Spherical Aberration-corrected Transmission Electron Microscopy Research Articles

Improvement of chemical mechanical polishing performance by introducing N–Si bond via external coating of cerium oxide with ZIF-8

Cerium oxide (CeO2) nanoparticles are widely used as abrasives in chemical mechanical polishing (CMP), an essential process for obtaining Si wafers with excellent surface quality. While CeO2 nanoparticles form Ce–O–Si bonds, which exhibit good CMP performance, their low activity results in a low removal rate. To address this issue, a two-step ultrasonic dispersion method was employed to prepare a uniform nuclear@shell CeO2@ZIF-8. The process of chemical bond generation in CMP was studied using spherical aberration-corrected transmission electron microscopy and X-ray photoelectron spectroscopy. In comparison to pure cerium oxide, the formation of Ce–O–Si bond was observed, and notably, the emergence of N–Si bond originating from the N of the ZIF-8 shell was identified. Coupled with the increased Ce3+ content, these developments enhanced CMP performance. The material removal rate (MRR) of the CeO2@ZIF-8 abrasives was 151.507 nm/min, which is 89.72% higher than that of the pure CeO2 abrasives. Moreover, the surface roughness, as determined via AFM testing, was reduced to 0.231 nm within the confines of a 5.0 × 5.0 μm² region. Therefore, this study introduces a novel concept for CMP research by developing a CeO2@ZIF-8 compound abrasive to create a new N–Si bond.

Influence of interface on the domain polarization orientation in ferroelectric Hf0.5Zr0·5O2 thin films

Ferroelectric HfO2-based materials have attracted extensive attention in the research of next-generation nonvolatile memories and logic devices due to their superior scaling properties and compatibility with the CMOS process. However, forming the ferroelectric metastable phase in the thin films of a few nanometers requires clamping of the top and bottom electrodes. As a result, the interface can significantly impact domain structure and polarization, leading to imprinting effects and non-uniform space charge distribution. Here, we have demonstrated how the interface affects the ferroelectric polarization in Hf0.5Zr0·5O2 (HZO) thin films produced by ALD and the physical mechanisms behind it. We found that by regulating the quantity of oxygen vacancy content at the top and bottom interfaces of TiN/HZO/TiN ferroelectric capacitors, it is possible to achieve inversion of polarization orientation. By using a spherical aberration-corrected transmission electron microscope and first-principles calculation, we identified that the internal electric field generated by the accumulation of oxygen vacancies at an interface and the asymmetrical electrostatic energy of the interfacial T-phase layer jointly play a role in the ferroelectric polarization orientation. Our results will significantly help the interface engineering of HfO2-based ferroelectric thin films and devices.

Rapid Plasma Electrolytic Oxidation Synthesis of Intermetallic PtBi/MgO/Mg Monolithic Catalyst for Efficient Removal of Organic Pollutants.

The intermetallic PtBi/MgO/Mg monolithic catalyst was first prepared using non-equilibrium plasma electrolytic oxidation (PEO) technology. Spherical aberration-corrected transmission electron microscope (ACTEM) observation confirms the successful synthesis of the PtBi intermetallic structure. The efficiency of PtBi/Mg/MgO catalysts in catalyzing the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4 was demonstrated. The activity factor for the catalyst is 31.8 s-1 g-1, which is much higher than reported values. In addition, the resultant catalyst also exhibits excellent catalytic activity in the organic pollutant reaction of p-nitrobenzoic acid (p-NBA) and methyl orange (MO). Moreover, benefiting from ordered atomic structures and the half-embedded PtBi nanoparticles (NPs), the catalyst demonstrates excellent stability and reproducibility in the degradation of 4-NP. This study provides an example of a simple method for the preparation of intermetallic structures as catalysts for organic pollutant degradation.

Enhanced catalytic oxidation of benzene though the synergistic Pt-Ni bimetallic single-atom catalyst

Excellent catalysts for the catalytic oxidation of benzene (C6H6) require active sites with robust oxidation capabilities. Heteronuclear dual-single atom catalysts can exhibit enhanced catalytic activity through synergistic interactions between the dual single atoms (SAs). Herein, we successfully engineered a bimetallic single-atom catalyst, Pt1-Ni1/TiO2-NW, which demonstrates exceptional performance in C6H6 oxidation. Various technologies, including X-ray absorption fine structure and Z-type spherical aberration-corrected transmission electron microscopy, were used to identify the bimetallic single atoms within the catalyst. The catalytic oxidation of C6H6 by Pt1-Ni1/TiO2-NW was comprehensively investigated. Finally, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) were employed to reveal the underlying mechanism for the synergetic effect of the bimetallic single atoms. The results indicate that the introduction of Ni SA favors the formation of Pt SA. The T90% of Pt1-Ni1/TiO2-NW reached 185 °C, a remarkable 40 °C lower than that of the single-atom catalyst Pt1/TiO2-NW. More importantly, the active center of Pt-O-Ni, formed by the collaboration of Pt SA and Ni SA with bridging oxygen, plays a crucial role in reducing the energy barrier during the conversion of C6H6 to phenol and further to benzoquinones. This research provides a valuable methodology for constructing heteronuclear bimetallic single-atom catalysts tailed specifically for C6H6 oxidation.

Cu@Co with Dilatation Strain for High-Performance Electrocatalytic Reduction of Low-Concentration Nitric Oxide.

Electrocatalytic reduction of nitric oxide (NO) to ammonia (NH3 ) is a clean and sustainable strategy to simultaneously remove NO and synthesize NH3 . However, the conversion of low concentration NO to NH3 is still a huge challenge. In this work, the dilatation strain between Cu and Co interface over Cu@Co catalyst is built up and investigated for electroreduction of low concentration NO (volume ratio of 1%) to NH3 . The catalyst shows a high NH3 yield of 627.20µg h-1 cm-2 and a Faradaic efficiency of 76.54%. Through the combination of spherical aberration-corrected transmission electron microscopy and geometric phase analyses, it shows that Co atoms occupy Cu lattice sites to form dilatation strain in the xy direction within Co region. Further density functional theory calculations and NO temperature-programmed desorption (NO-TPD) results show that the surface dilatation strain on Cu@Co is helpful to enhance the NO adsorption and reduce energy barrier of the rate-determining step (*NO to *NOH), thereby accelerating the catalytic reaction. To simultaneously realize NO exhaust gas removal, NH3 green synthesis, and electricity output, a Zn-NO battery with Cu@Co cathode is assembled with a power density of 3.08mW cm-2 and an NH3 yield of 273.37µg h-1 cm-2 .

Engineering Sub-Nanometer Hafnia-Based Ferroelectrics to Break the Scaling Relation for High-Efficiency Piezocatalytic Water Splitting.

Reversible control of ferroelectric polarization is essential to overcome the heterocatalytic kinetic limitation. This can be achieved by creating a surface with switchable electron density; however, owing to the rigidity of traditional ferroelectric oxides, achieving polarization reversal in piezocatalytic processes remains challenging. Herein, sub-nanometer-sized Hf0.5 Zr0.5 O2 (HZO) nanowires with a polymer-like flexibility are synthesized. Oxygen K-edge X-ray absorption spectroscopy and negative spherical aberration-corrected transmission electron microscopy reveal an orthorhombic (Pca21 ) ferroelectric phase of the HZO sub-nanometer wires (SNWs). The ferroelectric polarization of the flexible HZO SNWs can be easily switched by slight external vibration, resulting in dynamic modulation of the binding energy of adsorbates and thus breaking the "scaling relationship" during piezocatalysis. Consequently, the as-synthesized ultrathin HZO nanowires display superb water-splitting activity, with H2 production rate of 25687µmolg-1 h-1 under 40kHz ultrasonic vibration, which is 235and 41times higher than those of non-ferroelectric hafnium oxides and rigid BaTiO3 nanoparticles, respectively. More strikingly, the hydrogen production rates can reach 5.2µmolg-1 h-1 by addition of stirring exclusively.

Triethylenediamine cobalt complex encapsulated in a metal–organic framework cage to prepare a cobalt single-atom catalyst with a high Co-N4 density for an efficient oxygen reduction reaction

Transition metal single atom catalysts (TM SACs) are the most promising oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) and metal-air batteries. However, the low density of M-Nx active sites seriously hinders further improvement of the ORR electrocatalytic activity. Here, a strategy for encapsulating nitrogen-rich guest molecules (triethylenediamine cobalt complex, [Co(en)3]3+) was proposed to construct a high-performance cobalt single-atom catalyst (Co-encapsulated SAC/NC). With this strategy, the guest molecules are encapsulated into metal–organic framework (MOF) cages as an additional cobalt source to boost cobalt loading, while abundant nitrogen from guest molecules contributes to the formation of Co-N4 active sites. Remarkably, the resulting Co-encapsulated SAC/NC has a high cobalt loading amount of 4.03 wt%, and spherical aberration-corrected transmission electron microscopy (AC-TEM) has confirmed that most cobalt exists in a single-atom state. As a result, the Co-encapsulated SAC/NC exhibits excellent ORR catalytic performance with a half-wave potential of 0.88 V. Furthermore, Zn-air batteries employing Co-encapsulated SAC/NC as air cathode show high peak power density and excellent cycling stability. Density functional theory (DFT) calculations reveal that adjacent active sites have different rate-determining steps and lower reaction energy barriers than a single active site.

In situ observation of two-step crystallization of amorphous oxides via electron microscopy

The transformation from amorphous materials to crystals and the corresponding microscopic mechanism have become a research focus in materials science, condensed matter physics, and biophysics etc. In this paper, taking Y3Al5O12 (YAG) as an example, the crystallization process of amorphous YAG oxide is investigated from an atomic point of view through the combination of spherical aberration-corrected transmission electron microscopy (SCTEM) with an in situ heating method, and the morphology and structural evolution of the surface and interface are explored. It is observed that the crystallization proceeds on the surface and interior of the amorphous YAG separately. Through atomic-scale imaging, it is determined that semi-ordered structures are formed in a few atomic layers at the amorphous/crystalline interface and that the structural transformation process from the amorphous to the semi-ordered state and then to the crystal structure may be the main reason for the multiple stages of the crystallization of amorphous oxides. This study demonstrates the mechanism of the amorphous-to-crystal transformation on the atomic scale and contributes to gaining a better understanding of the crystallization process of bulk oxides, especially complex metal oxides.

Direct atomic-scale visualization of the 90° domain walls and their migrations in Hf0.5Zr0.5O2 ferroelectric thin films

Domain walls (DWs) play an essential role in altering the polarization and related properties of ferroelectric materials, and are regulated by the mechanism of changing atomic configuration. However, compared with perovskite ferroelectrics, the DWs in hafnia-based fluorite-structure ferroelectric is experimentally lacking, especially with regard to detailed studies of atomic-scale structures. The present work used spherical aberration-corrected transmission electron microscope combined with in-situ technique to ascertain the origin, atomic arrangements of 90° DWs in Hf0.5Zr0.5O2 thin films. Different types of 90° DWs were found to exhibit varying migration behaviors in response to the concentration of oxygen vacancies. Point defects and corresponding changes in the local strain field are proposed to drive DW switching in this material. These new insights into the atomic-scale characteristics of 90° DWs are expected to assist in elucidating the structure of DWs and improve our understanding of the ferroelectric characteristics of new type hafnia-based materials.

Guanosine-derived atomically dispersed Cu-N3-C sites for efficient electroreduction of carbon dioxide

Single-atom copper (Cu) embedded within carbon catalysts have demonstrated significant potential in the electrochemical reduction of carbon dioxide (CO2) into valuable chemicals and fuels. Herein, we develop a straightforward and template-free strategy for synthesizing atomically dispersed CuNC catalysts (CuG) by annealing the self-assembled guanosine. The CuG catalysts display two-dimensional morphology, tunable pore size and large surface areas that can be adjusted by changing carbonization temperature. Spherical aberration-corrected transmission electron microscopy reveals that single-atom Cu are homogeneously dispersed on the surface of carbon nanosheets. The optimum CuG-1000 catalysts achieve a high CO Faradaic efficiency (FEco) up to 99% and a high CO current density of 6.53 mA cm−2 (−0.65 V vs. RHE). Besides, the flow cell test of CuG-1000 shows a high current density up to 25.2 mA cm−2 and the FEco still exceeded 91% after more than 20 h of testing. Specifically, the existence of Cu-N3-C active sites was proved by extended X-ray absorption fine structure (EXAFS). Density functional theory evidences that tricoordinated copper with N can largely regulate the adsorption and desorption of key intermediates by transferring electrons to *COOH through Cu atoms, thereby improving selectivity toward CO. This work demonstrates the active origin of CuNC catalysts in CO2 electroreduction and offers a blueprint to construct atomically dispersed transition site catalysts by supramolecular self-assembly strategy.

Enhancing the chemoselective hydrogenation of nitroarenes: Designing a novel surface-strained carbon-based Pt nanocatalyst

Supported Pt nanoparticles (NPs) are highly active catalysts for heterogeneous catalytic hydrogenation reactions; however, controlling their selectivity remains the biggest challenge toward their applicability. Herein, we propose the formation of a highly selective and stable, surface-strained Pt-based nanocatalyst via a facile and scalable thermal reduction treatment. Spherical-aberration-corrected transmission electron microscopy (SACTEM) and various spectral analytic techniques reveal a strong metal-support interaction between the Pt NPs and carbon nanotubes (CNTs) support during the annealing process. Thereafter, a fraction of carbon atoms is etched from the carbon-coated Pt NPs, inducing a compressive strain on the surface of the Pt NPs. Notably, the chemoselectivity of the surface-strained Pt/CNTs-800H catalyst (where 800 represents the heat-treatment temperature; H represents a hydrogen atmosphere) is almost completely different compared to that of its pristine counterpart. This catalyst is used for the hydrogenation reactions of a styrene and nitrobenzene mixture as well as 4-nitrostyrene. Interestingly, similar findings were observed with 5 wt% Pt/C and Pt/rGO catalysts, confirming that this treatment could be generalized. Hence, it has great potential in the design and synthesis of carbon-based catalytic materials.

Unveiling positive impacts of fluorine anion doping on extraordinary catalytic activity of bifunctional-layered double perovskite electrodes for solid oxide fuel cells and electrolysis cells

To address the impacts of anion doping, F-doped layered double perovskites (Pr1.1Ba0.9Co2O5+δFx) have been synthesized, demonstrating bifunctional activity in both solid oxide fuel cell and solid oxide electrolysis cell models. The F doping induces increased oxygen vacancy concentration and enhanced electrical conductivity. Combined with spherical aberration-corrected transmission electron microscopy and X-ray photoelectron spectroscopy analyses, the F− anions are successfully introduced into the host lattice, promoting the oxygen surface exchange/diffusion rates. From the density functional theory calculations, the covalence between Co 3d and O 2p-orbital is enlarged in the F-doped model, and both oxygen reduction reaction and oxygen evolution reaction activities are improved. The area-specific resistance of the Pr1.1Ba0.9Co2O5+δF0.1 (P1.1BCOF0.1) electrode is as low as 0.033 Ω/cm2 at 700 °C. The P1.1BCOF0.1 cathode-based fuel cell delivers a peak power density of 1102 mW/cm2, along with an excellent operating stability at 700 °C. Moreover, the current density of 1335 mA/cm2 is achieved in the P1.1BCOF0.1 anode-based electrolysis cell at 1.8 V toward CO2 reduction reaction at 750 °C. These results here highlight the performance origin and bifunctional activity of fluorine-doped perovskite materials, which may help us rationally design the oxide catalysts.

Achieving superior cryogenic impact toughness and sufficient tensile properties in a novel high-Mn austenitic steel weld metal via cerium addition

The microstructure and mechanical properties of the novel cryogenic high-Mn austenitic steel weld metals with different cerium (Ce) contents were investigated via electron back-scattered diffraction, scanning and spherical aberration-corrected transmission electron microscopy, and cryogenic impact and tensile test. The results indicated that the crystallographic grain size was significantly refined, and the relative frequency of high-angle grain boundaries increased with the increase of Ce content. Ce element addition could modify the main inclusions from irregular Al2O3 into regular Ce-containing inclusions. Meanwhile, the inclusions gradually became fine and dispersed, and the volume fraction of inclusions decreased with the increase of Ce content from 0 to 870 ppm. The increase of Ce content continuously improved cryogenic impact toughness from 82 J to 116 J, and yield strength, tensile strength and total elongation all increased to different degrees. We found that in addition to the beneficial changes in inclusion modification, grain refinement and increase in the proportion of high-angle grain boundaries, the solid solution of Ce also played a crucial role in the mechanical properties.

Effect of vacancy ordering on the grain growth of Ge2Sb2Te5 film

Ge2Sb2Te5 (GST) is the most widely used matrix material in phase change random access memory (PCRAM). In practical PCRAM device, the formed large hexagonal phase in GST material is not preferred, especially when the size of storage architecture is continually scaling down. In this report, with the aid of spherical-aberration corrected transmission electron microscopy (Cs-TEM), the grain growth behavior during the in situ heating process in GST alloy is investigated. Generally, the metastable face-centered-cubic (f-) grain tends to grow up with increasing temperature. However, a part of f-phase nanograins with {111} surface plane does not grow very obviously. Thus, the grain size distribution at high temperature shows a large average grain size as well as a large standard deviation. When the vacancy ordering layers forms at the grain boundary area in the nanograins, which is parallel to {111} surface plane, it could stabilize and refine these f-phase grains. By elaborating the relationship between the grain growth and the vacancy ordering process in GST, this work offers a new perspective for the grain refinement in GST-based PCRAM devices.

Probing into the conduction band and type of carriers/traps on red/orange persistent phosphors in vacancy & solid-solution induced (Sr/Ba)1-xCaxGe4-yO9:Mn2.

Long-wavelength afterglow has been a key issue in the investigation of afterglow materials. Herein, the orange/red (Sr/Ba)1-xCaxGe4-yO9:0.01Mn2+ is prepared by the introduction of vacancy induction and solid solution. The (Sr/Ba)Ge4O9:Mn2+ hardly possesses an afterglow phenomenon and exhibits only red/orange photo-luminescence (PL) attributed to the d-d transition of Mn2+, while samples with reduction of the Ge4+ content and with replacement by Ca2+ show bright afterglow emission with the peak located at about 612 nm/620 nm. The emerging broadband peak comes from charge transfer involving Mn2+ and nearby defect clusters and the bottom of the conduction band (CB). The introduction of V''''Ge creates a defective energy level above the valence band, but the ground state energy difference with Mn2+ is too large (>1 eV) to allow hole transfer, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), spherical aberration-corrected transmission electron microscopy (AC-STEM), charge differential density (CDD) analysis, electron paramagnetic resonance (EPR) analysis, etc. With this achievement, we propose an original design strategy for long-wavelength afterglow and a more reasonable afterglow mechanism, which is of great importance for the investigation of long-wavelength afterglow materials.

Anomalous dielectric relaxation peak in Nb-doped SrTiO3 single crystals

Anomalous dielectric relaxation appears in Nb-doped SrTiO3 (SNT) single crystal with relaxation rate displaying unusual slowing down with increasing temperature. Dielectric measurements show that the resistivity increases with temperature. Raman spectrum and the spherical aberration-corrected transmission electron microscope (STEM) show the existence of polar nano regions (PNRs) in the crystal. The temperature-dependence of DC conductivity and current-voltage (I–V) curves suggest that this anomalous behavior happens at the interfaces of PNRs. The evolution of the anomalous relaxation peak in the annealed samples indicates that this effect is simultaneously affected by the carrier concentration and the interface structure. The anomalous dielectric relaxation is reminiscent of the positive temperature coefficient of resistance (PTCR) effect in a polycrystalline material. The resistive switching effect makes such materials to have potential application value in switches, sensors, and device miniaturization.

A stable LaB6 nanoneedle field-emission electron source for atomic resolution imaging with a transmission electron microscope

Field-induced electron emission offers the best performance point electron source for electron microscopes. Herewith we report a new cold field-emission point electron source utilizing a nanoneedle made of lanthanum hexaboride (LaB6) and its implementation in a spherical aberration-corrected transmission electron microscope (TEM). A sub-ångstrom resolution of 0.96 Å has been obtained with the TEM operated at an acceleration voltage of 60 kV and a smaller energy spread was also observed compared with the contemporary W(310) electron source. In particular, the LaB6 nanoneedle cold field-emission electron source required no thermal flashing for continuous operation of extended hours (>100 hours) while exhibiting and sustaining a high stability in emission current (<1%). We attribute the excellent performance of the LaB6 nanoneedle electron source to (i) a low work function of LaB6; (ii) excellent alignment of the nanoneedle emitter by the electron and ion dual-beam processing with nanoscale precision and (iii) robustness of the nanometric structure that suppressed mechanical vibrations of the LaB6 emitter even in a high electric field. This new LaB6 nanoneedle cold field-emission electron source enables stable high-resolution imaging in TEM and it will also benefit cryogenic electron microscopy, scanning electron microscopy, electron beam lithography, and other electron beam technologies.

Direct Observation of Cu Clusters and Dislocation Loops by Cs-Corrected STEM in Fe-0.6wt%Cu Alloy Irradiated in BR2

The neutron irradiation of Fe-based fusion and fission reactor materials leads to an increase in ductile-to-brittle transition temperature with a decrease in upper shelf energy. It is well known that Cu content has a strong influence on the embrittlement phenomenon, as Cu-rich clusters (CRPs) are thought to be directly responsible for embrittlement. In contrast, mechanical property studies for steels with different Cu levels exhibit dominant matrix defects in the embrittlement of both low-Cu steels and high-Cu steels at high fluences. To determine the effects of dislocation loops and CRPs on radiation hardening in those steels, neutron irradiation was conducted on Fe-0.6wt%Cu alloy. The neutron irradiation was performed in BR2 at 290 °C up to a dose of 4.1 × 1024 n/m2. After irradiation, the microstructure was observed and analyzed by spherical aberration-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with X-ray energy-dispersive spectroscopy, using a JEOL ARM200FC. This technique enabled simultaneous observation of ~10 nm CRPs and dislocation loops. Additional high-voltage electron irradiation was performed at room temperature, and the dislocation loops were identified as interstitial-type dislocation loops. Radiation-induced hardening due to neutron irradiation was estimated by measuring the size and density of the dislocation loops and the CRPs. These results suggest that simultaneous observation of dislocation loops and CRPs using the Cs-corrected STEM with EDS analysis is essential for the study of radiation-induced hardening in Fe-based alloys.

Realizing high thermoelectric performance in N-type Bi2Te3 compounds by Sb nano-precipitates

Driven by the prospective application of thermoelectric (TE) materials, massive efforts have been devoted to improving its performance. However, strong-coupled thermal and electrical transport is still the largest block for the promotion of TE techniques. Here, we report a peak ZT over 1.2 at 400 K in an n-type Bi2Te2.7Se0.3 sample doped by 5 wt. % Sb under the spark plasma sintering method. This high performance is attributed to the synergistically optimized thermal and electrical transport properties. Sb nano-precipitates, which have been observed directly by spherical aberration-corrected transmission electron microscopy, substantially decrease the lattice thermal conductivity of the sample, leading to a low value of 0.35 W m−1 K−1. Meanwhile, band bending caused by the nano-precipitates significantly enhances the Seebeck coefficient, resulting in a high PF of 35 μW cm−1 K−2. The study about optimizing electrical and thermal properties simultaneously opens the door to the high performance of Bi2Te3-based materials.

Rare-Earth Doping Graphitic Carbon Nitride Endows Distinctive Multiple Emissions with Large Stokes Shifts

Rare-Earth Doping Graphitic Carbon Nitride Endows Distinctive Multiple Emissions with Large Stokes Shifts