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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Possible Topological Hall Effect above Room Temperature in Layered Cr 1.2 Te 2 Ferromagnet

Topological Hall effect (THE) has been used as a powerful tool to unlock spin chirality in novel magnetic materials. Recent focus has been widely paid to THE and possible chiral spin textures in two-dimensional (2D) layered magnetic materials. However, the room-temperature THE has been barely reported in 2D materials, which hinders its practical applications in 2D spintronics. In this paper, we report a possible THE signal featuring antisymmetric peaks in a wide temperature window up to 320 K in Cr 1.2 Te 2 , a new quasi-2D ferromagnetic material. The temperature, thickness, and magnetic field dependences of the THE lead to potential spin chirality origin that is associated with the spin canting under external magnetic fields. Furthermore, our work holds promise for practical applications in future chiral spin-based vdW spintronic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Exceptional Radiation Absorption in a Pentagon-Based Si Allotrope

Excellent photovoltaic performance is predicted in a pentagonal covalent network of Si in a hollow structure exhibiting both thermal and dynamical stability. Consisting of a combination of sp 2 and sp 3 hybridized Si atomic orbitals, the GW0 computed band structure shows an indirect band gap near the zone edge and also a manifold of directly absorbing transitions at frequencies in the window of visible light, in distinction with conventional Si. Hydrogenation of a single sp 2 site is predicted to lead to a robust local magnetic moment. In this study, we find a low formation energy at low pressure that is compatible with other experimentally known phases, suggesting that a stable phase might be obtained.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Extending the Near-Infrared Emission Range of Indium Phosphide Quantum Dots for Multiplexed In Vivo Imaging

This report of the reddest emitting indium phosphide quantum dots (InP QDs) to date demonstrates tunable, near-infrared (NIR) photoluminescence (PL) as well as PL multiplexing in the first optical tissue window while avoiding toxic constituents. This synthesis overcomes the InP “growth bottleneck” and extends the emission peak of InP QDs deeper into the first optical tissue window using an inverted QD heterostructure, specifically ZnSe/InP/ZnS core/shell/shell nanoparticles. The QDs exhibit InP shell thickness-dependent tunable emission with peaks ranging from 515–845 nm. The high absorptivity of InP yields effective photoexcitation of the QDs with UV, visible, and NIR wavelengths. These nanoparticles extend the range of tunable direct-bandgap emission from InP-based nanostructures, effectively overcoming a synthetic barrier that has prevented InPbased QDs from reaching their full potential as NIR imaging agents. Further, multiplexed lymph node imaging in a mouse model demonstrates the potential of the NIR-emitting InP particles for in vivo imaging.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Defect-Enabled Phase Programming of Transition Metal Dichalcogenide Monolayers

The ability to tune the local electronic transport properties of group VI transition metal dichalcogenide (TMD) monolayers by strain-induced structural phase transformations (“phase programming”) has stimulated much interest in the potential applications of such layers as ultrathin programmable and dynamically switchable nanoelectronics components. In this manuscript, we propose a new approach toward controlling TMD monolayer phases by employing macroscopic in-plane strains to amplify heterogeneous strains arising from tailored, spatially extended defects within the monolayer. Furthermore, the efficacy of our proposed approach is demonstrated via numerical simulations of emerging domains localized around arrays of holes, grain boundaries, and compositional heterointerfaces. Quantitative relations between the macroscopic strains required, spatial resolution of domain patterns, and defect configurations are developed. In particular, the introduction of arrays of holes is identified as the most feasible phase programming route.

2D materials↗
DOE OSTI2021

Atomic-Scale Observation of O1 Faulted Phase-Induced Deactivation of LiNiO 2 at High Voltage

LiNiO 2 and cobalt-free ultrahigh-Ni content cathodes suffer from rapid capacity loss and severe chemomechanical degradation, especially when operated at high voltages. Here, by cycling LiNiO 2 up to 4.7 V, we report the atomic-scale observation of O1 faulted phase-induced deactivation of LiNiO 2 . We found that although a thin layer of O3 phase forms on the particle surface by reversible O3 → O1 transformation during discharge, the bulk interior still maintains the O1 faulted phase, leading to rapid capacity loss of LiNiO 2 . Moreover, the atomic configuration of the O1/O3 interface is investigated comprehensively. We found that the misfit along the c axes of the O1 and O3 phases results in the formation of misfit dislocations, whereby cation mixing is promoted at dislocation cores. Further, a transition zone with continuous shear along the a-b plane is uncovered between the O1 and O3 phases for the first time. Besides, severe oxygen loss-induced pore formation and concurrent rock salt transformation are also identified.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Kinetically Controlled Self-Assembly of Binary Polymer-Grafted Nanocrystals into Ordered Superstructures via Solvent Vapor Annealing

Polymer-inorganic nanocomposites based on polymer-grafted nanocrystals (PGNCs) are enabling technologically relevant applications owing to their unique physical, chemical, and mechanical properties. While diverse PGNC superstructures have been realized through evaporation-driven self-assembly, this approach presents multifaceted challenges in experimentally probing and controlling assembly kinetics. Here, we report a kinetically controlled assembly of binary superstructures from a homogeneous disordered PGNC mixture utilizing solvent vapor annealing (SVA). Using a NaZn 13 -type superstructure as a model system, we demonstrate that varying the solvent vapor pressure during SVA allows for exquisite control of the rate and extent of PGNC assembly, providing access to nearly complete kinetic pathways of binary PGNC crystallization. Characterization of kinetically arrested intermediates reveals that assembly follows a multistep crystallization pathway involving spinodal-like preordering of PGNCs prior to NaZn 13 nucleation. Our work opens up new avenues for the synthesis of multicomponent PGNC superstructures exhibiting multifunctionalities and emergent properties through a thorough understanding of kinetic pathways.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Resonance-Enhanced Excitation of Interlayer Vibrations in Atomically Thin Black Phosphorus

The strength of interlayer coupling critically affects the physical properties of 2D materials such as black phosphorus (BP), where the electronic structure depends sensitively on layer thickness. Rigid-layer vibrations reflect directly the interlayer coupling strength in 2D van der Waals solids, but measurement of these characteristic frequencies is made difficult by sample instability and small Raman scattering cross sections in atomically thin elemental crystals. Here, we overcome these challenges in BP by performing resonance-enhanced low-frequency Raman scattering under an argon-protective environment. Interlayer breathing modes for atomically thin BP were previously unobservable under conventional (nonresonant) excitation but became strongly enhanced when the excitation energy matched the sub-band electronic transitions of few-layer BP, down to bilayer thicknesses. The measured out-of-plane interlayer force constant was found to be 10.1 × 10 19 N/m 3 in BP, which is comparable to graphene. Accurate characterization of the interlayer coupling strength lays the foundation for future exploration of BP twisted structures and heterostructures.

interlayer interaction↗
DOE OSTI2021

Cross-Linking and Charging Molecular Magnetoelectronics

Magnetoelectrics are witnessing an ever-growing success toward the voltage-controlled magnetism derived from inorganic materials. However, these inorganic materials have predominantly focused on the ferroelectromagnetism at solid-to-solid interfaces and suffered several drawbacks, including the interface-sensitive coupling mediators, high-power electric field, and limited chemical tunability. Here, we report a promising design strategy to shift the paradigm of next-generation molecular magnetoelectrics, which relies on the integration between molecular magnetism and electric conductivity though an in situ cross-linking strategy. Following this approach, we demonstrate a versatile and efficient synthesis of flexible molecular-based magnetoelectronics by cross-linking of magnetic coordination networks that incorporate conducting chain building blocks. Furthermore, the as-grown compounds feature an improved critical temperature up to 337 K and a room-temperature magnetism control of low-power electric field. It is envisaged that the cross-linking of molecular interfaces is a feasible method to couple and modulate magnetism and electron conducting systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Photoinduced Rotation of Colloidal Semiconductor Nanocrystals in an Electric Field

In this work, we demonstrate that solution-phase semiconductor nanocrystals (NCs) undergo photoinduced rotation in an external electric field. Present measurements backed by theoretical calculations show that the rotation of colloidal NCs is driven by the excited-state dipole moment, which is counterbalanced by the solvent viscosity drag. Corresponding angular velocities range from 0.5°/ns for cubic CsPbBr 3 NCs to 3°/ns for nanoparticles with a large photoinduced charge separation (CdSe/CdS core–shell and dot-in-a-rod NCs). Because of photoinduced rotation, solution-phase semiconductor NCs exhibited an order-of-magnitude increase in the spectral changes caused by the quantum confined Stark effect (QCSE), compared to solid NC assemblies. The enhanced QCSE of colloidal NCs reflected their global alignment in solution, which could be retained in a solid environment by slow crystallization. Overall, we expect that the demonstrated phenomenon of the colloidal nanocrystal rotation in an electric field will open up new avenues for developing electro-optical and voltage-sensitive applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Ultrafast Carrier–Lattice Interactions and Interlayer Modulations of Bi 2 Se 3 by X-ray Free-Electron Laser Diffraction

We report as a 3D topological insulator, bismuth selenide (Bi 2 Se 3 ) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. Understanding the coupling with its topological phase requires studying the interactions of carriers with the lattice on time scales down to the sub-picosecond regime. Here we investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi 2 Se 3 thin films by time-resolved diffraction using an X-ray free-electron laser. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory (DFT) and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Our theory suggests that band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insights into the topological phase control by light-induced structural change on ultrafast timescales.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

The Buckling Spectra of Nanoparticle Surfactant Assemblies

Fine control over the mechanical properties of thin sheets underpins transcytosis, cell shape, and morphogenesis. Applying these principles to artificial, liquid-based systems has led to reconfigurable materials for soft robotics, actuation, and chemical synthesis. However, progress is limited by a lack of synthetic two-dimensional membranes that exhibit tunable mechanical properties over a comparable range to that seen in nature. In this study, we show that the bending modulus, B , of thin assemblies of nanoparticle surfactants (NPSs) at the oil–water interface can be varied continuously from sub- k B T to 10 6 k B T , by varying the ligands and particles that comprise the NPS. We find extensive departure from continuum behavior, including enormous mechanical anisotropy and a power law relation between B and the buckling spectrum width. Our findings provide a platform for shape-changing liquid devices and motivate new theories for the description of thin-film wrinkling.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Spatiotemporal Imaging of Thickness-Induced Band-Bending Junctions

van der Waals materials exhibit naturally passivated surfaces and an ability to form versatile heterostructures to enable an examination of carrier transport mechanisms not seen in traditional materials. Here, we report a new type of homojunction termed a "band-bending junction" whose potential landscape depends solely on the difference in thickness between the two sides of the junction. Using MoS 2 on Au as a prototypical example, we find that surface potential differences can arise from the degree of vertical band bending in thin and thick regions. Furthermore, by using scanning ultrafast electron microscopy, we examine the spatiotemporal dynamics of charge carriers generated at this junction and find that lateral carrier separation is enabled by differences in the band bending in the vertical direction, which we verify with simulations. As a result, band-bending junctions may therefore enable new optoelectronic devices that rely solely on band bending arising from thickness variations to separate charge carriers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Tuning Hybrid exciton–Photon Fano Resonances in Two-Dimensional Organic–Inorganic Perovskite Thin Films

As easy-to-grow quantum wells with narrow excitonic features at room temperature, two-dimensional (2D) Ruddleson–Popper perovskites are promising for realizing novel nanophotonic devices based on exciton–photon interactions. Here, we demonstrate a distinct hybrid exciton–photon Fano resonance in (C 4 H 9 NH 3 ) 2 PbI 4 thin films prepared via spin coating. Using a classical coupled-oscillator model and finite-difference time-domain simulations, we link the Fano interference to the coupling of the exciton with the Rayleigh-like scattering of the film microstructure. Combining colloidal plasmonic cavities with the 2D perovskite films, we demonstrate tuning of the Fano resonance. In combination with silver nanoparticles, the exciton–photon Fano interference couples to the in-plane plasmonic modes with indications of Rabi splitting. By creating a nanoparticle on mirror geometry, we address the out-of-plane excitonic component, reaching an intermediate coupling regime. Furthermore, these structures suggest possible photonic targets for biomolecular self-assembly applications.

2D perovskites↗
DOE OSTI2021

Controlled Formation of Stacked Si Quantum Dots in Vertical SiGe Nanowires

In this work, we demonstrate the ability to fabricate vertically stacked Si quantum dots (QDs) within SiGe nanowires with QD diameters down to 2 nm. These QDs are formed during high-temperature dry oxidation of Si/SiGe heterostructure pillars, during which Ge diffuses along the pillars’ sidewalls and encapsulates the Si layers. Continued oxidation results in QDs with sizes dependent on oxidation time. The formation of a Ge-rich shell that encapsulates the Si QDs is observed, a configuration which is confirmed to be thermodynamically favorable with molecular dynamics and density functional theory. The type-II band alignment of the Si dot/SiGe pillar suggests that charge trapping on the Si QDs is possible, and electron energy loss spectra show that a conduction band offset of at least 200 meV is maintained for even the smallest Si QDs. Our approach is compatible with current Si-based manufacturing processes, offering a new avenue for realizing Si QD devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Pseudoelasticity of SrNi2P2 Micropillar via Double Lattice Collapse and Expansion

The maximum recoverable strain of most crystalline solids is less than 1% because plastic deformation or fracture usually occurs at a small strain. In this work, we show that a SrNi 2 P 2 micropillar exhibits pseudoelasticity with a large maximum recoverable strain of ~14% under uniaxial compression via unique reversible structural transformation, double lattice collapse–expansion that is repeatable under cyclic loading. Its high yield strength (~3.8 ± 0.5 GPa) and large maximum recoverable strain bring out the ultrahigh modulus of resilience (~146 ± 19 MJ/m 3 ), a few orders of magnitude higher than that of most engineering materials. The double lattice collapse–expansion mechanism shows stress–strain behaviors similar to that of conventional shape-memory alloys, such as hysteresis and thermo-mechanical actuation, even though the structural changes involved are completely different. Our work suggests that the discovery of a new class of high-performance ThCr 2 Si 2 -structured materials will open new research opportunities in the field of pseudoelasticity.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Optical and Magneto-Optical Properties of Donor-Bound Excitons in Vacancy-Engineered Colloidal Nanocrystals

Controlled insertion of electronic states within the band gap of semiconductor nanocrystals (NCs) is a powerful tool for tuning their physical properties. One compelling example is II–VI NCs incorporating heterovalent coinage metals in which hole capture produces acceptor-bound excitons. To date, the opposite donor-bound exciton scheme has not been realized because of the unavailability of suitable donor dopants. Here, we produce a model system for donor-bound excitons in CdSeS NCs engineered with sulfur vacancies (V S ) that introduce a donor state below the conduction band (CB), resulting in long-lived intragap luminescence. V S -localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the V S are not magnetically coupled to the NC bands and that the polarization properties are determined by the spin of the valence-band photohole, whose spin flip is massively slowed down due to suppressed exchange interaction with the donor-localized electron.

77 NANOSCIENCE AND NANOTECHNOLOGY↗