Electron Energy-loss Analysis Research Articles

QUEELS: Software to calculate the energy loss processes in TEELS, REELS, XPS and AES including effects of the core hole

We present the user‐friendly and freely available software package QUEELS (QUantitative analysis of Electron Energy Losses at Surfaces) that allows to calculate effective inelastic scattering cross sections within the dielectric response description, for swift electrons travelling nearby surfaces in several environments. We briefly describe the underlying theoretical models and illustrate its use to evaluate the distribution of energy losses taking place in electron spectroscopies like transmission electron energy loss spectroscopy (TEELS), X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection electron energy loss spectroscopy (REELS), which are widely used for material analysis. This includes the intrinsic excitations due to the core hole in XPS and AES.

Optical Excitations with Electron Beams: ChallengesandOpportunities.

Free electron beams such as those employed in electron microscopes have evolved into powerful tools to investigate photonic nanostructures with an unrivaled combination of spatial and spectral precision through the analysis of electron energy losses and cathodoluminescence light emission. In combination with ultrafast optics, the emerging field of ultrafast electron microscopy utilizes synchronized femtosecond electron and light pulses that are aimed at the sampled structures, holding the promise to bring simultaneous sub-Å–sub-fs–sub-meV space–time–energy resolution to the study of material and optical-field dynamics. In addition, these advances enable the manipulation of the wave function of individual free electrons in unprecedented ways, opening sound prospects to probe and control quantum excitations at the nanoscale. Here, we provide an overview of photonics research based on free electrons, supplemented by original theoretical insights and discussion of several stimulating challenges and opportunities. In particular, we show that the excitation probability by a single electron is independent of its wave function, apart from a classical average over the transverse beam density profile, whereas the probability for two or more modulated electrons depends on their relative spatial arrangement, thus reflecting the quantum nature of their interactions. We derive first-principles analytical expressions that embody these results and have general validity for arbitrarily shaped electrons and any type of electron–sample interaction. We conclude with some perspectives on various exciting directions that include disruptive approaches to noninvasive spectroscopy and microscopy, the possibility of sampling the nonlinear optical response at the nanoscale, the manipulation of the density matrices associated with free electrons and optical sample modes, and appealing applications in optical modulation of electron beams, all of which could potentially revolutionize the use of free electrons in photonics.

Fundamentals and Prospects of Catalysis (Catalysis: Current and Future Developments – Volume 1)

The homogeneity of hydrogen termination on single-crystal CVD diamond surfaces exposed to air was studied with a new approach that encloses a combination of sheet resistance measurements, performed using four-point-probe method in Van Der Pauw configuration, and electron reflectivity measurements performed at intermediate kinetic energy to derive surface sensitive information on the chemical terminations (inelastic mean free path <0.8 nm). The coherence between the 2-dimensional spatial distributions of sheet resistance and electron reflectivity maps, as well as their similar dynamic range and consistency with an electron energy loss analysis, demonstrates that the proposed combined approach allows for a reliable and easy evaluation of both the effectiveness and homogeneity of the hydrogen-termination process of a diamond surface.

Combined electrical resistivity-electron reflectivity measurements for evaluating the homogeneity of hydrogen-terminated diamond surfaces

The homogeneity of hydrogen termination on single-crystal CVD diamond surfaces exposed to air was studied with a new approach that encloses a combination of sheet resistance measurements, performed using four-point-probe method in Van Der Pauw configuration, and electron reflectivity measurements performed at intermediate kinetic energy to derive surface sensitive information on the chemical terminations (inelastic mean free path <0.8 nm). The coherence between the 2-dimensional spatial distributions of sheet resistance and electron reflectivity maps, as well as their similar dynamic range and consistency with an electron energy loss analysis, demonstrates that the proposed combined approach allows for a reliable and easy evaluation of both the effectiveness and homogeneity of the hydrogen-termination process of a diamond surface.

Optical properties of molybdenum in the ultraviolet and extreme ultraviolet by reflection electron energy loss spectroscopy.

Optical properties of polycrystalline molybdenum are determined from ultraviolet up to extreme ultraviolet by reflection electron energy loss spectroscopy (REELS). Calculations are performed within the dielectric response theory by means of the quantitative analysis of electron energy losses at surfaces QUEELS-ϵ(k,ω)-REELS software [Surf. Interface Anal.36, 824 (2004)SIANDQ0142-242110.1002/sia.1774] that allows the simulation of inelastic scattering cross sections, using a parametric energy loss function describing the optical response of the material. From this energy loss function, the real and imaginary parts of the dielectric function, the refractive index, and the extinction coefficient are deduced and compared with previously published results.

Anionic substitution in LiMnPO4: The Li1-xMn1+x(PO4)1-y-z(VO4)y(OH)4z solid solutions prepared with a microwave-assisted hydrothermal method

Attempts to introduce vanadium doping into the olivine-structured LiMnPO4 cathode material for the Li-ion batteries resulted in the (y ​+ ​z)PO43− → yVO43− ​+ ​4zOH− (y ​< ​0.17, z ​≤ ​0.1) co-substitution and the formation of the hydrotriphylite-type Li1−xMn1+x(PO4)1−y−z(VO4)y(OH)4z solid solutions. Their crystal structure and chemical composition were studied with Rietveld structure refinement from synchrotron X-ray powder diffraction data, energy-dispersive X-ray analysis, infrared and Raman spectroscopy, electron energy loss analysis and mass spectrometry. Li0.96Mn1.04(PO4)0.74(VO4)0.16(OH)0.4 crystallizes in the space group Pnma and a ​= ​10.5150(2) Å, b ​= ​6.11773(9) Å and c ​= ​4.78555(9) Å with up to 5% of Mn in the Li position. Charge compensation is achieved through partial oxidation of the Mn cations to the oxidation state above +2. The heterovalent substitution in anionic sublattice and defectiveness of the cationic one cause blocking of Li-ion diffusion channels that drastically impede the electrochemical performance resulting in electrochemical capacity not exceeding 35 mAh g−1.

Helmholtz decomposition analysis of electron energy loss: differentiating resonances on polarization and radiation eigenmodes

Helmholtz decomposition is a powerful mathematical tool for investigation and analysis of vector fields, particularly in electrodynamic systems. In this paper, we apply it to electron energy loss spectroscopy of magnetically inactive structures to get insight into field composition of energy losses. The obtained results show that losses are incurred through two pathways—excitation of transverse/longitudinal fields by solenoidal/conservative currents. Separation of these processes gives us important information about the fields and the type of eigenmodes (polarization or radiation) being in resonance with the electron beam. Capabilities of such analysis are demonstrated in the study case of metal nanofilms and nanodiscs, where we perform differentiation of the resonances that have occurred on plasmons (polarization eigenmodes) and plasmon polaritons (radiation eigenmodes).

Novel near-infrared emission from crystal defects in MoS2 multilayer flakes.

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission.

Quantitative analysis of Ni 2p photoemission in NiO and Ni diluted in a SiO2 matrix

In X-ray excited photoelectron emission (XPS), besides the initial excitation process, the shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. To make an accurate quantitative interpretation of features observed in XPS, these effects must be included in the theoretical description of the emitted photoelectron spectra. It was previously shown [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17] that these three effects can be calculated by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS) in terms of effective energy-differential inelastic electron scattering cross-sections. The only input needed to calculate these cross-sections is the energy loss function of the media which is determined from analysis of Reflection Electron Energy Loss Spectra (REELS). The full XPS spectrum is then modeled by convoluting this energy loss cross-section with the primary excitation spectrum that accounts for all effects which are part of the initial photo-excitation process, i.e. lifetime broadening, spin–orbit coupling, and multiplet splitting. In this paper we apply the previously presented procedure to the study of Ni 2p photoemission in NiO and Ni diluted in a SiO2 matrix (Ni:SiO2), samples being prepared by reactive magnetron sputtering at room temperature. We observe a significant difference between the corresponding Ni 2p primary excitation spectra. The procedure allows quantifying the relative intensity of the c3d9L, c3d10L2, and c3d8 final states contributing to the Ni 2p photoemission spectra of the Ni2+ species in the oxide matrices. Especially, the intensity ratio in NiO between the non-local and local contributions to the 3d9L configuration is determined to be 2.5. Moreover the relative intensity ratio of the c3d9L/c3d10L2/c3d8 configurations is found to be 1.0/0.83/0.11 for both the NiO and Ni:SiO2 samples.

Co and C60 interaction under conditions of mixture

Mixture between Co and C 60 were prepared by simultaneous deposition of the components on (0001) α-Al 2 O 3 and (001) NaCl at room temperature in a vacuum of 10 -5 Pa. Atomic force microscopy show granule structure with average granule size of about 30nm. Transmission electron microscopy revealed mixed structure consisting of fcc-Co nanocrystals and C 60 -based substance, which contains diamond, graphite and cobalt carbides. Analysis of electron energy loss and Raman spectra indicate some chemical interaction between Co atoms and C 60 molecules in mixture.

Primary excitation spectra in XPS and AES of Cu, CuO: Relative importance of surface and core hole effects

Quantitative interpretation of structures observed in XPS and AES requires models to correct for various physical processes involved. Besides the initial excitation process in XPS and AES, the measured spectrum is affected by three additional effects: the corehole(s), transport to the surface region and passage through the surface and vacuum regions. These three effects can be calculated by the QUEELS-XPS software (Quantitative analysis of Electron Energy Losses at Surfaces) in terms of energy-differential inelastic electron scattering cross sections. From this and the QUASES software (Quantitative Analysis of Surfaces by Electron Spectroscopy), background contributions and primary excitation spectra are obtained for various transitions (Cu 2p from Cu or CuO and Cu L3M23M23) and we investigate the separate effect of bulk, surface, and core hole(s) excitations. We show that the shape of the XPS and AES primary spectra and background contributions are modified slightly by surface effects and very strongly by core hole(s) effects. For metals, the intrinsic excitations give rise to a prominent spike in the background close to the XPS-peak energy. This spike will be much reduced for wide band gap insulators. Moreover our method gives an easy procedure to obtain the true primary excitation spectra for XPS and AES.

LMM Auger primary excitation spectra of copper

The shape and intensity of measured Auger peaks are strongly affected by extrinsic excitations due to electron transport out of the surface and to intrinsic excitations induced by the sudden creation of the two static core holes. Following a method developed for XPS in a previous work [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17], we have calculated the effective energy-differential inelastic electron scattering cross-sections, including the effects of the surface and of the two core holes, within the dielectric response theory by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS). The Auger spectra are then modeled by convoluting this energy loss cross section with the primary excitation spectrum that accounts for all effects which are part of the initial Auger process, i.e. L–S coupling and vacancy satellite effects. The shape of this primary excitation spectrum is fitted to get close agreement between the theoretical and the experimental spectra obtained from X-ray excited Auger electron spectroscopy (XAES). We have performed these calculations of XAES spectra for various LMM Auger transitions of pure Cu (L3M45M45, L3M23M45, L3M23M23 and L2M45M45 transitions). We compare the resulting primary excitation spectra with theoretical results published in the literature and obtain reasonable quantitative agreement. In particular, we extract from experimental spectra quantitative intensities due to Coster–Kronig, shake-off and shake-up processes relative to the intensity from the “normal” Auger process.

Energy dependence of the energy loss function parametrization of indium in the Drude–Lindhard model

The energy loss function (ELF) is a key parameter for the calculations of energy losses undergone by electrons in matter. It is often the only input in the calculations performed within the models based on the semi‐classical dielectric response theory. Its perfect knowledge is thus of primordial importance. To evaluate the ELF, it is usual to consider as a model an expansion in Drude–Lindhard (DL) type oscillators with fixed values of the strength, width, energy and dispersion for the various oscillators. However, for materials that are characterized by a single sharp oscillator as aluminium or indium, it has been shown [Phys. Rev. B 46 (1992) 2486] that the damping parameter that corresponds in the DL model to the width of the oscillator increases for decreasing incident electron energy. To emphasize this effect, we perform in this work systematic calculation of the ELF for an indium target and for incident electron energies between 200 and 2000 eV. The ELF is determined by comparing REELS (reflection electron energy loss spectroscopy) experimental inelastic electron scattering cross sections with cross sections calculated within the semi‐classical dielectric response model, which is implemented in the QUEELS‐ε(k,ω)‐REELS software (Quantitative analysis of Electron Energy Losses at Surfaces) [Surf. Interface Anal. 36 (2004) 824]. We also perform measurements and calculations for varying incident and exit angles of the electron, namely for angles to the surface normal between 15o and 75o, to check the validity of our results for all geometries. Our results show that the damping parameter of indium in the DL model for electron energy of 200 eV is three times larger than for energy of 2000 eV. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Modeling of X‐ray photoelectron spectra: surface and core hole effects

The shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface and by intrinsic excitations induced by the sudden creation of the static core hole. Besides, elastic electron scattering may also be important. These effects should be included in the theoretical description of the emitted photoelectron peaks. To investigate the importance of surface and core hole effects relative to elastic scattering effect, we have calculated full XPS spectra for the Cu 2p emissions of Cu and CuO with the simulation of electron spectra for surface analysis (SESSA) software and with a convolution procedure using the differential inelastic electron scattering cross‐section obtained with the quantitative analysis of electron energy loss in XPS (QUEELS‐XPS) software. Surface and core hole effects are included in QUEELS‐XPS but absent in SESSA while elastic electron scattering effects are included in SESSA but absent in QUEELS‐XPS. Our results show that the shape of the XPS spectra are strongly modified because of surface and core hole effects, especially for energy losses smaller than about 20 eV. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy

Optimized conditions for imaging and spectroscopic/elemental mapping of thin perfluorosulfonic acid (PFSA) ionomer layers in fuel cell electrodes by scanning transmission electron microscopy (STEM) have been investigated. The proper conditions were first identified using model systems of either Nafion ionomer-coated nanostructured thin film catalysts or thin films on nanoporous Si. These analysis conditions were then applied in a quantitative study of the ionomer through-layer loading for two differently-prepared electrode catalyst layers using electron energy loss (EELS) and energy dispersive X-ray spectroscopy (EDS) in the STEM. The electron-beam induced damage to the PFSA ionomer was quantified by following the fluorine mass loss with electron dose/exposure and was mitigated by several orders of magnitude using cryogenic specimen cooling and a higher incident electron voltage. Multivariate statistical analysis was applied to the analysis of both EELS and EDS spectrum images for data de-noising and unbiased separation of the independent components related to the catalyst, ionomer, and support distributions within the catalyst layers.

Mineralogical variables that control the antibacterial effectiveness of a natural clay deposit

As antibiotic-resistant bacterial strains emerge and pose increased global health risks, new antibacterial agents are needed as alternatives to conventional antimicrobials. Naturally occurring antibacterial clays have been identified which are effective in killing antibiotic-resistant bacteria. This study examines a hydrothermally formed antibacterial clay deposit near Crater Lake, OR (USA). Our hypothesis is that antibacterial clays buffer pH and Eh conditions to dissolve unstable mineral phases containing transition metals (primarily Fe(2+)), while smectite interlayers serve as reservoirs for time release of bactericidal components. Model pathogens (Escherichia coli ATCC 25922 and Staphylococcus epidermidis ATCC 14990) were incubated with clays from different alteration zones of the hydrothermal deposit. In vitro antibacterial susceptibility testing showed that reduced mineral zones were bactericidal, while more oxidized zones had variable antibacterial effect. TEM images showed no indication of cell lysis. Cytoplasmic condensation and cell wall accumulations of <100 nm particles were seen within both bacterial populations. Electron energy loss analysis indicates precipitation of intracellular Fe(3+)-oxide nanoparticles (<10 nm) in E. coli after 24 h. Clay minerals and pyrite buffer aqueous solutions to pH 2.5-3.1, Eh > 630 mV and contain elevated level (mM) of soluble Fe (Fe(2+) and Fe(3+)) and Al(3+). Our interpretation is that rapid uptake of Fe(2+) impairs bacterial metabolism by flooding the cell with excess Fe(2+) and overwhelming iron storage proteins. As the intracellular Fe(2+) oxidizes, it produces reactive oxygen species that damage biomolecules and precipitates Fe-oxides. The ability of antibacterial clays to buffer pH and Eh in chronic non-healing wounds to conditions of healthy skin appears key to their healing potential and viability as an alternative to conventional antibiotics.

Determination of the Cu 2p primary excitation spectra for Cu, Cu2O and CuO

The shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. These effects must be included in the theoretical description of the emitted photoelectron spectra. We have calculated the effective energy-differential inelastic electron scattering cross section for XPS, including both surface and core hole effects, within the dielectric response theory by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS). The full XPS spectrum is then modeled by convoluting this energy loss cross section with the primary excitation spectrum that accounts for all effects which are part of the initial photo-excitation process, i.e. lifetime broadening, spin–orbit coupling, and multiplet splitting. The shape of this primary excitation spectrum is determined by requiring close agreement between the resulting theoretical spectrum and the experimental XPS spectrum. These calculations were performed for Cu 2p peaks of Cu, Cu2O, and CuO. For CuO, we compare the obtained primary excitation spectra with first principle calculations performed with the CTM4XAS software (Charge Transfer Multiplet program for X-ray Absorption Spectroscopy) for the corresponding emissions and we find good quantitative agreement.

The atomic and electronic structures of NiO(001)/Au(001) interfaces

The atomic and electronic structures of NiO(001)∕Au(001) interfaces were analyzed by high-resolution medium energy ion scattering (MEIS) and photoelectron spectroscopy using synchrotron-radiation-light. The MEIS analysis clearly showed that O atoms were located above Au atoms at the interface and the inter-planar distance of NiO(001)∕Au(001) was derived to be 2.30 ± 0.05 Å, which was consistent with the calculations based on the density functional theory (DFT). We measured the valence band spectra and found metallic features for the NiO thickness up to 3 monolayer (ML). Relevant to the metallic features, electron energy loss analysis revealed that the bandgap for NiO(001)∕Au(001) reduced with decreasing the NiO thickness from 10 down to 5 ML. We also observed Au 4f lines consisting of surface, bulk, and interface components and found a significant electronic charge transfer from Au(001) to NiO(001). The present DFT calculations demonstrated the presence of an image charge beneath Ni atoms at the interface just like alkali-halide∕metal interface, which may be a key issue to explain the core level shift and band structure.

Comparison between surface excitation parameter obtained from QUEELS and SESINIPAC

Surface excitations significantly influence the measured peak intensities in elastic peak electron spectroscopy. They are characterised by the surface excitation parameter (SEP) defined as the change in excitation probability of an electron caused by the presence of the surface in comparison with an electron moving in an infinite medium. It is thus important to have a large database of SEP values or to have the possibility to determine it with a user‐friendly software. Recently, Novák developed the programme Software for Electron Solid Inelastic Interaction Parameter Calculations (SESINIPAC) within the model of Tung, Chen, Kwei and Chou, which allows to determine inelastic mean free path, differential inelastic mean free path, SEP and differential SEP for various energy loss function models and dispersion relations with as only input the energy loss function of the material. Using SESINIPAC, we calculate SEP for 27 different types of materials (metals, semiconductors and insulators) and for various angles and energies. We compare these results with those obtained previously with the software Quantitative Analysis of Electron Energy Losses at Surfaces (QUEELS), which uses the Yubero‐Tougaard model. We show that the dependence on angle of emission and energy is quite similar for the two models. However, the absolute values calculated with SESINIPAC are generally larger than those calculated with QUEELS, and the mean relative difference is 20% for metals and semiconductors but exceeds 50% for insulators. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A photo-chemical method for the production of olivine nanoparticles as cosmic dust analogues

This paper describes a new experimental method to synthesise metal silicate particles in the laboratory with compositions and structures which reflect those likely to form in planetary atmospheres and in relatively cool regions of oxygen-rich stellar outflows. Fe–Mg-silicate nanoparticles of olivine composition were produced by the photo-oxidation of a mixture of Fe(CO)5, Mg(OC2H5)2 and Si(OC2H5)4 vapours in the presence of O3 at room temperature and atmospheric pressure. Transmission electron microscope X-ray and electron energy loss analysis of the particles from a number of experiments run with different precursor vapour mixture ratios show that Mg2xFe2−2xSiO4 particles can be produced ranging from x=0 to 1, where x is linearly proportional to the ratio of Mg(OC2H5)2/(Fe(CO)5+Mg(OC2H5)2). Electronic structure calculations with hybrid density functional/Hartree–Fock theory are then used to explore the pathways involved in producing olivine particles from the FeO3, MgO3 and SiO2 produced from the photolysis of the organometallic precursors in O3. These calculations indicate that highly exothermic reactions lead to the formation of Mg2SiO4, MgFeSiO4 and Fe2SiO4 molecules, which then polymerize. An alternative pathway, also strongly favoured thermodynamically, is the polymerization of MgSiO3 and FeSiO3 to form pyroxenes, which then undergo structural rearrangement to olivine and silica phases. The implications for metal silicate formation in planetary atmosphere and stellar outflows are then discussed.