Energy-loss Near-edge Structure Research Articles

Direct Evidence of Surface Reduction in Monoclinic BiVO4

Local measurements of the oxidation state of vanadium in monoclinic BiVO4 particles by means of electron energy-loss spectroscopy in scanning transmission electron microscopy reveal a pronounced surface reduction: within a 5-nm-thick shell, the oxidation state of vanadium is reduced from +5 to about +4. Thus, charge neutrality near the surface demands for ∼15% oxygen vacancies. Our results provide direct evidence for the segregation of oxygen vacancies at the surface of BiVO4. This observation is confirmed by X-ray photoelectron spectroscopy. The experimental findings are complemented with all-electron density functional theory based WIEN2k calculations of the density of electron states and of the electron energy-loss near-edge structure. The theoretical results provide further information on the electronic changes induced by the experimentally verified oxygen vacancies.

Nanoscale bonding between human bone and titanium surfaces: osseohybridization.

Until now, the chemical bonding between titanium and bone has been examined only through a few mechanical detachment tests. Therefore, in this study, a sandblasted and acid-etched titanium mini-implant was removed from a human patient after 2 months of placement in order to identify the chemical integration mechanism for nanoscale osseointegration of titanium implants. To prepare a transmission electron microscopy (TEM) specimen, the natural state was preserved as much as possible by cryofixation and scanning electron microscope/focused ion beam (SEM-FIB) milling without any chemical treatment. High-resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDS), and scanning TEM (STEM)/electron energy loss spectroscopic analysis (EELS) were used to investigate the chemical composition and structure at the interface between the titanium and bone tissue. HRTEM and EDS data showed evidence of crystalline hydroxyapatite and intermixing of bone with the oxide layer of the implant. The STEM/EELS experiment provided particularly interesting results: carbon existed in polysaccharides, calcium and phosphorus existed as tricalcium phosphate (TCP), and titanium existed as oxidized titanium. In addition, the oxygen energy loss near edge structures (ELNESs) showed a possibility of the presence of CaTiO3. These STEM/EELS results can be explained by structures either with or without a chemical reaction layer. The possible existence of the osseohybridization area and the form of the carbon suggest that reconsideration of the standard definition of osseointegration is necessary.

EELS study of Fe- or Co-doped titania nanosheets.

Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe(3+) and Co(2+) ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti(4+) and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe(3+) ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe(2+) ions in the reduction process by electron irradiation. In contrast, the chemical shift of the Ti-L edge of the Ti0.8Co0.2O2 nanosheets indicated that Ti(4+) ions were reduced. These results suggest that the substitution of 3d metals in titania nanosheets affects their crystal and electronic structures and material properties such as their long-range atomic configuration and reduction mechanism.

Quantitative use of electron energy-loss spectroscopy Mo-M2,3 edges for the study of molybdenum oxides.

Because of the large energy separation between O-K and Mo-L2,3 edges, extracting precise and reliable chemical information from core-loss EELS analyze of molybdenum oxides has always been a challenge. In this regard Mo-M2,3 edges represents an interesting alternative as they are situated close to the O-K edges. They should allow thus the extraction of a wealth of chemical information from the same spectra. However the main difficulty to overcome in order to work properly with these edges is the delayed maxima of the Mo-M4,5 edges which hinders the automated background subtraction with the usual inverse power low function. In this study we propose another background subtraction method specifically designed to overcome this obstacle and we apply it to the study of MoO3 and MoO2. We are able to show that quantitative chemical information can be precisely and accurately determined from the joined analyze of O-K and Mo-M2,3 edges. In particular k-factors are derived as a function of the integration window width and standard errors close to 2% are reported. The possibility to discriminate the two oxides thanks to chemical shifts and energy-loss near-edge structures is also investigated and discussed. Furthermore the M3/M2 ratios are derived and are found to be strongly dependent on the local chemical environment. This result is confirmed by multiplet calculations for which the crystal field parameters have been determined by ab initio calculations. The whole methodology as well as the conclusions presented in this paper should be easily transposable to any transitions metal oxides of the 4d family. This work should open a new and easier way regarding the quantitative EELS analyses of these compounds.

Interface composition between Fe3O4 nanoparticles and GaAs for spintronic applications

Recent interest in spintronic applications has necessitated the study of magnetic materials in contact with semiconductor substrates; importantly, the structure and composition of these interfaces can influence both device functionality and the magnetic properties. Nanoscale ferromagnet/semiconductor structures are of particular interest. In this study, the interface structure between a monolayer of ferromagnetic magnetite (Fe3O4) nanoparticles and a GaAs substrate was studied using cross-sectional transmission electron microscopy techniques. It was found that a continuous amorphous oxide interface layer separates the nanoparticles from the GaAs substrate, and that iron diffused into the interface layer forming a compositional gradient. Electron energy-loss near-edge fine structures of the O K absorption edge revealed that the amorphous oxide is composed of γ-Fe2O3 directly underneath the Fe3O4 nanoparticles, followed by a solid solution of Ga2O3 and FeO and mostly Ga2O3 when approaching the buckled oxide/substrate interface. Real-space density functional theory calculations of the dynamical form factor confirmed the experimental observations. The implication of the findings on the optimization of these structures for spin injection is discussed.

The effect of valence state and site geometry on Cr L3,2 electron energy-loss spectra of Cr-bearing oxidic compounds

Chromium L3,2 energy loss near-edge structures (ELNES) of nine chromium compounds were measured in a transmission electron microscope to study the effect of valence state and site geometry of chromium on spectral features. The compounds studied represent a set of standards with given valence state and coordination. The chromium coordination is either octahedral or tetrahedral with different degree of distortion. The distortion of polyhedra results in a loss of the fine detail of spectral features in Cr L3,2 ELNES spectra. The effect of valence state manifests in a systematic chemical shift of the ELNES spectra by about 2eV per oxidation state for octahedrally coordinated Cr and 0.4–1.4eV per oxidation state for tetrahedrally coordinated Cr. On the basis of collected Cr L3,2 ELNES spectra we propose a simple quantification technique for the determination of chromium oxidation state in oxidic compounds which is independent of the coordination.

Electron energy loss spectrum of graphane from first-principles calculations

In this study, the energy loss near edge structure (ELNES) of carbon atoms in chair and tricycle conformers of hydrogenated graphene, namely ‘graphane’, has been calculated in the density functional theory using FP-LAPW method, and then, it has been compared with that of graphite and graphene. Using ELNES from chair conformer, the carbon K-edge was found to have a few main features including electron transition from 1s orbital of carbon atom to π*, σ*, and a hybridization of these two states. The first feature in tricycle conformer, however, has contributions of both π* and σ* states. The comparison of ELNES and the unoccupied density of states in each structure also justifies this. The energy difference between π* and σ* features of graphane conformers was decreased relative to it in graphite and graphene. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in graphite and graphene, it may be essential as well for the ELNES spectra of graphane conformers.

Oxide Wizard: An EELS Application to Characterize the White Lines of Transition Metal Edges

Physicochemical properties of transition metal oxides are directly determined by the oxidation state of the metallic cations. To address the increasing need to accurately evaluate the oxidation states of transition metal oxide systems at the nanoscale, here we present "Oxide Wizard." This script for Digital Micrograph characterizes the energy-loss near-edge structure and the position of the transition metal edges in the electron energy-loss spectrum. These characteristics of the edges can be linked to the oxidation states of transition metals with high spatial resolution. The power of the script is demonstrated by mapping manganese oxidation states in Fe3O4/Mn3O4 core/shell nanoparticles with sub-nanometer resolution in real space.

Toward deep blue nano hope diamonds: heavily boron-doped diamond nanoparticles.

The production of boron-doped diamond nanoparticles enables the application of this material for a broad range of fields, such as electrochemistry, thermal management, and fundamental superconductivity research. Here we present the production of highly boron-doped diamond nanoparticles using boron-doped CVD diamond films as a starting material. In a multistep milling process followed by purification and surface oxidation we obtained diamond nanoparticles of 10-60 nm with a boron content of approximately 2.3 × 10(21) cm(-3). Aberration-corrected HRTEM reveals the presence of defects within individual diamond grains, as well as a very thin nondiamond carbon layer at the particle surface. The boron K-edge electron energy-loss near-edge fine structure demonstrates that the B atoms are tetrahedrally embedded into the diamond lattice. The boron-doped diamond nanoparticles have been used to nucleate growth of a boron-doped diamond film by CVD that does not contain an insulating seeding layer.

Impact of local strain on Ti-L2,3electron energy-loss near-edge structures of BaTiO3: a first-principles multiplet study

Identification of local strains is crucial because the local strains largely influence the ferroelectric property of BaTiO₃. The effects of local strains induced by external pressures on the Ti-L₂,₃ electron energy-loss near-edge structure (ELNES) of BaTiO₃ were theoretically investigated using first-principles multiplet calculations. We revealed that the effects appear in the position of the spectral threshold, namely the spectrum shifts to lower and higher energy sides by the tensile and compressive pressures, respectively. We concluded that conventional ELNES observations can identify only large strains induced by -10 GPa, and 0.1 eV energy resolution is required to identify ±2% of strains.

High-Resolution Spectroscopy of Bonding in a Novel BeP2N4Compound

The recently discovered compound BeP2N4 that crystallizes in the phenakite-type structure has potential application as a high strength optoelectronic material. Therefore, it is important to analyze experimentally the electronic structure, which was done in the present work by monochromated electron energy-loss spectroscopy. The detection of Be is challenging due to its low atomic number and easy removal under electron bombardment. We were able to determine the bonding behavior and coordination of the individual atomic species including Be. This is evident from a good agreement between experimental electron energy-loss near-edge structures of the Be-K-, P-L2,3-, and N-K-edges and density functional theory calculations.

Simulation of Probe Position-Dependent Electron Energy-Loss Fine Structure

We present a theoretical framework for calculating probe-position-dependent electron energy-loss near-edge structure for the scanning transmission electron microscope by combining density functional theory with dynamical scattering theory. We show how simpler approaches to calculating near-edge structure fail to include the fundamental physics needed to understand the evolution of near-edge structure as a function of probe position and investigate the dependence of near-edge structure on probe size. It is within this framework that density functional theory should be presented, in order to ensure that variations of near-edge structure are truly due to local electronic structure and how much from the diffraction and focusing of the electron beam.

First-principle calculations analysis of ELNES splitting for Mn3O4 spinels related to atomic local symmetry

By using a real space multiple scattering method (FEFF code) with a 2 × 2 × 2 cluster model, we investigated the effects of characteristic Jahn-Teller distortion on the electron energy loss near-edge structure (ELNES) of Mn3O4 spinel. In particular, we examined a correlation between the characteristics of the density of state and the ELNES spectral feature as a function of Jahn-Teller distortion. To this end, we introduced a geometrical variation approach to an Mn3O4 cluster model containing both Mn(3+) and Mn(2+) sites. Upon a prominent Jahn-Teller distortion of the Mn(3+)-octahedral site, we resolved the associated spectral features of Mn, comprising three peaks that merged upon increasing the symmetry of octahedral site from tetragonal (D4h) to cubic (Oh). We have also investigated the interplay between the Mn L-edge and corresponding O K-edge spectra.

Electron microscopy analysis of Ti-substituted Li2MnO3 positive electrode before and after carbothermal reduction

40%-titanium substituted Li2MnO3 (Li1+x(Ti0.4Mn0.6)1−xO2, 0 < x < 1/3) before and after carbothermal reduction was characterized by annular dark field scanning transmission electron microscopy (ADF-STEM) and electron energy-loss spectroscopy (EELS) to investigate the effects of the pre-reduction treatment. EELS spectra with high energy resolution using monochromator clearly reveal the appearance of reduced Mn ions after the reduction process. The spatial distribution of the reduced phase was visualized by multi-linear least-squares (MLLS) fitting method using Mn L-edge energy-loss near-edge structure (ELNES). The morphology change of a primary particle was observed around the Mn reduction phase.

A “Thickness Series”: Weak Signal Extraction of ELNES in EELS Spectra From Surfaces

We report a new simple but effective method to extract the weak surface signals from a "thickness series" of recorded electron energy-loss spectra. Using precise thickness measurements and energy-loss near-edge structures measured at increasing thicknesses, we are able to extract the surface and bulk components in the series. The electronic structure and bonding information from SrTiO3 (001) reconstructed surfaces have been successfully obtained by applying this approach. This approach can be applied to study many other cases including absorbed monolayers and beam-sensitive interfaces.

An EELS study of near edge structures of the oxygen K-edge in spinels

Spectra of seven aluminate, ferrite and chromate spinels were collected at the oxygen K-edge in order to examine the effect of octahedral ion composition on the near edge structures and explore whether these may shed some light on previously reported major variations in the optical absorption coefficient (e) for the spin-allowed d–d band transitions due to tetrahedrally coordinated Fe2+ in optical spectra of oxide spinels. Interpretation of the energy loss near edge structure (ELNES) spectra was aided using the multiple scattering code FEFF8.40. For the chromate samples spin polarised calculations were included. Gold’s iterative deconvolution method was utilized to improve on energy resolution, resulting in spectra equal in merit to those recorded by technically superior instruments. Results include absolute energy positions and interpretation of ELNES features in terms of transitions to available states. We conclude that the major causes of differences in the optical absorption coefficient mentioned above are hybridization between oxygen p- and octahedral metal orbitals. Our study does not support the idea presented by other authors that there exists a local antiferromagnetic ordering in MgCr2O4 at room temperature.

Oxygen 1sexcitation and tetragonal distortion from core-hole effect in BaTiO3

The accurate description of the O 1s excitation in BaTiO3 has been elusive so far. In this Rapid Communication, the electronic structure and the high-resolution electron energy-loss near-edge structures of the O K edge in tetragonal BaTiO3 are investigated using first-principles calculations. The results demonstrate a clear correlation between the broadening of the lower energy fine structure and the anisotropic effects induced by the core-hole potential, which are directly related to the structural distortion occurring in tetragonal BaTiO3. Moreover, we show that a significant improvement of the description of higher-lying structures can be obtained when correcting the energy position of the Ba 4f states. This demonstrates that the O 1s spectrum can be a very effective probe of the distortion and changes in the local electronic structure, and be used as a sensitive tool for studying new materials and ferroelectric transitions.

Combined study of the ground and unoccupied electronic states of graphite by electron energy-loss spectroscopy

Both the unoccupied and ground electronic states of graphite have been studied by electron energy-loss spectroscopy in a transmission electron microscope. Electron energy-loss near-edge structures of the K-edge of carbon have been investigated in detail for scattering angles from 0 to 2.8 mrad. The π* and σ* components were separated. The angular and energy dependences of the π* and σ* structures were in fair agreement with theory. Electron energy loss Compton spectra of graphite were recorded at scattering angles from 45 to 68 mrad. One Compton scattering spectrum was obtained in 1 min compared with several hours or days using photons. The contributions of core electrons were calculated by the exact Hartree-Slater method in the Compton scattering region. The electron Compton profile for graphite is in good agreement with other conventional Compton profile measurements, as well as with theory, thus establishing the validity of the technique.

Aberration‐corrected microscopy and spectroscopy analysis of pristine, nitrogen containing detonation nanodiamond

Aberration‐corrected transmission electron microscopy, electron energy‐loss spectroscopy, and density functional theory (DFT) calculations are used to solve several key questions about the surface structure, the particle morphology, and the distribution and nature of nitrogen impurities in detonation nanodiamond (DND) cleaned by a recently developed ozone treatment. All microscopy and spectroscopy measurements are performed at a lowered acceleration voltage (80/120 kV), allowing prolonged and detailed experiments to be carried out while minimizing the risk of knock‐on damage or surface graphitization of the nanodiamond. High‐resolution TEM (HRTEM) demonstrates the stability of even the smallest nanodiamonds under electron illumination at low voltage and is used to image the surface structure of pristine DND. High resolution electron energy‐loss spectroscopy (EELS) measurements on the fine structure of the carbon K‐edge of nanodiamond demonstrate that the typical π* pre‐peak in fact consists of three sub‐peaks that arise from the presence of, amongst others, minimal fullerene‐like reconstructions at the nanoparticle surfaces and deviations from perfect sp3 coordination at defects in the nanodiamonds. Spatially resolved EELS experiments evidence the presence of nitrogen within the core of DND particles. The nitrogen is present throughout the whole diamond core, and can be enriched at defect regions. By comparing the fine structure of the experimental nitrogen K‐edge with calculated energy‐loss near‐edge structure (ELNES) spectra from DFT, the embedded nitrogen is most likely related to small amounts of single substitutional and/or A‐center nitrogen, combined with larger nitrogen clusters.

Contribution of core-loss fine structures to the characterization of ion irradiation damages in the nanolaminated ceramic Ti3AlC2

The effect of low-energy ion irradiation on the nanolaminated Ti3AlC2 is investigated by means of X-ray diffraction, transmission electron microscopy, electron energy loss and X-ray absorption spectroscopy. The chemical sensitivity and local order probing from core-loss edges provide new insights into the structural modifications induced under irradiation. From the analysis of the C K energy loss near-edge structure and Al K X-ray absorption near-edge structure by ab initio calculations, the influence of the layered structure of this compound on the irradiation damage is demonstrated, and damage is found to be preferentially localized in the aluminum planes of the structure. On the basis of comparisons between calculations and experimental spectra, a structural model is proposed for the irradiated state. This study emphasizes the utility of core-loss fine structure analysis to enhance understanding of ion irradiation-induced damage in complex crystalline materials.