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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 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Symmetry breaking charge transfer as a means to study electron transfer with no driving force

Symmetry breaking charge transfer (SBCT) is a process where a symmetrically disposed pair of identical chromophores forms a charge transfer excited state with the hole and electron on different chromophores, i.e. chr–chr + hν → chr + –chr – . Herein we explore this process in two dipyrrin-based bichromophoric systems. One of these bisdipyrrins involved a pair of BODIPY chromophores linked by a single bond at their meso-positions (compound 1) and the other involved two dipyrrin ligands coordinated in a tetrahedral geometry at the Zn 2+ ion (compound 2). Both compounds show rapid SBCT in polar solvents and only dipyrrin based emission in nonpolar solvents, the latter arising from a dipyrrin localized excited sate (LE). By “tuning” the solvent polarity the equilibrium between the LE and SBCT states can be shifted to favor either state. Ultrafast transient absorption spectroscopy (TA) was used to probe the kinetics of the charge transfer for 2 in solvents where the electron transfer is endergonic, exergonic and has a ΔG close to zero. Our TA derived rates were used to predict fluorescence efficiencies in each of the different solvent systems and showed a good correspondence to measured values. Detailed density functional theory (DFT) and time dependent DFT were used to model the ground states as well as the LE and SBCT states of 1 and 2, in both polar and nonpolar media. Lastly, the ground and LE excited states show small dipole moments, while the SBCT states show dipole moments of 16.4 and 20.3 D for 1 and 2, respectively.

14 SOLAR ENERGY↗
DOE OSTI2022

Dynamic signatures of electronically nonadiabatic coupling in sodium hydride: a rigorous test for the symmetric quasi-classical model applied to realistic, ab initio electronic states in the adiabatic representation

Sodium hydride (NaH) in the gas phase presents a seemingly simple electronic structure making it a potentially tractable system for the detailed investigation of nonadiabatic molecular dynamics from both computational and experimental standpoints. The single vibrational degree of freedom, as well as the strong nonadiabatic coupling that arises from the excited electronic states taking on considerable ionic character, provides a realistic chemical system to test the accuracy of quasi-classical methods to model population dynamics where the results are directly comparable against quantum mechanical benchmarks. Here, using a simulated pump–probe type experiment, this work presents computational predictions of population transfer through the avoided crossings of NaH via symmetric quasi-classical Meyer–Miller (SQC/MM), Ehrenfest, and exact quantum dynamics on realistic, ab initio potential energy surfaces. The main driving force for population transfer arises from the ground vibrational level of the D 1 Σ + adiabatic state that is embedded in the manifold of near-dissociation C 1 Σ + vibrational states. When coupled through a sharply localized first-order derivative coupling most of the population transfers between t = 15 and t = 30 fs depending on the initially excited vibronic wavepacket. While quantum mechanical effects are expected due to the reduced mass of NaH, predictions of the population dynamics from both the SQC/MM and Ehrenfest models perform remarkably well against the quantum dynamics benchmark. Additionally, an analysis of the vibronic structure in the nonadiabatically coupled regime is presented using a variational eigensolver methodology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Ab-initio calculations of electronic, transport, and structural properties of boron phosphide

We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ultrafast laser and swift heavy ion irradiation: Response of Gd 2 O 3 and ZrO 2 to intense electronic excitation

In order to investigate the response of materials to extreme conditions, there are several approaches to depositing extremely high concentrations of energy into very small volumes of material, including ultrafast laser and swift heavy ion (SHI) irradiation. In this study, crystalline-to-crystalline phase transformations in cubic Gd 2 O 3 and monoclinic ZrO 2 have been investigated using ultrafast laser irradiation. The phases produced by the extreme conditions of irradiation were characterized by grazing incidence x-ray diffraction (GIXRD) and Raman spectroscopy. Gd 2 O 3 exhibited a cubic-to-monoclinic phase transformation, as evidenced by the appearance of the monoclinic (40$\bar{2}$), (003), (310), and (112$\bar{2}$) peaks in the GIXRD pattern and of four A g and three B g Raman modes. ZrO 2 underwent a monoclinic-to-tetragonal phase transformation, as evidenced by the emergence of the tetragonal (101) peak in the GIXRD pattern and of E g and A 1g Raman modes. The new phases formed by ultrafast laser irradiation are high temperature polymorphs of the two materials. No evidence of amorphization was seen in the GIXRD data, though Raman spectroscopy indicated point defect accumulation. These results are identical to those produced by irradiation with SHIs, which also deposit energy in materials primarily through electronic excitation. Finally, the similarity in damage process and material response between ultrafast laser and SHI irradiation suggests a fundamental relationship between these two techniques.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Engineering the mechanical properties of ultrabarrier films grown by atomic layer deposition for the encapsulation of printed electronics

Direct deposition of barrier films by atomic layer deposition (ALD) onto printed electronics presents a promising method for packaging devices. Films made by ALD have been shown to possess desired ultrabarrier properties, but face challenges when directly grown onto surfaces with varying composition and topography. Challenges include differing nucleation and growth rates across the surface, stress concentrations from topography and coefficient of thermal expansion mismatch, elastic constant mismatch, and particle contamination that may impact the performance of the ALD barrier. In such cases, a polymer smoothing layer may be needed to coat the surface prior to ALD barrier film deposition. We present the impact of architecture on the performance of aluminum oxide (Al{sub 2}O{sub 3})/hafnium oxide (HfO{sub 2}) ALD nanolaminate barrier films deposited on fluorinated polymer layer using an optical calcium (Ca) test under damp heat. It is found that with increasing polymer thickness, the barrier films with residual tensile stress are prone to cracking resulting in rapid failure of the Ca sensor at 50 °C/85% relative humidity. Inserting a SiN{sub x} layer with residual compressive stress between the polymer and ALD layers is found to prevent cracking over a range of polymer thicknesses with more than 95% of the Ca sensor remaining after 500 h of testing. These results suggest that controlling mechanical properties and film architecture play an important role in the performance of direct deposited ALD barriers.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic, transport, and optical properties of bulk and mono-layer PdSe 2

In this study, the electronic and optical properties of bulk and monolayer PdSe 2 are investigated using firstprinciples calculations. Using the modified Becke-Johnson potential, we find semiconductor behavior for both bulk and monolayer PdSe 2 with indirect gap values of 0.03 eV for bulk and 1.43 eV for monolayer, respectively. Our sheet optical conductivity results support this observation and show similar anisotropic feature in the 2D plane. We further study the thermoelectric properties of the 2D PdSe 2 using Blotzmann transport model and find interestingly high Seebeck coefficients (>200 μV/K) for both p- and n-type up to high doping level (–2 x 10 13 cm 2 ) with an anisotropic character in an electrical conductivity suggesting better thermoelectric performance along y direction in the plane.V

36 MATERIALS SCIENCE↗
DOE OSTI2016

Two-center three-electron bonding in ClNH{sub 3} revealed via helium droplet infrared laser Stark spectroscopy: Entrance channel complex along the Cl + NH{sub 3} → ClNH{sub 2} + H reaction

Pyrolytic dissociation of Cl{sub 2} is employed to dope helium droplets with single Cl atoms. Sequential addition of NH{sub 3} to Cl-doped droplets leads to the formation of a complex residing in the entry valley to the substitution reaction Cl + NH{sub 3} → ClNH{sub 2} + H. Infrared Stark spectroscopy in the NH stretching region reveals symmetric and antisymmetric vibrations of a C{sub 3v} symmetric top. Frequency shifts from NH{sub 3} and dipole moment measurements are consistent with a ClNH{sub 3} complex containing a relatively strong two-center three-electron (2c–3e) bond. The nature of the 2c–3e bonding in ClNH{sub 3} is explored computationally and found to be consistent with the complexation-induced blue shifts observed experimentally. Computations of interconversion pathways reveal nearly barrierless routes to the formation of this complex, consistent with the absence in experimental spectra of two other complexes, NH{sub 3}Cl and Cl–HNH{sub 2}, which are predicted in the entry valley to the hydrogen abstraction reaction Cl + NH{sub 3} → HCl + NH{sub 2}.

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

An atomic orbital based real-time time-dependent density functional theory for computing electronic circular dichroism band spectra

One of the challenges of interpreting electronic circular dichroism (ECD) band spectra is that different states may have different rotatory strength signs, determined by their absolute configuration. If the states are closely spaced and opposite in sign, observed transitions may be washed out by nearby states, unlike absorption spectra where transitions are always positive additive. To accurately compute ECD bands, it is necessary to compute a large number of excited states, which may be prohibitively costly if one uses the linear-response time-dependent density functional theory (TDDFT) framework. Here we implement a real-time, atomic-orbital based TDDFT method for computing the entire ECD spectrum simultaneously. The method is advantageous for large systems with a high density of states. In contrast to previous implementations based on real-space grids, the method is variational, independent of nuclear orientation, and does not rely on pseudopotential approximations, making it suitable for computation of chiroptical properties well into the X-ray regime.

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

On the geometric dependence of the molecular dipole polarizability in water: A benchmark study of higher-order electron correlation, basis set incompleteness error, core electron effects, and zero-point vibrational contributions

In this work, we investigate how geometric changes influence the static dipole polarizability ($α$) of a water molecule by explicitly computing the corresponding dipole polarizability surface (DPS) across 3125 total (1625 symmetry-unique) geometries using linear response coupled cluster theory including single, double, and triple excitations (LR-CCSDT) and the doubly augmented triple-$ζ$ basis set (d-aug-cc-pVTZ). Analytical formulae based on power series expansions of this $ab initio$ surface are generated using linear least-squares analysis and provide highly accurate estimates of this quantity as a function of molecular geometry (i.e., bond and angle variations) in a computationally tractable manner. An additional database, which consists of 25 representative molecular geometries and incorporates a more thorough treatment of both basis sets and core electron effects, is provided as a current benchmark for this quantity and the corresponding leading-order $C_6$ dispersion coefficient. This database has been utilized to assess the importance of these effects as well as the relative accuracy that can be obtained using several quantum chemical methods and a library of density functional approximations. In addition to high-level electron correlation methods (like CCSD) and our analytical least-squares formulae, we find that the SCAN0, PBE0, MN15, and B97-2 hybrid functionals yield the most accurate descriptions of the molecular polarizability tensor in H 2 O. Using first-order perturbation theory, we compute the zero-point vibrational correction to $α$ at the CCSDT/d-aug-cc-pVTZ level and find that this correction contributes approximately 3% to the isotropic ($α$ iso ) and nearly 50% to the anisotropic ($α$ aniso ) polarizability values. In doing so, we find that $α$ iso = 9.8307 bohr 3 , which is in excellent agreement with the experimental value of 9.83 ± 0.02 bohr 3 provided by Russell and Spackman. The DPS reported herein provides a benchmark-quality quantum mechanical estimate of this fundamental quantity of interest and should find extensive use in the development (and assessment) of next-generation force fields and machine-learning based approaches for modeling water in complex condensed-phase environments.

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

Electrical and thermal transport properties of the electron-doped cuprate Sm 2- x Ce x CuO 4- y system

Electrical and thermal transport measurements were performed on thin films of the electron-doped superconductor Sm 2- x Ce x CuO 4- y (x = 0.13 - 0.19) in order to study the evolving nature of the charge carriers from the under-doped to over-doped regime. A temperature versus cerium content (T - x) phase diagram has been constructed from the electrical transport measurements, yielding a superconducting region similar to that found for other electron-doped superconductors. Thermopower measurements show a dramatic change from the underdoped region (x < 0.15) to the overdoped region (x > 0.15). Application of the Fisher–Fisher–Huse (FFH) vortex glass scaling model to the magnetoresistance data was found to be insufficient to describe the data in the region of the vortex-solid to vortex-liquid transition. It was found instead that the modified vortex glass scaling model of Rydh, Rapp, and Anderson provided a good description of the data, indicating the importance of the applied field on the pinning landscape. A magnetic field versus temperature (H - T) phase diagram has also been constructed for the films with $x\geqslant 0.14$ , displaying the evolution of the vortex glass melting lines H g (T) across the superconducting regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗