Energy-loss Near-edge Structure Research Articles

Analysis of the Ag M4,5 EELS edge to study silver nanoparticle corrosion.

Electron energy loss spectra collected from fresh and corroded silver nanoparticles are compared with those from a number of reference materials, focusing on the M4,5 edge. Chemical shifts and changes in the energy loss near edge structure (ELNES) are described and found to be sufficient to distinguish metallic silver from chemically oxidised silver. The measurements, in conjunction with electron energy loss spectrum imaging, are used to assess the mechanisms for atmospheric corrosion of silver nanoparticles. We unambiguously assign the corrosion product under atmospheric conditions to be silver sulphide, but show the reaction process to be distinctly inhomogeneous, producing a variety of types of corroded particles. LAY DESCRIPTION: >Here, we use analytical electron microscopy to track the corrosion of silver nanoparticles and present chemical maps of the corrosion products. We show clear spectroscopic differences between metallic and corroded silver using the M4,5 electron energy loss spectral feature, which is not commonly studied. Our study shows that corrosion is due to interactions with sulphur in the atmosphere; and the corrosion is not uniform, but appears to develop from specific points on the surface of the nanoparticles.

Local valence analysis of 316L austenitic stainless steel produced by laser powder bed fusion

Ultra-rapid cooling of laser powder bed fusion (LPBF) generates a cellular solidification microstructure with submicron-sized periodicity accompanied by non-negligible segregation. In 316L austenitic stainless steel, an important corrosion-resistant alloy, the effect of segregation, particularly variations in Cr concentration, on the corrosion resistance of the LPBF product is unknown. Local valence analysis of the LPBF-produced 316L by electron energy-loss spectroscopy revealed no obvious changes in the energy-loss near-edge structures of Cr and Fe measured within the solidification cellular microstructure, at the cell boundary, or at the melt-pool boundary. This result indicates that solidification segregation in the LPBF-produced 316L is unlikely to affect the corrosion resistance of the material.

Spatially Resolved Functional Group Analysis of OLED Materials Using EELS and ToF-SIMS.

Electron energy-loss spectroscopy (EELS) is widely used in analyzing the electronic structure of inorganic materials at high spatial resolution. In this study, we use a monochromator to improve the energy resolution, allowing us to analyze the electronic structure of organic light-emitting diode (OLED) materials with greater precision. This study demonstrates the use of the energy-loss near-edge structure to map the nitrogen content of organic molecules and identify the distinct bonding characteristics of aromatic carbon and pyridinic nitrogen. Furthermore, we integrate EELS with time-of-flight secondary ion mass spectrometry for molecular mapping of three different bilayers composed of OLED materials. This approach allows us to successfully map functional groups in the by-layer OLED and measure the thickness of two OLED layers. This study introduces spatially resolved functional group analysis using electron beam spectroscopy and contributes to the development of methods for complete nanoscale analysis of organic multilayer architectures.

Theoretical Study of Carbon K‐Edge Energy‐Loss Near‐Edge Structure Spectra in the Ordered Mo2TiAlC2 MAX and Mo2TiC2 MXene

By means of density functional theory, the energy‐loss near‐edge structure (ELNES) of carbon K‐edge of Mo2TiAlC2 and corresponding MoTiC2 Mxene at orientational‐independent condition is dealt with. Compared to the MAX (M is transition metal, A is an elment from group 13–16, X is C or N) phase, the energy separations increase between the main spectral features at the C K edge of Mo2TiC2 MXene owing to the structural change and decreased bond length. The dispersions of the C K edge in both systems are similar to p‐symmetry densities of states. It is indicated that the source of the first fine structure at the C 1s edge in both phases mainly comes from the electron transfer to px + py‐like character. The other fine structures result from the transition to hybridization of pz and px + py states with the prominent contribution of px + py‐like character. Moreover, the comparison of C K‐edge ELNES spectra in three Mo‐based compounds reveals that, ongoing from Mo2TiAlC2 to Mo2TiC2 and then to Mo2C, the energy position of the fine structures is shifted to higher energies (blueshifted), due to the quantum confinement effects and the change of the chemical environment around the excited carbon.

The effect of nanometre-scale kinetic competition on the phase selection in Zr/Si superstructure

Nano structured coating system demonstrates outstanding performance stemming from unique structure evolution. Zr/Si coating was proposed as an ideal candidate for the protection of Zr alloy cladding. However, the phase selection remains a challenge to coordinate the complex requirement under the service and accident conditions of reactor core. In this work, a phase selection mechanism for amorphous Zr-Si-O (abbr. ZSO) was elucidated by the competitive consumption of Zr layer by silicidation and oxidation, which was also scale-dependent. The energy-loss near-edge fine structure and nanoindentation were also employed to characterize the unique scale effect in the two processes. Both the oxides kinetics and elastic modulus gain indicated an immediate passivation process under the subcritical condition after the in situ phase selection of the amorphous ZSO. Ab initio molecular dynamics (AIMD) was performed to disclose the origination of depressed O migration in ZSO. The multilayer structure also shows accident tolerance potential. The interfacial phase selection provided a strategy to regulate mesoscale structure originating from nanomter interface and meets the diversified requirement for “structure-performance relationship” under the service and accident conditions.

Carbon K edge spectra of functionalized molybdenum-based MXenes, Mo2CT2 (T = F, OH and O), from first-principles calculations

In this study, the effect of the termination groups (T = F, OH, O) on the energy loss near edge structure (ELNES) of carbon K edge in Mo2C MXene at orientation-independent conditions has been investigated using first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that within the YS-PBE0 functional, the Mo2CF2 is a semiconductor with an indirect band gap of 0.723 eV. For Mo2CO2, the indirect band gap increases to 0.17 eV within the screened hybrid functional. The calculation results of ELNES spectra with the affection of core-hole show that, in comparison to pristine Mo2C, as a fingerprint of termination groups, the spectral structures in Mo2CT2 are reproduced at higher energies. Moreover, the spectral features of Mo2CT2 are sensitive to the chemical nature and the location of the T groups on the pristine Mo2C MXene surface. When going from T = O to T = F and, further, to T = OH, the energy separation between the main peaks increases, which is a sign of decreasing the Mo-C bond length, respectively, from T = O to T = F and to T = OH. The comparison of ELNES spectra and the unoccupied densities of states (DOS) reveal that, the origin of the first structure at the carbon K edge of Mo2CT2 is mostly result from electron transition to pz state, while in pristine Mo2C, mainly due to the transition to px + py state. Other structures at higher energies mainly arise from electron transitions to px + py state and partially to pz state. The spectral decomposition of the ELNES into in-plane (l′ = 1, m′ = ± 1) and out-of-plane (l′ = 1, m′ = 0) components also confirms these results. Generally, in both Mo2C and Mo2CT2, the contribution of in-plane element in most of the structures is more considerable.

Structural andelectronic characterization offluorine-doped La0.5Sr0.5CoO3-δ using electron energy-loss spectroscopy.

Perovskite oxides, ABO3, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here, we characterized the crystal and electronic structures of fluorine-doped La0.5Sr0.5CoO3-δ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an energy-loss near-edge structure analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF2. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.

Electron energy loss spectra of sulfur in HgX2S4 (X = Al, Ga, In): A comparative theoretical study

By means of density functional theory based electronic structure calculations and energy loss near edge structure (ELNES) analysis, the present study calculates and describes the sulfur K and L2,3 edge spectra of mercury-based defective chalcopyrites (HgAl2S4 and HgGa2S4) and of its spinel structure (HgIn2S4), and compares with those of cadmium thiogallate (CdGa2S4) and the existing experimental spectra. The substitution of gallium by aluminium changes the sulfur K and L2,3 edge ELNES, decreasing the relative spectral intensities and shifting the main peaks to higher energies. The covalency power and the bond length play the considerable roles in the overall dispersions of the edge spectra.

Electronic structure characterization of TiO2-II with the α-PbO2 structure by electron-energy-loss-spectroscopy and comparison with anatase, brookite, and rutile

TiO2-II is a high pressure form of titania with a density about 2% larger than that of rutile. In contrast to the common polymorphs anatase, brookite and rutile its electronic structure and optical properties are poorly characterized. Here we report on a comparative electron-energy-loss-spectroscopy (EELS) study for which high resolution valence-loss and core-loss EELS data were acquired from nanocrystalline (<75 ​nm sized) titania particles with an energy resolution of about 0.2 ​eV. Electronic structure features revealed from titanium L3,2 and oxygen K electron energy loss near-edge structures show a strong similarity of TiO2-II with both rutile and brookite, which is attributed to similarities in the connectivity of octahedral TiO6 units with neighboring ones. From combined valence-loss EELS and UV-VIS diffuse reflectance spectroscopy data the band gap of TiO2-II was determined to be indirect and with a magnitude of ∼3.18 ​eV, which is very similar to anatase (indirect, ∼3.2 ​eV), and distinctly different from rutile (direct, ∼3.05 ​eV) and brookite (direct, ∼3.45 ​eV).

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn,Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

Unusual aliovalent doping effects on oxygen non-stoichiometry in medium-entropy compositionally complex perovskite oxides.

Aliovalent doping of perovskite oxides can tune the oxygen vacancy formation energy. This work discovers normal vs. abnormal aliovalent doping effects on redox behaviors in medium-entropy compositionally complex perovskite oxides (CCPOs) (La1-xSrx)(Mn1/3Fe1/3Ti1/3)O3-δ (LS_MFT) vs. (La1-xSrx)(Mn1/3Fe1/3Cr1/3)O3-δ (LS_MFC). In the LS_MFC series, the oxygen non-stoichiometry range Δδ (= δred - δox) linearly depends on the Sr molar ratio x, while the LS_MFT series shows a V-shape dependence of Δδ on x. This unusual observation is investigated and explained based on the analysis of the energy loss near edge structure (ELNES) in STEM electron energy loss spectroscopy, along with density functional theory (DFT) calculations. In LS_MFC, Cr-L2,3, Mn-L2,3 and Fe-L2,3 peaks have a similar linear shift to higher energy with increasing x, which indicates higher oxidation states of Cr, Mn, and Fe with lower oxygen vacancy formation energies. In LS_MFT, the V-shape of the Δδ vs. x curve is caused by the stable Ti4+ state and a V-shape Mn/Fe valency dependence on x. This study suggests the possible existence of different (including unexpected) coupled aliovalent doping effects in CCPOs with multiple B-site redox active elements.

Electron energy loss spectroscopy of K0.7Fe1.7Se2 superconductor

Electronic structure and microstructure of ternary selenide superconductor K0.7Fe1.7Se2 have been investigated by transmission electron microscopy (TEM) and energy band structure calculations. Electron energy-loss spectroscopy (EELS) was performed to study K0.7Fe1.7Se2 experimentally. The individual inter-bands transitions were identified through the comparison between the energy loss peak positions with the partial density of states (PDOS) obtained by first principle calculation. The electron energy-loss near-edge fine structure (ELNES) were analyzed, the core-hole effect was found to play a key role in the simulation of ELNES. The results can present some insight into the interaction between superconductivity and electronic structure in this group of iron-related superconductor.

Effect of site geometry on oxygen K, and silver and gallium L2,3 energy loss spectra of AgGaO2 polymorphs: A DFT study

By means of the ab initio calculations of the electron density of states and energy loss near edge structure (ELNES) analysis, the present study examines the effect of local coordination and site symmetry on the electron energy structure of AgGaO2 polymorphs. Difference in spectral dispersion and the energy peak positions of the O K, and Ag and Ga L2,3 ELNES spectra of three AgGaO2 polymorphs is attributed to the difference in local coordination, bond length, site symmetry, and electronic structure of the structures. In order to reproduce the edge spectra, it is inevitable to consider the core hole effect in calculations.

Analytical transmission electron microscopy investigation of electrochemically Li+–Na+ substituted LiNi0.5Mn1.5O4 electrode

The electrochemical Li+–Na+ alkaline cation substitution of a lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO), which is characterized via structural analysis, has previously been reported. However, detail characterizations of the chemical and electronic features are lacking. In this study, we characterized that by using analytical transmission electron microscopy. Energy dispersive X-ray spectroscopy (EDS) revealed uniform Na-insertion into a single particle. Electron energy-loss spectroscopy (EELS) analysis exhibited complete Li-extraction. Energy loss near edge structure (ELNES) of Mn and Ni L-edges indicated that only Ni contributes to the redox reaction as Ni2+ ​⇔ ​Ni4+ during the Li+–Na ​+ ​substitution. The elemental composition ratio analysis using both EDS and EELS indicated Na0.6Ni0.5Mn1.5O3.8 as a formula of the Na-substituted material. Oxygen desorption was provoked during the Na-insertion process, and oxygen defect formation was found to be essential for the Li+–Na+ substitution mechanism of the LNMO material.

Electron energy-loss spectra of functionalized titanium and zirconium MXene monolayers, including excitonic effects

Using the full-potential linearized augmented plane wave (FP-LAPW)-based energy-loss near-edge structure (ELNES), the present study deals with the electronic properties of titanium and zirconium MXene monolayers. Specifically, the carbon and nitrogen K-edges of M2CT2 and M2NT2 (M = Ti, Zr; T = F, O, OH) MXene monolayers have been calculated and compared with those of the existing experimental spectra. On going from the bulk systems to MXene monolayers, the main spectral features occur at higher energies as a result of the shorter M-X bond lengths in the monolayers. As fingerprints of the terminal groups, in the C K- and N K-edges of Ti- and Zr-based terminated MXenes, the oscillator strengths and overall spectral shapes are changed, and in comparison to bare MXenes, the main spectral features are red-shifted. Moreover, on going from O termination to OH and then to F, one can observe a slight red-shift in the K-edge spectra of Ti2C(N) and Zr2C(N). While the peaks originating from the px + py states are modified through the appending of terminal elements, showing an obvious red-shift, the terminal substituent has no significant effect on features engendered by pz character. In addition, including excitonic effects on the K-edge absorption spectra considerably changes the response of MXene semiconductors, resulting in red-shifts of the ELNES spectral features.

Several factors influencing energy-loss near-edge structure calculations using Wien2k.

Electron energy-loss spectroscopy (EELS) is widely applied combining with transmission electron microscopes with high spatial resolution, but its interpretation is a challenging task. One of the reasons is that the factors affecting EELS are very complicated. In this paper, we focus on the several factors involved in density functional theory (DFT) calculations. The sensitivity of calculated energy-loss near-edge structure (ELNES) to spin order, pressure and on-site Coulomb energy U has been discussed. Since EELS technique detects the local environment of atoms, the influence of spin order cannot be ignored. The chemical shifts and peak intensity of ELNES are also closely related to corresponding pressure. The correlation effects are very important for transition metal compounds and play a key role in EELS simulations. An overview of the effects of these factors on the ELNES is presented with the help of Wien2k code. The antiferromagnetic order results in the decreasing of intensities of related peaks and the moving of the peaks to high energy loss. The decreasing of lattice parameters causes the ELNES peaks to shift to high energy loss, and the peak shifts at the higher energy loss are more significant. The increase of correlation effect leads to the ELNES peaks to shift to high energy loss accompanied by the increase of the relative intensity of the peaks which locate at higher energy loss. Our work helps to understand how these factors affect EELS and to explain and predict the experimental EELS spectra. Through the discussion of these factors, we propose that some factors could not be ignored in EELS simulations.

Microstructure and Modifications of Cu/Al2O3 Interfaces

Abstract A combined approach of high-resolution transmission electron microscopy and electron energy-loss near-edge structure studies was employed to determine the atomic structure and bonding mechanisms at Cu/Al2O3 interfaces in dependence on the α-Al2O3 substrate orientation. The investigated specimens were prepared by molecular beam epitaxy using ultrahigh vacuum conditions, which led to atomically abrupt interfaces. The results show that intermetallic Cu–Al bonds occur at the investigated Cu/ (112̅0)Al2O3 interface, while ionic-covalent bonding contributions are observed at the Cu/(0001)Al2O3 interface. The interfacial microstructure of diffusion-bonded Cu/ (0001)Al2O3 samples was changed by annealing treatments under various oxygen partial pressures. Annealing resulted in the formation of a CuAlO2 reaction phase at the interface between Cu and Al2O3.

Atomic scale crystal field mapping of polar vortices in oxide superlattices

Polar vortices in oxide superlattices exhibit complex polarization topologies. Using a combination of electron energy loss near-edge structure analysis, crystal field multiplet theory, and first-principles calculations, we probe the electronic structure within such polar vortices in [(PbTiO3)16/(SrTiO3)16] superlattices at the atomic scale. The peaks in Ti L-edge spectra shift systematically depending on the position of the Ti4+ cations within the vortices i.e., the direction and magnitude of the local dipole. First-principles computation of the local projected density of states on the Ti 3d orbitals, together with the simulated crystal field multiplet spectra derived from first principles are in good agreement with the experiments.

Ab Initio modeling of Near-Edge EELS spectra for chemisorbed molecules

Electron energy loss spectroscopy (EELS) has recently been applied to probe chemisorbed molecules on metal nanostructures, but a fundamental understanding of the correlation between these spectra and the electronic structures of the adsorbates has been limited. We report here on the insights afforded by time-dependent density functional theory to decipher the energy loss near edge structure (ELNES) of EELS spectra associated with chemisorption. These first-principles calculations simulate the ELNES-EELS spectra for chemisorbed CO on various facets of Au and Pt. Computational predictions of key signatures such as the ‘red shift’ and reductions in the peak intensity of the 2π* and 6σ* peaks, as compared to free CO in the gas phase, are validated in comparison to experimentally collected EELS spectra. These signatures are revealed to arise from changes in the electronic structure in terms of unoccupied density of states associated with the chemisorption process. They are consistent with a Blyholder model that incorporates donation and back-donation of electrons. They are also characteristic of the chemisorption process, such as the choice of metal, site of adsorption and the coverage and distribution of adsorbates. Our simulations thus provide guidelines for the use of ELNES-EELS to characterize the atomic structure and adsorption property of nanostructured surfaces and facilitate the development of advanced nanomaterials for catalytic applications.

Cation-vacancy induced Li+ intercalation pseudocapacitance at atomically thin heterointerface for high capacity and high power lithium-ion batteries

It is challenging to create cation vacancies in electrode materials for enhancing the performance of rechargeable lithium ion batteries (LIBs). Herein, we utilized a strong alkaline etching method to successfully create Co vacancies at the interface of atomically thin Co3−xO4/graphene@CNT heterostructure for high-energy/power lithium storage. The creation of Co-vacancies in the sample was confirmed by high-resolution scanning transmission electron microscope (HRSTEM), X-ray photoelectron spectroscopy (XPS) and electron energy loss near-edge structures (ELNES). The obtained Co3−xO4/graphene@CNT delivers an ultra-high capacity of 1688.2 mAh g−1 at 0.2 C, excellent rate capability of 83.7% capacity retention at 1 C, and an ultralong life up to 1500 cycles with a reversible capacity of 1066.3 mAh g−1. Reaction kinetic study suggests a significant contribution from pseudocapacitive storage induced by the Co-vacancies at the Co3−xO4/graphene@CNT interface. Density functional theory confirms that the Co-vacancies could dramatically enhance the Li adsorption and provide an additional pathway with a lower energy barrier for Li diffusion, which results in an intercalation pseudocapacitive behavior and high-capacity/rate energy storage.