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Results for “density functional calculation”

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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Insight into the optoelectronic properties of designed solar cells efficient tetrahydroquinoline dye-sensitizers on TiO 2 (101) surface: first principles approach

Seven ‘lead’ dye-sensitizers from Tetrahydroquinoline (THQ) family were proposed and designed based on the structural attributes via quantitative-structure property relationship (QSPR) modeling. They were screened rationally through different computational approaches to explore their potential applications as photosensitizers for dye-sensitized solar cells (DSSCs). Compelling photophysical properties such as electron injection driving force, electron injection time, and dye regeneration were studied for the isolated dyes under the DFT and TD-DFT frameworks. Index of spatial extent (S, D, and Δq), the strength of charge transfer and separation along with the charge transfer process is explored. First principle approach including van der Waals density functional calculation of dye@TiO 2 interface indicates that all of the designed dyes have optimal interfacial behavior. Bader charge analysis, partial density of state (PDOS), charge density and electrostatic potential difference calculation confirms that THQ7 and THQ9 are the most efficient dye-sensitizers. The other five designed dyes also possess the required properties to emerge as effective dye-sensitizers potentially better than those already utilized.

14 SOLAR ENERGY↗
DOE OSTI2018

Structure, Elastic Constants and XRD Spectra of Extended Solids under High Pressure

We present results of evolutionary simulations based on density functional calculations of a potentially new type of energetic materials called extended solids: P-N and N-H. High-density structures with covalent bonds generated using variable and fixed concentration methods were analysed in terms of thermo-dynamical stability and agreement with experimental X-ray diffraction (XRD) spectra. X-ray diffraction spectra were calculated using a virtual diffraction algorithm that computes kinematic diffraction intensity in three-dimensional reciprocal space before being reduced to a two-theta line profile. Calculated XRD patterns were used to search for the structure of extended solids present at experimental pressures by optimizing data according to experimental XRD peak position, peak intensity and theoretically calculated enthalpy. Elastic constants has been calculated for thermodynamically stable structures of P-N system.

Batyrev, I. G.↗
DOE OSTI2017

Photo-induced reduction of Cr{sup 6+} by the hybrid systems “Cu{sup II} complex with Schiff base and TiO{sub 2}”: dependence on irradiation wavelength

The synthesis of novel Cu{sup II} complexes with a Schiff base obtained by condensation of salicylaldehyde and an L-aspartic acid ester are described. The physicochemical properties of the complexes were compared with those of related Cu{sup II} complexes obtained earlier. All the complexes studied were characterized by elemental analysis as well as by IR, UV-Vis, and EPR spectroscopies. The activity of the complexes and their hybrid systems (HS) with TiO{sub 2} in visible-light-driven photocatalysis in organic solvents was investigated. After irradiation with visible light, the complexes and corresponding HS reduce Cr{sup 6+} to Cr{sup 3+} more efficiently than bare TiO{sub 2}. To determine the molecular orbital compositions and energies and to explain the electronic spectra and redox properties of the systems studied, density functional calculations of the optimized structures of representative model complexes were performed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Ultrafast optical melting of trimer superstructure in layered 1T′-TaTe2

Abstract Quasi-two-dimensional transition-metal dichalcogenides are a key platform for exploring emergent nanoscale phenomena arising from complex interactions. Access to the underlying degrees-of-freedom on their natural time scales motivates the use of advanced ultrafast probes sensitive to self-organised atomic-scale patterns. Here, we report the ultrafast investigation of TaTe 2 , which exhibits unique charge and lattice trimer order characterised by a transition upon cooling from stripe-like chains into a (3 × 3) superstructure of trimer clusters. Utilising MeV-scale ultrafast electron diffraction, we capture the photo-induced TaTe 2 structural dynamics – exposing a rapid ≈ 1.4 ps melting of its low-temperature ordered state followed by recovery via thermalisation into a hot cluster superstructure. Density-functional calculations indicate that the initial quench is triggered by intra-trimer Ta charge transfer which destabilises the clusters, unlike melting of charge density waves in other TaX 2 compounds. Our work paves the way for further exploration and ultimately rapid optical and electronic manipulation of trimer superstructures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Qualitative breakdown of the unrestricted Hartree-Fock energy

The stretching of closed-shell molecules is a qualitative problem for restricted Hartree-Fock that is usually circumvented by the use of unrestricted Hartree-Fock (UHF). UHF is well known to break the spin symmetry at the Coulson-Fischer point, leading to a discontinuous derivative in the potential energy surface and incorrect spin density. However, this is generally not considered as a major drawback. In this work, we present a set of two electron molecules which magnify the problem of symmetry breaking and lead to drastically incorrect potential energy surfaces with UHF. These molecules also fail with unrestricted density-functional calculations where a functional such as B3LYP gives both symmetry breaking and an unphysically low energy due to the delocalization error. The implications for density functional theory are also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

The deep-acceptor nature of the chalcogen vacancies in 2D transition-metal dichalcogenides

Abstract Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/−1) and (−1/−2) transition levels occur in the gap, leading to paramagnetic charge states S = 1 / 2 and S = 1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Core Level Shifts of Hydrogenated Pyridinic and Pyrrolic Nitrogen in the Nitrogen-Containing Graphene-Based Electrocatalysts: In-Plane vs Edge Defects

A combination of N 1s X-ray photoelectron spectroscopy (XPS) and first principles calculations of nitrogen-containing model electrocatalysts was used to elucidate the nature of the nitrogen defects that contribute to the binding energy (BE) range of the N 1s XPS spectra of these materials above ~400 eV. Experimental core level shifts were obtained for a set of model materials, namely N-doped carbon nanospheres, Fe–N–carbon nanospheres, polypyrrole, polypyridine, and pyridinium chloride, and were compared to the shifts calculated using density functional theory. The results confirm that the broad peak positioned at ~400.7 eV in the N 1s XPS spectra of N-containing catalysts, which is typically assigned to pyrrolic nitrogen, contains contributions from other hydrogenated nitrogen species such as hydrogenated pyridinic functionalities. Namely, N 1s BEs of hydrogenated pyridinic-N and pyrrolic-N were calculated as 400.6 and 400.7 eV, respectively, using the Perdew–Burke–Ernzerhof exchange-correlation functional. A special emphasis was placed on the study of the differences in the XPS imprint of N-containing defects that are situated in the plane and on the edges of the graphene sheet. Density functional theory calculations for BEs of the N 1s of in-plane and edge defects show that hydrogenated N defects are more sensitive to the change in the chemical environment in the carbon matrix than the non-hydrogenated N defects. In conclusion, calculations also show that edge-hydrogenated pyridinic-N and pyrrolic-N defects only contribute to the N 1s XPS peak located at ~400.7 eV if the graphene edges are oxygenated or terminated with bare carbon atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Ab Initio Study of Structural, Electronic, and Magnetic Properties of A{sub 1−x}{sup III}Mn{sub x}B{sup VI}: In{sub 1−x}Mn{sub x}S-Diluted Magnetic Semiconductor

First-principles density functional calculations on the new class of diluted magnetic semiconductor A{sub 1−x}{sup III}Mn{sub x}B{sup VI} In{sub 1−x}Mn{sub x}S for x = 0.25 and 0.5 are investigated to study the structural, electronic, and magnetic properties, employing the full-potential linearized augmented plane wave method. Electronic band structures and density of states revealed a half-metallic character of In{sub 1−x}Mn{sub x}S and show the stability of anti-ferromagnetic states as compared with ferromagnetic states. The calculated exchange constants J{sub dd} are in good agreement with experimental and theoretical results on magnetic properties of single crystalline A{sub 1−x}{sup III}Mn{sub x}B{sup VI} in the anti-ferromagnetic case. Our predicated calculations on the s,p-d exchange constants N{sub 0}α and N{sub 0}β show that they are lower than in A{sub 1−x}{sup II}Mn{sub x}B{sup VI} DMS. The local environment is found tetrahedral as in the II–VI DMS and other (III,Mn) VI compounds. The total magnetic moment for In{sub 1−x}Mn{sub x}S for different concentrations is in accordance with the exact value 5 μ{sub B} and comes mainly from impurity Mn. The local magnetic moments of Mn ions are reduced from their free space charges values due to the p-d hybridization which produces small magnetic moments on the nonmagnetic In and S sites. The Curie temperature of In{sub 1−x}Mn{sub x}S is calculated within the mean field approximation and compared with other DMS systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electronic structure and the origin of the high ordering temperature in SrRu 2 O 6

SrRu 2 O 6 is a layered honeycomb-lattice material with an extraordinarily high magnetic ordering temperature. We investigated this material using density functional calculations. We find that the energy scales for moment formation and ordering are similar and high. Additionally, we find that the magnetic anisotropy is high and favors moments oriented along the c axis. This provides an explanation for the exceptionally high ordering temperature. Finally, the compound is found to be semiconducting at the bare density functional level, even without magnetic order. Lastly, we discuss experimental consequences of this scenario for the high ordering temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic structure and thermoelectric properties of pnictogen-substituted ASn{sub 1.5}Te{sub 1.5} (A = Co, Rh, Ir) skutterudites

Substituting group 14 and 16 elements on the pnictogen site in the skutterudite structure yields a class of valence-precise ternary AX{sub 1.5}Y{sub 1.5} compounds (A = Co, Rh, Ir, X = Sn, Ge, and Y = S, Se, Te), in which X and Y form an ordered sub-structure. Compared with unfilled binary skutterudites, pnictogen-substituted phases exhibit extremely low lattice thermal conductivity due to increased structural complexity. Here, we investigate the role of the transition metal species in determining the electronic structure and transport properties of ASn{sub 1.5}Te{sub 1.5} compounds with A = Co, Rh, Ir. Density functional calculations using fully ordered structures reveal semiconducting behavior in all three compounds, with the band gap varying from 0.2 to 0.45 eV. In CoSn{sub 1.5}Te{sub 1.5}, the electronic density of states near the gap is significantly higher than for A = Ir or Rh, leading to higher effective masses and higher Seebeck coefficients. Experimentally, Ir and Rh samples exhibit relatively large p-type carrier concentrations and degenerate semiconducting behavior. In contrast, CoSn{sub 1.5}Te{sub 1.5} shows mixed conduction, with n-type carriers dominating the Seebeck coefficient and light, high mobility holes dominating the Hall coefficient. zT values of up to 0.35 were obtained, and further improvement is expected upon optimization of the carrier concentration or with n-type doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Thermoelectric properties of CuAlCh{sub 2} (Ch = S, Se and Te)

Electronic and thermoelectric properties of ternary chalcopyrite-type CuAlCh{sub 2} (S, Se and Te) were studied using the first principles density functional calculations implemented in the full potential linear augmented plane wave (FP-LAPW) method. The thermoelectric properties are calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The calculated band gap using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of all the compounds are in good agreement with the available experiment and other theoretical reports. Thermoelectric properties like thermopower, electrical conductivity scaled by relaxation time are calculated as a function of carrier concentrations at different temperatures. The calculated thermoelectric properties are compared with the available experiment and other theoretical calculations of similar materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Tuning the Curie temperature of L1{sub 0} ordered FePt thin films through site-specific substitution of Rh

In structurally ordered magnetic thin films, the Curie temperature (T{sub C}) of ferromagnetic films depends on the exchange integral of the short range ordered neighboring atoms. The exchange integral may be adjusted by controlling the elemental substitutional concentration at the lattice site of interest. We show how to control the T{sub C} in high anisotropy L1{sub 0} Fe{sub 50}Pt{sub 50} magnetic thin films by substituting Rh into the Pt site. Rh substitution in L1{sub 0} FePt modified the local atomic environment and the corresponding electronic properties, while retaining the ordered L1{sub 0} phase. The analysis of extended x-ray Absorption Fine Structure spectra shows that Rh uniformly substitutes for Pt in L1{sub 0} FePt. A model of antiferromagnetic defects caused by controlled Rh substitution of the Pt site, reducing the T{sub C,} is proposed to interpret this phenomenon and its validity is further examined by ab initio density functional calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Microscopic Origin of Polarization Charges at Ga N / ( Al , Ga ) N Interfaces

Ga⁢N/(Al,Ga)⁢N heterojunctions are at the heart of high-electron-mobility transistors that are being adopted for high-power and high-frequency applications. The strong polarization fields present at this interface significantly enhance the density of the two-dimensional electron gas (2DEG) that is confined on the Ga⁢ N side of the junction. The microscopic origin of these electrons has been debated over the years: after excluding that they would be contributed by bulk donors, a model that identifies surface states on the (Al, Ga)⁢N surface as the source of electrons has become widely adopted. Recently, it has become clear, however, that the measured density of surface states is insufficient to account for the high electron density in the 2DEG. Here we demonstrate, based on state-of-the-art first-principles calculations, that the source of electrons is intrinsic to the overall structure and that the negative charge in the 2DEG is balanced by fixed charge on the surface. We perform a rigorous study of polarization, using our recently developed methodology for quantifying polarization fields within the finite-sized systems that can be addressed with density-functional calculations. The results show that the electrons that appear in the 2DEG originate locally at the interface, and that the net charge at the interface is predominantly compensated by fixed charge on the surface, rather than surface states. We elucidate the source of this fixed charge and associate it with surface reconstructions or the presence of heterovalent impurities (such as oxygen). Our results force a reassessment of the impact of surface states on the density of the 2DEG: rather than serving as a supply of electrons, the surface states mainly act to pin the Fermi level. Our conclusions allow a fresh interpretation of experimental observations and allow devising guidelines for optimizing carrier densities in the 2DEG.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Interlayer Interactions in Twisted WSe 2 /WS 2 Bilayer Heterojunctions: Synthesis, Characterization, and Modeling

Twisting adjacent layers in van der Waals solids can significantly alter their interlayer interactions for tunable optical and electronic properties. Here, we report theoretical calculations, fabrication, and detailed characterizations of WSe 2 /WS 2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of CVD-grown WS 2 and WSe 2 . Density functional calculations predicted the formation of type-II heterojunctions for the stamped bilayers, with band structures that strongly depend on the interlayer twist angle. Raman spectroscopy reveals strong interlayer coupling with the appearance of a layer-number sensitive mode of WS 2 at 311 cm -1 in WSe 2 /WS 2 bilayers. This strong interlayer coupling resulted in a 1~2 order of magnitude quenching of the photoluminescence. The broadening and shifts were observed in micro-absorption spectroscopy of WSe 2 /WS 2 bilayers, which resulted in a net ~10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed 24 4 meV broadening of the WSe 2 A-exciton absorption band in the bilayers provided an estimate on the rate of charge transfer between the layers that ranged from 23 to 33 fs, and was supported by direct femtosecond pump-probe measurements. These results indicate that interlayer exciton formation and non-radiative decay channels dominate optical properties in these bilayers, which may be important for tunable future photovoltaics and detector applications.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Water Interactions with Actinide Oxides from First Principles: A Computational Study - 16470

The interactions between water and the actinide oxides UO{sub 2} and PuO{sub 2} are important when considering the long-term storage of spent nuclear fuel. However, experimental studies in this area are severely limited by plutonium's intense radioactivity, and hence we have recently begun to investigate these interactions computationally. In this contribution we report the results of first principles calculations of the electronic and geometric structures of AnO{sub 2} bulk, AnO{sub 2} clusters, and AnO{sub 2} surfaces (An = U, Np, Pu), as well as the interaction of water with the latter. Strongly-correlated effects are taken into account using a Hubbard corrected potential, which enables us to perform efficacious plane-wave density functional calculations of extended systems. In particular, we compare results of water adsorption on UO{sub 2} (111) with the corresponding results on CeO{sub 2}, focusing on the energetics and structural properties of molecular versus dissociative adsorption, for both partially and fully covered surfaces. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Gamow-Teller Strength in Ca 48 and Ni 78 with the Charge-Exchange Subtracted Second Random-Phase Approximation

Here, we develop a fully self-consistent subtracted second random-phase approximation for charge-exchange processes with Skyrme energy-density functionals. As a first application, we study Gamow-Teller excitations in the doubly magic nucleus 48 Ca, the lightest double-β emitter that could be used in an experiment, and in 78 Ni, the single-beta-decay rate of which is known. The amount of Gamow-Teller strength below 20 or 30 MeV is considerably smaller than in other energy-density-functional calculations and agrees better with experiment in 48 Ca, as does the beta-decay rate in 78 Ni. These important results, obtained without ad hoc quenching factors, are due to the presence of two-particle–two-hole configurations. Their density progressively increases with excitation energy, leading to a long high-energy tail in the spectrum, a fact that may have implications for the computation of nuclear matrix elements for neutrinoless double-β decay in the same framework.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗