SEARCH · Search NASA
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Simulations on the aqueous green fluorescent protein (GFP) chromophore (in the equilibrium and non-equilibrium regimes) reveal that observed biexponential fluorescence originates from two competing torsional deactivation pathways.
The Z – E photoisomerization quantum yield of the HBDI − chromophore is a result of early, non-statistical dynamics around a less reactive I-twisted intersection and later, statistical behavior around the more reactive, near-enantiomeric counterpart.
Here, a new synthesis of Th(II) complexes has been identified involving addition of simple MX salts (M = Li, Na, K; X = H, Cl, Me, N 3 ) to Cp" 3 Th III [Cp" = [C 5 H 3 (SiMe 3 ) 2 ] in the presence of 18-crown-6 or 2.2.2-cryptand, forming [M(chelate)][Cp" 3 Th II ] and Cp'' 3 Th IV X. Cp tet 3 Th III (Cp tet = C 5 Me 4 H) reacts with KH to form Cp tet 3 Th IV H and the C–H bond activation product, [K(crypt)]{[Cp tet 2 Th IV H[η 1 :η 5 -C 5 Me 3 H(CH 2 )]}.
A combination of 19 F magic angle spinning (MAS) nuclear magnetic resonance (NMR) and density functional theory (DFT) were used to study the ordering of F atoms in Pb 2 Ti 4 O 9 F 2 .
Irradiation induced non-equilibrium point defect populations influence mass transport in oxides, which in turn affects their stability and performance in hostile environments. In this study a strong dose rate dependence is observed.
Synthesis of a series of hydrocarbon-soluble triarylamines bearing F, CF 3 , and Br substituents showing quasi-reversible redox events in the 0.59–1.32 V range is reported. A few have been isolated as [HCB 11 Cl 11 ] − salts.
Host-mediated liquid–liquid extraction is a convenient method for the separation of inorganic salts.
Molecular cages with three-dimensional cavities have garnered significant interest due to their enhanced encapsulation abilities.
Single crystals and polycrystalline powders of rare earth mixed chalcogenide iodides La 3 Si 2 Se 1.21 S 6.79 I, Ce 3 Si 2 Se 1.39 S 6.61 I, Pr 3 Si 2 Se 1.22 S 6.78 I, and Nd 3 Si 2 Se 1.18 S 6.82 I were prepared using the reactive flux-assisted boron–chalcogen mixture (BCM) method at 850 °C. All compounds crystallized in the monoclinic crystal system, space group C2/c (space group number 15). The series adopts the La 3 Si 2 O 8 Cl structure type, containing isolated SiQ 4 tetrahedra connected by REQ 8 (RE = La, Ce, Pr and Nd) polyhedra; this arrangement creates tunnels that are filled by I atoms. The partial substitution of S by Se was carried out to modulate the optical properties. Phase pure samples and uniform solid solutions were obtained for all compositions as determined using powder X-ray diffraction patterns. Polycrystalline powders were used for physical property measurements, including magnetic susceptibility and UV-Vis diffuse reflectance. The solid-state UV-Vis data for the polycrystalline La 3 Si 2 Se 1.21 S 6.79 I, Ce 3 Si 2 Se 1.39 S 6.61 I, and Pr 3 Si 2 Se 1.22 S 6.78 I samples revealed band gaps of E g = 2.5(1), 2.2(1), and 2.3(1) eV, typical of semiconductors. Magnetic measurements indicated that Ce 3 Si 2 Se 1.39 S 6.61 I and Nd 3 Si 2 Se 1.18 S 6.82 I exhibit paramagnetic behavior with slightly negative Weiss constants θ = −25 and −38. The photoluminescence spectrum of Ce 3 Si 2 Se 1.39 S 6.61 I exhibits a broad emission band around ∼493 nm.
Recently, transition metal perovskite chalcogenide materials have been introduced as possible candidates for solar cell applications. Here, we provide accurate theoretical calculations for BaZrS 3 and two phases of SrZrS 3 , which have been recently synthesized and their optical properties elaborated. In this study, we consider the substitution of S in BaZrS 3 with Se to form BaZrSe 3 . Evolutionary methods are used to find the optimal structure of this compound, and accurate calculations of its optoelectronic properties are presented. Using phonon frequency calculations and ab initio molecular dynamics, we assess the stability of this compound. We find that BaZrSe 3 is likely to be stable under typical conditions, with a low bandgap and high optical absorption coefficients. This indicates that BaZrSe 3 could be useful for solar cell applications.
Commercialization of halide perovskites in the semiconductor industry is hindered by their short-term stability. The instability of perovskites is closely interlinked with ionic diffusion. Historically, attempts to study diffusion in 2D perovskites mostly utilized electrical characterizations, but these characterizations pose a challenge in deconvoluting the impact of device architecture, interlayers, and ionic species. In this Perspective, we focus our attention on simple optical characterizations employed in the literature to investigate halide diffusion in 2D perovskites using lateral and vertical heterostructure platforms. We review the various synthesis techniques used for fabrication of halide perovskite heterostructures and discuss the qualitative and quantitative diffusion studies performed using these platforms. We discuss the numerical methods used to validate and supplement the experimental halide diffusion kinetics. Finally, we highlight the need to conduct further research on the impact of device operating conditions, lattice structure, and vacancy concentration on halide diffusion. Through this Perspective, we aim to emphasize the need of developing a comprehensive understanding of halide diffusion in perovskites for their successful deployment in optoelectronics.
Deep eutectic solvents (DESs) are considered tunable solvents because their specific properties can be achieved based on the choice of components and their relative concentrations in a mixture. In this article, we investigate the influence of the variation in halide ions (F − , Cl − , Br − , I − ) of choline salts used on the thermodynamic and physicochemical properties of choline halide-based DESs. Our findings show that the density of choline halide-based DESs decreases nonlinearly with an increasing mole fraction of water, following a trend based on the size of the halides, with choline iodide showing the highest density. Temperature-dependent density data reveal that the thermal expansion coefficient decreases slightly with increasing water content, indicating a more stable volume at a higher mole fraction of water. The excess molar volume (V E ) of the DES mixtures exhibits complex behavior depending on the choline halide used, with both negative and positive V E values observed across different water mole fractions. These variations are linked to the hydrogen bonding interactions between the DES components and water molecules. In addition, viscosities decrease with increasing water content, suggesting the disruption of hydrogen bonding networks and enhanced mobility of the ions, which contributes to the observed increase in conductivity. The excess molar Gibbs energies, enthalpies, and entropies of activation have also been determined.
Not Available
Here, the dynamics of dissociative electron attachment to ethanol is experimentally investigated at the Feshbach resonance formed with incident electron energies near 9.5 eV. Highly differential laboratory-frame momentum distributions of OH – fragments are measured for a series of energies spanning the resonance width, using the velocity-map-imaging technique. The OH – kinetic-energy distribution indicates that the C-O breaking dissociation process could either be a three-body dissociation or a two-body dissociation with significant rovibrational excited fragments. The small, but significant, anisotropy in the OH – angular distribution provides signatures of the molecular symmetry of the associated resonant state under the axial recoil approximation, which assumes the dissociation is much faster than any rotation of the dissociation axis. Within these assumptions, the 9.5-eV Feshbach resonance can be assigned to the electronic transition from the ($10a'$) orbital with its ground-state C s symmetry to the empty ($4a''$) level, involving the simultaneous electron attachment. This dynamics could be a model for C-O dissociation in larger alcohols and ethers.
At high energies, single-photon photodetachment of alkali negative ions populates final states where both the ejected electron and the residual valence electron possess high angular momenta. The photodetached electron interacts strongly with the anisotropic core, and thus the partial cross sections for these channels display non-Wigner threshold behavior reflecting these large, and occasionally repulsive, interactions. Our fully quantum-mechanical theoretical study enables a deeper interpretation of these partial cross sections. Comparisons of the behavior in different channels and between different atomic species—sodium, potassium, and cesium—show the critical role of near degeneracies in the energy spectrum and demonstrate that much of the behavior of the partial photodetachment cross sections stems from the permanent, rather than induced, electric dipole moments of these nearly degenerate channels. This provides a concrete example of a system where negative dispersion forces play a decisive role.
The high-temperature phase of LiBH 4 (HT-LiBH 4 ) exhibits a promisingly high lithium ion conductivity but is unstable at room temperature. We use density functional theory (DFT) calculations to investigate the stabilization effect of halogen and alkali cation/anion substitutions on HT-LiBH 4 as well the underlying mechanism for the high lithium ion conductivity. We find that increasing the substituent concentration enhances the stabilization of HT-LiBH 4 (i.e., the DFT energy difference between the low- and high-temperature forms of substituted LiBH 4 is reduced). Cation/anion substitution also leads to a higher Li defect (vacancy, interstitial, and Frenkel) formation energy, thereby reducing the Li defect (vacancy, interstitial, and Frenkel) concentrations. Using DFT migration barriers input into kinetic Monte Carlo simulations and the Materials INTerface (MINT) framework, here we calculate the room-temperature lithium ion conductivities for Li(BH 4 ) 1-x I x (x = 0.25 and 0.5) and Li 1-y K y BH 4 (y = 0.25). Our calculations suggest that the lower I concentration leads to a higher lithium ion conductivity of 5.7 x10 -3 S/cm compared with that of Li(BH 4 ) 0.5 I 0.5 (4.2 ×10 -5 S/cm) because of the formation of more Li-related defects. Based on our findings, we suggest that the stabilization of HT-LiBH 4 -based lithium ion conductors can be controlled by carefully tuning the cation/anion substituent concentrations to maximize the lithium ionic conductivities of the specific system.
In this paper, we report the computational investigation of a series of ternary $X_4Y_2Z$ and $X_5Y_2Z_2$ compounds with $X$ =Mg, Ca, Sr, Ba; $Y$ =P, As, Sb, Bi; and $Z$ =S, Se, Te. The compositions for these materials were predicted through a search guided by machine learning, while the structures were resolved using the minima hopping crystal structure prediction method. Based on ab initio calculations, we predict that many of these phases are thermodynamically stable. In particular, 21 of the $X_4Y_2Z$ compounds crystallize in a tetragonal structure with $I\bar{4}2d$ symmetry, and exhibit band gaps in the range of 0.8 and 1.8 eV, well suited for various energy applications. We show that several candidates (in particular $X_4Y_2$ Te and $X_4\mathrm{Sb_2Se}$) exhibit good photo absorption in the visible range, while others (e.g., $\mathrm{Ba_4Sb_2Se}$) show excellent thermoelectric performance due to high power factors and extremely low lattice thermal conductivities.