Annular Dark Field Detector Research Articles

Electron microscopic characterization of exhaust particles containing lead dibromide beads expelled from aircraft burning leaded gasoline

Piston powered aircraft burning leaded gasoline contribute ~70% of the lead in the atmosphere in the US. The physical size, composition, and structure of aircraft exhaust particles containing lead dibromide are poorly understood and heretofore have not been examined directly by electron microscopy (EM), in particular when captured from an aircraft in flight. To accomplish this, exhaust samples were trapped on EM supports within 10–15 ms of exiting the aircraft exhaust pipe. High angle annular detector dark field scanning EM revealed irregular particles with a mean diameter of 13 nm consisting of a 4 nm microcrystal of lead dibromide surrounded by a halo of hydrocarbons. In contrast, exhaust particles from an automobile burning leaded fuel averaged 35 nm in diameter and contained 5–10, 4 nm lead beads. Of significant concern, the smaller aircraft particles could penetrate mucosal barriers in the lung and be readily taken up by epithelial cells.

Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy

In this work, the effect of citric acid (CA) on the structure and activity of Ni-promoted WS2 particles supported on γ-Al2O3 were investigated by HRTEM (high resolution transmission electron microscopy), HR STEM-HAADF (high resolution scanning transmission electron microscopy equipped with high angular annular dark field detector), CO adsorption followed by infrared (IR) spectroscopy, and thiophene hydrodesulfurization (HDS) test. It is shown that CA addition increases the thiophene HDS activity. This may be directly ascribed to the increase in NiWS site concentration detected by IR spectroscopy. HRTEM emphasizes that the addition of CA to NiW catalysts decreases the average size of the particles. This is confirmed by HR STEM-HAADF that even evidences the presence of nanoclusters on NiW catalysts prepared with CA. Hence, local and global analysis of the NiW samples are in good agreement to account for the increase of NiWS site concentration by CA addition. Then the contrast and interatomic distances were analyzed in the atomic-scale images obtained by HR STEM-HAADF. These measurements allow identification of the nature of the atoms in the core and edge of the sulfided particles. The metal atoms of particle core are only W ones, whereas the edges of the particles could be either fully promoted with detection of only Ni atoms or partially promoted with detection of both Ni and W atoms. Accordingly, IR spectroscopy evidenced edge sites with different promotion degrees.

Tailoring thermoelectric properties of Zr0.43Hf0.57NiSn half-Heusler compound by defect engineering

The thermoelectric transport properties of Zr0.43Hf0.57NiSn half-Heusler compounds were investigated for samples sintered with different spark plasma sintering (SPS) periods: 8, 32 and 72 min. By means of scanning transmission electron microscopy with a high-angular annular dark-field detector (STEM-HAADF), it was found that sintering time affected the defect concentration, namely the amount of Ni interstitial atoms, and created locally ordered inclusions of full-Heusler phase. The structural information, phase composition and electrical transport properties could be consistently explained by the assumption that Ni interstitials give rise to an impurity band situated about 100 meV below the bottom of the conduction band via a self-doping behavior. The impurity band was found to merge with the conduction band for the sample with intermediate SPS time. The effect was ascribed to the gradual dissolution of full-Heusler phase inclusions and production of interstitial Ni defects, which eventually vanished for the sample with the longest sintering time. It was demonstrated that the modification of the density of states near the edge of the conduction band and enhanced overall charge carrier concentration provided by defect engineering led to overall 26% increase in the thermoelectric figure of merit (ZT) with respect to the other samples.

Rendering hydrophobic nanoclusters water-soluble and biocompatible.

Hydrophobic and hydrophilic nanoclusters embody complementary superiorities. The means to amalgamate these superiorities, i.e., the atomic precision of hydrophobic clusters and the water dissolvability of hydrophilic clusters, remains challenging. This work presents a versatile strategy to render hydrophobic nanoclusters water-soluble—the micellization of nanoclusters in the presence of solvent-conjoined Na+ cations—which overcomes the above major challenge. Specifically, although [Ag29(SSR)12(PPh3)4]3− nanoclusters are absolutely hydrophobic, they show good dissolvability in aqueous solution in the presence of solvent-conjoined Na+ cations (Na1(NMP)5 or Na3(DMF)12). Such cations act as both counterions of these nanoclusters and surface cosolvent of cluster-based micelles in the aqueous phase. A combination of DLS (dynamic light scattering) and aberration-corrected HAADF-STEM (high angle annular dark field detector scanning transmission electron microscopy) measurements unambiguously shows that the phase-transfer of hydrophobic Ag29 into water is triggered by the micellization of nanoclusters. Owing to the excellent water solubility and stability of [Ag29(SSR)12(PPh3)4]3−[Na1(NMP)5]3+ in H2O, its performance in cell staining has been evaluated. Furthermore, the general applicability of the micellization strategy has been verified. Overall, this work presents a convenient and efficient approach for the preparation of cluster-based, biocompatible nanomaterials.

Nano-architectural complexity of zinc oxide nanowall hollow microspheres and their structural properties

Zinc oxide (ZnO) hollow spheres with defined morphology and micro-/nanostructure are prepared by a hydrothermal synthesis approach. The materials possess fine-leaved structures at their particle surface (nanowall hollow micro spheres). Morphology control is achieved by citric acid used as an additive in variable relative quantities during the synthesis. The structure formation is studied by various time-dependent ex situ methods, such as scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The fine-leaved surface structure is characterized by high-resolution transmission electron microscopy techniques (HRTEM, STEM), using a high-angle annular dark field detector, as well as by differential phase contrast analysis. In-depth structural characterization of the nanowalls by drop-by-drop ex situ FE-SEM analysis provides insight into possible structure formation mechanisms. Further investigation addresses the thermal stability of the particle morphology and the enhancement of the surface-to-volume ratio by heat treatment (examined by N2 physisorption).

High-resolution STEM-HAADF microscopy on a γ-Al2O3 supported MoS2 catalyst—proof of the changes in dispersion and morphology of the slabs with the addition of citric acid

Atomic-scale images of MoS2 slabs supported on γ-Al2O3 were obtained by high-resolution scanning transmission electron microscopy equipped with a high angular annular dark field detector (HR STEM-HAADF). These observations, obtained for sulfide catalysts prepared with or without citric acid as a chelating agent, evidenced variations in morphology (shape) and size of the MoS2 nanoslabs, as detected indirectly by the adsorption of CO followed by infrared spectroscopy. Quantitative dispersion values and a morphology index (S-edge/M-edge ratio) were determined from the slabs observed. In this way, HR STEM-HAADF underlines that the addition of citric acid to Mo catalysts decreases the size of the particles and modifies the shape of the MoS2 nanoslabs from slightly truncated triangles to particles with a higher ratio of S-edge/M-edge. These finding are of great significance since tayloring the morphology of the slabs is a way to increase their catalytic activity and selectivity. Furthermore, this work demonstrated that the IR/CO method is a relevant approach to describe the MoS2 morphology of supported catalysts used in hydrotreatment processes for clean fuel production.

Spatially resolved structure and domain wall propagation in defect induced SmCo/Co exchange spring magnet

Abstract SmCo5/Co exchange-coupled films with high saturation magnetization, and hence high energy product were fabricated on MgO substrates by sequential deposition of elemental Sm and Co layers at 450 °C followed by subsequent annealing at the same temperature for 1 h. Formation of defects induced multi-layered structures and the composition of individual layers were confirmed through high resolution Scanning Transmission Electron Microscopy (HRSTEM) imaging acquired on high angle annular dark field (HAADF) detector and Energy Dispersive X-ray (EDX) spectroscopy. Micromagnetic simulations and magnetic measurements further confirmed that the observed structure is coupled in rigid phase between soft and hard magnetic layers and the enhancement in coercivity results due to the domain wall pinning at the defects sites. This leads to the magnetic energy density in the film of the order of 22.2 MGOe which is close to the bulk value and have potential applications towards Micro-Electro-Mechanical Systems.

Characterization of Palladium Nanoparticles Produced by Healthy and Microwave-Injured Cells of Desulfovibrio desulfuricans and Escherichia coli.

Numerous studies have focused on the bacterial synthesis of palladium nanoparticles (bio-Pd NPs), via uptake of Pd (II) ions and their enzymatically-mediated reduction to Pd (0). Cells of Desulfovibrio desulfuricans (obligate anaerobe) and Escherichia coli (facultative anaerobe, grown anaerobically) were exposed to low-dose radiofrequency (RF) radiation(microwave (MW) energy) and the biosynthesized Pd NPs were compared. Resting cells were exposed to microwave energy before Pd (II)-challenge. MW-injured Pd (II)-treated cells (and non MW-treated controls) were contacted with H2 to promote Pd(II) reduction. By using scanning transmission electron microscopy (STEM) associated with a high-angle annular dark field (HAADF) detector and energy dispersive X-ray (EDX) spectrometry, the respective Pd NPs were compared with respect to their mean sizes, size distribution, location, composition, and structure. Differences were observed following MWinjury prior to Pd(II) exposure versus uninjured controls. With D. desulfuricans the bio-Pd NPs formed post-injury showed two NP populations with different sizes and morphologies. The first, mainly periplasmically-located, showed polycrystalline Pd nano-branches with different crystal orientations and sizes ranging between 20 and 30 nm. The second NPpopulation, mainly located intracellularly, comprised single crystals with sizes between 1 and 5 nm. Bio-Pd NPs were produced mainly intracellularly by injured cells of E. coli and comprised single crystals with a size distribution between 1 and 3 nm. The polydispersity index was reduced in the bio-Pd made by injured cells of E. coli and D. desulfuricans to 32% and 39%, respectively, of the values of uninjured controls, indicating an increase in NP homogeneity of 30–40% as a result of the prior MWinjury. The observations are discussed with respect to the different locations of Pd(II)-reducing hydrogenases in the two organisms and with respect to potential implications for the catalytic activity of the produced NPs following injury-associated altered NP patterning.

Formation of hollow and solid carbon spheres in thermally stressed jet fuel in the low temperature autoxidation regime

We report the formation of hollow and solid carbon spheres and spherical nanostructures by self-assembly through thermal stressing of jet fuel (Jet A-1) in the autoxidation temperature range. The characterization of Jet A-1 deposits was conducted by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS). Additionally, high resolution mass spectrometry with electro spray ionization source (ESI-MS) have been used to identify large molecular weight compounds in the mass range 350 Da to 1000 Da in the thermally stressed fuel. Fourier transform infrared (FTIR) spectroscopy analysis revealed oxygenated functional groups in the jet fuel treated by flask tests. Compositional analysis of deposit by TEM EDS and scanning transmission electron microscopy (STEM) and a high-angle annular dark field (HAADF) detector identified multiple trace metallic elements such as Cu, Fe, Zn, Sn, Mg, Al and heteroatoms S, N, O. Hollow carbon sphere formation was due to the catalytic activity of trace metals. Spherical particle growth in this method also appeared to follow Ostwald ripening mechanism. Simple flask static tests in the low temperature regime with jet fuel as hydrocarbon source without externally added catalysts thus is a novel method for hollow and solid carbon sphere synthesis and presented in this paper.

AuPd/C core–shell and alloy nanoparticles with enhanced catalytic activity toward the electro-oxidation of ethanol in alkaline media

Carbon-supported Pd, Au@Pd core–shell and Au1–xPdx-alloyed nanoparticles were prepared by a chemical reduction method and characterized by different experimental techniques, including X-ray powder diffraction, transmission electron microscopy, scanning-transmission electron microscopy using bright-field and high-angle annular dark field detectors and X-ray energy dispersive spectroscopy. The catalytic mass activity toward ethanol oxidation was assessed by cyclic voltammetry and chronoamperometry at room temperature. The measurements showed that the addition of Au enhances remarkably the electrocatalytic activity of the material, due to the bifunctional effect of Au1–xPdx/C alloys, and the synergetic effect on Au@Pd/C, resulting in a dissolution resistance of core–shell catalysts at potentials of 1.5 V versus reversible hydrogen electrode. In situ Fourier transform infrared spectroscopy measurements showed that the mechanism for ethanol oxidation depends on the electrocatalyst structure and morphology. Acetate was identified as the main product of ethanol electro-oxidation on the studied electrocatalysts. However, the presence of a core–shell structure on Au@Pd/C resulted in enhanced ethanol oxidation selectivity toward CO2. The improvement of activity is attributed to the interaction between Pd shell and Au core.

A posteriori synchronization of scanning transmission electron microscopy signals with kilopixel per second acquisition rates

The stability and sensitivity of scanning transmission electron microscopes as well as detectors collecting e.g. electrons which suffered different scattering processes, or secondary radiation, have increased tremendously during the last decade. In order to fully exploit capabilities of simultaneously recording various signals with up to 1000 px/s acquisition rates the central issue is their synchronization. The latter is frequently a non-trivial problem without commercially available solution especially if detectors of different manufacturers are involved. In this paper, we present a simple scanning pattern enabling a posteriori synchronization of arbitrarily many signals being recorded entirely independently. We apply the approach to the simultaneous atomic-scale acquisition of signals from an annular dark-field detector and electron energy loss as well as energy-dispersive x-ray spectrometers. Errors emerging in scanning direction due to the independence of the respective processes are quantified and found to have a standard deviation of roughly half the pixel spacing. Since there are no intermediate waiting periods to maintain synchronicity, the proposed acquisition process is, in fact, demonstrated to be 12% faster than a commercial hardware-synchronized solution for identical sub-millisecond signal integration times and hence follows the trend in electron microscopy to extract more information per irradiating electron.

Quasi-parallel precession diffraction: Alignment method for scanning transmission electron microscopes

A general method to set illuminating conditions for selectable beam convergence and probe size is presented in this work for Transmission Electron Microscopes (TEM) fitted with µs/pixel fast beam scanning control, (S)TEM, and an annular dark field detector. The case of interest of beam convergence and probe size, which enables diffraction pattern indexation, is then used as a starting point in this work to add 100 Hz precession to the beam while imaging the specimen at a fast rate and keeping the projector system in diffraction mode. The described systematic alignment method for the adjustment of beam precession on the specimen plane while scanning at fast rates is mainly based on the sharpness of the precessed STEM image. The complete alignment method for parallel condition and precession, Quasi-Parallel PED-STEM, is presented in block diagram scheme, as it has been tested on a variety of instruments. The immediate application of this methodology is that it renders the TEM column ready for the acquisition of Precessed Electron Diffraction Tomographies (EDT) as well as for the acquisition of slow Precessed Scanning Nanometer Electron Diffraction (SNED). Examples of the quality of the Precessed Electron Diffraction (PED) patterns and PED-STEM alignment images are presented with corresponding probe sizes and convergence angles.

Quantification and optimization of ADF-STEM image contrast for beam-sensitive materials.

Many functional materials are difficult to analyse by scanning transmission electron microscopy (STEM) on account of their beam sensitivity and low contrast between different phases. The problem becomes even more severe when thick specimens need to be investigated, a situation that is common for materials that are ordered from the nanometre to micrometre length scales or when performing dynamic experiments in a TEM liquid cell. Here we report a method to optimize annular dark-field (ADF) STEM imaging conditions and detector geometries for a thick and beam-sensitive low-contrast specimen using the example of a carbon nanotube/polymer nanocomposite. We carried out Monte Carlo simulations as well as quantitative ADF-STEM imaging experiments to predict and verify optimum contrast conditions. The presented method is general, can be easily adapted to other beam-sensitive and/or low-contrast materials, as shown for a polymer vesicle within a TEM liquid cell, and can act as an expert guide on whether an experiment is feasible and to determine the best imaging conditions.

Effect of Storage Conditions on the Long-Term Stability of Bactericidal Effects for Laser Generated Silver Nanoparticles.

Silver nanoparticles (AgNPs) are widely used as antibacterial agents, but their antibacterial durability and the influence by storage conditions have not been thoroughly investigated. In this study, AgNPs were produced using a picosecond laser and stored under three different conditions: daylight, dark and cold (4 °C). The antibacterial effects of the laser AgNPs were examined against Escherichia coli in either a 14-day interval (frequent air exposure) or a 45-day interval (less frequent air exposure) using a well-diffusion method until the antibacterial effects disappeared. Results showed that the antibacterial activity of the laser generated AgNPs lasted 266 to 405 days. Frequent air exposure increased particle oxidation as measured by high-angle annular dark-field detector for scanning transmission electron microscopy (HAADF-STEM) and X-ray energy dispersive (EDX) spectroscopy, and reduced the antibacterial duration by about 13 weeks. Compared to the chemically produced AgNPs, the antibacterial effect of the laser AgNPs lasted over 100 days longer when tested in the 45-day interval, but was susceptible to oxidation when frequently exposed to the air. The laser generated AgNPs had lower antibacterial activity when stored in cold compared to that stored at room temperature. This study demonstrated the long lasting antibacterial durability of the laser generated AgNPs. Such information could help design future medical applications for the AgNPs.

Thin-Film Nanocomposite Membrane with the Minimum Amount of MOF by the Langmuir-Schaefer Technique for Nanofiltration.

An innovative procedure for positioning a monolayer of hydrophilic metal organic framework (MOF) MIL-101(Cr) (MIL, Materials of Institute Lavoisier) nanoparticles (NPs) in thin-film nanocomposite (TFN) membranes has been implemented by transferring a Langmuir-Schaefer (LS) film of the MOF in between the polyamide thin layer at the top and the cross-linked asymmetric polyimide (P84) support at the bottom. The presence and layout of the LS-MIL-101(Cr) monolayer in the TFN membrane was confirmed by scanning transmission electron microscopy imaging with a high-angle annular dark-field detector images and X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and atomic force microscopy analyses. This methodology requires the smallest amount of MOF reported to date, 3.8 μg cm-2, and promotes the formation of a defect-free ultrathin MOF film. Although conventional TFN membranes tend to show MOF agglomerates that could contribute to the formation of unselective defects, LS-TFN membranes, characterized by a homogeneous and continuous MOF coating, exhibit an optimal membrane performance, without a significant decrease in selectivity. Outstanding methanol permeances, one of the best results reported to date, of 10.1 ± 0.5 L m-2 h-1 bar-1 when filtering sunset yellow and of 9.5 ± 2.1 L m-2 h-1 bar-1 when filtering rose bengal have been achieved in LS-TFN membranes with a rejection higher than 90% in all cases. Methanol permeates through the polyamide and the LS-MIL-101(Cr) monolayer, greatly enhanced by the MOF pore system, in comparison to thin-film composite and conventional TFN membranes (7.5 ± 0.7 and 7.7 ± 1.1 L m-2 h-1 bar-1 when filtering sunset yellow), respectively, in which polyamide areas free of MOF NPs are present.

Pumping Metallic Nanoparticles with Spatial Precision within Magnetic Mesoporous Platforms: 3D Characterization and Catalytic Application.

The present work shows an efficient strategy to assemble two types of functional nanoparticles onto mesoporous MCM-41 silica nanospheres with a high degree of spatial precision. In a first stage, magnetite nanoparticles are synthesized with a size larger than the support pores and grafted covalently through a peptide-like bonding onto their external surface. This endowed the silica nanoparticles with a strong superparamagnetic response, while preserving the highly ordered interior space for the encapsulation of other functional guest species. Second, we report the finely controlled pumping of preformed Pt nanoparticles (1.5 nm) within the channels of the magnetic MCM-41 nanospheres to confer an additional catalytic functionality to the multiassembled nanoplatform. The penetration depth of the metallic nanoparticles can be explained as a result of the interplay between the particle-wall electrostatic attraction and the repulsive forces between neighboring Pt nanoparticles. A detailed transmission electron microscopy and a 3D high-resolution high-angle annular dark-field detector electron tomography study were carried out to characterize the material and to explain the assembly mechanism. Finally, the performance of these multifunctional nanohybrids as magnetically recoverable catalysts has been evaluated in the selective hydrogenation of p-nitrophenol, a well-known pollutant and intermediate in multiple industrial processes.

Microstructure evolution during heat treatment of Mg–Gd–Y–Zn–Zr alloy and its low-cycle fatigue behavior at 573 K

In as-cast Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr (mole fraction, %) alloy, lamellar microstructures that extend from grain boundaries to the interior of α-Mg grains are identified as clusters of γ′ using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector. Under a total strain-controlled low-cyclic loading at 573 K, the mechanical response and failure mechanism of Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr alloy (T6 peak-aging heat treatment) were investigated. Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and 573 K. The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures and α-Mg matrix and extend along the basal plane of α-Mg. The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.

Optical and TEM characterization of phase transformation in Zn ion implanted and thermal oxidized quartz

<table class="data" width="100%"><tbody><tr valign="top"><td class="value"><p>Metal and metal oxide nanoparticles (NPs) embedded in various matrices have been the object of increasing interest due to their peculiar physical properties significantly different from the corresponding ones of bulk material. Metallic Zn NPs can be used in UV photo-detectors. Zinc oxide NPs play an important role too, since ZnO has direct band gap of 3.37eV, large exiton binding energy of 60meV, sorption effect, room temperature ferromagnetism and others. So they can be used in varies perspective electron devices. In this paper the optical parameters and transmission electron microscopy (TEM) characterization of Zn implanted quartz during NP formation at annealing are reported.</p><p>Optical-grade high-pure (OH<sup><span style="font-size: small;">-</span></sup>: 50ppb) amorphous quartz slides were implanted by <sup><span style="font-size: small;">64</span></sup>Zn<sup><span style="font-size: small;">+</span></sup> ions with fluence of 5×10<sup><span style="font-size: small;">16</span></sup>/cm<sup><span style="font-size: small;">2</span></sup> and energy of 50keV. To avoid the magnificent substrate heating during implantation the ion current density was less than 0.5μA/cm<sup><span style="font-size: small;">2</span></sup>. Than the samples were subjected to isochronally during 1h oxidation in temperature ranges from 400 up to 800<sup><span style="font-size: small;">o</span></sup>C. The Zn contained phase creation and its thermal evolution were investigated by recording the optical transmittance spectra at room temperature in a spectral range of 200-800nm and by photoluminescence at temperatures of 10-300K in a spectral range of 350-800nm using an illumination of He-Cd laser with wavelength of 325nm. Visualization and identification of NPs was made by study of the cross section samples TEM fitted with electron diffraction, an EDAX detector attachment for X-ray energy dispersive spectroscopy (EDS), and with a high angle annular dark field detector (HAADF) for scanning operation. The EDS maps of the element distribution and the NP sizes were determined in the scanning TEM regime.</p><p>As a result of these studies it was found that after the quartz implantation by <sup><span style="font-size: small;">64</span></sup>Zn<sup><span style="font-size: small;">+</span></sup> ions with fluence of 5×10<sup><span style="font-size: small;">16</span></sup>/cm<sup><span style="font-size: small;">2</span></sup> and energy of 50keV the amorphous metallic Zn NPs with an average radius of 3 nm were created. During process of sequentially isochronally furnace annealing in oxygen atmosphere in 1h in temperature ranges from 400 up to 800<sup><span style="font-size: small;">o</span></sup>C in samples there was phase transformation from metallic Zn phase to its oxide form. After annealing at 800<sup><span style="font-size: small;">o</span></sup>C the Zn NPs transform to the ZnO or/and Zn<sub><span style="font-size: small;">2</span></sub>SiO<sub><span style="font-size: small;">4</span></sub> phase with average radius of 4,5nm.</p></td></tr></tbody></table><div id="indexing"><h4> </h4></div>

Dark-field image contrast in transmission scanning electron microscopy: Effects of substrate thickness and detector collection angle

An annular dark field (ADF) detector was placed beneath a specimen in a field emission scanning electron microscope operated at 30kV to calibrate detector response to incident beam current, and to create transmission images of gold nanoparticles on silicon nitride (SiN) substrates of various thicknesses. Based on the linear response of the ADF detector diodes to beam current, we developed a method that allowed for direct determination of the percentage of that beam current forward scattered to the ADF detector from the sample, i.e. the transmitted electron (TE) yield. Collection angles for the ADF detector region were defined using a masking aperture above the detector and were systematically varied by changing the sample to detector distance. We found the contrast of the nanoparticles, relative to the SiN substrate, decreased monotonically with decreasing inner exclusion angle and increasing substrate thickness. We also performed Monte Carlo electron scattering simulations, which showed quantitative agreement with experimental contrast associated with the nanoparticles. Together, the experiments and Monte Carlo simulations revealed that the decrease in contrast with decreasing inner exclusion angle was due to a rapid increase in the TE yield of the low atomic number substrate. Nanoparticles imaged at low inner exclusion angles (<150mrad) and on thick substrates (>50nm) showed low image contrast in their centers surrounded by a bright high-contrast halo on their edges. This complex image contrast was predicted by Monte Carlo simulations, which we interpreted in terms of mixing of the nominally bright field (BF) and ADF electron signals. Our systematic investigation of inner exclusion angle and substrate thickness effects on ADF t-SEM imaging provides fundamental understanding of the contrast mechanisms for image formation, which in turn suggest practical limitations and optimal imaging conditions for different substrate thicknesses.

Determination of three-dimensional atomic positions from tomographic reconstruction using ensemble empirical mode decomposition

Tomographic reconstruction from a tilt series of electron micrographs has raised great interest in materials, chemical, and condensed matters science because of its capability of revealing 3D local atomic structures within nanomaterials. Previous breakthroughs have demonstrated that the positions of individual atoms can not only be visualized but also determined by combining a scanning transmission electron microscope with a high-angle annular dark-field detector, equally sloped tomography, and the filtering/denoising method. However, the filtering/denoising approach—whether imposed on 2D projections or 3D reconstruction—raises concerns regarding the robustness of image processing, the accuracy of atomic positions, and the artificial atoms introduced during filtering. In this article, we report a general method that overcomes these limitations. By removing unphysical oscillations in 2D projections through ensemble empirical mode decomposition and applying a standard Wiener filter to the 3D reconstruction, we are able to determine atomic structures with higher accuracy.