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At least 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Electronic Structure Modification of Rectangular Phosphorene Quantum Dots Via Edge Passivation

Phosphorene is an elemental two-dimensional material that shows great promise as a metal-free photocatalyst owing to its high carrier mobility and direct band gap. Doping and edge functionalization of phosphorene can be exploited to tune its band gap and alter other properties. Herein, the electronic properties of a series of edge-passivated monolayer phosphorene quantum dots are investigated using an array of functional groups (OH, SH, NH 2 , SCH 3 , OCN, CN, and Cl) that are known to have electron-donating or electron-withdrawing capabilities. By employing density functional theory calculations, we demonstrate that edge functionalization can perturbatively shift the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels while simultaneously preserving the geometric structure and other favorable properties of pristine phosphorene. We use the average overall electrostatic potential of these systems to successfully predict the relative positioning of the HOMO and LUMO energy levels while presenting qualitative explanations on potential jumps introduced by the dipole layers formed due to edge functionalization. Furthermore, our results suggest that selective functionalization permits considerable control over the band edge positions, which could be utilized for applications in energy and optoelectronic devices.

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

Electronic, structural, and substrate effect properties of single-layer covalent organic frameworks

Recently synthesized two-dimensional covalent organic frameworks (COFs) exhibit high surface area, large pore size, and unique structural architectures, making them promising materials for various energy applications. Here, a total of nine COFs structures, including two deposited on a hexagonal boron nitride substrate, are investigated using density functional theory, quasi-particle many-body theory within the GW approximation, and an image charge model. The structures considered belong to two major families (thiophene-based COF-n (T-COF-n) and tetrakis (4-aminophenyl) porphyrin-x (TAPP-x)) differing from the presence of B—O or C=N linkers. While T-COF-n structures are shown to constitute planar networks, TAPP-x systems can display non-negligible corrugation due to the out-of-plane rotation of phenyl rings. We find that the electronic properties do not differ significantly when altering the chain molecules within each family. Many-body effects are shown to lead to large band-gap increase while the presence of the substrate yields appreciable reductions of the gaps, due to substrate polarization effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Identifying rhenium substitute candidate multiprincipal-element alloys from electronic structure and thermodynamic criteria

While rhenium has proven to be an ideal material in fast-cycling high-temperature applications such as rocket nozzles, its prohibitive cost limits its continued use and motivates a search for viable cost-effective substitutes. We show that a simple design principle that trades off average valence electron count and cost considerations proves helpful in identifying a promising pool of candidate substitute alloys: The Mo–Ru–Ta–W quaternary system. We demonstrate how this picture can be combined with a computational thermodynamic model of phase stability, based on high-throughput ab initio calculations, to further narrow down the search and deliver alloys that maintain rhenium’s desirable hcp crystal structure. Here, this thermodynamic model is validated with comparisons to known binary phase diagram sections and corroborated by experimental synthesis and structural characterization demonstrating multiprinciple-element hcp solid-solution samples selected from a promising composition range.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations

Energy loss spectra from fluorite-structured ZrO2, CeO2, and UO2 compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.

Aguiar, Jeff↗
DOE OSTI2020

Electronic Structure of Atomically Dispersed Supported Iridium Catalyst Controls Iridium Aggregation

Supported iridium complexes, Ir(C 2 H 4 ) 2 /support, were characterized by X-ray absorption spectroscopy during a temperature ramp to 120 °C in flowing H 2 . Iridium in complexes bonded to weak and moderate electron-donor supports, SiO 2 and γ-Al 2 O 3 , underwent aggregation, forming nanoparticles and clusters, respectively. When the support was a strong electron-donor (MgO), iridium remained site-isolated. Density functional theory calculations confirm the dependence of iridium–support bond strength on the support’s electron-donor character. Coating the SiO 2 -supported complexes with 1-n-ethyl-3-methyl-imidazolium acetate enhanced electron density on the iridium, hindering its aggregation. Finally, these results demonstrate opportunities for stabilizing atomically dispersed supported noble metals under reducing conditions by choice of support/ionic liquid sheath combinations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Photophysics and Electronic Structure of Lateral Graphene/MoS 2 and Metal/MoS 2 Junctions

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. In this work, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS 2 ) interface. A 10× larger photocurrent is extracted at the EG/MoS 2 interface when compared to the metal (Ti/Au)/MoS 2 interface. This is supported by semi-local density functional theory (DFT), which predicts the Schottky barrier at the EG/MoS 2 interface to be ~2× lower than that at Ti/MoS 2 . We provide a direct visualization of a 2D material Schottky barrier through combination of angle-resolved photoemission spectroscopy with spatial resolution selected to be ~300 nm (nano-ARPES) and DFT calculations. A bending of ~500 meV over a length scale of ~2-3 μm in the valence band maximum of MoS 2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1992

Evidence for endohedral muonium in K(x) C60 and consequences for electronic structure

Positive muons injected into solid C60, K4C60, and K6C60 form vacuumlike muonium with a (6-12) percent probability. Observation of coherent spin precession of muonium establishes that all three materials are nonmagnetic and nonconducting at low temperatures. From the temperature dependence of the signals we estimate the electronic band gaps in K4C60 and K6C60 to be considerably smaller than in C60. The similarity of the muonium centers supports a model in which a muonium atom is caged inside the C60 molecule in pure C60 or the C60 exp -x molecular ion in K(x)C60.

Kiefl, R. F.↗
DOE OSTI2014

Structural, electronic and optical properties of novel carbonate fluorides ABCO{sub 3}F (A=K, Rb, Cs; B=Ca, Sr)

In the present study, we performed first principles calculations on the electronic and optical properties of layered alkali–alkaline earth carbonate fluorides which attract attention in the domain of nonlinear optics. The calculated lattice parameters and volumes with and without inclusion of van der Waals (vdW) correction methods to standard density functional methods were compared with experiments. We observed that vdW interactions are predominant in RbCaCO{sub 3}F and CsCaCO{sub 3}F as compared with other computed compounds. The calculated bulk modulus from single crystal elastic constants reveals that these materials are all relatively harder than the KH{sub 2}PO{sub 4} (KDP) crystal. We also found that these materials are wide band gap insulators as obtained from Tran–Blaha modified Becke–Johnson potential. The linear optical properties such as dielectric function, refractive indices, birefringence and absorption spectra are presented. Finally, the calculated birefringence values indicate that these crystals could be promising for producing phase matching in the deep ultra-violet region. - Graphical abstract: The co- and anti-parallel alignment of CO{sub 3} groups leads to larger and smaller SHG coefficients in (a) KCaCO{sub 3}F, (b) KSrCO{sub 3}F, (c) RbSrCO{sub 3}F, (d) RbCaCO{sub 3}F, and (e) CsCaCO{sub 3}F. - Highlights: • Effect of van der Waals interactions on structural properties. • Single-crystalline elastic constants and mechanical stability. • Electronic properties. • Linear optical properties. • Comparison of present computed compounds with other well known materials in non-linear optical materials.

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

First-Principles Investigations of the Temperature Dependence of Electronic Structure and Optical Properties of Rutile TiO 2

To gain additional insight into high-temperature functional material properties for applications in optical gas sensing, the temperature effects on the band gap and optical properties of rutile TiO 2 are investigated using ab initio methods. By analyzing the contributions from electron–phonon interaction and lattice thermal expansion, we show that the electron–phonon interaction is the dominant factor for temperature band-gap renormalization. As the temperature increases, the band gap increases until 300 K and then narrows above 300 K. This behavior results from the acoustic phonons, which widen the band gap, dominate below 300 K, while the optical phonons, which narrow the band gap, dominate above 300 K. Our study suggests that the band gap is narrowed by about 138 meV at 1000 K. We also investigated the temperature effects on the dielectric constants, the refractive index, as well as the extinction coefficient. Both the rate of decrease of the refractive index at 650 and 800 nm as well as the experimentally derived band gap agree with experimentally measured data as the temperature increases. Furthermore, our results and computational methods are of interest for developing high-temperature functional materials with applications toward gas sensing.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Self-consistent implementation of meta-GGA functionals for the ONETEP linear-scaling electronic structure package

Accurate and computationally efficient exchange-correlation functionals are critical to the successful application of linear-scaling density functional theory (DFT). Local and semi-local functionals of the density are naturally compatible with linear-scaling approaches, having a general form which assumes the locality of electronic interactions and which can be efficiently evaluated by numerical quadrature. Presently, the most sophisticated and flexible semi-local functionals are members of the meta-generalized-gradient approximation (meta-GGA) family, and depend upon the kinetic energy density, τ, in addition to the charge density and its gradient. In order to extend the theoretical and computational advantages of τ-dependent meta-GGA functionals to large-scale DFT calculations on thousands of atoms, we have implemented support for τ-dependent meta-GGA functionals in the ONETEP program. In this paper we lay out the theoretical innovations necessary to implement τ-dependent meta-GGA functionals within ONETEP's linear-scaling formalism. We present expressions for the gradient of the τ-dependent exchange-correlation energy, necessary for direct energy minimization. We also derive the forms of the τ-dependent exchange-correlation potential and kinetic energy density in terms of the strictly localized, self-consistently optimized orbitals used by ONETEP. To validate the numerical accuracy of our self-consistent meta-GGA implementation, we performed calculations using the B97M-V and PKZB meta-GGAs on a variety of small molecules. Using only a minimal basis set of self-consistently optimized local orbitals, we obtain energies in excellent agreement with large basis set calculations performed using other codes. Lastly, to establish the linear-scaling computational cost and applicability of our approach to large-scale calculations, we present the outcome of self-consistent meta-GGA calculations on amyloid fibrils of increasing size, up to tens of thousands of atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The electronic structure of the actinide oxides and their singly and doubly charged cations: A ligand field approach

Abstract Ligand field theory (LFT) calculations of energy levels were performed for the neutral actinide monooxides ( An O) and their singly and doubly ionized cations ( An O + and An O 2+ ) by treating the molecular electronic states as An m+ free‐ion energy levels (where An ∈ Th through Lr and m = 1, 2, 3, or 4) perturbed by the electric field of O 2− or O − . LFT parameters obtained from fits to the energy levels of ThO, ThO + , UO, and UO + were used to compute molecular energy levels for the lowest energy (maximum S c , maximum L c ) 5 f ‐core states of An 4+ , An 3+ , An 2+ , and An + for the majority of the An 4+ O 2− , An 3+ O − , An 3+ O 2− , An 2+ O − , An 2+ O 2− , and An + O − electronic configurations. Simple linear relationships enabled predictions of the dissociation energies for An O, An O + and An O 2+ (where An ∈ Bk through Lr) and ionization energies for An O and An O + (where An ∈ Bk through Lr), mainly based on recent accurate experimental data for the ionization energies of An atoms (where An ∈ Fm, Md, No, and Lr) and correlations with the energetics of the atoms and ions.

Kaledin, Leonid A.↗
DOE OSTI2018

Strong Effect of Organic Ligands on the Electronic Structure of Metal-Chalcogenide Clusters

It is shown that multiple ionization energies of metal-chalcogenide clusters can be substantially reduced by adding ligands that form charge transfer complexes. In this report we demonstrate this intriguing phenomenon by considering metal-chalcogenide clusters including cases where a cluster has a filled electronic shell with a large gap between the occupied and unoccupied states reminiscent of stable species. The studies include a Co 6 Se 8 core ligated with tri-ethylphosphine (PEt 3 ) ligands forming a stable Co 6 Se 8 (PEt 3 ) 6 species. All of the ligated clusters have a first ionization energy in the range for alkali atoms and multiple ionization energies that are considerably lower than those for the non-ligated clusters. The change in electronic behavior upon ligation can be associated with a shift in the electronic spectrum via a crystal field like effect due to attaching ligands that form charge transfer complexes. We also show that metal-chalcogenide species can be programmed by proper ligand replacement to promote dimerization by first forming the Co 6 Se 8 (PEt 3 ) n (CO) 6-n (n = 0–6) clusters where the CO ligands could be replaced by diisocyanide (CNC 6 H 4 NC) ligands. The diisocyanide ligand acts as a rigid linker between the metallic cores, enabling the formation of a Co 6 Se 8 (PEt 3 ) 5 (CNC 6 H 4 NC)Co 6 Se 8 (PEt 3 ) 5 superatomic molecule (SM), and we examine the electronic and magnetic properties of the recently synthesized SM via studies on an analogous SM with smaller ligands.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Structural Electronic and Magnetic Properties of Quasi-1D Quantum Magnets [Ni(HF2)(pyz)2]X (pyz = pyrazine; X = PF6- SbF6-) Exhibiting Ni-FHF-Ni and Ni-pyz-Ni Spin Interactions

[Ni(HF{sub 2})(pyz){sub 2}]X {l_brace}pyz = pyrazine; X = PF{sub 6}{sup -} (1), SbF{sub 6}{sup -} (2){r_brace} were structurally characterized by synchrotron X-ray powder diffraction and found to possess axially compressed NiN{sub 4}F{sub 2} octahedra. At 298 K, 1 is monoclinic (C2/c) with unit cell parameters, a = 9.9481(3), b = 9.9421(3), c = 12.5953(4) {angstrom}, and {beta} = 81.610(3){sup o} while 2 is tetragonal (P4/nmm) with a = b = 9.9359(3) and c = 6.4471(2) {angstrom} and is isomorphic with the Cu-analogue. Infinite one-dimensional (1D) Ni-FHF-Ni chains propagate along the c-axis which are linked via {mu}-pyz bridges in the ab-plane to afford three-dimensional polymeric frameworks with PF{sub 6}{sup -} and SbF{sub 6}{sup -} counterions occupying the interior sites. A major difference between 1 and 2 is that the Ni-F-H bonds are bent (157{sup o}) in 1 but are linear in 2. Ligand field calculations (LFT) based on an angular overlap model (AOM), with comparison to the electronic absorption spectra, indicate greater {pi}-donation of the HF{sub 2}{sup -} ligand in 1 owing to the bent Ni-F-H bonds. Magnetic susceptibility data for 1 and 2 exhibit broad maxima at 7.4 and 15 K, respectively, and {lambda}-like peaks in dxT/dT at 6.2 and 12.2 K that are ascribed to transitions to long-range antiferromagnetic order (TN). Muon-spin relaxation and specific heat studies confirm these TN's. A comparative analysis of x vs T to various 1D Heisenberg/Ising models suggests moderate antiferromagnetic interactions, with the primary interaction strength determined to be 3.05/3.42 K (1) and 5.65/6.37 K (2). However, high critical fields of 19 and 37.4 T obtained from low temperature pulsed-field magnetization data indicate that a single exchange constant (J1D) alone is insufficient to explain the data and that residual terms in the spin Hamiltonian, which could include interchain magnetic couplings (J), as mediated by Ni-pyz-Ni, and single-ion anisotropy (D), must be considered. While it is difficult to draw absolute conclusions regarding the magnitude (and sign) of J and D based solely on powder data, further support offered by related Ni(II)-pyz compounds and our LFT and density-functional theory (DFT) results lead us to a consistent quasi-1D magnetic description for 1 and 2.

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

Electronic structure of U 2 PtC 2 and U 2 RhC 2

In this study, we present density functional theory calculations within the generalized gradient approximation of U 2 RhC 2 and U 2 PtC 2 . We find the calculated density of states are significantly less than that measured by specific heat indicating the need for electronic correlations. The mass enhancement found for U 2 PtC 2 is m*/m band ≈ 4.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Advanced Electronic Structure Theories for Strongly Correlated Ground and Excited States

The aim of this project was to create widely applicable, numerically robust, and systematically improvable multireference theories based on the Driven Similarity Renormalization Group (DSRG). The DSRG is a many-body formalism recently developed in our lab that maps a complex problem involving strongly and weakly interacting electrons to a simpler one in which a few electrons interact strongly via "renormalized" interactions. During this project, we focused our efforts on developing a multireference version of the DSRG (MR-DSRG) applicable to a wide range of problems in theoretical chemistry, including the computation of global ground and excited potential energy surfaces, spin-splittings in transition metal complexes and predicting the interaction of near-degenerate electronically excited states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Tuning Optical Properties of Conjugated Molecules by Lewis Acids: Insights from Electronic Structure Modeling

Understanding and controlling the optoelectronic properties of organic semiconductors at the molecular level remains a challenge due to the complexity of chemical structures and intermolecular interactions. A common strategy to address this challenge is to utilize both experimental and computational approaches. In this contribution, we show that density functional theory (DFT) calculation is a useful tool to provide insights into the bonding, electron population distribution, and optical transitions of adducts between conjugated molecules and Lewis acids (CM-LA). Adduct formation leads to relevant modifications of key properties, including a red shift in optical transitions and an increase in charge carrier density and charge mobility, compared to the parent conjugated molecules. We show that electron density transfer from the CM to the LA, which was hypothesized to cause the experimental red shift in absorption spectra upon LA binding, can be quantified and interpreted by population analysis. Experimental red shifts in optical transitions for all molecular families can also be predicted by time dependent DFT calculations with different density functionals. Finally, these detailed insights help to optimize a priori design guidelines for future applications.

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

Electronic Structure Calculations for Improving Catalyst Design for Use in the Hybrid Sulfur Cycle

The hybrid sulfur cycle is a promising means for efficient production of large quantities of hydrogen with a small carbon footprint. However, many challenges remain in designing catalysts that are stable at high temperatures and have high sulfur tolerance. Theoretical calculations can provide fundamental insights at the electronic scale to improve our understanding of reaction mechanisms and surface energetics, thereby facilitating improved catalyst design. Here, density functional theory was used to explore the reaction energetics of the hybrid sulfur cycle on Pt, Au, and Pt - Au surfaces. A database of information has been built that contains calculations from which adsorption energetics and density of states information can be extracted.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of heterostructure engineering on electronic structure and transport properties of two-dimensional halide perovskites

Organic-inorganic halide perovskite solar cells have attracted much attention due to their low-cost fabrication, flexibility, and high-power conversion efficiency. More recent efforts show that the reduction from three- to two-dimensions (2D) of organic–inorganic halide perovskites promises an exciting opportunity to tune their electronic properties. Here, we explore the effect of reduced dimensionality and heterostructure engineering on the intrinsic material properties, such as energy stability, bandgap and transport properties of 2D hybrid organic–inorganic halide perovskites using first-principles density functional theory. We show that the energy stability of engineered perovskite heterostructures is significantly enhanced. The heterostructures with improved stability also show excellent transport properties similar to their bulk counterparts. Furthermore, these layered chemistries demonstrate the advantage of a broad range of tunable bandgaps and high-absorption coefficient in the visible spectrum. The proposed 2D heterostructured material holds potential for nano-optoelectronic devices as well as for effective photovoltaics.

36 MATERIALS SCIENCE↗