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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 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Sequential oligodiacetylene formation for progressive luminescent color conversion via co-micellar strategy

Design and construction of organic systems that can enhance internal quantum efficiency to fully make use of radiative decay during the excitation process is highly desirable for the development of new-generation emissive materials. In practice, to employ those materials with an effective tuning of triplet-state involved multicolor luminescence is quite a considerable issue. This work takes advantage of a diphenyl-diacetylene- based co-micellar nano-ensemble that can undergo a sequential photocrosslinking to form a corresponding trimeric oligodiacetylene and a dimeric oligodiacetylene. Their emissive bands individually cover the visible-light spectral region and such a controllable dual band characteristic can result in a progressive luminescent color conversion from light-green to white and finally to purplish blue. In addition, the oligodiacetylenes possess an ultrafast intersystem crossing characteristic with a small S 1 –T 1 energy gap. The leading thermally activated delayed fluorescence and phosphorescence can be employed to achieve long-life broadband emissions without chromatic aberration in this single organic system, towards the creation of emitting materials with high quantum efficiency. The synthesis of the sub-components is simple and the co-micellar strategy is extremely straightforward. Such a material design and approach may find potential uses in the microfabrication of novel organic light-emitting diodes and sophisticated controllable optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Direct-write liquid phase transformations with a scanning transmission electron microscope

The highly energetic electron beam from a scanning transmission electron microscope (STEM) can induce local changes in the state of matter, ranging from local knock-out and atomic movement, to amorphization/crystallization, and chemical/electrochemical reactions occuring at localized liquid-solid and gas-solid interfaces. To date, fundamental studies of e-beam induced phenomena and practical applications have been limited by conventional e-beam rastering modes that allow only for uniform e-beam exposures. Here we develop an automated liquid phase nanolithography method that is capable of directly writing nanometer scaled features within silicon nitride encapsulated liquid cells. An external beam control system, connected to the scan coils of an aberration-corrected STEM, is used to precisely control the position, dwell time, and scan velocity of a sub-nanometer STEM probe. Site-specific locations in a sealed liquid cell containing an aqueous solution of H 2 PdCl 4 are irradiated to controllably deposit palladium onto silicon nitride membranes. We determine the threshold electron dose required for the radiolytic deposition of metallic palladium, explore the influence of electron dose on the feature size and morphology of nanolithographically patterned nanostructures, and propose a feedback-controlled monitoring method for active control of the nanofabricated structures through STEM detector signal monitoring. As a result, this approach enables both fundamental studies of electron beam induced interactions with matter, as well as opens a pathway to fabricate nanostructures with tailored architectures and chemistries via shape-controlled nanolithographic patterning from liquid phase precursors.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Revealing the semiconductor–catalyst interface in buried platinum black silicon photocathodes

Nanoporous 'black' silicon semiconductors interfaced with buried platinum nanoparticle catalysts have exhibited stable activity for photoelectrochemical hydrogen evolution even after months of exposure to ambient conditions. The mechanism behind this stability has not been explained in detail, but is thought to involve a Pt/Si interface free from SiOx layer that would adversely affect interfacial charge transfer kinetics. In this paper, we resolve the chemical composition and structure of buried Pt/Si interfaces in black silicon photocathodes from a micron to sub-nanometer level using aberration corrected analytical scanning transmission electron microscopy. Through a controlled electrodeposition of copper on samples aged for one month in ambient conditions, we demonstrate that the main active catalytic sites are the buried Pt nanoparticles located below the 400-800 nm thick nanoporous SiOx layer. Though hydrogen production performance degrades over 100 h under photoelectrochemical operating conditions, this burying strategy preserves an atomically clean catalyst/Si interface free of oxide or other phases under air exposure and provides an example of a potential method for stabilizing silicon photoelectrodes from oxidative degradation in photoelectrochemical applications.

14 SOLAR ENERGY↗
DOE OSTI2018

Rational design and observation of the tight interface between graphene and ligand protected nanocrystals

Heterostructures constructed of graphene and colloidal nanocrystals provide a unique way to exploit the coupled physical properties of the two functional building blocks. Studying the interface structure between the two constituent materials is important to understand the formation mechanism and the resulting physical and chemical properties. Along with ab initio calculations, we elucidate that the bending rigidity and the strong van der Waals interaction of graphene to the metal surface guide the formation of a tight and conformal interface. Using theoretical foundations, we construct colloidal nanocrystal-graphene heterostructures with controlled interfacial structures and directly investigate the cross-sectional structures of them at high resolution by using aberration-corrected transmission electron microscopy. Furthermore the experimental method and observations we present here will link the empirical methods for the formation of nanocrystal-graphene heterostructures to the mechanistic understanding of their properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Atomistic manipulation of reversible oxidation and reduction in Ag with an electron beam

Employing electrons for direct control of a nanoscale reaction is highly desirable since it enables fabrication of nanostructures with different properties at atomic resolution and with flexibility of dimensions and location. Here, applying in situ transmission electron microscopy, we show the reversible oxidation and reduction kinetics in Ag, well controlled by changing the dose rate of the electron beam. Aberration-corrected high-resolution transmission electron microscopy observation reveals that O atoms are preferably inserted and extracted along the {111} close-packed planes of Ag, leading to the nucleation and decomposition of nanoscale Ag 2 O islands on the Ag substrate. By controlling the electron beam size and dose rate, we demonstrated the fabrication of an array of 3 nm Ag 2 O nanodots in an Ag matrix. Our results open a new pathway to manipulate an atomistic reaction with an electron beam towards the precise fabrication of nanostructures for device applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Random anion distribution in MS x Se 2- x (M = Mo, W) crystals and nanosheets

The group VIb dichalcogenides (MX 2 , M = Mo, W; X= S, Se) have a layered molybdenite structure in which M atoms are coordinated by a trigonal prism of X atoms. Ternary solid solutions of MS x Se 2– x were synthesized, microcrystals were grown by chemical vapor transport, and their morphologies and structures were characterized by using synchrotron X-ray diffraction, Rietveld refinement, DIFFaX simulation of structural disorder, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Double aberration corrected scanning transmission electron microscopy was used to determine the anion distributions in single-layer nanosheets exfoliated from the microcrystals. These experiments indicate that the size difference between S and Se atoms does not result in phase separation, consistent with earlier studies of MX 2 monolayer sheets grown by chemical vapor deposition. However, stacking faults occur in microcrystals along the layering axis, particularly in sulfur-rich compositions of MS x Se 2– x solid solutions.

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

The sol–gel autocombustion as a route towards highly CO 2 -selective, active and long-term stable Cu/ZrO 2 methanol steam reforming catalysts

The adaption of the sol-gel autocombustion method to the Cu/ZrO 2 system opens new pathways for the specific optimisation of the activity, long-term stability and CO 2 selectivity of methanol steam reforming (MSR) catalysts. Calcination of the same post-combustion precursor at 400 °C, 600 °C or 800 °C allows accessing Cu/ZrO 2 interfaces of metallic Cu with either amorphous, tetragonal or monoclinic ZrO 2 , influencing the CO 2 selectivity and the MSR activity distinctly different. While the CO 2 selectivity is less affected, the impact of the post-combustion calcination temperature on the Cu and ZrO 2 catalyst morphology is more pronounced. A porous and largely amorphous ZrO 2 structure in the sample, characteristic for sol-gel autocombustion processes, is obtained at 400 °C. This directly translates into superior activity and long-term stability in MSR compared to Cu/tetragonal ZrO 2 and Cu/monoclinic ZrO 2 obtained by calcination at 600 °C and 800 °C. The morphology of the latter Cu/ZrO 2 catalysts consists of much larger, agglomerated and non-porous crystalline particles. Based on aberration-corrected electron microscopy, we attribute the beneficial catalytic properties of the Cu/amorphous ZrO 2 material partially to the enhanced sintering resistance of copper particles provided by the porous support morphology.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Single atoms and small clusters of atoms may accompany Au and Pd dendrimer-encapsulated nanoparticles

Here we report the presence of small clusters of atoms (<1 nm) (SCs) and single atoms (SAs) in solutions containing 1–2 nm dendrimer-encapsulated nanoparticles (DENs). Au and Pd DENs were imaged using aberration-corrected scanning transmission electron microscopy (ac-STEM), and energy dispersive spectroscopy (EDS) was used to identify and quantify the SAs/SCs. Two main findings have emerged from this work. First, the presence or absence of SAs/SCs depends on both the terminal functional group of the dendrimer (–NH 2 or –OH) and the elemental composition of the DENs (Au or Pd). Second, dialysis can be used to remove the majority of SAs/SCs in cases where a high density of SAs/SCs are present. The foregoing conclusions provide insights into the mechanisms for Au and Pd DEN synthesis and stability. Ultimately, these results demonstrate the need for careful characterization of systems containing nanoparticles to ensure that SAs/SCs, which may be below the detection limit of most analytical methods, are taken into consideration (especially for catalysis experiments).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Building workflows for an interactive human-in-the-loop automated experiment (hAE) in STEM-EELS

Exploring the structural, chemical, and physical properties of matter on the nano- and atomic scales has become possible with the recent advances in aberration-corrected electron energy-loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM). However, the current paradigm of STEM-EELS relies on the classical rectangular grid sampling, in which all surface regions are assumed to be of equal a priori interest. However, this is typically not the case for real-world scenarios, where phenomena of interest are concentrated in a small number of spatial locations, such as interfaces, structural and topological defects, and multi-phase inclusions. One of the foundational problems is the discovery of nanometer- or atomic-scale structures having specific signatures in EELS spectra. Herein, we systematically explore the hyperparameters controlling deep kernel learning (DKL) discovery workflows for STEM-EELS and identify the role of the local structural descriptors and acquisition functions in experiment progression. In agreement with the actual experiment, we observe that for certain parameter combinations the experiment path can be trapped in the local minima. We demonstrate the approaches for monitoring the automated experiment in the real and feature space of the system and knowledge acquisition of the DKL model. Based on these, we construct intervention strategies defining the human-in-the-loop automated experiment (hAE). This approach can be further extended to other techniques including 4D STEM and other forms of spectroscopic imaging. The hAE library is available on Github at https://github.com/utkarshp1161/hAE/tree/main/hAE.

Pratiush, Utkarsh [Univ. of Tennessee, Knoxville, ↗
DOE OSTI2013

Experimental energy resolution of a paracentric hemispherical deflector analyzer for different entry positions and bias

A specially designed hemispherical deflector analyzer (HDA) with 5-element input lens having a movable entry position R{sub 0} suitable for electron energy analysis in atomic collisions was constructed and tested. The energy resolution of the HDA was experimentally determined for three different entry positions R{sub 0}= 84, 100, 112 mm as a function of the nominal entry potential V(R{sub 0}) under pre-retardation conditions. The resolution for the (conventional) entry at the mean radius R{sub 0}= 100 mm was found to be a factor of 1.6-2 times worse than the resolution for the two (paracentric) positions R{sub 0}= 84 and 112 mm at particular values of V(R{sub 0}). These results provide the first experimental verification and a proof of principle of the utility of such a paracentric HDA, while demonstrating its advantages over the conventional HDA: greater dispersion with reduced angular aberrations resulting in better energy resolution without the use of any additional fringing field correction electrodes. Supporting simulations of the entire lens plus HDA spectrometer are also provided and mostly found to be within 20%-30% of experimental values. The paracentric HDA is expected to provide a lower cost and/or more compact alternative to the conventional HDA particularly useful in modern applications utilizing a position sensitive detector.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Antisite Defects In La0.7Sr0.3MnO3 and La0.7Sr0.3FeO3

Complex oxide thin films and superlattices with the perovskite ABO3 structure have been found to possess novel functional properties. Here, we present our discovery of antisite defects, La ions in Fe(Mn) sites (denoted as LaB), in a La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 superlattice. The antisite defect was directly characterized by atomic resolution Z-contrast imaging and the composition and electronic structure was analyzed by electron energy loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Density functional theory was used to calculate the formation energy, showing that the formation of the detected antisite defects is a consequence of the slightly reducing conditions during sample growth.

Gu, Meng↗
DOE OSTI2013

Atomic-scale chemical quantification of oxide interfaces using energy-dispersive X-ray spectroscopy

Atomic-scale quantification of chemical composition across oxide interfaces is important for understanding physical properties of epitaxial oxide nanostructures. Energy-dispersive X-ray spectroscopy (EDS) in an aberration-corrected scanning transmission electron microscope was used to quantify chemical composition across the interface of ferromagnetic La{sub 0.7}Sr{sub 0.3}MnO{sub 3} and antiferromagnetic BiFeO{sub 3} quantum structure. This research demonstrates that chemical composition at atomic columns can be quantified by Gaussian peak-fitting of EDS compositional profiles across the interface. Cation diffusion was observed at both A- and B-sublattice sites; and asymmetric chemical profiles exist across the interface, consistent with the previous studies.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Low-energy inverse photoemission spectroscopy using a high-resolution grating spectrometer in the near ultraviolet range

An inverse photoemission spectroscopy (IPES) apparatus using a Czerny-Turner grating spectrometer is demonstrated. Previous IPES instruments based on grating spectrometers used a concave grating and operated in the vacuum ultraviolet range. The reflectance of such gratings is lower than 20% and the aberration cannot be finely corrected leading to an energy resolution of up to 0.1 eV. In the present study, employing the low energy IPES regime [H. Yoshida, Chem. Phys. Lett. 539–540, 180 (2012)], incident electrons with a kinetic energy below 5 eV are used, while photon emission in the range of between 250 and 370 nm is analyzed with a 10-cm Czerny-Turner grating spectrometer. The signal intensity is at least 30 times higher than the previous apparatus. The resolution of photon detection is set at 0.07 eV though the ultimate resolution is one order of magnitude higher. The experiment is performed both by sweeping the electron energy (isochromat mode) and by simultaneously analyzing the photon of whole wavelength range (tunable photon energy mode)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Research Updates: Epitaxial strain relaxation and associated interfacial reconstructions: The driving force for creating new structures with integrated functionality

Here, we report detailed strain mapping analysis at heterointerfaces of a new multiferroic complex oxide Bi{sub 3}Fe{sub 2}Mn{sub 2}O{sub x}(BFMO322) supercell and related layered structures. The state-of-the-art aberration corrected scanning transmission electron microscopy (Cs-corrected STEM) and the modified geometric phase analysis (GPA) have been used to characterize the self-assembled transitional layers, misfit defects, and, in particular, the biaxial lattice strain distributions. We found that not only a sufficient lattice misfit is required through substrate selection and to be preserved in initial coherent epilayer growth, but also an appropriate interfacial reconstruction is crucial for triggering the growth of the new BFMO322 supercell structure. The observation of new transitional interfacial phases behaving like coherent film layers within the critical thickness challenges the conventional understanding in existing epitaxial growth model.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The pristine atomic structure of MoS{sub 2} monolayer protected from electron radiation damage by graphene

Materials can, in principle, be imaged at the level of individual atoms with aberration-corrected transmission electron microscopy. However, such resolution can be attained only with very high electron doses. Consequently, radiation damage is often the limiting factor when characterizing sensitive materials. Here, we demonstrate a simple and an effective method to increase the electron radiation tolerance of materials by using graphene as protective coating. This leads to an improvement of three orders of magnitude in the radiation tolerance of monolayer MoS{sub 2}. Further on, we construct samples in different heterostructure configurations to separate the contributions of different radiation damage mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Compositional analysis of GaAs/AlGaAs heterostructures using quantitative scanning transmission electron microscopy

We demonstrate a method for compositional mapping of Al{sub x}Ga{sub 1–x}As heterostructures with high accuracy and unit cell spatial resolution using quantitative high angle annular dark field scanning transmission electron microscopy. The method is low dose relative to spectroscopic methods and insensitive to the effective source size and higher order lens aberrations. We apply the method to study the spatial variation in Al concentration in cross-sectioned GaAs/AlGaAs core-shell nanowires and quantify the concentration in the Al-rich radial band and the AlGaAs shell segments.

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

The effect of strain induced by Ag underlayer on saturation magnetization of partially ordered Fe{sub 16}N{sub 2} thin films

Partially ordered Fe-N thin films were grown by a facing target sputtering process on the surface of a (001) Ag underlayer on MgO substrates. It was confirmed by x-ray diffraction that the Ag layer enlarged the in-plane lattice of the Fe-N thin films. Domains of the ordered α″-Fe{sub 16}N{sub 2} phase within an epitaxial (001) α′-Fe{sub x}N phase were identified by electron diffraction and high-resolution aberration-corrected scanning transmission electron microscopy (STEM) methods. STEM dark-field and bright-field images showed the fully ordered structure of the α″-Fe{sub 16}N{sub 2} at the atomic column level. High saturation magnetization(Ms) of 1890 emu/cc was obtained for α″-Fe{sub 16}N{sub 2} on the Ag underlayer, while only 1500 emu/cc was measured for Fe-N on the Fe underlayer. The results are likely due to a tensile strain induced in the α″-Fe{sub 16}N{sub 2} phase by the Ag structure at the interface.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Multiple species beam production on laser ion source for electron beam ion source in Brookhaven National Laboratory

Extracted ion beams from the test laser ion source (LIS) were transported through a test beam transport line which is almost identical to the actual primary beam transport in the current electron beam ion source apparatus. The tested species were C, Al, Si, Cr, Fe, Cu, Ag, Ta, and Au. The all measured beam currents fulfilled the requirements. However, in the case of light mass ions, the recorded emittance shapes have larger aberrations and the RMS values are higher than 0.06 π mm mrad, which is the design goal. Since we have margin to enhance the beam current, if we then allow some beam losses at the injection point, the number of the single charged ions within the acceptance can be supplied. For heaver ions like Ag, Ta, and Au, the LIS showed very good performance.

43 PARTICLE ACCELERATORS↗