Transmission Electron Energy Loss Spectroscopy Research Articles

Nanophase Iron Particles Derived From Fayalitic Olivine Decomposition in Chang'E‐5 Lunar Soil: Implications for Thermal Effects During Impacts

AbstractSurface‐correlated nanophase iron particles (npFe0) alter the reflectance spectrum characteristics of airless bodies, thus making it an essential aspect of studying space weathering. Vapor deposition has been the only strongly proven npFe0 formation mechanism owing to the long exposure time of Apollo samples, whereas other formation mechanisms remain questioned. Newly returned younger Chang'E‐5 samples provide an opportunity to study the incipient formation mechanism of npFe0. Here, we combined transmission electron microscopy and electron energy loss spectroscopy to characterize the microscopic features of Chang'E‐5 olivine rims. The uppermost layer of these grains exhibits the simultaneous coexistence of npFe0 with Si‐rich material overlying an Mg‐rich layer, as well as numerous irregular vesicles containing oxygen‐rich (SiO and O2) components embedded in the npFe0. These microscopic features collectively suggest subsolidus olivine decomposition during (micro)impact‐induced fragmentation or local heating processes, which may be the essential agent to alter the reflectance spectrum of airless bodies.

Two-step implantation of gold into graphene

As a one-atom thick, mechanically strong, and chemically stable material with unique electronic properties, graphene can serve as the basis for a large number of applications. One way to tailor its properties is the controlled introduction of covalently bound heteroatoms into the lattice. In this study, we demonstrate efficient implantation of individual gold atoms into graphene up to a concentration of 1.7 × 1011 atoms cm−2 via a two-step low-energy ion implantation technique that overcomes the limitation posed by momentum conservation on the mass of the implanted species. Atomic resolution scanning transmission electron microscopy imaging and electron energy-loss spectroscopy reveal gold atoms occupying double vacancy sites in the graphene lattice. The covalently bound gold atoms can sustain intense electron irradiation at 60 kV during the microscopy experiments. At best, only limited indication of plasmonic enhancement is observed. The method demonstrated here can be used to introduce a controlled concentration of gold atoms into graphene, and should also work for other heavier elements with similar electronic structure.

Shifts in valence states in bimetallic MXenes revealed by electron energy-loss spectroscopy (EELS)

MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti2−y Nb y CT x , Nb2−y V y CT x , and Ti2−y V y CT x ) were investigated with scanning transmission electron microscopy and electron energy-loss spectroscopy to determine the localized valence states of metals at the nanoscale. The analysis demonstrates changes in the electronic configuration of V upon modification of the overall composition and within individual MXene flakes. These shifts of oxidation state can explain the nonlinear optical and electronic features of solid-solution MXenes. Vanadium appears to be particularly sensitive to modification of the valence state, while titanium maintains the same oxidation state in Ti–Nb and Ti–V MXenes, regardless of stoichiometry. The study also explains Nb’s influential role in the previously observed electronic properties in the Nb–V and Nb–Ti systems.

Insights into tribofilm formation on Ti-6V-4Al in a bioactive environment: Correlation between surface modification and micro-mechanical properties

Ti-6Al-4V has been used as a surgical implant material for a long time because of its combination of strength, corrosion resistance and biocompatibility. However, there remains much that is not understood about how the surface reacts with the environment under tribocorrosion conditions. In particular, the conditions under which tribofilms form and their role on friction and wear are not clear. To evaluate the complicated nature of the dynamic surface microstructural changes on the wear track, high resolution transmission electron microscopy (TEM), scanning transmission electron microscope (STEM) and electron energy loss spectroscopy (EELS) have been used to characterise the structure and chemical composition of the tribofilm. Detailed analysis of the formation and structure of the tribofilm and the metal surface deformation behaviour were studied as a function of applied potential and the role of proteins in the lubricant. For the first time, graphitic and onion-like carbon structures from wear debris were found in the testing solution. The presence of carbon nanostructures in the tribocorrosion process and the formation of the tribofilm leads to an improved tribocorrosion behaviour of the system, in particular a reduction in wear and friction. A detailed, quantitative, analysis of surface deformation was undertaken, in particular, the geometrically necessary dislocation (GND) density was quantified using precession electron diffraction (PET). A clear correlation between applied potential, tribofilm formation and the surface strain was established. Statement of significanceThe formation of tribofilm and microstructure modification of the Ti-6Al-4V surface during tribocorrosion in a physiological environment is not fully understood. In particular, the correlation between microstructural changes and electrochemical conditions is not clear. This study presents a detailed investigation of the structure and chemical composition of tribofilms at the nanoscale during tribocorrosion tests in simulated body fluid and gives a detailed and quantitative description of the evolved surface structure. A clear correlation between applied potential, tribofilm formation and the surface strain was established. Moreover, particular attention is paid to the wear debris particles captured from the lubricating solution, including nanocarbon onion structures. The implications for tribocorrosion of the alloy in its performance as an implant are discussed.

Epitaxial Growth of CaMnO3–y Films on LaAlO3(112¯0) by Pulsed Direct Current Reactive Magnetron Sputtering

CaMnO3 is a perovskite with attractive magnetic and thermoelectric properties. CaMnO3 films are usually grown by pulsed laser deposition or radio frequency magnetron sputtering from ceramic targets. Herein, epitaxial growth of CaMnO3–y (002) films on a ‐oriented LaAlO3 substrate using pulsed direct current reactive magnetron sputtering is demonstrated, which is more suitable for industrial scale depositions. The CaMnO3–y shows growth with a small in‐plane tilt of <≈0.2° toward the (200) plane of CaMnO3–y and the () with respect to the LaAlO3 substrate. X‐ray photoelectron spectroscopy of the electronic core levels shows an oxygen deficiency described by CaMnO2.58 that yields a lower Seebeck coefficient and a higher electrical resistivity when compared to stoichiometric CaMnO3. The LaAlO3 substrate promotes tensile‐strained growth of single crystals. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal antiphase boundaries composed of Ca on Mn sites along <101> and <002>, forming stacking faults.

Improvement of Fermi-Level Pinning and Contact Resistivity in Ti/Ge Contact Using Carbon Implantation.

Effects of carbon implantation (C-imp) on the contact characteristics of Ti/Ge contact were investigated. The C-imp into Ti/Ge system was developed to reduce severe Fermi-level pinning (FLP) and to improve the thermal stability of Ti/Ge contact. The current density (J)-voltage (V) characteristics showed that the rectifying behavior of Ti/Ge contact into an Ohmic-like behavior with C-imp. The lowering of Schottky barrier height (SBH) indicated that the C-imp could mitigate FLP. In addition, it allows a lower specific contact resistivity (ρc) at the rapid thermal annealing (RTA) temperatures in a range of 450–600 °C. A secondary ion mass spectrometry (SIMS) showed that C-imp facilitates the dopant segregation at the interface. In addition, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping showed that after RTA at 600 °C, C-imp enhances the diffusion of Ge atoms into Ti layer at the interface of Ti/Ge. Thus, carbon implantation into Ge substrate can effectively reduce FLP and improve contact characteristics.

Effects of Carbon Incorporation on Electrical Characteristics and Thermal Stability of Ti/TiO2/n-Ge MIS Contact

The effects of carbon incorporation on the thermal stability of the interfacial TiO₂ layer and the electrical characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were investigated. The improved thermal stability and contact characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were characterized in terms of Schottky barrier height (SBH) and specific contact resistivity (&#x03C1;_<i>c</i>) using the Schottky diode and circular transmission line model (CTLM). The values of SBH and &#x03C1;_<i>c</i> increased after the rapid thermal annealing (RTA) above 550 &#x00B0;C. The current density&#x2013;bias voltage (<i>J&#x2013;V</i>) curves of the Schottky diode showed a change of contact characteristics from Ohmic-like behavior to rectifying. This thermal instability was mainly caused by the decomposition of the interfacial TiO₂ layer after high-temperature annealing. The structural degradation was confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses. When carbon ions were incorporated into the interfacial TiO₂ layer, the SBH and &#x03C1;_<i>c</i> values showed relatively stable characteristics as the RTA temperature increased up to 600 &#x00B0;C. The EELS mapping showed that the diffusion of oxygen from the interfacial TiO₂ layer was effectively suppressed thanks to the incorporation of carbon. Thus, the carbon incorporation can improve the thermal stability of the interfacial TiO₂ layer and the metal&#x2013;insulator&#x2013;semiconductor contact characteristics for Ge-based device applications.

Probing the Thermal Behavior and Stability of Metal‐Fe3O4 Heterodimer Nanoparticles Utilizing In Situ Pulsed Laser Heating TEM

In situ laser heating with transmission electron microscopy (ILH‐TEM) is utilized as a neoteric method to probe the thermal behavior and stability of metal‐ (Pt, Au‐) metal oxide (Fe3O4) heterodimer (HD) nanoparticles. Two different supporting membrane types are used for the laser heating: pure‐carbon and silicon‐nitride, requiring different amounts of laser power to induce morphological changes. Diffraction pattern analysis and scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) are utilized to analyze reduction of the iron oxide under laser heating, which has differing effects depending on the support film heating mode and the heterodimer particle. A unique wetting behavior of Pt and Au onto Fe3O4 is observed in both heterodimers of Au and Pt. However, this wetting behavior is found to be partially reversible only in the case of the Au‐Fe3O4 HD system, whereas for the Pt‐Fe3O4 HD system, this wetting process was found to be irreversible under the applied experimental conditions. This observation of a novel wetting behavior has future implications for understanding the performance of oxide supported metal nanoparticles in high temperature applications, such as catalysis.

Analysis of structural transformation in nanocrystalline Y2O3 during high energy ball milling

In-depth understanding of the structural transformation mechanism in nanocrystalline yttria (Y2O3) is crucial to design advanced materials satisfying a wide range of technical demands. Present study investigates the structural modification in cubic Y2O3 nanopowders induced by high energy ball-milling as a function of milling duration. Y2O3 nanopowder with an initial crystallite size of 25–50 nm was subjected to high energy ball milling (1000 rpm) for incremental time periods up to 30 h. X-ray diffraction (XRD) and Rietveld analysis were employed to study the phase evolution, crystallite size, and strain at every stages of milling. High Resolution Transmission Electron Microscopy (HR TEM) and Electron Energy Loss Spectroscopy (EELS) is utilized to realize the mechanisms involved in phase transformation. It is observed that as milling duration progresses, the cubic Y2O3 gets refined and gradually transforms to the monoclinic phase. It is also apparent that at finer sizes, the monoclinic phase is preferred over the cubic one. The critical crystallite size for the monoclinic phase to stabilize is observed to be ~ 13 nm. Results show the complete structural modification after 30 h of milling duration.

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.

Phase Change Ge-Rich Ge-Sb-Te/Sb2Te3 Core-Shell Nanowires by Metal Organic Chemical Vapor Deposition.

Ge-rich Ge–Sb–Te compounds are attractive materials for future phase change memories due to their greater crystallization temperature as it provides a wide range of applications. Herein, we report the self-assembled Ge-rich Ge–Sb–Te/Sb2Te3 core-shell nanowires grown by metal-organic chemical vapor deposition. The core Ge-rich Ge–Sb–Te nanowires were self-assembled through the vapor–liquid–solid mechanism, catalyzed by Au nanoparticles on Si (100) and SiO2/Si substrates; conformal overgrowth of the Sb2Te3 shell was subsequently performed at room temperature to realize the core-shell heterostructures. Both Ge-rich Ge–Sb–Te core and Ge-rich Ge–Sb–Te/Sb2Te3 core-shell nanowires were extensively characterized by means of scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman microspectroscopy, and electron energy loss spectroscopy to analyze the surface morphology, crystalline structure, vibrational properties, and elemental composition.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates.

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form SW and S2W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS2 monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS2 . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS2 monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Metallic capped quasi-two-dimensional electron gas in a SrTiO3-based heterostructure

Two-dimensional electron gas (2DEG) in SrTiO3 (STO)-based heterostructures has been a subject of intense scientific interest in recent years. In this work, the metallic transition metal dichalcogenides ZrTe2 was grown on STO by pulsed laser deposition and AlN was subsequently deposited as a protection layer. The high-resolution transmission electron microscopy and electron energy loss spectroscopy results demonstrated the system as a multilayer structure of AlN/ZrTe2/ZrO2/STO due to interface redox reactions and implied a conductive STO surface. The remarkable Shubnikov–de Haas oscillations detected by angular dependent magnetotransport measurements reveal clear evidence of a high mobility quasi-2DEG in the STO-based interface. Moreover, evidence for extra carriers with three-dimensional features is observed implying the multiband contributions, which provide an explanation for some anomalous behavior in STO-based heterostructures. In addition, the thickness dependence study suggests the charge transfer effect between the capping metallic topological material ZrTe2 and the interfacial 2DEG. This work provides insight into the intrinsic electronic structure of STO-based heterostructures, and the integrated systems can serve as a platform for studying the interplay of the 2DEG with attractive materials as well as developing practical device applications.

Atomic-scale characterization of structural and electronic properties of Hf doped β-Ga2O3

In this Letter, we investigate the atomic and electronic structure of a Hf-doped beta-gallium oxide (β-Ga2O3) single crystal using high resolution scanning transmission electron microscopy imaging and electron energy loss spectroscopy. Ultraviolet-visible (UV-Vis)-near-infrared absorption measurements and density functional theory calculations are performed to further connect the nanoscale observation to the macroscale properties arising from the atomic structure. The Hf-doped sample was grown from the melt with a nominal Hf concentration of 0.5 at. %. We show that the Hf dopants prefer to occupy octahedral over tetrahedral sites by 0.68 eV and have some resistance to form precipitates due to a repulsive interaction of 0.17 eV between Hf atoms on neighboring sites. Also, the presence of Hf atoms on either tetrahedral or octahedral sites do not significantly affect the crystal structure of β-Ga2O3. Finally, the bandgap values of the Hf doped β-Ga2O3 obtained by electron energy loss spectroscopy and UV-Vis-spectroscopy were Eg = 4.83 ± 0.1 and 4.75 ± 0.02 eV, respectively, similar to the values reported for unintentionally doped β-Ga2O3 crystals. All these results make Hf an excellent dopant candidate for β-Ga2O3.

Thermal near-field tuning of silicon Mie nanoparticles

Abstract Tunable high-refractive-index nanostructures are highly desired for realizing photonic devices with a compact footprint. By harnessing the large thermo-optic effect in silicon, we show reversible and wide thermal tuning of both the far- and near-fields of Mie resonances in isolated silicon nanospheres in the visible range. We perform in situ heating in a transmission electron microscope and electron energy-loss spectroscopy to show that the Mie resonances exhibit large spectral shifts upon heating. We leverage the spectral shifts to demonstrate near-field tuning between different Mie resonances. By combining electron energy-loss spectroscopy with energy-dispersive X-ray analysis, we show a reversible and stable operation of single silicon nanospheres up to a temperature of 1073 K. Our results demonstrate that thermal actuation offers dynamic near-field tuning of Mie resonances, which may open up applications in tunable nonlinear optics, Raman scattering, and light emission.

Hydroxyapatite and lead-substituted hydroxyapatite near-surface structures: Novel modelling of photoemission lines from X-ray photoelectron spectra

X-ray photoelectron spectroscopy (XPS) is one of the main tools for hydroxyapatite (HA) surface characterization in developing materials for biomedical and heterogeneous catalysis. Despite the XPS technique's potential to correlate binding energies with existing photo-emitter sites on near-surfaces, few previous studies analyzed this aspect for HA and metal-substituted HA surfaces. In this work, we theoretically reconstructed the XPS spectra of stoichiometric HA and lead-substituted hydroxyapatite (PbCaHA, Ca10-xPbx(PO4)6(OH)2; x = 2, 10) using a first-principles linear combination of atomic orbitals embedded cluster approach and periodic supercell band structures within the framework of Density Functional Theory (DFT). We take into account photoemission lines contributions from Ca(1), Ca(2), Pb(1) and Pb(2) sites located on surface and near-surface depths (up to ∼15 Å) along the (001) and (100) surfaces. The calculated DFT spectra of HA and PbCaHA were compared with high-resolution XPS spectra previously characterized by synchrotron X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). A combined theoretical and experimental approach enables decoding of the complex structure of HA and PbCaHA in XPS spectra. It was found that XPS binding energies profiles depend significantly on photo-emitters from near-surface sites and surface crystallographic orientation. The main Ca 2p3/2 envelope peak in HA is predominantly from Ca(1) and Ca(2) sites (∼347.4 eV), while the weaker peak is due to the Ca(2) site only (∼345.0 eV). Variations on HA nanoparticle morphology could be a critical factor for changes in XPS binding energies' profile.

A semi-classical theory of magnetic inelastic scattering in transmission electron energy loss spectroscopy

The feasibility of detecting magnetic excitations using monochromated electron energy loss spectroscopy in the transmission electron microscope is examined. Inelastic scattering cross-sections are derived using a semi-classical electrodynamic model, and applied to AC magnetic susceptibility measurements and magnon characterization. Consideration is given to electron probes with a magnetic moment, such as vortex beams, where additional inelastic scattering can take place due to the change in magnetic potential energy of the incident electron in a non-uniform magnetic field. This so-called ‘Stern-Gerlach’ energy loss can be used to enhance the strength of the scattering by increasing the orbital angular momentum of the vortex beam, and enables separation of magnetic from non-magnetic (i.e. dielectric) energy losses, thus providing a promising experimental route for detecting magnons. AC magnetic susceptibility measurements are however not feasible using Stern-Gerlach energy losses for a vortex beam.

Nature of optical excitations and bandgap of Re x Mo1−x S2 alloy at nanoscale probed from high resolution low loss electron energy loss spectroscopy

The two-dimensional (2D) transitional metal dichalcogenides (TMDS) have become an intensive research topic recently. The alloys of these TMDs have offered continuous tunability of the bandstructure and carrier concentration, providing a new opportunity for various device applications. Here the rich variations in optical excitations in Re x Mo1−x S2 alloy at the nanoscale region are shown. The alloy bandgap and charge response are probed by low-loss high-resolution transmission electron energy loss spectroscopy (HR-EELS). Concurrent density functional theory calculations revealed many electronic structures from n-type semiconductors to metallic and p-type semiconducting nature with band bowing effect. The alloying-induced Peierls distortion leads to a change in crystal symmetry and decreased interlayer coupling. These alloys undergo indirect to direct bandgap transition with the function of Re concentration. These unique correlated structural and electronic properties of these 2D alloys can be potentially applicable for various electronic and optoelectronic devices.

Phosphazene-Based Covalent Organic Polymer Decorated with NiCo2O4 Nanocuboids as a Trifunctional Electrocatalyst: A Unique Replacement for the Conventional Electrocatalysts

Developing nonprecious metal-based electrocatalysts to convert water into green fuels (H2 and O2) is key to address urgent climate and energy challenges. We have prepared an electrocatalyst by the immobilization of NiCo2O4 on a phosphazene-based covalent organic polymer (P-COP) through a facile hydrothermal method. The elemental composition of the P-COP showed the presence of a greater amount of heteroatoms N (6.62%) and P (5.62%) throughout the polymer support. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were utilized to determine the atomic structure of the nanocuboids, which depicted the formation of an inverse spinel structure. A NiCo2O4-P-COP-based electrode was simultaneously used for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), and it displayed a minimum overpotential of 270 and 130 mV (V vs RHE), respectively, at a current density of 10 mA cm–2. In addition, it acted as an oxygen reduction catalyst with a half-wave potential of 0.83 V (V vs RHE) and a maximum current density of 4.5 mA cm–2. The electrocatalytic activity is comparable with that of the commercially available Pt and RuO2 catalysts. The combined experimental and computational studies confirm that the catalytic centers formed through the interaction between the heteroatoms (N and P) in the phosphazene matrix and metal oxides (Co and Ni) play an important role in its improved durability and electrocatalytic activity.

Hard carbon as a negative electrode material for potassium-ion batteries prepared with high yield through a polytetrafluoroethylene-based precursor

A new additive (polytetrafluoroethylene, PTFE) to typical sugar precursors for hard carbon (HC) preparation via hydrothermal carbonization has been proposed and investigated. The HC samples obtained from sugars (D-glucose and pectin) with and without PTFE were characterized with X-ray powder diffraction, Raman spectroscopy, scanning and transmission electron microscopy, energy-dispersive X-ray and electron energy loss spectroscopy, specific surface area measurements. Adding PTFE to sugars significantly improves the HC yield from <10% to 28% per total precursor mass as PTFE pyrohydrolysis generates small hollow HC nanospheres of 200-250 nm in diameter which then act as seeds for carbonization of sugars. Employing the PTFE additives improves discharge capacity (285 mAh/g at C/10 charge/discharge rate), enhances rate capability (232 mAh/g at 1C charge/discharge rate) and cycling stability of HC as a negative electrode material for potassium-ion batteries that has been tested in both potassium half-cell and potassium-ion full cell configurations.