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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Small, solubilized platinum nanocrystals consist of an ordered core surrounded by mobile surface atoms

In situ structures of Platinum (Pt) nanoparticles (NPs) can be determined with graphene liquid cell transmission electron microscopy. Atomic-scale three-dimensional structural information about their physiochemical properties in solution is critical for understanding their chemical function. We here analyze eight atomic-resolution maps of small (<3 nm) colloidal Pt NPs. Their structures are composed of an ordered crystalline core surrounded by surface atoms with comparatively high mobility. 3D reconstructions calculated from cumulative doses of 8500 and 17,000 electrons/pixel, respectively, are characterized in terms of loss of atomic densities and atomic displacements. Less than 5% of the total number of atoms are lost due to dissolution or knock-on damage in five of the structures analyzed, whereas 10–16% are lost in the remaining three. Less than 5% of the atomic positions are displaced due to the increased electron irradiation in all structures. The surface dynamics will play a critical role in the diverse catalytic function of Pt NPs and must be considered in efforts to model Pt NP function computationally.

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

Modulation of crystal growth and structure within cerium-based metal–organic frameworks

The intriguing catalytic properties of cerium-based materials have motivated the development of Ce-based metal–organic frameworks (MOFs). However, the controlled crystallization of Ce MOFs remains nascent due to complications with the high reduction potential of Ce 4+ species. Modulators offer a route in other well-studied coordination networks to slow down crystallization processes to allow for corrective, uniform crystal growth. Herein, we report an investigation of modulator identity and concentration on the synthesis of a Ce-UiO-type MOF with 2,6-naphthalenedicarboxylic acid (NDC) as a linker. At low concentrations using both benzoic acid and trifluoroacetic acid, we observed a mononuclear Ce 3+ MOF (NU-350) through single-crystal X-ray diffraction studies. Higher modulator concentrations yielded pure-phase Ce-UiO-NDC, with uniform particle sizes observed with utilizing benzoic acid as a modulator. Moreover, we demonstrated the transferability of this synthesis through the pure-phase synthesis of Ce-UiO-66 with benzoic acid as a modulator. High-resolution transmission electron microscopy showed a single crystalline domain within Ce-UiO-NDC.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Non-Equilibrium Doping of Poly-Si:Ga/SiOx Passivating Contacts Solar Cells

In this contribution, we show that Ga-hyperdoped poly-Si/SiOx contacts are realized by pulsed laser annealing and post-hydrogenation. The motivation behind this work is to improve the inferior performance of boron-doped poly- Si/SiOx contacts when compared to the phosphorus-doped contacts. Here we used Ga as a novel p-type dopant to replace B, as B is known to accumulate at the interface between the oxide and the crystalline silicon and cause recombination. Due to a high diffusivity of Ga in oxide and a much higher segregation coefficient (Si:SiOx), Ga is shown to be immune to the oxide pileup phenomenon. However, Ga has a low solid solubility limit in Si (5x10^19 cm-3), which is not enough to form a low resistivity contact. To overcome this problem, we explored pulsed laser annealing via excimer laser by varying the laser energy densities and pulses. The passivation and electrical properties are studied systematically using photoluminescence (PL), secondary ion mass spectrometry (SIMS), and Van der Pauw-Hall measurements. We show an active Ga doping concentration above 1x10^20 cm^-3, which is above the solid solubility limit of Ga in Si. Good passivation quality is demonstrated with the highest implied Voc (iVoc) of 721 mV on single side polished samples. Furthermore, a low diode contact resistivity of ~22 m..omega.. cm^2 is obtained using Ti/Ag contacts, with minimal metal-to-semiconductor contact resistivity of 0.9 m..omega.. cm^2.

non-equilibrium doping↗
DOE OSTI2024

High temperature stability of regrown and alloyed Ohmic contacts to AlGaN/GaN heterostructure up to 500 °C

This Letter reports the stability of regrown and alloyed Ohmic contacts to AlGaN/GaN-on-Si high electron mobility transistors (HEMTs) for high temperature applications up to 500 °C. Transfer length method (TLM) measurements from 25 to 500 °C in air show that the regrown contacts appear to be stable up to 500 °C during short term (approximately 1 h) testing, while alloyed contacts appear to decrease in contact resistance from 300 to 500 °C though increases in the error bounds due to increase sheet resistance make it difficult to conclude definitely. Additionally, longer term testing shows both technologies remain stable at least up to 48 h at 500 °C, after which the large increase in sheet resistance makes the measurement uncertainty too large to conclude definitively. Advanced microscopy images indicate both the regrown and alloyed contact regions remain structurally intact after prolonged high temperature exposure with no visible degradation in crystallinity or metal composition.

Physics↗
DOE OSTI2020

Designing electrolytes with polymerlike glass-forming properties and fast ion transport at low temperatures

Significance Liquid electrolytes with thermophysical properties analogous to solid polymers, but with exceptional liquidlike ionic conductivities, are formed spontaneously when moderate amounts (≤1 M) of inorganic salts coordinate strongly with small molecules in a conventional aprotic solvent. Specifically, we report that electrolytes composed of the cyclic liquid ether, dioxolane (DOL), and containing the simple salt LiNO 3 are able to completely bypass the liquid → crystalline solid thermal transition, and to exhibit abnormally high bulk and interfacial ionic conductivities down to temperatures as low as −50 °C. Through physical, spectroscopic, and ion-transport measurements it is shown that strong interactions between LiNO 3 and DOL distort bonds in DOL, couple motions of individual solvent molecules, and lower the thermodynamic activity of the electrolyte.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Environment-imposed selection rules for nuclear-spin conversion of H 2 in molecular crystals

Nuclear-spin conversion in molecular hydrogen is governed by strict symmetry rules that typically require magnetic fields or catalytic surfaces to break. Here we demonstrate that the intrinsic tensor composition of a non-magnetic molecular crystal field can impose and relax these rules without external fields. High-resolution infrared spectra of H 2 in crystalline CO 2 reveal large rank-2 (quadrupolar) crystal-field splittings of the 𝑚 sublevels, while nuclear-spin conversion occurs only through 𝛥𝑚 = 0 channels. Replacing CO 2 with polar N 2 O introduces rank-1 (dipole) components that partially open 𝛥⁢𝑚 ≠ 0 pathways, while incorporation of paramagnetic NO 2 fully lifts the restriction. Furthermore, these results establish a direct correspondence between crystal-field tensor rank and nuclear-spin dynamics, introducing a general symmetry-based framework for designing and controlling spin-isomer populations and quantum-state connectivity in molecular solids.

McLane, Nathan [University of Maryland, College Pa↗
DOE OSTI2022

First-principles insight into all-optical spin switching in the half-metallic Heusler ferrimagnet Mn 2 RuGa

All-optical spin switching (AOS) represents a new frontier in magnetic storage technology—spin manipulation without a magnetic field—but its underlying working principle is not well understood. Many AOS ferrimagnets such as GdFeCo are amorphous and renders the high-level first-principles study unfeasible. The crystalline half-metallic Heusler Mn 2 RuGa presents an opportunity. Here we carry out hitherto the comprehensive density functional investigation into the material properties of Mn 2 RuGa, and introduce two concepts—the spin anchor site and the optical active site—as two pillars for AOS in ferrimagnets. In Mn 2 RuGa, Mn(4a) serves as the spin anchor site, whose band is below the Fermi level and has a strong spin moment, while Mn(4c) is the optical active site whose band crosses the Fermi level. Our magneto-optical Kerr spectrum and band structure calculation jointly reveal that the delicate competition between the Ru-4d and Ga-4p states is responsible for the creation of these two sites. Furthermore these two sites found here not only present a unified picture for both Mn 2 RuGa and GdFeCo, but also open the door for future applications. Specifically, we propose a Mn 2 Ru x Ga-based magnetic tunnel junction where a single laser pulse can control magnetoresistance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Mesoporous optically clear heat insulators for sustainable building envelopes

Mesoporous materials exhibit highly controlled nanoscale structures, often templated by liquid crystalline assemblies of surfactants, with emergent and often designable physical properties. However, scaling their fabrication to be suitable for uses like envelopes of buildings is challenging. Here we describe fabrication of flexible square-meter-sized films and multi-centimeter-thick slabs made of three-dimensional spatial graphs of mesopore tubes having all structural features under 50 nanometers. Solution-based kinetic fabrication process templates growing networks of cylindrical surfactant micelles with thin tubes of polysiloxane forming gel networks and, upon replacing surfactants and solvents with air, yields lightweight materials with >99% visible-range optical transparency and ~10 mW K-1m-1 thermal conductivity. Such predesigned metamaterials enable transparent thermal barriers for wall-grade insulated glass units, square-meter window retrofits and unconcentrated solar thermal energy harnessing.

14 SOLAR ENERGY↗
DOE OSTI2024

Quantum barriers engineering toward radiative and stable perovskite photovoltaic devices

Efficient photovoltaic devices must be efficient light emitters to reach the thermodynamic efficiency limit. Here, we present a promising prospect of perovskite photovoltaics as bright emitters by harnessing the significant benefits of photon recycling, which can be practically achieved by suppressing interfacial quenching. We have achieved radiative and stable perovskite photovoltaic devices by the design of a multiple quantum well structure with long (~3 nm) organic spacers with oleylammonium molecules at perovskite top interfaces. Our L-site exchange process (L: barrier molecule cation) enables the formation of stable interfacial structures with moderate conductivity despite the thick barriers. Compared to popular short (~1 nm) Ls, our approach results in enhanced radiation efficiency through the recursive process of photon recycling. This leads to the realization of radiative perovskite photovoltaics with both high photovoltaic efficiency (in-lab 26.0%, certified to 25.2%) and electroluminescence quantum efficiency (19.7 % at peak, 17.8% at 1-sun equivalent condition). Furthermore, the stable crystallinity of oleylammonium-based quantum wells enables our devices to maintain high efficiencies for over 1000 h of operation and >2 years of storage.

14 SOLAR ENERGY↗
DOE OSTI2026

Nanocluster Rearrangement Forms a Family of Ordered Cerium–Titanium Bimetallic Metal–Organic Frameworks with Three Different Nodes, Nanocavities, and Thermal Stabilities

Metal–organic frameworks (MOFs) provide a versatile platform for incorporating multiple metal ions within a single crystalline framework, yet achieving spatial and stoichiometric order in heterometallic nodes remains a synthetic challenge. Building on our previously reported, highly thermally stable Ce/Ti bimetallic MOF NU-3000, we identified and isolated two additional crystalline phases, NU-2998 and NU-2999, that arise from the same Ce/Ti nanocluster precursor under modified solvothermal conditions. Systematic variation of reaction temperature, time, solvent ratio, and modulator concentration directs the assembly of these distinct frameworks. Structural analysis and comprehensive characterization studies reveal that these MOFs each feature an unreported nodal geometry with nanocavities of different sizes. NU-2998 even adopts an unreported topology, denoted nui , that features an elongated pore spanning 4 nm. Together, these findings establish a synthesis route that starts with a nanocluster and ends with a set of bimetallic MOFs, offering a glimpse into the pathway-dependent assembly of multimetallic porous materials. Finally, we evaluated the thermal stability of each additional analogue and compared them to NU-3000, providing further insight into material stability. NU-3000 maintained the highest thermal stability and was evaluated as a catalyst for CO oxidation at elevated temperatures.

bimetallic MOFs↗
DOE OSTI2025

Electrical and structural characterization of in situ MOCVD Al2O3/β-Ga2O3 and Al2O3/β-(AlxGa1−x)2O3 MOSCAPs

This study investigates the electrical and structural properties of metal–oxide–semiconductor capacitors (MOSCAPs) with in situ metal-organic chemical vapor deposition-grown Al2O3 dielectrics deposited at varying temperatures on (010) β-Ga2O3 and β-(AlxGa1−x)2O3 films with different Al compositions. The Al2O3/β-Ga2O3 MOSCAPs exhibited a strong dependence of electrical properties on Al2O3 deposition temperature. At 900 °C, reduced voltage hysteresis (∼0.3 V) with improved reverse breakdown voltage (74.5 V) was observed, corresponding to breakdown fields of 5.01 MV/cm in Al2O3 and 4.11 MV/cm in β-Ga2O3 under reverse bias. In contrast, 650 °C deposition temperature resulted in higher voltage hysteresis (∼3.44 V) and lower reverse breakdown voltage (38.8 V) with breakdown fields of 3.69 and 2.87 MV/cm in Al2O3 and β-Ga2O3, respectively, but exhibited impressive forward breakdown field, increasing from 5.62 MV/cm at 900 °C to 7.25 MV/cm at 650 °C. High-resolution scanning transmission electron microscopy (STEM) revealed improved crystallinity and sharper interfaces at 900 °C, contributing to enhanced reverse breakdown performance. For Al2O3/β-(AlxGa1−x)2O3 MOSCAPs, increasing Al composition (x) from 5.5% to 9.2% reduced net carrier concentration and improved reverse breakdown field contributions from 2.55 to 2.90 MV/cm in β-(AlxGa1−x)2O3 and 2.41 to 3.13 MV/cm in Al2O3. The electric field in Al2O3 dielectric under forward bias breakdown also improved from 5.0 to 5.4 MV/cm as Al composition increased from 5.5% to 9.2%. The STEM imaging confirmed the compositional homogeneity and excellent stoichiometry of both Al2O3 and β-(AlxGa1−x)2O3 layers. These findings demonstrate the robust electrical performance, high breakdown fields, and excellent structural quality of Al2O3/β-Ga2O3 and Al2O3/β-(AlxGa1−x)2O3 MOSCAPs, highlighting their potential for high-power electronic applications.

Bhuiyan, A. F. M. Anhar Uddin (ORCID:0000000334212↗
DOE OSTI2022

Highly stable and tunable peptoid/hemin enzymatic mimetics with natural peroxidase-like activities

Abstract Developing tunable and stable peroxidase mimetics with high catalytic efficiency provides a promising opportunity to improve and expand enzymatic catalysis in lignin depolymerization. A class of peptoid-based peroxidase mimetics with tunable catalytic activity and high stability is developed by constructing peptoids and hemins into self-assembled crystalline nanomaterials. By varying peptoid side chain chemistry to tailor the microenvironment of active sites, these self-assembled peptoid/hemin nanomaterials (Pep/hemin) exhibit highly modulable catalytic activities toward two lignin model substrates 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) and 3,3’,5,5’-tetramethylbenzidine. Among them, a Pep/hemin complex containing the pyridyl side chain showed the best catalytic efficiency ( V max / K m = 5.81 × 10 −3 s −1 ). These Pep/hemin catalysts are highly stable; kinetics studies suggest that they follow a peroxidase-like mechanism. Moreover, they exhibit a high efficacy on depolymerization of a biorefinery lignin. Because Pep/hemin catalysts are highly robust and tunable, we expect that they offer tremendous opportunities for lignin valorization to high value products.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Benzotriazole‐Based Nonfused Ring Acceptors for Efficient and Thermally Stable Organic Solar Cells

Abstract Nonfused ring acceptors (NFRAs) have attracted significant attention for nonfullerene organic solar cells (OSCs) owing to their chemical tunability and facile synthesis. In this study, a benzotriazole‐based NFRA with chlorinated end groups (Triazole‐4Cl) is developed to realize highly efficient and thermally stable NFRA‐based OSCs; an analogous NFRA with nonchlorinated end groups (Triazole‐H) is synthesized for comparison. Triazole‐4Cl film exhibits the high‐order packing structure and the near‐infrared absorption capability, which are advantageous in charge transport and light harvesting of the resulting OSCs. In particular, the strong crystalline behavior of Triazole‐4Cl results in enhanced self‐aggregation, leading to high charge carrier mobility. Owing to these properties, a PBDB‐T (polymer donor):Triazole‐4Cl OSC demonstrates a high short‐circuit current, fill factor, and power conversion efficiency (PCE = 10.46%), outperforming a PBDB‐T:Triazole‐H OSC (PCE = 7.65%). In addition, the thermal stability of a PBDB‐T:Triazole‐4Cl OSC at an elevated temperature of 120 °C exceeds that of a PBDB‐T:Triazole‐H OSC. This is mainly attributed to the significantly higher cold crystallization temperature of Triazole‐4Cl (205.9 °C). This work provides useful guidelines for the design of NFRAs to achieve efficient and thermally stable NFRA‐based OSCs.

Polymer Science↗
DOE OSTI2023

Symmetry Control of Unconventional Spin–Orbit Torques in IrO 2

Abstract Spin–orbit torques generated by a spin current are key to magnetic switching in spintronic applications. The polarization of the spin current dictates the direction of switching required for energy‐efficient devices. Conventionally, the polarizations of these spin currents are restricted to be along a certain direction due to the symmetry of the material allowing only for efficient in‐plane magnetic switching. Unconventional spin–orbit torques arising from novel spin current polarizations, however, have the potential to switch other magnetization orientations such as perpendicular magnetic anisotropy, which is desired for higher density spintronic‐based memory devices. Here, it is demonstrated that low crystalline symmetry is not required for unconventional spin–orbit torques and can be generated in a nonmagnetic high symmetry material, iridium dioxide (IrO 2 ), using epitaxial design. It is shown that by reducing the relative crystalline symmetry with respect to the growth direction large unconventional spin currents can be generated and hence spin–orbit torques. Furthermore, the spin polarizations detected in (001), (110), and (111) oriented IrO 2 thin films are compared to show which crystal symmetries restrict unconventional spin transport. Understanding and tuning unconventional spin transport generation in high symmetry materials can provide a new route towards energy‐efficient magnetic switching in spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

High-energy micrometre-scale pixel direct conversion X-ray detector

The objective of this work was to fabricate and characterize a new X-ray imaging detector with micrometre-scale pixel dimensions (7.8 µm) and high detection efficiency for hard X-ray energies above 20 keV. A key technology component consists of a monolithic hybrid detector built by direct deposition of an amorphous selenium film on a custom designed CMOS readout integrated circuit. Characterization was carried out at the synchrotron beamline 1-BM-B at the Advanced Photon Source of Argonne National Laboratory. The direct conversion detector demonstrated micrometre-scale spatial resolution with a 63 keV modulation transfer function of 10% at Nyquist frequency. In addition, spatial resolving power down to 8 µm was determined by imaging a transmission bar target at 21 keV. X-ray signal linearity, responsivity and lag were also characterized in the same energy range. Finally, phase contrast edge enhancement was observed in a phase object placed in the beam path. This amorphous selenium/CMOS detector technology can address gaps in commercially available X-ray detectors which limit their usefulness for existing synchrotron applications at energies greater than 50 keV; for example, phase contrast tomography and high-resolution imaging of nanoscale lattice distortions in bulk crystalline materials using Bragg coherent diffraction imaging. Here, the technology will also facilitate the creation of novel synchrotron imaging applications for X-ray energies at or above 20 keV.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Localized electronic and vibrational states in amorphous diamond

Amorphous diamond structures are generated by quenching high-density high-temperature liquid carbon using tight-binding molecular-dynamics simulations. We show that the generated amorphous diamond structures are predominated by strong tetrahedral bonds with the sp 3 bonding fraction as high as 97%, thus exhibit an ultra-high incompressibility and a wide band gap close to those of crystalline diamond. A small amount of sp 2 bonding defects in the amorphous sample contributes to localized electronic states in the band gap while large local strain gives rise to localization of vibrational modes at both high and low frequency regimes.

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

Robust and Highly Conductive Water-Stable Copper Iodide-Based Hybrid Single Crystals

High conductivity, carrier mobility as well as long diffusion length are critical for highly efficient solar cells. Here, we report new types of crystalline inorganic-organic hybrid semiconductors, 2D-Cu 2 I 2 (L) [L = benzo[c][1,2,5]thiadiazole (bs) and benzo[c][1,2,5] selenadiazole (bse)], composed of one-dimensional (1D) copper iodide staircase-like chain and benzodiazole-based ligands. These compounds possess narrow and tunable band gaps and high conductivity. High-quality large single crystals of 2D-Cu 2 I 2 (bse) were grown and used to evaluate the charge transport properties of the material. State-of-the-art time-resolved microwave conductivity (TRMC) techniques were used to investigate the charge carrier dynamics, from which a high conductivity of 2 × 10 -4 S/cm, a carrier mobility of ~1 cm 2 V -1 s -1 , a maximum doping/carrier density of 2 +/- 1 × 10 15 cm -3 , and a diffusion length of ~0.17 um were obtained. We report the excellent water/chemical stability combined with high conductivity makes this I-VII-based hybrid material family promising candidates for potential optoelectronic applications.

36 MATERIALS SCIENCE↗