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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 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Influence of heterovalent doping on tetrahedral N interstitial formation in dilute GaAsN alloys

Dilute alloying of GaAs with N enables bandgap tuning for near-infrared to mid-infrared optoelectronic devices. However, non-substitutional N incorporation has been linked to lower absorption and emission efficiencies in dilute-nitride-alloy-based devices, especially in those containing heterovalent dopants. Here, in this work, we examine the influence of heterovalent dopants on N incorporation mechanisms in dilute GaAs 1−x N x alloys with N composition intentionally below the threshold composition for the formation of tetrahedral N interstitials (N tetra ) in undoped GaAs 1−x N x . For undoped GaAs 1−x N x , 20% of the N incorporates in non-substitutional sites, as (N-N) As and (N-As) split interstitials. Interestingly, Si dopants induce the formation of N tetra , while Be doping has a negligible effect on the interstitial type. Although elastic interactions due to opposite signs of the misfit volumes of N tetra and N As contribute to N tetra incorporation above a threshold N composition, Si dopants reduce the threshold composition, due to the Fermi level-dependent stability of N tetra .

36 MATERIALS SCIENCE↗
DOE OSTI2017

Chemical vapor deposition of monolayer MoS 2 directly on ultrathin Al 2 O 3 for low-power electronics

Monolayer MoS 2 has recently been identified as a promising material for high-performance electronics. However, monolayer MoS 2 must be integrated with ultrathin high-κ gate dielectrics in order to realize practical low-power devices. In this paper, we report the chemical vapor deposition (CVD) of monolayer MoS 2 directly on 20 nm thick Al 2 O 3 grown by atomic layer deposition (ALD). The quality of the resulting MoS 2 is characterized by a comprehensive set of microscopic and spectroscopic techniques. Furthermore, a low-temperature (200 °C) Al 2 O 3 ALD process is developed that maintains dielectric integrity following the high-temperature CVD of MoS 2 (800 °C). Field-effect transistors (FETs) derived from these MoS 2 /Al 2 O 3 stacks show minimal hysteresis with a sub-threshold swing as low as ~220 mV/decade, threshold voltages of ~2 V, and current I ON /I OFF ratio as high as ~10 4 , where I OFF is defined as the current at zero gate voltage as is customary for determining power consumption in complementary logic circuits. Finally, the system presented here concurrently optimizes multiple low-power electronics figures of merit while providing a transfer-free method of integrating monolayer MoS 2 with ultrathin high-κ dielectrics, thus enabling a scalable pathway for enhancement-mode FETs for low-power applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Impact of Isotropic Strain on Magnetic Properties of Monolayer MoSe{sub 2}-Doped with Nonmagnetic Metal and Nonmetal Atoms

Electronic and magnetic properties of MoSe{sub 2} monolayer doped with different nonmagnetic metal (Ag, Al, Li, Mg, and Na) and nonmetal (B, C, F, N, and O) elements have been studied by using the density functional theory calculations. Magnetism has been induced by Ag, Al, and Mg doping. Then, we further have studied the effect of strain on the magnetism in Al-doped MoSe{sub 2} monolayer. We have applied an isotropic strain on the system. On the one hand, the magnetism disappeared while the compressive strain was applied. On the other hand, the magnetism kept stable under the tensile strain. Moreover, we investigated the magnetic coupling in two-Al-doped MoSe{sub 2} structures under the tensile strain. Interestingly, a tunable magnetic coupling was observed. As the tensile strain increased, the magnetic coupling between two Al atoms switched from nonmagnetic (NM) to ferromagnetic (FM). The analysis of the PDOS showed that the p-p hybridization between the Al atom and its neighboring Se atoms resulted in such magnetic behavior. Our results showed some potential applications in MoSe{sub 2}-based nanoscale devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phase evolution during spark plasma sintering of novel Si3N4-doped TiB2–SiC composite

Highlights: • The effect of Si{sub 3}N{sub 4} addition on microstructure of TiB{sub 2}–SiC composite was studied. • Fully dense TiB{sub 2}-based composite was obtained by addition of SiC and Si{sub 3}N{sub 4}. • TiC, B{sub 4}C and SiO{sub 2} were formed in Si{sub 3}N{sub 4}-free and Si{sub 3}N{sub 4}-doped TiB{sub 2}–SiC ceramics. • TiN and BN were in-situ formed only in Si{sub 3}N{sub 4}-doped TiB{sub 2}–SiC sample. - Abstract: Spark plasma sintering process was employed to fabricate TiB{sub 2}–SiC and novel Si{sub 3}N{sub 4}-doped TiB{sub 2}–SiC ceramic composites at 1900 °C for 7 min under 40 MPa. The microstructural development of TiB{sub 2}-based composites was significantly influenced by the addition of SiC as a reinforcement phase and Si{sub 3}N{sub 4} as a sintering aid. Thermodynamic assessment, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy disclosed the in-situ formation of TiC, B{sub 4}C and SiO{sub 2} in both samples as well as TiN and BN in the Si{sub 3}N{sub 4}-doped TiB{sub 2}–SiC sample through the chemical reactions of SiC and Si{sub 3}N{sub 4} additives with the surface oxide impurities of TiB{sub 2} particles (TiO{sub 2} and B{sub 2}O{sub 3}). The elimination of harmful oxide layers and the in-situ formation various secondary phases had remarkable influences on the sinterability and final microstructure of TiB{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Impurity-generated non-Abelions

Two classes of topological superconductors and Majorana modes in condensed matter systems are known to date: one in which disorder induced by impurities strongly suppresses topological superconducting gap and is detrimental to Majorana modes, and another where Majorana fermions are protected by a disorder-robust topological superconductor gap. Observation and control of Majorana fermions and other non-Abelions often requires a symmetry of an underlying system leading to a gap in the single-particle or quasiparticle spectra. In semiconductor structures, impurities that provide charge carriers introduce states into the gap and enable conductance and proximity-induced superconductivity via the in-gap states. Thus a third class of topological superconductivity and Majorana modes emerges, in which topological superconductivity and Majorana fermions appear exclusively when impurities generate in-gap states. We show that impurity-enabled topological superconductivity is realized in a quantum Hall ferromagnet, when a helical domain wall is coupled to an 𝑠-wave superconductor. As an example of emergence of topological superconductivity in quantum Hall ferromagnets, we consider the integer quantum Hall effect in Mn-doped CdTe quantum wells. Recent experiments on transport through the quantum Hall ferromagnet domain wall in this system indicated a vital role of impurities in the conductance, but left unresolved the question whether impurities preclude generation of Majorana fermions and other non-Abelions in such systems in general. Here, solving a general quantum-mechanical problem of impurity bound states in a system of spin-orbit coupled Landau levels, we demonstrate that impurity-induced Majorana modes emerge at boundaries between topological and conventional superconducting states generated in a domain wall due to proximity to an 𝑠 superconductor. We consider both short-range disorder and a smooth random potential. The phase diagram of the system is defined by characteristic disorder, gate voltage induced angular momentum splitting of impurity levels, and by a proximity superconducting gap. The phase diagram exhibits two ranges of gate voltage with conventional superconducting order separated by a gate voltage range with topological superconductivity. We show that electrostatic control of domain walls in an integer quantum Hall ferromagnet allows manipulation of Majorana fermions. Ferromagnetic transitions in the fractional quantum Hall regime may lead to the formation and electrostatic control of higher order non-Abelian excitations.

Edge states↗
DOE OSTI2022

Zn‐Doped P‐Type InAs Nanocrystal Quantum Dots

Abstract Doped heavy metal‐free III–V semiconductor nanocrystal quantum dots (QDs) are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p–n homojunction devices. InAs nanocrystals (NCs), that are of particular relevance for short‐wave IR detection and emission applications, manifest heavy n‐type character poising a challenge for their transition to p‐type behavior. The p‐type doping of InAs NCs is presented with Zn – enabling control over the charge carrier type in InAs QDs field effect transistors. The post‐synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p‐type doping is achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn 2+ replacing In 3+ is established by X‐ray absorption spectroscopy analysis. Furthermore, enhanced near infrared photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high‐resolution electron microscopy corroborated by X‐ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short‐wave infrared region utilizing heavy‐metal free nanocrystal building blocks.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Topological Surface Superconductivity via Josephson Coupling in Bi 2 Te 3 /Nb

Since discoveries of protected conducting surface states, topological superconducting qubits have enchanted quantum science as prime elements in future fault-tolerant devices, particularly those based on Josephson junctions containing topological insulators. Still, Josephson coupling is often eclipsed by other proximity effects that can dilute topological superconducting pairing at the nontrivial insulator’s boundaries. Here, however, using an ultra-low-temperature scanning tunneling microscope, we detect Josephson physics in topological Bi 2 Te 3 films on superconducting Nb. At low temperatures, a previously undetected proximity gap varies little with Bi 2 Te 3 thickness and the density of states exhibits normal and superconducting components. Such observations are rationalized via Josephson pair tunneling through the (nearly) insulating Bi 2 Te 3 bulk, creating a rare, pure topological superconducting sheet. Our findings establish routes toward accessible topological superconducting states in qubits.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Nitrogen-Doped Carbon Flowers with Fe and Ni Dual Metal Centers for Effective Electroreduction of Oxygen

Carbon-based nanocomposites have been attracting extensive attention as high-performance catalysts in alkaline media towards the electrochemical reduction of oxygen. Herein, polyacrylonitrile nanoflowers are synthesized via a free-radical polymerization route and used as a structural scaffold and precursor, whereby controlled pyrolysis leads to the ready preparation of carbon nanocomposites (FeNi-NCF) doped with both metal (Fe and Ni) and nonmetal (N) elements. Transmission electron microscopy studies show that the FeNi-NCF composites retain the flower-like morphology, with the metal species atomically dispersed into the flaky carbon petals. Remarkably, despite a similar structure, elemental composition, and total metal content, the FeNi-NCF sample with a high Fe:Ni ratio exhibits an electrocatalytic performance towards oxygen reduction reaction (ORR) in alkaline media that is similar to that by commercial Pt/C, likely due to the Ni to Fe electron transfer that promotes the adsorption and eventual reduction of oxygen, as evidenced in X-ray photoelectron spectroscopic measurements. Results from this study underline the importance of the electronic properties of metal dopants in the manipulation of the ORR activity of carbon nanocomposites.

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

Observation of Quantum Anomalous Hall Effect and Exchange Interaction in Topological Insulator/Antiferromagnet Heterostructure

Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall e?ect in a magnet-ically-doped topological insulator grown on the antiferromagnetic insulator Cr?O?. The exchange coupling between the two materials is investigated using ?eld-cooling-dependent magnetometry and polarized neutron re?ec-tometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr?O? and the magnetic topological insulator, manifested as an exchange bias when the sample is ?eld-cooled under an out-of-plane magnetic ?eld, and an exchange spring-like magnetic depth pro?le when the system is magnetized within the ?lm plane. These results identify antiferromagnetic insulators as suitable candidates for the manipula-tion of magnetic and topological order in topological insulator ?lms.

Pan, Lei↗
DOE OSTI2024

Electrochemical Control of the Ultrafast Lattice Response of a Layered Semimetal

The unique layer-stacking in two-dimensional (2D) van der Waals materials facilitates the formation of nearly degenerate phases of matter and opens novel routes for the design of low-power, reconfigurable functional materials. Electrochemical ion intercalation between stacked layers offers a promising approach to stabilize bulk metastable phases and to explore the effects of extreme carrier doping and strain. However, in situ characterization methods to study the structural evolution and dynamical functional properties of these intercalated materials remains limited. Here a novel experimental platform is presented capable of simultaneously performing electrochemical lithium-ion intercalation and multimodal ultrafast characterization of the lattice using both electron diffraction and nonlinear optical techniques. Using the layered semimetal WTe 2 as a model system, the interlayer shear phonon mode that modulates stacking between 2Dlayers is probed, showing that small amounts of lithiation enhance the amplitude and lifetime of the phonon, contrary to expectations. This results from the dynamically fluctuating and anharmonic structure between nearly degenerate phases at room temperature, which can be stabilized by electronic carriers accompanying the inserted lithium ions. At high lithiation, the T d ’ structure emerges and quenches the phonon response. This work defines new approaches for using electrochemistry to engineer the dynamic structure of 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhancing the Rate Capability of Niobium Oxide Electrode through Rare‐Earth Doping Engineering

Abstract Niobium oxide is considered one of the most promising electrode materials for ultrafast electrochemical energy storage devices with high energy density. In this work, we first found that trace amounts (<1 %) of Yb 3+ doping remarkably enhanced the rate capability of T−Nb 2 O 5 , which was further proved to be because of the formation of a thin layer of amorphous YbNbO x . The capacity of YbNbO x ‐coated Nb 2 O 5 reached 124 mAh g −1 at 20 C, which is the highest value reported among pristine Nb 2 O 5 electrodes. Besides, we found that doping with other rare‐earth elements such as Nd 3+ , Sm 3+ , and Gd 3+ can also improve the rate capability of T−Nb 2 O 5 . It can be concluded that rare‐earth doping can be an effective and robust method to enhance the intrinsic rate performance of electrode materials without complicated modifications.

Yin, Dandan↗
DOE OSTI2018

Observation of Magnetic Phase Transition and Magnetocaloric Effect in Ba{sub 1−x}Sr{sub x}MnO{sub 3−δ}

A series of perovskite materials (BaMnO{sub 3−δ} and Ba{sub 1−x}Sr{sub x}MnO{sub 3}) were synthesized by a solid-state reaction method. The content of oxygen vacancy can be controlled by verifying the sintering temperature and atmosphere. Two obvious divarications between zero-field-cooling curves and field-cooling curves were observed in a high-temperature region of 250–300 K and at a low temperature of 40 K, respectively. It is found that the different oxygen vacancy contents in BaMnO{sub 3−δ} cause the change of crystalline structure and variation of the divarication temperature at a high-temperature region. The factor that controls the negative magnetic susceptibility was discussed. The superexchange interaction between Mn ions can give rise to the coexistence of the ferromagnetic phase and antiferromagnetic phase in the system. We further performed the study of magnetocaloric effect in Sr-doped BaMnO{sub 3}. However, the values of magnetic entropy change and refrigeration coefficient are much smaller than those of rare-earth manganites due to the weak magnetocrystalline anisotropy in Ba{sub 1−x}Sr{sub x}MnO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of the Milling Conditions on the Properties of ZnO Doped with Fe

(Zn{sub 0.9}Fe{sub 0.1})O powders were produced with different ball to powder weight ratios (BPR) by ball milling of ZnO and Fe elemental powders. The BPR used were 15:1, 20:1, 25:1, and 30:1. The prepared samples were studied by x-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectroscopy (MS). The XRD patterns showed a zinc oxide (ZnO) single phase with wurtzite structure, except for the 15:1 ratio, which exhibits an additionally bcc structure associated to α-Fe. The mean crystallite sizes were found to be around 100 Å. The Mössbauer spectra were fitted using two doublets, associated to the incorporation of Fe ions into the ZnO lattice, which correspond to the Fe{sup 3+} and Fe{sup 2+} oxidation states except for the 15:1 ratio, for which an additional sextet was necessary to obtain a good fit, this sextet was associated to segregated Fe into the sample. The increase of the BPR allowed that all Fe penetrated into the ZnO matrix. The magnetization measurement using VSM reveals that the system exhibits ferromagnetic behavior at room temperature. For sample with BPR 15:1, the ferromagnetism is due to the segregated Fe, and for higher BPRs, from 20:1 up to 30:1, we propose that their ferromagnetism is due to the percolation of polarons. These polarons appear due to the oxygen vacancies (detected by XRD) which are occupied by electrons, which appear in the charge balance. Exchange interaction between the electron spins located in the vacancies or holes and those of the magnetic impurities (Fe atoms) leads to the formation of bound magnetic polarons, which percolate, producing in this way magnetic behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Surface modification and optical properties of metal doped indium sulfide thin films

Indium sulfide (In{sub 2}S{sub 3}) thin films were prepared by chemical bath deposition method in same deposition conditions, and then doped by metals including Ag, Al and Cu. Produced films post annealed at 400 °C temperature about 1 h. The effects of different metal-doping on structural, morphological and optical properties of the films were investigated by means of X-ray diffraction, scanning electron microscopy, atomic force microscopy and UV–Vis spectroscopy analyses. Produced indium sulfide thin film was amorphous and doping process lead to shifts peaks in lower angles. Morphology and nanostructure of doped films changed completely. Compact doped films have been formed. Optical constants of the produced thin films were calculated using Kramers–Kronig relations on reflectivity curves. Different doping impurities changed optical curves and band gaps.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Sol-gel derived silicate-phosphate glass SiO 2 –P 2 O 5 –CaO–TiO 2 : The effect of titanium isopropoxide on porosity and thermomechanical stability

Despite of several decades lasting extensive research of bioactive and bioresorbable glasses the systematic parametrization and determination of the key factors affecting porosity and thermomechanical characteristics still remains challenging. Here, we present silica-phosphate glasses, with the composition 70SiO 2 - 20P 2 O 5 - (10-x)CaO-xTiO 2 (mol%; x = 0, 2.5, 5, and 7.5), prepared by sol -gel method and reinforced by titanium dioxide via titanium isopropoxide (TTIP) incorporation which demonstrated tunable variation of porosity from micro-to macro -region and superb mechanical integrity during the calcination process. The presence of 7.5 mol% TiO 2 promotes dimensional stability up to 1000°C as investigated by thermomechanical analysis. The XRD showed the dominant presence of silicon phosphate [Si(P 2 O 7 )], titanium phosphate [Ti(P 2 O 7 )] and calcium phosphates [β-Ca(P 2 O 6 ) and γ- Ca 2 (P 2 O 7 )]. The effect of TiO 2 doping on the multiscale morphology and porosity was investigated by means of SEM, MIP, μCT, N 2 adsorption and USAXS/SAXS. Increasing TiO 2 content leads to the formation of open porosity up to 70vol% and drives the formation of a refined interconnected macroporosity of 2-30 μm. In contrast, mesoporosity with a dominance of 3-6nm pores decreases in all samples with increasing TiO 2 content. USAXS/SAXS revealed an increase in primary particle size with increasing TiO 2 content which is in good agreement with the nitrogen physisorption analysis showing that microporosity decreases with increasing TiO2 content.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Shock-induced phase transitions and stacking fault formation in additively manufactured eutectic high-entropy alloy Ni 40 Co 20 Fe 10 Cr 10 Al 18 W 2

Ni 40 Co 20 Fe 10 Cr 10 Al 18 W 2 additively manufactured using laser powder bed fusion (LPBF) is among the toughest as-built alloys reported and is a promising candidate for use in extreme environments. However, its behavior under multi-megabar pressure regimes remains unexplored. We used femtosecond in situ X-ray diffraction to investigate the shock response of LPBF Ni40Co20Fe10Cr10Al18W2 under laser-driven shock compression and release. Our results reveal that the initial dual-phase face-centered + body-centered structure transforms to a single face-centered phase over a wide pressure range of 84±10 to 277±55 GPa, followed by a transition to a single body-centered phase at 431±48 GPa. We establish the Hugoniot equation-of-state of LPBF Ni 40 Co 20 Fe 10 Cr 10 Al 18 W 2 and compare it to the benchmark alloy AlCoCrFeNi 2.1 , demonstrating the effects of W-doping and increased Al content. High stacking fault probabilities, close to those measured in Au and Ag, are observed upon compression. A portion of the stacking faults are annihilated upon release to ambient pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electrochemical Oxidation of Metal–Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of Fe 2 TiO 5 Photoanodes

A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Since catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type Fe 2 TiO 5 photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in Fe 2 TiO 5 . Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe 2 TiO 5 has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of Fe 2 TiO 5 . Here, the combined experimental and theoretical results presented in this study greatly improve our understanding of Fe 2 TiO 5 for use as a photoanode.

14 SOLAR ENERGY↗
DOE OSTI2018

Ab Initio Simulation Explains the Enhancement of Catalytic Oxygen Evolution on CaMnO 3

Experimental results have shown promising catalytic activity for the oxygen evolution reaction (OER) on the perovskite-type material CaMnO 3 . Through density functional theory investigations, we study the OER mechanism on CaMnO 3 , on the basis of a thermodynamic stability approach. Our results reveal that the formation of Mn vacancies caused by the solubility of Mn enhances lattice oxygen activity, which then reduces the energy of the adsorbate *OOH and therefore loosens the lower overpotential limit predicted from the “scaling relationship”. This effect suggests that, by doping manganite oxides with soluble elements, we could enhance their OER catalytic activities due to the high surface lattice oxygen activity induced by surface metal ion vacancies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗