Scanning Transmission Electron Microscopy Energy-dispersive X-ray Research Articles

Distribution Analysis of Iron and Copper by STEM-EDX Spectroscopy of Hemosiderin Particles in the Liver of Rats Overloaded With Iron.

Our recent studies have indicated that trace copper co-existed with iron in hemosiderin particles of human genetic iron overload. To understand this phenomenon, we analyzed hemosiderin particles in iron-overloaded rat liver by using scanning transmission electron microscopy - energy-dispersive X-ray (STEM-EDX) spectroscopy. Samples for STEM-EDX spectroscopy were prepared from the liver of rats administered an intraperitoneal injection of dextran iron. The micro-domain analysis with STEM-EDX spectroscopy showed that dense bodies contained high levels of iron and trace copper. Quantitative analysis of copper levels in the liver specimen using atomic spectrophotometry showed that copper concentration in the liver was not increased by iron overload. These findings suggest that the overload of iron induced distribution of trace copper to hemosiderin particles without changing cellular copper levels. Co-existence of copper with iron was observed in hemosiderin particles of the liver of an experimental model of iron overload, suggesting that iron overload induced distribution of trace copper into hemosiderin particles.

Noise Reduction of Low-Count STEM-EDX Data by Low-Rank Regularized Spectral Smoothing.

Statistically weighted principal component analysis (wPCA) is widely used to reduce the noise of scanning transmission electron microscopy-energy-dispersive X-ray (STEM-EDX) spectroscopic data. It is beneficial to retain the spatial resolution of observation in each step of the analysis, but the direct application of wPCA without preprocessing, such as spatial averaging, often fails against low-count STEM-EDX data. To enhance the applicability of wPCA while retaining spatial resolution, a step-by-step noise reduction method is considered in this study. Specifically, a numerical optimization is developed to simultaneously characterize the smoothness of EDX spectra and the low rankness of the data. In the presented approach, low-count data are first spectrally smoothed by solving this optimization problem, and then further denoised by using wPCA to project onto a subspace rigorously spanned by a small number of components. A challenging example is provided, and the improved noise reduction performance is demonstrated and compared to existing spectral smoothing techniques.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) for Analysis of Surface and Interface Chemistry of Porous Transport Layers

As the United States energy infrastructure moves towards the integration of hydrogen energy, the reliable production of hydrogen through water electrolyzers is imperative. In proton exchange membrane water electrolyzes (PEMWE’s), the porous transport layer (PTL) plays an important role. When choosing PTL material, one should consider corrosion resistance, electrical conductivity, and ability to support the structure of the electrochemical cell. Due to these requirements, titanium is the current state-of-the-art anode PTL material. However, titanium quickly forms a layer of titanium oxide which significantly decreases conductivity of the PTL and respectively decreases the overall efficiency of the PEMWE system. Coatings are commonly applied to the PTL to combat this complication, but the use of noble metals leads to cost concerns. Additionally, the degradation of coated PTLs is not yet well known.Advanced physicochemical characterization of PTLs at various stages of fabrication and testing is vital to understand their properties and the impact on electrochemical performance in order to improve durability and meet cost targets. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is currently used to cross section areas of the PTL for subsequent analysis with Scanning Transmission Electron MicroscopyEnergy-dispersive X-ray Spectroscopy (STEM-EDS) analysis to visualize the elemental information of the PTL materials and PTL coatings, specifically looking at detrimental oxide layer formation. Unfortunately, this approach is time-consuming and difficult, motivating the development of alternative approaches that allow the characterization of wide sample sets more efficiently. Additionally, STEM-EDS analysis only provides elemental information, so if several oxide layers preside, it can be difficult to differentiate them. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiling is a very powerful technique that has been commonly used to characterize thin films and buried interfaces. Unlike the FIB-SEM and STEM-EDS combination, ToF-SIMS can be performed relatively quickly in several areas of the PTL, provides chemical information, and is sensitive to trace elements. An added advantage of ToF-SIMS is ability to do 2D and 3D chemical analysis on different scales. This enables visualization of elemental and chemical distribution on both larger scale (70x70 um) areas, and smaller areas (20x10 um) which provides more detailed surface and interface information. This allows for detailed tracking of all major constituents of the PTL and coating. It also allows to follow changes in oxide layers upon fabrication and after electrochemical testing, as well as track homogeneity of the surface and oxide layer interfaces throughout different parts of the PTL. This presentation will demonstrate capabilities of this technique for characterization of PTLs, along with progress and potential challenges. ToF-SIMs results will be compared to TEM analysis of cross-sections obtained with FIB-SEM. This study will highlight similarities and differences between the techniques, technique optimization for these morphologically challenging samples, and paths for future investigation moving forward.

Ferroelectric and piezoelectric properties of 100 nm-thick CeO2-HfO2 epitaxial films

Scaling up the film thickness of HfO2-based ferroelectrics is an important factor leading to their potential application in piezoelectric devices. In this paper, we report the ferroelectric and piezoelectric properties of 100 nm-thick 0.1CeO2-0.9HfO2 films, epitaxially grown on (001) indium-tin-oxide//(001)yttria-stabilized zirconia substrates. The crystal structure was investigated using x-ray diffraction and scanning transmission electron microscopy (STEM). These analyses revealed that the polar orthorhombic phase was stabilized, even at a thickness of 100 nm, whereas the formation of a nonpolar monoclinic phase was suppressed. In addition, the elemental mappings obtained by STEM–energy dispersive x-ray spectroscopy revealed that the film was compositionally uniform across its thickness. The chemical state of Ce in the polar orthorhombic domain was investigated using STEM–electron energy loss spectroscopy, which revealed the coexistence of Ce4+ and Ce3+. In addition, pinched polarization-electric field loops were observed, and their shapes were found to remain unaltered even after 109 electric field cycles. The strain-electric field curves originating from ferroelectricity were observed before and after the electric field cycling, and the high-field observed strain, Smax/Emax, was found to be approximately 7.2 pm/V. These results demonstrate that thicker CeO2-HfO2 ferroelectric films are promising candidates for use as piezoelectric materials.

Surface segregation of 3 mol % yttria-doped tetragonal zirconia particle studied by atomic-resolution scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy

Atomic structure of the surfaces in commercial 3 mol % yttria-doped tetragonal zirconia (TZP) nano-particulate powders were studied by atomic resolution scanning transmission electron microscopy (STEM), and the surface segregation of Y were investigated by STEM energy-dispersive X-ray spectroscopy (EDS) at atomic scale. STEM-EDS observations revealed that the Y cations tend to segregate on the top surface layer, forming a monolayer segregation structure. Static lattice calculations revealed that the surface segregation of Y is energetically favorable. These results indicate that Y surface segregation occurs in the TZP powders, which might partially account for the grain boundary segregation in the sintered body of TZP ceramics.

Characterizing electrical breakdowns upon reoxidation atmosphere for reliable multilayer ceramic capacitors

Electrical breakdowns of multilayer ceramic capacitors (MLCCs) manifest an increase in leakage current and are characterized as a function of atmospheric reoxidation. The atmospheric reoxidation is controlled with respect to the theoretical oxygen partial pressure for the oxidation of Ni internal electrodes. The breakdowns are characterized by a Maxwell–Wagner polarization technique, which dominantly exhibits space-charge-limited and Poole–Frenkel currents for all measured samples. The threshold voltage for the transition between these two conduction modes is suggested as an index for the robustness of the grain boundary resistance of BaTiO3; therefore, the breakdown voltage. The reoxidation atmosphere, which prevents the Ni oxidation, increases the threshold voltage, dramatically enhancing the breakdown voltage and insulation resistance. Impedance spectroscopy and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy reveal that the cation distribution throughout BaTiO3 grains and grain boundaries changes during the reoxidation, including Ni cations from the internal electrodes, which affects the grain boundary resistance and determines the breakdown voltage of MLCCs with Ni internal electrodes. These observations emphasize that the reoxidation should be concurrently optimized in terms of the cation redistribution and elimination of oxygen vacancies.

Silica-Supported PdGa Nanoparticles: Metal Synergy for Highly Active and Selective CO2-to-CH3OH Hydrogenation.

The direct conversion of CO2 to CH3OH represents an appealing strategy for the mitigation of anthropogenic CO2 emissions. Here, we report that small, narrowly distributed alloyed PdGa nanoparticles, prepared via surface organometallic chemistry from silica-supported GaIII isolated sites, selectively catalyze the hydrogenation of CO2 to CH3OH. At 230 °C and 25 bar, high activity (22.3 molMeOH molPd–1 h–1) and selectivity for CH3OH/DME (81%) are observed, while the corresponding silica-supported Pd nanoparticles show low activity and selectivity. X-ray absorption spectroscopy (XAS), IR, NMR, and scanning transmission electron microscopy–energy-dispersive X-ray provide evidence for alloying in the as-synthesized material. In situ XAS reveals that there is a dynamic dealloying/realloying process, through Ga redox, while operando diffuse reflectance infrared Fourier transform spectroscopy demonstrates that, while both methoxy and formate species are observed in reaction conditions, the relative concentrations are inversely proportional, as the chemical potential of the gas phase is modulated. High CH3OH selectivities, across a broad range of conversions, are observed, showing that CO formation is suppressed for this catalyst, in contrast to reported Pd catalysts.

Clustering of oxygen point defects in transition metal nitrides

Point defects create exotic properties in materials such as defect-induced luminescence in wide-bandgap semiconductors, magnetism in nonmagnetic materials, single-photon emission from semiconductors, etc. In this article, oxygen defect formation in metallic TiN and semiconducting rock salt-(Al,Sc)N is investigated with a combination of first-principles density functional theory, synchrotron-based x-ray absorption spectroscopy (XAS) analysis, and scanning transmission electron microscopy–energy-dispersive x-ray spectroscopy mapping. Modeling results show that oxygen in TiN and rock salt-(Al,Sc)N prefers to be in the defect complex of substitutional and interstitial oxygen (nON + Oi) types. While in TiN, the preferential interstitial sites of oxygen in ON + Oi are at the tetrahedral site, in rock salt-(Al,Sc)N, a split interstitial site along the [111] direction was found to be energetically preferable. Simulations performed as a function of the oxygen partial pressure show that under experimental growth conditions, four oxygen atoms at the substitutional sites of nitrogen (4ON), along with four Ti atoms, decorate around an interstitial oxygen atom at the tetrahedral site (Oi) in the energetically favored configuration. However, in rock salt-(Al,Sc)N, n in nON + Oi was found to vary from two to four depending on the oxygen partial pressure. Theoretical predictions agree well with the experimentally obtained XAS results. These results are not only important for a fundamental understanding of oxygen impurity defect behavior in rock salt nitride materials but will also help in the development of epitaxial metal/semiconductor superlattices with efficient thermionic properties.

Metallization of Al2O3 ceramic with Mg by friction stir spot welding

The surfaces of Al2O3 ceramic plates were metallized with by a friction stir spot welding method. Tensile tests revealed that the adhesive strength between the Mg film and Al2O3 ceramic plate was 23.3 MPa. To investigate the origin of this strong adhesion, the interface structure was examined by X-ray diffraction, scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The results indicated the formation of a microcrystalline MgO layer at the interface. Furthermore, the surfaces of two metallized Al2O3 ceramic plates separated by Ni foil were bonded together by hot pressing at 1100 °C for 180 min under vacuum. The resulting cross tensile strength of the bonding was 2.1 MPa, indicating strong adhesion between the Al2O3 ceramic plates.

Remarkable bactericidal traits of a metal-ceramic composite coating elated by hierarchically structured surface.

SummaryA ceramic-based coating with a hierarchical surface structure was synthesized via solid-state reaction enabled by a double cathode glow discharge technique. This innovative coating comprises two distinct layers, specifically an outer layer with a well-aligned micro-pillar array and a dense inner layer. Both are composed of a face-centered cubic Cu(Co,Ni,Fe) solid solution phase together with a spinel-type Fe(Al,Cr)2O4 oxide. This coating exhibits superhydrophobicity and, yet, a very strong adhesion to water, i.e., the so-called “rose petal effect”. This coating also exhibits highly efficient antibacterial ability against both Staphylococcus aureus and Escherichia coli bacteria under both dark and visible light conditions. The excellent antibacterial property originates from the synergistic effects through the release of Cu ions coupled with photothermal activity upon light activation.

TEM and STEM-EDS evaluation of metal nanoparticle encapsulation in GroEL/GroES complexes according to the reaction mechanism of chaperonin

Abstract Escherichia coli chaperonin GroEL, which is a large cylindrical protein complex comprising two heptameric rings with cavities of 4.5 nm each in the center, assists in intracellular protein folding with the aid of GroES and adenosine triphosphate (ATP). Here, we investigated the possibility that GroEL can also encapsulate metal nanoparticles (NPs) up to ∼5 nm in diameter into the cavities with the aid of GroES and ATP. The slow ATP-hydrolyzing GroELD52A/D398A mutant, which forms extremely stable complexes with GroES (half-time of ∼6 days), made it possible to analyze GroEL/GroES complexes containing metal NPs. Scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy analysis proved distinctly that FePt NPs and Au NPs were encapsulated in the GroEL/GroES complexes. Dynamic light scattering measurements showed that the NPs in the GroEL/GroES complex were able to maintain their dispersibility in solution. We previously described that the incubation of GroEL and GroES in the presence of ATP·BeFx and adenosine diphosphate·BeFx resulted in the formation of symmetric football-shaped and asymmetric bullet-shaped complexes, respectively. Based on this knowledge, we successfully constructed the football-shaped complex in which two compartments were occupied by Pt or Au NPs (first compartment) and FePt NPs (second compartment). This study showed that metal NPs were sequentially encapsulated according to the GroEL reaction in a step-by-step manner. In light of these results, chaperonin can be used as a tool for handling nanomaterials.

Correlation between Optical and Structural Characteristics in Coaxial GaInN/GaN Multiple Quantum Shell Nanowires with AlGaN Spacers

High crystalline quality coaxial GaInN/GaN multiple quantum shells (MQSs) grown on dislocation-free nanowires are highly in demand for efficient white-/micro-light-emitting diodes (LEDs). Here, we propose an effective approach to improve the MQS quality during the selective growth by metal-organic chemical vapor deposition. By increasing the growth temperature of GaN barriers, the cathodoluminescent intensity yielded enhancements of 0.7 and 3.9 times in the samples with GaN and AlGaN spacers, respectively. Using an AlGaN spacer before increasing the barrier temperature, the decomposition of GaInN quantum wells was suppressed on all planes, resulting in a high internal quantum efficiency up to 69%. As revealed by scanning transmission electron microscopy (STEM) characterization, the high barrier growth temperature allowed to achieve a clear interface between GaInN quantum wells and GaN quantum barriers on the c-, r-, and m-planes of the nanowires. Moreover, the correlation between the In incorporation and structure features in MQS was quantitatively assessed based on the STEM energy-dispersive X-ray spectroscopy mapping and line-scan profiles of In and Al fractions. Ultimately, it was demonstrated that the unintentional In incorporation in GaN barriers was induced by the evaporation of predeposited In-rich particles during low-temperature growth of GaInN wells. Such residual In contamination was sufficiently inhibited by inserting low Al fraction (∼6%) AlGaN spacers after each GaInN well. During the growth of AlGaN spacers, AlN polycrystalline particles were deposited on the surrounding dummy substrate, which suppressed the evaporation of the predeposited In-rich particles. Thus, the presence of AlGaN spacers certainly improved the uniformity of In fraction through five GaInN quantum wells and reduced the diffusion of point defects from n-core to MQS active structures. The superior coaxial GaInN/GaN MQS structures with the AlGaN spacer are supposed to improve the emission efficiency in white-/micro-LEDs.

Au-Cu@PANI Alloy Core Shells for Aerobic Fibrin Degradation under Visible Light Exposure.

Fibrin plays a critical role in wound healing and hemostasis, yet it is also the main case of cardiovascular diseases and thrombosis. Here, we show the unique design of Au-Cu@PANI alloy core-shell rods for fibrin clot degradation. Microscopic (transmission electron microscopy (TEM), scanning transmission electron microscopy-energy-dispersive X-ray (STEM-EDX)) and structural characterizations (powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS)) of the Au-Cu@PANI hybrid material reveal the formation of Au-Cu heterogeneous alloy core rods (aspect ratio = 3.7) with thin Cu2O and PANI shells that create a positive surface charge (ζ-potential = +22 mV). This architecture is supported by the survey XPS spectrum showing the presence of Cu 2p, N 1s, and C 1s features with binding energies of 934.8, 399.7, and 284.8 eV, respectively. Upon photolysis (λ ≥ 495 or 590 nm), these hybrid composite nanorods provide sufficient excited-state redox potential to generate reactive oxygen species (ROS) for degradation of model fibrin clots within 5-7 h. Detailed scanning electron microscopy (SEM) analysis of the fibrin network shows significant morphology modification including formation of large voids and strand termini, indicating degradation of fibrin protofibril by Au-Cu@PANI. The dye 1,3-diphenylisobenzofuran (DPBF) used to detect the presence of 1O2 shows a 27% bleaching of the absorption at λ = 418 nm within 75 min of irradiation of an aqueous Au-Cu@PANI solution in air. Moreover, electron paramagnetic resonance (EPR) spin-trapping experiments reveal a hyperfine-coupled triplet signature at room temperature with intensities 1:1:1: and g-value = 2.0057, characteristic of the reaction between the spin probe 4-Oxo-TEMP and 1O2 during irradiation. Controlled 1O2 scavenging experiments by NaN3 show 82% reduction in the spin-trapped EPR signal area. Both DPBF bleaching and EPR spin trapping indicate that in situ generated 1O2 is responsible for fibrin strand scission. This unique nanomaterial function via use of ubiquitous oxygen as a reagent could open creative avenues for future in vivo biomedical applications to treat fibrin clot diseases.

Construction of Flexible‐on‐Rigid Hybrid‐Phase Metal–Organic Frameworks for Controllable Multi‐Drug Delivery

Multi-component MOFs contain multiple sets of unique and hierarchical pores, with different functions for different applications, distributed in their inter-linked domains. Herein, we report the construction of a class of precisely aligned flexible-on-rigid hybrid-phase MOFs with a unique rods-on-octahedron morphology. We demonstrated that hybrid-phase MOFs can be constructed based on two prerequisites: the partially matched topology at the interface of the two frameworks, and the structural flexibility of MOFs with acs topology, which can compensate for the differences in lattice parameters. Furthermore, we achieved domain selective loading of multiple guest molecules into the hybrid-phase MOF, as observed by scanning transmission electron microscopy-energy-dispersive X-ray spectrometry elemental mapping. Most importantly, we successfully applied the constructed hybrid-phase MOF to develop a dual-drug delivery system with controllable loading ratio and release kinetics.

First results from in situ transmission electron microscopy studies of all-solid-state fluoride ion batteries

A focused ion beam (FIB) system is used to fabricate a micron-sized all-solid-state fluoride ion cell from a bulk battery for in situ transmission electron microscopy (TEM) testing. The bulk battery is based on a La0·9Ba0·1F2.9 solid-state electrolyte with a nanocomposite of Cu/C as a cathode and a nanocomposite of MgF2, Mg, La0·9Ba0·1F2.9 and C as an anode. The evolution of the morphology, structure, and composition of the electrodes and their interfaces with the electrolyte is characterized using in-situ TEM during electrochemical cycling. The high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy-energy dispersive X-ray (STEM-EDX) analysis of the cathode-electrolyte interface reveal the expected formation of CuF2 phase during charging. During cycling, grain growth of Cu in the cathode ingredients and Cu diffusion from the cathode into the electrolyte are observed in addition to void formation.

Structural and electrical characterization of hydrothermally deposited piezoelectric (K,Na)(Nb,Ta)O3 thick films

(K0.89Na0.11)(Nb0.85Ta0.15)O3 thick films were epitaxially grown at 200 °C on (001)La:SrTiO3 and (001)cSrRuO3//(001)SrTiO3 substrates by hydrothermal method, and their crystal structures and electrical properties were investigated. Film thickness increased with deposition time and reached 6 µm in 10 h. High-temperature X-ray diffraction measurement showed that successive phase transitions from orthorhombic to tetragonal and from tetragonal to cubic phases take place at 120 and 400 °C, respectively. Microstructure analyses were performed by using electron microscopy, which revealed the existence of two types of stripe patterns with a width of 100 nm or less. In addition, scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy elemental mapping showed that Nb/(Nb + Ta) ratio of the deposited films abruptly changed around 700 nm in thickness. Annealing at 500 °C led to the reduction in leakage current density from 102 to 10–5 A/cm2 at 30 kV/cm, showing that annealing is an effective way to improve insulation. Relative dielectric constant (er) decreased linearly with increasing frequency, reaching 450 at 10 kHz. Polarization–electric field hysteresis loop and field-induced stain curve were measured by piezoelectric force microscopy, which showed remanent polarization (Pr) of 30 µC/cm2 and piezoelectric constant (d33,PFM) of 70 pm/V. These results demonstrate that (K,Na)(Nb,Ta)O3 thick films with superior electrical properties can be fabricated by the low-temperature deposition technique.

Direct Visualization of Arsenic Binding on Green Rust Sulfate.

"Green rust" (GR), a redox-active Fe(II)-Fe(III) layered double hydroxide, is a potential environmentally relevant mineral substrate for arsenic (As) sequestration in reduced, subsurface environments. GR phases have high As uptake capacities at circum-neutral pH conditions, but the exact interaction mechanism between the GR phases and As species is still poorly understood. Here, we documented the bonding and interaction mechanisms between GR sulfate and As species [As(III) and As(V)] under anoxic and circum-neutral pH conditions through scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray (EDX) spectroscopy and combined it with synchrotron-based X-ray total scattering, pair distribution function (PDF) analysis, and As K-edge X-ray absorption spectroscopy (XAS). Our highly spatially resolved STEM-EDX data revealed that the preferred adsorption sites of both As(III) and As(V) are at GR crystal edges. Combining this data with differential PDF and XAS allowed us to conclude that As adsorption occurs primarily as bidentate binuclear (2C) inner-sphere surface complexes. In the As(III)-reacted GR sulfate, no secondary Fe-As phases were observed. However, authigenic parasymplesite (ferrous arsenate nanophase), exhibiting a threadlike morphology, formed in the As(V)-reacted GR sulfate and acts as an additional immobilization pathway for As(V) (∼87% of immobilized As). We demonstrate that only by combining high-resolution STEM imaging and EDX mapping with the bulk (differential) PDF and extended X-ray absorption fine structure (EXAFS) data can one truly determine the de facto As binding nature on GR surfaces. More importantly, these new insights into As-GR interaction mechanisms highlight the impact of GR phases on As sequestration in anoxic subsurface environments.

High-temperature structural change and microtexture formation of sillimanite and its phase relation with mullite

Abstract Synchrotron powder X-ray diffraction (XRD) experiments and transmission electron microscopy (TEM) observations of heat-treated sillimanite at various pressures were conducted to clarify the detailed phase relation between sillimanite and mullite. Under TEM, heat-treated sillimanite frequently showed anti-phase boundary (APB)-like textures with a displacement vector of ½[001]sil. Additional scanning TEM energy-dispersive X-ray spectroscopy analysis of regions with APB-like texture showed that they were clearly enriched in Al and accompanied by very fine, Si-rich glass inclusions, which indicates that the APB-like textures are composed of fine mullite. Moreover, synchrotron XRD patterns of these samples clearly showed double peaks of newly formed mullite and remnant sillimanite, indicating that the compositional transformation from sillimanite to mullite and glass is discontinuous. We separately determined the cell parameters of the sillimanite and mullite from the XRD pattern and found that the b axial length of the sillimanite increased with the treatment temperature, reflecting disordering of tetrahedral Al and Si in the sillimanite. In contrast, the positions of the deconvoluted mullite peaks indicated that the a axial length of mullite decreased as experimental pressure increased, owing to enrichment of the Si component. By projecting the cell parameters onto the a–b axial plane, the detailed changes in the crystallographic state of the sillimanite and mullite could be easily and comprehensively identified. On the basis of our results, we propose a new P-T diagram for the Al2SiO5 system that shows the transformation boundary between sillimanite and mullite + SiO2-rich melt and the contour of the Al/Si order parameter of sillimanite.

Obtaining Structural Parameters from STEM–EDX Maps of Core/Shell Nanocrystals for Optoelectronics

Characterization efforts of core/shell and core/multishell nanocrystals have struggled to quantitatively evaluate the interface width between the core and shell materials despite its importance in their optoelectronic properties. Here, we demonstrate a scanning transmission electron microscopy (STEM) method for measuring the radial elemental composition of two spherical core/shell nanocrystal systems, Ge/Si core/shell and CdSe/CdS/ZnS core/double-shell nanocrystals. By fitting model-based radial distributions of elements to measured STEM–energy-dispersive X-ray (EDX) maps, this method yields reliable and accurate measurements of interface broadening as well as core and shell sizes, surface roughness, and the fraction of core material in the shell. The direct evaluation of the structural parameters is an important step toward improving the synthesis of core/shell nanocrystals and optimizing their optoelectronic properties.

Studies of x-ray localization and thickness dependence in atomic-scale elemental mapping by STEM energy-dispersive x-ray spectroscopy using single-frame scanning method

The delocalization of x-ray signals limits the spatial resolution in atomic-scale elemental mapping by scanning transmission electron microscopy (STEM) using energy-dispersive x-ray spectroscopy (EDS). In this study, using a SrTiO3 [001] single crystal, we show that the x-ray localization to atomic columns is strongly dependent on crystal thickness, and a thin crystal is critical for improving the spatial resolution in atomic-scale EDS mapping. A single-frame scanning technique is used in this study instead of the multiple-frame technique to avoid peak broadening due to tracking error. The strong thickness dependence is realized by measuring the full width at half maxima (FWHM) as well as the peak-to-valley (P/V) ratio of the EDS profiles for Ti K and Sr K + L, obtained at several crystal thicknesses. A FWHM of about 0.16 nm and a P/V ratio of greater than 7.0 are obtained for Ti K for a crystal thickness of less than 20 nm. With increasing crystal thickness, the FWHM and P/V ratio increases and decreases, respectively, indicating the advantage of using a thin crystal for high-resolution EDS mapping.