Resolution Scanning Transmission Electron Microscopy Research Articles

Investigation of structural defects and beam induced transitions in MgV2O4 nanocrystals using atomic resolved scanning transmission electron microscopy.

Investigation of structural defects and beam induced transitions in MgV₂O₄ nanocrystals using atomic resolved scanning transmission electron microscopy.

Single indium atoms and few-atom indium clusters anchored onto graphene via silicon heteroatoms

Single atoms and few-atom nanoclusters are of high interest in catalysis and plasmonics, but pathways for their fabrication and stable placement remain scarce. We report here the self-assembly of room-temperature-stable single indium (In) atoms and few-atom In clusters (2-6 atoms) that are anchored to substitutional silicon (Si) impurity atoms in suspended monolayer graphene membranes. Using atomically resolved scanning transmission electron microscopy (STEM), we find that the exact atomic arrangements of the In atoms depend strongly on the original coordination of the Si anchors in the graphene lattice: Single In atoms and In clusters with 3-fold symmetry readily form on 3-fold coordinated Si atoms, whereas 4-fold symmetric clusters are found attached to 4-fold coordinated Si atoms. All structures are produced by our fabrication route without the requirement for electron-beam induced materials modification. In turn, when activated by electron beam irradiation in the STEM, we observe in situ the formation, restructuring and translation dynamics of the Si-anchored In structures: Hexagon-centered 4-fold symmetric In clusters can (reversibly) transform into In chains or In dimers, whereas C-centered 3-fold symmetric In clusters can move along the zig-zag direction of the graphene lattice due to the migration of Si atoms during electron-beam irradiation, or transform to Si-anchored single In atoms. Our results provide a novel framework for the controlled self-assembly and heteroatomic anchoring of single atoms and few-atom clusters on graphene.

Effect of Sc-Segregation on Transport and Durability Properties of Solid Oxide Fuel Cells

In solid oxide fuel cells (SOFC), strontium zirconate (SrZrO3) insulating phase formation between Sr-containing cathode and yttria-stabilized zirconia (YSZ) electrolyte was a long-standing unsolved problem. Though a bilayer strategy with gadolinium doped ceria (GDC), or the use of samarium doped ceria (SDC) as a protective layer have been reported to improve stability and performance, the effectiveness of these processes at preventing secondary phase formation is still open to debate. In this study, focusing on Sc and Ce co-doped ZrO2 (ScCeSZ), we found that nanometer-scale Sc-segregation layers are present at co-sintered GDC/ScCeSZ interfaces. Utilizing atomic-column resolved scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), and electrochemical measurements, we successfully demonstrate that SOFC cells consisting of Sc-segregated interfaces show a remarkable increment of long-term durability.

Phase-Selective Synthesis of Ultrathin FeTe Nanoplates by Controllable Fe/Te Atom Ratio in the Growth Atmosphere.

Phase controllable synthesis of 2D materials is of significance for tuning related electrical, optical, and magnetic properties. Herein, the phase-controllable synthesis of tetragonal and hexagonal FeTe nanoplates has been realized by a rational control of the Fe/Te ratio in a chemical vapor deposition system. Using density functional theory calculations, it has been revealed that with the change of the Fe/Te ratio, the formation energy of active clusters changes, causing the phase-controllable synthesis of FeTe nanoplates. The thickness of the obtained FeTe nanoplates can be tuned down to the 2D limit (2.8nm for tetragonal and 1.4nm for hexagonal FeTe). X-ray diffraction pattern, transmission electron microscopy, and high resolution scanning transmission electron microscope analyses exhibit the high crystallinity of the as-grown FeTe nanoplates. The two kinds of FeTe nanoflakes show metallic behavior and good electrical conductivity, featuring 8.44 × 104 S m-1 for 9.8nm-thick tetragonal FeTe and 5.45 × 104 S m-1 for 7.6nm-thick hexagonal FeTe. The study provides an efficient and convenient route for tailoring the phases of FeTe nanoplates, which benefits to study phase-sensitive properties, and may pave the way for the synthesis of other multiphase 2D nanosheets with controllable phases.

A half-shear-half-shuffle mechanism and the single-layer twinning dislocation for [formula omitted] mode in hexagonal close-packed titanium

Among the twinning modes in hexagonal close-packed (HCP) metals, the mechanism for {112¯2}〈112¯3¯〉 mode is particularly confusing and controversial. In the literature reports, there are three possible second invariant planes, i.e. the K2 planes for {112¯2}〈112¯3¯〉 twinning mode: {112¯4¯} which has been widely accepted and corresponds to a three-layer zonal twinning dislocation; {112¯2¯} that is deemed unfavorable; and (0002) which has only been observed in atomistic simulations and corresponds to a single-layer twinning dislocation. {112¯4¯} was predicted by classical twinning theory and the experimentally measured magnitude of twinning shear s in titanium and zirconium seemed to agree well with the prediction. However, {112¯4¯} has never been verified in simulations which show that (0002) should be the K2 plane. This conflict has not been resolved due to the lack of experimental observation of the structure of twinning dislocations. In this work, scanning transmission electron microscopy (STEM) observations are conducted to resolve the twin boundary structure in deformed pure titanium on the atomic scale, combined with atomistic simulations. Atomic resolution STEM unambiguously shows that the twinning dislocation only involves a single twinning plane and the K2 plane is (0002), which is consistent with the atomistic simulations. The STEM results also reveal a half-shear-half-shuffle process which is manifested by a unique twin boundary structure generated by the glide of single-layer twinning dislocations. To explain these results, the lattice correspondences of all three K2 planes are examined in great detail. In particular, shear and shuffle required in the lattice transformations are analyzed inside the framework of classical theory. These analyses explain well why (0002) is the more favorable K2 plane than {112¯4¯} and {112¯2¯}, and properly resolve the conflict between the prediction of the classical twinning theory and the simulation results.

Picometer-Precision Atomic Position Tracking through Electron Microscopy.

The modern aberration-corrected scanning transmission electron microscopes (AC-STEM) have successfully achieved direct visualization of atomic columns with sub-angstrom resolution. With this significant progress, advanced image quantification and analysis are still at the early stages. In this work, we present the complete pathway for the metrology of atomic resolution scanning transmission electron microscopy (STEM) images. This includes (1) tips for acquiring high-quality STEM images; (2) denoising and drift-correction for enhancing measurement accuracy; (3) obtaining initial atomic positions; (4) indexing the atoms based on unit cell vectors; (5) quantifying the atom column positions with either 2D-Gaussian single peak fitting or (6) multi-peak fitting routines for slightly overlapping atomic columns; (7) quantification of lattice distortion/strain within the crystal structures or at the defects/interfaces where the lattice periodicity is disrupted; and (8) some common methods to visualize and present the analysis. Furthermore, a simple self-developed free MATLAB app (EASY-STEM) with a graphical user interface (GUI) will be presented. The GUI can assist in the analysis of STEM images without the need for writing dedicated analysis code or software. The advanced data analysis methods presented here can be applied for the local quantification of defect relaxations, local structural distortions, local phase transformations, and non-centrosymmetry in a wide range of materials.

Picometer-Precision Atomic Position Tracking through Electron Microscopy

The modern aberration-corrected scanning transmission electron microscopes (AC-STEM) have successfully achieved direct visualization of atomic columns with sub-angstrom resolution. With this significant progress, advanced image quantification and analysis are still at the early stages. In this work, we present the complete pathway for the metrology of atomic resolution scanning transmission electron microscopy (STEM) images. This includes (1) tips for acquiring high-quality STEM images; (2) denoising and drift-correction for enhancing measurement accuracy; (3) obtaining initial atomic positions; (4) indexing the atoms based on unit cell vectors; (5) quantifying the atom column positions with either 2D-Gaussian single peak fitting or (6) multi-peak fitting routines for slightly overlapping atomic columns; (7) quantification of lattice distortion/strain within the crystal structures or at the defects/interfaces where the lattice periodicity is disrupted; and (8) some common methods to visualize and present the analysis. Furthermore, a simple self-developed free MATLAB app (EASY-STEM) with a graphical user interface (GUI) will be presented. The GUI can assist in the analysis of STEM images without the need for writing dedicated analysis code or software. The advanced data analysis methods presented here can be applied for the local quantification of defect relaxations, local structural distortions, local phase transformations, and non-centrosymmetry in a wide range of materials.

Calibration of high magnification in the measurement of critical dimension by AFM and SEM

In the measurement of critical dimension (CD) by scanning electron microscopy (SEM) and atomic force microscopy (AFM), the measured linewidths of the CD patterns are different from the real values due to the edge effect of secondary electrons and the radius and half cone angle of the tips for AFM. In order to solve this problem, a certified reference material (CRM) with a series of CD lines having different linewidth from 20 nm to 60 nm was designed to calibrate the magnification. The linewidths of the CRM could be traceably certified by high resolution scanning transmission electron microscope from the lattice constant of Si(1 1 1) plane in the Si(1 0 0) substrate. As a result, the magnification of AFM and SEM can be calibrated from the certified linewidths of the CRM. The magnification correction factors of AFM and SEM showed the relative standard uncertainties of 1.3% and 0.9%, respectively. The linewidths of the CD lines in the target samples can be traceably determined from the calibrated magnification after calibrating the instruments using the CRM and a calibration program.

Microstructure formation and high temperature oxidation behavior of Ti-Al-Cr-Y-Si coatings on TiAl

The paper presents the application of Closed Hollow Cathode Physical Vapor Deposition (CHC-PVD) method for the depostion of Ti-Al-Cr-Y-Si coatings on γ-TiAl 48-2-2 alloy for high temperature oxidation protection. The study concerned the analysis of the coating’s growth mechanism, initial microstructure as well as phase transformations investigations using high resolution Scanning Transmission Electron Microscopy (STEM) and high temperature X-ray diffraction (HT-XRD). The coated alloy was subjected to high temperature oxidation test at 850 °C where a fivefold lower mass gain compared to bare 48-2-2 alloy was observed. Detailed microstructural investigations allowed to characterize the thermally grown oxide scale, which was found to be composed of nanometric layers of titania, equiaxed (Al,Cr)2O3 and columnar alumina. These investigations provided microstructural evidence for the Cr effect on the formation of Al2O3, which was postulated previously. Yttrium was found to segregate to the grain boundaries of alumina oxide scale during high temperature oxidation, indicating the occurence of the reactive element effect.

Lattice-Match Stabilization and Magnetic Properties of Metastable Epitaxial Permalloy-Disilicide Nanostructures on a Vicinal Si(111) Substrate.

We report the epitaxial formation of metastable γ-(FexNi1−x)Si2 nanostructure arrays resulting from the reaction of Ni80Fe20 permalloy with vicinal Si(111) surface atoms. We then explore the effect of structure and composition on the nanostructure’s magnetic properties. The low-temperature annealing (T < 600 °C) of a pre-deposited permalloy film led to solid-phase epitaxial nucleation of compact disk-shaped island nanostructures decorating <110> ledges of the stepped surface, with either (2 2) or () R30° reconstructed flat top faces. High resolution scanning transmission electron microscopy analysis demonstrated fully coherent epitaxy of the islands with respect to the substrate, consistent with a well-matched CaF2-prototype structure associated with γ-FeSi2, along perfect atomically sharp interfaces. Energy dispersive spectroscopy detected ternary composition of the islands, with Fe and Ni atoms confined to the islands, and no trace of segregation. Our magnetometry measurements revealed the superparamagnetic behavior of the silicide islands, with a blocking temperature around 30 K, reflecting the size, shape, and dilute arrangement of the islands in the assembly.

Resolving few-layer antimonene/graphene heterostructures

Two-dimensional (2D) antimony (Sb, “antimonene”) is of interest in electronics and batteries. Sb however exhibits a large allotropic structural diversity, which is also influenced by its support. Thus, Sb heterostructure formation is key in 2D Sb integration. Particularly, 2D Sb/graphene interfaces are important. We thus study here few-layered 2D Sb/graphene heterostructures with atomic resolution (scanning) transmission electron microscopy. We find two Sb morphologies to coexist: first, a 2D morphology of layered β-Sb with β-Sb(001)||graphene(001) texture. Second, one-dimensional Sb nanowires which can be matched to β-Sb[2-21]⊥graphene(001) and are closely related to cubic Sb(001)||graphene(001). Importantly, both Sb morphologies show rotational van-der-Waals epitaxy with graphene. Both are resilient against oxidation, although superficial Sb-oxide formation merits consideration, including epitaxial Sb2O3(111)/β-Sb(001) heterostructures. Exact Sb growth behavior depends on processing and substrate properties including, notably, the support underneath the graphene. Our work elucidates the rich phase and epitaxy landscape in 2D Sb and 2D Sb/graphene heterostructures.

Position-controlled remote epitaxy of ZnO for mass-transfer of as-deployed semiconductor microarrays

We report the site-selective remote epitaxial growth of mechanically transferable ZnO microrod (MR) and microdisk (MD) arrays via hydrothermal growth. To designate the growth sites, a hole-patterned poly(methyl methacrylate) mask layer is formed on the graphene-coated GaN substrate. ZnO microarrays are exclusively grown to be either MR or MD on graphene-exposed patterned areas via the remote epitaxy. The remote heteroepitaxial relation between ZnO and GaN across graphene is observed by atomic resolution scanning transmission electron microscopy. The non-covalent remote epitaxial interface allows the mechanical lift-off of the ZnO microarrays and mass-transfer onto a surface of interest using a sticky tape as those arrays are well maintained. The donor substrate is refurbished for repetitive position-controlled remote epitaxy. This study provides a simple method of fabricating mass-transferable microarrays of semiconductors that can maintain the addressable spatial arrays of semiconductors to an arbitrary receiver substrate for ease of heterogeneous integration without an additional assembly process for position control.

TEM characterization of dislocation loops in proton irradiated single crystal ThO2

• This paper reports the full characterization of dislocation loops in single crystal ThO 2 after 0.47 dpa proton irradiation at 600 o C. • Dislocation loops were found to be mainly edge type with different variants of faulted 1 / 3 〈 111 〉 Frank loops along { 111 } habit plane. • Inside-outside contrast technique and atomic resolution STEM-HAADF imaging revealed the interstitial nature of dislocation loops. This work focuses on the full characterization of dislocation loops induced by proton irradiation in single-crystal ThO 2 . Irradiation was performed using 2 MeV H + ions with sample temperature at 600 o C and a dose of up to 0.47 displacements per atom (dpa). Transmission electron microscopy (TEM) characterization was performed on a large number of dislocation loops. Burgers vector ( b → ) analysis using standard g → . b → = 0 invisibility criterion revealed different variants of 1 / 3 〈 111 〉 type dislocation loops. TEM analysis of edge-on dislocation loops was used to determine habit planes as { 111 } type. The nature of dislocation loops was revealed using the inside-outside contrast method as interstitial type. Rel-rod dark field images were obtained by selecting streak at g → = 1 / 2 [ 31 1 ¯ ] in diffraction pattern to identify the Frank loops present in the microstructure. Subsequent analysis of a single dislocation loop using atomic resolution scanning transmission electron microscopy (STEM) confirmed the interstitial nature of the loop with { 111 } habit plane.

Observations of multi-component boride precipitates in ultrahard boron carbide

Boron carbide (B4C) often consists of intrinsic secondary phase precipitates that are formed during the reaction-sintering process. These precipitates not only play an important role during the densification process but also affect the mechanical properties of B4C. In the present study, the structural determination of multicomponent secondary phases that are precipitated out in monolithic B4C was investigated using atomic resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT). Here we found two types (Type-I and II) of multicomponent boride nano-precipitates in B4C i.e. (i) elongated precipitates (type-I), identified as (Cr(1-x-y)-Fex-Wy)3B4 phase, rich in chromium (Cr), Iron (Fe), tungsten (W), and boron (B) (ii) equiaxial (type-II) precipitates that are rich in titanium (Ti), Cr, and B in form of Ti(1-x)CrxB2 phase. The estimated reduced modulus and hardness using depth sensitive nanoindentation on type-I precipitates were 335 ± 6 GPa and 32 ± 2 GPa and that in type-II precipitates were 268 ± 13 GPa and 22 ± 3 GPa, respectively. These results underscore the importance of multicomponent solid solution hardening in ultra-hard B4C material.

Epitaxial growth and transport properties of compressively-strained Ba2IrO4 films* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774153, 11861161004, 51772143, 11974163, and 51672125), the National Key Research and Development Program of China (Grant No. 2016YFA0201104), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 0213-14380167 and 0213-14380198), and the Hong Kong Research Grants Council

Ba2IrO4 is a sister compound of the widely investigated Sr2IrO4 and has no IrO6 octahedral rotation nor net canted antiferromagnetic moment, thus it acts as a system more similar to the high-T c cuprate. In this work, we synthesize the Ba2IrO4 epitaxial films by reactive molecular beam epitaxy and study their crystalline structure and transport properties under biaxial compressive strain. High resolution scanning transmission electron microscopy and x-ray diffraction confirm the high quality of films with partial strain relaxation. Under compressive epitaxial strain, the Ba2IrO4 exhibits the strain-driven enhancement of the conductivity, consistent with the band gap narrowing and the stronger hybridization of Ir-t2g and O-2p orbitals predicted in the first-principles calculations.

Predictability as a probe of manifest and latent physics: The case of atomic scale structural, chemical, and polarization behaviors in multiferroic Sm-doped BiFeO3

The predictability of a certain effect or phenomenon is often equated with the knowledge of relevant physical laws, typically understood as a functional or numerically derived relationship between the observations and known states of the system. Correspondingly, observations inconsistent with prior knowledge can be used to derive new knowledge on the nature of the system or indicate the presence of yet unknown mechanisms. Here, we explore the applicability of Gaussian processes (GP) to establish predictability and uncertainty of local behaviors from multimodal observations, providing an alternative to this classical paradigm. Using atomic resolution scanning transmission electron microscopy (STEM) of multiferroic Sm-doped BiFeO3 across a broad composition range, we directly visualize the atomic structure and structural, physical, and chemical order parameter fields for the material. GP regression is used to establish the predictability of the local polarization field from different groups of parameters, including the adjacent polarization values and several combinations of physical and chemical descriptors, including lattice parameters, column intensities, etc. We observe that certain elements of microstructure, including charged and uncharged domain walls and interfaces with the substrate, are best predicted with specific combinations of descriptors, and this predictability and associated uncertainties are consistent across the composition series. The associated generative physical mechanisms are discussed. It is also found that certain parameter combinations tend to predict the orthorhombic phase in the cases where rhombohedral phase is observed, suggesting a potential role of clamping and confinement phenomena in phase equilibrium in Sm-BiFeO3 system close to morphotropic phase boundary. We argue that predictability and uncertainty in observational data offer a new pathway to probe the physics of condensed matter systems from multimodal local observations.

New Order in (BiS)1.19(Bi1/3Cr2S4) Misfit Layer Compound.

AbstractIn a sample with nominal composition BiCr2S5 a new misfit layer compound has been synthesized and studied via transmission electron microscopy (TEM). It presents a composite modulated structure with a composition that can be formulated as (BiS)1.19[(CrS2)‐Bi1/3‐(CrS2)]. It consists of the periodic stacking of a Q layer (BiS) with two H layers (CrS2), …QHHQHH…, with additional bismuth atoms inserted between the H layers. Selected area electron diffraction (SAED) patterns show an almost commensurate fit between the pseudo‐tetragonal Q (BiS) and the pseudo‐orthohexagonal H (Bi1/3Cr2S4) subcells along the misfit direction a, being b the same for both sublattices and c the layers stacking direction. In addition to the characteristic misfit modulation between the two sub‐structures, a second modulation is observed in the H sub‐structure produced by the ordering of bismuth atoms within the Van der Waals gap that occurs between consecutive H layers. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) images show ordered stacking sequences between the (BiS) and (Bi1/3Cr2S4) layers. Besides, disordered intergrowths have been occasionally found as well as different kinds of twinning defects.

Au101-rGO nanocomposite: immobilization of phosphine-protected gold nanoclusters on reduced graphene oxide without aggregation.

Graphene supported transition metal clusters are of great interest for potential applications, such as catalysis, due to their unique properties. In this work, a simple approach to deposit Au101(PPh3)21Cl5 (Au101NC) on reduced graphene oxide (rGO) via an ex situ method is presented. Reduction of graphene oxide at native pH (pH ≈ 2) to rGO was performed under aqueous hydrothermal conditions. Decoration of rGO sheets with controlled content of 5 wt% Au was accomplished using only pre-synthesised Au101NC and rGO as precursors and methanol as solvent. High resolution scanning transmission electron microscopy indicated that the cluster size did not change upon deposition with an average diameter of 1.4 ± 0.4 nm. It was determined that the rGO reduction method was crucial to avoid agglomeration, with rGO reduced at pH ≈ 11 resulting in agglomeration. X-ray photoelectron spectroscopy was used to confirm the deposition of Au101NCs and show the presence of triphenyl phosphine ligands, which together with attenuated total reflectance Fourier transform infrared spectroscopy, advocates that the deposition of Au101NCs onto the surface of rGO was facilitated via non-covalent interactions with the phenyl groups of the ligands. Inductively coupled plasma mass spectrometry and thermogravimetric analysis were used to determine the gold loading and both agree with a gold loading of ca. 4.8–5 wt%. The presented simple and mild strategy demonstrates that good compatibility between size-specific phosphine protected gold clusters and rGO can prevent aggregation of the metal clusters. This work contributes towards producing an agglomeration-free synthesis of size-specific ligated gold clusters on rGO that could have wide range of applications.

Effect of hot deformation on microstructure and quenching-induced precipitation behavior of Al-Zn-Mg-Cu alloy

The effect of hot deformation on microstructure and quenching-induced precipitation behavior of an Al-Zn-Mg-Cu alloy 7055 was investigated by electron backscattered diffraction (EBSD), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM). In the solution heat treated and slowly-quenched alloy without hot deformation, there are equiaxed grains with low dislocation density, and there is major quenching-induced η (MgZn2) phase located at grain boundaries and on some Al3Zr dispersoids in the interior of grains and minor T(Al2Zn3Mg3) phase on Al3Zr dispersoids inside grains. While in the solution heat treated and slowly-quenched alloy after hot rolling, there are recrystallized grains and subgrains with higher dislocation density, and there is a larger amount of quenching-induced η and T phase on incoherent Al3Zr dispersoids inside recrystallized grains and at (sub) grain boundaries; moreover, quenching-induced S (Al2CuMg) phase and Zn-Cu rich Y phase are observed in the interior of subgrains. The precipitation behavior of these quenching-induced phases has been discussed primarily based on their chemical compositions, nucleation sites and different microstructure features in the solution heat treated alloy with and without hot deformation.

Green palladium nanoparticles prepared with glycerol and supported on maghemite for dye removal application

This study aims to synthesize palladium nanoparticles (PdNPs) in presence of glycerol as green reducing/stabilizing agent, supported on γ-Fe2O3 (maghemite), as catalysts for dye removal. Three materials were prepared with different Pd loadings, from 0.43 to 0.63 wt%. PdNPs at the surface were composed of Pd0, Pd2+ and Pd4+ species, at average oxidation state of +1.6 and Pd2+:Pd0 atomic ratio up to 1:0.9, which showed that glycerol was able to partially reduce Pd2+. PdNPs of spherical shaped clumps morphology, predominantly well dispersed on the γ-Fe2O3 surface were formed. High resolution scanning transmission electron microscopy images of the materials with higher Pd content revealed PdNPs smaller than 5 nm with crystal interplanar distance compatible with Pd(0). The catalysts were applied in the adsorption as well as in the heterogeneous Fenton degradation of the azo dye methyl orange with 90% of decolourization after 240 min. It was possible to observe an improvement in the catalytic activity with the Pd content increase as well as with the PdNPs size distribution. The dye removal followed pseudo-first-order kinetics. The effect of catalyst dosage and temperature were evaluated. The acidic character of the iron compounds as well as of the glycerol acidic hydrogens constituents of the catalyst can be released in the reaction medium, being responsible to adjust the reaction pH to 3, which is the optimum pH for Fenton process.