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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Size and Stoichiometry Effects on the Reactivity of MoCy Nanoparticles Towards Ethylene

Molybdenum carbides are promising catalysts alternative to Pt-group metals for the hydrogenation of unsaturated hydrocarbons. Nanostructuring has been shown to be an efficient way to boost the catalytic activity of these materials with MoCy nanoparticles (NPs) exhibiting a good performance when encapsulated inside zeolites or dispersed on inert supports such as carbon or gold. In this work, we focus on a systematic DFT study of the interaction of MoCy NPs with ethylene (C 2 H 4 ), as a general and simple approach for examining binding and activation of C≡C bonds. Models for 14 NPs, with a Mo/C ratio in the 0.67 to 2.00 range, have been built following a cascade procedure. Several chemical descriptors, including the adsorption energy, structural NPs distortion, C≡C deformation, and C 2 H 4 attachment energy have been analyzed along with a meticulous geometric and electronic characterization of bare NPs and C2H4 binding. The present results show that 1:1 stoichiometric Mo 6 C 6 , Mo 12 C 12 , and Mo 24 C 24 , and the non-stochiometric Mo 4 C 6 , Mo 8 C 12 (MetCar), and Mo 14 C 13 (Nanocube) are excellent systems for the binding and activation of ethylene, exhibiting a much bigger reactivity than a bulk δ-MoC(001) surface with a similar Mo:C ratio. In addition, C 2 H 4 binding on the NPs with a Mo/C < 1.08 is advantageous since, apart from a rather large adsorption energy, implies low energy values for NPs deformation (from 0.00 to 0.31 eV), C≡C distortion (from 0.30 to 0.52 eV), and C 2 H 4 attachment (from -2.12 to -2.58 eV). These theoretical results point to the ideal MoCy size and composition for C 2 H 4 binding, providing a background for further experimental studies aimed at the preparation of MoCy NPs as hydrogenation catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Charge Carrier Transport in Iron Pyrite Thin Films: Disorder-Induced Variable-Range Hopping

Here, the origin of p-type conductivity and the mechanism responsible for low carrier mobility were investigated in pyrite (FeS 2 ) thin films. Temperature-dependent resistivity measurements (10-400 K) were performed on polycrystalline and nanostructured thin films prepared by three different methods: (1) spray pyrolysis, (2) hot-injection synthesized and spin-coated nanocubes, and (3) pulsed laser deposition. The films have a high hole density (10 18 -10 19 ) cm -3 and low mobility (0.1-4 cm 2 V -1 s -1 ) regardless of the method used for their preparation. The charge transport mechanism is determined to be thermally activated conduction (TAC) at near room temperature, with Mott-type variable-range hopping (VRH) of holes via localized states occurring at lower temperatures. The density functional theory (DFT) predicts that sulfur vacancy induces localized defect states within the band gap and the charge remains localized around the defect. The data indicates that the electronic properties including hopping transport in pyrite thin films can be correlated to sulfur vacancy-related defects. The results provide insights into the electronic properties of pyrite thin films and their implications for charge transport.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tip-Enhanced Multipolar Raman Scattering

We record nanoscale chemical images of thiobenzonitrile (TBN)-functionalized plasmonic gold nanocubes via tip-enhanced Raman spectroscopy (TERS). The spatially averaged optical response is dominated by conventional (dipolar) TERS scattering from TBN but also contains weaker spectral signatures in the 1225-1500 cm-1 region. The weak optical signatures dominate some of the recorded single pixel spectra. We can uniquely assign these Raman forbidden transitions to multipolar Raman scattering, which implicates spatially varying enhanced electric field gradients at plasmonic tip-sample nanojunctions. Specifically, we can assign observations of tip-enhanced electric dipole-magnetic dipole as well as electric dipole-electric quadrupole driven transitions. These signals need to be understood and accounted for in the interpretation of TERS spectral images, particularly in on-going quests aimed at chemical reaction mapping via TERS.

Wang, Chih-Feng↗
DOE OSTI2021

Mapping Molecular Adsorption Configurations with < 5 nm Spatial Resolution through Ambient Tip-Enhanced Raman Imaging

We interrogate para-mercaptobenzoic acid (MBA) molecules chemisorbed onto plasmonic silver nanocubes through tip-enhanced Raman (TER) spectral nano-imaging. Through a detailed examination of the spectra, aided by correlation analysis and density functional theory calculations, we find that MBA chemisorbs onto the plasmonic particles with at least two distinct configurations: S- and CO2-bound. High spatial resolution TER mapping allows us to distinguish between the distinct adsorption geometries with a pixel-limited (< 5 nm) spatial resolution under ambient laboratory conditions.

Gabel, Matthew L.↗
DOE OSTI2021

Imaging Plasmons with Sub-2 nm Spatial Resolution via Tip-Enhanced Four-Wave Mixing

Four-wave mixing at plasmonic tip-sample nanojunctions may be used to visualize plasmonic fields with sub-2nm spatial resolution under ambient laboratory conditions. We illustrate the latter using a gold-coated atomic force microscopy probe irradiated with a pair of near-IR femtosecond laser pulses and used to interrogate plasmonic gold nanoplates and silver nanocubes. Through diagnostic polarization-dependent tip-only measurements, we illustrate that the 4-wave mixing signal is localized to the tip apex. The apex-bound signal is further enhanced when the tip is located near plasmonic nanoparticles. Overall, this work paves the way for visualizing coherent electronic and vibrational dynamics with joint femtosecond temporal and few-nanometer spatial resolution under ambient conditions.

Wang, Chih-Feng↗
DOE OSTI2026

Anisotropic Exciton Transport in a Lamellar CsPbBr 3 Nanocrystal Superlattice

Colloidal self-assembly is one strategy for engineering anisotropic properties into otherwise isotropic materials. In this work, we demonstrate anisotropic exciton transport in an A 2 B-type superlattice containing columns of 5.3 nm CsPbBr 3 nanocubes assembled into a hexagonal lattice around 6.5 nm LaF 3 nanodisks. Using transient photoluminescence microscopy, we determined that the exciton diffusivity along the fast axis of the superlattice is more than twice as large as that along the slow axis at T = 5 K, but that anisotropy is greatly suppressed at room temperature. Calculations of the diffusivity anisotropy ratio based on Förster theory overestimate the measured values, highlighting the limitations of this theory in completely describing exciton transport. Overall, our results demonstrate how self-assembly of colloidal nanocrystals can be used to engineer directional energy transport, and raise more questions about the microscopic nature of dipole coupling in CsPbBr 3 NC superlattices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Density Functional Tight Binding Insights into Plasmonic Silver–Platinum Nanoparticles and Alloys for Enhanced Photocatalysis

Developing accurate and efficient Slater-Koster (SK) tight-binding parameter sets is essential for quantum plasmonic studies of alloyed metal nanoparticles, as conventional time dependent density functional theory (TD-DFT) calculations are computationally prohibitive for larger clusters. In this work, we develop and validate density functional tight binding (DFTB) parameter sets for both ground state (GS-SK) and excited state (ES-SK) calculations to study the structural, electronic, and optical properties of silver (Ag), platinum (Pt), and Ag–Pt nanoalloys. Our investigation of the ground state properties demonstrates that the GS-SK parameters enable DFTB to closely reproduce the electronic structures of platinum clusters with diverse sizes and geometries – showing qualitative agreement with DFT for density of states (DOS) profiles and energy levels. The ES-SK parameters accurately describe excited-state properties compared to TD-DFT reference calculations, including the broad, featureless absorption profiles of Pt that are dominated by interband transitions. Using the ES-SK parameters within a real-time TD-DFTB framework, we compute size-dependent optical absorption spectra of Ag, Pt and Ag-Pt nanocubes containing up to 1099 atoms (size ∼4.18 nm). A detailed study of Ag–Pt and Pt-Ag core–shell nanoparticles shows quenching of the Ag plasmon resonance even at monolayer coverage for Ag-Pt, but not for Pt-Ag. We also show how to define submonolayer Ag-core Pt-shell cubic structures that have similar optical properties to those generated experimentally for much larger particles, which offers potential for describing plasmon-enhanced photocatalysis. Collectively, the GS-SK and ES-SK parameter sets provide an accurate, computationally efficient approach for modeling the complex optical and electronic behavior of noble–transition metal nanostructures and their alloys.

SPR↗
DOE OSTI2021

Large-Area, Highly Crystalline DNA-Assembled Metasurfaces Exhibiting Widely Tunable Epsilon-Near-Zero Behavior

Metasurfaces prepared via bottom-up nanoparticle assembly enable the deliberate manipulation of light in the optical regime, resulting in media with unprecedented optical responses. Here, we report a scalable method to grow highly crystalline 2D metasurfaces comprised of colloidal gold nanocubes, over macroscopic areas, using DNA-mediated assembly under equilibrium conditions. Using an effective medium description, we predict that these plasmonic metasurfaces behave as dielectric media with high refractive indices that can be dynamically tuned by tuning DNA length. Furthermore, we predict that when coupled with an underlying thin gold film, the real permittivity of these metasurfaces exhibits a crossover region between positive and negative values, known as the epsilon-near-zero (ENZ) condition, which can be tuned between 1.5 - 2.6 μm by changing DNA length. Optical characterization performed on the DNA-assembled metasurfaces reveals that the predicted optical properties agree well with measured response. Overall, we propose an efficient method for realizing large-area plasmonic metasurfaces that enable dynamic control over unique optical characteristics. High-index and ENZ metasurfaces operating in the telecommunications regime could have significant implications in high-speed optical computing, optical communications, optical imaging, and other areas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

2D Semiconductor Nanosheets Supported on Colloidal Quantum Cubes

Two-dimensional (2D) colloidal nanocrystals bridge the physics of epitaxial quantum wells with the synthetic scalability of solution-processed nanomaterials, making them an attractive platform for future LEDs, lasers, and photodetectors. However, their strongly anisotropic 2D morphology complicates the formation of close-packed films required for electrically interfaced devices. Here, we address this limitation by introducing semiconductor quantum cubes (QCs) in which 2D CdSe nanosheets are conformally grown on six faces of CdS cubic scaffolds. This architecture preserves excitonic physics of quasi-2D nanosheets while leveraging the mechanical rigidity and self-assembly of nanocubes, enabling close-packed films with improved electronic coupling and electrical conductivity. By extending CdS/CdSe QCs into a CdS/CdSe/CdS core/shell/shell structure, we realize 2D quantum wells that exhibit unusual multiexciton behavior. In particular, CdS/CdSe/CdS QCs show repulsive exciton–exciton interactions, which decouple CdSe facets into quasi-independent emitters. This leads to the suppressed Auger recombination, long multiexciton lifetimes, and broadband optical gain. The demonstrated combination of unusual multiexciton photophysics and favorable film self-assembly characteristics in QCs presents opportunities for solution-processed optoelectronic devices.

perovskites↗
DOE OSTI2026

Atomic Evolution of Hydrogen Intercalation Wave Dynamics in Palladium Nanocrystals Revealed by Liquid-Phase Transmission Electron Microscopy

Solute-intercalation-induced phase separation creates spatial heterogeneities in host materials, a phenomenon ubiquitous in batteries, hydrogen storage, and other energy devices. Despite many efforts, probing intercalation processes at the atomic scale has been a significant challenge. By utilizing liquid-phase transmission electron microscopy (TEM), we study hydrogen (de)intercalation in palladium nanocrystals as a model system and have achieved unprecedented atomic-resolution imaging of hydrogen intercalation wave dynamics. Our observations reveal that intercalation wave mechanisms, instead of shrinking-core mechanisms, prevail at ambient temperature for palladium nanocubes ranging from ∼60 nm down to ∼10 nm. Systematic image analysis uncovers the atomic evolution of the hydrogen intercalation wave, transitioning from nonplanar and inclined boundaries to those closely aligned with {100} planes. Our kinetic Monte Carlo simulations demonstrate that the observed intercalation wave dynamics correspond to sorption pathways minimizing the lattice mismatch strain at the phase boundary. In conclusion, unveiling the atomic intercalation pathways holds profound implications for engineering intercalation-mediated devices and advancements in energy sciences.

Lee, Daewon [Lawrence Berkeley National Laboratory↗
DOE OSTI2024

Operando Raman spectroscopy uncovers hydroxide and CO species enhance ethanol selectivity during pulsed CO 2 electroreduction

Pulsed CO 2 electroreduction (CO 2 RR) has recently emerged as a facile way to in situ tune the product selectivity, in particular toward ethanol, without re-designing the catalytic system. However, in-depth mechanistic understanding requires comprehensive operando time-resolved studies to identify the kinetics and dynamics of the electrocatalytic interface. Here, we track the adsorbates and the catalyst state of pre-reduced Cu 2 O nanocubes (~30 nm) during pulsed CO 2 RR using sub-second time-resolved operando Raman spectroscopy. By screening a variety of product-steering pulse length conditions, we unravel the critical role of co-adsorbed OH and CO on the Cu surface next to the oxidative formation of Cu-O ad or CuO x /(OH) y species, impacting the kinetics of CO adsorption and boosting the ethanol selectivity. However, a too low OH ad coverage following the formation of bulk-like Cu 2 O induces a significant increase in the C 1 selectivity, while a too high OH ad coverage poisons the surface for C-C coupling. Thus, we unveil the importance of co-adsorbed OH on the alcohol formation under CO 2 RR conditions and thereby, pave the way for improved catalyst design and operating conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Operando studies reveal active Cu nanograins for CO 2 electroreduction

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals. Although Cu enables CO 2 -to-multicarbon product (C 2+ ) conversion, the nature of the active sites under operating conditions remains elusive. Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu 2 O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO 2 reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C–C coupling. Quantitative structure–activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C 2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C 2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. Importantly, the correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Operando imaging reveals nanocatalyst reconstruction

Here, understanding the nature of structural evolution of Cu nanocatalysts during CO 2 electroreduction is a long-standing question in catalysis science. Now, operando electron microscopy techniques correlated with X-ray spectroscopy and first-principles modelling demonstrate that the reaction products themselves drive the structural evolution in Cu nanocubes.

Characterization and analytical techniques↗
DOE OSTI2021

Planar defect-driven electrocatalysis of CO 2 -to-C 2 H 4 conversion

The selectivity towards a specific C 2+ product, such as ethylene (C 2 H 4 ), is sensitive to the surface structure of copper (Cu) catalysts in carbon dioxide (CO 2 ) electro-reduction. The fundamental understanding of such sensitivity can guide the development of advanced electrocatalysts, although it remains challenging at the atomic level. In this work, we demonstrated that planar defects, such as stacking faults, could drive the electrocatalysis of CO 2 -to-C 2 H 4 conversion with higher selectivity and productivity than Cu(100) facets in the intermediate potential region (–0.50 ~ –0.65 V vs. RHE). The unique right bipyramidal Cu nanocrystals containing a combination of (100) facets and a set of parallel planar defects delivered 67% faradaic efficiency (FE) for C 2 H 4 and a partial current density of 217 mA cm –2 at –0.63 V vs. RHE. In contrast, Cu nanocubes with exclusive (100) facets exhibited only 46% FE for C 2 H 4 and a partial current density of 87 mA cm –2 at an identical potential. Both ex situ CO temperature-programmed desorption and in situ Raman spectroscopy analysis implied that the stronger *CO adsorption on planar defect sites facilitates CO generation kinetics, which contributes to a higher surface coverage of *CO and in turn an enhanced reaction rate of C–C coupling towards C 2+ products, especially C 2 H 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Inverse size-dependent Stokes shift in strongly quantum confined CsPbBr 3 perovskite nanoplates

Colloidal semiconductor nanocrystals (NCs) are used as bright chromatic fluorophores for energy-efficient displays. Here we focus here on the size-dependent Stokes shift for CsPbBr 3 nanocrystals. The Stokes shift, i.e., the difference between the wavelengths of absorption and emission maxima, is crucial for display application, as it controls the degree to which light is reabsorbed by the emitting material reducing the energetic efficiency. One major impediment to the industrial adoption of NCs is that slight deviations in manufacturing conditions may result in a wide dispersion of the product's properties. A data-driven analysis of over 2000 reactions comparing two data sets, one produced via standard colloidal synthesis and the other via high-throughput automated synthesis is discussed. We show that differences in the reaction conditions of colloidal CsPbBr 3 nanocrystals yield nanocrystals with opposite Stokes shift size-dependent trends. These match the morphologies of two-dimensional nanoplatelets (NPLs) and nanocrystal cubes. The Stokes shift size dependence trend of NPLs and nanocubes is non-monotonic indicating different physics is at play for the two nanocrystal morphologies. For nanocrystals with cubic shape, with the increase of edge length, there is a significant decrease in Stokes shift values. However, for NPLs with the increase of thickness (1–4 ML), Stokes shift values will increase. The study emphasizes the transition from a spectroscopic point of view and relates the two Stokes shift trends to 2D and 0D exciton dimensionalities for the two morphologies. Our findings highlight the importance of CsPbBr 3 nanocrystal morphology for Stokes shift prediction.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Morphology and property tuning in ZnO–Ni hybrid metamaterials in vertically aligned nanocomposite (VAN) form

ZnO thin films have attracted significant interest in the past decades owing to their unique wide band gap properties, piezoelectric properties, non-linearity and plasmonic properties. Recent efforts have been made in coupling ZnO with secondary phases to enhance its functionalities, such as Au–ZnO nanocomposite thin films with tunable optical and plasmonic properties. In this work, magnetic nanostructures of Ni are incorporated in ZnO thin films in a vertically aligned nanocomposite (VAN) form to couple magnetic and plasmonic response in a complex hybrid metamaterial system. Nickel (Ni) is of interest due to its ferromagnetic and plasmonic properties along with gold (Au) which is also plasmonic. Therefore, two approaches, namely, tuning of the deposition pressure and use of a ZnO–Au seeding layer have been attempted to achieve unique Ni nanostructures in addition to tuning of the microstructure. Together, both approaches demonstrate a range of microstructures such as core–shell, nanodisk, nanocup, and nanocube-like morphologies not previously attempted. Additionally, there is effective tuning of properties. Specifically, the seeding layer thickness causes hyperbolic behavior as well as redshift in the surface plasmon resonance (SPR) wavelength. The addition of the ZnO–Au seeding layer directly influences the optical properties. Plus, regardless of the different approaches, the films demonstrate magnetic anisotropy based on the composition and microstructure of the film which impacted the saturation magnetization and coercivity. This study demonstrates the potential of ZnO-based complex hybrid metamaterials with coupled electro-magneto-optical properties for integrated photonic devices.

Bhatt, Nirali A. [Purdue Univ., West Lafayette, IN↗
DOE OSTI2025

Formation of open ruthenium branched structures with highly exposed active sites for oxygen evolution reaction electrocatalysis

The formation of exposed active sites that have high activity and stability for oxygen evolution reaction (OER) catalysis is a significant opportunity for improving water electrolysers. Low-index facets surface Ru can achieve both high activity and stability for OER. Here, we present a new catalyst design where low-index faceted Ru branches are grown off the corners of Pt nanocubes, forming open Ru branched nanoparticles. This open branched structure, exposing low-index facets on its length-tunable branch, enables a high electrochemically active surface area (ECSA), achieving high activity and stability for OER. This design strategy and synthetic control provide a principle for achieving high-performance OER nanocatalysts.

Xiao, Sa [The University of New South Wales, Sydne↗
DOE OSTI2023

Nucleation-mediated reshaping of facetted metallic nanocrystals: Breakdown of the classical free energy picture

Shape stability is key to avoiding degradation of performance for metallic nanocrystals synthesized with facetted non-equilibrium shapes to optimize properties for catalysis, plasmonics, and so on. Reshaping of facetted nanocrystals is controlled by the surface diffusion-mediated nucleation and growth of new outer layers of atoms. Here, Kinetic Monte Carlo (KMC) simulation of a realistic stochastic atomistic-level model is applied to precisely track the reshaping of Pd octahedra and nanocubes. Unexpectedly, separate constrained equilibrium Monte Carlo analysis of the free energy profile during reshaping reveals a fundamental failure of the classical nucleation theory (CNT) prediction for the reshaping barrier and rate. Why? Nucleation barriers can be relatively low for these processes, so the system is not locally equilibrated before crossing the barrier, as assumed in CNT. This claim is supported by an analysis of a first-passage problem for reshaping within a master equation framework for the model that reasonably captures the behavior in KMC simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗