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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 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Ab initio study of H, He, Li and Be impurity effect in tungsten Σ3{112} and Σ27{552} grain boundaries

Density functional theory calculations were performed to study the effects of H, He, Li and Be on the cohesion of W Σ3<110>{112} and Σ27<110>{552} grain boundaries (GBs). In Σ3, Li causes the strongest embrittlement, while in Σ27 it is He. In both GBs, H slightly reduces the cohesion. Compared to He and Li, H exhibits much stronger attractive binding with W that may inhibit subsequent segregation of Li and He to the GBs. In Σ3, Be decreases the cohesion. However, in Σ27, Be strengthens the cohesion in several interstitial positions by increasing the bonding across the interface or by inducing GB restructuring.

Ab Initio↗
DOE OSTI2022

Mechanistic Insights and Rational Design of Ca-Doped CeO 2 Catalyst for Acetic Acid Ketonization

Carboxylic acid ketonization has recently gained significant attention to produce biomass-derived hydrocarbon fuels as it not only removes the highly reactive carboxylic functional group but also increases the size of the carbon chain. In this work, Ca-doped CeO 2 -based catalysts were investigated for acetic acid ketonization using a combined experimental and computational approach. Acetic acid conversion was performed across a range of temperatures including higher temperatures relevant to catalytic hot gas filtration (450 °C). Ca addition slightly decreases overall acetic acid ketonization reactivity yet stabilizes the catalyst at the higher temperatures necessary for catalytic hot gas filtration. From density functional theory calculations of the ketonization reaction mechanism, the C–C coupling and water formation steps are identified as two of the most energy-consuming steps on a CeO 2 surface with a proximal oxygen vacancy and the presence of a Ca dopant stabilizes the key intermediates. Calculations predict an optimal structure comprising three Ca ensembles to minimize the reaction free energies for C–C coupling and water formation steps. These findings provide a priori information to guide future experiments for ketonization catalyst design and development.

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

Large dielectric constant, high acceptor density, and deep electron traps in perovskite solar cell material CsGeI 3

Here we report that many metal halides that contain cations with the ns 2 electronic configuration have recently been discovered as high-performance optoelectronic materials. In particular, solar cells based on lead halide perovskites have shown great promise as evidenced by the rapid increase of the power conversion efficiency. In this paper, we show density functional theory calculations of electronic structure and dielectric and defect properties of CsGeI 3 (a lead-free halide perovskite material). The potential of CsGeI 3 as a solar cell material is assessed based on its intrinsic properties. We find anomalously large Born effective charges and a large static dielectric constant dominated by lattice polarization, which should reduce carrier scattering, trapping, and recombination by screening charged defects and impurities. Defect calculations show that CsGeI 3 is a p-type semiconductor and its hole density can be modified by varying the chemical potentials of the constituent elements. Despite the reduction of long-range Coulomb attraction by strong screening, the iodine vacancy in CsGeI3 is found to be a deep electron trap due to the short-range potential, i.e., strong Ge–Ge covalent bonding, which should limit electron transport efficiency in p-type CsGeI 3 . This is in contrast to the shallow iodine vacancies found in several Pb and Sn halide perovskites (e.g., CH 3 NH 3 PbI 3 , CH 3 NH 3 SnI 3 , and CsSnI 3 ). The low-hole-density CsGeI 3 may be a useful solar absorber material but the presence of the low-energy deep iodine vacancy may significantly reduce the open circuit voltage of the solar cell. Still, on the other hand, CsGeI 3 may be used as an efficient hole transport material in solar cells due to its small hole effective mass, the absence of low-energy deep hole traps, and the favorable band offset with solar absorber materials such as dye molecules and CH 3 NH 3 PbI 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Density functional theory study on the coupling and reactions of diferuloylputrescine as a lignin monomer

Diferuloylputrescine has been found in a variety of plant species, and recent work has provided evidence of its covalent bonding into lignin. Results from nuclear magnetic resonance spectroscopy revealed the presence of bonding patterns consistent with homo-coupling of diferuloylputrescine and the possibility of cross-coupling with lignin. In the present work, density functional theory calculations have been applied to assess the energetics associated with radical coupling, rearomatization, and dehydrogenation for possible homo-coupled dimers of diferuloylputrescine and cross-coupled dimers of diferuloylputrescine and coniferyl alcohol. Furthermore, the values obtained for these reaction energetics are consistent with those reported for monolignols and other novel lignin monomers. As such, this study shows that there would be no thermodynamic impediment to the incorporation of diferuloylputrescine into the lignin polymer and its addition to the growing list of non-canonical lignin monomers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Solution phase high repetition rate laser pump x-ray probe picosecond hard x-ray spectroscopy at the Stanford Synchrotron Radiation Lightsource

We present a dedicated end-station for solution phase high repetition rate (MHz) picosecond hard x-ray spectroscopy at beamline 15-2 of the Stanford Synchrotron Radiation Lightsource. A high-power ultrafast ytterbium-doped fiber laser is used to photoexcite the samples at a repetition rate of 640 kHz, while the data acquisition operates at the 1.28 MHz repetition rate of the storage ring recording data in an alternating on-off mode. The time-resolved x-ray measurements are enabled via gating the x-ray detectors with the 20 mA/70 ps camshaft bunch of SPEAR3, a mode available during the routine operations of the Stanford Synchrotron Radiation Lightsource. As a benchmark study, aiming to demonstrate the advantageous capabilities of this end-station, we have conducted picosecond Fe K-edge x-ray absorption spectroscopy on aqueous [FeII(phen)3]2+, a prototypical spin crossover complex that undergoes light-induced excited spin state trapping forming an electronic excited state with a 0.6–0.7 ns lifetime. In addition, we report transient Fe Kβ main line and valence-to-core x-ray emission spectra, showing a unique detection sensitivity and an excellent agreement with model spectra and density functional theory calculations, respectively. Notably, the achieved signal-to-noise ratio, the overall performance, and the routine availability of the developed end-station have enabled a systematic time-resolved science program using the monochromatic beam at the Stanford Synchrotron Radiation Lightsource.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Nucleation and Initial Stages of Growth during the Atomic Layer Deposition of Titanium Oxide on Mesoporous Silica

A chemical approach to the deposition of thin films on solid surfaces is highly desirable but prone to affect the final properties of the film. To better understand the origin of these complications, the initial stages of the atomic layer deposition of titania films on silica mesoporous materials were characterized. Adsorption-desorption measurements indicated that the films grow in a layer-by-layer fashion, as desired, but initially exhibit surprisingly low densities, about one-quarter of that of bulk titanium oxide. Electron microscopy, X-ray diffraction, UV/visible, and X-ray absorption spectroscopy data pointed to the amorphous nature of the first monolayers, and EXAFS and 29 Si CP/MAS NMR results to an initial growth via the formation of individual tetrahedral Ti-oxide units on isolated Si-OH surface groups with unusually long Ti-O bonds. Here, density functional theory calculations were used to propose a mechanism where the film growth starts at the nucleation centers to form an open 2D structure.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh 2 ) 2 , DTS(F2BTTh 2 ) 2 , DTS(PTTh 2 ) 2 , DTG(FBTTh 2 ) 2 and DTG(F2BTTh 2 ) 2 ) with the fullerene derivative PC 61 BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh 2 ) 2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh 2 ) 2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. As a result, the higher BET triplet exciton population in the DTS(PTTh 2 ) 2 blend is in accordance with the slower charge separation dynamics observed in this blend.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Enhanced CO 2 Reactive Capture and Conversion Using Aminothiolate Ligand–Metal Interface

Metallic catalyst modification by organic ligands is an emerging catalyst design in enhancing the activity and selectivity of electrocatalytic carbon dioxide (CO 2 ) reactive capture and reduction to value-added fuels. However, a lack of fundamental science on how these ligand–metal interfaces interact with CO 2 and key intermediates under working conditions has resulted in a trial-and-error approach for experimental designs. With the aid of density functional theory calculations, we provided a comprehensive mechanism study of CO 2 reduction to multicarbon products over aminothiolate-coated copper (Cu) catalysts. Our results indicate that the CO 2 reduction performance was closely related to the alkyl chain length, ligand coverage, ligand configuration, and Cu facet. The aminothiolate ligand–Cu interface significantly promoted initial CO 2 activation and lowered the activation barrier of carbon–carbon coupling through the organic (nitrogen (N)) and inorganic (Cu) interfacial active sites. Experimentally, the selectivity and partial current density of the multicarbon products over aminothiolate-coated Cu increased by 1.5-fold and 2-fold, respectively, as compared to the pristine Cu at –1.16 V RHE , consistent with our theoretical findings. Furthermore, this work highlights the promising strategy of designing the ligand–metal interface for CO 2 reactive capture and conversion to multicarbon products.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2024

Unraveling the structural dependency of Weyl nodes in Co 2 MnGa

Conventionally, the modulation of the intrinsic Weyl nodes in Weyl semimetals is challenging, due to topological protection. Here we report the structural dependence of the Weyl nodes in a Co2⁢MnGa Heusler thin film via a temperature-dependent tetragonal distortion. The ability to manipulate these Weyl nodes allows for the control of the intrinsic electromagnetic properties. Temperature-dependent x-ray diffraction (XRD) measurements identify a compressive tetragonal distortion with decreasing temperature from 300 to 20 K. The calculated Weyl properties can be directly compared with experimental parameters through the temperature-dependent XRD measurements which show the intrinsic correlation between Weyl properties and important magnetic parameters. Further, the microscopic momentum space properties of Weyl nodes such as the distance (d W ), solid angle (Ω W ), tilt (φ W ), and nodal point energy (E W ) directly affect the macroscopic observable properties such as exchange stiffness (A), magnetization (M), and effective anisotropy field $H^{eff}_{K}$, as shown via structure-dependent density functional theory calculations. These predictions are experimentally observed as large variations in the bulk magnetization and effective anisotropy field as a function of temperature. These results highlight a unique degree of freedom in the control of macroscopic magnetic properties via the modulation of the intrinsic properties of Weyl nodes through structural distortions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Computational descriptor for electrochemical currents of carbon dioxide reduction on Cu facets

Computation screening is crucial for designing efficient electrochemical catalysts for carbon dioxide (CO 2 R) reduction that produce valuable hydrocarbons and oxygenates. In this work, leveraging density functional theory calculations for the CO adsorption energy ΔE CO on seventeen Cu terminations, we discover a strong linear correlation between ΔE CO and the experimentally measured CO 2 R electrochemical currents (ACS Catal. 2022, 12, 11, 6578–6588). Examining ab initio thermodynamics of early critical intermediates CO*, COH*, and CHO*, we find that CO* → CHO* is the thermodynamically controlling step. Beyond the general CO adsorption energy that only shows a linear trend with CO 2 R activity, we show that the reaction free energy of CO* → CHO* is the descriptor for the overall CO 2 R activity for Cu facets, as it displays a volcano relationship with the experimental current. Importantly, we show that high step and kink density of the Cu terminations not only enhances CO adsorption strength but also modulates the CO* → CHO* pathway, as respectively exemplified in the (941) and (741) facets. In addition, we explain that the high activity of (741) is due to its relatively low hydrogen evolution reaction activity compared with the other Cu surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Band energy control of molybdenum oxide by surface hydration

The application of oxide buffer layers for improved carrier extraction is ubiquitous in organic electronics. However, the performance is highly susceptible to processing conditions. Notably, the interface stability and electronic structure is extremely sensitive to the uptake of ambient water. In this study we use density functional theory calculations to asses the effects of adsorbed water on the electronic structure of MoO{sub x}, in the context of polymer-fullerene solar cells based on PCDTBT. We obtain excellent agreement with experimental values of the ionization potential for pristine MoO{sub 3} (010). We find that IP and EA values can vary by as much as 2.5 eV depending on the oxidation state of the surface and that adsorbed water can either increase or decrease the IP and EA depending on the concentration of surface water.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

POLYCYCLIC AROMATIC HYDROCARBONS WITH ARMCHAIR EDGES AND THE 12.7 μm BAND

In this Letter, we report the results of density functional theory calculations on medium-sized neutral polycyclic aromatic hydrocarbon (PAH) molecules with armchair edges. These PAH molecules possess strong C-H stretching and bending modes around 3 μm and in the fingerprint region (10-15 μm), and also strong ring deformation modes around 12.7 μm. Perusal of the entries in the NASA Ames PAHs Database shows that ring deformation modes of PAHs are common, although generally weak. Therefore, we propose that armchair PAHs with N{sub C} > 65 are responsible for the 12.7 μm aromatic infrared band in H II regions and discuss the astrophysical implications in the context of the PAH life cycle.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Copper dimer interactions on a thermomechanical superfluid {sup 4}He fountain

Laser induced fluorescence imaging and frequency domain excitation spectroscopy of the copper dimer (B{sup 1}Σ{sub g}{sup +}←X{sup 1}Σ{sub u}{sup +}) in thermomechanical helium fountain at 1.7 K are demonstrated. The dimers penetrate into the fountain provided that their average propagation velocity is ca. 15 m/s. This energy threshold is interpreted in terms of an imperfect fountain liquid-gas interface, which acts as a trap for low velocity dimers. Orsay-Trento density functional theory calculations for superfluid {sup 4}He are used to characterize the dynamics of the dimer solvation process into the fountain. The dimers first accelerate towards the fountain surface and once the surface layer is crossed, they penetrate into the liquid and further slow down to Landau critical velocity by creating a vortex ring. Theoretical lineshape calculations support the assignment of the experimentally observed bands to Cu{sub 2} solvated in the bulk liquid. The vibronic progressions are decomposed of a zero-phonon line and two types of phonon bands, which correlate with solvent cavity interface compression (t < 200 fs) and expansion (200 < t < 500 fs) driven by the electronic excitation. The presented experimental method allows to perform molecular spectroscopy in bulk superfluid helium where the temperature and pressure can be varied.

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

Theoretical Investigation of 2D Conductive Microporous Coordination Polymers as Li-S Battery Cathode with Ultrahigh Energy Density

Even though tremendous achievement has been made experimentally in the performance of lithium–sulfur (Li–S) battery, theoretical studies in this area are lagging behind due to the complexity of the Li–S systems and the effects of solvent. Because of this, a new methodology is developed for investigating the 2D hexaaminobenzene-based coordination polymers (2D-HAB-CPs) as cathode candidate materials for Li–S batteries via density functional theory calculations in combination with an in-house developed charge polarized solvent model and a genetic algorithm structure global search code. With high ratios of transition metal atoms and two-coordinated nitrogen atoms, excellent electric conductivity, and structural porosity, the 2D-HAB-CP is able to address all of the three main challenges facing Li–S batteries: confining the lithium polysulfides from dissolution, facilitating the electron conductivity and buffering the volumetric expansion during the lithiation process. In addition, the theoretical energy density of this system is as high as 1395 Wh kg -1 . These results demonstrate that the 2D-HAB-CP is a promising cathode material for Li–S batteries. The proposed computational framework not only opens a new avenue for understanding the key role played by solution and liquid electrolytes in Li–S batteries, but also can be generally applied to other processes with liquids involved.

25 ENERGY STORAGE↗
DOE OSTI2015

Probing the mechanism of CO 2 capture in diamine-appended metal–organic frameworks using measured and simulated X-ray spectroscopy

Diamine-appended metal–organic frameworks display great promise for carbon capture applications, due to unusual step-shaped adsorption behavior that was recently attributed to a cooperative mechanism in which the adsorbed CO 2 molecules insert into the metal–nitrogen bonds to form ordered ammonium carbamate chains. We present a detailed study of this mechanism by in situ X-ray absorption spectroscopy and density functional theory calculations. Distinct spectral changes at the N and O K-edges are apparent upon CO 2 adsorption in both mmen-Mg 2 (dobpdc) and mmen-Mn 2 (dobpdc), and these are evaluated based upon computed spectra from three potential adsorption structures. The computations reveal that the observed spectral changes arise from specific electronic states that are signatures of a quasi-trigonal planar carbamate species that is hydrogen bonded to an ammonium cation. This eliminates two of the three structures studied, and confirms the insertion mechanism. Here, we note the particular sensitivity of X-ray absorption spectra to the insertion step of this mechanism, underpinning the strength of the technique for examining subtle chemical changes upon gas adsorption.

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

Dielectronic satellite emission from a solid-density Mg plasma: Relationship to models of ionization potential depression

Here, we report on experiments where solid-density Mg plasmas are created by heating with the focused output of the Linac Coherent Light Source x-ray free-electron laser. We study the K-shell emission from the helium- and lithium-like ions using Bragg crystal spectroscopy. Observation of the dielectronic satellites in lithium-like ions confirms that the M-shell electrons appear bound for these high charge states. An analysis of the intensity of these satellites indicates that when modeled with an atomic-kinetics code, the ionization potential depression model employed needs to produce depressions for these ions which lie between those predicted by the well known Stewart-Pyatt and Ecker-Kroll models. These results are largely consistent with recent density functional theory calculations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Revisiting the structures and phase transitions of Ba 2 NaNb 5 O 15

The room-temperature and low-temperature structure(s) of Ba 2 NaNb 5 O 15 (BNN) have been debated since the structure was proposed in the 1960s. This work revisits the structures and phase transitions of BNN, combining high-resolution X-ray and neutron powder diffraction with density functional theory calculations. Temperature-dependent high-resolution X-ray powder diffraction patterns are collected from 4 to 918 K, and sequential batch Rietveld refinement using a symmetry mode approach to describe the structure is used to extract the main structural changes as a function of temperature. The data show that the average structure of BNN is best described by the Ama2 space group, and no other structural phase transitions were observed below the ferroelastic transition. The symmetry mode analysis, combining results from diffraction and density functional theory, shows significant octahedral tilting and corrugations of both the A1 and A2 sites along the c direction. A strong correlation between the spontaneous strain and the octahedral tilting was observed, and a potential connection with emerging microstructure at low temperatures is proposed, all enabled by the symmetry mode approach used in this work.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

VASP calculations on Chicoma: CPU vs. GPU [Slides]

Density functional theory (DFT) calculations with the VASP code provide insight into materials behavior. In particular, the behavior of Pu still poses challenges to our understanding. Calculations for a 108-atom system that represents delta phase Pu with one substitutional Ga show a significant speedup running on GPUs vs CPUs. The gain in speed (2.7), however, remains well below the ratio of SU per node hour conversions, 540/108 = 5.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗