Combination Of Scanning Transmission Electron Microscopy Research Articles

The role of clay minerals in the concentration and distribution of critical metals in lateritic palaeosols from NE Iberia

The intense chemical weathering processes that lead to the formation of laterites and bauxites result in the enrichment of critical metals (Sc, V, Co, Ga, Ge, Nb, In, Hf, Ta, W, and REEs), which are of great economic interest and are directly involved in the manufacture of various devices essential for the technology transition. This work focuses on combined geochemical and scanning transmission electron microscopy (STEM) studies of lateritic palaeosols developed in continental profiles (Lower Cretaceous, NE Iberia) in order to evaluate the distribution and concentration of critical metals and their possible relationship with weathering processes. In this study, we show that during the process of laterization the highest weathering produces the highest critical metal concentrations (reaching concentrations of 1914.80 ppm in the lateritic palaeosols and 926.30 ppm in the ferruginous macropisoids). Concentrations normalized to the upper continental crust (Taylor and McLennan, 1985) indicate that both the lateritic palaeosols and the macropisoids are enriched in LREEs (256.81 and 223.01 ppm, respectively) and HREEs (107.11and 110.53, respectively). In the lateritic palaeosols, the (La/Sm)c values are higher than those of (Gd/Yb)c, indicating that the LREEs exhibit higher fractionation with weathering, whereas the opposite occurs for the macropisoids, in which the HREEs have higher fractionation. These data indicate that the HREEs are less mobile than the LREEs during laterization. The critical metals present a good positive correlation with Fe, Ti, and Al, but a good negative correlation with K. STEM images and EDS analyses allowed us to identify REE-phases (including monazite, xenotime, and La, Ce and Nd oxides) and nanoparticles of gold adsorbed on the surface of kaolinite crystals. This shows not only that during the laterization process Fe and Ti oxides act as scavengers for the critical metals, but also that kaolinite controls their distribution.

Microscopy aided detection of the self-intercalation mechanism and in situ electronic properties in chromium selenide.

Two-dimensional (2D) chromium-based self-intercalated materials Cr1+nX2 (0 ≤ n ≤ 1, X = S, Se, Te) have attracted much attention because of their tunable magnetism with good environmental stability. Intriguingly, the magnetic and electrical properties of the materials can be effectively tuned by altering the coverage and spatial arrangement of the intercalated Cr (ic-Cr) within the van der Waals gap, contributing to different stoichiometries. Several different Cr1+nX2 systems have been widely investigated recently; however, those with the same stoichiometric ratio (such as Cr1.25Te2) were reported to exhibit disparate magnetic properties, which still lacks explanation. Therefore, a systematic in situ study of the mechanisms with microscopy techniques is in high demand to look into the origin of these discrepancies. Herein, 2D self-intercalated Cr1+nSe2 nanoflakes were synthesized as a platform to conduct the characterization. Combining scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM), we studied in depth the microscopic structure and local electronic properties of the Cr1+nSe2 nanoflakes. The self-intercalation mechanism of ic-Cr and local stoichiometric-ratio variation in a Cr1+nSe2 ultrathin nanoflake is clearly detected at the nanometer scale. Scanning tunneling spectroscopy (STS) measurements indicate that Cr1.5Se2/Cr2Se2 and Cr1.25Se2 exhibit conductive and semiconductive behaviors, respectively. The STM tip manipulation method is further applied to manipulate the microstructure of Cr1+nSe2, which successfully produces clean zigzag-type boundaries. Our systematic microscopy study paves the way for the in-depth study of the magnetic mechanism of 2D self-intercalated magnets at the nano/micro scale and the development of new magnetic and spintronic devices.

Atomically Unraveling Highly Crystalline Self-Intercalated Tantalum Sulfide with Correlated Stacking Registry-Dependent Magnetism.

In self-intercalated two-dimensional (ic-2D) materials, understanding the local chemical environment and the topology of the filling site remains elusive, and the subsequent correlation with the macroscopically manifested physical properties has rarely been investigated. Herein, highly crystalline gram-scale ic-2D Ta1.33S2 crystals were successfully grown by the high-pressure high-temperature method. Employing combined atomic-resolution scanning transmission electron microscopy annular dark field imaging and density functional theory calculations, we systematically unveiled the atomic structures of an atlas of stacking registries in a well-defined √3(a) × √3(a) Ta1.33S2 superlattice. Ferromagnetic order was observed in the AC' stacking registry, and it evolves into an antiferromagnetic state in AA/AB/AB' stacking registries; the AA' stacking registry shows ferrimagnetic ordering. Therefore, we present a novel approach for fabricating large-scale highly crystalline ic-2D crystals and shed light on a powerful means of modulating the magnetic order of ic-2D systems via stacking engineering, i.e., stackingtronics.

High-resolution chemical and structural characterization of the native oxide scale on a Mg-based alloy

The structure and composition of the native oxide forming on the basal plane (0001) of the magnesium-based alloy Mg-2Al-0.1Ca was investigated by combining scanning transmission electron microscopy (STEM) and atom probe tomography (APT). While STEM measurements demonstrated the growth of a (111) MgO oxide layer with 3–4 nm thickness on the basal (0001) plane of the atom probe specimen, APT data further revealed the formation of an aluminum-rich region between bulk magnesium and the native oxide. The aluminum enrichment of up to ∼20 at% at the interface is consistent with an inward growth of the oxide scale.

Ti-Doping in Silica-Supported PtZn Propane Dehydrogenation Catalysts: From Improved Stability to the Nature of the Pt-Ti Interaction.

Propane dehydrogenation is an important industrial reaction to access propene, the world's second most used polymer precursor. Catalysts for this transformation are required to be long living at high temperature and robust toward harsh oxidative regeneration conditions. In this work, combining surface organometallic chemistry and thermolytic molecular precursor approach, we prepared well-defined silica-supported Pt and alloyed PtZn materials to investigate the effect of Ti-doping on catalytic performances. Chemisorption experiments and density functional calculations reveal a significant change in the electronic structure of the nanoparticles (NPs) due to the Ti-doping. Evaluation of the resulting materials PtZn/SiO2 and PtZnTi/SiO2 during long deactivation phases reveal a stabilizing effect of Ti in PtZnTi/SiO2 with a kd of 0.015 h-1 compared to PtZn/SiO2 with a kd of 0.022 h-1 over 108 h on stream. Such a stabilizing effect is also present during a second deactivation phase after applying a regeneration protocol to the materials under O2 and H2 at high temperatures. A combined scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory study reveals that this effect is related to a sintering prevention of the alloyed PtZn NPs in PtZnTi/SiO2 due to a strong interaction of the NPs with Ti sites. However, in contrast to classical strong metal-support interaction, we show that the coverage of the Pt NPs with TiOx species is not needed to explain the changes in adsorption and reactivity properties. Indeed, the interaction of the Pt NPs with TiIII sites is enough to decrease CO adsorption and to induce a red-shift of the CO band because of electron transfer from the TiIII sites to Pt0.

On the faulting and twinning mediated strengthening and plasticity in a γʹ strengthened CoNi-based superalloy at room temperature

Microscopic defects and their mutual interactions are fundamentally important in deciding the load carrying capacity of structural materials. In this context, the present work reports an unprecedented precipitate shearing mechanism in CoNi-based superalloys at room temperature where L12 ordered γʹ precipitates are embedded within a disordered CoCrNi-rich matrix. A combined electron channelling contrast imaging (ECCI) and scanning transmission electron microscopy (STEM) analysis revealed stacking fault (SF) and SF coupled nano-twin formation that rendered a significant work hardening rate. The observed twinning induced plasticity (TWIP) effect correlates with an excellent combination of yield strength, ultimate tensile strength (UTS) and plasticity of 910 ± 30 MPa, 1360 ± 40 MPa and ∼ 18.5 (%), respectively, as exhibited by the peak aged Co-30Ni-12Cr-7Al-4Ti-2Nb-0.006B alloy at room temperature. Rigorous center of symmetry (COS) analysis on atomic resolution STEM images from the deformed samples unambiguously establishes precipitate shearing via single a/6⟨112⟩{111} partial dislocations resulting in twin formation, succeeding the intrinsic stacking fault (ISF) and complex stacking fault (CSF) of γ and γʹ, respectively. Through thermodynamic and first principles density functional theory calculations, it is demonstrated that the experimentally observed deformation mechanism is directly correlated with the negative stacking fault energy of γ matrix and low CSF energy of the γʹ precipitates. Based on these new findings, the present work demonstrates the flexibility of tuning the composition of CoCrNi containing superalloys to widen the alloy spectrum for designing alloys with improved properties.

Development of a Novel, Impurity-Scavenging, Corrosion-Resistant Coating for Ni-Based Superalloy CMSX-4

Sulfur, a common impurity arising from atmospheric and environmental contamination, is highly corrosive and detrimental to the lifespan of nickel superalloys in jet engines. However, sulfur-scavenging coatings have yet to be explored. Our study presents the successful development of a stable, uniform, impurity-scavenging Ni-Mn coating on Ni-based superalloy CMSX-4, through electroplating. The coating was characterised via combined scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. An optimal coating thickness of > 600 nm was deposited. The coated alloy was exposed to corrosive salt mixture 98% Na2SO4–2% NaCl at 550 °C for 100 h, mimicking engine exposure conditions, thereby proving that the coating successfully trapped sulfur and prevented its diffusion into an underlying alloy. This work presents a promising development for the prevention of sulfur-induced corrosion in industrial setting such as gas turbine engine, where the effects of sulfur diffusion into the bulk alloy could lead to premature failure.Graphical

Plasmonic properties of aluminium nanowires in amorphous silicon

Plasmonic structures can help enhance optical activity in the ultraviolet (UV) region and therefore enhancing photocatalytic reactions and the detection of organic and biological species. Most plasmonic structures are composed of Ag or Au. However, producing structures small enough for optical activity in the UV region has proved difficult. In this study, we demonstrate that aluminium nanowires are an excellent alternative. We investigated the plasmonic properties of the Al nanowires as well as the optoelectronic properties of the surrounding a − Si matrix by combining scanning transmission electron microscopy imaging, electron energy loss spectroscopy and electrodynamic modelling. We have found that the Al nanowires have distinct plasmonic modes in the UV and far UV region, from 0.75 eV to 13 eV. In addition, simulated results found that the size and spacing of the Al nanowires, as well as the embedding material were shown to have a large impact on the type of surface plasmon energies that can be generated in the material. Using electromagnetic modelling, we have identified the modes and illustrated how they could be tuned further.

Structure and electronic properties of domain walls and stacking fault defects in prospective photoferroic materials bournonite and enargite

Bournonite (CuPbSbS3) and enargite (Cu3AsS4) have recently been used as absorber layers in thin-film photovoltaic devices due to their ideal bandgap and ferroelectric properties. An understanding of the ferroelectric domain structure in these materials is required so that the benefits of the internal depolarizing electric fields can be fully exploited. Here, the atomic structure and electronic properties of domain walls (DWs) are elucidated through a combined aberration-corrected scanning transmission electron microscopy and density functional theory study. ∼90° and 180° DWs are observed in bournonite. As the 180° DW is charge neutral, it cannot contribute to the anomalous photovoltaic effect that leads to high open circuit voltages. The ∼90° DW shows a slight offset across the boundary, but the contributions of this to the anomalous photovoltaic effect are negligible. The DWs are also electrically passive, i.e., they do not result in significant recombination and do not block charge carrier transport. A high density of stacking faults (SF) was, however, observed in enargite. The SFs have a large number of defect states within the bandgap, which would lower the device efficiency through Shockley–Read–Hall recombination.

First-principles insights into complex interplays among nano-phases in an Al-Cu-Li-Zr alloy

Multiple nano-phases and nano-complexes feature the typical microstructures of current commercial Al-Li alloys. We present a combined first-principles and scanning transmission electron microscopy study of various nano-features in an aged Al-Cu-Li-Zr model alloy, in attempt to elucidate the fundamental aspects of their formation and interaction. Our results suggest that highly-diffusive excess Li can segregate to the early-precipitated β’(L12-Al3Zr) nano-phase, to develop a tentative δ’(L12-Al3Li) shell at the early-aging stage. The δ’ shell promotes GP-zone precipitation along the (100) facets but is metastable only, and can eventually dissolve into β’ with time. Consequently, GP-zones can evolve into θ” and θ’ independently under the prolonged aging. Excess Li also weakly segregates to θ’ interfaces to develop an “in-phase” or “anti-phase” δ’ shell. The gained insights help clarify various experimental observations, and their implications for Al-Li alloy design are also discussed.

High-Performance Electrostrictive Relaxors with Dispersive Endotaxial Nanoprecipitations.

Ultrahigh-precision manufacturing and detection have highlighted the importance of investigating electrostrictive materials with a weak stimulated extrinsic electric field and a simultaneous large hysteresis-free strain. In this study, a new type of electrostrictive relaxor ferroelectric is designed by constructing a complex inhomogeneous local structure to realize excellent electrostrictive properties. A remarkably large electrostrictive coefficient, M33 (8 × 10-16 m2 V-2 ) is achieved. Through a combined atomic-scale scanning transmission electron microscopy and advanced in situ high-energy synchrotron X-ray diffraction analysis, it is observed that such superior electrostrictive properties can be ascribed to a special domain structure that consists of endotaxial nanoprecipitations embedded in a polar matrix at the phase boundary of the rhombohedral/tetragonal/cubic phases. The matrix contributes to the high strain response under the weak extrinsic electric field because of the highly flexible polarization and randomly dispersed endotaxial nanoprecipitations with a nonpolar central region, which provide a strong restoring force that reduces the strain hysteresis. The approach developed in this study is widely applicable to numerous relaxor ferroelectrics, as well as other dielectrics, for further enhancing their electrical properties, such as electrostriction and energy-storage capacity.

Structure stabilization effect of vacancies and entropy in hexagonal WN.

The structural stability of hexagonal tungsten mononitride (WN) has been studied combining scanning transmission electron microscopy and first-principles calculations. The results show that the WC-type WN with vacancies of 6∼8 at% is more stable than the previously proposed MnP-type and NiAs-type structures. Due to the larger vibrational entropy of the WC-type WN, the vacancy concentration required to stabilize the WC-type structure is lower at high temperatures. The results demonstrate the importance of vacancies and configurational and vibrational entropies in the structural stability of compounds synthesized at high temperatures.

Dynamic hetero-metallic bondings visualized by sequential atom imaging

Traditionally, chemistry has been developed to obtain thermodynamically stable and isolable compounds such as molecules and solids by chemical reactions. However, recent developments in computational chemistry have placed increased importance on studying the dynamic assembly and disassembly of atoms and molecules formed in situ. This study directly visualizes the formation and dissociation dynamics of labile dimers and trimers at atomic resolution with elemental identification. The video recordings of many homo- and hetero-metallic dimers are carried out by combining scanning transmission electron microscopy (STEM) with elemental identification based on the Z-contrast principle. Even short-lived molecules with low probability of existence such as AuAg, AgCu, and AuAgCu are directly visualized as a result of identifying moving atoms at low electron doses.

Nanotwinning-assisted dynamic recrystallization at high strains and strain rates.

Grain refinement is a widely sought-after feature of many metal production processes and frequently involves a process of recrystallization. Some processing methods use very high strain rates and high strains to refine the grain structure into the nanocrystalline regime. However, grain refinement processes are not clear in these extreme conditions, which are hard to study systematically. Here, we access those extreme conditions of strain and strain rate using single copper microparticle impact events with a laser-induced particle impact tester. Using a combined dictionary-indexing electron backscatter diffraction and scanning transmission electron microscopy approach for postmortem characterization of impact sites, we systematically explore increasing strain levels and observe a recrystallization process that is facilitated by nanotwinning, which we term nanotwinning-assisted dynamic recrystallization. It achieves much finer grain sizes than established modes of recrystallization and therefore provides a pathway to the finest nanocrystalline grain sizes through extreme straining processes.

Electron microscopy for polymer structures

Abstract This paper reviews recent advances and perspectives of electron microscopy and its application to polymer hierarchical structures. Of the various kinds of hierarchical polymer structures, we placed particular emphasis on polymer nanocomposites and polymer crystals based mainly on our recent results. In those nanocomposites, the chemical bonding between the nanometer-size fillers and rubber matrix, a key contributor to the mechanical properties of the material, has been investigated by combining scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS). The position-dependent EELS spectrum with high spatial resolution of STEM successfully provided revealed the presence/absence of the chemical bonds across the interface. The mechanical properties and fracture mechanism of nanocomposites have been studied by combining structural observations made using transmission electron microscopy (TEM) with simulations. They have been further investigated using in situ TEM with a newly designed stretching holder, in which morphological changes, including cavity formation, were visualized and analyzed in terms of local strain distribution. The fracture processes of nanocomposite have been observed at nanometer resolution. The fundamental reinforcement mechanisms have been elucidated from morphological studies of nanocomposites under tensile deformation and during the fracture process. Moreover, nano-diffraction imaging, a position-resolved electron diffraction imaging with STEM, has been applied to a polymer crystal to evaluate the orientation of lamellar crystals at nanometer resolution. All these recent successes with radiation-sensitive polymer materials stemmed from developments made in electron optics and super-sensitive cameras used for advanced electron microscopy.

STEM-EELS Spectrum Imaging of an Aerosol-Deposited NASICON-Type LATP Solid Electrolyte and LCO Cathode Interface

All-solid-state batteries (ASSBs) are promising candidates for application as next-generation high-power supply and storage devices in electric vehicles. ASSBs offer excellent safety and a high energy density; however, the high interfacial resistance between the positive electrode and solid electrolyte due to solid–solid contact reactions at elevated temperatures limits their applications. To address these issues, the effect of thermal annealing on the interfacial structure between a sodium super ionic conductor (NASICON)-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte and a LiCoO2 (LCO) cathode in an ASSB fabricated by aerosol deposition was investigated experimentally. Specifically, spectrum imaging was conducted by combining scanning transmission electron microscopy and electron energy loss spectroscopy. Metastable degraded low-density transition layers were formed between LATP and LCO in the as-deposited sample. A significant reduction in interfacial resistance was achieved after thermal annealing at 250–300 °C, which was mainly attributed to structural recovery in this temperature range. However, thermal annealing at 400 °C resulted in increased interfacial resistance due to the formation of a Co3O4-like spinel blocking layer at the LATP/LCO interface. These findings provided valuable insights into the electronic properties of the ASSB composite under investigation and were consistent with theoretical predictions of Li and O transfer between the layers due to thermal annealing.

Hybrid molecular beam epitaxy growth of BaTiO3 films

The ability to reproducibly synthesize thin films with precise composition and controlled structure is essential for fundamental study and mass production. Here, we demonstrate the hybrid molecular beam epitaxy (MBE) growth of epitaxial, single crystalline BaTiO3 films with different thicknesses on Nb-doped SrTiO3 substrates with atomically smooth surfaces. By combining scanning transmission electron microscopy, temperature-dependent high-resolution x-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy, we study the effect of growth conditions and the interplay between stoichiometry and epitaxial strain on the resulting structure. Furthermore, we demonstrate a close to bulk-like ferroelectric phase transition in thicker films and highlight the effect of strain on the phase transition temperature. This work establishes the hybrid MBE approach for the growth of heteroepitaxial BaTiO3 films on conducting substrates with scalable thickness and controlled stoichiometry.

Vacancy and solute co-segregated η1 interface in over-aged Al-Zn-Mg alloys

We present a combined first-principles and scanning transmission electron microscopy (STEM) study of the η1/Al interfaces in an over-aged Al-Zn-Mg alloy. As one of the major η-MgZn2 precipitate types, η1 tends to form highly coherent interfaces with Al matrix, along with various solute segregation patterns on its interfacial layers. The interface phase diagram suggests either an Mg-rich or a Zn-rich interface, depending on the chemical potential range of Zn. Further segregation calculations, however, strongly suggest the STEM Z-contrast imaged interface to be a vacancy and solute co-stabilized Mg-rich structure that is favored at relatively low Zn chemical potentials. Based on our experimental and calculation results, the profound thermodynamics origin of the η1/Al interface and its intricate segregation behavior is clarified and described using a generalized structure model. The η1/Al interface structure and segregation at higher Zn chemical potentials are also predicted for future experimental validation.

Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO3.

The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.

Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds.

In this work, we report the presence of surface-densified phases (β-Ni5O8, γ-Ni3O4, and δ-Ni7O8) in LiNiO2 (LNO)- and LiNi0.8Al0.2O2 (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These surface phases form upon electrochemical aging at high state of charge corresponding to a fully delithiated state. A unique feature of these phases is the periodic occupancy by Ni2+ in the Li layer. This periodic Ni occupancy gives rise to extra diffraction reflections, which are qualitatively similar to those of the LiNi2O4 spinel structure, but these surface phases have a lower Ni valence state and cation content than spinel. These experimental results confirm the presence of thermodynamically stable surface phases and provide new insights into the phenomena of surface phase formation in Ni-rich layered structures.