Z-contrast Imaging Research Articles

Superior piezoelectricity in lead-free barium titanate piezoceramics

Since the 21st century, increasing environmental protection and human health concern have been the driving force to develop lead-free piezoelectric materials with enhanced performances, and phase engineering strategy has been validated to be a viable method in numerous methodologies. Here, we gained a superb d33∼(637 ± 30) pC/N in lead-free (1–x)(Ba0.93Ca0.07)(Sn0.08Ti0.92)O3-x(Sb0.5Li0.5)TiO3 [abbreviated as (1–x)BCST-xSLT, 0 ≤ x ≤ 0.4% (in mole)] piezoelectrics utilizing chemical doping. To illustrate the relationship among composition-structure-performance, microstructure characterization, electrical properties measurement, first-principles calculation, and phase-field simulations were performed. Atomic-resolved polarization mapping of z-contrast imaging manifests the ferroelectric three phases (RO–T) coexist at the nanoscale with nanoscale polarization switching among them. Theoretical calculations and simulations confirm that the high-density nano-domain boundary bridges the polyphase coexisting nano-domains, which makes the polarization reversal easy, thus significantly reducing the energy barrier and polarization anisotropy among different phases.

Reliable phase quantification in focused probe electron ptychography of thin materials

Electron ptychography provides highly sensitive, dose efficient phase images which can be corrected for aberrations after the data has been acquired. This is crucial when very precise quantification is required, such as with sensitivity to charge transfer due to bonding. Drift can now be essentially eliminated as a major impediment to focused probe ptychography, which benefits from the availability of easily interpretable simultaneous Z-contrast imaging. However challenges have remained when quantifying the ptychographic phases of atomic sites. The phase response of a single atom has a negative halo which can cause atoms to reduce in phase when brought closer together. When unaccounted for, as in integrating methods of quantification, this effect can completely obscure the effects of charge transfer. Here we provide a new method of quantification that overcomes this challenge, at least for 2D materials, and is robust to experimental parameters such as noise, sample tilt.

Experimental reconstructions of 3D atomic structures from electron microscopy images using a Bayesian genetic algorithm

We introduce a Bayesian genetic algorithm for reconstructing atomic models of monotype crystalline nanoparticles from a single projection using Z-contrast imaging. The number of atoms in a projected atomic column obtained from annular dark field scanning transmission electron microscopy images serves as an input for the initial three-dimensional model. The algorithm minimizes the energy of the structure while utilizing a priori information about the finite precision of the atom-counting results and neighbor-mass relations. The results show promising prospects for obtaining reliable reconstructions of beam-sensitive nanoparticles during dynamical processes from images acquired with sufficiently low incident electron doses.

Isolated-Oxygen-Vacancy Hardening in Lead-Free Piezoelectrics.

Defect engineering is a well-established approach to customize the functionalities of perovskite oxides. In demanding high-power applications of piezoelectric materials, acceptor doping serves as the state-of-the-art hardening approach, but inevitably deteriorates the electromechanical properties. Here, a new hardening effect associated with isolated oxygen vacancies for achieving well-balanced performances is proposed. Guided by theoretical design, a well-balanced performance of mechanical quality factor (Qm ) and piezoelectric coefficient (d33 ) is achieved in lead-free potassium sodium niobate ceramics, where Qm increases by over 60% while d33 remains almost unchanged. By atomic-scale Z-contrast imaging, hysteresis measurement, and quantitative piezoresponse force microscopy analysis, it is revealed that the improved Qm results from the inhibition of both extrinsic and intrinsic losses while the unchanged d33 is associated with the polarization contributions being retained. More encouragingly, the hardening effect shows exceptional stability with increasing vibration velocity, offering potential in material design for practical high-power applications such as pharmaceutical extraction and ultrasonic osteotomes.

Characterization and energy calculation of the S/Al interface of Al–Cu–Mg alloys: Experimental and first-principles calculations

In this work, the S/Al interface properties are investigated by aberration-corrected HAADF-STEM observations and first-principles calculations. Four interface atomic model is built from the images observed by transmission electron microscopy and the interfacial energy, adhesion work and electronic properties of the four interfaces are calculated. The calculations results demonstrate that the CuMg-terminated interface has the lowest interfacial energy and the highest adhesion work when compared to the other three interfaces. Among the candidate interfaces, the CuMg-terminated S(021)/Al(014) interface is the most stable in thermodynamics. By analyzing the density of states and charge density difference of the interface, it is found that strong Al–Cu covalent bonds are formed at the interface, making the S precipitated phase and Al matrix interface combination is more stable. Furthermore, energy calculations are applied to explain the rearrangement phenomenon of Cu atoms in the Z-contrast images of the experiment. Meanwhile, the fundamental data in this paper will be useful for comprehending various behaviors of the S/Al interfaces.

Nanoscale bubble domains with polar topologies in bulk ferroelectrics

Multitudinous topological configurations spawn oases of many physical properties and phenomena in condensed-matter physics. Nano-sized ferroelectric bubble domains with various polar topologies (e.g., vortices, skyrmions) achieved in ferroelectric films present great potential for valuable physical properties. However, experimentally manipulating bubble domains has remained elusive especially in the bulk form. Here, in any bulk material, we achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, especially skyrmions, as validated by direct Z-contrast imaging. This phenomenon is driven by the interplay of bulk, elastic and electrostatic energies of coexisting modulated phases with strong and weak spontaneous polarizations. We demonstrate reversable and tip-voltage magnitude/time-dependent donut-like domain morphology evolution towards continuously and reversibly modulated high-density nonvolatile ferroelectric memories.

Incorporation mechanism of structurally bound gold in pyrite: Insights from an integrated chemical and atomic-scale microstructural study

Abstract Pyrite is one of the most important carriers for Au in hydrothermal deposits, but the incorporation mechanism of structurally bound Au has long been a matter of debate. This is due to a poor understanding of the chemical state and coordination environments of Au in the pyrite structure. Arsenian pyrite is the dominant Au-hosting mineral from the Shanggong Au deposit (130 t Au) in the southern margin of the North China Craton. Our EPMA and LA-ICP-MS results show that the arsenian pyrite has remarkably high Au (0.05–0.78 wt%) and As (0.39–4.60 wt%) contents. Au is negatively correlated with Fe but positively correlated with As. Z-contrast HAADF-STEM imaging reveals that Au atoms sit at the Fe atom sites in the arsenian pyrite structure. Our μ-XANES results and previously reported data suggest that Au and As in arsenian pyrite are predominant of chemically bound Au+ and As–, respectively. We thus propose that Au+ and As– are both structurally bound and sit at the Fe and S atom sites of pyrite, respectively. As may not be necessary but favorable for the incorporation of structurally bound Au in pyrite. These results have significant implications for understanding the enrichment mechanism of Au from ore-forming fluids with low-Au and low-As concentrations into arsenian pyrite and for revealing the role of As in Au mineralization of hydrothermal deposits.

Symmetry of the Underlying Lattice in (K,Na)NbO3-Based Relaxor Ferroelectrics with Large Electromechanical Response.

The piezoelectric constant (d33) and converse piezoelectric coefficient (d33*) of a piezoelectric material are critically important parameters for sensors and actuators. Here, we simultaneously achieve a high d33 of 595 ± 10 pC/N and a large d33* of ∼776 ± 20 pm/V in (K,Na)NbO3 (KNN)-based ceramics, which exceed those of PZT5H and PZT4 ceramics, presenting good potential for practical piezoelectric applications. Moreover, the ceramic exhibits a relaxor-like and diffuse dielectric behavior due to the occurrence of local heterogeneity. According to the experiments and atomic resolution polarization mapping by Z-contrast imaging, hierarchical architecture of nanodomains and even smaller polar nanoregions with multiphase coexistence caused by compositional modification is the structural origin of the enhanced piezoelectric properties in this work. This work would pave a practical way to future applications of lead-free KNN-based ceramics.

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.

Ultrafine microstructure and hardness in Fe-Cr-Co alloy induced by spinodal decomposition under magnetic field

Application of an external magnetic field during heat treatment affects the hardness of magnetic Fe-Cr-Co alloys. The microstructure and composition at atomic scale, as well as the hardening mechanisms need extensive studies. Using atomic resolution STEM, we investigated the effect of a 3 T magnetic field on the spinodal decomposition, as well as microstructure and hardness in step-aged Fe27wt. %Cr15wt. %Co samples. Spinodal decomposition resulted in a homogeneous α phase transforming into an Fe-Co rich α1 phase and a Cr rich α2 phase. Although α1 and α2 showed distinct contrast at low magnification Z-contrast images, close examination at atomic scale of the samples showed no sharp α1/α2 interfaces. Inside each phase, composition fluctuations occurred. A 3 T external magnetic field during step aging increased the size of the α1 phase and introduced microstructural anisotropy, which is desired for permanent magnet applications, In addition, the spinodal decomposition increased the hardness. Annealing in a 3 T magnetic field decreased the hardness but increased the ductility, which is desirable for manufacturing permanent magnets. The change in hardness values is attributed to the composition fluctuations of Fe, Cr and Co, and we further discuss the mechanisms for composition fluctuations and hardness.

In-plane and vertical heterostructures from 1T′/2H transition-metal dichalcogenides

Abstract An avalanche of research has been carried out on two-dimensional (2D) transition metal dichalcogenides (TMDs) due to their potential applications in advanced electronics and flexible devices. To take full use of the emerging 2D TMDs materials, their in-plane/vertical heterostructures have been explored, enabling effective tuning of their physical and chemical properties. However, structural differences between the various phases impede the formation of functional heterostructures. Therefore, robust synthesis strategies for heterostructures with different phases have been explored in this study. A chemical vapor deposition process has been proposed in which the key parameters like reaction sources and deposition sites have been carefully adjusted, trying to achieve simultaneous synthesis of 1T′/2H in-plane and vertical heterostructures. Consequently, 2D in-plane RexMo1−xS2/MoS2 and vertical ReS2/MoS2 heterostructures have been produced in different regions at the same time. Atomic-resolution Z-contrast images reveal the detailed atomic structure of the 1T′/2H interfaces. The lateral interface is found to contain Mo atoms with only 5-fold coordination with S due to the phase mismatch. This work demonstrates a route to exploit heterostructures of different phases and opens the possibility to build more complicated 2D heterostructures using CVD.

Quantification of Protodolomite Using a Combination of XRD, EDS, Z-contrast Imaging and Simulation

Quantification of Protodolomite Using a Combination of XRD, EDS, Z-contrast Imaging and Simulation

Nature and Distribution of Cu and Pd Species in CuPd/TiO2-Na Bimetallic Catalysts for Glycerol Hydrodeoxygenation.

We demonstrated recently that CuPd/TiO2–Na bimetallic catalysts synthesized by sequential wet impregnation are active, selective, and stable for the hydrodeoxygenation (HDO) of glycerol into propylene glycol at low H2 pressure. The present study reports on the nature and distribution of Cu and Pd surface species in CuPd/TiO2–Na bimetallic catalysts using different scanning transmission electron microscopy techniques that supply cluster-specific alloying details. In particular, we used atomic-resolution Z-contrast imaging, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. We also include X-ray photoelectron spectroscopy results. Our analysis shows that the metallic nanoparticles adopt mainly five different structures according to how the Cu and Pd atoms coordinate among themselves: a homogeneous CuPd alloy structure (45–61%), a Cu shell/CuPd core (15–23%), a smaller number of particles formed by Cu on the surface and Pd in the nucleus (10–17%), and there are also nanoparticles formed only by Pd (4–7%) or by Cu (8–13%). We determined that there is a inhomogeneous distribution of Cu and Pd in the bimetallic nanoparticles, with Cu being predominant on the surface (between 76 and 90% of the total area analyzed for each particle). Most bimetallic nanoparticles have sizes below 6 nm, the Pd monometallic nanoparticles are in the 2–4 nm range, whereas the monometallic Cu nanoparticles are larger than 8 nm. Bimetallic nanoparticles with sizes smaller than 6–7 nm are fundamentally made up of Cu0–Pd0 and Cu1+–Pd0. The nanoparticles with sizes greater than 7 nm consist of Cu2+ and Cu2+–Pd2+. Our obtained results also help describe reports about the activation of HDO by Pd–Cu in the absence of H2, an effect apparently not observed with other bimetallic systems.

The effect of nano-twins on the thermoelectric properties of Ga2O3(ZnO)m (m = 9, 11, 13 and 15) homologous compounds

Ga2O3(ZnO)m (m = integer) homologous compounds are naturally occurring nanostructured materials. Their intrinsically low thermal conductivity makes them attractive for thermoelectric applications. High density Ga2O3(ZnO)m (m = 9, 11, 13, and 15) single phase ceramics were prepared by solid-state reaction. Nano-sized, twin-like V-shaped boundaries parallel to b-axis (apex angle ∼ 60°) were observed for all compositions. Atomic resolution Z-contrast imaging and EDS analysis for m = 15 showed segregation of Ga ions at the interface of V-shaped twin boundaries. Thermal and charge transport properties depend on the value of m. Compositions with m = 9 exhibited very low lattice thermal conductivity of 2 to 1.5 W/m.K at 300 K–900 K; compositions with m=15 showed improved power factor of 140 μW/m. K2 at 900 K leading to a thermoelectric figure of merit (ZT value) of 0.055. This study explores the structural variants and routes to improve the thermoelectric properties of these materials

A decagonal quasicrystal with rhombic and hexagonal tiles decorated with icosahedral structural units.

The structure of a decagonal quasicrystal in the Zn58Mg40Y2 (at.%) alloy was studied using electron diffraction and atomic resolution Z-contrast imaging techniques. This stable Frank-Kasper Zn-Mg-Y decagonal quasicrystal has an atomic structure which can be modeled with a rhombic/hexagonal tiling decorated with icosahedral units at each vertex. No perfect decagonal clusters were observed in the Zn-Mg-Y decagonal quasicrystal, which differs from the Zn-Mg-Dy decagonal crystal with the same space group P10/mmm. Y atoms occupy the center of 'dented decagon' motifs consisting of three fat rhombic and two flattened hexagonal tiles. About 75% of fat rhombic tiles are arranged in groups of five forming star motifs, while the others connect with each other in a 'zigzag' configuration. This decagonal quasicrystal has a composition of Zn68.3Mg29.1Y2.6 (at.%) with a valence electron concentration (e/a) of about 2.03, which is in accord with the Hume-Rothery criterion for the formation of the Zn-based quasicrystal phase (e/a = 2.0-2.15).

Ultrahigh-Loading of Ir Single Atoms on NiO Matrix to Dramatically Enhance Oxygen Evolution Reaction.

Engineering single-atom electrocatalysts with high-loading amount holds great promise in energy conversion and storage application. Herein, we report a facile and economical approach to achieve an unprecedented high loading of single Ir atoms, up to ∼18wt%, on the nickel oxide (NiO) matrix as the electrocatalyst for oxygen evolution reaction (OER). It exhibits an overpotential of 215 mV at 10 mA cm-2 and a remarkable OER current density in alkaline electrolyte, surpassing NiO and IrO2 by 57 times and 46 times at 1.49 V vs RHE, respectively. Systematic characterizations, including X-ray absorption spectroscopy and aberration-corrected Z-contrast imaging, demonstrate that the Ir atoms are atomically dispersed at the outermost surface of NiO and are stabilized by covalent Ir-O bonding, which induces the isolated Ir atoms to form a favorable ∼4+ oxidation state. Density functional theory calculations reveal that the substituted single Ir atom not only serves as the active site for OER but also activates the surface reactivity of NiO, which thus leads to the dramatically improved OER performance. This synthesis method of developing high-loading single-atom catalysts can be extended to other single-atom catalysts and paves the way for industrial applications of single-atom catalysts.

Dealloying Kinetics of AgAu Nanoparticles by In Situ Liquid-Cell Scanning Transmission Electron Microscopy.

Understanding the formation and evolution of bicontinuous nanoporous structure during dealloying has been one of the most challenging subjects of dealloying research. However, previous in situ investigations either suffer from insufficient spatial resolution (e.g., X-ray tomography) or lack morphology visualization and mass information (e.g., scanning tunneling microscopy). In this work, we report the kinetics of the whole course of dealloying by utilizing liquid-cell aberration-corrected scanning transmission electron microscopy. With Z-contrast imaging analysis, the in situ sub-nanoscale characterization reveals two new phenomena, an initial period of dealloying indicative of an initial length scale for bulk dealloying and a large volume shrinkage in a nanoscale alloy precursor. We explain the particle-size-dependent volume shrinkage with the formation of a dense shell and quantify the dependence with a simple geometric model. These insights into the mechanisms of dealloying will enable deliberate designs of nanoporous structures.

High-performance potassium sodium niobate piezoceramics for ultrasonic transducer

Ultrasonic transducers can be used as medical diagnostic imaging and nondestructive testing. This work presents phase structure engineered high-performance potassium sodium niobate [(K,Na)NbO3, KNN]-based ceramics and its potential application on ultrasonic transducer. A connection among structural engineering, electrical properties, and prototype device designing was established in lead-free KNN-based ceramics. Structural elucidation through atomic resolution polarization mapping by Z-contrast imaging indicates that the nanoscale balanced multiple phase coexistence (R-O-T) is the origin of both enhanced piezoelectricity and improved temperature stability: high d33 of 500 pC/N and improved in-situ temperature stability of d33 (less than 24% variation for d33 from 25 to 100 °C), together with the improvement of electromechanical coupling coefficient (kp~0.5 and kt~0.55). Based on the high-performance ceramics, a 1–3 composite ultrasonic transducer with a center frequency of 5 MHz is produced, and a bandwidth of 81% at −6 dB is substantially higher than that of PZT-based transducers at the same center frequency. We believe the novel way to improve electrical properties through structural engineering could open a new road to promote the practical application of KNN-based ceramics.

Direct Observation of Modulation Structure in Room Temperature Multiferroic Bi <sub>4.2</sub>K <sub>0.8</sub>Fe <sub>2</sub>O <sub>9+δ</sub>

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.

SM-1 Identification of Z-contrast image of twinned catalytic gold nanoparticles processed by Hough transformation using convolutional neural network

SM-1 Identification of Z-contrast image of twinned catalytic gold nanoparticles processed by Hough transformation using convolutional neural network