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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Preferential hole defect formation in monolayer W Se 2 by electron-beam irradiation

Monolayer transition-metal dichalcogenides (TMDCs) have been extensively studied due to their wide range of physical properties and applications. It has been demonstrated that the electron beam in a transmission electron microscope (TEM) or scanning TEM (STEM) generates chalcogen vacancies that agglomerate into dispersed linelike or holelike defects. Here we employ a STEM electron beam and demonstrate that, in WSe 2 , beam-induced chalcogen vacancies initially form certain dispersed multivacancy structures, as seen in TMDCs in prior work. However, with suitable control of the STEM focused electron beam, these multivacancies gradually evolve into a dense network of ten-, 12-, 14-, and 16-member ring round holes, whereas the same process leads predominantly to chalcogen-vacancy line defects in other trigonal-prismatic TMDCs. Density functional theory calculations find that the underlying atomic-scale processes lead preferentially to defect structures that lower the total energy so that we are able to track the formation of the observed multivacancy complexes, which then lead to the formation of dense large round holes in WSe 2 . The same processes in WS 2 , however, are energetically unfavorable, while linear multivacancy defects are preferred, as observed. The demonstrated control of the formation of unique high-density round holes in WSe 2 has potential for applications such as atomic and molecular sieving.

36 MATERIALS SCIENCE↗
DOE OSTI2025

La 2 ⁢O 3 ⁢Mn 2 ⁢Se 2 : A correlated insulating layered d-wave altermagnet

Altermagnets represent a new class of magnetic phases without net magnetization, invariant under a combination of rotation and time reversal. Unlike conventional collinear antiferromagnets (AFM), altermagnets could lead to new correlated states and important material properties deriving from their nonrelativistic spin-split band structure. Indeed, they serve as the magnetic analogue of unconventional superconductors and can yield spin-polarized electrical currents in the absence of external magnetic fields, making them promising candidates for next-generation spintronics. Here, we report altermagnetism in the correlated insulator, magnetically ordered tetragonal oxychalcogenide, L⁢a 2 ⁢O 3⁢ M⁢n 2 ⁢S⁢e 2 . Symmetry analysis reveals a 𝑑 𝑥 2 −𝑦 2 -wave-like spin-momentum locking arising from the M⁢n 2 ⁢O Lieb lattice, supported by density functional theory (DFT) calculations. Magnetic measurements confirm the AFM transition below ∼166K while neutron pair distribution function analysis reveals a 2D short-range magnetic order that persists above the Néel temperature. Single crystals are grown and characterized using x-ray diffraction, optical and electron microscopy, and micro-Raman spectroscopy to confirm the crystal structure, stoichiometry, and uniformity. Furthermore, our findings establish L⁢a 2⁢ O 3 ⁢M⁢n 2⁢ S⁢e 2 as a model altermagnetic system realized on a Lieb lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Tuning the charge density wave quantum critical point and the appearance of superconductivity in Ti Se 2

The transition metal dichalcogenide TiSe 2 is an ideal correlated system for studying the interplay between superconductivity (SC) and a charge density wave (CDW) because both symmetry-breaking phases can be easily controlled by either Cu intercalation or physical pressure. SC appears in proximity to a CDW quantum critical point (QCP) induced by both Cu intercalation and applied pressure, raising the possibility of CDW-driven SC. Here, we report tuning the CDW QCP by simultaneously controlling Cu intercalation and external pressure and the appearance of a SC dome centered on the tunable QCP. When subjected to pressure, CDW ordering of Cu-intercalated Cu 0.025 TiSe 2 is completely suppressed at 2.3 GPa, where the residual resistivity and the resistivity-temperature exponent decrease sharply, indicating the presence of the CDW QCP. The upper critical field of Cu 0.025 TiSe 2 is 3.51 kOe, 16 times larger than that of pristine TiSe 2 , and its temperature dependence is linear, indicating that SC of TiSe 2 is switched from the two-dimensional- to anisotropic three-dimensional-like by Cu intercalation. These discoveries show that the simultaneous application of Cu intercalation and pressure move the CDW QCP and that the highest SC transition temperature is pinned to the QCP, suggesting that the SC in TiSe 2 is strongly correlated with CDW quantum criticality.

2-dimensional systems↗
DOE OSTI2023

First-principles derivation of magnetic interactions in the triangular quantum spin liquid candidates KYb C h 2 ( C h = S , Se , Te ) and A YbSe 2 ( A = Na , Rb )

The AYb Ch 2 (A=alkali metal, Ch=chalcogen) delafossites are a family of crystals which have a triangular lattice of effective S=1/2 moments on their Yb atoms. They hold great promise for the realization of the triangular quantum spin liquid state because of their defect-minimized growth and the ability to interchange chemical constituencies among the family. Here we use ab initio computations to evaluate the exchange couplings of four realized (and one theoretical) rhombohedral delafossite structures and examine the influence of chemical substitution on promoting the development of a quantum spin liquid state. We find good agreement with experiment regarding the antiferromagnetic nearest-neighbor exchange J 1 , but our calculations underestimate J 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Vapor Treatment and In-situ Recrystallization by Copper Chloride on Cu(In,Ga)Se 2 Thin Film

Deposition of CIGS semiconductor thin films was performed at low temperature and high rate by three-stage coevaporation process on molybdenum coated glass substrate. Here, a vapor treatment was done in between the second and third stage by flashing CuCl 2 for 5 mins at 400 °C. A large change in morphology and crystal structure was observed after the treatment. XRD and SEM showed that small grains transformed into large grains. A smoother Ga profile was observed by SIMS measurements for the treated films as compared to as-deposited films. Furthermore, the Na profile was also modified in the recrystallized samples, with a lower content after recrystallization.

14 SOLAR ENERGY↗
DOE OSTI2021

Growth of topological insulator Bi 2 Se 3 particles on GaAs via droplet epitaxy

The discovery of topological insulators (TIs) and their unique electronic properties has motivated research into a variety of applications, including quantum computing. It has been proposed that TI surface states will be energetically discretized in a quantum dot nanoparticle. These discretized states could then be used as basis states for a qubit that is more resistant to decoherence. In this work, prototypical TI Bi2Se3 nanoparticles are grown on GaAs (001) using the droplet epitaxy technique, and we demonstrate the control of nanoparticle height, area, and density by changing the duration of bismuth deposition and substrate temperature. Within the growth window studied, nanoparticles ranged from 5 to 15 nm in height with an 8–18 nm equivalent circular radius, and the density could be relatively well controlled by changing the substrate temperature and bismuth deposition time.

Materials Science↗
DOE OSTI2020

Paleotopography controls weathering of Cambrian-age profiles beneath the Great Unconformity, St. Francois Mountains, SE Missouri, U.S.A.

ABSTRACT Precambrian (1.4 to 1.5 Ga) granite and rhyolite in the St. Francois Mountains at the northeast corner of the Ozark Plateau in Missouri has been altered down to a depth up to 8 meters below the Great Unconformity (the contact between Paleozoic sedimentary rock and underlying Precambrian). Petrographic, geochemical, and mineralogic data indicate that at least two events generated this alteration: 1) surficial weathering due to subaerial exposure of the granite before Cambrian burial—this material is preserved as a paleosol; and 2) alteration due to reaction with basinal fluids channeled along the unconformity from nearby sedimentary basins long after burial by Paleozoic strata. To assess the variation between surficial weathering and basinal fluid alteration, we measured and sampled for petrologic, geochemical, and mineralogic data in the rock at and just below the Great Unconformity at three paleoelevations. Whole-rock geochemical oxide and X-ray diffraction data indicate that K-metasomatism and highly crystalline illite occurred in each profile. The K increase reflects crystallization of authigenic feldspar and illite from basinal fluids that were channeled along the Great Unconformity during younger Paleozoic fluid-flow events. Each profile also exhibits an upward increase in altered feldspar crystals and highly crystalline kaolinite, and an upward decrease in Ca and Na. Such changes reflect soil formation due to reaction with meteoric water before Cambrian burial, indicating that the altered granite was a paleosol before Paleozoic basinal fluid-flow events. Notably, the paleosol at the highest paleoelevation displays the least amount of paleoweathering and the paleosol at the lowest displays the greatest amount of paleoweathering. These results demonstrate that not only can characteristics of the paleosol just below the Great Unconformity be recognized in the St. Francois Mountains, despite subsequent alteration, but also it is possible to detect variations in soil thickness that were controlled by slope steepness and, therefore, water availability and/or soil creep or failure. This spatial relationship is compatible with studies of modern soils which indicate that soil character varies with position on a slope.

Geology↗