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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,801 records · Page 100
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DOE OSTI2015

First principles study of electronic structure for cubane-like and ring-shaped structures of M{sub 4}O{sub 4}, M{sub 4}S{sub 4} clusters (M = Mn, Fe, Co, Ni, Cu)

Spin-polarized DFT has been used to perform a comparative study of the geometric structures and electronic properties for isolated M{sub 4}X{sub 4} nano clusters between their two stable isomers - a planar rhombus-like 2D structure and a cubane-like 3D structure with M = Mn, Fe, Co, Ni, Cu ; X = O, S. These two structural patterns of the M{sub 4}X{sub 4} clusters are commonly found as building blocks in several poly-nuclear transition metal complexes in inorganic chemistry. The effects of the van der Waals corrections to the physical properties have been considered in the electronic structure calculations employing the empirical Grimme’s correction (DFT+D2). We report here an interesting trend in their relative structural stability - the isolated M{sub 4}O{sub 4} clusters prefer to stabilize more in the planar structure, while the cubane-like 3D structure is more favorable for most of the isolated M{sub 4}S{sub 4} clusters than their planar 2D counterparts. Our study reveals that this contrasting trend in the relative structural stability is expected to be driven by an interesting interplay between the s-d and p-d hybridization effects of the constituents’ valence electrons.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Strain effect on electronic structure and thermoelectric properties of orthorhombic SnSe: A first principles study

Strain effect on thermoelectricity of orthorhombic SnSe is studied using density function theory. The Seebeck coefficients are obtained by solving Boltzmann Transport equation (BTE) with interpolated band energies. As expected from the crystal structure, calculated Seebeck coefficients are highly anisotropic, and agree well with experiment. Changes in the Seebeck coefficients are presented, when strain is applied along b and c direction with strength from -3% to +3%, where influence by band gaps and band dispersions are significant. Moreover, for compressive strains, the sign change of Seebeck coefficients at particular direction suggests that the bipolar transport is possible for SnSe.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The electronic structure and thermoelectric properties of BiTl{sub 9}Te{sub 6} and SbTl{sub 9}Te{sub 6}: First-principles calculations

The electronic structure and thermoelectric properties of MTl{sub 9}Te{sub 6} (M = Bi, Sb) were studied using density functional theory and the semiclassical Boltzmann theory. It is found that the band gaps of BiTl{sub 9}Te{sub 6} and SbTl{sub 9}Te{sub 6} are equal to 0.59 eV and 0.72 eV, respectively. The relative large band gap and strong coupling between Sb s and Te p are helpful to the thermoelectric properties of SbTl{sub 9}Te{sub 6}. Near the bottom of the conduction bands, the number of band valleys of SbTl{sub 9}Te{sub 6} is four and is larger than that of BiTl{sub 9}Te{sub 6} (two band valleys), which will increase its Seebeck coefficient. Although BiTl{sub 9}Te{sub 6} has a larger electrical conductivity relative to relaxation time (σ/τ) along the z-direction than that of SbTl{sub 9}Te{sub 6}, the results show that the transport properties of SbTl{sub 9}Te{sub 6} are better than those of BiTl{sub 9}Te{sub 6} possibly due to its large Seebeck coefficient. The maximum value of power factor relative to relaxation time (S{sup 2}σ/τ) for SbTl{sub 9}Te{sub 6} reaches 4.30 × 10{sup 11 }W/K{sup 2} m s at 900 K, that is, originated from its relatively large Seebeck coefficient, suggesting its promising thermoelectric performance at high temperature.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Anisotropic lattice thermal conductivity in chiral tellurium from first principles

Using ab initio based calculations, we have calculated the intrinsic lattice thermal conductivity of chiral tellurium. We show that the interplay between the strong covalent intrachain and weak van der Waals interchain interactions gives rise to the phonon band gap between the lower and higher optical phonon branches. The underlying mechanism of the large anisotropy of the thermal conductivity is the anisotropy of the phonon group velocities and of the anharmonic interatomic force constants (IFCs), where large interchain anharmonic IFCs are associated with the lone electron pairs. We predict that tellurium has a large three-phonon scattering phase space that results in low thermal conductivity. The thermal conductivity anisotropy decreases under applied hydrostatic pressure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A self-consistent first-principle based approach to model carrier mobility in organic materials

Transport through thin organic amorphous films, utilized in OLEDs and OPVs, has been a challenge to model by using ab-initio methods. Charge carrier mobility depends strongly on the disorder strength and reorganization energy, both of which are significantly affected by the details in environment of each molecule. Here we present a multi-scale approach to describe carrier mobility in which the materials morphology is generated using DEPOSIT, a Monte Carlo based atomistic simulation approach, or, alternatively by molecular dynamics calculations performed with GROMACS. From this morphology we extract the material specific hopping rates, as well as the on-site energies using a fully self-consistent embedding approach to compute the electronic structure parameters, which are then used in an analytic expression for the carrier mobility. We apply this strategy to compute the carrier mobility for a set of widely studied molecules and obtain good agreement between experiment and theory varying over several orders of magnitude in the mobility without any freely adjustable parameters. The work focuses on the quantum mechanical step of the multi-scale workflow, explains the concept along with the recently published workflow optimization, which combines density functional with semi-empirical tight binding approaches. This is followed by discussion on the analytic formula and its agreement with established percolation fits as well as kinetic Monte Carlo numerical approaches. Finally, we skatch an unified multi-disciplinary approach that integrates materials science simulation and high performance computing, developed within EU project MMM@HPC.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First-principles study of the structural and dynamic properties of the liquid and amorphous Li–Si alloys

We have performed density functional theory calculations and ab initio molecular dynamics to investigate the structures and dynamic properties of the liquid and amorphous Li{sub x}Si alloys over a range of composition from x = 1.0 − 4.8. Our results show that Si atoms can form a variety of covalently bonded polyanions with diverse local bonding structures in the liquid alloys. Like in c-LiSi, Si atoms can form a continuous bond network in liquid Li{sub 1.0}Si at 1050 K, while it gradually disintegrates into many smaller Si polyanions as the Li content increases in the alloys. The average sizes of Si polyanions in these liquid alloys were found to be relatively larger than those in their crystalline counterparts, which can even persist in the highly lithiated Li{sub 4.81}Si alloy at 1500 K. Our results also show that amorphous Li{sub x}Si alloys have similar local bonding structures but a largely increased short-range order as compared to their liquid counterparts. The differences between the average coordination number of each atomic pair in amorphous solids and that in the liquids are less than 1.1. Furthermore, our calculations reveal that Li and Si atoms can exhibit very distinct dynamic behaviors in the liquids and their diffusivities appear to be largely dependent on the chemical composition of the alloys. The diffusivity of Li was found to increase with the Li content in the alloys primarily because of the reduced interactions between Li and Si atoms, while the Si diffusivity also increases due to the gradual disintegration of the strongly interconnected Si bond network. The diffusivity of Li in amorphous Li{sub x}Si was predicted to lie in the range between 10{sup −7} and 10{sup −9} cm{sup 2}/s at 300 K, which is more than 20-fold larger than that of Si over the composition range considered. Our calculations further show that the diffusivities of both Li and Si can increase by two orders of magnitude as x increases from 1.0 to 3.57 in amorphous Li{sub x}Si, indicating a more profound dependence on the alloy composition than those in the liquid state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Anisotropy induced Kondo splitting in a mechanically stretched molecular junction: A first-principles based study

The magnetic anisotropy and Kondo phenomena in a mechanically stretched magnetic molecular junction are investigated by combining the density functional theory (DFT) and hierarchical equations of motion (HEOM) approach. The system is comprised of a magnetic complex Co(tpy–SH){sub 2} sandwiched between adjacent gold electrodes, which is mechanically stretched in experiments done by Parks et al. [Science 328, 1370 (2010)]. The electronic structure and mechanical property of the stretched system are investigated via the DFT calculations. The HEOM approach is then employed to characterize the Kondo resonance features, based on the Anderson impurity model parameterized from the DFT results. It is confirmed that the ground state prefers the S = 1 local spin state. The structural properties, the magnetic anisotropy, and corresponding Kondo peak splitting in the axial stretching process are systematically evaluated. The results reveal that the strong electron correlations and the local magnetic properties of the molecule magnet are very sensitive to structural distortion. This work demonstrates that the combined DFT+HEOM approach could be useful in understanding and designing mechanically controlled molecular junctions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Origin of the performances degradation of two-dimensional-based metal-oxide-semiconductor field effect transistors in the sub-10 nm regime: A first-principles study

The impact of the scaling of the channel length on the performances of metal-oxide-semiconductor field effect transistors, based on two-dimensional (2D) channel materials, is theoretically investigated, using density functional theory combined with the non-equilibrium Green's function method. It is found that the scaling of the channel length below 10 nm leads to strong device performance degradations. Our simulations reveal that this degradation is essentially due to the tunneling current flowing between the source and the drain in these aggressively scaled devices. It is shown that this electron tunneling process is modulated by the effective mass of the 2D channel material, and sets the limit of the scaling in future transistor designs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A generalized Poisson solver for first-principles device simulations

Electronic structure calculations of atomistic systems based on density functional theory involve solving the Poisson equation. In this paper, we present a plane-wave based algorithm for solving the generalized Poisson equation subject to periodic or homogeneous Neumann conditions on the boundaries of the simulation cell and Dirichlet type conditions imposed at arbitrary subdomains. In this way, source, drain, and gate voltages can be imposed across atomistic models of electronic devices. Dirichlet conditions are enforced as constraints in a variational framework giving rise to a saddle point problem. The resulting system of equations is then solved using a stationary iterative method in which the generalized Poisson operator is preconditioned with the standard Laplace operator. The solver can make use of any sufficiently smooth function modelling the dielectric constant, including density dependent dielectric continuum models. For all the boundary conditions, consistent derivatives are available and molecular dynamics simulations can be performed. The convergence behaviour of the scheme is investigated and its capabilities are demonstrated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Improved thermoelectric performance of (Fe,Co)Sb{sub 3}-type skutterudites from first-principles

Skutterudite materials have been considered as promising thermoelectric candidates due to intrinsically good electrical conductivity and tailorable thermal conductivity. Options for improving thermal-to-electrical conversion efficiency include identifying novel materials, adding filler atoms, and substitutional dopants. Incorporating filler or substitutional dopant atoms in the skutterudite compounds can enhance phonon scattering, resulting in reduction of thermal conductivity, as well as improving electrical conductivity. The structures, electronic properties, and thermal properties of double-filled Ca{sub 0.5}Ce{sub 0.5}Fe{sub 4}Sb{sub 12} and Co{sub 4}Sb{sub 12−2x}Te{sub x}Ge{sub x} compounds (x = 0, 0.5, 1, 2, 3, and 6) have been studied using density functional theory-based calculations. Both Ca/Ce filler atoms in FeSb{sub 3} and Te/Ge substitution in CoSb{sub 3} cause a decrease in lattice constant for the compounds. As Te/Ge substitution concentration increases, lattice constant decreases and structural distortion of pnictogen rings in the compounds occurs. This indicates a break in cubic symmetry of the structure. The presence of fillers and substitutions cause an increase in electrical conductivity and a gradual decrease in electronic band gap. A transition from direct to indirect band-gap semiconducting behavior is found at x = 3. Phonon density of states for both compounds indicate phonon band broadening by the incorporation of fillers and substitutional atoms. Both systems are also assumed to have acoustic-mode-dominated lattice thermal conductivity. For the Co{sub 4}Sb{sub 12−2x}Te{sub x}Ge{sub x} compounds, x = 3 has the lowest phonon dispersion gradient and lattice thermal conductivity, agreeing well with experimental measurements. Our results exhibit the improvement of thermoelectric properties of skutterudite compounds through fillers and substitutional doping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First principles studies on the impact of point defects on the phase stability of (Al{sub x}Cr{sub 1−x}){sub 2}O{sub 3} solid solutions

Density Functional Theory applying the generalised gradient approximation is used to study the phase stability of (Al{sub x}Cr{sub 1−x}){sub 2}O{sub 3} solid solutions in the context of physical vapour deposition (PVD). Our results show that the energy of formation for the hexagonal α phase is lower than for the metastable cubic γ and B1-like phases–independent of the Al content x. Even though this suggests higher stability of the α phase, its synthesis by physical vapour deposition is difficult for temperatures below 800 °C. Aluminium oxide and Al-rich oxides typically exhibit a multi-phased, cubic-dominated structure. Using a model system of (Al{sub 0.69}Cr{sub 0.31}){sub 2}O{sub 3} which experimentally yields larger fractions of the desired hexagonal α phase, we show that point defects strongly influence the energetic relationships. Since defects and in particular point defects, are unavoidably present in PVD coatings, they are important factors and can strongly influence the stability regions. We explicitly show that defects with low formation energies (e.g. metal Frenkel pairs) are strongly preferred in the cubic phases, hence a reasonable factor contributing to the observed thermodynamically anomalous phase composition.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles studies of the atomic, elastic and vibrational properties of BaNbO{sub 3}

We have employed the ab initio pseudopotential method, within a generalized gradient approximation of the density functional theory, a linear response approach, to determine atomic, elastic and lattice dynamical properties of the BaNbO{sub 3}. The obtained structural and elastic parameter are in good agreement with previous experimental and theoretical results. The analysis of B/G ratio indicates that BaNbO{sub 3} is a ductile material. The calculated lattice dynamical properties compared with phonon spectrum of the similar structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Heterostructures of phosphorene and transition metal dichalcogenides for excitonic solar cells: A first-principles study

Using the many-body perturbation GW theory, we study the quasiparticle conduction-band offsets of phosphorene, a two-dimensional atomic layer of black phosphorus, and transition-metal dichalcogenides (TMDs). The calculated large exciton binding energies of phosphorene and TMDs indicate that their type-II heterostructures are suitable for excitonic thin-film solar cell applications. Our results show that these heterojunctions have a potential maximum power conversion efficiency of up to 12%, which can be further enhanced up to 20% by strain engineering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Determination of the electronic, dielectric, and optical properties of sillenite Bi{sub 12}TiO{sub 20} and perovskite-like Bi{sub 4}Ti{sub 3}O{sub 12} materials from hybrid first-principle calculations

Density functional theory calculation was conducted to determine the optoelectronic properties of bismuth titanate sillenite (Bi{sub 12}TiO{sub 20}) and perovskite-like (Bi{sub 4}Ti{sub 3}O{sub 12}) structures. The lattice parameters were experimentally obtained from Rietveld analysis. The density functional perturbation theory approach was used with the standard Perdew–Burke–Ernzerhof functional and screened Coulomb hybrid Heyd–Scuseria–Ernzerhof functional to investigate the electronic structure and absorption coefficient. Both compounds have good carrier transport properties, low effective hole and electron masses, high dielectric constant, and low exciton binding energy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Properties of samarium nitride: First principle calculations

The tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA has been used to calculate structural and electronic properties of samarium nitride at ambient and high pressure. As a function of volume, the total energy is evaluated. The phase transition pressure for this compound was found to be 6.0 GPa and it is transform from NaCl to CsCl-type structure. Apart from this, the lattice parameter (a{sub 0}), bulk modulus (B{sub 0}), band structure (BS) and density of states (DOS) are calculated. From energy band diagram we observed metallic behaviour in SmN compound. The values of equilibrium lattice constants and bulk modulus are agreed well with the available data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First-principles study of electronic states in Mg{sub 2}C

The electronic properties of magnesium methanide Mg{sub 2}C are reported. Investigations are carried out applying the linear combination of atomic orbitals method. After settling ground state of the antifluorite structure the electronic band structure calculations are undertaken using PBE-GGA exchange and correlation functional and PBE0 hybrid functional. All calculations show that Mg{sub 2}C has indirect band gap with conduction band minimum at Γ and valence band maximum at the X point. The values of band gap from PBE and the PBE0 are 2.05 and 3.46 eV respectively. In comparison to beryllium methanide the electronic bands are flat suggesting ionic nature of bonding.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First-principles study of electronic and elastic properties of LuAl{sub 3}

A systematic theoretical study of electronic structure of rare earth intermetallic LuAl{sub 3} has been carried out using full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT) within the generalized gradient approximation(GGA) for exchange and correlation potential. The ground state properties such as lattice constant (a{sub o}), bulk modulus (B) and pressure derivative of bulk modulus (B′) were evaluated. LuAl{sub 3} has the cubic AuCu{sub 3} type crystal structure. The electronic properties of this compound have been analyzed quantatively from band structure and DOS. It is clear from band structure that this compound is metallic in nature. The calculated elastic constants infer that this compound is mechanically stable.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A first principle study of the pressure dependent elastic properties of monazite LaPO{sub 4}

DFT based ab-initio simulations have been performed to study the effect of pressure on the elastic properties of monazite LaPO{sub 4} which is a promising host material for immobilization of high level nuclear waste. The phase is found to be stable up to 30 GPa. The calculated polycrystalline bulk, shear and Young moduli show an increasing trend as a function of pressure. The ductility and anisotropy in shear modulus of the material have been found to increase with pressure; whilethe bulk modulus anisotropy decreases with pressure.

36 MATERIALS SCIENCE↗

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