Experimental Electron Energy Loss Spectra Research Articles

The role of plasmons and interband transitions in the color of AuAl2, AuIn2, and AuGa2

First principles calculations of the optical properties of the intermetallic compounds AuAl2, AuIn2, and AuGa2 have been performed. Analysis of the dielectric functions showed that AuAl2 is unique because a bulk plasmon is seen in the optical region and contributes to the purple color of this material. An experimental electron energy-loss spectrum showed excellent agreement with the theoretical prediction and confirmed the presence of the bulk plasmon.

Calculated and experimental electron energy-loss spectra of La2O3, La(OH)3, and LaOF nanophases in high permittivity lanthanum-based oxide layers

Using first principles methods, the O K energy-loss near-edge structure of cubic and hexagonal La2O3, La(OH)3, and LaOF phases have been calculated. These calculations support the identification of nanocrystalline phases evidenced experimentally by electron energy-loss spectroscopy (EELS) performed in a transmission electron microscope. The nanocrystals have been observed in atomic layer deposited La2O3 thin films developed for advanced metal-oxide-semiconductor field effect transistor applications. The presence of the nanophases can be explained by the hygroscopicity and the particular reactivity with fluorine of La2O3. These calculations provide a set of EELS fingerprints which will be useful for local phase identification in La2O3-based materials.

Revisiting lithium K and iron M 2,3 edge superimposition: The case of lithium battery material LiFePO 4

Experimental electron energy-loss spectra are presented for FePO 4, LiFePO 4 and NaFePO 4 from 0 to 80 eV. With the help of the NaFePO 4 spectrum in the 50–80 eV range, the double peak observed in LiFePO 4 could be ascribed to the presence of Fe II and not to the Li K edge, contrary to what was thought previously. Crystal field multiplet calculations confirm this attribution. Using VASP programme based on density functional theory, dielectric response calculations including local field effects in the Hartree approximation are then proven to properly simulate the fine structures due to the lithium K edge. By comparing absolute spectrum intensities, it is shown that the lithium K edge cannot be used to quantify lithium in such compounds. This detailed comparison between theoretical calculations and experimental spectra helps defining the relevant parameters governing intensities in the 50–80 energy range.

Comparison between Monte Carlo and experimental aluminum and silicon electron energy loss spectra

A Monte Carlo (MC) simulation is described and used to calculate the energy distribution spectra of backscattered electrons from Al and Si. For the simulations, elastic scattering cross sections are calculated by numerically solving the Dirac equation in a central field. Inelastic scattering cross sections are computed within the dielectric response theory developed by Ritchie, and by Tung et al. Extension from the optical case to non-zero momentum transfer is done according to Ritchie and Howie. To evaluate surface and bulk contributions to the spectra, the Monte Carlo model treats the surface excitations according to the Werner differential surface and volume excitation probability theory. The Monte Carlo calculations are compared with the experimental reflection electron energy loss (REEL) spectra acquired in our laboratory.

Indirect interband transitions in graphite with a wide quasigap

Sets of fundamental optical functions in the energy range of 0–40 eV for longitudinal indirect interband transitions in graphite with a quasi-gap up to 2.6 eV have been determined for the first time. Their structure and parameters depend strongly on the electron wave vector transfer q. The nature of the peaks and shoulders of these functions is explained within the model of theoretical shifted bands for four electron wave vector transfers q in a volume characteristic electron-energy losses for the Γ-P direction in the Brillouin zone. The calculations were performed on the basis of the known experimental volume characteristic electron-energy loss spectra of graphite in the range of 0–40 eV for |q| = 0.00, 0.375, 0.625, and 1.00 A−1 using a special software package.

Optical and electron energy loss spectra of cadmium dichloride and lead dichloride crystals

Experimental electron energy loss spectra of cadmium dichloride and reflection spectra of lead dichloride are used to calculate the complete sets of fundamental optical functions of the two highly ionic materials in a wide energy range (0–30 eV). Their cumulative permittivity and volume electron energy loss curves are decomposed into transverse and longitudinal elementary components of transitions, and their main parameters are evaluated. The effect of the nature of the cation on the key features of the optical spectra is examined.

Relativistic electron energy loss spectroscopy of solid and core-shell nanowires

Near-field optical spectroscopy with nanometer spatial resolution using transition radiation from a focused, relativistic electron beam is demonstrated for Ge nanowires. The excitation of the waveguide modes is absent in theoretical treatments that neglect retardation because the coupling of the swift electron to the optical modes of the nanowire is a consequence of its relativistic nature. Here, relativistic energy loss probabilities for aloof electron trajectories near a perpendicularly oriented single nanowire are analytically calculated using a local dielectric theory. Multimode and zero-mode Ge nanowires are experimentally measured in this geometry with a $\ensuremath{\sim}2\text{-nm}$-size electron probe. Observations fall in excellent agreement with the retarded calculations. Optical eigenmodes are identified from comparison of the experimental electron energy loss spectra to theoretical dispersion maps. Adding a Drude-type metallic shell to a dielectric nanowire provides surface-plasmon modes in the same energy range as the dielectric waveguide, leading to strongly coupled hybrid modes that can be efficiently excited by the incident electron. Such results demonstrate a powerful solution for optical studies of nanosystems with nanometer spatial resolution over a broadband energy range.

Theoretical Study on Inner Shell Electron Impact Excitation of Lithium

Cross sections for electron impact excitation of lithium from the ground state 1s22s to the excited states 1s2s2, 1s2p2, 1s2snp (n = 2–5), 1s2sns (n = 3–5), 1s2pns (n = 3–5), and 1s2pnp (n = 3–5) are calculated by using a full relativistic distorted wave method. The latest experimental electron energy loss spectra for inner-shell electron excitations of lithium at a given incident electron energy of 2500 eV [Chin. Phys. Lett. 25(2008)3649] have been reproduced by the present theoretical investigation excellently. At the same time, the structures of electron energy loss spectra of lithium at low incident electron energy are also predicted theoretically, it is found that the electron energy loss spectra in the energy region of 55–57eV show two-peak structures.

The role of carbon impurities on the Si(0 0 1)-c(4 × 4) surface reconstruction: Theoretical calculations

In this work we employ the state-of-the-art pseudopotential method, within a generalized gradient approximation to the density functional theory, combined with a recently developed method for the calculation of HREELS spectra to study a series of different proposed models for carbon incorporation on the silicon (0 0 1) surface. A fully discussion on the geometry, energetics and specially the comparison between experimental and theoretical STM images and electron energy loss spectra indicate that the Si(1 0 0)-c(4 × 4) is probably induced by Si–C surface dimers, in agreement with recent experimental findings.

Structural and vibrational properties of two-dimensionalMnxOylayers on Pd(100): Experiments and density functional theory calculations

Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd(100) at (sub)monolayer coverage. Amongst the multitude of phases experimentally observed we focus our attention on four structures which can be classified into two distinct regimes, characterized by different building blocks. Two oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers, whereas the other two have a lower oxygen content and are based on a MnO(100)-like monolayer structure. The excellent agreement between calculated and experimental scanning tunneling microscopy images and vibrational electron energy loss spectra allows for a detailed atomic description of the explored models.

Quantitative study of the Si/SiO 2 phase separation in substoichiometric silicon oxide films

A systematic study on the synthesis of silicon nanoclusters formed in plasma-enhanced chemical vapor deposited substoichiometric silicon oxide films by annealing at 1100 °C for 1/2 h as a function of different deposition parameters, has been performed. The clustered Si volume fraction is deduced from a fit to the experimental electron energy loss spectrum. At any deposition condition, the clustered silicon concentration is significantly lower than the initial silicon excess concentration, demonstrating that high temperature anneal of SiO x films induces only partially the phase separation between Si and SiO 2. This effect, explained by taking into account the strain energy associated with the different atomic densities of Si and SiO 2, has been confirmed through a detailed study of the host material by scanning a 2 Å electron beam among agglomerated silicon dots, showing that the matrix it is not a stoichiometric SiO 2 but still a substoichiometric silicon oxide film whose excess of silicon is the one after to be subtracted by the agglomerated one. This stress the evidence that the phase separation between Si and SiO 2, in PECVD deposited films, is not complete even after high temperature annealing for relatively long time.

REEL spectra from aluminium: experiment and Monte Carlo simulation using two different dielectric functions

AbstractWithin the framework of the dielectric theory, the spectral response of a solid irradiated by an electron beam is described by the energy loss function.In the limit of zero momentum transfer, the dielectric function is generally known from optical data for many materials. The main problem, when electron beams are involved, concerns the extension of the dielectric function from the optical limit to non‐zero momentum transfer.This extension requires introducing a dielectric function, which depends on both energy loss and momentum transfer.Using Al–representative of a dilute gas of free electrons–as a case study, the present work compares two Monte Carlo (MC) simulations with experimental reflection electron energy loss (REEL) spectra excited by 2 keV electrons. To obtain the two MC simulated spectra, the dielectric function was modelled, respectively, according to Ashley (1988) and according to an early suggestion by Ritchie and Howie (1977), later reconsidered by Yubero and Tougaard (1992).MC simulated REEL spectra obtained according to the latter model reproduce the experimental REEL spectra better than those obtained using the Ashley dielectric function. The comparison involves not only the single scattering plasmon peak (bulk and surface) but also multiple scattering peaks. Copyright © 2008 John Wiley & Sons, Ltd.

Interaction of low-energy electrons with linear diphenylethynyl derivatives in the gas phase

The gas phase electron impact spectroscopy has been used to study the relative efficiency of excitation into singlet states and energies of singlet-triplet transitions for two electroactive organic materials, anthracene and biphenyl-containing diphenylethynyl derivatives. The probability of the lowest singlet-triplet transition in anthracene-containing molecule was found to be much higher than that in anthracene which is connected with triple bonds. No noticeable contribution of the triple bonds into singlet spectra of the studied molecules was observed. There are a number of intense transitions in the range higher than 10 eV. The optical spectrum calculated using the density functional theory is in good agreement with experimental electron energy loss and optical absorption spectra.

Joint experimental and computational study of aluminum electron energy loss spectra

Experimental reflection electron energy loss (REEL) spectra are measured from aluminum for primary energies ranging from 130 eV to 2 keV. A Monte Carlo simulation is shortly described and used to calculate the same spectra. The focus is on reproducing the variable weight of surface and bulk losses as the surface sensitivity of spectra changes by changing the primary electron energy. The intensity of surface losses in the simulations is modulated by the thickness of the region where surface excitations occur. Simulations based either on a constant or an energy-dependent thickness for this layer are considered. In both cases, simulated spectra reproduce the experimental trend as a function of energy, though the correct surface-to-bulk intensity ratio for each energy is either underestimated or overestimated.

Joint experimental and computational study of silicon dioxide electron energy loss spectra

Experimental reflection electron energy loss spectra from silicon dioxide are excited by electrons with energy ranging from $90\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. A Monte Carlo simulation, shortly described, is utilized to calculate the same spectra. The comparison between simulated and experimental spectra shows substantial agreement, particularly at high exciting energies. Differences at low exciting energies are mainly ascribed to surface effects.

Computational and experimental study of π and π + σ plasmon loss spectra for low energy (

A Monte Carlo simulation is described and used to calculate the energy distribution spectra of low primary energy E 0 (lower than 1000 eV) electrons impinging on HOPG. The simulated spectra are compared with experimental electron energy loss (EEL) spectra. Similarities and differences between experimental and simulated data are discussed.

The theoretical consistency of the experimental EELS of the fcc phase of carbon

Recently, Konyashin et al. [I. Konyashin, V. Khvostov, V. Babaev, M. Guseva, J. Mayer, A. Sirenko, A new hard allotropic form of carbon: dream or reality? International Journal of Refractory Metals and Hard Materials (article in press)] have reported electron energy loss spectra (EELS) of face centered cubic (fcc) carbon films that indicate unique electronic properties of this particular crystal phase of carbon. Motivated by that, we have investigated the optical properties of carbon in its fcc phase ( Fm3¯ m) by means of a first principles calculation. In this work we present the calculated dielectric tensor and EELS, which are in good agreement with the experimental spectra. In the EELS of fcc carbon there exists only one peak located at 22–23 eV, we have reproduced this by the density functional calculation at 22 eV. The electronic density of states and band structure is additionally shown to get a deeper understanding of the optical response. In order to check the accuracy of our calculations analogous results obtained for the diamond phase of carbon are included in this work.

Red-Shift of Carbon Nanotube Surface Plasmon EEL Features Utilising a Dielectric Filling

Simulations of the dynamical polarisability of a MWCNT (multi-wall carbon nanotube) by primary STEM electrons were performed. The simulations demonstrated eigenenergy modification (red-shifting), ascribed to the presence of a MWCNT dielectric filling, of coupled π-surface plasmon EEL (electron energy-loss) features. Experimental EEL spectra of MWCNTs were acquired utilising a Gatan Enfina system attached to a dedicated STEM. These spectra confirmed the eigenenergy modification predicted by the model. In a filled MWCNT, an eigenenergy modification of a coupled π-surface plasmon feature, redshifted from ∼5.7 eV to ∼4.4 eV, was experimentally validated. Red-shifting of this feature down to the visible frequency regime was not possible, owing to eigenenergy modification not occurring at the coupled limit. The eigenenergy modification phenomenon is remarkable since it is only one factor, namely whether or not a MWCNT is filled with a dielectric, that allows the plasmon energy to be red-shifted.

Absolute determination of inelastic mean-free paths and surface excitation parameters by absolute reflection electron energy loss spectrum analysis

An analytical approach was proposed for simultaneously determining an inelastic mean-free path (IMFP) and a surface excitation parameter (SEP) with absolute units by the analysis of an absolute experimental reflection electron energy loss spectrum. The IMFPs and SEPs in Ni were deduced for electrons of 300 to 3000 eV. The obtained IMFPs were in good agreement with those calculated using the TPP-2M equation. The Chen-type empirical formula was proposed for determining the SEP. The results confirmed the applicability of the present approach for determining the IMFP and SEP for medium-energy electrons.

Electronic structure and electron energy-loss spectra of Sr 0.35CoO 2

The electronic structure of Sr 0.35CoO 2, structurally analogous to the layered Na x CoO 2, has been evaluated using the local spin density approximation (LSDA). We find that evident c-dispersions appear in both the e g ′ and a 1g Co-derived bands, demonstrating the existence of a notable interplanar interaction in Sr 0.35CoO 2. The LSDA+ U calculation reveals that the electronic structure, in particular the band splitting between the spin-up and spin-down electrons, changes evidently along with the increase of the effective on-site Coulomb interaction U. Analysis of theoretical and experimental electron energy-loss spectra (EELS) for the oxygen K-edge and the Co L 23-edge suggests that the on-site interaction ( U) in Sr 0.35CoO 2 is less than 3 eV which is noticeably weaker than the estimated value (from 5 to 8 eV) in the Na x CoO 2 materials.