High-resolution High-angle Annular Dark Field Research Articles

Fabrication and Excellent Performances of Bismuth Telluride-Based Thermoelectric Devices.

The barrier layer between thermoelectric (TE) legs and electrodes has crucial impact on the electrothermal conversion efficiency of the TE device; however, the interfacial reaction of the Ni metal barrier layer with TE legs in traditional Bi2Te3-based devices is harmful to the device performance. Herein, a high-quality barrier layer of a Ni-based alloy has been fabricated on both n-type and p-type Bi2Te3-based TE legs by the electroplating method. The in situ XRD results indicate that the as-prepared Bi2Te3-based TE legs with a Ni-based alloy barrier layer remain stable even at 300 °C. The high-resolution high-angle annular dark field scanning transmission electron microscopy images reveal that the Ni-based alloy barrier layer has more excellent stability than that of the Ni metal barrier layer. The Bi2Te3-based TE devices with excellent structural and performance stabilities were assembled with the as-grown high-performance n-type and p-type Bi2Te3-based leg with a Ni-based alloy barrier layer, which have lower internal resistance and higher cooling and power generation performances. A maximum cooling temperature difference over 65 K and a maximum cooling capacity of 55 W were obtained for the high-performance Bi2Te3-based TE devices. This work provides a new strategy for high-temperature applications of commercial Bi2Te3-based TE devices.

Understanding the formation of antiphase boundaries in layered oxide cathode materials and their evolution upon electrochemical cycling

Understanding the formation of antiphase boundaries in layered oxide cathode materials and their evolution upon electrochemical cycling

Structural atomic study in platinum heterogeneous catalyst by aberration-corrected STEM

The atomic and structural analysis of platinum nanoparticles (PtNPs) heterogeneous catalysts was carried out using the scanning transmission electron microscopy with spherical aberration-corrected (Cs-STEM) technique. The corrected spherical aberration coefficients in the convergent electron beam under focus and over focus made it possible to obtain subangstrom resolution to perfectly identify positions and atomic columns in PtNPs supported on magnetite (Fe3O4). The images formed in high-angle annular dark field (HAADF), allowed to observe PtNPs < 2 nm, crystalline defects, steps, twin boundaries and disorder in the atomic columns, as well as an estimate of the number of atoms contains in a cluster of PtNPs according to our 3D simulation. Hence, the perspective of this work is to study high resolution HAADF images to extract as much information as possible according to the challenges and opportunities involved the use and development of new systems used in nano-catalysis.

HR-STEM investigation of atomic lattice defects in different types of η precipitates in creep-age forming Al–Zn–Mg–Cu aluminium alloy

High-resolution (HR) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has revealed the atomic lattice defects in different types of η precipitates in the Al–Zn–Mg–Cu aluminium alloy subjected to creep-age forming treatment (with a constant stress lower than its room-temperature yield strength during ageing). Along the zone axes of [110] Al //[2 1 ‾ 1 ‾ 0]η of η 1 and η 12 , [112] Al //[2 1 ‾ 1 ‾ 0]η of η 2 and [100] Al //[2 1 ‾ 1 ‾ 0]η of η 13 , atomic projections of (2 1 ‾ 1 ‾ 0)η have been investigated. In those types of η, elongated hexagonal lattice defects (labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings. Furthermore, in η 12 , elongated hexagonal lattice defects with a much higher aspect ratio (labelled as Type II defects) are uniquely observed. These atomic lattice defects are presumably pertinent to the lattice accommodation in the course of creep-age forming. Additionally, in η 1 and η 12 , the features of a Penrose tiling defect connecting with Type I defects are observed, and these complex defects obviously affect the growth direction of the precipitate, resulting in a nearly spherical morphology. Alternatively, several entirely-passed faulted layers in a new type of precipitate, η 14 , consequently bring about a new orientation relationship: (51 3 ‾ ) Al //(0001)η 14 and [112] Al //[2 1 ‾ 1 ‾ 0]η 14 . Moreover, in an atomic STEM image of η 14 , the significant Z-contrast gradient adjacent to the transformation front of η 14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {1 1 ‾ 1 ‾ } Al planes at the interface. • Along the [2110]η zone axis, the edge-on atomic configurations of η 1 , η 2 , η 12 , η 13 and η 14 precipitates have been investigated to explore their atomic lattice defects. • Within the spherical-like morphologies of η 1 and η 12 , Penrose tiling features jointed with elongated hexagonal lattice defects were identified. • In η 12 precipitates, two types of elongated hexagonal lattice defects of different sizes were found. • Differently-oriented elongated hexagonal lattice defects presumably affect the local stackings, bringing about differently-oriented types of η precipitates. • In the newly-typed η 14 precipitate, some entirely-passed stacking fault layers presumably cause a new specific orientation relationship with respect to the Al matrix.

Tuning the electromechanical properties and polarization of Aluminium Nitride by ion beam-induced point defects

We report the evolution of uniaxial strain, resulting in an expansion of the c-axis in the wurtzite structure by up to 1 %, without significant degradation of the crystal structure of 30 keV Zr+ implanted epitaxial AlN films, grown on Si substrates. Raman and X-ray absorption spectroscopies demonstrated that the dominant defects are ZrAl, VAl and VN. The uniaxial strain can be attributed to a weakening of the Metal-N π bond along the c-axis. Monte Carlo simulations further predict the formation of a cation-rich region within the Zr implantation range, along with a buried anion-rich layer for all investigated fluences. The anion-rich layer undergoes a polarity inversion, which was experimentally confirmed by high-resolution high-angle annular dark field scanning transmission electron microscopy. Those microstructural changes influence the macroscopic electromechanical properties of AlN. The effective piezoelectric coefficient, d33, reduces from (7.0 ± 0.5) pm/V to (5.2 ± 0.5) pm/V at a fluence of 1015 at./cm2 Zr+. At higher fluences AlN undergoes a strain- and compositionally-induced polarity change, and the piezoelectric coefficient decreases due to crystalline damage. These results provide a pathway to optimise the performance of AlN by ion implantation for applications such as energy harvesting, light-emitting diodes and acoustic wave devices. In addition, the capability to engineer a buried polarisation inversion after AlN growth may enable novel device design.

Triangular Single Shockley Stacking Fault Analyses on 4H-SiC PiN Diode with Forward Voltage Degradation

The structure of partial dislocations (PDs), which surround triangular single Shockley stacking faults (1SSFs) expanded during electroluminescence observation in a 4H-SiC PiN diode, is investigated by photoluminescence (PL) imaging, PL spectroscopy, plan-view transmission electron microscopy (TEM) imaging, g·b analysis, cross-sectional bright-field scanning TEM (BF-STEM) imaging, and high-resolution high-angle annular dark-field (HAADF)-STEM imaging. The zigzag configuration is found on the upstanding side of the triangular 1SSF by plan-view TEM imaging, while it is observed as a single line by PL imaging. Using g·b analysis, the zigzag line is confirmed to be constructed by 30° and 90° PDs. Cross-sectional BF-STEM observation proved that the expanding 1SSF penetrates through the p–n interface whose position was estimated from secondary-ion mass spectrometry (SIMS) depth profile measurements of the aluminum and nitrogen concentrations. This provides experimental evidence that the 1SSF loses its driving force for recombination-enhanced dislocation glide when it stops somewhere in the p-layer where sufficient electrons no longer exist. Using cross-sectional HAADF-STEM observation, core species of the PDs were analyzed and the Burgers vector for the dislocation loop uniquely determined. Also, the first experimental observation of a 90° silicon-core PD, reported so far, is confirmed. According to these PD analyses, the possible atomic arrangement around the 1SSF is examined and the way tetrahedrons face across each PD is summarized, based on HAADF-STEM analysis of the 1SSF line.

Direct Observation of Modulation Structure in Room Temperature Multiferroic Bi &lt;sub&gt;4.2&lt;/sub&gt;K &lt;sub&gt;0.8&lt;/sub&gt;Fe &lt;sub&gt;2&lt;/sub&gt;O &lt;sub&gt;9+δ&lt;/sub&gt;

Microstructure features in correlation with the incommensurate modulation and potassium position in Bi4.2K0.8Fe2O9+δ (BKFO) nanobelts were studied by scanning transmission electron microscopy (STEM). Atomic displacements following the modulation wave were well characterized by a wave function for each atomic layer. The potassium position in perovskite layers can be directly observed with the incoherent Z-contrast imaging technique. Present results show that high-resolution high-angle annular dark-field (HAADF) imaging provides a powerful tool to study chemical and structural nature of this material on the atomic scale.

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation.

The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties of following in real time the heterogeneous nucleation and predicting structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the heteronucleation process involved in one-pot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution high-angle annular dark field (HAADF)-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets, or hourglass-like particles) were grown by reducing cobalt carboxylate in liquid polyols in the presence of iridium or ruthenium chloride as the nucleating agent. A reaction scheme was established by monitoring the H2 evolution resulting from the decomposition of metal hydrides, formed in situ by β-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds made evident two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps, and the microstructure is discussed.

The maximum a posteriori probability rule for atom column detection from HAADF STEM images

Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images. The method combines statistical parameter estimation and model-order selection using a Bayesian framework and has been shown to be especially useful for the analysis of the structure of beam-sensitive nanomaterials. In order to avoid beam damage, images of such materials are usually acquired using a limited incoming electron dose resulting in a low contrast-to-noise ratio (CNR) which makes visual inspection unreliable. This creates a need for an objective and quantitative approach. The present paper describes the methodology of the MAP probability rule, gives its step-by-step derivation and discusses its algorithmic implementation for atom column detection. In addition, simulation results are presented showing that the performance of the MAP probability rule to detect the correct number of atomic columns from HAADF STEM images is superior to that of other model-order selection criteria, including the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Moreover, the MAP probability rule is used as a tool to evaluate the relation between STEM image quality measures and atom detectability resulting in the introduction of the so-called integrated CNR (ICNR) as a new image quality measure that better correlates with atom detectability than conventional measures such as signal-to-noise ratio (SNR) and CNR.

Atomic scale analyses of {\bb Z}-module defects in an NiZr alloy.

Some specific structures of intermetallic alloys, like approximants of quasicrystals, have their unit cells and most of their atoms located on a periodic fraction of the nodes of a unique {\bb Z}-module [a set of the irrational projections of the nodes of a (N > 3-dimensional) lattice]. Those hidden internal symmetries generate possible new kinds of defects like coherent twins, translation defects and so-called module dislocations that have already been discussed elsewhere [Quiquandon et al. (2016). Acta Cryst. A72, 55-61; Sirindil et al. (2017). Acta Cryst. A73, 427-437]. Presented here are electron microscopy observations of the orthorhombic phase NiZr - and its low-temperature monoclinic variant - which reveal the existence of such defects based on the underlying {\bb Z}-module generated by the five vertices of the regular pentagon. New high-resolution electron microscopy (HREM) and scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) observations demonstrate the agreement between the geometrical description of the structure in five dimensions and the experimental observations of fivefold twins and translation defects.

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

Summary The most striking difference between the trivalent rare-earth cations and their alkali and alkaline-earth peers is the presence of localized 4 f electrons. Placing trivalent rare-earth cations inside fullerene molecules or in between blocks of intermetallics has given rise to a plethora of fascinating properties and materials. A long-sought-after yet undiscovered material is a semiconducting compound with rare-earth cations situated inside the oversized polyhedral cages of a three-dimensional framework. In this work, we present a synthesis of unconventional clathrates, Ba 8−x R x Cu 16 P 30 , with the smallest pentagonal dodecahedral cages encapsulating La 3+ and Ce 3+ . Their composition and crystal structure were unambiguously determined by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances hole mobility and reduces hole concentration, resulting in a drastic (700%) increase in the thermoelectric performance.

Surface relaxation of strained Ga(P,As)/GaP heterostructures investigated by HAADF STEM.

The surfaces of thin transmission electron microscopy (TEM) specimens of strained heterostructures can relax. The resulting bending of the lattice planes significantly influences high-angle annular dark field (HAADF) measurements. We investigate the impact by evaluating the intensities measured at the atomic columns as well as their positions in high-resolution HAADF images. In addition, the consequences in the diffraction plane will be addressed by simulated position averaged convergent beam electron diffraction (PACBED) patterns. The experimental column intensities and positions acquired from a strained Ga(P,As) quantum well (QW) embedded in a in a GaP matrix agree very well with frozen phonon contrast simulations, if the surface relaxation is taken into account by finite element relaxation. Neglecting the surface relaxation the As content of the QW can be significantly underestimated. Taking the effects into account correctly, we find that the lower interface of the investigated Ga(P,As) QW is atomically abrupt whereas the upper one is smeared out.

Aberration corrected STEM by means of diffraction gratings

In the past 15 years, the advent of aberration correction technology in electron microscopy has enabled materials analysis on the atomic scale. This is made possible by precise arrangements of multipole electrodes and magnetic solenoids to compensate the aberrations inherent to any focusing element of an electron microscope. Here, we describe an alternative method to correct for the spherical aberration of the objective lens in scanning transmission electron microscopy (STEM) using a passive, nanofabricated diffractive optical element. This holographic device is installed in the probe forming aperture of a conventional electron microscope and can be designed to remove arbitrarily complex aberrations from the electron's wave front. In this work, we show a proof-of-principle experiment that demonstrates successful correction of the spherical aberration in STEM by means of such a grating corrector (GCOR). Our GCOR enables us to record aberration-corrected high-resolution high-angle annular dark field (HAADF-) STEM images, although yet without advancement in probe current and resolution. Improvements in this technology could provide an economical solution for aberration-corrected high-resolution STEM in certain use scenarios.

Nucleation and thermal stability of an icosahedral nanophase during the early crystallization stage in Zr-Co-Cu-Al metallic glasses

The early stage of crystallization in amorphous phases is a key step in the overall microstructural evolution. In a Zr-based amorphous alloy the formation of metastable nanophase was observed at the earliest crystallization stage, with an ultra-high nucleation density of 6.5 × 1024 m−3 and a size of ∼1–2 nm. The nanophase has an icosahedral structure and exhibits an extremely high thermal stability up to 150 K above the crystallization onset. The chemical and structural heterogeneity in the early stage of the glass crystallization were observed using high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), which can be applied to study materials with atomic resolution. A nucleation kinetics analysis of the nanophase formation accounts for the high product number density and indicates a low interface energy value between the metastable clusters and the amorphous matrix, σa,i of 14 mJ/m2. The thermal stability and resistance to size change by growth or coarsening are attributed to a stalled growth due to solute depletion and the low σa,i value. With the high nucleation rate and the low barrier, it is remarkable that the kinetics follow conventional nucleation theory.

P–n Crossed Nanojunctions from Electroless-Etched Si Nanowires

Electroless etching was used to prepare p- and n-Si nanowire (NW) arrays from Si wafers, with the lengths of the Si NW arrays varied by controlling the etching time. The etching rate was ∼150 nm/min up to 300 min but decreased notably, to 32 nm/min, thereafter. Transmission electron microscopy confirmed that the Si NWs were single-crystalline and aligned along the [001] direction. The dumbbell separation of 0.13 nm, observed using high-resolution high-angle annular-dark field scanning transmission electron microscopy, corresponded to Si atom arrangements projected along the [1–10] zone axis. A statistical study revealed that the average carrier concentrations of the p- and n-Si NW devices were 1.15 × 1018 and 2.61 × 1017 cm–3, respectively, while the average carrier mobilities were 6.66 × 10–3 and 5.41 × 10–3 cm2 V–1 s–1, respectively. A subsequently prepared p–n Si NW crossed nanojunction exhibited rectifying behavior typical of a p–n junction. Thus the preparation of Si NW arrays from Si wafers through ...

Influence of severe plastic deformation on intermetallic particles in Mg-12 wt.%Zn alloy investigated using transmission electron microscopy

The in-depth microstructural characterization of intermetallic particles in an Mg-12wt.%Zn binary alloy subjected to a severe plastic deformation is presented. The alloy was processed by four passes via equal channel angular pressing with an applied back pressure at a gradually decreasing temperature and analyzed using transmission electron microscopy techniques to observe the influence of processing on intermetallic particles. The results are compared with the initial state of the material prior to severe plastic deformation.The microstructural evolution of the α-Mg matrix and the Mg21Zn25, Mg51Zn20 and MgZn2 was analyzed using bright field imaging, selected area electron diffraction, high-resolution transmission electron microscopy and high-angle annular dark field imaging in scanning mode. The plastic deformation process influenced the α-Mg matrix and each type of intermetallic particle. The α-Mg matrix consisted of two types of areas. The first type of area had a highly deformed structure, and the second type of area had a partially recrystallized structure with an average grain size of approximately 250nm. The Mg21Zn25 microparticles exhibited distinct forms in the α-Mg matrix that were characterized as a single-crystalline form, a nano-crystalline form and a broken up form. No evidence of Mg51Zn20 nanoparticles within the α-Mg matrix was found in the microstructure, which indicates their dissolution or phase transformation during the deformation process. MgZn2 nanoparticles exhibited different behavior in both types of α-Mg matrix. Two orientation relationships toward the highly deformed α-Mg matrix were observed; however, there was no relationship toward the partially recrystallized α-Mg matrix. Additionally, the growth of the MgZn2 nanoparticles was different in the two types of α-Mg matrix. The Mg51Zn20 nanoparticles inside Mg21Zn25 microparticles exhibited a distinct behavior within the single-crystalline or nano-crystalline form of the parent Mg21Zn25 microparticles. The Mg21Zn25+Mg51Zn20 eutectic compound was confirmed for both forms of the parent Mg21Zn25 microparticles, and the growth of Mg51Zn20 nanoparticles is discussed.

Quantitative Two-Dimensional (2D) Morphology–Selectivity Relationship of CoMoS Nanolayers: A Combined High-Resolution High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HR HAADF-STEM) and Density Functional Theory (DFT) Study

Cobalt-promoted and nonpromoted MoS2 nanolayers supported on alumina are prepared and activated under various sulfidation (temperature/pressure (T, P)) conditions which induce the formation of nanolayers with two-dimensional (2D) morphology of MoS2 tuned by the presence of the promoter and by the sulfidation conditions. An unprecedented high selectivity is found for the CoMoS nanolayers. The origin of this selectivity is explained by 2D morphology effects quantified by high-resolution scanning transmission electron microscopy in high-angle annular dark field mode (HR HAADF-STEM) and density functional theory (DFT) calculations. A quantitative structure–selectivity relationship is identified between the 2D shape index of CoMoS nanolayers and their selectivity performances. This 2D shape index is determined by statistical analysis of the CoMoS nanolayers identified after principal component analysis processing of HR HAADF-STEM images. It is shown that this shape index, reflecting the isotropic/anisotropic d...

MOVPE growth studies of Ga(NAsP)/(BGa)(AsP) multi quantum well heterostructures (MQWH) for the monolithic integration of laser structures on (001) Si-substrates

High-quality, pseudomorphically strained Ga(NAsP)/(BGa)(AsP)-multiple quantum well heterostructures (MQWH) have been deposited on exactly oriented (001) Si-substrate by metal organic vapour phase epitaxy (MOVPE) in a wide temperature range between 525°C and 700°C. The individual atomic incorporation efficiencies, growth rates as well as nanoscale material properties have been clarified by applying detailed high-resolution X-ray diffraction (HR-XRD), photoluminescence (PL) spectroscopy and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) studies. An almost constant N-incorporation efficiency is obtained for a wide growth temperature range from 550°C up to 650°C. The P-incorporation is steadily increasing with increasing growth temperature reaching values at high temperatures in excess of the applied gas phase ratio. While the lower interface from the binary GaP- to the quaternary Ga(NAsP)-material system is very sharp, the upper interface is significantly rougher with a roughness scale of ±0.43nm in quantum well thickness variation at a growth temperature of 525°C. This roughness scale increases steadily with increasing growth temperature. No indication of any phase separation effects is detected in the Ga(NAsP)-material system even at the highest growth temperature of 700°C. The obtained experimental results are briefly discussed with respect to the anticipated metastable character of the novel dilute-nitride Ga(NAsP)-material system grown lattice-matched to (001) Si-substrate.

Strain mapping accuracy improvement using super-resolution techniques.

Super-resolution (SR) software-based techniques aim at generating a final image by combining several noisy frames with lower resolution from the same scene. A comparative study on high-resolution high-angle annular dark field images of InAs/GaAs QDs has been carried out in order to evaluate the performance of the SR technique. The obtained SR images present enhanced resolution and higher signal-to-noise (SNR) ratio and sharpness regarding the experimental images. In addition, SR is also applied in the field of strain analysis using digital image processing applications such as geometrical phase analysis and peak pairs analysis. The precision of the strain mappings can be improved when SR methodologies are applied to experimental images.

Quantification of Bi distribution in MOVPE-grown Ga(AsBi) via HAADF STEM

The importance of dilute bismide III/V semiconductors increases and their physical properties open up a wide range for applications. Therefore, high quality layers are required, what is difficult to achieve, as these alloys are highly metastable and phase separation can occur. We use HAADF (high angle annular dark field) imaging in aberration-corrected STEM (scanning transmission electron microscopy) to quantify the Bi distribution in MOVPE (metal organic vapor phase epitaxy) grown material at large length scales as well as down to the nanoscale. This is done for different Bi fractions in the solid, which are achieved by changing the MOVPE growth conditions. The composition of the Ga(AsBi) was determined by comparing frozen lattice annular dark field simulations of different Ga(AsBi) supercells with the experimental HAADF STEM images. The derived compositions are in quantitative agreement with results of HR-XRD (high resolution X-ray diffraction) and SIMS (secondary ion mass spectroscopy) of the same samples. We furthermore show a homogeneous Bi distribution for the investigated samples, which contain up to 5% Bi. By separating the group III intensities from the group V intensities in high resolution HAADF STEM images, we can investigate the group V intensity distribution only. Moreover from the statistical evaluation, we conclude that the Bi is distributed homogeneously across the group V lattice positions, confirming the excellent structural quality of the layers. This result is also important for device applications of dilute bismide alloys, as homogeneous layers are a prerequisite for optimizing optoelectronic applications.