While recent efforts have shown how local structure plays an essential role in the dynamic heterogeneity of homogeneous glass-forming materials, systems containing interfaces such as thin films or composite materials remain poorly understood. It is known that interfaces perturb the molecular packing nearby, however, numerous studies show the dynamics are modified over a much larger range. Here, we examine the dynamics in polymer nanocomposites (PNCs) using a combination of simulations and experiments and quantitatively separate the role of polymer packing from other effects on the dynamics, as a function of distance from the nanoparticle surfaces. After showing good qualitative agreement between the simulations and experiments in glassy structure and creep compliance, we use a machine-learned structure indicator, softness, to decompose polymer dynamics in our simulated PNCs into structure-dependent and structure-independent processes. With this decomposition, the free energy barrier for polymer rearrangement can be described as a combination of packing-dependent and packing-independent barriers. We find both barriers are higher near nanoparticles and decrease with applied stress, quantitatively demonstrating that the slow interfacial dynamics is not solely due to polymer packing differences, but also the change of structure-dynamics relationships. Finally, we present how this decomposition can be used to accurately predict strain-time creep curves for PNCs from their static configuration, providing additional insights into the effects of polymer-nanoparticle interfaces on creep suppression in PNCs.

Journal Article Understanding Nucleation of Mesophase Pitch Tactoids using 4D-STEM Get access Robert Colby, Robert Colby Research, ExxonMobil Technology and Engineering Company, Annandale, NJ Search for other works by this author on: Oxford Academic Google Scholar Kazem Edmond, Kazem Edmond Research, ExxonMobil Technology and Engineering Company, Annandale, NJ Search for other works by this author on: Oxford Academic Google Scholar Daniella Mendez, Daniella Mendez Research, ExxonMobil Technology and Engineering Company, Annandale, NJ Search for other works by this author on: Oxford Academic Google Scholar Stuart Smith Stuart Smith Research, ExxonMobil Technology and Engineering Company, Annandale, NJ Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 274–275, https://doi.org/10.1093/micmic/ozad067.125 Published: 22 July 2023

Electron energy loss spectroscopy (EELS) is a proven tool for probing materials chemistry at high spatial resolution. Core-loss EELS fine structure should allow measurement of local polymer chemistry. For organic materials, sensitivity to radiolysis is expected to limit the resolution achievable with EELS: but core-loss EELS has proven difficult at any resolution, yielding inconsistent spectra that compare unfavorably with theoretically analogous x-ray absorption spectra. Many of the previously identified shortcomings should not be limiting factors on modern equipment. This study establishes that EELS can generate identifiable carbon K-edge spectra for a range of common polymer types and chemistry, and demonstrates fine structure features matching prior x-ray absorption spectra. EELS fine structure features broaden intuitively with the instrument's energy resolution, and beam-induced features are readily differentiated by collecting spectra at a series of doses. The results are demonstrated with spectrum images of a model polymer blend, and used to estimate practical pixel sizes that can be used for mapping core-loss EELS as a function of electron dose.

A key step towards realizing the promise of macroscopic carbon nanotube (CNT) articles in high-performance structural applications is polymer infiltration into the porous CNT structure, which improves stress transfer and promotes long-term material integrity. However, infiltration is often found to be sub-optimal -a significant impediment to the scaling and adoption of CNT-article composites that has thus far received little systematic attention. In this study, a unique combination of inverse gas chromatography (IGC) and a statistical thermodynamic model is used to accurately quantify the energetic driving force for infiltration into CNT articles for the first time. This is measured to have a very low value. Using gas chromatography-mass spectroscopy, IGC analysis and electron microscopy, this low energy is found to result from near-complete surface coverage by non-graphitic pyrolysis byproducts. The surface energy is improved by plasma treatment as confirmed by IGC analysis. The effects of the surface treatment and modifications to the porous structure on the infiltration rate of model liquids are studied by optical imaging. Accurate surface energy measurements and image analysis are coupled to shed light on the critical parameters in these CNT articles that dictate the infiltration physics, which can be tuned to maximize the rate and extent of space filling.

Nominally anhydrous minerals (NAM) such as olivine are important reservoirs for water in the Earth's upper mantle, although the association of water with crystallographic defects is poorly understood. Here, hydrated regions at the atomic scale in fayalite (Fe2SiO4) are directly resolved using atom probe tomography. Site-specific analyses reveal regions that are either compositionally homogenous or contain nanometer-long channels that are consistent with hydrated defects. Na as a minor element is also enriched within the interlayers, indicating the formation environment is associated with Na geochemically. The ability to directly visualize and characterize hydrated regions at the nanometer scale can provide critical insight into the material properties of NAM and their impact on planetary-scale processes, such as the influence of water-bearing minerals on mantle rheology and geodynamics.

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Metal organic framework (MOF) copper benzene-1,3,5-tricarboxylate (Cu3(btc)2 or HKUST-1) is one of the most broadly studied MOFs, considered for a diverse range of applications including gas storage and separation. Like many MOFs, HKUST-1 has a known sensitivity to water vapor, with performance in most applications degrading under ambient humidity. Prior work on the effect of moisture has focused on performance metrics or changes at the scale of the copper sites and the lattice. While it is generally appreciated that moisture leads to framework collapse, the consequences for the MOF microstructure remain largely unconsidered.

Chemical analysis at the nanoscale is critical to advance our understanding of materials and systems from medicine and biology to material science and computing. Macroscale-observed phenomena in these systems are in the large part driven by processes that take place at the nanoscale and are highly heterogeneous. Therefore, there is a clear need to develop a new technology that enables correlative imaging of material functionalities with nanoscale spatial and chemical resolutions that will enable us to untangle the structure-function relationship of functional materials. Therefore, here, we report on the analytical figures of merit of the newly developed correlative chemical imaging technique of helium ion microscopy coupled with secondary ion mass spectrometry (HIM-SIMS) that enables multimodal topographical/chemical imaging of organic and inorganic materials at the nanoscale. In HIM-SIMS, a focused ion beam acts as a sputtering and ionization source for chemical analysis along with simultaneous high-resolution surface imaging, providing an unprecedented level of spatial resolution for gathering chemical information on organic and inorganic materials. In this work, we demonstrate HIM-SIMS as a platform for a next-generation tool for an in situ material design and analysis capable of down to 8 nm spatial resolution chemical imaging, layered metal structure imaging in depth profiling, single graphene layer detection, and spectral analysis of metals, metal oxides, and polymers.

Using a known organosilane building block (1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane), materials with unexpectedly high surface area (>1200 m(2)/g) and porosity with narrow pore-size distributions were synthesized in the total absence of pore-templating agents. The properties of these mesoporous organosilicas (MOS), closely resembling those of surfactant-templated mesoporous silicas, are found to vary over a wide range depending on synthesis gel pH and gelation temperature and time. Considerable microporosity is observed at short 90 degrees C gelation times at pH 12.5, which diminishes to a very minor component of the porosity at times > 23 h. Under the gelation conditions used here (90 degrees C, pH 12.5), pore volume and pore diameter rise sharply with time, ultimately leveling off at 1.2 cm(3)/g and 70 angstrom at times > 70 h. This building block is shown to be stable in acidic gels but undergoes gradual ring opening at gelation temperatures and times in excess of "standard" conditions employed for alkaline gels.The synthesis and properties of these materials, called EMA-2, are compared with materials made from the same precursor in a templated synthesis and from other organosilane precursors in nontemplated syntheses. Comparative TEM and SAXS analysis of products from identical preparations using tetraethylorthosilicate and the 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane precursors indicates that the latter forms a highly interconnected network gel lacking discrete particle aggregates.

Physical synthesis employing magnetron sputtering and gas aggregation in a modified commercial source has been coupled with size-selection and ion soft landing to prepare bare nanoparticles on surfaces with controlled coverage, size, composition, and morphology. Employing atomic force microscopy (AFM) and scanning electron microscopy (SEM), it is demonstrated that the size and coverage of nanoparticles on flat and stepped surfaces may be controlled using a quadrupole mass filter and the length of deposition, respectively. AFM shows that nanoparticles bind randomly to flat surfaces when soft landed at relatively low coverage (4 × 10(4) ions μm(-2)). On stepped surfaces at intermediate coverage (4 × 10(5) ions μm(-2)) nanoparticles bind along step edges forming extended linear chains. At the highest coverage (2 × 10(6) ions μm(-2)) nanoparticles form a continuous film on flat surfaces. On one surface with sizable defects, the presence of localized imperfections results in agglomeration of nanoparticles onto these features and formation of neighboring zones devoid of particles. Employing high resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) the customized magnetron sputtering/gas aggregation source is demonstrated to produce bare single metal particles with controlled morphology as well as bimetallic alloy nanoparticles with defined core-shell structures of that are of interest to catalysis.

Invented in 1967 1 , atom probe tomography (APT) was designed to visualize chemical heterogeneity present in a small volume of a specimen with a sub-nanometer spatial resolution 2-5 .Numerical simulation plays an important role in studying the sample geometry evolution and corresponding electrical field changes during field evaporation of needle shaped specimens in APT.The level set method can provide sub-grid accuracy on tracking the solid-vacuum interface and much higher computational efficiency than other models that require an atomic level grid.The modeling details are introduced in our previous work 6 .The proposed approach is applied to predict the tip evaporation of the specimen, which is (100) chromium (Cr) single crystalline thin film grown on a (100) single crystalline magnesium oxide (MgO) substrate.Through comparison with experimental observation, the level set method shows the capability of providing atomic level accuracy while using a relatively coarse simulation grid that is about 5 times the atomic cell.This leads to a huge advantage in computational efficiency.The scanning transmission electron microscopy (STEM) image of the tip geometry before and after evaporation is shown in Fig. 1(a), and the comparison between STEM image and simulation results is shown in Fig. 1(b).Additionally level set simulations were used to investigate the dynamic tip shape evolution of oxide multilayer materials.Due to their interesting interfacial properties oxide multilayered structures have acquired a lot of attention in several scientific and industrial fields, such as magnetic storage media and microelectronics.The dynamic tip shape evolution of a composite material consisting of alternating layers of CeO 2 and ZrO 2 in two orientations (topdown and side-ways) was studied by the numerical simulations.In order to study how the sample geometry and electrical field changes affect the species density distribution on the detector, a trajectory simulation model by Runge-Kutta method is integrated into the level set solid-vacuum interface tracking model.Because the grid size of the level set method is around 5 times the atomic volume, a local grid refinement method is used for initialization of the starting positions of evaporated atoms for the trajectory calculation.Fig. 2 shows the simulation results for the CeO 2 and ZrO 2 horizontal multilayer sample evaporation.Fig. 2(a) and (b) show the tip geometry and electric field evolution during evaporation after 0 and 5000 atoms are evaporated respectively.Fig. 2(c) shows the end position of atoms flying over the counter electrode, and Fig. 2(d) shows the atom density distribution on the selected region as shown on Fig. 2(c).Because the evaporation strength of ZrO 2 is about double that of CeO 2 , CeO 2 is evaporated much faster and easier than ZrO 2 , which causes concaves for CeO 2 on the tip surface.The concaves on the tip surface lead to higher density of electrical field lines, and make the trajectories of Ce closer to each other.This is consistent with the narrower band of Ce and wider band of Zr in the density distribution as shown in Fig. 2(d), which is fairly well matched to the experiment observations and measurements.Similarly, Fig. 3 shows the simulation results for the CeO 2 and ZrO 2 vertical multilayer sample evaporation.

Polarization-induced weak ferromagnetism has been predicted a few years back in compounds MTiO3 (M=Fe, Mn, Ni) (Fennie, 2008). We set out to stabilize this metastable, distorted perovskite structure by growing NiTiO3 epitaxially on sapphire Al2O3 (001) substrate, and to control the polar and magnetic properties via strain. Epitaxial Ni1−xTi1−yO3 films of different Ni/Ti ratios and thicknesses were deposited on Al2O3 substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy shows that substrate-induced strain has a marked effect on the structure and crystalline quality of the films. Physical property measurements reveal a dependence of the Néel transition and lattice polarization on strain, and highlight our ability to control the ferroic properties in NiTiO3 thin films by film stoichiometry and thickness.

In atom probe tomography (APT), accurate reconstruction of the spatial positions of field evaporated ions from measured detector patterns depends upon a correct understanding of the dynamic tip shape evolution and evaporation laws of component atoms. Artifacts in APT reconstructions of heterogeneous materials can be attributed to the assumption of homogeneous evaporation of all the elements in the material in addition to the assumption of a steady state hemispherical dynamic tip shape evolution. A level set method-based specimen shape evolution model is developed in this study to simulate the evaporation of synthetic layered-structured APT tips. The simulation results of the shape evolution by the level set model qualitatively agree with the finite element method and the literature data using the finite difference method. The asymmetric evolving shape predicted by the level set model demonstrates the complex evaporation behavior of heterogeneous tip and the interface curvature can potentially lead to the artifacts in the APT reconstruction of such materials. Compared with other APT simulation methods, the new method provides smoother interface representation with the aid of the intrinsic sub-grid accuracy. Two evaporation models (linear and exponential evaporation laws) are implemented in the level set simulations and the effect of evaporation laws on the tip shape evolution is also presented.

Rare earth iron perovskites RFeO3, where R is a rare earth cation, exhibit an array of magnetic, catalytic, optical, and electrochemical properties. Here we study EuFeO3 films synthesized by molecular beam epitaxy to improve our understanding of the optical properties of ferrites. A combination of X-ray diffraction, X-ray reflectivity, Rutherford backscattering spectroscopy, and scanning transmission electron microscopy was used to characterize the film structure and cation composition. X-ray absorption spectroscopy confirms the nominal 3+ valence states of Eu and Fe. The optical properties of EuFeO3 were investigated using variable-angle spectroscopic ellipsometry between the photon energies of 1.25 and 5 eV. We find that EuFeO3 is a semiconductor with an onset of optical absorption near 2.5 eV. The absorption spectrum of EuFeO3 is blue-shifted with respect to LaFeO3 films, a result that is attributed to the structural differences between the two materials.

The mixed electronic and ionic conductivity of perovskite oxides has enabled their use in diverse applications such as automotive exhaust catalysts, solid oxide fuel cell cathodes, and visible light photocatalysts. The redox chemistry at the surface of perovskite oxides is largely dependent on the oxidation state of the metal cations as well as the oxide surface stoichiometry. In this study, LaFeO3 (LFO) thin films grown on yttria-stabilized zirconia (YSZ) was characterized using both bulk and surface sensitive techniques. A combination of in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) demonstrated that the film is primarily textured in the [1 0 0] direction and is stoichiometric. High-resolution transmission electron microscopy measurements show regions that are dominated by [1 0 0] oriented LFO grains that are oriented with respect to the substrates lattice. However, selected regions of the film show multiple domains of grains that are not [1 0 0] oriented. The film was annealed in an ultra-high vacuum chamber to simulate reducing conditions and studied by angle-resolved X-ray photoelectron spectroscopy (XPS). Iron was found to exist as Fe(0), Fe(II), and Fe(III) depending on the annealing conditions and the depth within the film. A decrease in the concentration of surface oxygen species was correlated with iron reduction. These results should help guide and enhance the design of LFO materials for catalytic applications. (C) 2015 Elsevier B.V. All rights reserved.

Magnetron sputtering of two independent Pt and Ru targets coupled with inert gas aggregation in a modified commercial source has been combined with soft landing of mass-selected ions to prepare bare 4.5 nm diameter PtRu nanoparticles on glassy carbon electrodes with controlled size and morphology for electrochemical reduction of oxygen in solution. Employing atomic force microscopy (AFM) it is shown that the nanoparticles bind randomly to the glassy carbon electrode at a relatively low coverage of 7 × 10(4) ions μm(-2) and that their average height is centered at 4.5 nm. Scanning transmission electron microscopy images obtained in the high-angle annular dark field mode (HAADF-STEM) further confirm that the soft-landed PtRu nanoparticles are uniform in size. Wide-area scans of the electrodes using X-ray photoelectron spectroscopy (XPS) reveal the presence of both Pt and Ru in atomic concentrations of ∼9% and ∼33%, respectively. Deconvolution of the high energy resolution XPS spectra in the Pt 4f and Ru 3d regions indicates the presence of both oxidized Pt and Ru. The substantially higher loading of Ru compared to Pt and enrichment of Pt at the surface of the nanoparticles is confirmed by wide-area analysis of the electrodes using time-of-flight medium energy ion scattering (TOF-MEIS) employing both 80 keV He(+) and O(+) ions. The activity of electrodes containing 7 × 10(4) ions μm(-2) of bare 4.5 nm PtRu nanoparticles toward the electrochemical reduction of oxygen was evaluated employing cyclic voltammetry (CV) in 0.1 M HClO4 and 0.5 M H2SO4 solutions. In both electrolytes a pronounced reduction peak was observed during O2 purging of the solution that was not evident during purging with Ar. Repeated electrochemical cycling of the electrodes revealed little evolution in the shape or position of the voltammograms indicating high stability of the nanoparticles supported on glassy carbon. The reproducibility of the nanoparticle synthesis and deposition was evaluated by employing the same experimental parameters to prepare nanoparticles on glassy carbon electrodes on three occasions separated by several days. Surfaces with almost identical electrochemical behavior were observed with CV, demonstrating the highly reproducible preparation of bare nanoparticles using physical synthesis in the gas-phase combined with soft landing of mass-selected ions.

Oxide-supported metal nanoparticles are widely used in heterogeneous catalysis. The increasingly detailed design of such catalysts necessitates three-dimensional characterization with high spatial resolution and elemental selectivity. Laser-assisted atom probe tomography (APT) is uniquely suited to the task but faces challenges with the evaporation of metal/insulator systems. Correlation of APT with aberration-corrected scanning transmission electron microscopy (STEM), for Au nanoparticles embedded in MgO, reveals preferential evaporation of the MgO and an inaccurate assessment of nanoparticle composition. Finite element field evaporation modeling is used to illustrate the evolution of the evaporation front. Nanoparticle composition is most accurately predicted when the MgO is treated as having a locally variable evaporation field, indicating the importance of considering laser-oxide interactions and the evaporation of various molecular oxide ions. These results demonstrate the viability of APT for analysis of oxide-supported metal nanoparticles, highlighting the need for developing a theoretical framework for the evaporation of heterogeneous materials.