Transmission Electron Energy Loss Spectroscopy Research Articles

Thermal stability and diffusion characteristics of ultrathin amorphous carbon films grown on crystalline and nitrogenated silicon substrates by filtered cathodic vacuum arc deposition

Amorphous carbon (a-C) films are widely used as protective overcoats in many technology sectors, principally due to their excellent thermophysical properties and chemical inertness. The growth and thermal stability of sub-5-nm-thick a-C films synthesized by filtered cathodic vacuum arc on pure (crystalline) and nitrogenated (amorphous) silicon substrate surfaces were investigated in this study. Samples of a-C/Si and a-C/SiNx/Si stacks were thermally annealed for various durations and subsequently characterized by high-resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The TEM images confirmed the continuity and uniformity of the a-C films and the 5-nm-thick SiNx underlayer formed by silicon nitrogenation using radio-frequency sputtering. The EELS analysis of cross-sectional samples revealed the thermal stability of the a-C films and the efficacy of the SiNx underlayer to prevent carbon migration into the silicon substrate, even after prolonged heating. The obtained results provide insight into the important attributes of an underlayer in heated multilayered media for preventing elemental intermixing with the substrate, while preserving the structural stability of the a-C film at the stack surface. An important contribution of this investigation is the establishment of an experimental framework for accurately assessing the thermal stability and elemental diffusion in layered microstructures exposed to elevated temperatures.

Development of a gas environmental heating specimen holder system using differential pumping.

We developed a gas environmental heating specimen holder system by applying differential pumping effect to a specimen holder for the insitu transmission electron microscopy observation and electron energy loss spectroscopy (EELS) analysis of catalytic materials. In the insitu experiments, using two small orifices and O-rings, the maximum formed gas pressure was ∼20 Pa. Also, using a heater membrane, the maximum obtained heating temperature was ∼1000°C. We could actually observe/analyze the Pt and Ni nanoparticles with an atomic scale using a double-aberration-corrected microscope and an EELS instrument in the reaction gases at high temperatures.

Formation of Vanadium Nanoparticles by Laser Ablation in Reductive Aqueous Solution

Vanadium nanoparticles were produced by laser ablation of a vanadium metal plate in a reductive aqueous solution. Transmission electron microscopy and electron energy-loss spectroscopy revealed tha...

Grain boundaries contribute to highly efficient lithium‐ion transport in advanced LiNi0.8Co0.15Al0.05O2 secondary sphere with compact structure

AbstractLiNi0.8Co0.15Al0.05O2 (NCA) secondary particles with high tap density have a great potential for high volumetric energy density lithium (Li)‐ion power battery. However, the ionic conductivity mechanism of NCA with compact structure is still a suspense, especially the function of grain boundaries. Herein, we systematically investigate the Li‐ion transport behavior in both the primitive NCA (PNCA) secondary sphere densely grown by single‐crystal primary grains and ball‐milled NCA (MNCA) nanosized particle to reveal the role of grain boundaries for Li‐ion transport. The PNCA and MNCA have comparable Li‐ion diffusion coefficients and rate performance. Moreover, the graphene nanosheet conductive additive only mildly affects the Li‐ion diffusion in PNCA cathode, while which severely blocks the Li‐ion transport in MNCA cathode. Through high‐resolution transmission electron microscopy and electron energy loss spectroscopy, we clearly observe Li‐ion depletion at lower state of charge (SOC) and Li‐ion aggregation at high SOC along the grain boundaries of PNCA secondary particles during high‐rate lithiation process. The grain boundaries can construct an interconnected Li‐ion transport network for highly efficient Li‐ion transport, which contributes to excellent high‐rate performance of compact PNCA secondary particles. These findings present new strategy and deep insight in designing compact materials with excellent high‐rate performance.

Subnanoscale Investigation of the Interface Between c-Si and PECVD AlOxand Its Effect on Surface Passivation

We present a subnanometer scale study of the interface between crystalline silicon (c-Si) and aluminum oxide (AlOx) deposited by an industrial type, remotely plasma-enhanced chemical vapor deposition (PECVD) system. We investigate the origin of the negative charge density within the AlOx layer and the interface defect density. In order to elucidate the source of the negative charge in subnanoscale, we study various AlOx layers with different properties, such as total charge ( Q tot) and interface defect density ( Dit ) in both as-deposited and thermally treated conditions using scanning transmission electron microscopy and electron energy loss spectroscopy (EELS). The spatially resolved EELS analysis of aluminum (Al) at the c-Si/AlOx interface reveals changes in its chemical configuration through the thermal process. It is found that the ratio of tetrahedrally- to octahedrally-coordinated Al at the c-Si/AlOx interface is a key factor that linearly correlates with the resulting Q tot. However, the ratio in the AlOx layer (far from the interface) stays constant and is the same for both as-deposited and thermally-treated samples, which seems to show a different aspect of PECVD AlOx layers to those presented in previous reports regarding plasma-assisted atomic layer deposited Al2O3. We also highlight the possibility of the combined impact on Dit from the interfacial layer's thickness and the ratio of silicon dioxide to the tetrahedral Al peaks.

Mapping Modified Electronic Levels in the Moiré Patterns in MoS2/WSe2 Using Low-Loss EELS.

Hybrid/moiré interlayer and intralayer excitons have been realized in twisted two-dimensional transition metal chalcogenides (2D-TMD) due to variation in local moiré potential within a moiré supercell. Though moiré excitons have been detected in TMD heterostructures by macroscopic spectroscopic techniques, their spatial distribution is experimentally unknown. In the present work, using high-resolution scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS), we explore the effect of the twist angle in MoS2/WSe2 heterostructures. We observe weak interaction between the layers at higher twist angles (>5°) and stronger interaction for lower twist angles. The optical response of the heterostructure varies within the moiré supercell, with a lower energy absorption peak appearing in regions with the AA stacking.

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.

Low-Temperature Exothermic Reactions in Al/CuO Nanothermites Producing Copper Nanodots and Accelerating Combustion

Several Al/CuO nanocomposite reactive materials, prepared recently by arrested reactive milling, were found to exhibit a distinct low-temperature exothermic peak around 600 K seen by differential scanning calorimetry. This work is an experimental study aimed to establish whether this low-temperature reaction affects combustion and why it is observed in some but not all materials with identical compositions. The peak is only observed when aluminum is initially separately milled in acetonitrile. Electron microscopy showed resulting composite powders to be porous, unlike fully dense powders obtained without premilling aluminum. The as-prepared and partially reacted powders recovered after heating just past the peak to 650 K were examined using high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy. Additionally, X-ray diffraction measurements were performed for all materials. Finally, the reactive powders were ignited using plasma and shock generated by an electrostatic discharge. Results show that the low-temperature exothermic peak is associated with a redox reaction pathway involving release of oxygen by CuO that is not in direct contact with Al and free-molecular transport of that oxygen to the nearby surface of Al. This reaction pathway inhibits the formation of Al/Cu intermetallic phases. Instead, nanometer-scale metallic Cu particles are formed due to CuO reduction. It is also found that this reaction pathway accelerates ignition of reactive powders, which in turn leads to a higher burn rate of aerosolized powder.

Enhanced ferroelectric switching speed of Si-doped HfO2 thin film tailored by oxygen deficiency

Investigations concerning oxygen deficiency will increase our understanding of those factors that govern the overall material properties. Various studies have examined the relationship between oxygen deficiency and the phase transformation from a nonpolar phase to a polar phase in HfO2 thin films. However, there are few reports on the effects of oxygen deficiencies on the switching dynamics of the ferroelectric phase itself. Herein, we report the oxygen- deficiency induced enhancement of ferroelectric switching properties of Si-doped HfO2 thin films. By controlling the annealing conditions, we controlled the oxygen deficiency concentration in the ferroelectric orthorhombic HfO2 phase. Rapid high-temperature (800 °C) annealing of the HfO2 film accelerated the characteristic switching speed compared to low-temperature (600 °C) annealing. Scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) revealed that thermal annealing increased oxygen deficiencies, and first-principles calculations demonstrated a reduction of the energy barrier of the polarization flip with increased oxygen deficiency. A Monte Carlo simulation for the variation in the energy barrier of the polarization flipping confirmed the increase of characteristic switching speed.

Phase formation and thermoelectric properties of Zn1+Sb binary system

The phase formation and thermoelectric (TE) properties in the central region of the Zn−Sb phase diagram were analyzed through synthesizing a series of Zn1+xSb (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) materials by reacting Zn and Sb powders below the solidus line of the Zn−Sb binary phase diagram followed by furnace cooling. In this process, the nonstoichiometric powder blend crystallized in a combination of ZnSb and β-Zn4Sb3 phases. Then, the materials were ground and hot pressed to form dense ZnSb/β-Zn4Sb3 composites. No traces of Sb and Zn elements or other phases were revealed by X-ray diffraction, high resolution transmission electron microscopy and electron energy loss spectroscopy analyses. The thermoelectric properties of all materials could be rationalized as a combination of the thermoelectric behavior of ZnSb and β-Zn4Sb3 phases, which were dominated by the main phase in each sample. Zn1.3Sb composite exhibited the best thermoelectric performance. It was also found that Ge doping substantially increased the Seebeck coefficient of Zn1.3Sb and led to significantly higher power factor, up to 1.51 mW·m−1·K−2 at 540 K. Overall, an exceptional and stable TE figure of merit (ZT) of 1.17 at 650 K was obtained for Zn1.28Ge0.02Sb.

Cadmium tolerance and bioremediation potential of filamentous fungus Penicillium chrysogenum FMS2 isolated from soil

Cadmium-tolerant fungus, designated as strain FMS2, was isolated from metal-polluted soil collected from Lahore, Pakistan, and its role in cadmium bioremediation was investigated. Strain FMS2 was identified as Penicillium chrysogenum through sequence analysis of the internal transcribed spacer (ITS) regions. Physicochemical growth factors and maximum tolerance level to cadmium were determined. Cadmium uptake and bioremoval potential were analyzed by atomic absorption spectrophotometry, scanning electron microscopy and energy-dispersive spectroscopic analysis, whereas its distribution inside the fungal cell was detected by scanning transmission electron microscopy and electron energy loss spectroscopy. P. chrysogenum FMS2 was found to be resistant to cadmium with a maximum tolerance level up to 1000 mg l−1. Strain FMS2 can grow under a wide range of temperature (15–35 °C) and pH conditions (4.0–12.0). The cadmium removal capacity of strain FMS2 was observed as 49 ± 0.8% within 15 days after exposure. Distinct white patches were observed in scanning electron micrographs with clear peaks of cadmium in energy-dispersive spectroscopic spectra indicating cadmium biosorption. The scanning transmission electron micrographs and electron energy loss spectra of strain FMS2 cells revealed cytoplasmic localization of cadmium.

A Sequence of Phase Transformations and Phases in NiCoFeCrGa High Entropy Alloy

The present investigation is directed to phase transitions in the equimolar NiCoFeCrGa high entropy alloy, which is a mixture of face-centered cubic (FCC) and body-centered cubic (BCC) crystalline phases. The microstructure of the samples was investigated by using scanning electron microscopy (SEM), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), transmission electron microscopy-based energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), as well as X-ray diffraction (XRD) measurements. Based on the phases observed in different temperature ranges, a sequence of the phase transitions can be established, showing that in a realistic process, when freely cooling the sample with the furnace from high to room temperature, a microstructure having spinodal-like decomposition can also be expected. The elemental mapping and magnetic behaviors of this decomposed structure are also studied.

Formation of Highly Doped Nanostripes in 2D Transition Metal Dichalcogenides via a Dislocation Climb Mechanism.

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4-15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2 . Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

Modification of the van der Waals interaction at the Bi2Te3 and Ge(111) interface

We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.

Two-dimensional iodine-monofluoride epitaxy on WSe2

Interhalogen compounds (IHCs) are extremely reactive molecules used for halogenation, catalyst, selective etchant, and surface modification. Most of the IHCs are unstable at room temperature especially for the iodine-monofluoride (IF) whose structure is still unknown. Here we demonstrate an unambiguous observation of two-dimensional (2D) IF bilayer grown on the surface of WSe2 by using scanning transmission electron microscopy and electron energy loss spectroscopy. The bilayer IF shows a clear hexagonal lattice and robust epitaxial relationship with the WSe2 substrate. Despite the IF is known to sublimate at −14 °C and has never found as a solid form in the ambient condition, but surprisingly it is found stabilized on a suitable substrate and the stabilized structure is supported by a density functional theory. This 2D form of IHC is actually a byproduct during a chemical vapor deposition growth of WSe2 in the presence of alkali metal halides as a growth promoter and requires immediate surface passivation to sustain. This work points out a great possibility to produce 2D structures that are unexpected to be crystallized or cannot be obtained by a simple exfoliation but can be grown only on a certain substrate.

Constructing Li‐Rich Artificial SEI Layer in Alloy–Polymer Composite Electrolyte to Achieve High Ionic Conductivity for All‐Solid‐State Lithium Metal Batteries

To achieve high ionic conductivity for solid electrolyte, an artificial Li-rich interface layer of about 60nm thick has been constructed in polymer-based poly(ethylene oxide)-lithium bis(trifluoromethanesulfonyl)imide composite solid electrolyte (briefly noted as PEOm ) by adding Li-based alloys. As revealed by high-resolution transmission electron microscopy and electron energy loss spectroscopy, an artificial interface layer of amorphous feature is created around the Li-based alloy particles with the gradient distribution of Li across it. Electrochemical analysis and theoretical modeling demonstrate that the interface layer provides fast ion transport path and plays a key role in achieving high and stable ionic conductivity for PEOm -Li21 Si5 composite solid electrolyte. The PEOm -5%Li21 Si5 composite electrolyte exhibits an ionic conductivity of 3.9 × 10-5 Scm-1 at 30°C and 5.6 × 10-4 Scm-1 at 45°C. The LiFePO4 | PEOm -5%Li21 Si5 | Li all-solid-state batteries could maintain a stable capacity of 129.2mAhg-1 at 0.2 C and 30°C after 100 cycles, and 111.3mAhg-1 after 200 cycles at 0.5 C and 45°C, demonstrating excellent cycling stability and high-rate capability.

Co oxidation state at LSC-YSZ interface in model solid oxide electrochemical cell

Oxidation states of cations in a perovskite structure play an important role for conductivity in solid oxide electrochemical cells. For bulk materials oxidation states can be calculated for specific conditions, but is not well understood for micro−/nanostructured materials and at interfaces between materials. We present a fundamental study of interfaces between La0.6Sr0.4CoO3-δ (LSC) and yttria-stabilized zirconia (YSZ) in symmetric model solid oxide electrochemical cells. Nanoscale morphology as well as the chemical state of the LSC are investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The experiments were performed in situ at temperatures up to 600 °C in high vacuum (ca. 10−7 mbar) and in 2 mbar oxygen. The measured LSC Co oxidation at room temperature is lower than that expected for bulk LSC, indicating a high oxygen vacancy density and possibly high ionic conductivity. However, the Co oxidation state increases with increasing temperature, both in oxygen and in vacuum. The results shows that the Co oxidation state approach that expected for bulk LSC for typical solid oxide fuel or electrolysis cell operation temperatures.

Spatial distribution of metallic heteroatoms in soot nanostructure mapped by aberration-corrected STEM-EELS

Soot from the thermochemical conversion of solid and liquid fuels can be infused with metallic heteroatoms originating from the fuel – these heteroatoms alter the nanostructure and the reactivity of the soot. Here, we investigate the spatial distribution of metallic heteroatoms in soot generated by biomass gasification, using aberration-corrected Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). The technique allowed for the mapping of heteroatom distribution in soot at the nanoscale, and thereby for the direct correlation of heteroatom concentration with the graphitic nanostructure. Spherical soot particles were coated with a thin layer of silicon, possibly in the form of quartz that may be linked to minor distortions of the nanostructure of the graphitic shell of the particles. Further results on non-spherical soot and inorganic-carbon fused aggregates suggest that the chemistry of formation was affected by the presence of gaseous ash-forming elements, especially calcium, with carbon-oxygen functional groups forming as intermediates in the graphite-inorganic reaction; i.e., prior to the formation of the thermodynamically stable carbonate bonds. The analytical approach demonstrated here can potentially help select fuel additives or aid in the design of fuel blends that minimize the formation of similar, hybrid carbon nanoparticles in combustion or gasification systems.

Flash sintering for BaTiO<sub>3</sub> with square alternating current field including zero-field duration

The shrinkage behavior of barium titanate (BaTiO3) green compacts during flash sintering with a square alternating current (AC) field including a zero-field duration (AC-pulse wave) was investigated. It was found that an AC-pulse wave is effective for the densification of BaTiO3 green compacts without reduction and discharge, which is confirmed by transmission electron microscopy and electron energy loss spectroscopy. BaTiO3 sintered polycrystal was obtained at a furnace temperature of 1000 °C by the optimal AC-pulse conditions of 285 V/cm at 250 Hz and a duty ratio of 30 %. A waveform with alternating polarity and the inclusion of a zero-field duration is shown to be possibly useful for flash sintering of oxide materials including altervalent cations.

Resolving Heterogeneity in Battery Interphases By Cryogenic Electron Microscopy

The stability of lithium batteries is tied to the physicochemical properties of the solid-electrolyte interphase (SEI), a surface passivation layer on the battery anode. However, owing to the difficulty in characterizing this sensitive interphase with nanoscale resolution, the distribution of SEI components at the nanoscale is poorly understood. Here, we use cryogenic scanning transmission electron microscopy (cryo-STEM) and electron energy loss spectroscopy (EELS) to map the spatial distribution of sensitive SEI components across the metallic Li anode. We reveal that LiF, an inorganic SEI component widely believed to play an important role in battery passivation, is absent within the compact SEI film (~15 nm) in direct contact with the active material. Instead, LiF particles (100-400 nm) are found to precipitate across the electrode surface independently from compact SEI formation. Based on these observations, we conclude that LiF cannot be a dominant contribution to anode passivation nor does it influence Li+ transport across the compact SEI film. Using cryo-STEM and EELS, we refine the traditional view of the SEI structure derived from ensemble-averaged characterizations and nuance the role of SEI components on battery performance.