Reflection Electron Energy Loss Research Articles

Analysis of Plasmon Loss Peaks of Oxides and Semiconductors with the Energy Loss Function.

This paper highlights the use and applications of the energy loss function (ELF) for materials analysis by using electron energy loss spectroscopy (EELS). The basic Drude-Lindhart theory of the ELF is briefly presented along with reference to reflection electron energy loss (REELS) data for several dielectric materials such as insulating high-k binary oxides and semiconductors. Those data and their use are critically discussed. A comparison is made to the available ab initio calculations of the ELF for these materials. Experimental, high-resolution TEM-EELS data on Si, SiC, and CeO2 obtained using a high-resolution, double-Cs-corrected transmission electron microscope are confronted to calculated spectra on the basis of the ELF theory. Values of plasmon energies of these three dielectric materials are quantitatively analyzed on the basis of the simple Drude's free electron theory. The effects of heavy ion irradiation on the TEM-EELS spectra of Si and SiC are addressed. In particular, the downward shifts of plasmon peaks induced by radiation damage and the subsequent amorphization of Si and SiC are discussed. TEM-EELS data of CeO2 are also analyzed with respect to the ELF data and with comparison to isostructural ZrO2 and PuO2 by using the same background and with reference to ab initio calculations.

Decreasing charge recombination by magnetic trap of iron‐carbon (Fe‐AC) composite for enhanced photocatalytic performance

Electronic properties of composite Fe and activated carbon (AC) were determined by using X‐ray photoelectron (XPS) and reflection electron energy loss (REELS), structural properties by X‐ray diffraction (XRD), and photo‐catalytic performance by ultra violet visible (UV‐Vis) spectroscopy. The intensity of low loss spectrum shows surface state at 5.42 eV is decreased with increasing the amount of AC indicated the Fe successfully oxidizes to form iron oxide. The bandgap was increased from 2.2 eV for 10% AC to 4.3 eV for 25% AC indicated the absorption wavelength shifted from the visible region to ultra‐violet region. The magnetic properties and porosity were increased with increasing the amount of AC in composites and show successfully used, as a magnetic trap of the charge (electron and hole) for producing radical atoms. The magnetic pore transformed from the disordered to ordered nature in the periodic structure was used to accelerate the degradation from 70.15% for 10% AC to 80% for 25% AC for 60 min. This study shows magnetic trap of composite Fe‐AC materials useful for enhanced photocatalytic performance.

Formation of graphitic films on Cu(111) via electron beam induced deposition

Electron beam induced deposition using ethylene as precursor gas is used to generate carbonaceous films on Cu(111). The study was performed in an ultrahigh vacuum chamber for two different precursor gas pressures, and two substrate temperatures. Auger electron spectroscopy and reflection electron energy loss spectroscopy were used to characterize the film growth process in-situ. These techniques allowed us to determine how the deposited film covers the Cu(111) substrate as a function of the ethylene exposure, obtaining a final coverage of up to 0.8 monolayer, depending on the growing conditions. Based on the CKLL Auger line shape and on the reflection electron energy loss (REELS) spectra we can conclude on the graphitic characteristics of the grown film and on the optimum growing conditions: lower substrate temperature and higher ethylene pressure. Raman spectroscopy and scanning tunneling spectroscopy were used as ex-situ techniques to assess the properties of the grown film. Results from both techniques indicate that the film consists of small (10–20 nm in size) nanocrystals of few (1–2) layers graphene. Once optimized, this method may allow growing graphene with a predesigned pattern, without the need to heat the substrate at high temperatures.

A comparison between Monte Carlo method and the numerical solution of the Ambartsumian-Chandrasekhar equations to unravel the dielectric response of metals

In this work we describe two different models for interpreting and predicting Reflection Electron Energy Loss (REEL) spectra and we present results of a study on metallic systems comparing their respective computational cost and accuracy. These approaches are the Monte Carlo (MC) method and the Numerical Solution (NS) of the Ambartsumian-Chandrasekhr equations. The former is based on a statistical algorithm to sample the electron trajectories within the target material for describing the electron transport. The latter relies on the numerical solution of the Ambartsumian-Chandrasekhar equations using the invariant embedding method. Both methods receive the same input parameters to deal with the elastic and inelastic electron scattering. To test their capability of describing REEL experimental spectra, we use copper, silver, and gold as case studies. Our simulations include both bulk and surface plasmon contributions to the energy loss spectrum by using the effective electron energy loss functions and the relevant extensions to finite momenta. The agreement between MC and NS theoretical spectra with experimental data is remarkably good. Nevertheless, while we find that these approaches are comparable in accuracy, the computational cost of NS is several orders of magnitude lower than the widely used MC.Inputs, routines and data are enclosed with this manuscript via the Mendeley database.

Stopping power and inelastic mean free path of 300 eV–50 keV electrons for lanthanum aluminate

SP (Stopping Power) and IMFP (Inelastic Mean Free Path) of lanthanum alumínate (La2O3)x (Al2O3)1–x was varied with x = 0.25, 0.34, and 0.5. for electron energy from 300 eV to 50 keV have been determined by using composite Simpson from the equation of Born Ochkur correction. The main parameter in determining the SP and IMFP is Energy Loss Function (ELF) which was obtained from the experimental reflection electron energy loss (REELS) spectra. The method in this study is convenient methods for determining SP for binary compound lanthanum aluminate.

Absolute determination of optical constants by reflection electron energy loss spectroscopy

We present an absolute extraction method of optical constants of metal from the measured reflection electron energy loss (REELS) spectra by using the recently developed reverse Monte Carlo (RMC) technique. The method is based on a direct physical modeling of electron elastic and electron inelastic scattering near the surface region where the surface excitation becomes important to fully describe the spectrum loss feature intensity in relative to the elastic peak intensity. An optimization procedure of oscillator parameters appeared in the energy loss function (ELF) for describing electron inelastic scattering due to the bulk- and surface-excitations was performed with the simulated annealing method by a successive comparison between the measured and Monte Carlo simulated REELS spectra. The ELF and corresponding optical constants of Fe were obtained from the REELS spectra measured at incident energies of 1000, 2000 and 3000 eV. The validity of the present optical data has been verified with the f- and ps-sum rules showing the accuracy and applicability of the present approach. Our data are also compared with previous optical data from other sources.

Effect of Chlorine Plasma Treatment on Electronic Properties of GIZO Thin Film Grown on SiO2/Si Substrate

The effect of chlorine plasma treatment on electron ic properties of GIZO grown on SiO2/Si by RF magnetron sputtering was investigated usi ng X- ray photoelectron spectroscopy (XPS), reflection el ectron energy loss spectroscopy (REELS), and secondary ion mass spectroscopy (SIMS). SIMS depth profiles indicated that the concentration of InO and ZnO on the surface was decreased after Cl 2 plasma treatment. REELS data showed that the band gap increased from 3.4 to 3.7 eV. XPS showed that Ind 5/2 and Zn2p 3/2 shifted to the higher binding energies by 0.5 eV and 0.3 eV, respe ctively. These phenomena were caused by oxygen deficiency and hydrocarbon contamination reduction as indicated by Cl atom bonding with In and Zn cations that are present on the surface after Cl 2 plasma treatment.

Correlation between carbon nanotube microstructure and their catalytic efficiency towards the p-coumaric acid degradation

Pristine and annealed multi-walled C nanotubes are preliminarily tested as catalysts towards the degradation of p-coumaric acid with better performances than the widely used activated carbon. In order to clarify the origin of their different catalytic efficiency, the samples are analysed by means of high-resolution transmission electron microscopy, X-ray photoemission, reflection electron energy loss and micro-Raman spectroscopy. Chemical composition, mass density, bonding configurations and threefold- to fourfold-coordinated carbon bonding ratios are determined. The comparative discussion of the results obtained leads to attribute the higher catalytic activity of the pristine nanotubes to their larger sp2/sp3 carbon bonding fraction and greater amount of oxygen containing functional groups.

ZnO nanostructures produced by laser ablation in water: Optical and structural properties

ZnO nanostructures have been prepared in distilled water by pulsed laser ablation, by varying the laser pulse energy between 20 and 150mJ and for an ablation time of 10min. The structural properties of the samples have been studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS), reflection electron energy loss (REELS), UV–vis and photoluminescence (PL) spectroscopies. The nanoparticles in colloidal suspension have shown a good degree of stoichiometry, as deduced by the analysis of the O 1s and Zn2p3/2 photoemission spectra. The diffraction pattern and the interplanar spacings well match to the standard diffraction pattern of wurtzite zinc oxide (ZnO). The optical band gap calculated by the absorption spectra is found to be size dependent showing an increase for a decreasing particle size. As estimated from the analysis of TEM images and confirmed by the XRD data, the diameter of the nanocrystals within the greater nanostructures is mainly in the range of 1–10nm. A large number of oxygen vacancies have been put in evidence by our PL measurements. This supports the lower density value of 6.12g/cm3, estimated by REELS in the free electron approximation, with respect to the one generally reported for thin film and crystal phases.

Partial intensity approach for quantitative analysis of reflection-electron-energy-loss spectra

We have considered a formalism, known as partial intensity approach (PIA), previously developed to quantitatively analyze reflection electron energy loss (REEL) spectra [1,2]. The aim of the approach is, in particular, to recover the single scattering distribution of energy losses and to separate it into bulk and surface contributions, respectively referred to as the differential inverse inelastic mean free path (DIIMFP) and the differential surface excitation parameter (DSEP). As compared to [1] and [2], we have implemented a modified approach, and we have applied it to the specific geometry of the cylindrical mirror analyzer (CMA), used to acquire the REEL spectra shown here. Silicon, a material with well-defined surface and bulk plasmons, is taken as a case study to investigate the approach as a function of electron energy over the energy range typical of REELS, i.e. from 250 eV to 2 keV. Our goal is, on the one hand, to examine possible limits for the applicability of the approach and, on the other hand, to test a basic assumption of the PIA, namely that a unique DIIMFP and a unique DSEP account for REEL spectra, whatever the acquisition conditions (i.e. electron energy or angle of surface crossing) are. We find that a minimum energy exists below which the PIA cannot be applied and that the assumption of REEL spectra accounted for by unique DIIMFP and DSEP is indeed an approximation.

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.

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.

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.

Analysis of the principal components for REEL spectra of indium

The reflection electron energy loss (REEL) spectra of indium for 300 eV energy of the primary electrons are analyzed with change of the experimental geometry conditions. It is demonstrated that the principal component analysis allows to describe the spectra as linear combinations of the three principal components within the range of scattering angles 94–146°. It is shown that the shapes of surface and volume plasmon peaks do not change with change of the geometric conditions of the experiment. At the same time, the electron scattering angle considerably and differently influences the relative intensity of the surface and volume plasmon peaks. Intensity of the volume plasmon peak (normalized by intensity of the peak of the elastically reflected electrons) changes almost three times in the magnitude. This considerably exceeds possible changes of the intensity due to changes of the angle of primary electron incidence and exit angle for the detected electrons.

Electronic and vibrational excitations in carbon nanotubes

We investigated the electronic and vibrational properties of single wall carbon nanotubes by reflection electron energy loss and X-ray absorption spectroscopies. We report on single particle excitations measured at the C-1s edge and on collective excitations of the valence band region. The comparison between the two techniques allows us to locate empty electronic states of the carbonaceous sample. Loss spectra taken in the infrared region reveal two loss features at 90 and 170 meV assigned to the excitation of optical phonon modes.

A new approach to analysis of reflection electron energy loss spectra

A new approach is proposed for the analysis of reflection electron energy loss (REEL) spectra that were obtained for different electron-scattering configurations. A distinctive feature of the approach is that a REEL spectrum can be separated into two components that change in relative intensity with changes in the experimental configuration. No assumption is made about the number of peaks in the component spectrum and their shape. The suggested method is demonstrated on REEL spectra of aluminium, which is a traditional test object. It is established that in the range of energy losses 0–50 eV, the A1 REEL spectrum can be described by the sum of two components. The components of the spectra measured at different angles of primary electron incidence and exit angles for the detected electrons but with constant scattering angle have the same shape. The relative intensities of the two components define the spectral shape for different geometric conditions of the experiment. The shape of the components and their changes in relative intensity with change of primary electron energy give new information useful for development and further improvement of models used for studying medium energy (0.1–1.0 keV) electron scattering in the surface region of solids.

Characterization of pulsed laser deposited a-C films by means of reflection electron energy loss spectroscopy

Amorphous carbon films have been deposited by ablating a high purity graphite target using a KrF excimer pulsed laser. Both the laser energy density (up to 15 J/cm 2) and the substrate temperature (up to 900 K) were varied. Reflection electron energy loss spectra have been carried out in an ultra high vacuum surface analysis apparatus. The loss spectra were processed by removing the elastic peak contribution and the secondary electron background. The optical functions were deduced using a procedure based on the assumption that surface effects are negligible with respect to the bulk ones. The loss spectra are dominated by the presence of the π and π+σ plasma resonances, whose energy positions and shapes are significantly affected by the different preparation conditions. In particular, it has been made possible to determine, on the basis of reflection electron energy loss (REEL) data only, the sp 2/sp 3 bonding ratio. Overall, these results are also relevant for the correlation between the details of the preparation method and the material electronic properties.

Influence of Different Carbon Structures on Diamond Synthesis by Chemical Vapour Deposition

Structurally different carbon films selectively generated at the surface of glassy carbon, titanium, and silica glass substrates are tested as nucleating layers for diamond synthesis by hot filament chemical vapour deposition (HFCVD). Before the CVD process, the substrates are treated in different ways in order to produce carbonaceous surface layers with specific structures. On glassy carbon specimens, surface structural modifications are induced by pulsed ruby laser irradiation (λ = 694.3 nm, energy density in the range 100 to 500 mJ/cm2). The polycrystalline titanium and silica glass substrates are conversely precoated with amorphous carbon layers produced by electron beam irradiation. The structural analysis of the modified substrate surfaces and of the CVD deposits is performed by means of reflection high energy electron diffraction (RHEED) and electron energy loss spectroscopy (EELS). The effects due to both structure and thickness of the various carbon layers on the CVD synthesis of the diamond phase are discussed.

Electron and ion radiation damage of a-C:H films detected by energy loss spectroscopy of reflected and transmitted electrons

Reflection electron energy loss and transmission electron energy loss spectra were recorded to investigate the electron- and ion-beam damage in amorphous hydrogenated carbon (a-C:H) films. The irradiation energy was chosen to be 3 keV for argon ions and 1 keV for electron bombardment, which are typical energies used in surface analysis. Changes in density and bonding of a-C:H films were observed for doses up to 6 × 10 18 cm −2 for electrons and up to 2 × 10 17 cm −2 for ions. In this work, the two types of electron energy loss spectroscopy were compared to identify surface and bulk effects due to irradiation.

In situ quantitative analysis of GeXSi1-X alloys during growth by reflection EELS

The universality of reflection high energy electron diffraction (RHEED) as a structural tool during film growth by molecular beam epitaxy (MBE) brings with it the possibility for in situ surface chemical analysis via spectroscopy of the accompanying inelastically scattered electrons. We have modified a serial electron energy loss spectrometer typically used on an electron microscope to work with a 30 keV RHEED-equipped MBE growth chamber in order to determine the composition of GexSi1-x alloys by reflection electron energy loss (REELS) experiments. Similar work done in transmission electron microscopes has emphasized the surface sensitivity of this technique even though these experiments have never been done under ultra-high vacuum conditions. In this work, we are primarily concerned with the accuracy with which core losses can be used to determine composition during MBE growth.