical electronic structure simulations.

The primary goal of the proposed research was to study and develop alloying concepts for understanding intermetallic alloys as derived from a first principles quantum mechanical approach. Thus, a major part of our effort was to study and determine ductilizing effects of alloying elements in several intermetallic compounds and to work closely with experimental efforts to evaluate the applicability of the theoretical approach to alloy design. Specifically, highly precise all-electron quantum mechanical electronic structure methods were applied to the study of a number of materials problems in order to obtain from first principles information of relevance to alloy stability on the design of structural materials. Using our recently developed state-of-the-art all-electron self-consistent total energy methods which gives precise solutions of the local density equations, fundamental information was sought about the structural and electronic properties of these alloys in order to predict stable and metastable phases and how alloying effects bonding, crystal ordering and crystal symmetry. The first principles approach used here addresses questions of a metallurgical nature, such as phase stability, crystal structure, equilibrium lattice constants and mechanical properties including the effect of atomic relaxation.

The primary goal of the proposed research is to study and develop alloying concepts for understanding intermetallic alloys as derived from a first principles quantum mechanical approach. Thus, a major part of our effort will be to study and determine ductilizing effects of alloying elements in several intermetallic compounds and to work closely with experimental efforts to evaluate the applicability of the theoretical approach to alloy design. Specifically, highly precise all-electron quantum mechanical electronic structure methods will be applied to the study of a number of materials problems in order to obtain from first principles information of relevance to alloy stability and the design of structural materials. Using our recently developed state-of-the-art all-electron self-consistent total energy methods which give precise solutions of the local density equations, fundamental information will be sought about the structural and electronic properties of these alloys in order to predict stable and metastable phases and how alloying affects bonding, crystal ordering and crystal symmetry. The first principles approach used here will address questions of a metallurgical nature, such as phase stability, crystal structure, equilibrium lattice constants, and mechanical properties including the effect of atomic relaxation. The proposed research seeks to explore a new capability for modeling materials and their properties on the computer which have not yet been made in practice.

A new technique for determining the electronic structure of bulk solids has been developed by solving Poisson's equation for a general charge density and potential. All contributions to the potential are completely taken into account in the Hamiltonian matrix elements. The high accuracy of this approach allows us to go beyond traditional treatments and to consider the total energy which allows the accurate determination of structural information (lattice parameters, force constants, etc.) and total energy determinations of the lowest energy phase. The problems treated during this period include: (i) excited state properties of rare-earths and their compounds, (ii) room temperature superconductivity in metallic hydrogen under high pressure and the nature of the molecular to metal phase transition in hydrogen under high pressure, (iii) total energy theory of mixed valence, (iv) electronic structure and superconductivity in the high Tc A15 compounds, and (v) superconductivity and magnetism of C15 intermetallic compounds.

Our primary goal is to study and develop alloying concepts for understanding intermetallic alloys as derived from a highly precise quantum mechanical approach. Thus, a major part of our effort is to study and determine ductilizing effects and to work closely with experimental efforts to evaluate the applicability of the theoretical approach to alloy design. Specifically we have studied a number of materials problems to obtain first principles information of relevance to alloy stability and design of structural materials. Fundamental information has been obtained about the structural and electronic properties in order to predict stable and metastable phases and how alloying affects bonding, crystal order and crystal symmetry. Our approach addresses questions of a metallurgical nature, such as phase stability, crystal structure, equilibrium lattice constants, and mechanical properties. For the ordered compounds, tailoring the chemistry of these alloys to obtain higher symmetry (and thus more ductile) crystalline phases is extremely important. The research seeks to explore a new capability for modelling materials and their properties on the computer which have not yet been made in practice.

The electronic interactions responsible for the observed electronically induced phase transformations in several important classes of materials were investigated theoretically in close collaboration with experimental programs at other institutions. These include destruction of superconductivity and onset of a normal magnetic state at low temperatures in ternary rare-earth superconducting compounds and the possible occurrence of a new mixed phase, the origin of the phase transition to the superconducting phase in the Chevrel compounds, the possibility of excitonic superconductivity in CuCl, the origin of phonon. Keywords: Copper; Chloride.

Our recent development of self-consistent local (spin) density energy band approaches have provided a powerful theoretical/computational tool for determining the electronic structure and properties of complex materials. The importance of charge transfer between constituent atom species has been demonstrated as has its inclusion by means of accurate self-consistent solutions. Some examples of areas in which progress has been achieved include: the high field superconductivity in Chevrel phase compounds; the unusual magnetic and superconducting properties of some C15 compounds including a better understanding of their relationship between the electronic, lattice and superconducting properties of these materials; self-consistent energy band calculations including all electrons and all atoms in the 16 atoms per unit cell of the linear chain transition metal trichalcogenide, TaSe3; and a detailed assessment of theoretical determination of the electron phonon coupling parameter in metals and intermetallic compounds.

The electronic interactions responsible for the observed electronically induced phase transformations in several important classes of materials were investigated theoretically in close collaboration with experimental programs at other institutions. These include the high field superconductivity of Chevrel phase compounds and the first theoretical demonstration that the Jaccarino-Peter mechanism is responsible for the large increase in the upper critical field when large concentrations of Eu magnetic impurities are added to SnMo6S8, an extensive study of the unusual magnetic and superconducting properties of some C15 compounds, self-consistent energy band calculations including all electrons and all atoms in the 16 atoms per unit cell of the linear chain transition metal trichalcogenide, TaSe3, and the first demonstration of the onset of a metamagnetic transition in Pd Metal. (Author)

The electronic interactions responsible for the observed electronically induced phase transformations in several important classes of materials are being investigated theoretically in close collaboration with experimental programs at other institutions. These include destruction of superconductivity and onset of a normal magnetic state at low temperatures in ternary rare-earth superconducting compounds, charge density waves in layered transition metal dichalcogenides, origin of phonon anomalies in Pd and Pt metals, local density formalism for electronic bands, and metal-insulator phase transition in Nb02. (Author)

A tunable laser radar system was designed to detect the water vapor content in the atmosphere by remote sensing. The basic physical process involved is the molecular resonance adsorption of water molecules. The method proposed is the differential adsorption method which compares the backscattering signal for a wavelength off of a water adsorption line to the signal for a wavelength on a water adsorption line. (Author)

The electronic interactions responsible for the unusual properties of several important classes of materials (including the highly anisotropic layered dichalcogenides, and network and cage structure materials and pseudobinary alloys) have been investigated experimentally and theoretically. The unique ability of NMR to probe the local electronic properties of the various constituents of an intermetallic compound or alloy has provided important new information including correlations with observed changes in superconducting transition temperatures. Novel theoretical models (including relativistic effects) are found to yield energy band structures, Fermi surfaces, transport properties, charge and spin densities, generalized magnetic susceptibilities, and optical properties in very good agreement with experimental determinations of these observable phenomena. (Author)

The purpose of this project has been to investigate theoretically the origin of the magnetism observed in magnetic materials and to obtain a detailed understanding of the unusual magnetic, electric and optical properties of these systems. Quantum theoretical approaches to the study of the electronic properties of magnetic materials and of solids in general have traditionally followed somewhat different views, the localized model and the itinerant model. This research follows the philosophy that a fundamental description of the physics of electrons in solids is found in an approach which lies somewhere in between these two extremes. The results of these theoretical calculations are closely correlated with data obtained from experiments such as Mossbauer effect, NMR, neutron diffraction and Fermi surface studies. The investigations fall into three main areas, studies of the electronic band structure of magnetic materials like the actinide metals and their compounds, studies of covalency and superexchange in antiferromagnetic insulating materials involving 3d transition metal, 4f rare-earth, and 5f actinide metal ions, and development of a tight binding method in order to include into one scheme the principal advantages developed. (Author)

During the contract period important improvements in the high field magnet facility were made. Continuous fields of 255 kilogauss in a very small volume, 225 kilogauss in a 1-1/4 inch bore and 205 kilogauss in a 2 inch bore are available. These are the highest continuous fields produced anywhere. More reliable superconducting magnets and high field, larger volume pulsed field magnets have also been developed. The Laboratory's research program in solid state and low temperature physics has improved understanding of the electrical, magnetic, optical and acoustic properties of matter. New and more sensitive techniques for magneto-optical studies such as electro-reflectance and piezo-reflectance and the use of laser sources have been developed and used to gain new information on the band structure of semiconductors. Studies of the new type II high field superconductors by a variety of techniques have revealed the factors which limit the usefulness of these materials for high field magnets. The magnetic properties of many solids have been explored by high field magnetization apparatus giving temperatures down to three millidegrees above absolute zero has been developed. Theoretical studies of band structure, crystal field theory, superconductivity and other topics have been carried on in close conjunction with the experimental program. (Author)