Low Energy Loss Region Research Articles

A theoretical and experimental investigation of Bragg's rule for energy-loss straggling in low mean energy loss regime in air and its constituents

A theoretical and experimental investigation of Bragg's rule in low energy loss region where charge exchange straggling can be ignored is presented. Energy-loss straggling of 5.486 MeV alpha particles in air and 5N purity constituent gases was measured using high-resolution alpha spectroscopy system. Measured values are compared with Bohr, Bethe-Livingston and Yang et al. formulations. An experimental investigation of Bragg's rule to estimate the energy-loss straggling in air using the experimental data of constituents is discussed.

Spatially Resolved Characterization of Phases in LiFePO4 Battery Cathodes Using Low Loss Electron Energy-loss Spectroscopy

Lithium iron phosphate (LiFePO4) has considerable potential for automotive applications due to its high rate capability, reasonable energy density and environmentally benign nature [1]. However, performance degradation seen after thousands of cycles at high charging-rates (C-rates) has been a point of major concern [2]. Studies of the aging mechanism suggest that phases (LiFePO4/FePO4) formed in the cathode during discharge influence the aging profile [3]. These phases have been investigated recently using x-ray and neutron diffraction [4, 5]. While these methods provide insights into crystal structure and unit cell volume changes associated with phase changes between LiFePO4 and FePO4, their ability to provide spatially resolved measurements of quantities such as Li and Fe concentration at the nanoscale is severely limited. Such measurements are key to confirming the presence or absence of the phases mentioned above and solid solution phases (LixFePO4). With recent advances in aberration corrected electron microscopy, electron energy-loss spectroscopy (EELS) performed in the scanning transmission electron microscope (STEM) has emerged as a leading technique to obtain spatially resolved measurements of chemistry, structure and bonding. Previously, researchers have used shifts observed in core-loss peaks of iron and oxygen to study the delithiation of FePO4 host lattice [6-8]. However, these ionization edges have relatively small inelastic scattering cross sections, and consequently, a relatively high electron dose is required to achieve acceptable signal-to-noise ratios in the data resulting in significant electron beam damage to the specimen. The low energy-loss region of the spectrum (0-40 eV) can be very useful since (i) the intensity is much larger compared to the coreloss spectrum and (ii) lesser collection time is required which significantly lowers the risk of beam damage to the material. In this study, we explore the use of low loss EELS to characterize the nominally pure LiFePO4 powder with trace amounts of FePO4 which is typical of phase mixtures that is present during phase transformation within the LiFePO4 battery cathode at the nanoscale during aging.

Proton Energy Straggling Measurement in Metal Foils by Using a Si(Li) Detector

Measurements of energy loss and transmitted energy straggling were performed using a Si(Li) detector for 28.6 MeV protons incident on thin foil targets of Al, Cu and Au in air. The straggling widths were studied as functions of the target thickness and the target species. The initial proton beam energy from the MC-50 cyclotron was 45 MeV and it was decreased to 28.6 MeV by passing it through a 2-mm-thick Al window, 34.5 cm of air, a 2-mm-thick Al degrader and 144.5 cm of air. This 28.6 MeV proton beam was incident on Al, Cu and Au foils of various thicknesses installed in front of a Si(Li) detector. As a result, the energy loss strongly depended on the target thickness and species and the transmitted energy straggling was not related to the target species. Compared with the calculated results based on a simple Bohr's expression, the measured values agreed with the calculated ones in the low energy loss region less than 60 % of the incident proton beam energy. In the high-loss region, there was big mismatch of more than 25 %.

Electron Spectroscopy of COx Magnetron Sputtered Films

COx films were deposited by magnetron sputtering of a graphite target within two different routes: (i) by sputtering in an Ar+O2 gas mixture or (ii) by sputtering in pure Ar followed by annealing at 300 °C in air. Surface composition and chemical bonding was studied by X-ray induced photoelectron and Auger electron spectroscopy (XPS/XAES), and electron energy loss spectroscopy (EELS) in a low energy loss region. Oxygen content reached ∼20 at. % in the COx layers, independent on the route used. The C 1s spectral line shapes indicate C–O and C=O bonding states, regardless dominating C–C and CHx bonds. Substantial differences were found in the XAES and reflection electron energy loss spectra (REELS) recorded from surface regions of the samples. The both spectra indicate dominating sp2 bonding of carbon atoms in an analyzed volume of COx films sputtered in Ar and oxidized in air at elevated temperature whereas for those deposited in an Ar+O2 mixture the sp3 bonding prevails over sp2.

Study of the order–disorder transition and martensitic transformation in a Cu–Al–Be alloy by EELS

Changes in 3d states occupancy associated with order–disorder transition and martensitic transformation in a Cu–Al–Be alloy was investigated by electron energy loss spectroscopy (EELS) in both high energy and low energy loss regions. From the high energy loss region, the Cu L 2,3 white-line intensities, which reflect the unoccupied density of states in 3d bands, was measured for three states of the alloy: disordered austenite, ordered austenite and martensite. It was found that the white-line intensity remains the same during order–disorder transition but appears slightly smaller in martensite, indicating that some electrons left Cu 3d bands or some hybridization took place during phase transformation. From the low energy loss region, the optical joint density of states (OJDS) was obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions, these spectra can be used to probe changes in the electronic band structure. The analysis shows that during the martensitic transformation, the peaks positions and relative intensities in the OJDS spectra undergoes noticeable changes, which are associated with interband transitions.

First-principles study of electronic structure and electron energy-loss-spectroscopy (EELS) of transition-metal aluminates as high- k gate dielectrics

We study the electronic structure and electron energy loss function of different transition-metal (Y, Sc, Zr, Hf and Ta) aluminates using density functional theory (DFT) and local density approximation (LDA). The transition metals are substituted favorably at the octahedral Al sites in the α-alumina system. Y and Sc introduced shallow unoccupied d-states near the conduction band edge. Using the calculated orbital resolved partial p-DOS, we have investigated the oxygen K-edge and compared to the experimental results. In addition, the energy-loss-functions were calculated in the low energy-loss region directly reflects the collective electron excitations (plasmons) and single valence electron excitation in to unoccupied states of the conduction band.

Initial oxidation process of Mg films characterized by AES, EELS, and UPS

Initial oxidation process of Mg films was investigated by Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A polycrystalline Mg film was exposed to oxygen at room temperature. An inspection of EELS reveals that the initial oxidation process could be classified according to the curve profile in the low energy loss region. The oxide formation is initiated after oxygen exposure of 3 L (Langmuir).

Modulation electron energy loss spectroscopy and its application of quantitative analysis

Electron energy loss spectroscopy (EELS) gives an inportant insight into the variety of excitations a sample may undergo when irradiated by an electron beam. The focus of this work was to simulate electronic excitations within the energy range from a few to several hundred eV. Our recently developed modulation scheme, combines both convolution and deconvolution techniques, to provide quantitative information about elementary inelastic scattering processes without knowledge of sample parameters such as thickness or optical constants.In the low energy loss region of the spectrum the primary excitation mechanisms include interband transitions, and surface and bulk plasmons. In general these individual excitation events overlap in the spectrum. A FFT convolution procedure was developed where the basic inelastic processes may be represented by the dielectric theory . The dielectric function ε is used to describe both single excitations and collective excitations, whereHere ωp2=4πNe2/m is the bulk plasmon frequency, N is number of free electrons per unit volume, e and m are the charge and mass of the electron respectively and ω0is a constant which is finite for a bound state but zero for a free electron.

Structure of amorphous hydrogenated carbon film prepared from rf plasma deposition

An amorphous hydrogenated carbon (a-C : H) film was prepared from rf plasma deposition, and the structure was investigated by means of FT-IR, TEM and electron energy-loss spectrometry (EELS). The proportions among aromatic, aliphatic sp 2- and sp 3-coordinated carbons were determined to be 1 : 4 : 10 by curve-separation technique of FT-IR absorption peaks. The diameters of the hexagonal planes and the cluster of sp 3-coordinated carbons were made to be about 7 Å with the electron diffraction pattern of the a-C : H. The low energy-loss region of the EELS spectrum for the a-C : H was analyzed by the use of the energy-loss function and separated into one π plasmon peak and two plasmon peaks, each of which was associated with sp 2- and sp 3-coordinated carbons, respectively. The ratio of electrons contributing to the peaks was calculated using the theory of the effective number of electrons. The σ electrons associated with the peak by sp 2-coordinated carbons ( σ g ) were distinguished from those with sp 3-coordinated carbons ( σ d ). Considering the π electrons (Π), the relative numbers of electrons were estimated to be π : σ g : σ d = 1 : 6 : 4. One of the possible structures for the a-C : H used was exemplified on the basis of these quantitative data.

Optimization of quantitative electron energy loss spectroscopy in the low loss region: Phosphorus L-edge

The purpose of this study was to optimize quantitative electron energy loss spectroscopy (EELS) of elements that have characteristic edges in the low energy loss region and are components of organic matrices. The optimum parameters for phosphorus L 2,3-edge (at 135 eV) detection were determined by numerical analysis of computer-generated, Poisson-noisy spectra and by experimental measurements (at 80 keV) of films of the phosphoprotein, phosvitin. When the first, second and third valence electron/plasmon scatterings are included in the multiple least-squares (MLS) fit, the background subtraction of (first-difference) spectra is significantly more accurate than that obtained with the “inverse power law” method, even for a specimen thickness of only 0.25λ. Taking into account the effects of plural scattering, the optimal thickness for P quantitation is approximately 0.3λ. Signal-to-noise ( S / N) ratio decreases rapidly with thickness, and at 1.0λ, it is only about 60% of the optimum S / N. The combined effects of the statistical uncertainty of measurements and of the systematic error due to gain variations of the parallel detector were evaluated, and the relative sensitivities of the no-difference (raw spectrum), first-difference and second-difference methods were compared. For channel-to-channel gain variations greater than 0.1% and up to 0.8%, the first-difference method results in the lowest uncertainty of P measurements. In the absence of gain variations, direct fitting provides the greatest sensitivity (least uncertainty), whereas at larger gain variations it may be necessary to use the second-difference method. The optimum energy shift for collecting a first-difference spectrum, approximately 15 eV, did not show any great variation between 5 and 25 eV, and is, in general, specimen dependent. Quantitation with EELS showed excellent correlation with simultaneous electron probe X-ray microanalysis, but, for the detection of P in a 0.25λ thick specimen, EELS was approximately five to six times more sensitive than X-ray. The minimal detectable P concentration, with 0.5 nA beam current for 100 s in a 0.25λ thick specimen, was 8.4 mmol/kg (0.01 at%) at the 99% confidence level, equivalent to 34 phosphorus atoms for a 15 nm probe. This value is close to the theoretical prediction of 7.5 mmol/kg, and can be improved only by further reducing the gain variation and directly fitting the non-difference spectrum. Appropriate reduction of the gain variations to less than 0.1% would result in a further, approximately two-fold, improvement in the parallel EELS detection system. However, in order to obtain the optimal specimen thickness for phosphorus quantitation in cryosections, it will be necessary to use higher (approximately 200 keV) excitation voltages.

Small metallic particles studied by optical and electron-optical spectroscopy

Electron energy loss spectroscopy (EELS) in a transmission electron microscope (TEM) and photoacoustic spectroscopy (PAS) has been used to study the electronic structure of small Ag, In and Au particles, deposited on thin carbon foils, quartz supports or as self-supporting layers, respectively.In the low energy-loss region the EELS spectra of Ag particles with diameters of ca. 50 nm are different from the spectra of the homogeneous Ag layers, mainly owing to the influence of the carbon foil. By decreasing the packing density of the Ag particles, a shift of the plasma losses to lower energies is observed in the energy region 10 eV. The optical behaviour of homogeneous metallic and small particle layers on thin quartz supports was studied by PAS in the ultraviolet/visible (UV/VIS) spectral region. The homogeneous layers, e.g. of Ag, show, in the UV range, the well known change of reflectance due to an interband transition of bare silver. For Ag particle layers surface plasma resonances are excited. With increasing packing density of the particles the absorption peak observed is red-shifted and becomes broader, whereas the results of EELS in the low energy-loss region revealed a shift to higher energies.To obtain additional information investigations of In and Au particles were performed.

Effect of finite volume on core loss fine structure in small particles

During the past several years, experimental and theoretical results have established that collective excitations in the low energy loss region are affected strongly by the relative size and geometry of the probe and the probed volumes. Non-dipole transitions have also been found to play an important part. In contrast, core loss scattering has been shown to be dominated by dipole excitations to localized final states showing little sensitivity to nearby surfaces. Thus, quantitative elemental concentrations may be obtained from simple intensity sums.We now have electron spectrometers which are capable of obtaining the near edge fine structure in the STEM. In large volumes the results are still explained by the standard theory. In small volumes, however, the near edge results are quite different.

Valence electron energy loss spectroscopy in reflection geometry

The low energy loss region of a EELS spectrum carries information about the valence electron excitation processes (e.g., collective excitations for free electron like materials and interband transitions for insulators). The relative intensities and the positions of the interband transition energy loss peaks observed in EELS spectra are determined by the joint density of states (DOS) of the initial and final states of the excitation processes. Thus it is expected that EELS in reflection mode could yield information about the perturbation of the DOS of the conduction and valence bands of the bulk crystals caused by the termination of the three dimensional periodicity at the crystal surfaces. The experiments were performed in a Philipps 400T transmission electron microscope operated at 120 kV. The reflection EELS spectra were obtained by a Gatan 607 EELS spectrometer together with a Tracor data acquisition system and the resolution of the spectrometer was about 0.8 eV. All the reflection spectra are obtained from the specular reflection spots satisfying surface resonance conditions.

Energy loss extreme — near-edge and pre-edge structure

During the past several years we have begun to understand many features of the core loss excitations including absolute crosssections, extended fine structure, (EXELFS) and near edge fine structure. (ELNES) In electronic materials near interfaces or defects, we expect the valence and conduction bands to be modified by the existence of isolated resonances, broadening of the band edges, or a completely filled gap region. These electronic changes have been observed with high resolution EELS in the low energy loss region (0-3eV) where direct inter-band transitions are likely. The present work is aimed at determining if pre-edge structure near the core loss scattering may reflect local changes in the electronic structure as well. There are two major experimental difficulties: 1) the necessary energy resolution is of order 0.1-0.3eV in order to define the shape of the core edge; and 2) the energy differential scattering cross section is small (of order 10-23cm2eV-1). At IBM I have addressed these problems by constructing a high resolution energy loss spectrometer system with parallel recording to allow signal collection in reasonable times.

Structure Studies of Thin Amorphous Films by Synchrotron X-Ray Absorption and Analytical Electron Microscopy.

ABSTRACTWe combine the techniques of EXAFS and XANES using synchrotron x-rays, and the corresponding techniques using electron energy loss spectroscopy (EXELFS and ELNES) to characterize and study the local atomic arrangement in thin amorphous films. These probes are complementary in that x-rays are best suited for absorption edges above 3 keV and electron energy loss spectroscopy for low energy edges, and thus this combination is very useful for amorphous materials containing both low and high Z elements. We use in addition transmission electron microscopy, electron diffraction, and electron energy loss spectroscopy in the low energy loss region. Results from amorphous T1O2 and amorphous SnO2 made by reactive sputtering are presented.

Low Energy Loss Structure of Small Aluminum Particles

Usually, configuration effects are ignored in microanalytical work. That is, the energy loss mechanisms in small volumes are expected to be similar to those in large volumes. However, it is shown elsewhere in these proceedings1 that this assumption is not valid for bulk plasmons in small Al spheres. In fact, ‘configurational’ energy loss effects can dominate the low energy loss region when probing small volumes. These effects include: surface plasmons, bulk plasmon spatial quantization, Schottky barriers, and any other mechanisms that rely on the close proximity of surfaces or interfaces. We have examined the energy loss structure for ˜200Å Al particles (formed as explained in Ref. 1) with the VG Microscopes, Ltd. HB5 scanning transmission electron microscope at 100KeV. The particles consist of single crystal Al cores covered by ˜40Å of Al2O3, and possibly further covered by 5-15Å of carbon contamination. We obtained electron energy loss spectra (EELS), and inelastic scattering images of the particles to correlate the observed EELS structure with the spatial structure and to make a tentative identification of the losses. The spectra were recorded with a probe angular diameter of 8mR (1.4Å−1) and an energy loss resolution of 0.7eV. The images were recorded with an energy loss resolution of 2eV.

Energy loss spectroscopy (ELS) on the Si-Au system

We report on the energy loss spectroscopy obtained on the clean cleaved Si(111) covered with different gold thickness. The results clearly indicate that in the Si-Au system the interface is characterized by a Si rich phase with a well defined electronic transition at 7.5 eV energy loss. Increasing Au thickness the main changes in the spectrum arise in the low energy loss region and a gold-like behaviour is observed for a gold coverage of 60 monolayers. The effects of annealing at 350°C are reported.

Energieverlustspektren der Alkalihalogenide und der Metalle Cu, Ag und Au und Vergleich mit optischen Messungen

Energy loss spectra of 50 keV electrons have been measured in transmission of the alkali halides and of Cu, Ag, and Au at zero scattering angle. These spectra are compared with the quantity −Im(1/e) determined from optical measurements. The general agreement is good, especially in the low energy loss region. In the case that the foil thicknesses could be measured, the absolute intensity of −Im(1/e) is found to be equal or up to three times larger than from optical data. In addition to the sharp low energy peaks (excitons), characteristic sharp peaks are found at higher energies: 60 eV for the Li-halides, 33 eV for the Na-halides, and 20 eV, 17 eV, and 13 eV for the K-, Rb-, and Cs-halides respectively. These peaks are very little shifted by the different anions in the crystal lattice and may be due to excitons caused by alkali band transitions. We have further examined several peaks in the loss spectra of LiF, KCl, KBr and the Na-halides: 1. The angular dependence of the loss intensity is in agreement with theory. 2. The dispersion of the peaks is much smaller than for plasma losses. 3. The temperature dependence of the peak energy has been found quite different for different peaks.