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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

The Piezoresponse in WO3 Thin Films Due to N2-Filled Nanovoids Enrichment by Atom Probe Tomography

Tungsten trioxide (WO3) is a versatile n-type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d33 = 35 pmV−1 on WO3 thin films ~200 nm deposited using RF-sputtering onto alumina (Al2O3) substrate with post-deposit annealing treatment of 400 °C in a 3% H2/N2-forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO3 with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N2-filled nanovoids that acts as strain centers producing a local deformation of the WO3 lattice into a non-centrosymmetric structure, which is related to piezoresponse observations.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Mechanical properties of zeolite-templated carbons from approximate density functional theory calculations

Zeolite-templated carbon (ZTC) is a unique porous carbonaceous material whose structure is ordered at the nanometre scale, enabling a representative periodic description at the atomistic level. Utilizing an existing, well-defined reference model for ZTCs, a structural library of varying compositions was developed by refinement using density-functional tight-binding (DFTB) potentials parameterized for materials science applications. We first determined the quantum chemical-refined structures of models with CH, CHO, CHON, CHOB, and CHOBN compositions with various degrees of heteroatom substitution. These structural models comprise the characteristic morphological features of highly porous carbon materials, such as open-blade surfaces, edges, saddles, and closed-strut formations, spanning a range of curvatures and characteristic sizes. Second, we carried out alternating compression and expansion of the CHO model unit cell to determine the lowest energy structure as well as to obtain its bulk modulus in order to demonstrate a close connection between macroscopic observations and atomic-scale structures. Further, the agreement between experimental measurements and the computational model is remarkable and demonstrates the power of approximate density functional theory as a cost-effective computational tool with chemical accuracy for the investigation of structure/property relationships in real-world carbon-based solids.

03 NATURAL GAS↗
DOE OSTI2026

Mechanisms of Alkali Ionic Transport in Amorphous Oxyhalides Solid State Conductors

Amorphous oxyhalides have attracted significant attention due to their relatively high ionic conductivity (1 mS cm –1 ), excellent chemical stability, mechanical softness, and facile synthesis routes via standard solid‐state reactions. These materials exhibit an ionic conductivity that is almost independent of the underlying chemistry, in stark contrast to what occurs in crystalline conductors. In this work, we employ machine learning interatomic potentials to construct large‐scale molecular dynamics trajectories encompassing hundreds of nanoseconds to obtain statistically converged transport properties. We find that the amorphous state consists of chain fragments of metal‐anion tetrahedra of various lengths. By analyzing the residence time of alkali cations migrating around tetrahedrally‐coordinated metals, we find that oxygen anions limit alkali diffusion. By computing the full Einstein expression of the ionic conductivity, we demonstrate that the alkali transference number of these materials is strongly influenced by distinct‐particles correlations, while alkali transport is dictated by uncorrelated self‐diffusion. By extending this analysis to chemical compositions AMX 2.5 O 0.75 , spanning different alkaline (A = Li, Na, K), metallic (M = Al, Ga, In), and halogen (X = Cl, Br, I) species, we clarify why the diffusion properties of these materials remain largely insensitive to variations in atomic isovalent chemistry.

amorphous materials↗
DOE OSTI2020

Redox Stability Controls the Cellular Uptake and Activity of Ruthenium‐Based Inhibitors of the Mitochondrial Calcium Uniporter (MCU)

Abstract The mitochondrial calcium uniporter (MCU) is the ion channel that mediates Ca 2+ uptake in mitochondria. Inhibitors of the MCU are valuable as potential therapeutic agents and tools to study mitochondrial Ca 2+ . The best‐known inhibitor of the MCU is the ruthenium compound Ru360. Although this compound is effective in permeabilized cells, it does not work in intact biological systems. We have recently reported the synthesis and characterization of Ru265, a complex that selectively inhibits the MCU in intact cells. Here, the physical and biological properties of Ru265 and Ru360 are described in detail. Using atomic absorption spectroscopy and X‐ray fluorescence imaging, we show that Ru265 is transported by organic cation transporter 3 (OCT3) and taken up more effectively than Ru360. As an explanation for the poor cell uptake of Ru360, we show that Ru360 is deactivated by biological reductants. These data highlight how structural modifications in metal complexes can have profound effects on their biological activities.

Woods, Joshua J.↗
DOE OSTI2020

Redox Stability Controls the Cellular Uptake and Activity of Ruthenium-Based Inhibitors of the Mitochondrial Calcium Uniporter (MCU)

The mitochondrial calcium uniporter (MCU) is the ion channel that mediates Ca 2+ uptake in mitochondria. Inhibitors of the MCU are valuable as potential therapeutic agents and tools to study mitochondrial Ca 2+ . The best-known inhibitor of the MCU is the ruthenium compound Ru360. Although this compound is effective in permeabilized cells, it does not work in intact biological systems. We have recently reported the synthesis and characterization of Ru265, a complex that selectively inhibits the MCU in intact cells. In this work, the physical and biological properties of Ru265 and Ru360 are described in detail. Using atomic absorption spectroscopy and X-ray fluorescence imaging, we show that Ru265 is transported by organic cation transporter 3 (OCT3) and taken up more effectively than Ru360. As an explanation for the poor cell uptake of Ru360, we show that Ru360 is deactivated by biological reductants. These data highlight how structural modifications in metal complexes can have profound effects on their biological activities.

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

Orientation-dependent structural properties during growth and growth mechanism of CoO films

The orientation-dependent local structural properties of CoO films on sapphire substrates during growth were investigated through linearly-polarized extended X-ray absorption fine structure (EXAFS) measurements. Specifically, CoO(1 1 1) and (100) crystals with a rock-salt structure (Fm3m) were epitaxially grown on alpha-Al2O3(0001) and (1012) substrates, respectively, at 700 degrees C using a radio-frequency sputtering system. The local structural properties of CoO films in the in-plane and out-of-plane orientations were quantitatively determined using linearly-polarized EXAFS at the Co K-edge during growth. The EXAFS analysis revealed that during the initial stages of growth, the local structural properties exhibit significant differences compared to thick films, with short atomic distances and large (small) Debye-Waller factors observed in the out-of-plane (in-plane) orientations. The local structural strain mostly diminished when approximately 20 CoO layers accumulated on the substrate. Density functional theory (DFT) calculations further supported these findings by confirming that cobalt atoms initially form stable bonds with the sapphire surface, leading to the simultaneous growth of oxygen and cobalt layers in a coordinated manner through layer-by-layer growth.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Alpha-decay width of a near proton-threshold resonance in the 7 Li( α, α ) channel

We investigate the alpha-decay width of a near proton-threshold resonance in 11 B by the excitation functions of 7 Li(α, α) and 7 Li(α, α’) reactions. This resonance is an example of loosely bound atomic nuclei, understood as small open quantum systems, that exhibit properties significantly influenced by their coupling to the continuum. The present experiment focuses on the formation of a specific state in 11 B at excitation energy in the region near 11.4 MeV by bombarding a 7 Li target with a 4 He beam at energies ranging from 3.92 to 4.56 MeV in the laboratory frame. The study provides data complementary to previous observations of the proton emission in the β-decay of the neutron-rich halo nucleus 11 Be. An R-matrix fit to the data provides resonance energy and partial widths consistent with J π =1/2 + for a narrow near-threshold state in 11 B at 11.400(20) MeV, that is 171(20) keV above the proton emission threshold, with partial α-decay width Γ α =2.5$^{+3.5}_{–1.5}$ keV in elastic scattering and Γ α' =15.8$^{+1.9}_{–0.4}$ keV in inelastic scattering. These findings contribute to the understanding of the structure of this open quantum system resonance.

11B↗
DOE OSTI2021

Efimov-like states and quantum funneling effects on synthetic hyperbolic surfaces

Engineering lattice models with tailored inter-site tunnelings and onsite energies could synthesize essentially arbitrary Riemannian surfaces with highly tunable local curvatures. Here, we point out that discrete synthetic Poincaré half-planes and Poincaré disks, which are created by lattices in flat planes, support infinitely degenerate eigenstates for any nonzero eigenenergies. Such Efimov-like states exhibit a discrete scaling symmetry and imply an unprecedented apparatus for studying quantum anomaly using hyperbolic surfaces. Furthermore, all eigenstates are exponentially localized in the hyperbolic coordinates, signifying the first example of quantum funneling effects in Hermitian systems. As such, any initial wave packet travels towards the edge of the Poincaré half-plane or its equivalent on the Poincaré disk, delivering an efficient scheme to harvest light and atoms in two dimensions. Furthermore, our findings unfold the intriguing properties of hyperbolic spaces and suggest that Efimov states may be regarded as a projection from a curved space with an extra dimension.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Solution-Grown Ternary Semiconductors: Nanostructuring and Stereoelectronic Lone Pair Distortions in I–V–VI 2 Materials

Alkali pnictogen dichalcogenides–I–V–VI 2 or APnCh 2 –have been identified as promising semiconducting materials for energy conversion devices. However, the controlled nanoscale synthesis and our understanding of the effects of cation ordering and stereochemically active lone pairs on the structures of these ternary compounds remain underdeveloped. Here, we use solution-phase chemistry to synthesize a family of APnCh 2 materials, including LiSbSe 2 , NaSbS 2 , NaSbSe 2 , NaBiS 2 , and NaBiSe 2 . Our approach utilizes alkali metal hydrides (AH) or carboxylates, A(O 2 CR), PnPh 3 , and elemental chalcogens as synthetic precursors and oleylamine or 1-octadecene as solvents. Synthetic manipulation via fine-tuning of reaction temperature enables control over the degree of ordering caused by the Sb 5s 2 lone pair-induced distortions in NaSbS 2 . Pair distribution function analysis demonstrates that the structure of the Sb-containing phases deviates much more from a disordered rock salt structure than that of the Bi-containing phases. This local distortion, induced by the Sb lone pair, leads to a previously unreported noncentrosymmetric NaSbS 2 crystal structure, which is additionally supported by second-harmonic generation measurements. Infrared and multinuclear solid-state NMR spectroscopies show that oleylamine or chelating carboxylates and, in some cases, unreacted precursors (LiH and PnPh 3 ) remain bound to the nanocrystalline surfaces. Further, a deeper understanding of the local atomic environment, long-range ordering, surface chemistry, and optoelectronic properties of these materials may speed up their fundamental study and application.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Consequences and Control of Multiscale Order/Disorder in Chiral Magnetic Textures

Transition metal intercalated transition metal dichalcogenides (TMDs) are promising platforms for next-generation spintronic devices based on their wide range of electronic and magnetic phases, which can be tuned by varying the host lattice or intercalant’s identity, stoichiometry, or spatial order. Some of these compounds host a chiral magnetic phase in which the helical winding of magnetic moments propagates along a high-symmetry crystalline axis. Previous studies have demonstrated that variation in intercalant concentrations can have a dramatic effect on the formation of chiral domains and ensemble magnetic properties. However, a systematic and comprehensive study of how atomic-scale order and disorder impact these chiral magnetic textures is so far lacking. Here, we leverage a combination of imaging modes in the (scanning) transmission electron microscope (S/TEM) to directly probe (dis)order across multiple length scales and show how subtle changes in the atomic lattice can tune the mesoscale spin textures and bulk magnetic response in Cr 1/3 NbS 2 , with direct implications for the fundamental understanding and technological implementation of such compounds.

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

Quantum-Confined Lifshitz Transition on Weyl Semimetal T d -MoTe 2

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal T d -MoTe 2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Supramolecular Assembly of U(IV) Clusters and Superatoms with Unconventional Countercations

Superatoms are nanometer-sized molecules or particles that form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in lattices of quantum dots, p-block clusters, and fullerenes. In this report, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters: acidity and the lanthanide or transition-metal countercation. In acid, larger Ln III (Ln = La–Ho) link hexamer (U 6 ) oxoclusters into body-centered cubic frameworks, while smaller Ln III (Ln = Er–Lu and Y) promote linking of 14 U 6 clusters into hollow superclusters (U 84 superatoms). U 84 assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations and U 6 clusters. Divalent transition metals (TM = Mn II and Zn II ) charge-balance and promote the fusion of 10 U 6 and 10 U monomers into a wheel-shaped cluster (U 70 ). Dissolution of U 70 in organic media shows (by small-angle X-ray scattering) that differing supramolecular assemblies are accessed, controlled by TM II -linking of U 70 clusters. Magnetic measurements of these assemblies reveal Curie–Weiss behavior at high temperatures, without pairing of the 5f 2 -electrons down to 2 K.

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

All-Inorganic Halide Perovskites as Potential Thermoelectric Materials: Dynamic Cation off-Centering Induces Ultralow Thermal Conductivity

Halide perovskites are anticipated to impact next generation high performance solar cells because of their extraordinary charge transport and optoelectronic properties. However, their thermal transport behavior has received limited attention. In this work, we studied the thermal transport and thermoelectric properties of the CsSnBr 3–x I x perovskites. We find a strong correlation between lattice dynamics and an ultralow thermal conductivity for series CsSnBr 3–x I x reaching 0.32 Wm –1 K –1 at 550 K. The CsSnBr 3–x I x also possess a decent Seebeck coefficient and controllable electrical transport properties. The crystallography data and theoretical calculations suggest the Cs atom deviates from its ideal cuboctahedral geometry imposed by the perovskite cage and behaves as a heavy atom rattling oscillator. This off-center tendency of Cs, together with the distortion of SnX 6 (X = Br or I) octahedra, produces a highly dynamic and disordered structure in CsSnBr 3–x I x , which gives rise to a very low Debye temperature and phonon velocity. Moreover, the low temperature heat capacity data suggests strong coupling between the low frequency optical phonons and heat carrying acoustical phonons. In conclusion, this induces strong phonon resonance scattering that induces the ultralow lattice thermal conductivity of CsSnBr 3–x I x .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Atomically Resolved Electrically Active Intragrain Interfaces in Perovskite Semiconductors

Deciphering the atomic and electronic structures of interfaces is key to developing state-of-the-art perovskite semiconductors. However, conventional characterization techniques have limited previous studies mainly to grain-boundary interfaces, whereas the intragrain-interface microstructures and their electronic properties have been much less revealed. Herein using scanning transmission electron microscopy, we resolved the atomic-scale structural information on three prototypical intragrain interfaces, unraveling intriguing features clearly different from those from previous observations based on standalone films or nanomaterial samples. These intragrain interfaces include composition boundaries formed by heterogeneous ion distribution, stacking faults resulted from wrongly stacked crystal planes, and symmetrical twinning boundaries. The atomic-scale imaging of these intragrain interfaces enables us to build unequivocal models for the ab initio calculation of electronic properties. Our results suggest that these structure interfaces are generally electronically benign, whereas their dynamic interaction with point defects can still evoke detrimental effects. This work paves the way toward a more complete fundamental understanding of the microscopic structure-property-performance relationship in metal halide perovskites.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Electronic Configurational Transformation of Network Modifiers in Aluminate Glass above Megabar Pressures

Electronic responses of glasses under extreme pressures differ from those of crystalline analogs. Their distinct electronic environments are found in network formers with well-defined, covalent-bonded coordination environments (e.g., [4] Si and [4] Al) and in network modifiers with more disordered, ionic-bonded configurations (e.g., [5,6,7] Ca). Deciphering the evolution of the bonding environment of network modifier cations upon compression provides atomic insights into the pressure-driven hardening and transport properties of glasses. Despite the importance, in contrast to extensive efforts to uncover how network formers behave under pressure, considerable structural disorder around network modifiers makes it challenging to probe their electronic bonding environments under compression. Our understanding of the evolution of network modifiers above megabars is currently absent. Here, we report a discovery of highly densified electronic configurations of network modifier Ca in aluminate glass under extreme compression via the first inelastic X-ray scattering at the Ca L-edge up to 140 GPa. As evidenced by the prominent pressure-driven increases in electronic dispersion and delocalization, densified calcium environments are characterized by a decreased average Ca–O distance, the formation of highly coordinated calcium, a broader distribution of topological variables, and a greater distortion of Ca polyhedra above megabars. The spectral features for the Ca environments reveal significant electronic and bonding modifications, including pressure-driven increases in the ligand field interaction, the covalence characteristic of the Ca–O bond, and the electron–hole Coulomb interaction. These densification paths identify the electronic adaptation of network modifiers above megabars, shedding light on the origins of enhanced electron transport and the electron-storing capacity of glasses under pressure.

amorphous materials↗
DOE OSTI2020

Machine-learned interatomic potentials by active learning: amorphous and liquid hafnium dioxide

We propose an active learning scheme for automatically sampling a minimum number of uncorrelated configurations for fitting the Gaussian Approximation Potential (GAP). Our active learning scheme consists of an unsupervised machine learning (ML) scheme coupled with a Bayesian optimization technique that evaluates the GAP model. We apply this scheme to a Hafnium dioxide (HfO 2 ) dataset generated from a "melt-quench" ab initio molecular dynamics (AIMD) protocol. Our results show that the active learning scheme, with no prior knowledge of the dataset, is able to extract a configuration that reaches the required energy fit tolerance. Further, molecular dynamics (MD) simulations performed using this active learned GAP model on 6144 atom systems of amorphous and liquid state elucidate the structural properties of HfO 2 with near ab initio precision and quench rates (i.e., 1.0 K/ps) not accessible via AIMD. The melt and amorphous X-ray structural factors generated from our simulation are in good agreement with experiment. In addition, the calculated diffusion constants are in good agreement with previous ab initio studies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Frustration- and doping-induced magnetism in a Fermi–Hubbard simulator

Geometrical frustration in strongly correlated systems can give rise to a plethora of novel ordered states and intriguing magnetic phases, such as quantum spin liquids. Promising candidate materials for such phases can be described by the Hubbard model on an anisotropic triangular lattice, a paradigmatic model capturing the interplay between strong correlations and magnetic frustration. However, the fate of frustrated magnetism in the presence of itinerant dopants remains unclear, as well as its connection to the doped phases of the square Hubbard model. Here we investigate the local spin order of a Hubbard model with controllable frustration and doping, using ultracold fermions in anisotropic optical lattices continuously tunable from a square to a triangular geometry. At half-filling and strong interactions U/t ≈ 9, we observe at the single-site level how frustration reduces the range of magnetic correlations and drives a transition from a collinear Néel antiferromagnet to a short-range correlated 120° spiral phase. Away from half-filling, the triangular limit shows enhanced antiferromagnetic correlations on the hole-doped side and a reversal to ferromagnetic correlations at particle dopings above 20%, hinting at the role of kinetic magnetism in frustrated systems. Furthermore, this work paves the way towards exploring possible chiral ordered or superconducting phases in triangular lattices and realizing t–t' square lattice Hubbard models that may be essential to describe superconductivity in cuprate materials.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Revealing quantum effects in highly conductive δ-layer systems

Abstract Thin, high-density layers of dopants in semiconductors, known as δ -layer systems, have recently attracted attention as a platform for exploration of the future quantum and classical computing when patterned in plane with atomic precision. However, there are many aspects of the conductive properties of these systems that are still unknown. Here we present an open-system quantum transport treatment to investigate the local density of electron states and the conductive properties of the δ -layer systems. A successful application of this treatment to phosphorous δ -layer in silicon both explains the origin of recently-observed shallow sub-bands and reproduces the sheet resistance values measured by different experimental groups. Further analysis reveals two main quantum-mechanical effects: 1) the existence of spatially distinct layers of free electrons with different average energies; 2) significant dependence of sheet resistance on the δ -layer thickness for a fixed sheet charge density.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗