Energy Loss Spectroscopy Research Articles

Enhancing Ion Migration in Grain Boundaries of Hybrid Organic–Inorganic Perovskites by Chlorine

Ionicity plays an important role in determining material properties, as well as optoelectronic performance of organometallic trihalide perovskites (OTPs). Ion migration in OTP films has recently been under intensive investigation by various scanning probe microscopy (SPM) techniques. However, controversial findings regarding the role of grain boundaries (GBs) associated with ion migration are often encountered, likely as a result of feedback errors and topographic effects common in to SPM. In this work, electron microscopy and spectroscopy (scanning transmission electron microscopy/electron energy loss spectroscopy) are combined with a novel, open‐loop, band‐excitation, (contact) Kelvin probe force microscopy (BE‐KPFM and BE‐cKPFM), in conjunction with ab initio molecular dynamics simulations to examine the ion behavior in the GBs of CH3NH3PbI3 perovskite films. This combination of diverse techniques provides a deeper understanding of the differences between ion migration within GBs and interior grains in OTP films. This work demonstrates that ion migration can be significantly enhanced by introducing additional mobile Cl− ions into GBs. The enhancement of ion migration may serve as the first step toward the development of high‐performance electrically and optically tunable memristors and synaptic devices.

Consequences of the CMR effect on EELS in TEM

Double perovskite oxides have gained in importance and exhibit negative magnetoresistance, which is known as colossal magnetoresistance (CMR) effect. Using a La2CoMnO6 (LCM) thin film layer, we proved that the physical consequences of the CMR effect do also influence the electron energy loss spectrometry (EELS) signal. We observed a change of the band gap at low energy losses and were able to study the magnetisation with chemical sensitivity by employing energy loss magnetic chiral dichroism (EMCD) below the Curie temperature TC, where the CMR effect becomes significant.

Precision measurements of cross-sections for inelastic processes in collisions of alkali metal ions with atoms of rare gases

A multifaceted experimental study of collisions of Na[Formula: see text] and K[Formula: see text] ions in the energy range of 0.5–10 keV with He and Ar atoms is presented. Absolute cross-sections for charge-exchange, ionization, stripping and excitation processes were measured using a refined version of the transfer electric field method, angle- and energy-dependent collection of product ions, energy loss and optical spectroscopy methods. The experimental data and the schematic correlation diagrams are employed to analyze and determine the mechanisms for these processes.

Atomic Insights into the Enhanced Surface Stability in High Voltage Cathode Materials by Ultrathin Coating

Surface properties of electrode materials play a critical role in the function of batteries. Therefore, surface modifications, such as coatings, have been widely used to improve battery performance. Understanding how these coatings function to improve battery performance is crucial for both scientific research and applications. In this study the electrochemical performance of coated and uncoated LiNi0.5Mn1.5O4 (LNMO) electrodes is correlated with ensemble‐averaged soft X‐ray absorption spectroscopy (XAS) and spatially resolved scanning transmission electron microscopy‐electron energy loss spectroscopy (STEM‐EELS) to illustrate the mechanism of how ultrathin layer Al2O3 coatings improve the cycle life of LiNi0.5Mn1.5O4. Mn2+ evolution on the surface is clearly observed in the uncoated sample, which results from the reaction between the electrolytic solution and the surfaces of LiNi0.5Mn1.5O4 particles, and also possibly atomic structure reconstructions and oxygen loss from the surface region in LiNi0.5Mn1.5O4. The coating effectively suppresses Mn2+ evolution and improves the battery performance by decelerating the impedance buildup from the surface passivation. This study demonstrates the importance of combining ensemble‐averaged techniques (e.g., XAS) with localized techniques (e.g., STEM‐EELS), as the latter may yield unrepresentative information due to the limited number of studied particles, and sheds light on the design of future coating processes and materials.

Energy-loss spectroscopy ofC60fullerenes with twisted electrons: Influence of orbital-angular-momentum transfer on plasmon generation

Recent experimental progress in creating and controlling singular electron beams that carry orbital angular momentum allows for new types of local spectroscopies. We theoretically investigate the twisted-electron energy loss spectroscopy (EELS) from the C60 fullerene. Of particular interest are the strong multipolar collective excitations and their selective response to the orbital angular momentum of the impinging electron beam. Based on ab-initio calculations for the collective response we compute EELS signals with twisted electron beams and uncover the interplay between the plasmon polarity and the amount of angular momentum transfer.

Transition radiation in EELS and cathodoluminescence

The excitation probability of transition radiation is measured for varying beam energies in a transmission electron microscope once using optical spectrometry of the emitted light and second using electron energy loss spectrometry. In both cases similar results are found being in good agreement with theory. The knowledge about this probability enables us to judge whether or not transition radiation has to be considered in EELS and CL data interpretation. Additionally it is shown that the emission of transition radiation happens at the sample surfaces only, when the electron passes the vacuum/sample interface and thus feeling the change of its dielectric environment. We demonstrate that in the case of aluminum the influence of transition radiation on the low loss EELS spectrum is only minor and conclude that it might be negligible for many other materials.

Valence EELS below the limit of inelastic delocalization using conical dark field EFTEM or Bessel beams

In this experimental work we present novel methods to increase the spatial resolution of valence electron energy loss spectrometry (VEELS) investigations below the limit given by the inelastic delocalization. For this purpose we analyse a layer stack consisting of silicon/silicon-oxide/silicon-nitride/silicon-oxide/silicon (SONOS) with varying layer thickness down to the 2nm level. Using a combination of a conical illumination and energy filtered transmission electron microscopy we are able to identify the layers by using low energy losses. Employing Bessel beams we demonstrate that VEELS can be performed in dark-field conditions while simultaneously the Bessel beam increases the spatial resolution of the elastic image due to less sensitivity to the spherical aberration of the condenser lens system. The dark-field conditions also guarantee that only electrons are collected that have neither undergone an energy loss being due to the Cˇerenkov effect, nor due to the excitation of transition radiation or light guiding modes. We consequently are able to measure the optical properties of a 2.5nm thin oxide being sandwiched by the silicon substrate and a silicon-nitride layer.

Nanoparticulated spinel-type iron oxides obtained in supercritical water and their electrochemical performance as anodes for Li ion batteries

Nanoparticulated iron oxide-hydroxide samples are synthetized in supercritical water in continuous regime. Freshly prepared iron(II) acetate solutions are used as precursors and experiments at different flow rates and reactor temperatures are carried out. Samples composition is determined by thermogravimetric and energy loss spectroscopy. The obtained powdered samples are structurally characterized through Rietveld refinements of X-ray diffraction data. They mainly consist of FeO1.5−z spinel-type phases, though minority oxyhydroxide and hematite phases are obtained at low temperatures and total flow rates.Analysis by electron microscopy reveals mean particles sizes around 30 nm and roughly spherical morphology in most cases.The electrochemical response, cycling performance and Coulombic efficiency are evaluated from discharge-charge and cycling experiments at different current densities. The obtained response seems to depend on the presence or absence of oxyhydroxide phases and on both microstructural homogeneity at nanoscale and relative concentration of supercritical water at the mixing point. The 7-3-200 electrode sample shows superior rate capability, with capacity values higher than graphite theoretical one, even at high rates (500 Ah kg−1 at 2C after 100 cycles). Besides, the capacity values are restored when returning to low rates, reaching 1200 Ah kg−1 at C/5 and 800 Ah kg−1 at C/2.

Effects of uniaxial stress on the electrical structure and optical properties of Al-doped n-type ZnO

To analyze the effect of uniaxial stress on the Al-doped ZnO, we investigated the parameters of electronic structure and optical properties such as band structure, density of states (DOS), dielectric constant, absorption coefficient and reflection spectra under different uniaxial stresses by using first-principles based on density functional theory (DFT). The results demonstrated that the lattice constants (a, c) vary linearly with changes of uniaxial stress. The band gaps are broadened with the decrease in the uniaxial tensile stress and with the increase in the uniaxial compressive stress. When the uniaxial stress is 5GPa, the crystal field splitting energy reaches a minimum value. Meanwhile, the imaginary part of the dielectric function decreases with the increase in the uniaxial compressive stress with the energy lower than 5.9eV. And it increases with the increase in the uniaxial compressive stress and appears a blueshift when the energy is higher than 5.9eV. With the increase in the uniaxial compressive stress, absorption coefficient α(E), reflectance R(E), refractive index n(E), and energy loss spectroscopy L(E) all move to the higher energy direction (blueshift) and have a larger spectral peaks. This may imply a certain theoretical reference for the modulation of Al-doped n-type ZnO.

Investigation of Intermixing/Implantation in Pulsed Laser Deposited Metallic Cr/Ni80Fe20 and Ni80Fe20/Polypyrrole Multilayer Thin Films

Cr/Ni80Fe20 and Ni80Fe20/Polypyrrole multilayer thin films were prepared by pulsed laser deposition in vacuum and in Argon background gas at different pressures. Intermixing/implantation at their interfaces were investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analysis. Surface morphology (step height and roughness) of these samples were carried out by atomic force microscopy (AFM) and magnetic property (saturation magnetic flux) measurement by hysteresis loop tracer. The magnetic and surface morphology characterisations were used to calculate the interfacial mixing and/or implantation depth which have been found to agree with direct measured values. A correlation of these intermixing depths with Monte Carlo simulations of ion bombardment in the deposition process, using Stopping range of ion in matter (SRIM) incorporated with Transport of ion in matter (TRIM) gave the energy of the most implanted ionic species in a very simple way. Under high vacuum pressure, intermixing/implantation depth at the interfaces was high and occurs due to the presence of energetic species in the ablated plasma plume. With Argon gas background of increasing pressure, a reduction in energetic species energy is followed with a decrease in intermixing/implantation depth and the scattering of the ablated species leads to a decrease in the growth rate of Cr. An Argon background gas pressure of 20 mTorr gave the kinetic energy of the most energetic Cr species as 150 eV (from simulation) with 0.61 nm intermixing/implantation depth, while the vacuum pressure of 1.3 × 10− 7Torr gave the most energetic Cr energy as 3.2 keV with 2.38 nm depth of intermixing. Vacuum pressure 5 × 10− 7Torr with deposition of Permalloy (Py) Ni80Fe20 into polypyrrole gave measured intermixing/implantation depths of 6.81 nm with Ni80Fe20 energy of about 800 eV. The surface of the Py/polypyrrole (Ppy) was very rough at vacuum pressure 5 × 10− 3 Torr deposition as confirmed by the atomic force microscope measurement and STEM image than under background gas pressure 1 × 10− 3 Torr deposition. This is helpful for stoichiometric preparation of metallic multilayer thin films with well defined individual sharp interfaces and surface.

Investigation of the Composition of the Si/SiO2 Interface in Oxide Precipitates and Oxide Layers on Silicon by STEM/EELS

In a previous work, we found that both the interface between an oxide precipitate and the surrounding silicon matrix and the interface between a silicon substrate and a thermal oxide layer are of the same nature [1]. In both cases, between SiO2 existing in the center of the precipitate and in the oxide layer and Si of the matrix and the substrate a suboxide region of 2-3 nm exists. These results were obtained by electron energy loss spectrometry (EELS) carried out by scanning transmission electron microscopy (STEM). In the low loss region, it is possible to distinguish between Si, SiO, and SiO2 which all exhibit different maxima of the plasmon loss energy. By deconvolution, the local composition of the phase can be determined with the help of reference spectra of the three components. The stoichiometry of the oxide precipitates (SiO x ) was debated for a long time and application of different methods on different samples containing oxygen precipitates resulted in different values for x ranging from 1 to 2. Recently, it was demonstrated by applying several direct and indirect methods that the oxide precipitates consist of SiO2 [2]. Inspired by the results in Ref. 1 and 2, a layer model was proposed explaining the different values for x for oxide precipitates of different geometry [3]. Especially, for plate-like precipitates with a large surface the lower x values could be explained. Now the question came up if the width of the interface between Si and SiO2 is constant for all growth temperatures because from interface physics it could be expected that its width increases with temperature. Another point is, if the thickness of oxide layer or oxide precipitates plays a role. For this reason, we again used thermal oxide layers on (100) silicon of different thickness grown at different temperature to investigate the composition profile across the interface. We also compared the results with profiles from plate-like oxide precipitates grown at different temperatures. A scanning transmission electron microscope (STEM) FEI Tecnai Osiris equipped with tools for energy dispersive X-ray spectroscopy (EDX) analysis and EELS was used to analyze the interface and composition of a plate-like precipitates located at a very thin place of the lamella prepared from the sample by grinding, polishing, and ion milling. The profiles of the x values across the interfaces of the oxide layers and the precipitates were determined from the deconvolution of the Si, SiO, and SiO2 EELS spectra. The results of these investigations demonstrate that the stoichiometry of SiO2 (x=2) cannot be reached if the oxide layer thickness is lower than 10 nm for thermal oxides grown at 900 °C. The lower the thickness of the layer is the lower is the maximum x value. This is due to an interface layer of equal maximum slope for all oxide layers. The slope was obtained from fitting by sigmoid functions. Such interface layers were also found for the oxide precipitates but the maximum slope is lower. It was found to increase with increasing diffusion length of interstitial oxygen. The maximum of the x profiles also depends on the thickness of the plate-like oxide precipitates. The results will be discussed with respect to the phase separation process leading to precipitation of interstitial oxygen in silicon.

Practical analysis of Li distribution by EELS

Practical measurement of the Li distribution in Li‐ion battery electrode materials is demonstrated using a scanning transmission electron microscopy‐electron energy loss spectroscopy (EELS) spectrum imaging method. Lithium distribution images were obtained for LiCoO2 positive electrode material using conventional software, Digital Micrograph (Gatan), for STEM‐EELS spectrum imaging data. Some data processing methods using Li K‐edge EELS spectra were examined to obtain a Li distribution image for LiCoO2. A quantitative method was successfully applied to Li2TiO3, while it was not suitable for LiCoO2. Issues regarding the measurement of Li distributions in electrode materials are discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Microstructural Evolution Of Iron Oxyfluoride/Carbon Nanocomposites Upon Electrochemical Cycling

High electrochemical performance iron oxyfluoride conversion electrode undergoes complex electrochemical reaction mechanisms upon cycling. In this work, a combination of selected area electron diffraction (SAED) and scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS) analysis techniques have been used to understand the conversion-reconversion mechanisms of FeO0.7F1.3/C upon cycling. Considerable changes have been observed with cycling. For the fully delithiated electrodes, the nanometer scale intermixing of amorphous rutile and nanocrystalline rocksalt phases is stable up to 20 cycles; however, upon further cycling the amount of amorphous rutile phase decreased and amount of rocksalt phase increased gradually, implying incomplete reconversion reactions with increasing cycle number. In addition, a progressive growth of solid electrolyte interphase (SEI) layer was observed with cycling, which is mainly composed of LiF. Interestingly, Fe2+ and Fe nanoparticles were found t...

Core-Shell Vanadium Modified Titania@β-In2S3 Hybrid Nanorod Arrays for Superior Interface Stability and Photochemical Activity.

Core-shell rutile TiO2@β-In2S3 and modified V-TiO2@β-In2S3 were synthesized to develop bilayer systems to uphold charge transport via an effective and stable interface. Morphological studies revealed that β-In2S3 was deposited homogeneously on V-TiO2 as compared to unmodified TiO2 nanorod arrays. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectrometry studies verified the presence of various oxidation states of vanadium in rutile TiO2 and the vanadium surface was utilized for broadening the charge collection centers in host substrate layer and hole quencher window. Subsequently, X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectra confirmed the rutile phases of TiO2 and modified V-TiO2 along with the phases of crystalline β-In2S3. XPS valence band study explored the interaction of valence band quazi Fermi levels of β-In2S3 with the conduction band quazi Fermi levels of modified V-TiO2 for enhanced charge collection at the interface. Photoelectrochemical studies show that the photocurrent density of V-TiO2@β-In2S3 is 1.42 mA/cm(2) (1.5AM illumination). Also, the frequency window for TiO2 was broadened by the vanadium modification in rutile TiO2 nanorod arrays, and the lifetime of the charge carrier and stability of the interface in V-TiO2@β-In2S3 were enhanced compared to the unmodified TiO2@β-In2S3. These findings highlight the significance of modifications in host substrates and interfaces, which have profound implications on interphase stability, photocatalysis and solar-fuel-based devices.

Understanding the Stability for Li‐Rich Layered Oxide Li2RuO3 Cathode

Lithium‐rich layered oxides are considered as promising cathode materials for Li‐ion batteries with high energy density due to their higher capacity as compared with the conventional LiMO2 (e.g., LiCoO2, LiNiO2, and LiNi1/3Co1/3Mn1/3O2) layered oxides. However, why lithium‐rich layered oxides exhibit high capacities without undergoing a structural collapse for a certain number of cycles has attracted limited attention. Here, based on the model of Li2RuO3, it is uncovered that the mechanism responsible for the structural integrity shown by lithium‐rich layered oxides is realized by the flexible local structure due to the presence of lithium atoms in the transition metal layer, which favors the formation of O22−‐like species, with the aid of in situ extended X‐ray absorption fine structure (EXAFS), in situ energy loss spectroscopy (EELS), and density functional theory (DFT) calculation. This finding will open new scope for the development of high‐capacity layered electrodes.

Reduction in sintering temperature for flash-sintering of yttria by nickel cation-doping

1 mol% Ni2+-doped Y2O3 shows flash-sintering at DC fields greater than 300 V/cm. When compared to the undoped Y2O3, the onset temperature for the flash event is reduced by about 200 °C due to the Ni-doping. The electric conductivity during the sintering experiments for the Ni-doped Y2O3 begins to increase above those of the undoped Y2O3 at temperatures greater than 700 °C, then steeply rises corresponding to the flash event. Electron energy loss spectrometry (EELS) measurements indicated that Y2O3 is highly reduced by the combination of the Ni-doping and flash-sintering as well as by conventional sintering in a reducing atmosphere. The applied electric field must involve the formation of oxygen anion vacancies and liberated electrons, and consequently, enhances the diffusional mass transport and electronic conduction in the Y2O3. The enhanced flash-sintering in the Ni-doped Y2O3 must be attributed to an avalanche of the vacancy-electron pairs facilitated by the Ni-doping.

Linear and nonlinear optical response in 3-nitroaniline: A DFT study for comparison between molecular and bulk phases

We have studied the electronic structure and optical responses of both molecular and bulk phases of 3-nitroaniline (m-NA) using density functional theory (DFT). We have developed our calculations within the framework of band structure theory for both phases. The results of current simulation are also useful for comparison between solids and vapors. Findings show that the crystalline m-NA has an indirect bandgap which is smaller than its molecular counterpart. Due to the weak intermolecular interactions, the crystalline band structure shows very low dispersions and hence the crystalline spectra are very similar to those of molecules, especially at lower energies. This resemblance is extended to lower wavelengths in linear regime, but limited to higher wavelengths in nonlinear one. This study shows that the substituent groups play major roles in the band structure and charge transfer excitations. The optical spectra show higher intensity and more splitting, especially in nonlinear regime, when we go from molecular phase to bulk phase. Findings show that the electron energy loss structures in vapor phase are very close to [Formula: see text], hence energy loss spectroscopy is a direct way for measuring dielectric function of vapors. According to our results, the crystalline phase exhibits plasmon resonances at much higher energies compared to those of vapor phase. For example, the energy loss spectrum of m-NA molecule shows plasmon peak around 17[Formula: see text]eV which is about 10[Formula: see text]eV lower than the crystalline counterpart. The comparison between nonlinear spectra and the linear spectra (as functions of both [Formula: see text] and 2[Formula: see text]) reveals the significant resemblance between linear and nonlinear structures in both phases. This study shows that the molecular-based models for investigating optical properties of organic crystals are only suitable for low energy regions. Finally, our simulation reproduces the experimental results very well.

Cerium oxide nanoparticles induce oxidative stress in the sediment-dwelling amphipod Corophium volutator

Cerium oxide nanoparticles (CeO2 NPs) exhibit fast valence exchange between Ce(IV) and Ce(III) associated with oxygen storage and both pro and antioxidant activities have been reported in laboratory models. The reactivity of CeO2 NPs once they are released into the aquatic environment is virtually unknown, but this is important to determine for assessing their environmental risk. Here, we show that amphipods (Corophium volutator) grown in marine sediments containing CeO2 NPs showed a significant increase in oxidative damage compared to those grown in sediments without NPs and those containing large-sized (bulk) CeO2 particles. There was no exposure effect on survival, but significant increases in single-strand DNA breaks, lipid peroxidation and superoxide dismutase activity were observed after a 10-day exposure to 12.5 mg L−1 CeO2. Characterisation of the CeO2 NPs dispersed in deionised or saline exposure waters revealed that more radicals were produced by CeO2 NPs compared with bulk CeO2. Electron energy loss spectroscopy (EELS) analysis revealed that both CeO2 NPs were predominantly Ce(III) in saline waters compared to deionised waters where they were predominantly Ce(IV). In both types of medium, the bulk CeO2 consisted mainly of Ce(IV). These results support a model whereby redox cycling of CeO2 NPs between Ce(III) and Ce(IV) is enhanced in saline waters, leading to sublethal oxidative damage to tissues in our test organism.

On the Localized Nature of the Structural Transformations of Li2MnO3 Following Electrochemical Cycling

Although the Li‐excess layered‐oxide Li2MnO3 has a high theoretical capacity, structural transformations within the oxide during electrochemical cycling lead to relatively low experimental capacities, hindering its use in practical applications. Here, aberration‐corrected scanning transmission electron microscopy/electron energy loss spectroscopy and high‐resolution X‐ray diffraction are used to characterize the oxide following electrochemical cycling. Microscopy reveals the coexistence of regions with local monoclinic, spinel, and rock‐salt symmetries, indicating localized and inhomogeneous structural evolutions. Crystal structure transformations are observed both at the particle surface and in the bulk. At the surface, these transformed regions resemble spinel Mn3O4 or rock‐salt MnO, consistent with oxygen loss. In the bulk, the regions resemble defect spinels, such as the layered‐spinel LixMn4/3O4, which suggest a partial phase transformation consistent with oxygen retention. Both microscopy and diffraction data of the cycled sample indicate areas of pristine Li2MnO3; the presence of such areas, in close proximity to LixMn4/3O4 areas, suggests that the layered to spinel structure transformation is partially reversible. Spinel, disordered rock salt, and pristine areas are also observed in Li2MnO3 samples intentionally damaged by electron beam irradiation. This observation indicates that the dynamic processes resulting in phase transformations can be studied for a variety of oxide systems by a judicious selection of irradiation conditions.

The Čerenkov limit of Si, GaAs and GaP in electron energy loss spectrometry

Since the advent of monochromated electron energy loss spectrometry (EELS) the experimental detection of band gaps in semiconducting materials is of great importance. In the non-relativistic limit of this technique the onset of the inelastic signal represents the band gap. But due to relativistic energy losses, like Čerenkov losses and the corresponding light guiding modes, appearing at high beam energies the band gap is usually hidden. The highest beam energy, which does not excite relativistic losses in a certain material, is called the Čerenkov limit of the material. In this work the low loss EELS signals of Si, GaAs and GaP are measured at various beam energies and the calculated Čerenkov limits are experimentally confirmed.