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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Suppression of Stacking Order with Doping in 1T-TaS 2–x Se x

In 1T-TaS 2–x Se x , the charge density wave (CDW) state features a star of David lattice that expands across layers as the system becomes commensurate upon cooling. The layers can also order along the c-axis, and different stacking orders have been proposed. For this work, using neutron scattering on powder samples, we compared the stacking order previously observed in 1T-TaS 2 when the system is doped with Se. While at low temperature, a 13c layer sequence stacking was observed in TaS 2 ; this type of ordering was not evident with doping. Doping with Se results in a metallic state in which the Mott transition is suppressed, which may be linked to the absence of layer stacking.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Hierarchical Hybrid Multifunctional Materials through Interface Engineering

This project focuses on the development of stimuli-responsive hybrid multifunctional materials. We place emphasis on the design, synthesis, structural characterization, evaluation of functional properties (electronic, thermal and optical) of several (1-x)Cu 2 Se/(x)WBGS hierarchical bulk composites between Cu 2 Se, a narrow band gap semiconductor (NBGS), with a range of wider band gap semiconductors (WBGS) such as CuMSe 2 (M = Al, Ga, In, Fe, Cr) and Cu 4 TiSe 4 . Cu2Se is a well-studied NBGS with excellent thermoelectric properties (high electrical conductivity, large thermopower, etc.) while CuMSe 2 and Cu 4 TiSe 4 are high performance solar absorber materials (large band gap, large absorption coefficient, etc.). Our primary objectives are (i) to demonstrate the ability to integrate dissimilar functional properties such as large optical absorption coefficient and high electronic conductivity, within (1-x)Cu 2 Se/(x)WBGS composite; and (ii) to establish the correlation between the hierarchical structural entanglement of Cu 2 Se with WBGS (CuMSe 2 or Cu 4 TiSe 4 ) phase, the interactions between native electronic defects within the coexisting phases in the resulting (1-x)Cu 2 Se/(x)WBGS bulk composites , and the impacts on their electronic conductivity, thermal transport and optical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Neuromorphic Photonics Based on Phase Change Materials

Neuromorphic photonics devices based on phase change materials (PCMs) and silicon photonics technology have emerged as promising solutions for addressing the limitations of traditional spiking neural networks in terms of scalability, response delay, and energy consumption. In this review, we provide a comprehensive analysis of various PCMs used in neuromorphic devices, comparing their optical properties and discussing their applications. We explore materials such as GST (Ge 2 Sb 2 Te 5 ), GeTe-Sb 2 Te 3 , GSST (Ge 2 Sb 2 Se 4 Te 1 ), Sb 2 S 3 /Sb 2 Se 3 , Sc 0.2 Sb 2 Te 3 (SST), and In 2 Se 3 , highlighting their advantages and challenges in terms of erasure power consumption, response rate, material lifetime, and on-chip insertion loss. By investigating the integration of different PCMs with silicon-based optoelectronics, this review aims to identify potential breakthroughs in computational performance and scalability of photonic spiking neural networks. Further research and development are essential to optimize these materials and overcome their limitations, paving the way for more efficient and high-performance photonic neuromorphic devices in artificial intelligence and high-performance computing applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bond Dissociation Energies in Heavy Element Chalcogen and Halogen Small Molecules

Thermodynamic properties including bond dissociation energies, heats of formation, and gas phase acidities for the hydrides and dimers of chalcogens and halogens: H 2 Y, HX, Y 2 and X 2 for Y = Se, Te, At and X = Br, I, and At have been predicted using the Feller-Peterson-Dixon composite correlated molecular orbital theory approach. A full 4-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se 2 where the AoC-DHF value was used due to strong multireference effects in Se 2 for the SO calculations. The calculated results show that the At 2 bond dissociation energy is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin orbit effects. H 2 Po is not predicted to be stable to dehydrogenation to Po + H 2 in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than H 2 SO 4 . Here, the current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.

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

Reversible electrochemical conversion from selenium to cuprous selenide

Using elemental selenium as an electrode, the redox-active Cu 2+ /Cu + ion is reversibly hosted via the sequential conversion reactions of Se → CuSe → Cu 3 Se 2 → Cu 2 Se. Here, the four-electron redox process from Se to Cu 2 Se produces a high initial specific capacity of 1233 mA h g –1 based on the mass of selenium alone or 472 mA h g –1 based on the mass of Cu 2 Se, the fully discharged product.

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

Templated Growth and Passivation of Vertically Oriented Antimony Selenide Thin Films for High-Efficiency Solar Cells in Substrate Configuration

Antimony selenide (Sb 2 Se 3 ) is a promising low-cost photovoltaic material with a 1D crystal structure. The grain orientation and defect passivation play a critical role in determining the performance of polycrystalline Sb 2 Se 3 thin-film solar cells. In this work, a seed layer is introduced on a molybdenum (Mo) substrate to template the growth of a vertically oriented, columnar Sb 2 Se 3 absorber layer by closed space sublimation. By controlling the grain orientation and compactness of the Sb 2 Se 3 seeds, obtain high-quality Sb 2 Se 3 absorber layers with passive Sb 2 Se 3 /Mo interfaces is obtained, which in turn improve the transport of photoexcited charge carriers through the absorber layer and its interfaces. Post-deposition annealing of absorber layers in ambient air is further utilized to passivate the defects in Sb 2 Se 3 and enhance the quality of the front heterojunction. As a result of systematic processing optimization, Sb 2 Se 3 planar heterojunction solar cells are fabricated in substrate configuration with a champion power conversion efficiency of 8.5%.

14 SOLAR ENERGY↗
DOE OSTI2022

Tunable and reconfigurable atomically thin heterostructures

Heterocrystals of metal dichalcogenides and Bi 2 S 3 , Bi 2 Se 3 or Bi 2 Te 3 are presented, in which the metal dichalcogenides and Bi 2 S 3 , Bi 2 Se 3 or Bi 2 Te 3 do not largely retain their independent properties. These heterocrystals exhibit electronic and optical changes, which make them attractive for beyond-silicon electronics and optoelectronics. Particularly, these heterocrystals can be re-configured in a manner that allows bit writing and pattern drawing. Embodiments of these heterocrystals, methods of forming these heterocrystals, methods of reconfiguring the heterocrystals, information storage devices, optoelectronic circuits and photonic crystals are presented.

Vargas, Anthony↗
DOE OSTI2025

MXene/graphitic carbon nitride-supported metal selenide for all-solid-state flexible supercapacitor and oxygen evolution reaction

We report a new type of combination of rare earth metal selenides (Ce 2 Se 3 and Er 2 Se 3 ) with a Ti 3 C 2 T x /S-doped graphitic carbon nitride heterostructure for bifunctional application in flexible supercapacitors and oxygen evolution reactions. The incorporation of S-doped graphitic carbon nitride in MXenes reduced the layer stacking tendency of both two-dimensional sheets and eliminated volume expansion by forming a heterostructure. Cerium and erbium rare earth metal centers induce reactive surface sites, whereas binary layers of Ti 3 C 2 T x /S-doped graphitic carbon nitride provide a conducting matrix for the homogeneous growth of the metal selenides. The assembled all-solid-state flexible asymmetric supercapacitor exhibited a high specific capacitance of 60 F g –1 , an energy density of 10.1 W h kg –1 (volumetric energy density: 0.9 mW h cm –3 ) at 2 A g –1 , and 100% capacitance retention after 10 000 charge–discharge cycles with good flexibility for real-time applications. Furthermore, the optimum nanohybrid showed a low overpotential of 280 mV and a Tafel slope of 99 mV dec –1 with durable electrocatalytic performance. Furthermore, this work is the first to investigate the bifunctional energy efficiency of rare earth metal selenides grown over MXene materials.

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

Unleashing the Potential of Fast Charging Batteries: Leveraging Anion Redox Chemistry in Ni- and Co-Free Cathodes

Designing Li-ion battery cathodes free from critical raw materials such as Co and Ni has a huge technological and societal impact. Though anion redox-based Li-rich oxide cathodes allow designing Co and Ni free cathode compositions, the Li-rich oxides witnessed voltage fade, voltage hysteresis, and irreversible oxygen release despite their high capacity. Conversely, anion redox through highly covalent chalcogenides (S/Se) is emerging due to the improved covalency between metal d and ligand p bands. Here, we investigate the tuning of multi-chalcogen (S/Se) p-band and redox-active metal d-band in a model Li-rich chalcogen composition Li 1.13 Ti 0.57 Fe 0.3 S 2-y Se y (y = 0 - 1) through in-depth electrochemical, X-ray spectroscopy, and DFT-based electronic structure investigations. Introducing the appropriate amount of Se p band character in anion redox sulfides increases interlayer distance and metal - ligand covalency without modifying the original crystal structure, promoting significant electrochemical reversibility through mixed anionic (Se 2- /Se n- , S 2- /S n- , wherein n<2) and cationic (Fe 2+ /Fe 3+ ) redox reactions. Here we show the detailed Fe, S, and Se redox contributions during Li insertion/extraction through X-ray Absorption (XAS) and Hard X-ray Photoemission Spectroscopy (HAXPES) measurements. The orbital tuning approach improves rate capability for more than 10 C charge-discharge rate, exhibiting more than 50% of its original capacity obtained at C/20 rate. The buffer cation in the lattice (Ti 4+ ) remains electrochemically inactive even after significant Se p-band introduction in the sulfide framework. Overall, this work takes advantage of multi-anion redox chemistry to uncover practically demanding fast charging-discharging characteristics in intercalation cathodes. The obtained knowledge of this design can be extended to other oxide and chalcogen cathodes for high performance Li-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2020

Evolution of nontrivial Fermi surface features in the band structures of the homologous members Pb 5 Bi 6 Se 14 and Pb 5 Bi 12 Se 23

High-quality single crystals of (PbSe) 5 (Bi 2 Se 3 ) 3m were grown and analyzed by nuclear magnetic resonance (NMR) spectroscopy. We report on 77 Se and 207 Pb NMR shifts and nuclear spin-lattice relaxation measurements in the naturally formed heterostructure homology (PbSe) 5 (Bi 2 Se 3 ) 3m with m = 1 (Pb 5 Bi 6 Se 14 ) and m = 2 (Pb 5 Bi 12 Se 23 ). A distinct site-specific contribution has been detected for both nuclei as a function of temperature, which reveals an electronic changeover from a semiconducting (Pb 5 Bi 6 Se 14 ) to a semimetalliclike (Pb 5 Bi 12 Se 23 ) system with nontrivial band structure features near the Fermi level. The temperature dependences of the relaxation rates are dominated by significant changes in the topology of energy dispersions accompanied with band edges and crossings in the region of the Fermi surface. These results, which interrogate nuclear spin interactions from selected atomic sites, clearly expose the effects of the added Bi 2 Se 3 layer on the crystal and electronic structure of (Pb 5 Bi 12 Se 23 ). These findings provide direct microscopic insight into the unconventional and dual nature of the electronic structure of these homologous thermoelectric and topologically nontrivial compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Design Strategies of Spinel Oxide Frameworks Enabling Reversible Mg-Ion Intercalation

In this study, reversible Mg 2+ intercalation in metal oxides frameworks is a key enabler for an operational Mg ion battery with high energy density needed for the next generation of energy storage technologies. While functional Mg-ion batteries have been achieved in structures with soft anions (e.g., S 2- and Se 2- ), they do not meet energy density requirements to compete with the current rechargeable lithium-ion batteries due to their low insertion potentials, emphasizing the necessity of finding an oxide-based cathode that operates at high potentials. A leading hypothesis to explain the limited availability of oxide Mg-ion cathodes is the belief that Mg 2+ has sluggish diffusion kinetics in oxides due to strong electrostatic interactions between the Mg 2+ ions and oxide anions in the lattice. From this assessment, it can be hypothesized that such rate limiting kinetic shortcomings can be mitigated by tailoring an oxide framework through creating less stable Mg 2+ - O 2- coordination.

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

Importance of van der Waals interactions for ab initio studies of topological insulators

We investigate the lattice and electronic structures of the bulk and surface of the prototypical layered topological insulators Bi 2 Se 3 and Bi 2 Te 3 using ab initio density functional methods, and systematically compare the results of different methods of including van der Waals (vdW) interactions. We show that the methods utilizing semi-empirical energy corrections yield accurate descriptions of these materials, with the most precise results obtained by properly accounting for the long-range tail of the vdW interactions. The bulk lattice constants, distances between quintuple layers and the Dirac velocity of the topological surface states (TSS) are all in excellent agreement with experiment. In Bi 2 Te 3 , hexagonal warping of the energy dispersion leads to complex spin textures of the TSS at moderate energies, while in Bi 2 Se 3 these states remain almost perfectly helical away from the Dirac point, showing appreciable signs of hexagonal warping at much higher energies, above the minimum of the bulk conduction band. Furthermore, our results establish a framework for unified and systematic self-consistent first principles calculations of topological insulators in bulk, slab and interface geometries, and provides the necessary first step toward ab initio modeling of topological heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Out-of-Plane Magnetic Anisotropy Engineered via Band Distortion in Two-Dimensional Materials

We unveil the connection between magnetic anisotropy and band structures in Γ-specialized In 2 Se 3 /Fe 3 GeTe 2 heterostructure, through first-principles calculations. A significant band distortion occurs near the Γ point with a formed valley. As the band-distortion-caused valley moves relative to the quasivalence band maximum, the magnetic anisotropy switches between in-plane and out-of-plane magnetic anisotropy. Such a rule applies to either the strained or polarization-switched In 2 Se 3 /Fe 3 GeTe 2 heterostructure. Furthermore, these findings demonstrate the feasibility of predicting the magnetic anisotropy by band structure modification in two-dimensional magnetic systems.

2-dimensional systems↗
DOE OSTI2020

Crystal Structure and Atomic Vacancy Optimized Thermoelectric Properties in Gadolinium Selenides

Thermoelectric materials enable the energy conversion of waste heat into electricity, helpful to relieve global energy crisis. Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd 3-x Se 4 (x = 0.16, 0.21 and 0.25) and orthorhombic Gd 2 Se 3-y (y = 0.02, 0.06 and 0.08). High energy synchrotron x-ray diffraction and total scattering have been used to investigate the crystallographic and local structures. Atomic-scale clusters of Gd vacancy in the cubic phase are observed by employing the reverse Monte Carlo simulation. For cubic Gd 3-x Se 4 , adjusting Gd vacancy triggers the effect of multiple conduction bands, confirmed by the increase in effective masses and theoretical calculations. A reasonable peak zT value of 0.27 is achieved at 850 K for Gd 3-x Se 4 (x = 0.16). On the other hand, tuning Se vacancy enables the optimization of electron concentration for the orthorhombic Gd 2 Se 3-y . More significantly, its low deformation potential (Ξ = 12eV) gives rise to enhanced electron mobility and higher peak zT value of 0.54 at 850 K for Gd 2 Se 3-y (y = 0.02). Intriguingly, a higher zT of 1.2 at 1200 K is reasonably predicted by quality factor analysis. Finally, this work extends the scope of high-temperature thermoelectric materials and facilitates the exploration of novel high-temperature thermoelectric materials

36 MATERIALS SCIENCE↗
DOE OSTI2022

20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

Bandgap gradient is a proven approach for improving the open-circuit voltages (V OC s) in Cu(In,Ga)Se 2 and Cu(Zn,Sn)Se 2 thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers. We show that the introduction of the bandgap gradient reduces the hole density in the front junction region and introduces a small spike in the band alignment between this and the absorber regions, effectively suppressing the nonradiative recombination therein and leading to improved VOCs in Cd(Se,Te) solar cells using commercial SnO 2 buffers. A champion device achieves an efficiency of 20.03% with a V OC of 0.863 V.

14 SOLAR ENERGY↗
DOE OSTI2022

Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures

Interfacing topological insulators (TIs) with magnetic insulators (MIs) have been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of the magnetic proximity effect (MPE) imposed to TIs. Here, in this work, we report the observation of the nonlinear planar Hall effect (NPHE) in Bi 2 ⁢Se 3 films grown on MI thulium and yttrium-iron-garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi 2 ⁢Se 3 grown on nonmagnetic gadolinium-gallium-garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi 2 ⁢Se 3 depends linearly on the external magnetic field, while that in YIG/Bi 2 ⁢Se 3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate that the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Quantum oscillation studies of the nodal line semimetal Ni 3 In 2 S 2-x Se x

Ternary shandite compounds with the general formula T 3 M 2 X 2 (T = Ni, Co, Rh or Pd; M = Sn, In, or Pb; and X = S or Se) have emerged as a large pool of topological semimetals. This family of compounds hosts different topological phases for various combinations of T, M and X. This paper reports the observation of quantum oscillations under the high magnetic fields in Ni 3 In 2 S 2-x Se x single crystals. Angular dependence of oscillation frequency suggests an evolution of the Fermi surface from three-dimensional to two-dimensional on Se substitution for S in Ni 3 In 2 S 2 . Here, the effective mass obtained for each composition by fitting the oscillation amplitude with the Lifshitz-Kosevich formula, shows no significant change, suggesting that the topological phase might be relatively robust against enhanced spin-orbit coupling upon Se doping in Ni 3 In 2 S 2 .

36 MATERIALS SCIENCE↗