163 research outputs found
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Whole-Cell Analysis of Low-Density Lipoprotein Uptake by Macrophages Using STEM Tomography
Nanoparticles of heavy materials such as gold can be used as markers in quantitative electron microscopic studies of protein distributions in cells with nanometer spatial resolution. Studying nanoparticles within the context of cells is also relevant for nanotoxicological research. Here, we report a method to quantify the locations and the number of nanoparticles, and of clusters of nanoparticles inside whole eukaryotic cells in three dimensions using scanning transmission electron microscopy (STEM) tomography. Whole-mount fixed cellular samples were prepared, avoiding sectioning or slicing. The level of membrane staining was kept much lower than is common practice in transmission electron microscopy (TEM), such that the nanoparticles could be detected throughout the entire cellular thickness. Tilt-series were recorded with a limited tilt-range of 80Ā° thereby preventing excessive beam broadening occurring at higher tilt angles. The 3D locations of the nanoparticles were nevertheless determined with high precision using computation. The obtained information differed from that obtained with conventional TEM tomography data since the nanoparticles were highlighted while only faint contrast was obtained on the cellular material. Similar as in fluorescence microscopy, a particular set of labels can be studied. This method was applied to study the fate of sequentially up-taken low-density lipoprotein (LDL) conjugated to gold nanoparticles in macrophages. Analysis of a 3D reconstruction revealed that newly up-taken LDL-gold was delivered to lysosomes containing previously up-taken LDL-gold thereby forming onion-like clusters
Elucidating the structural composition of a Fe-N-C catalyst by nuclear and electron resonance techniques
FeāNāC catalysts are very promising materials for fuel cells and metalāair batteries. This work gives fundamental insights into the structural composition of an FeāNāC catalyst and highlights the importance of an inādepth characterization. By nuclearā and electronāresonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of Ī±āiron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in FeāNāC catalysts prepared by pyrolysis at 900āĀ°C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with endāon bonded oxygen as one of the axial ligands
Carrier-envelope phase control over pathway interference in strong-field dissociation of H
The dissociation of an H molecular-ion beam by linearly polarized,
carrier-envelope-phase-tagged 5 fs pulses at 4W/cm with a
central wavelength of 730 nm was studied using a coincidence 3D momentum
imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission
direction of H fragments relative to the laser polarization were observed.
These asymmetries are caused by interference of odd and even photon number
pathways, where net-zero photon and 1-photon interference predominantly
contributes at H+H kinetic energy releases of 0.2 -- 0.45 eV, and
net-2-photon and 1-photon interference contributes at 1.65 -- 1.9 eV. These
measurements of the benchmark H molecule offer the distinct advantage
that they can be quantitatively compared with \textit{ab initio} theory to
confirm our understanding of strong-field coherent control via the
carrier-envelope phase
Observing the morphology of single-layered embedded silicon nanocrystals by using temperature-stable TEM membranes
We use high-temperature-stable silicon nitride membranes to investigate single layers of silicon nanocrystal ensembles by energy filtered transmission electron microscopy. The silicon nanocrystals are prepared from the precipitation of a silicon-rich oxynitride layer sandwiched between two SiO diffusion barriers and subjected to a high-temperature annealing. We find that such single layers are very sensitive to the annealing parameters and may lead to a significant loss of excess silicon. In addition, these ultrathin layers suffer from significant electron beam damage that needs to be minimized in order to image the pristine sample morphology. Finally we demonstrate how the silicon nanocrystal size distribution develops from a broad to a narrow log-normal distribution, when the initial precipitation layer thickness and stoichiometry are below a critical value
Towards FIB-SEM Based Simulation of Pore-Scale Diffusion in SCR Catalyst Layers
The diffusivity in the upper Cu-Chabazite layer of a dual layer ammonia oxidation catalyst with a lower Pt layer was investigated. In a first step, the pore structure of the upper Cu-Chabazite catalyst layer was determined by Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) slice&view tomography. From the FIB-SEM data the 3D pore structure of the catalyst was reconstructed and diffusion simulations were performed on the reconstructed pore geometry, resulting in an estimated effective diffusivity of D/Dā=ā0.31. To validate the FIB-SEM derived estimates of the diffusivity, measurements of CO oxidation on the dual layer catalyst were performed, where the CO was oxidized in the lower Pt-layer while the upper SCR layer served as an inactive diffusion barrier. In this way, the effective diffusivity can be determined from the measured CO conversion. An effective diffusion coefficient of D/Dā=ā0.11 was obtained from the CO oxidation measurements, three times lower than the value obtained from the FIB-SEM data, but in line with previous literature data for the effective diffusivity in monolith washcoat layers. Additional NH oxidation experiments were performed on the dual layer catalyst. The results were well reproduced by a reactor model applying the effective diffusion coefficient obtained by the CO oxidation experiments. The origin of this apparent inconsistency is currently not understood and requires further investigation
Dislocation-mediated and twinning-induced plasticity of CoCrFeMnNi in varying tribological loading scenarios
Coarse-grained, metallic materials undergo microstructure refinement during
tribological loading. This in turn results in changing tribological properties,
so understanding deformation under tribological load is mandatory when
designing tribological systems. Single-trace experiments were conducted to
understand the initiation of deformation mechanisms acting in various
tribological systems. The main scope of this work was to investigate the
influence of normal and friction forces as well as crystal orientations on the
dominating deformation mechanism in a face-centred cubic concentrated solid
solution. While varying the normal force is easily realised, varying friction
forces were achieved by using several counter body materials paired against
CoCrFeMnNi. The subsurface deformation layer was either mediated through
dislocation slip or twinning, depending on the grain orientation and on the
tribological system. A layer dominated by dislocation-based deformation is
characterised by lattice rotation, the formation of a dislocation trace line or
subgrain formation. Such behaviour is observed for tribological systems with a
low friction coefficient. For systems dominated by deformation twinning, three
types of twin appearance were observed: small twins interacting with the
surface, large twins and grains with two active twin systems. Two different
twinning mechanisms are discussed as responsible for these characteristics
Dislocation-mediated and twinning-induced plasticity of CoCrFeMnNi in varying tribological loading scenarios
Porosity and Structure of Hierarchically Porous Ni/Al2O3 Catalysts for CO2 Methanation
CO2 methanation is often performed on Ni/Al2O3 catalysts, which can suffer from mass transport limitations and, therefore, decreased efficiency. Here we show the application of a hierarchically porous Ni/Al2O3 catalyst for methanation of CO2. The material has a well-defined and connected meso- and macropore structure with a total porosity of 78%. The pore structure was thoroughly studied with conventional methods, i.e., N2 sorption, Hg porosimetry, and He pycnometry, and advanced imaging techniques, i.e., electron tomography and ptychographic X-ray computed tomography. Tomography can quantify the pore system in a manner that is not possible using conventional porosimetry. Macrokinetic simulations were performed based on the measures obtained by porosity analysis. These show the potential benefit of enhanced mass-transfer properties of the hierarchical pore system compared to a pure mesoporous catalyst at industrially relevant conditions. Besides the investigation of the pore system, the catalyst was studied by Rietveld refinement, diffuse reflectance ultraviolet-visible (DRUV/vis) spectroscopy, and H2-temperature programmed reduction (TPR), showing a high reduction temperature required for activation due to structural incorporation of Ni into the transition alumina. The reduced hierarchically porous Ni/Al2O3 catalyst is highly active in CO2 methanation, showing comparable conversion and selectivity for CH4 to an industrial reference catalyst
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