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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

A Comparison of Single-Molecule Emission in Aluminum and Gold Zero-Mode Waveguides

We characterized the effect of gold and aluminum zero-mode waveguides (ZMWs) on the brightness of immobilized single emitters by probing fluorophores that absorb in the green and red regions of the visible spectrum. Aluminum ZMWs enhance the emission of Atto565 fluorophores upon green excitation, but they do not enhance the emission of Atto647N fluorophores upon red excitation. Gold ZMWs increase emission of both fluorophores with Atto647N showing enhancement that is threefold higher than that observed for Atto565. Our work indicates that 200 nm gold ZMWs are better suited for single-molecule fluorescence studies in the red region of the visible spectrum, while aluminum appears more suited for the green region of the visible spectrum.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Probing Biomolecular Interactions with Gold Nanoparticle-Decorated Single-Walled Carbon Nanotubes

Hybrid nanomaterials comprising metal–graphitic interfaces are uniquely suitable to probe molecular interactions and the associated phenomena such as charge transfer and adsorbate spillover effects. In this paper, we study the modulation of the electronic and chemical properties of gold nanoparticle-decorated single-walled carbon nanotubes (SWCNT) using Raman spectroscopy and measurements of field-effect transistor (FET) characteristics. SWCNT are extremely sensitive to changes in the local electronic environment and therefore gold-analyte interactions may be probed both through changes in FET characteristics (as an electrical transducer) and in surface-enhanced Raman scattering (as a chromophore). We study these changes both experimentally and theoretically in order to elucidate the electronic structure of complex nanocomposites, and the information gathered from these experiments is applied to the study of biomolecular interactions with gold nanoparticle-decorated SWCNT. Lastly, this study, in addition to providing deeper understanding of metal–graphitic interfaces, will offer a combined approach to SWCNT biosensing methodology based on the dual monitoring of the FET–Raman characteristics, which we demonstrate through detection of glutathione.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electron–Hole-Pair-Induced Vibrational Energy Relaxation of Rhenium Catalysts on Gold Surfaces

A combination of time-resolved vibrational spectroscopy and density functional theory techniques have been applied to study the vibrational energy relaxation dynamics of the Re(4,4'-dicyano-2,2'-bipyridine)(CO)3Cl (Re(CO)3Cl) catalyst for CO2 to CO conversion bound to gold surfaces. The kinetics of vibrational relaxation exhibits a biexponential decay including an ultrafast initial relaxation and complete recovery of the ground vibrational state. Ab initio molecular dynamics simulations and time-dependent perturbation theory reveal the former to be due to vibrational population exchange between CO stretching modes and the latter to be a combination of intramolecular vibrational relaxation (IVR) and electron–hole pair (EHP)-induced energy transfer into the gold substrate. EHP-induced energy transfer from the Re(CO)3Cl adsorbate into the gold surface occurs on the same time scale as IVR of Re(CO)3Cl in aprotic solvents. Therefore, it is expected to be particularly relevant to understanding the reduced catalytic activity of the homogeneous catalyst when anchored to a metal surface.

Ge, Aimin↗
DOE OSTI2020

Creation of Single-Photon Emitters in WSe 2 Monolayers Using Nanometer-Sized Gold Tips

Due to their tunable bandgaps and strong spin-valley locking, transition metal dichalcogenides constitute a unique platform for hosting single-photon emitters. Here, we present a versatile approach for creating bright single-photon emitters in WSe 2 monolayers by the deposition of gold nanostars. Our molecular dynamics simulations reveal that the formation of the quantum emitters is likely caused by the highly localized strain fields created by the sharp tips of the gold nanostars. The surface plasmon modes supported by the gold nanostars can change the local electromagnetic fields in the vicinity of the quantum emitters, leading to their enhanced emission intensities. Moreover, by correlating the emission energies and intensities of the quantum emitters, we are able to associate them with two types of strain fields and derive the existence of a low-lying dark state in their electronic structures. The findings here are highly relevant for the development and understanding of single-photon emitters in transition metal dichalcogenide materials.

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

Nonequilibrium Thermodynamics of Colloidal Gold Nanocrystals Monitored by Ultrafast Electron Diffraction and Optical Scattering Microscopy

Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. In this paper, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron–phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron–phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Size Dependence of Atomically Precise Gold Nanoclusters in Chemoselective Hydrogenation and Active Site Structure

We here investigate the catalytic properties of water-soluble Aun(SG)m nanocluster catalysts (H-SG = glutathione) of different sizes, including Au15(SG)13, Au18(SG)14, Au25(SG)18, Au38(SG)24, and captopril-capped Au25(Capt)18 nanoclusters. These Aun(SR)m nanoclusters (-SR represents thiolate generally) are used as homogeneous catalysts (i.e., without supports) in the chemoselective hydrogenation of 4-nitrobenzaldehyde (4-NO2PhCHO) to 4-nitrobenzyl alcohol (4-NO2PhCH2OH) in water with H2 gas (20 bar) as the hydrogen source. These nanocluster catalysts, except Au18(SG)14, remain intact after the catalytic reaction, evidenced by UV-vis spectra which are characteristic of each sized nanoclusters and thus serve as spectroscopic fingerprints . We observe a drastic size-dependence and steric effect of protecting ligands on the gold nanocluster catalysts in the hydrogenation reaction. Density functional theory (DFT) modeling of the 4-nitrobenzaldehyde adsorption shows that both the CHO and NO2 groups are in close interact with the S-Au-S staples on the gold nanocluster surface; the adsorption of the 4-nitrobenzaldehyde molecule on the four different sized Aun(SR)m nanoclusters are moderately strong and similar in strength. The DFT results suggest that the catalytic activity of the Aun(SR)m nanoclusters is primarily determined by the surface area of the Au nanocluster, consistent with the observed trend of the conversion of 4-nitrobenzaldehyde versus the cluster size. Overall, this work offers the molecular insight into the hydrogenation of 4-nitrobenzaldehyde and the catalytically active site structure on gold nanocluster catalysts.

Au nanocluster↗
DOE OSTI2014

Dendritic Assembly of Gold Nanoparticles during Fuel-Forming Electrocatalysis

We report on the dendritic assembly of alkanethiol-capped gold nanoparticles on a glassy carbon support during electrochemical reduction of protons and CO 2 . We discovered that the primary mechanism by which surfactant-ligated gold nanoparticles lose surface area is by taking a random walk along the support, colliding with their neighbors, and fusing to form dendrites, a type of fractal aggregate. A random walk model reproduces the fractal dimensionality of the dendrites observed experimentally. The rate at which the dendrites form is strongly dependent on the solubility of the surfactant in the electrochemical double layer under the conditions of electrolysis. Since alkanethiolate surfactants reductively desorb at potentials close to the onset of CO 2 reduction, they do not poison the catalytic activity of the gold nanoparticles. While catalyst mobility is typically thought to be limited for room-temperature electrochemistry, our results reflect that nanoparticle mobility is significant under conditions at which they electrochemically catalyze gas evolution, even in the presence of a high surface area carbon and binder. A careful understanding of the electrolyte- and polarization-dependent nanoparticle aggregation kinetics informs strategies for maintaining catalyst dispersion during fuel-forming electrocatalysis.

dendrons↗
DOE OSTI2015

Silica-Supported Cationic Gold(I) Complexes as Heterogeneous Catalysts for Regio- and Enantioselective Lactonization Reactions

An effective approach for the synthesis of heterogeneous gold catalysts has been developed. These catalysts were easily assembled from readily available silica materials and gold complexes. The heterogeneous catalysts reflected superior reactivity in various reactions where protodeauration is the rate-limiting step. Dramatic enhancement in regio- and enantioselectivity was observed when compared to the homogeneous unsupported gold catalyst. The catalysts are easily recovered and recycled up to 11 times without loss of enantioselectivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Influence of surface potential on the adhesive force of radioactive gold surfaces

Radioactive particles may acquire surface potential through self-charging, and thus can behave differently from natural aerosols in atmospheric systems with respect to aggregation, deposition, resuspension, and transport to areas surrounding a radioactive source. Here, this work focuses on the adhesive force between radioactive particles and metallic surfaces, which relates to the deposition and resuspension of particles on surrounding surfaces. Scanning surface potential microscopy was employed to measure the surface potential of radioactive gold foil. Atomic force microscopy was used to investigate the adhesive force for gold that acquired surface charge either by irradiation or by application of an equivalent electrical bias. Overall, the adhesive force increases with increasing surface potential or relative humidity. However, a behavior that does not follow the general trend was observed for the irradiated gold at a high decay rate. A comparison between experimental measurements and calculated values revealed that the surface potential promotes adhesion. The contribution of the electrostatic force at high levels of relative humidity was lower than the one found using theoretical calculations due to the effects caused by enhanced adsorption rate of water molecules under a high surface charge density. Lastly, the results of this study can be used to provide a better understanding of the behavior of radioactive particles in atmospheric systems.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2017

Alloy-assisted deposition of three-dimensional arrays of atomic gold catalyst for crystal growth studies

Large-scale assembly of individual atoms over smooth surfaces is difficult to achieve. A configuration of an atom reservoir, in which individual atoms can be readily extracted, may successfully address this challenge. In this work, we demonstrate that a liquid gold-silicon alloy established in classical vapor-liquid-solid growth can deposit ordered and three-dimensional rings of isolated gold atoms over silicon nanowire sidewalls. Here, we perform ab initio molecular dynamics simulation and unveil a surprising single atomic gold-catalyzed chemical etching of silicon. Experimental verification of this catalytic process in silicon nanowires yields dopant-dependent, massive and ordered 3D grooves with spacing down to similar to 5 nm. Finally, we use these grooves as self-labeled and ex situ markers to resolve several complex silicon growths, including the formation of nodes, kinks, scale-like interfaces, and curved backbones.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ion irradiation induced phase transformation in gold nanocrystalline films

Gold is a noble metal typically stable as a solid in a face-centered cubic (FCC) structure under ambient conditions; however, under particular circumstances aberrant allotropes have been synthesized. In this work, we document the phase transformation of 25 nm thick nanocrystalline (NC) free-standing gold thin-film via in situ ion irradiation studied using atomic-resolution transmission electron microscopy (TEM). Utilizing precession electron diffraction (PED) techniques, crystallographic orientation and the radiation-induced relative strains were measured and furthermore used to determine that a combination of surface and radiation-induced strains lead to an FCC to hexagonal close packed (HCP) crystallographic phase transformation upon a 10 dpa radiation dose of Au 4+ ions. Contrary to previous studies, HCP phase in nanostructures of gold was stabilized and did not transform back to FCC due to a combination of size effects and defects imparted by damage cascades.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Improved CO 2 reduction activity towards C 2+ alcohols on a tandem gold on copper electrocatalyst

Here, the discovery of materials for the electrochemical transformation of carbon dioxide into liquid fuels has the potential to impact large-scale storage of renewable energies and reduce carbon emissions. Here, we report the discovery of an electrocatalyst composed of gold nanoparticles on a polycrystalline copper foil (Au/Cu) that is highly active for CO 2 reduction to alcohols. At low overpotentials, the Au/Cu electrocatalyst is over 100 times more selective for the formation of products containing C–C bonds versus methane or methanol, largely favouring the generation of alcohols over hydrocarbons. A combination of electrochemical testing and transport modelling supports the hypothesis that CO 2 reduction on gold generates a high CO concentration on nearby copper, where CO is further reduced to alcohols such as ethanol and n-propanol under locally alkaline conditions. The bimetallic Au/Cu electrocatalyst exhibits synergistic activity and selectivity superior to gold, copper or AuCu alloys, and opens new possibilities for the development of CO 2 reduction electrodes exploiting tandem catalysis mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Visualization of expanding warm dense gold and diamond heated rapidly by laser-generated ion beams

With the development of several novel heating sources, scientists can now heat a small sample isochorically above 10,000 K. Although matter at such an extreme state, known as warm dense matter, is commonly found in astrophysics (e.g., in planetary cores) as well as in high energy density physics experiments, its properties are not well understood and are difficult to predict theoretically. This is because the approximations made to describe condensed matter or high-temperature plasmas are invalid in this intermediate regime. A sufficiently large warm dense matter sample that is uniformly heated would be ideal for these studies, but has been unavailable to date. We have used a beam of quasi-monoenergetic aluminum ions to heat gold and diamond foils uniformly and isochorically. For the first time, we visualized directly the expanding warm dense gold and diamond with an optical streak camera. Furthermore, we present a new technique to determine the initial temperature of these heated samples from the measured expansion speeds of gold and diamond into vacuum. We anticipate the uniformly heated solid density target will allow for direct quantitative measurements of equation-of-state, conductivity, opacity, and stopping power of warm dense matter, benefiting plasma physics, astrophysics, and nuclear physics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Undecylprodigiosin conjugated monodisperse gold nanoparticles efficiently cause apoptosis in colon cancer cells in vitro

Bacterial pigment undecylprodigiosin (UP) was produced using Streptomyces sp. JS520 and conjugated to monodisperse gold nanoparticles (UP-Au). Both UP and UP-Au showed cytocidal activity towards melanoma (A375), lung carcinoma (A549), breast cancer (MCF-7) and colon cancer (HCT-116) cells, inducing apoptosis with IC50 values ranging from 0.4 to 4 mu g ml(-1). Unconjugated UP had a tendency to lose its activity over time and to change biophysical characteristics over pH. The loss of the pigment potency was overcome by conjugation with gold nanoparticles. UP-Au exhibited high stability over pH 3.8 to 7.4 and its activity remained unaffected in time. Nano-packing changed the mechanism of UP toxicity by converting the intracellular signals from a mitochondrial dependent to a mitochondrial independent apoptotic process. The availability of nonpyrogenic UP in high amounts, together with specific anticancer activity and improved stability in the complex with gold nanoparticles, presents a novel platform for further development of UP-Au complexes as an anticancer drug suitable for clinical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Gold nanotriangles decorated with superparamagnetic iron oxide nanoparticles: a compositional and microstructural study

The combination of iron oxide and gold in a single nanoparticle results in both magnetic and plasmonic properties that can stimulate novel applications in bio-sensing, medical imaging, or therapeutics. Microwave assisted heating allows the fabrication of multi-component, multi-functional nanostructures by promoting selective heating at desired sites. Recently, we reported a microwave-assisted polyol route yielding gold nanotriangles decorated with iron oxide nanoparticles. Here, we present an in-depth microstructural and compositional characterization of the system by using scanning transmission electron microscopy (STEM) and electron energy loss (EELS) spectroscopy. A method to remove the iron oxide nanoparticles from the gold nanocrystals and some insights on crystal nucleation and growth mechanisms are also provided.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Green and cytocompatible carboxyl modified gold–lysozyme nanoantibacterial for combating multidrug-resistant superbugs

The dissemination of multi-drug resistant (MDR) superbugs in hospital environments, communities and food animals and the very dynamic bacterial mutation frequency require the development of prolonged therapeutic strategies to gain mastery over antibiotic resistance. A AuNP–lysozyme nanoantibacterial was fabricated by the conjugation of AuNPs-C 6 H 4 -4-COOH with lysozyme via green reduction of aryldiazonium gold(III) salt [HOOC-4-C 6 H 4 N≡N]AuCl 4 . Results from molecular docking calculations aimed at revealing the binding mode of benzoic acid with the lysozyme structure clearly showed the lowest energy conformation with benzoic acid bound in the deep buried hydrophobic cavity of the protein active site through strong hydrogen bonding and hydrophobic interactions, thus validating the experimental outcomes of the current study which also exhibited the binding of –COOH functional groups in the interior of the protein structure. The superiority of the lysozyme bioconjugate against superbugs was demonstrated by the enhanced and broadened lysozyme antibacterial activities of 98–99% against extended spectrum beta lactamase (ESBL) producing Escherichia coli and imipenem-resistant Pseudomonas aeruginosa clinical isolates and a selection of Gram-negative and Gram-positive standard ATCC strains. Selective toxicity against bacteria was confirmed by the high viability of HeLa and fibroblast cell lines and the outstanding hemocompatibility at the minimum bacterial inhibitory concentrations (MICs). Turbidimetric enzyme kinetic assay showed the enhancement of the lysozyme hydrolytic activity by gold nanoparticles on the Micrococcus lysodeikticus bacterial substrate. Using gel electrophoresis, the induced cell wall breakdown was confirmed by detecting the leaked-out bacterial genomic DNA. The integrity and morphology changes of the E. coli bacteria were investigated using a scanning electron microscope after one hour of contact with the lysozyme–gold bioconjugate. The antibacterial functionalities showed little or no damage to healthy human cells and can be applied to wound dressings and medical devices.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Spectroscopic observation and structure-insensitivity of hydroxyls on gold

Here, the O–H stretching vibration of surface hydroxyls remained at 3691 cm –1 for gold structures ranging in size from clusters to nanoparticles, to non-flat bulk surfaces. In contrast, this vibration was not observed on flat gold surfaces. Therefore, this vibration can serve as an indicator of the roughness of the gold surface and associated functional properties, such as catalytic activity.

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

The pH-dependent orientation of N3 dye on a gold substrate is revealed using heterodyne-detected vibrational sum frequency generation spectroscopy

We report on systematic changes to the adsorption geometry of the dye N3 {[cis-bis(isothiocyanato)bis(2,2' -bipyridyl-4,4' -dicarboxylato ruthenium(II)]} on a gold substrate as the pH of the deposition environment is altered. The protonation states of the four --COOH groups of the N3 dye change according to the modified pH conditions, thus affecting the number of --COOH and --NCS functional groups that participate in the adsorption to gold. Here, we use heterodyne detected vibrational sum frequency generation (HD-VSFG) spectroscopy to obtain surface specific vibrational information on both --COOH and --NCS groups as a function of pH of the deposition conditions. Polarization-dependent HD-VSFG yields sets of complex χ (2) spectra, enabling us to perform a simultaneous fitting procedure to the polarization-dependent real and imaginary components and thus extract detailed structural information of the N3/gold interface. Overall, our results show that N3 preferentially adsorbs to gold either with two --COOH groups and one --NCS group in more acidic conditions or with one --COOH group and two --NCS groups in more basic conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗