White-line Intensity Ratio Research Articles

(Invited) In Situ Electrochemical STEM As a Platform for Interpreting Electrochemical Phenomena

In situ electrochemical STEM (ec-STEM) is a multi-modal characterization technique that allows for the direct interpretation of nanoscale mechanisms and kinetics during electrochemical operation. The major advantage of this method is the ability to perform quantitative electrochemical measurements while simultaneously characterizing nanoscale structural and chemical changes that occur at site-specific electrode/electrolyte interfaces using high spatial resolution imaging, spectroscopy, and/or diffraction; all within a scanning transmission electron microscope (STEM). ec-STEM is being used for battery research by studying the formation mechanisms of the solid electrolyte interphase (SEI) and growth mechanisms of lithium metal dendrites within an organic electrolyte (1.2M LiPF6 EC:DMC). The initial stage of lithium metal dendrite nucleation and growth is observed on the surface and edge of the glassy carbon working electrode during cyclic voltammetry experiments below -3.0V vs. Ptpseudo reference electrode. The Li dendrites grow with a globular morphology during a sequence of bursts, whereby newly formed lithium metal deposits are passivated by the SEI followed by additional growth events associated with localized regions of SEI breakdown. Electron energy loss spectroscopy (EELS) confirms that the deposits are lithium metal with surfaces composed of inorganic compounds commonly found in the SEI. The EEL spectra reveal a Li metal plasmon peak at 7.5eV and a characteristic LiF spectral feature on the Li K-edge at 55eV. Furthermore, a framework for obtaining and analyzing quantitative EELS measurements is demonstrated to directly determine the oxidation state of battery electrodes (LiMn2O4 and Li4Ti5O12) using the “white-lines” of the L2,3 core loss ionization edges of Mn and Ti. By simultaneously acquiring low-loss and core-loss EEL spectra, plural scattering effects caused by scattering through the electrolyte can be removed to quantify the oxidation state of Mn and Ti using the white-line intensity ratio method. RF magnetron sputtering was used to deposit a thin layer of LiMn2O4, a commonly used battery cathode material, directly onto the silicon nitride membrane of the ec-STEM cell. Simultaneous low-loss and core-loss EEL spectra are acquired with and without the presence of dimethyl carbonate (DMC) in the liquid cell (Figure 3c). The low-loss portion of the EEL spectra confirms the presence of the DMC within the liquid cell and aides in the quantification of fluid layer thickness. If the fluid layer thickness is small, it is possible to extract and analyze the Mn L2,3-edge for quantification of the Mn oxidation state using the “white-line” intensity ratio method. Mixed Mn3+/4+ oxidation is measured in the as-deposited LiMn2O4, which shifts to a lower oxidation state when DMC is present in the cell, suggesting that LiMn2O4 may be prone to electron beam induced chemical reduction when immersed in a battery electrolyte during in situ ec-STEM experiments. The information obtained from these studies can help provide a deeper understanding of how batteries function at the nanoscale. Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Effects of thickness on electronic structure of titanium thin films

Effects of thickness on the electronic structure of e-beam evaporated thin titanium films were studied using near-edge X-ray absorption fine structure (NEXAFS) technique at titanium L 2,3 edge in total electron yield (TEY) mode and transmission yield mode. Thickness dependence of L 2,3 branching ratio (BR) of titanium was investigated and it was found that BR below 3·5 nm shows a strong dependence on film thickness. Mean electron escape depth (λ) in titanium, an important parameter for surface applications, was determined to be λ = 2·6 ± 0·1 nm using L 2,3 resonance intensity variation as a function of film thickness. The average L 3 /L 2 white line intensity ratio of titanium was obtained as 0·89 from the ratio of amplitudes of each L 3 and L 2 peaks and 0·66 from the integrated area under each L 3 and L 2 peaks. In addition, a theoretical calculation for pure titanium was presented for comparison with experimental data.

Investigation of thickness dependence on electronic structures of iron and nickel thin films by L-edge X-ray absorption spectroscopy

We have studied the effect of the film thickness on the electronic structure of pure nickel and iron thin films. Series of the thin films were evaporated by e-beam evaporation on SiN substrates. The electronic structure of the thin films was investigated using X-ray absorption near edge structure (XANES) spectroscopy. We have showed the thickness dependent variation of the experimental branching ratio (BR) and full-width at half-maximum (FWHM) at the L3 and L2 edges for both thin films. A strong thickness dependence of the L2,3 BR and FWHM was found. We have also focused on the deviation of L3 to L2 ratio from its statistical value. The average L3/L2 white-line intensity ratio was calculated to be 3.4 and 3.0 from peak height and integrated area under each L3 and L2 peaks, respectively for iron. Additionally, a theoretical L2,3 edge calculation for nickel was presented. The obtained results were consistent with the general view of the L2,3 BR and FWHM of iron and nickel transition metals.

EELS compositional and valence mapping in iron fluoride–carbon nanocomposites

Iron fluoride–carbon nanocomposites (FeF2–C and FeF3–C) are newly developed materials for positive electrodes in Li–ion batteries. These new composites represent high-capacity and reversible conversion electrode materials with capacity close to theoretical value and four times that of lithium cobalt oxide; the most widely used compounds as positive electrode. In this paper, we report detailed chemical and structural analysis of FeF2–C nanocomposite electrode at different discharge states. The FeF2–C nanocomposite was prepared by high-energy ball milling at ambient conditions and further processed in helium environment. The as synthesized electrode consists of FeF2 nanodomains with crystallite size ranging from 2 to 18 nm in an amorphous carbon matrix. The electrode material was analyzed by the electron energy loss spectroscopy (EELS) compositional imaging with nanometer resolution, and selected area electron diffraction (SAED) at 36 %, 75 %, and fully discharged states. The high-resolution EELS elemental maps and intensity line profiles of Fe and F were obtained to investigate phase transformation during discharge. Valence maps of Fe were also produced based on the Fe L3/L2 white line intensity ratios. The 2-nanometer scale EELS elemental maps reveal that Fe in the as processed electrode consists of a mixed valence states (2+ and 3+). The discharged samples at 36 and 75 % demonstrate compositional modulation with F-rich and F-depleted areas with the fully discharged material consisting of nanoscale regions of LiF and Fe in agreement with the cathodic conversion reaction.

Influence of strain on the electronic structure of the TbMnO3/SrTiO3 epitaxial interface

Understanding the magnetotransport properties of epitaxial strained thin films requires knowledge of the chemistry at the interface. We report on the change in Mn electronic structure at the epitaxially strained TbMnO3/SrTiO3 interface. Scanning transmission electron microscopy shows an abrupt interface with a bright contrast, indicating the presence of misfit strain. Electron energy loss spectroscopy displays a chemical shift of the Mn L2,3 edge together with a high white line intensity ratio revealing a reduction in the nominal Mn oxidation state in the first 3–4 monolayers. These observations indicate misfit strain significantly changes the electronic structure at the interface.

Phase separation in the iron chalcogenide superconductor Fe1+yTexSe1−x

We present direct evidence for phase separation and chemical inhomogeneity in Fe1+yTexSe1−x single crystals from scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) data. In STEM, images recorded using a high-angle annular dark field (HAADF) detector show characteristic nanometer-scale patterns of phase separation from the Z-dependent image contrast. The separation was observed both in non-superconducting samples with excess iron and in superconducting samples. Using EELS, we determined ∼20% (or lower) fluctuation in Te concentration from the average local compositions by integrating the intensity of the Te-M4,5 edge. The energy-loss near-edge structure (ELNES) of the Fe-L2,3 edge changes with varying composition, especially the L3/L2 white-line intensity ratio, which is sensitive to the d-state occupancy of the Fe. The results suggest a miscibility gap in the Fe1+yTexSe1−x system and changes in the d-electron states at the nanometer scale from the separated phases.

Determination of Local Oxidations States in Ni-NiO Core-shell Structures Using White Line Intensity Ratios

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Manganese valence imaging in Mn minerals at the nanoscale using STEM-EELS

Electron energy loss spectroscopy (EELS) was used with scanning transmission electron microscopy (STEM) to quantify the average Mn valence in natural minerals at the nanometer scale. A method was developed to calibrate the energy-loss scale accurately, providing a comparison between STEM-EELS and the X-ray absorption spectroscopy methods that investigate the L -edge chemical shift as Mn valence changes. The chemical-shift measurements were consistent with data reported by previous researchers from both X-ray and electron energy-loss spectroscopy. The L 3 /L 2 white-line intensity ratios also were consistent with previous work. A calibration curve for Mn valence was produced using the L 3 /L 2 white-line intensity ratios from measurements of synthetic standards. The average Mn valence was determined because it is not possible to distinguish Mn3+ from mixtures of Mn2+ and Mn4+ using either method. The white-line intensity method was implemented in automated software that allows for rapid processing of point spectra, and 1-D and 2-D spectrum images. Point analyses of two natural pyrolusite samples indicated a Mn valence of 4.0, and point analyses of romanechite and manganite gave values of 3.8 and 3.4, respectively. An interface between braunite and bementite was used to illustrate 1-D and 2-D spectrum-imaging capabilities. The measured valence of Mn in the braunite and bementite was 2.9 and 2.0, respectively; both consistent with theoretical values. The braunite-bementite sample demonstrated the heterogeneity of Mn valence common to natural minerals and the advantages of acquiring quantitative valence information in a known spatial context.

Determination of manganese valency in La 1− xSr xMnO 3 using ELNES in the (S)TEM

In this article several experimentally identified Mn valence-sensitive ELNES quantities for the La 1− x Sr x MnO 3 compound class are presented, namely the energy separations between Mn-L 3 and O-K a, between O-K b and O-K a edges, the Mn-L 2,3 white line intensity ratio, and the Mn-L 3 line width. Valence sensitivities of these quantities are evaluated, and possible additional influences on them are considered. At high signal-to-noise ratio the two energy separations display most sensitively changes of the Mn valency. An experiment-based estimation of the total uncertainties of the quantities indicates that at low signal-to-noise ratio, which is the case when studying interface effects at high spatial resolution, again both energy separations allow to resolve the smallest valency changes.

Energy-loss near-edge fine structures of iron nanoparticles

Body centered cubic (bcc) Fe nanoparticles were fabricated by in situ decomposition of iron fluoride films in a transmission electron microscope. Electron energy-loss near edge structure (ELNES) was used to characterize this exposure process. In particular, the L 3/L 2 white-line intensity ratio (WLR) was used to monitor the iron valence state during exposure, and as an indicator of other properties of the iron nanoparticles. Iron nanoparticles with sizes between 2 and 20 nm exhibit a constant WLR, whose value is same as that for a continuous bcc iron film, suggesting little or no dependence of the local magnetic moment or structure on the particle size. A broad but prominent peak which occurs 40 eV after the L 3-ionization threshold in the iron fluoride, is absent for a metallic iron film but reappears when the iron is converted to an oxide. Long-range ferromagnetic coupling was observed in samples densely populated with iron nanoparticles. Because there is little interaction between particles and the supporting carbon substrate, these samples provide an ideal model system for studying the influence of particle size and interparticle distance on magnetic properties.

Oxidation states of Mn and Fe in various compound oxide systems

Energy-loss near-edge structure (ELNES) data of Mn-L2,3 and Fe-L2,3 ionization edges have been measured by means of quantitative electron energy-loss spectroscopy (EELS) for two series of Mn and Fe oxides with known formal cation oxidation states. In both series the absolute energy positions of Mn-L2,3 and Fe-L2,3 white-lines, as well as the white-line intensity ratio (L3/L2) vary with cation oxidation states. Additionally, spin–orbit spitting, i.e. the energy difference ΔE(L2−L3) between Mn-L2,3 white-lines decreases with increasing Mn oxidation state. With these data from known standards calibration curves on white-line intensity ratio Mn(L3/L2) vs. Mn oxidation state, and Fe(L3/L2) vs. Fe oxidation state were established.EELS measurements on Mn and Fe doped ZnO thin films showed that the valence states of the dopants can unambiguously be determined by calibrating the Mn-L2,3 and Fe-L2,3 ELNES data against the measured standards. It is revealed that Mn in ZnO adopt a divalent state, thus Mn2+ ions substitute for Zn2+, whereas Fe dopants retain a trivalent oxidation state in the ZnO host lattice. Measurements on (Ba, Fe, Mn)-oxides revealed that both Fe and Mn cations are in a trivalent state. Thus, it is assumed that Mn3+ can partially be substituted for Fe3+ in barium hexaferrites.

Extraction of EELS white-line intensities of manganese compounds: Methods, accuracy, and valence sensitivity

This article evaluates the valence sensitivity and accuracy of selected EELS white-line extraction methods based on the white-line intensity ratio in case of manganese oxides, predominantly in the valency range [ Mn 3 + ; Mn 4 + ]. For this purpose Mn– L 2 , 3 ionization edges of several Mn oxides were measured and four different methods were applied to extract the intensities. The obtained ratios have been analyzed in terms of their errors, their scatter, and their sensitivity on manganese valency. We found that the maximum-intensity as well as the Pearson and Walsh–Dray methods show larger relative changes of the ratio between Mn 3 + and Mn 4 + than curve-fitting. Taking the scatter into account, Walsh–Dray distinguishes more clearly between those two valencies than curve-fitting. Thus the Walsh–Dray method promises the most accurate determination of small valency shifts.

Surface Chemistry of Combustion-Synthesized Iron Oxide Nanoparticles Determined by Electron Energy-Loss Spectroscopy

ABSTRACTParticulate matter (PM) that is composed of a complex mixture of organic compounds, soot, transition metals, sulfates, nitrates, and other trace elements has been associated with a range of adverse health effects. The effect of the separate components of PM and their interaction with each other is important in order to understand the origins of toxicity in these materials. Iron and soot are common environmental contaminants occurring in ultrafine particulate matter in the air, and have been experimentally generated, in a manner simulating high-temperature, industrial processes, through the combustion of iron pentacarbonyl in a mixture of acetylene and ethylene. Nanoprobe electron energy-loss spectroscopy (EELS) in the transmission electron microscope was used to collect Fe L3:L2 white line-intensity ratios from the interiors and surfaces of iron oxide nanoparticles present in samples generated under low and high soot conditions. This ratio is sensitive to any change in the iron oxidation state, and indicated that iron oxide nanoparticles co-generated with large quantities of soot during combustion processes show a subsurface layer (of thickness 2–3 nm) of decreased iron oxidation state. A quantitative analysis of the iron L32 EELS ionization edge measured in nanoprobe mode indicated that in these particles, the oxidation state of iron at the surface is decreased from Fe3+ to Fe2+, possibly explaining the toxicity of these materials in the respiratory tract. The application of EELS to understanding the nanoscale distribution of the various components and their chemical interactions in these samples, and correlation of these results with acute respiratory toxicity studies of the laboratory-generated particulate matter, are discussed.

Mapping The Valence States of Transition Metals Across Interfaces By Energy-Filtered Tem

Some properties of transition metal oxides are related to the presence of elements with mixed valences. In electron energy-loss spectroscopy (EELS), the L or M ionization edges of transition-metal, rare-earth and actinide elements usually display sharp threshold peaks known as white-lines. EELS experiments have shown that a change in cation valence state introduces a significant change in the White-line intensity ratio [1]. With the use of valence state information provided by the white lines, an experimental approach is demonstrated here to map the valence state distributions of Mn and Co using an energy-filtered transmission electron microscope (TEM). A spatial resolution of ˜ 2 nm has been achieved. This technique should be particularly useful in studying valence states of cations in magnetic oxides.To map the distribution of ionization states, an energy window of ˜ 10 eV in width is required to isolate the L3 from L2 white lines (Figure 1).

Quantitative determination of iron oxidation states in minerals using Fe L 2,3 -edge electron energy-loss near-edge structure spectroscopy

The Fe L2,3-edge spectra for a range of natural minerals and synthetic solid solutions have been measured using the technique of parallel electron energy-loss spectroscopy (PEELS) recorded in a transmission electron microscope (TEM). The Fe L2,3-edges of the minerals are characterised by two white-line features and exhibit electron energy-loss near-edge structure (ELNES) characteristic of Fe valence state. For divalent iron, the Fe L3-edge spectra are dominated by a sharp peak (white-line) at ca. 707.8 eV, followed by a broader and less intense peak at ca. 710.5 eV. The ELNES on the Fe L3-edge of trivalent iron consists of a white-line with its maximum at ca. 709.5 eV and a preceeding peak at ca. 708.0 eV. Mineral solid solutions that contain both Fe2+ and Fe3+ exhibit an Fe L3-edge shape that is composed of Fe L3-edges from the respective Fe2+- and Fe3+-bearing end members. The integral Fe L2,3-edge white-line intensity ratios I(L3)/I(L2) show clear differences for Fe2+ and Fe3+. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by determining the integral Fe L2,3-edge white-line intensity ratios I(L3)/I(L2) as a function of the ferric iron concentration resulting in an universal curve within the experimental errors. The application of the universal curve combined with the high spatial resolution using the PEELS/TEM allows the quantification of the ferric iron concentration on a scale down to 10 nm, which is illustrated from a sample of ilmenite containing hematite exsolution lamellae that shows different Fe L2,3-edge shapes consistent with variations in the Fe2+-Fe3+ ratio over distances of ca. 100 nm.

Electron energy loss spectroscopy studies of CoCrTaPt/Cr thin films for high-density longitudinal magnetic recording media

The effects of Ta content in CoCrTaPt thin films on the near-edge structure of Co and Cr L 2.3 absorption edges have been studied using parallel electron energy loss spectroscopy (PEELS). It was found that the addition of Ta decreased the 3d band occupancy in CoCrTaPt thin films. The addition of Ta was also observed to decrease the L 3/L 2 white line intensity ratio, which showed a qualitative correlation with Curie temperature measurements. This result indicates a decrease in the ferromagnetic exchange interaction between neighboring Co atoms which may account for the low noise of CoCrTaPt thin film media.

Determining Manganese Oxidation State in Soils Using X‐ray Absorption Near‐Edge Structure (XANES) Spectroscopy

AbstractThe redox chemistry of soil Mn is important in Mn uptake by plants, the movement of trace elements absorbed on or occluded in Mn‐oxides, and the etiology of some soil‐borne plant fungal diseases. The objectives of this study were to develop calibration curves for determining Mn oxidation states in moist soil samples using micro‐x‐ray absorption near‐edge structure spectroscopy and to test the method on selected soils. Oxidation‐state standards were prepared by mixing dry powders of MnSO4·H2O (Mn2+) and a synthetic Na‐birnessite, Na4Mn14O27·9H2O (Mn4+) to obtain various Mn2+ mole fractions, f(Mn2+), where f(Mn2+) = Mn2+/(Mn2+ + Mn4+). Corundum (α‐Al2O3) was used as a diluent to prepare mixtures containing 50, 1000, and 100 000 mg Mn kg−1. Quantitative analysis of Mn oxidation state is possible using either the energy of the pre‐edge peak at 6540 eV or a ratio based on the intensity of the Mn2+ white line peak at 6552.6 eV divided by the intensity of the Mn4+ white line peak at 6560.9 eV. The white line intensity ratio is more sensitive, with an estimated lower detection limit of 20 mg Mn kg−1 using a 300 by 300 µm spot, and predicts f(Mn2+) with an accuracy of ±0.1 mole fraction (95% confidence interval) through most of its range. The f(Mn2+) for four air‐dried Indiana surface soils ranged from 0.26 to 0.44 ± 0.1. Saturation and reduction for 5 d resulted in complete reduction of Mn in the soil matrix.

Electron-energy-loss core-edge structures in manganese oxides.

Oxygen K and manganese ${\mathit{L}}_{2,3}$ edges electron-energy-loss spectra have been measured for a series of manganese oxides with different nominal oxidation states for the cation (MnO, ${\mathrm{Mn}}_{3}$${\mathrm{O}}_{4}$, ${\mathrm{Mn}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{MnO}}_{2}$, and ${\mathrm{BaMnO}}_{4}$). Spectra have been processed for quantitative elemental analysis and for the evaluation of manganese ${\mathit{L}}_{2,3}$ white-line intensity ratios and normalized total intensities. Prepeaks on the oxygen K edge are sensitive to the nature of local chemical bonds and their detailed substructures are interpreted with the support of a molecular-orbital picture. As for the cation point of view, the analysis of the white-line intensities provides a satisfactory measurement of the effective 3d orbital occupancy as well as a hint of the local magnetic properties. The present study also confirms that the hybridization between oxygen 2p and manganese 3d orbitals plays an important role when considering the electronic structures of these oxides.

L 3/L 2 white-line intensity ratios in the electron energy-loss spectra of 3d transition-metal oxides

The L 3/L 2 white-line intensity ratio in transition-metal oxides deviates widely from the statistical value of 2 : 1 but shows interesting systematics. In a series of oxides of a given metal, the ratio reaches a maximum for the d 5 configuration (e.g. MnO) and a minimum for the d 0 configuration (e.g. KMnO 4). In a series of monoxides, sesquioxides and dioxides of different metals, the ratio is again a maximum at the d 5 configuration and decreases as the configuration changes towards d 0 or d 10. Our results, obtained by electron energy-loss spectroscopy, carried out in an electron microscope, are interpreted on an atomic mechanism involving spin-spin coupling. According to this model, the L 2 transition probability decreases in the progression d 0 to d 5 whereas the L 3 transition probability decreases beyond d 5.

Study of theL23edges in the3dtransition metals and their oxides by electron-energy-loss spectroscopy with comparisons to theory

Excitations of the $2p$ subshell in the $3d$ transition metals and their oxides have been studied by inelastic scattering of 75-keV electrons. The ${L}_{23}$ which arise from dipole transitions to unoccupied $d$ states have been investigated in terms of their threshold energies, widths, and intensity ratios. Shifts in the ${L}_{3}$ threshold energy between the metal and oxide are different from the chemical shifts measured by x-ray photoemission spectroscopy and this suggests the importance of relaxation effects. Single-particle calculations for the ${L}_{3}$ spectra are also discussed. Measured ${L}_{3}$ linewidths are generally larger than those predicted by suitably broadened theory. A variation from the statistical ${L}_{3}$-to-${L}_{2}$ white-line intensity ratio of 2:1 has been observed across the $3d$ transition row, with values ranging between 0.8:1 for Ti to 5:1 for FeO. This behavior appears to be associated with the white lines since Cu with a filled $3d$ band exhibits the statistical results. It is suggested that the anomalous ratios may be explained by a breakdown of $j\ensuremath{-}j$ coupling caused by an exchange mechanism. Finally, the extended x-ray absorption fine-structure-type structure extending several hundred eV above the white lines is analyzed for Cr to provide the radial distribution function.