Search NASA⌕ Search

SEARCH · Search NASA

Results for “substrate interactions”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Single-Molecule Interfacial Electron Transfer

This project is focused on the use of single-molecule high spatial and temporal resolved techniques to study molecular dynamics in condensed phase and at interfaces, especially, the complex reaction dynamics associated with electron and energy transfer rate processes. The complexity and inhomogeneity of the interfacial ET dynamics often present a major challenge for a molecular level comprehension of the intrinsically complex systems, which calls for both higher spatial and temporal resolutions at ultimate single-molecule and single-particle sensitivities. Combined single-molecule spectroscopy and electrochemical atomic force microscopy approaches are unique for heterogeneous and complex interfacial electron transfer systems because the static and dynamic inhomogeneities can be identified and characterized by studying one molecule at a specific nanoscale surface site at a time. The goal of our project is to integrate and apply these spectroscopic imaging and topographic scanning techniques to measure the energy flow and electron flow between molecules and substrate surfaces as a function of surface site geometry and molecular structure. We have been primarily focusing on studying interfacial electron transfer under ambient condition and electrolyte solution involving both single crystal and colloidal TiO 2 and related substrates. The resulting molecular level understanding of the fundamental interfacial electron transfer processes will be important for developing efficient light harvesting systems and broadly applicable to problems in fundamental chemistry and physics. We have made significant advancement on deciphering the underlying mechanism of the complex and inhomogeneous interfacial electron transfer dynamics in dyesensitized TiO 2 nanoparticle systems that strongly involves with and regulated by molecule-surface interactions. We have studied interfacial electron transfer on TiO 2 nanoparticle surfaces by using ultrafast single-molecule spectroscopy and electrochemical AFM metal tip scanning microscopy, focusing on understanding the interfacial electron transfer dynamics at specific nanoscale electron transfer sites with high-spatially and temporally resolved topographic-and-spectroscopic characterization at individual molecule basis, characterizing single-molecule rate processes, reaction driving force, and molecule-substrate electronic coupling. One of the most significant characteristics of our new approach is that we are able to interrogate the complex interfacial electron transfer dynamics by actively pin-point energetic manipulation of the surface interaction and electronic couplings, beyond the conventional excitation and observation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Bending stiffness and interlayer shear modulus of few-layer graphene

Interlayer shear deformation occurs in the bending of multilayer graphene with unconstrained ends, thus influencing its bending rigidity. Here, we investigate the bending stiffness and interlayer shear modulus of few-layer graphene through examining its self-folding conformation on a flat substrate using atomic force microscopy in conjunction with nonlinear mechanics modeling. The results reveal that the bending stiffness of 2–6 layers graphene follows a square-power relationship with its thickness. The interlayer shear modulus is found to be in the range of 0.36–0.49 GPa. The research findings show that the weak interlayer shear interaction has a substantial stiffening effect for multilayer graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer

We report on the systematic investigation of optoelectronic properties of tin (IV) sulfide (SnS) van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Energy band simulation using commercial CASTEP code indicates that SnS has an indirect bandgap of size 0.982 eV. Furthermore, our simulation shows that elemental Cu can be used as a p-type dopant for the material. Growth of high quality SnS thin films is accomplished by MBE technique using graphene as the buffer layer. We observed significant reduction in the rocking curve FWHM over the existing published values. Crystallite size in the range of 2-3 {mu}m is observed which is also significantly better than the existing results. Measurement of the absorption coefficient, {alpha}, is performed using a Hitachi U-4100 Spectrophotometer system which demonstrate large values of {alpha} of the order of 10{sup 4} cm{sup -1}. Sharp cutoff in the values of {alpha}, as a function of energy, is observed for the films grown using a graphene buffer layer indicating low concentration of localized states in the bandgap. Cu-doping is achieved by co-evaporation technique. It is demonstrated that the hole concentration of the films can be controlled between 10{sup 16} cm{sup -3} and 5 x 10{sup 17}cm{sup -3} by varying the temperature of the Cu K-cell. Hole mobility as high as 81 cm{sup 2}V{sup -1}s{sup -1} is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the surface for both SnS and the graphene buffer layer. Consequently, the interaction between the SnS thin films and the graphene buffer layer is dominated by van der Waals force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Self-Organization of Detonation-Diamond Particles on a Substrate in Carbon Condensation from the Vapor–Gas Phase

Processes in which the island structure is ordered occur in composite carbon films produced by preliminary population of an amorphous substrate with a detonation-diamond system and subsequent condensation of carbon from the vapor–gas phase. These processes, observed already upon filling of the substrate with diamond-growth centers, are manifested in that there appears a structural periodicity of the islands. It was found that the condensation of carbon on the populated substrate is accompanied by the evolution of the island structure of primary growth centers. This consists in that a hexagonal packing of the islands is formed, with the size of these islands increasing by two orders of magnitude. All these structural features of how a composite diamond-carbon film is formed indicate that self-organization processes occur in the system of diamond islands when carbon atoms are condensed and interact with primary diamond crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Influence of viscoelastic property on laser-generated surface acoustic waves in coating-substrate systems

Taking account of the viscoelasticity of materials, the pulsed laser generation of surface acoustic waves in coating-substrate systems has been investigated quantitatively by using the finite element method. The displacement spectra of the surface acoustic waves have been calculated in frequency domain for different coating-substrate systems, in which the viscoelastic properties of the coatings and substrates are considered separately. Meanwhile, the temporal displacement waveforms have been obtained by applying inverse fast Fourier transforms. The numerical results of the normal surface displacements are presented for different configurations: a single plate, a slow coating on a fast substrate, and a fast coating on a slow substrate. The influences of the viscoelastic properties of the coating and the substrate on the attenuation of the surface acoustic waves have been studied. In addition, the influence of the coating thickness on the attenuation of the surface acoustic waves has been also investigated in detail.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Single-molecule imaging at high fluorophore concentrations by local activation of dye

Single-molecule fluorescence microscopy is a powerful approach to observe biomolecular interactions with high spatial and temporal resolution. Detecting fluorescent signals from individual, labeled proteins above high levels of background fluorescence remains challenging, however. For this reason, the concentrations of labeled proteins in in vitro assays are often kept low compared to their in vivo concentrations. Here, we present a new fluorescence imaging technique by which single fluorescent molecules can be observed in real time at high, physiologically relevant concentrations. The technique requires a protein and its macromolecular substrate to be labeled each with a different fluorophore. Then, making use of short-distance energy-transfer mechanisms, the fluorescence from only those proteins bound to their substrate are selectively activated. This approach is demonstrated by labeling a DNA substrate with an intercalating stain, exciting the stain, and using energy transfer from the stain to activate the fluorescence of only those labeled DNA-binding proteins bound to the DNA. Such an experimental design allowed us to observe the sequence-independent interaction of Cy5-labeled interferon-inducible protein 16 (IFI16) with DNA and the sliding via one-dimensional diffusion of Cy5-labeled adenovirus protease (pVIc-AVP) on DNA in the presence of a background of hundreds of nM Cy5 fluorophore.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2013

Magneto-infrared study of electron-hole system in strained semimetallic HgTe quantum wells

Magneto infrared absorption measurements have been performed on HgTe/CdHgTe quantum wells with different thicknesses grown on (013) GaAs substrate. Cyclotron resonance effective masses, inter-Landau-level transition energies and their dependence on magnetic field are measured. The measured intersubband energies are in good agreement with the theoretically calculated values. Strong spin-orbit interaction is responsible for cyclotron resonance splitting in asymmetric quantum wells. We demonstrate that the increase of the quantum well thickness leads to a semimetallic state, allowing for simultaneous observation of holes and electron transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Nacre-like MXene/Polyacrylic Acid Layer-by-Layer Multilayers as Hydrogen Gas Barriers

MXenes are a promising class of 2D nanomaterials and are of particular interest for gas barrier applications due to their high aspect ratio. However, MXene nanosheets naturally bear a negative charge, which prevents assembly with negatively charged polymers, such as polyacrylic acid (PAA), into gas barrier coatings. Here, we present MXene- and PAA-based layer-by-layer (MXene/ PAA LbL) multilayers formed by leveraging hydrogen bonding interactions. When assembled in acidic conditions, MXene/PAA LbL multilayers exhibit conformal, pinhole-free, nacre-like structures. The MXene/PAA LbL multilayers yield high blocking capability and low permeability (0.14 ± 0.01 cc·mm·m −2 ·day −1 · MPa −1 ) for hydrogen gas which is over 9000 times lower than uncoated niobium (Nb) substrate. These nacre-like structures are also electronically conductive (σ DC , up to 370 ± 30 S cm −1 ). Because these multilayers utilize hydrogen bonding, their properties are highly sensitive to the pH of the assembly and its external environment. Specifically, the reversible deconstruction of these multilayers under basic conditions is experimentally verified. This study shows that hydrogen bonding interactions can be leveraged to form MXene LbL multilayers as gas barriers, electronically conductive coatings, and deconstructable thin films via pH control.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Loss tangent imaging: Theory and simulations of repulsive-mode tapping atomic force microscopy

An expression for loss tangent measurement of a surface in amplitude modulation atomic force microscopy is derived using only the cantilever phase and the normalized cantilever amplitude. This provides a direct measurement of substrate compositional information that only requires tuning of the cantilever resonance to provide quantitative information. Furthermore, the loss tangent expression incorporates both the lost and stored energy into one term that represents a fundamental interpretation of the phase signal in amplitude modulation imaging. Numerical solutions of a cantilever tip interacting with a simple Voigt modeled surface agree with the derived loss tangent to within a few percent.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Critical assessment of the issues in the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals

This paper presents a review on the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals. The ablation of a target is the key process that determines the amount of material to be deposited; while, the plasma expansion governs the characteristics of the deposited material. The modeling of ablation process involves a study of two complex phenomena: (i) laser-target interaction and (ii) plasma formation and subsequent shielding of the incoming radiation. The laser-target interaction is a function of pulse duration, which is captured by various models that are described in this paper. The plasma produced as a result of laser-target interaction, further interacts with the incoming radiation, causing the shielding of the target. The shielding process has been modeled by considering the various photon absorption mechanisms operative inside the plasma, namely: inverse Bremsstrahlung, photoionization, and Mie absorption. Concurrently, the plasma expands freely until the ablated material gets deposited on the substrate. Various models describing the plasma expansion process have been presented. The ability of the theoretical models in predicting various ablation and plasma characteristics has also been compared with the relevant experimental data from the literature. The paper concludes with identification of critical issues and recommendations for future modeling endeavors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Conformational flexibility related to enzyme activity: evidence for a dynamic active-site gatekeeper function of Tyr 215 in Aerococcus viridans lactate oxidase

L-Lactate oxidase (LOX) belongs to a large family of flavoenzymes that catalyze oxidation of α-hydroxy acids. How in these enzymes the protein structure controls reactivity presents an important but elusive problem. LOX contains a prominent tyrosine in the substrate binding pocket (Tyr 215 in Aerococcus viridans LOX) hat is partially responsible for securing a flexible loop which sequesters the active site. To characterize the role of Tyr 215 , effects of substitutions of the tyrosine (Y215F, Y215H) were analyzed kinetically, crystallographically and by molecular dynamics simulations. Enzyme variants showed slowed flavin reduction and oxidation by up to 33-fold. Pyruvate release was also decelerated and in Y215F, it was the slowest step overall. A 2.6-Å crystal structure of Y215F in complex with pyruvate shows the hydrogen bond between the phenolic hydroxyl and the keto oxygen in pyruvate is replaced with a potentially stronger hydrophobic interaction between the phenylalanine and the methyl group of pyruvate. Residues 200 through 215 or 216 appear to be disordered in two of the eight monomers in the asymmetric unit suggesting that they function as a lid controlling substrate entry and product exit from the active site. Substitutions of Tyr 215 can thus lead to a kinetic bottleneck in product release.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Experimental and theoretical investigations on the adherent behaviors of high viscosity liquid: The effect of surface topography

Wetting test and classical molecular dynamics (MD) simulations were employed to investigate the adherent behaviors of hydroxyl-terminated polybutadiene (HTPB), a typical high viscosity liquid widely used as a binder in rocket motor propellants, onto the copper (Cu) surfaces with different topographies. Experimental data show that the microstructures protruding above the substrate surfaces can be ‘sensed’ by the HTPB droplet, which has been indicated by the smaller contact angles and higher interface tension for this specific liquid on the rough surfaces (featured with sinusoidal structures or square pillars) than that on the smooth surface. Simulation results reveal the atomic details of spreading process of HTPB chains, with diverse diffusion propensities presented on different surfaces. The HTPB droplet spreads widely on the smooth surface, exhibiting a roughly isotropic trend. On the contrary, the state of this high viscosity liquid on the sinusoidal-structured surface is featured by wide contact diffusion along the run of groove, but relatively restricted extension in the direction perpendicular to the groove, as a result of the barriers exerted by the nearest neighbouring peaks. The existence of nanoscale pillars increases the exterior surface area of Cu substrate, therefore the HTPB chains can penetrate deep to envelop the peripheries of the pillars, forming abundant direct contacts with Cu surface and producing a highest HTPB–Cu interaction energy, which well accords with the experimental results about the adherent strength of HTPB.

Wu, Chunya↗
DOE OSTI2017

Atomistic modeling of nanoparticle generation in short pulse laser ablation of thin metal films in water

Laser ablation in liquids is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms of the laser ablation in liquids and the key processes that control the nanoparticle structure, composition, and size distribution, however, are not yet fully understood. In this paper, we report the results of first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of the liquid environment (parameterized for water), a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions is developed and applied for simulation of laser ablation of a thin silver film deposited on a silica substrate. The simulations, performed at two laser fluences in the regime of phase explosion, predict a rapid deceleration of the ejected ablation plume and the formation of a dense superheated molten layer at the water-plume interface. The water in contact with the hot metal layer is brought to the supercritical state and transforms into an expanding low density metal-water mixing region that serves as a precursor for the formation of a cavitation bubble. Two distinct mechanisms of the nanoparticle formation are predicted in the simulations: (1) the nucleation and growth of small (mostly ≤10nm) nanoparticles in the metal-water mixing region and (2) the formation of larger (tens of nm) nanoparticles through the breakup of the superheated molten metal layer triggered by the emergence of complex morphological features attributed to the Rayleigh-Taylor instability of the interface between at the superheated metal layer and the supercritical water. The first mechanism is facilitated by the rapid cooling of the growing nanoparticles in the supercritical water environment, resulting in solidification of the nanoparticles located in the upper part of the mixing region on the timescale of nanoseconds. The computational prediction of the two mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes is consistent with experimental observations of two distinct nanoparticle populations appearing at different stages of the ablation process.

Chemistry↗
DOE OSTI2015

Utilizing Structures of CYP2D6 and BACE1 Complexes To Reduce Risk of Drug–Drug Interactions with a Novel Series of Centrally Efficacious BACE1 Inhibitors

In recent years, the first generation of β-secretase (BACE1) inhibitors advanced into clinical development for the treatment of Alzheimer’s disease (AD). However, the alignment of drug-like properties and selectivity remains a major challenge. Here in this paper, we describe the discovery of a novel class of potent, low clearance, CNS penetrant BACE1 inhibitors represented by thioamidine 5. Further profiling suggested that a high fraction of the metabolism (>95%) was due to CYP2D6, increasing the potential risk for victim-based drug–drug interactions (DDI) and variable exposure in the clinic due to the polymorphic nature of this enzyme. To guide future design, we solved crystal structures of CYP2D6 complexes with substrate 5 and its corresponding metabolic product pyrazole 6, which provided insight into the binding mode and movements between substrate/inhibitor complexes. Guided by the BACE1 and CYP2D6 crystal structures, we designed and synthesized analogues with reduced risk for DDI, central efficacy, and improved hERG therapeutic margins.

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

The Role of Electronic and Phononic Excitation in the Optical Response of Monolayer WS 2 after Ultrafast Excitation

We study transient changes of the optical response of WS 2 monolayers by femtosecond broadband pump–probe spectroscopy. Time-dependent absorption spectra are analyzed by tracking the line width broadening, bleaching, and energy shift of the main exciton resonance as a function of time delay after the excitation. Two main sources for the pump-induced changes of the optical response are identified. Specifically, we find an interplay between modifications induced by many-body interactions from photoexcited carriers and by the subsequent transfer of the excitation to the phonon system followed by cooling of the material through the heat transfer to the substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Arrays of Functionalized Graphene Chemiresistors for Selective Sensing of Volatile Organic Compounds

Real-time detection of volatile organic compounds (VOCs) is being spotlighted for its importance in environmental monitoring and disease detection from the breath. However, the selective detection of VOCs in a small form factor gas sensor remains a challenge. Here, we present the VOC sensing performance of phthalocyanine (Pc)-functionalized graphene chemiresistors supported by the theoretical prediction of Pc's VOC interactions. Pc functionalization, with and without metalation, was studied parametrically with varying deposition parameters to explore impacts on the gas sensing response toward VOCs as a function of substrate coverage. Sensing data were collected to establish fingerprint signatures for a variety of VOCs. In conclusion, improvement in selectivity is shown by successfully differentiating the VOCs from each other when combined into a gas sensor array tested with acetone, ethanol, formaldehyde, and toluene.

2D materials↗
DOE OSTI2014

Dynamic magnetic interaction in La{sub 2/3}Sr{sub 1/3}MnO{sub 3}/BiFeO{sub 3} heterostructure

The La{sub 2/3}Sr{sub 1/3}MnO{sub 3}/BiFeO{sub 3} (LSMO/BFO) heterostructure is studied as probed by temperature-dependent ferromagnetic resonance (FMR). Consistent with the structure analysis, two FMR modes are found for both a single LSMO layer and a LSMO/BFO bilayer grown on DyScO{sub 3}, corresponding to the strained and relaxed phases of LSMO. The FMR linewidth intensity vs temperature curve shows two maxima at 200 K and 140 K in the strained phase of LSMO/BFO bilayer, which is related to the magnetic excitations in BFO. Our results demonstrate a dynamic spin interaction between LSMO and BFO, suggesting an effective route to manipulate the magnetic properties of oxide heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Differences in substrate use linked to divergent carbon flow during litter decomposition

ABSTRACT Discovering widespread microbial processes that create variation in soil carbon (C) cycling within ecosystems may improve soil C modeling. Toward this end, we screened 206 soil communities decomposing plant litter in a common garden microcosm environment and examined features linked to divergent patterns of C flow. C flow was measured as carbon dioxide (CO2) and dissolved organic carbon (DOC) from 44-days of litter decomposition. Two large groups of microbial communities representing ‘high’ and ‘low’ DOC phenotypes from original soil and 44-day microcosm samples were down-selected for fungal and bacterial profiling. Metatranscriptomes were also sequenced from a smaller subset of communities in each group. The two groups exhibited differences in average rate of CO2 production, demonstrating that the divergent patterns of C flow arose from innate functional constraints on C metabolism, not a time-dependent artefact. To infer functional constraints, we identified features – traits at the organism, pathway or gene level – linked to the high and low DOC phenotypes using RNA-Seq approaches and machine learning approaches. Substrate use differed across the high and low DOC phenotypes. Additional features suggested that divergent patterns of C flow may be driven in part by differences in organism interactions that affect DOC abundance directly or indirectly by controlling community structure.

59 BASIC BIOLOGICAL SCIENCES↗