Composition measurement in substitutionally disordered materials by atomic resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy

Direct observation of space-charge-induced electric fields at oxide grain boundaries

Space charge layers (SCLs) formed at grain boundaries (GBs) are considered to critically influence the properties of polycrystalline materials such as ion conductivities. Despite the extensive researches on this issue, the presence of GB SCLs and their relationship with GB orientations, atomic-scale structures and impurity/solute segregation behaviors remain controversial, primarily due to the difficulties in directly observing charge distribution at GBs. In this study, we directly observe electric field distribution across the well-defined yttria-stabilized zirconia (YSZ) GBs by tilt-scan averaged differential phase contrast scanning transmission electron microscopy. Our observation clearly reveals the existence of SCLs across the YSZ GBs with nanometer precision, which are significantly varied depending on the GB orientations and the resultant core atomic structures. Moreover, the magnitude of SCLs show a strong correlation with yttrium segregation amounts. This study provides critical insights into the complex interplay between SCLs, orientations, atomic structures and segregation of GBs in ionic crystals.

Enhancing the Mechanical Properties of AM60B Magnesium Alloys through Graphene Addition: Characterization and Regression Analysis

The light weight and high strength of magnesium alloys have garnered significant attention, rendering them suitable for various applications across industries. Nevertheless, to meet industrial requirements, the mechanical properties must be improved. This investigation explores the potential of graphene addition to enhance the mechanical properties of AM60B magnesium alloy. Tests were conducted on samples with different weight percentages (wt.%) of graphene (0 wt.%, 0.1 wt.%, and 0.2 wt.%) using stir casting. The elongation and tensile strength of the composite materials were also assessed. The phase composition, particle size, and agglomeration phenomena were analyzed using characterization techniques such as X-ray diffraction, optical microscopy, and SEM-EDS. The yield strength of the magnesium alloy was enhanced by approximately 13.4% with the incorporation of 0.1 wt.% graphene compared to the alloy without graphene. Additionally, an 8.8% increase in elongation was observed. However, the alloy tensile properties were reduced by adding 0.2 wt.% graphene. The tensile fractography results indicated a higher probability of brittle fracture with 0.2 wt.% graphene. Furthermore, regression analysis employing machine learning techniques revealed the potential of predicting the stress-strain curve of composite materials.

Composition and Element Distribution Mapping of γ' and γ″ Phases of Inconel 718 by High-Resolution Scanning Transmission Electron Microscopy and X-ray Energy-Dispersive Spectrometry

The main strengthening mechanism for Inconel 718 (IN718), a Ni-based superalloy, is precipitation hardening by γ' and γ″ particles. It is thus essential, for good alloy performance, that precipitates with the desired chemical composition have adequate size and dispersion. The distribution of the γ' and γ″ phases and their chemical composition were investigated in the nickel-based Inconel 718 superalloy by taking advantage of the new capabilities of scanning transmission electron microscopy and energy-dispersive X-ray spectrometry using a windowless multiple detector, a high-brightness Schottky electron gun, and a spherical aberration corrector in the illumination probe optics. A small routine was developed to deconvolute the respective compositions of γ' and γ″ nanoprecipitates embedded in the γ matrix. Keeping the electron probe current low enough-a few hundred pA-prevented excessive irradiation damage during the acquisition of element maps and brought their spatial resolution down to the atomic column level to track their element compositions. The present results agree with and complement atomic probe tomography observations and Thermo-Calc predictions from the literature. The presence of an Al enrichment at the γ'/γ″ interface-which may control the γ″ phase coarsening-is observed in the last row of Al-Nb-Ti columns along this interface. In addition, a few columns with similar composition changes are found randomly distributed in the γ' phase.

Element Specific Atom Counting at the Atomic Scale by Combining High Angle Annular Dark Field Scanning Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy

A new methodology is presented to count the number of atoms in multimetallic nanocrystals by combining energy dispersive X-ray spectroscopy (EDX) and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). For this purpose, the existence of a linear relationship between the incoherent HAADF STEM and EDX images is exploited. Next to the number of atoms for each element in the atomic columns, the method also allows quantification of the error in the obtained number of atoms, which is of importance given the noisy nature of the acquired EDX signals. Using experimental images of an Au@Ag core-shell nanorod, it is demonstrated that 3D structural information can be extracted at the atomic scale. Furthermore, simulated data of an Au@Pt core-shell nanorod show the prospect to characterize heterogeneous nanostructures with adjacent atomic numbers.

Optimizing Experimental Conditions for Accurate Quantitative Energy-Dispersive X-ray Analysis of Interfaces at the Atomic Scale

The invention of silicon drift detectors has resulted in an unprecedented improvement in detection efficiency for energy-dispersive X-ray (EDX) spectroscopy in the scanning transmission electron microscope. The result is numerous beautiful atomic-scale maps, which provide insights into the internal structure of a variety of materials. However, the task still remains to understand exactly where the X-ray signal comes from and how accurately it can be quantified. Unfortunately, when crystals are aligned with a low-order zone axis parallel to the incident beam direction, as is necessary for atomic-resolution imaging, the electron beam channels. When the beam becomes localized in this way, the relationship between the concentration of a particular element and its spectroscopic X-ray signal is generally nonlinear. Here, we discuss the combined effect of both spatial integration and sample tilt for ameliorating the effects of channeling and improving the accuracy of EDX quantification. Both simulations and experimental results will be presented for a perovskite-based oxide interface. We examine how the scattering and spreading of the electron beam can lead to erroneous interpretation of interface compositions, and what approaches can be made to improve our understanding of the underlying atomic structure.

Atomic-Resolution EDX, HAADF, and EELS Study of GaAs1-xBix Alloys

The distribution of alloyed atoms in semiconductors often deviates from a random distribution which can have significant effects on the properties of the materials. In this study, scanning transmission electron microscopy techniques are employed to analyze the distribution of Bi in several distinctly MBE grown GaAs1-xBix alloys. Statistical quantification of atomic-resolution HAADF images, as well as numerical simulations, are employed to interpret the contrast from Bi-containing columns at atomically abrupt (001) GaAs-GaAsBi interface and the onset of CuPt-type ordering. Using monochromated EELS mapping, bulk plasmon energy red-shifts are examined in a sample exhibiting phase-separated domains. This suggests a simple method to investigate local GaAsBi unit-cell volume expansions and to complement standard X-ray-based lattice-strain measurements. Also, a single-variant CuPt-ordered GaAsBi sample grown on an offcut substrate is characterized with atomic scale compositional EDX mappings, and the order parameter is estimated. Finally, a GaAsBi alloy with a vertical Bi composition modulation is synthesized using a low substrate rotation rate. Atomically, resolved EDX and HAADF imaging shows that the usual CuPt-type ordering is further modulated along the [001] growth axis with a period of three lattice constants. These distinct GaAsBi samples exemplify the variety of Bi distributions that can be achieved in this alloy, shedding light on the incorporation mechanisms of Bi atoms and ways to further develop Bi-containing III-V semiconductors.

Transition-Metal Distribution in Brownmillerite Ca2FeCoO5

Ca₂Fe_2-xCo_xO₅ (0 ≤ x ≤ 1) with higher Co content, which crystallizes in a brownmillerite-type structure, is currently one of the best oxygen-evolution-reaction (OER) catalysts. Identifying the Fe/Co occupancies at the octahedral (O_h) and tetrahedral (T_d) sites in the structure is the foundation for the understanding of the role of cobalt in each site and the exploration of further improvement in the OER activity. Here, we investigate the Fe/Co distribution in Ca₂FeCoO₅ by means of atomic-resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy and dynamical image simulations combined with systematic density functional theory calculations. Our careful microscopic study reveals the absence of long-range Fe/Co order within the transition-metal (TM) layers, but cobalt is slightly enriched at the T_d and O_h sites in the as-synthesized (1100 °C) and 800 °C annealed for a month samples, respectively. The observed Co site preferences are interpretable from the viewpoints of TM ionic size effect and ligand field effect, which are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We also elucidate that the as-synthesized sample with Co enrichment at the T_d site shows the better OER activity, and the optimum annealing temperature for more OER active Ca₂FeCoO₅ should be higher than the crossover temperature.

Accuracy and precision of thickness determination from position-averaged convergent beam electron diffraction patterns using a single-parameter metric

Position-averaged convergent beam electron diffraction patterns are formed by averaging the transmission diffraction pattern while scanning an atomically-fine electron probe across a sample. Visual comparison between experimental and simulated patterns is increasingly being used for sample thickness determination. We explore automating the comparison via a simple sum square difference metric. The thickness determination is shown to be accurate (i.e. the best-guess deduced thickness generally concurs with the true thickness), though factors such as noise, mistilt and inelastic scattering reduce the precision (i.e. increase the uncertainty range). Notably, the precision tends to be higher for smaller probe-forming aperture angles.