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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Light–Induced Frenkel Defect Pair Formation Can Lead to Phase–Segregation of Otherwise Miscible Halide Perovskite Alloys

Alloys of ABX 3 halide perovskites (HP) exhibit unique phase behavior compared to traditional III-V and II-VI semiconductor alloys used in solar cells. While the latter typically have good mutual miscibility when their mixed components are size matched, and phase-segregate when size mismatched, HP alloys show good miscibility in the dark but can phase-segregate under light. Quantum mechanical calculations described herein reveal light-induced defect formation and migration hold the key. Specifically, the interaction between a halogen vacancy V X with halogen interstitial X i forming together a Frenkel-pair defect emerges as the enabler for phase-segregation in HP alloys. At a threshold bromine composition in the Br-I alloys, the photogenerated holes in the valence band localize, creating thereby a doubly-charged iodine Frenkel-pair (V I + I i ) 2+ . Faster migration of iodine over bromine interstitial into the vacant iodine V I site leads to the formation of iodine-rich and iodine-depleted regions, establishing phase-segregation. Removal of the mobile defects–the agent of segregation–by dark thermal annealing, supplies the opposing force, leading to reversal of phase-segregation. Furthermore, this atomistic understanding can enable some control of the phase-segregation by selecting substituting elements on the B site–such as replacing some Pb by Sn–that are unable to form stable Frenkel defects.

14 SOLAR ENERGY↗
DOE OSTI2022

Charge Reservoirs in an Expanded Halide Perovskite Analog: Enhancing High‐Pressure Conductivity through Redox‐Active Molecules

Abstract As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. Herein, we describe a novel strategy of using redox‐active organic molecules as stoichiometric electron acceptors. The cavities in the new expanded perovskite analogs (dmpz)[Sn 2 X 6 ], (X=Br − ( 1Br ) and I − ( 1I )) are occupied by dmpz 2+ ( N , N ′‐dimethylpyrazinium), with the LUMOs lying ca. 1 eV above the valence band maximum (VBM). Compressing the metal‐halide framework drives up the VBM in 1I relative to the dmpz LUMO. The electronic conductivity increases by a factor of 10 5 with pressure, reaching 50(17) S cm −1 at 60 GPa, exceeding the high‐pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by dmpz 2+ reduction. This work elevates the role of organic cations in 3D metal‐halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

Matheu, Roc↗
DOE OSTI2022

Charge Reservoirs in an Expanded Halide Perovskite Analog: Enhancing High–Pressure Conductivity through Redox–Active Molecules

As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. Herein, we describe a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors. The cavities in the new expanded perovskite analogs (dmpz)[Sn 2 X 6 ], (X=Br – (1Br) and I – (1I)) are occupied by dmpz 2+ (N,N'-dimethylpyrazinium), with the LUMOs lying ca. 1 eV above the valence band maximum (VBM). Compressing the metal-halide framework drives up the VBM in 1I relative to the dmpz LUMO. The electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm –1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by dmpz 2+ reduction. Lastly, this work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Elucidating the Role of Hydrogen at c‐In 2 O 3 /a‐In 2 O 3− x Interface

Ab initio molecular dynamics simulations and hybrid functional electronic structure calculations are employed to determine the formation, the structural and electronic properties, and the dynamics of covalent (In—OH) and ionic (In—H—In) hydrogen defects at crystalline-In 2 O 3 /amorphous-In 2 O 3−x interface. This comprehensive computational study considers (i) various interstitial and substitutional hydrogen site locations within the crystalline, amorphous, and interfacial regions; (ii) several oxygen-to-hydrogen ratios; and (iii) possible defect charged states. The results reveal hydrogen's inability to fully passivate the undercoordinated under-shared in atoms in amorphous highly substoichiometric oxide, giving rise to the formation of deep electron traps even in net-charge neutral structures. These trap defects are found to be sensitive to photoexcitation, in contrast to In—OH with H's electronic states located below the valence band and to In—H—In, where H is found to maintain its charge state upon illumination. Nevertheless, H plays a critical role in photoinduced conductivity and its relaxation by promoting In—O coordination transformations at the interfacial region, including deterioration of the crystalline layer. Finally, the results help identify mobile H species and metastable H defect complexes (such as In—H—H—In, In—OH–H—In, and In—OH–O—In) that are responsible for long relaxation times of the conductivity decay.

H defects↗
DOE OSTI2020

Orbital Hybridization and Magnetic Coupling at Cuprate–Manganite Interfaces Driven by Manganite Doping

Fundamental understanding of interface mechanisms that generate unexpected physical properties in the cuprate/manganite heterostructures is essential for possible applications as spintronic devices. In this study, CaCuO 2 /La 1–x Sr x MnO 3 (CCO/LSMO) superlattices are investigated, where the infinite-layer cuprate CCO does not have apical oxygen for Cu in the Ca-plane so that the Mn–O–Cu superexchange coupling can be only present at the MnO 2 –(La,Sr)O–CuO 2 interface. Two different doping states for the manganites are studied, namely x = 0.1 and x = 0.3, corresponding to the ferromagnetic insulating and ferromagnetic metallic states, respectively, in the manganite phase diagram. Linear and circular dichroism in X-ray absorption by synchrotron radiation at Cu and Mn L-edges clearly demonstrate that in the absence of apical oxygen in the cuprate block, the magnetic coupling between the LSMO and CCO is weakly ferromagnetic when LSMO is metallic, while it is antiferromagnetic when LSMO is insulating, along with an increased Cu 3d(3z 2 –r 2 ) and Mn 3d(3z 2 –r 2 ) orbital occupation. It is proposed that the Mn 3d valence band upward shift driven by the enhancement of Mn 3+ content in the underdoped sample enhances the orbital hybridization. The stronger hybridization at the interface MnO 2 –(La,Sr)O–CuO 2 gives rise to the antiferromagnetic coupling between cuprate and underdoped manganite.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ti‐Modified Imogolite Nanotubes as Promising Photocatalyst 1D Nanostructures for H 2 Production

Imogolite nanotubes (INTs) are predicted as a unique 1D material with spatial separation of conduction and valence band edges but their large band gaps have inhibited their use as photocatalysts. The first step toward using these NTs in photocatalysis and exploiting the polarization-promoted charge separation across their walls is to reduce their band gap. Here, the modification of double-walled aluminogermanate INTs by incorporation of titanium into the NT walls is explored. The precursor ratio x = [Ti]/([Ge]+[Ti]) is modulated between 0 and 1. Structural and optical properties are determined at different scales and the photocatalytic performance is evaluated for H 2 production. Although the incorporation of Ti atoms into the structure remains limited, the optimal condition is found around x = 0.4 for which the resulting NTs reveal a remarkable hydrogen production of ≈1500 µmol g −1 after 5 h for a noble metal-free photocatalyst, a 65-fold increase relative to a commercial TiO 2 -P25. This is correlated to a lowering of the recombination rate of photogenerated charge carriers for the most active structures. These results confirm the theoretical predictions regarding the potential of modified INTs as photoactive nanoreactors and pave the way for investigating and exploiting their polarization properties for energy applications.

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

Electronic structural and lattice thermodynamic properties of MAlO2 and M5AlO4 (M = Li, Na, K) sorbents for CO2 capture applications

Abstract The electronic properties and thermal stabilities of MAlO 2 and M 5 AlO 4 (M = Li, Na, K) are investigated by density functional theory and lattice phonon dynamics. Based on the calculated electronic and lattice thermodynamic properties, their abilities to capture CO 2 as solid sorbents are analyzed. The calculated electronic structural properties of MAlO 2 and M 5 AlO 4 indicate that all these alkali aluminates are semiconductors with a bandgap range of 2.4 ~ 6.4 eV. The 1st valence bands of these alkali aluminates are located 0 ~ − 6 eV under Fermi levels and are mainly contributed by p orbitals of O, s and p orbitals of Al and M. The phonon vibrational frequencies of M 5 AlO 4 spread at a lower frequency range compared to their MAlO 2 phases. With increasing temperature, the calculated phonon free energies of M 5 AlO 4 decrease faster than their corresponding MAlO 2 while their entropies have opposite trends. The reaction 2MAlO 2 + CO 2 = M 2 CO 3 + Al 2 O 3 has higher reaction heat and Gibbs free energy change than those of corresponding reaction 2 / 5 M 5 AlO 4 + CO 2 = M 2 CO 3 + 1 / 5 Al 2 O 3 , which shows the former reaction possesses lower turnover temperature. Among the alkali aluminates studied, the β-NaAlO 2 , lt-KAlO 2 , and γ-LiAlO 2 are better candidates that could be applied for CO 2 capture technologies. Graphical Abstract

36 MATERIALS SCIENCE↗
DOE OSTI2020

A plausible method of preparing the ideal p-n junction interface of a thermoelectric material by surface doping

Recent advances in two-dimensional (2D) crystals make it possible to realize an ideal interface structure that is required for device applications. Specifically, a p-n junction made of 2D crystals is predicted to exhibit an atomically well-defined interface that will lead to high device performance. Using angle-resolved photoemission spectroscopy, a simple surface treatment was shown to allow the possible formation of such an interface. Furthermore, Ta adsorption on the surface of a p-doped SnSe shifts the valence band maximum towards the higher binding energy due to the charge transfer from Ta to SnSe that is highly localized at the surface due to the layered structure of SnSe. As a result, the charge carriers of the surface are changed from holes of its bulk characteristics to electrons, while the bulk remains as a p-type semiconductor. This observation suggests that the well-defined interface of a p-n junction with an atomically thin n-region is formed between Ta-adsorbed surface and bulk.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Unraveling spectral shapes of adventitious carbon on gold using a time-resolved high-resolution X-ray photoelectron spectroscopy and principal component analysis

To extract chemical information reliably, we propose a new data processing method based both on the creation of information vectors and on a vector base change. The originality of this method is the combination of the different core XPS peaks, Auger and/or valence bands in a single vector. We show that the measurement of a sample upon its progressive chemical change, such as accumulation of the adventitious carbon, allows us to create a new vector basis using linear algebra and Principal Component Analysis. In this work, we demonstrate the application of this method using adventitious carbon films on Au foil utilizing Au 4f, C 1s and O 1s spectral envelopes. This method expands the possibilities of XPS measured chemical environment analysis and is an operator-unbiased solution for materials for which the reference XPS spectra are not available. In this demonstration, the emphasis is placed on identifying changes in a C 1s peak assumed to be adventitious carbon and highlight uncertainties associated with calibrating the binding energy scale using a simple peak model to identify a component C 1s assumed to be saturated hydrocarbon in origin.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Evaluation of PbTe and SnTe as Ohmic Contact Layers in CdTe Solar Cell Devices

For solar cells based on CdTe, the choice of a suitable back contact material is limited by CdTe's deep work function. Here, we explored p-type PbTe and SnTe as ohmic contacts to CdTe. These contact layers were grown on single crystal CdTe substrates by molecular beam epitaxy, and the valence band offset between film and substrate was measured using X-ray photoemission spectroscopy. Polycrystalline device structures were also grown by sublimation to assess performance improvements. Doping was achieved in PbTe by thallium incorporation. Only the highest Tl doped PbTe resulted in a desirable band alignment with the CdTe, forming an electron reflector and no hole barrier. Time-resolved photoluminescence measurements also revealed significant photocarrier lifetime improvements for only the highest doped PbTe. Consequentially, devices incorporating the highest doped PbTe layers showed increased power conversion efficiency, primarily from increased fill factor. Doping of the PbTe was measured via Hall effect with variable magnetic field, which was required due to the formation of an n-type parasitic interface layer. To properly interpret the variable field Hall measurements, we derived an expression for the magnetic field-dependent conductivity tensor of an L-valley semiconductor.

anisotropic band valley↗
DOE OSTI2026

Electrochemical Characterization of Photo-Driven Hole-Scavenging by Cadmium Sulfide Quantum Dot-Nitrogenase Biohybrid Complexes

Light-driven biohybrid systems that couple semiconductor nanocrystals with enzymes offer a promising strategy for solar-to-chemical energy-conserving reduction reactions, yet are often limited by inefficient hole scavenging. Hole scavenging is critical for maintaining charge separation in the light-absorbing electron donor molecule and, thus, for sustaining catalytic turnover, making quantification essential to improving system efficiency. Here, we introduce a photoelectrochemical approach for real-time monitoring of hole scavenging in cadmium sulfide (CdS) quantum dot (QD)-nitrogenase MoFe protein biohybrids that catalyze the reduction of H+ and N2. Using hydroquinone (HQ) as the sacrificial electron donor (SED), oxidation of HQ to benzoquinone (BQ) by photogenerated valence-band holes is coupled to the electrochemical reduction of BQ at an electrode, enabling quantification of the hole-scavenging reaction via chronoamperometry. This approach provides a continuous, real-time readout of charge-transfer dynamics under illumination. Systematic variation of light intensity and donor concentration reveals that hole scavenging scales with photon flux and exhibits a non-linear dependence on SED concentration, with an optimum at intermediate HQ concentrations. These results establish chronoamperometry as a powerful tool for probing photoinduced charge transfer, advancing the quantitative understanding of hole-scavenging in biohybrid systems, and offering a generalizable framework for optimizing solar-driven biocatalysis.

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

GFCCLib: Scalable and efficient coupled-cluster Green's function library for accurately tackling many-body electronic structure problems

Coupled-cluster Green’s function (GFCC) calculation has drawn much attention in the recent years for targeting the molecular and material electronic structure problems from a many-body perspective in a systematically improvable way. However, GFCC calculations on scientific computing clusters usually suffer from expensive higher di- mensional tensor contractions in the complex space, expensive inter-process communi- cation, and severe load imbalance, which limits it’s routine use for tackling electronic structure problems. Here we present a numerical library prototype that is specifically designed for large-scale GFCC calculations. The design of the library is focused on a systematically optimal computing strategy to improve its scalability and efficiency. The performance of the library is demonstrated by the relevant profiling analysis of running GFCC calculations on remote giant computing clusters. The capability of the library is highlighted by computing a wide near valence band of a fullerene C60 molecule for the first time at the GFCCSD level that shows excellent agreement with the experimental spectrum.

Peng, Bo↗
DOE OSTI2025

Unveiling the nature of Ga-based chalcogenides for electrical switching selectors

Three-dimensional phase-change memory with stackable crossbar architecture is a promising technology to meet the urgent demands for high-density storage and rapid information processing in the era of explosive data growth. The performance depends strongly on the properties of ovonic threshold switching (OTS) selectors, which control the on/off states of memory units. Amorphous GaS serves as an outstanding OTS material, distinguished by its sizable mobility gap and high crystallization temperature, while the underlying mechanism continues to be inadequately comprehended. Here, in this work, we systematically studied the structural and electronic properties of amorphous Ga-X (X = S/Se/Te) using first-principles calculations. The results show that Ga atoms adopt tetrahedral motifs, while S/Se/Te atoms predominantly exhibit the structure of a distorted triangular pyramid. This structural arrangement is ascribed to the substantial dative bonds formed by the lone-pair electrons of the anions and the vacant sp3 orbitals around Ga atoms. Large mobility gaps (e.g., GaS: 2.43 eV, GaSe: 1.76 eV, GaTe: 1.26 eV) and distinct mid-gap states (e.g., ∼0.66 eV above valence band tail) ensure that these three chalcogenide glasses can be switched on under an external electric field while effectively suppressing leakage current without a bias, and the defect electronic states originate from short, robust Ga-Ga bonds due to the formation of distorted chain-like local structures. Our research elucidates the mechanisms of amorphous Ga-X as OTS materials, enriching the spectrum of electrical switching selectors by incorporating III-VI chalcogenides. This inclusion offers novel opportunities for the refinement and optimization of high-density integrated memory systems.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Locating anionic hydrogen in Ba 3 (Yb,Lu) 2 O 5 H 2 : A combined approach of X-ray diffraction, crystal chemistry, and DFT calculations

By a combination of x-ray diffraction, structural chemistry, and DFT calculations, the presence and location of anionic hydrogen in the two new, layered lanthanide oxyhydrides, Ba 3 Ln 2 O 5 H 2 (Ln ​= ​Yb, Lu) is inferred. Single crystals of the compounds have been synthesized from a molten barium flux with the addition of small amounts of BaH 2 . These phases crystallize in space group I4/mmm (#139, Z ​= ​2) with lattice parameters a ​= ​4.3336(2) Å and c ​= ​22.7197(6) Å, and a ​= ​4.3291(1) Å and c ​= ​22.597(1) Å, respectively. The Ba 3 Ln 2 O 5 H 2 phases comprise two different structural moieties: a perovskite double layer of stoichiometry Ba 2 Ln 2 O 5 H – formed by corner-connected LnO 5 tetragonal bi-pyramids with a terminating hydrogen anion, and a puckered rocksalt-type (BaH) + layer that is stretched along the c-axis. DFT calculations were used to arrive at hydrogen positions that minimize energy and are consistent with structural chemistry principles. Furthermore, the calculations show that the valence band edge is dominated by oxygen 2p orbitals with hydrogen 1s states admixed. The conduction band is formed by barium 5d-orbitals and Lu (Yb) 5d-orbitals. These are characteristics of materials with anionic H – . These new phases are isostructural with the Ba 3 Ln 2 O 5 Cl 2 (Ln ​= ​Gd–Lu) family of compounds with the chlorine atom in the same apical position as the hydrogen atom. Finally, steric effects limit the size of the lanthanide ion for Ba 3 Ln 2 O 5 H 2 .

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

How silicon and boron dopants govern the cryogenic scintillation properties of N -type GaAs

This work is the first report describing how the concentrations of silicon and boron govern the cryogenic scintillation properties of n-type GaAs. It shows that valence band holes are promptly trapped on radiative centers and then combine radiatively with silicon donor band electrons at rates that increase with the density of free carriers. It also presents the range of silicon and boron concentrations needed for efficient light emission under X-ray excitation, which along with its low band gap and apparent absence of afterglow, make scintillating GaAs suitable for the detection of rare, low-energy electronic excitations from interacting dark matter particles. A total of 29 samples from four different suppliers were studied. Luminosities and timing responses were measured for the four principal emission bands centered at 860, 930, 1070, and 1335 nm, and for the total emissions. Excitation pulses of 40 kVp X-rays were provided by a light-excited X-ray tube driven by an ultra-fast laser. Scintillation emissions from 800 to 1350 nm were measured using an InGaAs photomultiplier. Within the concentration ranges of free carriers from 2 x 10 16 /cm 3 to 6 x 10 17 /cm 3 and boron from 1.5 × 10 18 /cm 3 to 6 x 10 18 /cm 3 , nine samples have luminosities > 70 photons/keV and two have luminosities > 110 photons/keV. Other samples in that range have lower luminosities due to higher concentrations of non-radiative centers. The decay times decrease by typically a factor of ten with increasing free carrier concentrations from 10 17 /cm 3 to 2 x 10 18 /cm 3 .

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

What can data science tell us about finding new superconductors?

Can data science guide researchers toward understanding superconductivity or discover new superconductors? We examine this question in light of a study in this issue of Patterns by Liu et al., who find that the superconducting transition temperature and certain computed energy intervals of the valence band are correlated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Zr 3 Mn 3 Sn 4 Ga: A new hetero-kagome bilayer antiferromagnet

Here, we report the magnetic and electrical transport properties of single-crystalline Zr 3 Mn 3 Sn 4 Ga, featuring two distinct kagome lattices: a non-magnetic breathing Zr 3 Sn 4 lattice and a magnetic intact Mn 3 Ga lattice. The material undergoes an antiferromagnetic phase transition at T N = 87 K, with neutron diffraction confirming commensurate ordering characterized by k = (1/3,1/3,0). Transport measurements show metallic behavior, a resistivity anomaly near T N , and 12% magnetoresistance at 2 K under 9 T. Deviations from the conventional second-order power law, along with negative magnetoresistance and nonlinear Hall slope variations near T N , suggest strong magneto-electronic coupling. Resonant photoemission spectroscopy identifies Zr 4d and Mn 3d orbitals as dominant contributors to the valence band, linking the material’s unique electronic properties to its kagome layers. Zr 3 Mn 3 Sn 4 Ga offers a valuable platform to study interactions between magnetism and topological electronic bands in hetero-kagome systems using the co- existence of magnetic and non-magnetic kagome layers and its tunable electronic structure.

Crystal structure↗