Scanning Transmission Electron Microscopy Observations Research Articles

Superconducting Boride Nb2IrB2 with Variable and Tunable Stacking Behaviors of Two-Dimensional [Nb-Ir-Nb] Triangular Lattices.

In structures with special geometry lattices, variations in stacking sequences are ubiquitous, yielding many novel structures and functionalities. Despite a wealth of intriguing properties and wide-ranging applications, there remains a considerable gap in understanding the correlation between special geometry lattices and functionalities in borides. Here, we design and synthesize a new superconducting boride Nb2IrB2, with a body-centered orthorhombic structure, consisting of alternating two-dimensional [Nb-Ir-Nb] triple-triangular-lattice-layers and B fragment layers. Advanced aberration-corrected scanning transmission electron microscopy observations show variable stacking configurations between [Nb-Ir-Nb] triple-triangular-lattice layers that can be tuned through synthesis conditions. Density functional theory calculations reveal that the coherent low-energy boundary interface plane of {101} between [11̅1] and [010] domains is responsible for the variable stacking behaviors. Energetically favorable structures are thereby reasonably proposed, based on nanoscale imperfect structure units. These findings provide valuable insights for designing and exploring new structures and functionalities within boride systems involving special geometry lattices.

Grain rotation mechanisms in nanocrystalline materials: Multiscale observations in Pt thin films.

Near-rigid-body grain rotation is commonly observed during grain growth, recrystallization, and plastic deformation in nanocrystalline materials. Despite decades of research, the dominant mechanisms underlying grain rotation remain enigmatic. We present direct evidence that grain rotation occurs through the motion of disconnections (line defects with step and dislocation character) along grain boundaries in platinum thin films. State-of-the-art in situ four-dimensional scanning transmission electron microscopy (4D-STEM) observations reveal the statistical correlation between grain rotation and grain growth or shrinkage. This correlation arises from shear-coupled grain boundary migration, which occurs through the motion of disconnections, as demonstrated by in situ high-angle annular dark-field STEM observations and the atomistic simulation-aided analysis. These findings provide quantitative insights into the structural dynamics of nanocrystalline materials.

Flame made low Pt loading catalysts supported on different metal oxides for catalytic combustion of CO and CH4

Catalytic combustion is an effective approach to remove air pollutants from various emission sources. For this purpose, supported noble metal catalysts are preferred in commercial applications due to their outstanding catalytic activity for eliminating CO, hydrocarbon compounds and NOx. In this paper, we employ the flame spray pyrolysis method to prepare a series of Pt-based catalysts with four different supports (TiO2, ZrO2, MgO and ZnO) and variable low Pt loadings for catalytic combustion of CO and CH4. The performance of 0.5 Pt/TiO2 is the best in all samples, in which the T90 temperatures are 107 and 500 °C for 90% conversion of CO and CH4, respectively. To examine its thermal stability, a time-on-stream test at 700 °C for 420 min is carried out, resulting in a decrease of about 5% in the final conversion of CH4. The X-ray diffraction results show that TiO2 support is a mixed phase with a major amount of anatase and a small amount of rutile other than a pure phase of ZrO2, MgO and ZnO. Furthermore, X-ray photoelectron spectroscopy analysis and high-angle annular dark-field scanning transmission electron microscopy observation show that when the Pt loading is low, the Pt species exist as highly dispersed single atoms on the surface of the TiO2 support. As the Pt loading gradually increases, the state of the Pt species transitions from single atoms to Pt clusters, resulting in a decrease in dispersion. Ultimately, the Pt can successfully accumulate on the surface of the TiO2 nanoparticles, providing abundant active sites for efficient catalytic combustion reactions.

Unraveling the Unique Strong Metal-Support Interaction in Titanium Dioxide Supported Platinum Clusters for the Hydrogen Evolution Reaction.

Strong metal-support interaction (SMSI) is crucial to modulating the nature of metal species, yet the SMSI behaviors of sub-nanometer metal clusters remain unknown due to the difficulties in constructing SMSI at cluster scale. Herein, we achieve the successful construction of the SMSI between Pt clusters and amorphous TiO2 nanosheets by vacuum annealing, which requires a relatively low temperature that avoids the aggregation of small clusters. In situ scanning transmission electron microscopy observation is employed to explore the SMSI behaviors, and the results reveal the dynamic rearrangement of Pt atoms upon annealing for the first time. The originally disordered Pt atoms become ordered as the crystallizing of the amorphous TiO2 support, forming an epitaxial interface between Pt and TiO2. Such a SMSI state can remain stable in oxidation environment even at 400 °C. Further investigations prove that the electron transfer from TiO2 to Pt occupies the Pt 5d orbitals, which is responsible for the disappeared CO adsorption ability of Pt/TiO2 after forming SMSI. This work not only opens a new avenue for constructing SMSI at cluster scale but also provides in-depth understanding on the unique SMSI behavior, which would stimulate the development of supported metal clusters for catalysis applications.

Unraveling the Unique Strong Metal‐Support Interaction in Titanium Dioxide Supported Platinum Clusters for the Hydrogen Evolution Reaction

AbstractStrong metal‐support interaction (SMSI) is crucial to modulating the nature of metal species, yet the SMSI behaviors of sub‐nanometer metal clusters remain unknown due to the difficulties in constructing SMSI at cluster scale. Herein, we achieve the successful construction of the SMSI between Pt clusters and amorphous TiO2 nanosheets by vacuum annealing, which requires a relatively low temperature that avoids the aggregation of small clusters. In situ scanning transmission electron microscopy observation is employed to explore the SMSI behaviors, and the results reveal the dynamic rearrangement of Pt atoms upon annealing for the first time. The originally disordered Pt atoms become ordered as the crystallizing of the amorphous TiO2 support, forming an epitaxial interface between Pt and TiO2. Such a SMSI state can remain stable in oxidation environment even at 400 °C. Further investigations prove that the electron transfer from TiO2 to Pt occupies the Pt 5d orbitals, which is responsible for the disappeared CO adsorption ability of Pt/TiO2 after forming SMSI. This work not only opens a new avenue for constructing SMSI at cluster scale but also provides in‐depth understanding on the unique SMSI behavior, which would stimulate the development of supported metal clusters for catalysis applications.

Enhancing atomic ordering, magnetic and transport properties of Mn2VGa Heusler alloy thin films toward negatively spin-polarized charge injection

Magnetic materials with negative spin polarization have attracted attention for their potential to increase the design freedom of spintronic devices. This study investigated the effects of off-stoichiometry on the atomic ordering, microstructure, and magneto-transport properties in Mn2+xV1−xGa (x = −0.2, 0, +0.2, +0.4) Heusler alloy films, which are predicted to have large negative spin polarization derived from a pseudo band gap in the majority spin channel. The Mn2+xV1−xGa films epitaxially grown on MgO(001) substrates exhibits variations of B2 and L21 order with the Mn concentration. A high-quality L21 ordered film was achieved in the Mn-rich composition (x = +0.2) with B2 and L21 order parameters of 0.97 and 0.86, respectively, and a saturation magnetization of 1.4 μB/f.u., which agrees with the Slater-Pauling rule. Scanning transmission electron microscopy observations showed that B2 and L21 phases coexist in Mn-poor and stoichiometric films, while the L21 phase is dominant in the Mn-rich film with small amounts of Mn–V and Mn–Ga disorders, as revealed by laboratory and anomalous X-ray diffraction. Combined first-principles calculations and anisotropic magnetoresistance analysis confirms that the addition of excess Mn preserves the high spin polarization by suppressing the formation of detrimental antisites of V atoms occupying Mn sites. Therefore, the Mn-rich composition is promising for negatively spin-polarized charge injection in Mn2VGa-based spintronic applications.

Atomic and electronic structures of domain boundaries in LaTiO3 thin films

Domain boundaries in perovskite oxides often exhibit abundant physical properties and phenomena. Here, epitaxial LaTiO3 thin films on (100) SrTiO3 substrates are prepared by pulsed-laser deposition. X-ray diffraction and transmission electron microscopy investigations reveal that the epitaxial LaTiO3 thin films have good crystallinity but a high density of domain boundaries. Atomic-scale scanning transmission electron microscopy observations reveal that two types of domain boundaries are formed in the LaTiO3 thin films. The type I domain boundaries are formed on the {100} crystal planes, while the type II domain boundaries on the {110} crystal planes. Electron energy-loss spectroscopy analyses suggest that the valence states of Ti ions at the type I domain boundaries are +3, while those at the type II domain boundaries are +4. First-principles calculations reveal that the bandgap decreases at both domain boundaries compared to the bulk. The carrier concentration at the type I domain boundaries is significantly higher than that of the bulk, while the carrier concentration at the type II domain boundaries is lower. These findings suggest that domain boundaries play an important role in tailoring the electrical properties of the LaTiO3 thin films, thereby promoting the potential applications and property modulation of related materials and devices.

Optimized carbon extraction replicas for transmission electron microscopy of nanoprecipitates in microalloyed low-carbon steels

Nanoprecipitates play a decisive role in strengthening of microalloyed low-carbon steels. The direct carbon extraction replica (CER) method is a promising way to extract the nanoprecipitates from the surrounding iron matrix for a detailed characterization of their structure and chemistry using electron microscopy. Better results can generally be obtained by electron microscopy when the magnetic matrix is removed. However, there are challenges in the preparation of relatively thin CERs, especially during stripping and cleaning stages of the procedure. In this contribution, a systematic study of different parameters associated with the direct CER preparation method was performed on a vanadium-microalloyed low-carbon model steel to improve the replication procedure. Relatively low nanoscale surface kurtosis of the iron matrix with respect to the thickness of the deposited carbon layer was found to be important for successful replications from different crystallographic planes of the matrix during the stripping stage. Furthermore, modified water-based stripping and cleaning solutions were introduced that minimized wrinkling, coalescence, and disintegration of ∼8 nm thick CERs. These results are supported by (scanning) transmission electron microscopy observations, which showed successful extraction of V-rich nanoprecipitates in a size range of 1–50 nm from both the ferritic and bainitic microstructures. The modified method improves the common direct CER preparation procedure and can be used to study precipitates in a wide range of microalloyed low-carbon steels.

Enhancement of superconductivity and phase diagram of Ta-doped Kagome superconductor CsV3Sb5

Kagome superconductors AV3Sb5 (A = K, Rb, and Cs) have attracted enormous interest due to the coexistence of charge density wave (CDW) order, unconventional superconductivity (SC) and anomalous Hall effect (AHE). In this paper, we reported an intensive investigation on Cs(V1−xTax)3Sb5 single crystals with systematic Ta doping. Ta was confirmed to be doped into V-site in the Kagome layer from both single crystal X-ray diffraction structural refinement and scanning transmission electron microscopy observation. The highest Ta doping level was found to be about 16%, which is more than twice as much as 7% in Nb-doped CsV3Sb5. With the increase of Ta doping, CDW order was gradually suppressed and finally vanished when the doping level reached to more than 8%. Meanwhile, superconductivity was enhanced with a maximum critical temperature (Tc) of 5.3 K, which is the highest Tc in the bulk crystal of this Kagome system at ambient pressure so far. The μ0Hc2(T) behavior demonstrates that the system is still a two-band superconductor after Ta doping. Based on the electrical transport measurement, a phase diagram was set up to exhibit the evolution of CDW and SC in the Cs(V1−xTax)3Sb5 system. These findings pave a new way to search for new superconductors with higher Tc in the AV3Sb5 family and establish a new platform for tuning and controlling the multiple orders and superconducting states.

Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Visualization of the structural transformation of NiO/YSZ/BZY nanocomposite particles using in situ gas environmental transmission electron microscopy.

This study focused on investigating the dynamic structural transformations of spherical NiO/YSZ/BZY triple-phase nanocomposite particles, commonly employed for cermet anodes, during the hydrogen reduction reaction. We utilized both spherical aberration (Cs) corrected transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation modes under a controlled gaseous environment. The environmental gas pressure was set to 1 atm (760 Torr), mirroring real-world conditions. To elucidate pre- and post-hydrogen reduction compositional alterations, we conducted elemental mapping using energy-dispersive X-ray spectroscopy (EDS). Our findings indicated that NiO nanoparticles underwent reduction to Ni particles upon heat treatments in an environment containing H2 gas. Significantly, this reduction of NiO led to the migration of Ni along the external surface of each composite particle, ultimately resulting in the agglomeration at the interparticle spaces among the three NiO/YSZ/BZY nanocomposite particles.

Growth of metastable 2H-CaSi2 films on Si(111) substrates with ultrathin SiO2 films by solid phase epitaxy

The Si-nano dot substrates formed using the ultrathin silicon oxide films were applied to fabricate CaSi2 films. The CaSi2 formed by this process was identified as the metastable phase 2H as the main component, and the 1H structure existed partially at the grains of the 2H phase. Although no experimental reports exist for the formation of 2H-CaSi2 crystal, the Si-nano dot substrates are considered as the high-entropy substrate to form the metastable phases. We experimentally determined the lattice parameter of the 2H phase by the annular dark field–scanning transmission electron microscopy observations using the Si as an internal standard sample.

High-Resolution Electron Microscopic Characterization of Surfaces and Interfaces of Calcium in Calcium-Sulfur Batteries

As we globally realize large-scale electrical energy storage coupled with the rapid increase in global electromobility, the challenge of securing a consistent supply of critical raw materials utilized in lithium-ion batteries (LIB) continues to grow. Consequently, the search for alternative systems delivering comparable energy densities has become an important topic. Multivalent metals, renowned for their long-term stability, emerge as promising candidates in this regard.The combination of calcium and sulfur represents a potent alternative, attributed to their abundant availability and recent research delineating the appreciable stability of their electrolyte system 1. Nevertheless, to enhance fabrication efficiency, performance, and stability, a deeper understanding of the electrochemical dynamics at the anode surface, where the electrolyte interfaces with calcium, is essential. The work in the Casino project aims for an understanding and optimization of key aspects towards its application.Outer surfaces, shared with the electrolyte system are central to battery health, dictating its longevity and efficiency, making their degradation a critical aspect affecting the overall battery degradation. Therefore, investigating all galvanic surfaces stands paramount to this study. Drawing parallels with LIB systems, the application of artificial or potential optimization of natural Solid-Electrolyte Interphases (SEI) presents itself as a viable strategy to further stability within the full cells of Ca-S batteries. Envisioning the manufacturing, galvanic processes may offer significant merits for battery production, regarding its ability to build natural SEI layers.This work leans on a detailed exploration of battery systems sharing similar electrochemical foundations with galvanic systems, asserting that an investigation into galvano systems can furnish vital insights not only into the fabrication process but also unravel the undercurrent processes active during battery charging. A focal point of this research is the internal surfaces, which potentially delivers data to the understanding the electrochemical reactions, which produces the particle-like structures.In our pursuit to further unravel the intricacies of the internal particle surfaces, we used Focused Ion Beam (FIB) as preparation method to craft lamellas (see Fig.) from galvanic plating samples generated using both pulsed and constant currents. Despite the soft nature of the material posing challenges in FIB preparation, refined techniques were utilized to mitigate these issues. The analysis extended to Scanning Transmission Electron Microscopy (STEM) observations, intending to offer spectroscopic insights. Energy-Dispersive X-ray Spectroscopy (EDX) coupled with Electron Energy Loss Spectroscopy (EELS) for a detailed chemical characterization. This approach aimed to unravel bonding configurations and potential electronic transitions during the battery's operational life and to identify the potential variations in material phases or the presence of different chemical components. This approach afforded a closer look at the atomic arrangements potentially influencing the electrochemical properties.The project was financially supported by the Bundesministerium für Bildung und Forschung (BMBF), under project CaSino, Grant number (Förderkennzeichen) 03XP0487D.[1] Li, Z., Fuhr, O., Fichtner, M. & Zhao-Karger, Z. Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries. Energy Environ. Sci. 12, 3496–3501 (2019). Figure 1

Investigation of the CdZnTe (2 1 1) and (1 3 3) films grown on GaAs (2 1 1) controlled by temperature: Experiment and first-principles calculations

The origin and evolution of dual epitaxy in CdZnTe epilayer grown on GaAs substrates by close spaced sublimation has been studied, both (211) and (133) films are suitable for HgCdTe growth. And the effect of growth temperature on the orientation and crystalline quality of CdZnTe films has been investigated, more uniform orientation of adjoining growth islands in the growth of (133) film leads to a smoother surface with less threading dislocations and roughness. The interfacial properties of CdZnTe films were studied by the first-principles calculations combined with scanning transmission electron microscopy observations. By calculation the work of adhesion between the CdTe films and GaAs (211), obtain the CdTe (211)/GaAs (211) interface is the more stable interface structure at the initial nucleation stage. Combining to the results of experiment, at higher growth temperature, only (133) nucleus can reach the critical size and thus stabilize the growth. More importantly, the results on the dual epitaxy of CdZnTe films are expected to play a crucial role in guiding the dual epitaxy growth of other zincblende-type crystal.

III–V gain region/Si waveguide hybrid lasers by chip-on-wafer hydrophilic bonding process using UV-ozone treatment

The direct bonding process of InP chips on a silicon-on-insulator (SOI) wafer is investigated using surface hydrophilization by UV-ozone treatment. The influence of the treatment on surface roughness is observed by atomic force microscopy, and found to be negligibly small. A high-quality III–V/Si bonding interface without crystal defects is verified in a scanning transmission electron microscope observation. The III–V layers remaining on the SOI wafer after the removal process of the InP substrate show uniform photoluminescence intensity over the whole region of the chip, indicating a bonding interface without the influence of lateral etching and peeling-off. From this bonding process, Fabry–Perot lasers with III–V gain and Si waveguide regions are fabricated, and cw operation is successfully achieved at a stage temperature from 20 °C to 85 °C. Stable operation is also confirmed from the changes of threshold current in the aging test (200 mA at 85 °C) after 2000 h.

Creep Behavior and Deformation Mechanism of a Third-Generation Single Crystal Ni-Based Superalloy at 980 °C

As the primary choice for aero-engine turbine blades, creep resistance is an important mechanical property for the developing third-generation single crystal Ni-based superalloys. The creep behavior of the superalloy in the [001] orientation was studied at 980 °C under a series of stress levels, accompanied with scanning electron microscope (SEM) and transmission electron microscope (TEM) observation to investigate the microstructure and deformation mechanism. The deformation mechanism of the alloy is found to be dislocation gliding, propagating and forming a dislocation network in the γ/γ′ interface. Dislocation networks could hinder the movement of dislocation and decrease the creep rate to a constant during the steady-creep stage. The formation of dislocation networks was analyzed due to the interaction of <110> {111} dislocations. Then dislocations cut into γ′ phases as individual <110> super-dislocations, anti-phase boundary dislocation pairs, and stacking faults. The <110> super-dislocation in the γ′ phase may cross-slip into the {001} plane from the {111} plane to form Kear–Wilsdorf locks, which could inhibit dislocations from gliding or cross-slipping and then enhance the creep resistance.

Non-Stoichiometric <i>fcc</i>-Base GdO<i><sub>x</sub></i> Precipitations in a Mg–Zn–Gd Alloy

We have found that, during in-situ scanning transmission electron microscopy observations, heating of a Mg97Zn1Gd2 (at.%) alloy at 623K leads to dynamic precipitations of face-centered-cubic (fcc)-based Gd nanoparticles. With the aid of density-functional theory (DFT) calculations, the observed lattice constant of 5.32 Å, which is larger than that expected for pure fcc-Gd of 5.06 Å, is likely to be due to oxygen atoms inserted at tetrahedral interstitial sites with essentially a fractional occupation. Systematic DFT calculations show that the fcc-base GdOx can be stable configurations in a wide composition range, where the O atoms behave like a solute up to x ~ 1.5 to provide continuously the non-stoichiometric GdOx solid-solution phase.

Corrosion behavior of pure Al under Fe rust deposition: Decrease in the corrosion resistance of Al caused by γ-FeOOH

This study investigates the effect of γ-FeOOH on the corrosion behavior of pure Al. Polarization measurements in 0.5 wt% NaCl revealed that when the Al surface was entirely covered with γ-FeOOH, the corrosion resistance decreased. Grow discharge optical emission spectrometry and scanning transmission electron microscopy observations showed that Fe was incorporated into the surface film of Al, rendering the film inhomogeneous and decreasing in its protective ability. In addition, because γ-FeOOH readily causes the reduction reaction (cathodic reaction), the anodic reaction of Al was accelerated to balance with this increased cathodic reaction.

Observation of comet-shaped defect as killer defect in halide vapor phase epitaxial (001) β-Ga2O3 and its impact on Schottky barrier diodes

We observed killer defects that served as reverse leakage current paths for halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diodes, which resulted in a leakage current of −0.46 μA at reverse bias of −100 V. Synchrotron X-ray topography revealed comet-shaped contrasts extending along [010] which are induced from the strain field surrounding the defects. They consisted of perfect (100)-cracks and horizontal-oriented (001) cracks. The cross-sectional scanning transmission electron microscopy observation showed (100)-cracks on the surface and dislocations along [100] beneath the surface. This defect was speculated to be generated from damage during the chemical-mechanical polishing.

Cooperative Effect of Ni-Decorated Monolayer WS2, NiO, and AC on Improving the Flame Retardancy and Mechanical Property of Polypropylene Blends.

Improving the residual char of polypropylene (PP) is difficult due to the preferential complete combustion. Here, we designed a combination catalyst that not only provides physical barrier effects, but also dramatically promotes catalytic charring activity. We successfully synthesized WS2 monolayer sheets decorated with isolated Ni atoms that bond covalently to sulfur vacancies on the basal planes via thiourea. Subsequently, PP blends composed of 8 wt.% Ni-decorated WS2, NiO, and activated carbon (AC) were obtained (ENi-SWS2-AC-PP). Combining the physical barrier effects of WS2 monolayer sheets with the excellent catalytic carbonization ability of the ENi-SWS2-AC combination catalyst, the PP blends showed a remarkable improvement in flame retardancy, with the yield of residual char reaching as high as 41.6 wt.%. According to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, it was revealed that the microstructure of residual char contained a large number of carbon nanotubes. The production of a large amount of residual char not only reduced the release of pyrolytic products, but also formed a thermal shield preventing oxygen and heat transport. Compared to pure PP, the peak heat release rate (pHRR) and total heat release rate (THR) of ENi-SWS2-AC-PP were reduced by 46.32% and 26.03%, respectively. Furthermore, benefiting from the highly dispersed WS2, the tensile strength and Young's modulus of ENi-SWS2-AC-PP showed similar values to pure PP, without sacrificing the toughness.