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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 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Ultrafast excited-state dynamics in shape- and composition-controlled gold–silver bimetallic nanostructures

In this paper, we have examined the ultrafast dynamics of shape- and composition-controlled bimetallic Au/Ag core/shell nanostructures with transient absorption spectroscopy (TAS) as a function of Ag layer thickness (0–15 nm) and pump excitation fluence (50–500 nJ/pulse). Our synthesis approach generated both bimetallic nanocubes and nanopyramids with distinct dipolar plasmon resonances and plasmon dephasing behavior at the resonance. Lifetimes obtained from TAS at low powers (50 nJ/pulse) demonstrated minimal dependence on the Ag layer thickness, whereas at high power (500 nJ/pulse) a rise in electron–phonon coupling lifetime (τ 1 ) was observed with increasing Ag shell thickness for both nanocubes and nanopyramids. This is attributable to the stronger absorption of the 400 nm pump pulse with higher Ag content, which induced higher electron temperatures. The phonon–phonon scattering lifetime (τ 2 ) also rises with increasing Ag layer, contributed both by the increasing size of the Au/Ag nanostructures as well as by surface chemistry effects. Further, we observed that even the thinnest, 2 nm, Ag shell strongly impacts both τ 1 and τ 2 at high power despite minimal change in overall size, indicating that the nanostructure composition also strongly impacts the thermalization temperature following absorption of 400 nm light. We also observed a shape-dependent trend at high power, where τ 2 increased for the nanopyramids with increasing Ag shell thickness and nanostructure size, but bimetallic nanocubes demonstrated an unexpected decrease in τ 2 for the thickest, 15 nm, Ag shell. This was attributed to the larger number of corners and edges in the nanocubes relative to the nanopyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Theoretical Insights into Methane C–H Bond Activation on Alkaline Metal Oxides

Here, we investigate the role of alkaline metal oxides (AMO) (MgO, CaO, and SrO) in activating the C–H bond in methane. We also use Density Functional Theory (DFT) and microkinetic modeling to study the catalytic elementary steps in breaking the C–H bond in methane and creating the methyl radical, a precursor prior to creating C2 products. We also study the effects of surface geometry on the catalytic activity of AMO by examining terrace and step sites. We observe that the process of activating methane depends strongly on the structure of the AMO. When the AMO surface is doped with an alkali metal, the transition state (TS) structure has a methyl radical-like behavior, where the methyl radical interacts weakly with the AMO surface. In this case, the TS energy scales with the hydrogen binding energy. On pure AMO, the TS interacts with AMO surface oxygen as well as the metal atom on the surface, and consequently the TS energy scales with the binding energy of hydrogen and methyl. We study the activity of AMO using a mean-field microkinetic model. The results indicate that terrace sites have similar catalytic activity, with the exception of MgO(100). Step sites bind hydrogen more strongly, making them more active, and this confirms previously reported experimental results. We map the catalytic activity of AMO using a volcano plot with two descriptors: the methyl and the hydrogen binding energies, with the latter being a more significant descriptor. The microkinetic model results suggest that C–H bond dissociation is not always the rate-limiting step. At weak hydrogen binding, the reaction is limited by C–H bond activation. At strong hydrogen binding, the reaction is limited due to poisoning of the active site. We found an increase in activity of AMO as the basicity increased. Finally, the developed microkinetic model allows screening for improved catalysts using simple calculations of the hydrogen binding energy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Size-Dependent Asymmetric Auger Interactions in Plasma-Produced n- and p-Type-Doped Silicon Nanocrystals

Nonradiative Auger recombination (AR) tends to dominate carrier dynamics in charged, quantum-confined structures. Consequently, it complicates the practical realization of many semiconductor nanocrystal (NC)-based devices such as light-emitting diodes, photovoltaic cells, and single-photon emitters, in which charged exciton states often occur. To this end, extensive experimental studies on direct band gap NCs have investigated the trion components (both positive and negative) that construct the total AR rate. However, such an analysis has remained elusive for indirect band gap Si NCs. In this study, we investigate AR decay of non-thermal plasma-produced n- and p-type-doped Si NCs. We expand the study over a large NC size range (D NC ~ 3-8 nm), in which n- and p-type doping is achieved by either a substitutional or surface doping effect, respectively. First, we monitor the AR of charge-neutral multiexcitons by measuring the biexciton lifetime (t X X) as a function of the NC size and doping configuration. We show that this method can be used to determine the presence of free carriers for any doped NC system, regardless of the presence/absence of defect channels in the carrier dynamics. Second, we develop a photophysical fitting model to determine the Auger lifetime of the simplest charged states in Si NCs: the negative (t X - ) and positive (t X + ) trions. Trion lifetimes shorten with increasing quantum confinement, as expected from (1) closer spatial proximity of the interacting charges and (2) increased relaxation of the momentum conservation rule. While both t X - and t X + are in the nanosecond time regime (and both therefore completely dominate the carrier dynamics), AR with excess holes is faster. This asymmetry is explained by a higher density of valence band states in comparison to the conduction band states, due to effective mass differences between electrons and holes.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Phosphorene: Fabrication, properties, and applications

Phosphorene, the single- or few-layer form of black phosphorus, was recently rediscovered as a two-dimensional layered material holding great promise for applications in electronics and optoelectronics. Research into its fundamental properties and device applications has since seen exponential growth. In this Perspective, we review recent progress in phosphorene research, touching upon topics on fabrication, properties, and applications; we also discuss challenges and future research directions. We highlight the intrinsically anisotropic electronic, transport, optoelectronic, thermoelectric, and mechanical properties of phosphorene resulting from its puckered structure in contrast to those of graphene and transition-metal dichalcogenides. The facile fabrication and novel properties of phosphorene have inspired design and demonstration of new nanodevices; however, further progress hinges on resolutions to technical obstructions like surface degradation effects and nonscalable fabrication techniques. We also briefly describe the latest developments of more sophisticated design concepts and implementation schemes that address some of the challenges in phosphorene research. As a result, it is expected that this fascinating material will continue to offer tremendous opportunities for research and development for the foreseeable future.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Structure and Dynamics of CO 2 at the Air–Water Interface from Classical and Neural Network Potentials

The accurate description of the structure and dynamics of CO 2 at the instantaneous air–water interface, along with the effects of surface fluctuations on the CO 2 -transport processes, is essential for the development of negative emission technologies aimed at minimizing climate change. In this study, we performed molecular dynamics simulations of CO 2 at the air–water interface using neural network potentials (NNPs) trained on ab initio data generated through density-functional-theory-based molecular dynamics simulations. We compared these results with classical force fields to assess their performance in modeling interfacial CO 2 behavior. Our findings revealed that the asymmetric interactions, coupled with thermal surface fluctuations at the air–water interface, significantly influence CO 2 transport into the aqueous phase. The simulations demonstrate that classical force fields underestimate both the free energy of CO 2 transport and the strength of its interactions at the interface compared with the neural network potentials. In conclusion, the free energy and the interfacial dynamics of CO 2 are primarily influenced by the distribution of water within the instantaneous interfacial water layer, responsible for creating an asymmetric intermolecular interaction environment within the interfacial region.

Ab initio molecular dynamics↗
DOE OSTI2026

Understanding Solvent-Induced Glass Transition in Polymer Thin Films Using Absorption–Desorption Isotherms

The fundamental thermodynamic and mechanical underpinnings of polymer thin films exposed to solvent vapor are critical for the development of advanced nanolithography and high-performance coatings. This work investigates the solvent− polymer interactions of glassy thin films by using the solvent absorption−desorption isotherms. An analogous relationship to the Flory−Fox equation was observed between solvent−induced glass transition, swelling, Flory−Huggins interaction parameter, and molecular weight. Isothermal swelling measurements revealed that the glass transition trends are more robust in the absorption curve compared to desorption, contrary to previous reports. Excess osmotic pressure analysis of the isotherm provides a measure of the degree of physical aging in thin films annealed below the glass transition. This is further validated in the ordering of block copolymer (BCP) films annealed at low solvent activity. In agreement with the thermal analysis, free-surface plasticization effects become the most prominent below 100 nm. However, solvent annealing is largely dependent on solvent mass transport, as made evident by the strong dependence on solvent viscosity. From these observations, four general types of isotherms are identified that graphically capture distinct solvent−polymer interaction regimes. More broadly, these results inform solvent vapor annealing-induced self-assembly, sequential infiltration synthesis, membrane-based separations, adsorptive processes, and swelling-based responsive materials design.

Hendeniya, Nayanathara [Iowa State Univ., Ames, IA↗
DOE OSTI2018

Physical Stability and Dissolution Behavior of Ketoconazole–Organic Acid Coamorphous Systems

In an earlier investigation, coamorphous systems of ketoconazole (KTZ) prepared with each oxalic (OXA), tartaric (TAR), citric (CIT), and succinic (SUC) acid, revealed drug–acid ionic or hydrogen bonding interactions in the solid-state (Fung et al, Mol. Pharmaceutics, 2018, 15 (3), 1052–1061). We showed that the drug–acid interactions in KTZ–TAR were the strongest, followed by KTZ–OXA, KTZ–CIT, and KTZ–SUC. In this study, we investigated the crystallization propensity and dissolution behavior of the KTZ–acid coamorphous systems. When in contact with water (either as water vapor or as aqueous phosphate buffer), while KTZ–CIT and KTZ–TAR were physically stable and resisted crystallization, KTZ–SUC and KTZ–OXA crystallized more readily than KTZ alone. The dissolution performances of the coamorphous systems were compared using the area under the curve (AUC) obtained from the concentration–time profiles. KTZ–OXA exhibited the highest AUC, while it was about the same for KTZ–TAR and KTZ–CIT and the lowest for KTZ–SUC. The enhancement in dissolution appeared to become more pronounced as the strength of the acid (OXA > TAR > CIT > SUC) increased. Coamorphization with acid caused at least a twofold increase in AUC when compared with amorphous KTZ. The decrease in pH of the diffusion layer of the dissolving solid, brought about by the acid, is at least partially responsible for the dissolution enhancement. In addition, the particles of KTZ– OXA, KTZ–TAR, and KTZ–CIT were much smaller than those of KTZ–SUC. The consequent effect on surface area could be another contributing factor to the initial dissolution behavior.

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

Engineering of Exciton Spatial Distribution in CdS Nanoplatelets

Zinc-blende CdS nanoplatelets with atomically flat and very large {100} basal planes terminated solely by one type of element (either Cd or S atoms) are synthesized. Optical spectroscopy, X-ray diffraction, X-ray absorption, and transmission electron microscopy confirm that the surface structures of newly developed S-terminated CdS nanoplatelets are at least as well-defined as the original Cd-terminated nanoplatelets. Band gaps of the nanoplatelets are found to depend on not only the quantum-confined dimension (thickness) but also the nature of the surface termination. The facet structure dictates the packing of the ligands (carboxylate for Cd-terminated nanoplatelets and alkyl for S-terminated nanoplatelets), which causes a difference in the lattice strain and significantly affects the optical spectral width. Experimental and theoretical results reveal that engineering the exciton spatial distribution by the tailored synthesis of semiconductor nanocrystals with a precisely controlled surface structure is fully possible, which should open a new door for delivering the long-promised potential of semiconductor nanocrystals.

Cadmium sulfide↗
DOE OSTI2025

Imaging the Acceptor Wave Function Anisotropy in Silicon

We present the first scanning tunneling microscopy (STM) image of hydrogenic acceptor wave functions in silicon. These acceptor states appear as square-ring-like features in STM images and originate from near-surface defects introduced by high-energy bismuth implantation into a silicon (001) wafer. Scanning tunneling spectroscopy confirms the formation of a p-type surface. Effective-mass and tight-binding calculations provide an excellent description of the observed square-ring-like features, confirming their acceptor character and attributing their symmetry to the light- and heavy-hole band degeneracy in silicon. A detailed understanding of the energetic and spatial properties of acceptor wave functions in silicon is essential for engineering large-scale acceptor-based quantum devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Probing the Reactivity of the Active Material of a Li-Ion Silicon Anode with Common Battery Solvents

Calculations and modeling have shown that replacing the traditional graphite anode with silicon can greatly improve the energy density of lithium-ion batteries. However, the large volume change of silicon particles and high reactivity of lithiated silicon when in contact with the electrolyte lead to rapid capacity fading during charging/discharging processes. In this report, we use specific lithium silicides (LS) as model compounds to systematically study the reaction between lithiated Si and different electrolyte solvents, which provides a powerful platform to deconvolute and evaluate the degradation of various organic solvents in contact with the active lithiated Si-electrode surface after lithiation. Nuclear Magnetic Resonance (NMR) characterization results show that a cyclic carbonate such as ethylene carbonate is chemically less stable than a linear carbonate such as ethylmethyl carbonate, fluoroethylene carbonate, and triglyme as they are found to be more stable when mixed with LS model compounds. Furthermore, guided by the experimental results, two ethylene carbonate (EC)-free electrolytes are studied, and the electrochemical results show improvements with graphite-free Si electrodes relative to the traditional ethylene-carbonate-based electrolytes. More importantly, the study contributes to our understanding of the significant fundamental chemical and electrochemical stability differences between silicon and traditional graphite lithium-ion battery (LIB) anodes and suggests a focused development of electrolytes with specific chemical stability vs lithiated silicon which can passivate the surface more effectively.

25 ENERGY STORAGE↗
DOE OSTI2020

Single-Facet Dominant Anatase TiO 2 (101) and (001) Model Catalysts to Elucidate the Active Sites for Alkanol Dehydration

Alkanol dehydration on Lewis acid-base pairs of transition metal oxide catalysts is a reaction of importance in oxygen removal from biomass-derived feedstocks and their conversion to chemicals in general. However, catalysts with a high degree of structural heterogeneity, such as commercial TiO 2 powders, are not well-suited to establish rigorous structure-function relationships at an atomic level. Here, we provide compelling evidence for the effects of surface orientation of TiO 2 catalyst on elimination reactions of alcohols. Two anatase titania model catalysts, with preferential exposure of (101) and (001) facets, were synthesized and studied for 2-propanol dehydration using kinetic, isotopic, microscopic, and spectroscopic measurements, coupled with DFT calculations. Surface Lewis acid sites were found to be active for 2-propanol dehydration and (101) facets are more reactive than (001) facets under the reaction conditions studied. On both anatase surfaces, 2-propanol was found to dehydrate via concerted E2 elimination pathways, but with different initial states and thus also different intrinsic activation barriers. Molecular 2-propanol dehydration dominates on TiO2 (101) while on TiO 2 (001), 2-propanol simultaneously converts to more stable 2-propoxide before dehydration, which then requires higher activation energies for E2 elimination.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Tantalum Oxide Electron-Selective Heterocontacts for Silicon Photovoltaics and Photoelectrochemical Water Reduction

Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes for water splitting and renewable production of hydrogen. Here, we demonstrate a nanoscale tantalum oxide (TaO x , ~6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst. The PV application of TaO x is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%. In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%. These results represent a 2% and 3.8% absolute enhancement over control devices without a TaO x interlayer, respectively. The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications.

14 SOLAR ENERGY↗
DOE OSTI2020

Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si 6 H 12 ) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Here, through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

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

Quantitative Infrared-to-Terahertz Nanospectroscopy of Semiconductors

Semiconductor technology now employs few-nanometer features, necessitating tools probing electronic properties on the same length scale. While the concentration of free charge carriers is routinely measured, the scattering rate remains challenging to access at the nanoscale. Here, we present ultrabroadband (5–50 THz) synchrotron infrared nanospectroscopy as a quantitative metrology tool for semiconductors. This technique can determine both the charge carrier concentration and scattering rate with percent-level accuracy, and it is inherently capable of ∼10 nm spatial resolution. We study silicon with different doping levels and confirm the method’s accuracy by statistical analysis and comparison with established far-field infrared spectroscopy. Near-field measurements systematically reveal charge-carrier concentrations ∼30% lower than far-field values, consistent with increased surface sensitivity and surface depletion. Our work establishes synchrotron infrared nanospectroscopy as a precise tool for quantitative nanoscale semiconductor characterization and paves the way toward all-optical characterization of surface depletion effects.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Multilevel Computational Studies Reveal the Importance of Axial Ligand for Oxygen Reduction Reaction on Fe–N–C Materials

The systematic improvement of Fe–N–C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe–N–C materials is the formation of the Fe III –OOH intermediate. They also show that an active site model with a water molecule axially ligated to the iron center throughout the catalytic cycle produces results that are consistent with the experimental measurements. In particular, reliable prediction of the ORR onset potential and the Fe(III/II) redox potential associated with the conversion of Fe III –OH to Fe II and desorbed H 2 O requires an axial H 2 O co-adsorbed to the iron center. The observation of a five-coordinate rather than four-coordinate active site has significant implications for the thermodynamics and mechanism of ORR. Importantly, these findings highlight the importance of solvent–substrate interactions and surface charge effects for understanding the PCET reaction mechanisms and transition-metal redox couples under realistic electrochemical conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗