Bulk Energy Loss Research Articles

The more accurate determination of surface excitation parameters, the more accurate quantitative surface analysis

The accurate quantitative surface analysis for the study of nanostructures practically used for e.g. photocatalysis, electrocatalysis, energy storage and sensing applications is of high importance. Moreover, the accurate quantitative electron spectroscopy and analytical microscopy of semiconducting nanomaterials such as nano-photocatalysts relies on the accurate determination of surface excitation parameters (SEP). Reflection electron energy loss spectroscopy (REELS) as a surface sensitive technique could be applied for the determination of SEP. The chemical, mechanical, thermal stability, optical and dielectric properties of Ni6MnO8 as binary metal oxide makes such structure suitable for practical applications. Therefore, here Ni6MnO8 due to its great importance in its special applications was typically studied by REELS using the Yubero-Tougaard algorithm, which is based on dielectric response theory. After an extrapolation process applied to the obtained REELS spectrum, the energy gap (Eg) of this material was obtained to be 3 eV. The surface and bulk energy loss functions of the material were also determined. The values of inelastic mean free path (IMFP) of the electrons of different energies in the range of 500–4000 electron volts travelled in Ni6MnO8 were determined. In addition, from the obtained energy loss function (ELF), by using the Kramers-Kronig transformations of the real and imaginary parts of dielectric function (ε), the important optical parameters of Ni6MnO8 such as refractive index (n), attenuation coefficient (k) and absorption coefficient (μ) were determined. Finally, as a critical parameter in quantitative electron spectroscopy, the SEP and its angular distribution were determined. It was found that the Pauli-Tougaard model requires a modification to make it better descriptive to well fit the SEP data for a given material. Therefore, a more accurate model was obtained to make a better surface analysis and analytical microscopy.

Effect of local structure on the optical and dielectric behaviour of Sm doped GeSbTe phase change material

Abstract Ge2Sb2Te5 (GST) is considered to be one of the most prominent phase change materials for data storage applications. The UV–Vis–NIR transmittance spectra of (Ge2Sb2Te5)100-xSmx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) thin films are utilized to analyze the effect of rare earth element Sm on the optical behaviour of GST. The linear and nonlinear optical studies of GST on Sm incorporation provide the understanding of electron transport properties and band structural details. The optical parameters such as absorption coefficient (α) and the refractive index are obtained through transmission data and are analyzed for Sm doped GST thin films. There is significant change of the absorption coefficient near the optical band gap for (Ge2Sb2Te5)100-xSmx thin films. The optical dispersion parameters are analyzed to understand the material's structure. The optical band gap, Urbach energy, and disorder of Sm doped GST thin films are analyzed for band structure modification details on Sm addition. The free career concentration to electron effective mass ratio and high-frequency dielectric constant for (Ge2Sb2Te5)100-xSmx thin films are analyzed. Optical conductivity, dielectric constant, surface and bulk energy loss function and tan δ loss function for (Ge2Sb2Te5)100-xSmx thin films are obtained. The optical non-linear parameters are analyzed to understand the dependence of the polarizability of GST on Sm addition. The incorporation of Sm leads to the change in local bonding environment which may improve the switching mechanism of phase change memory devices.

On the properties of dissipative shocks in the relativistic accretion flows

ABSTRACT In this work, we study the properties of dissipative shocks for fully relativistic accretion flows around spinning black holes. In an accretion flow harbouring a dissipative shock (formally known as radiative shock), a significant portion of the thermal energy may get released from the post-shock corona. A stellar-mass black hole may therefore emit hard X-rays from the inner edge of the disc. If the bulk energy loss is significant, post-shock pressure drops, and shock moves forward towards the black hole compressing the size of the post-shock corona, resulting an enhancement of the corona temperature and compression ratio. The dynamical properties of the radiative shocks are therefore systematically investigated to understand accurately the radiative loss processes, temporal variations, and the spectral properties. We notice that the range of flow parameters (e.g. energy and angular momentum) responsible for the formation of ‘shocks in accretion (SA)’ is identical for both the cases of standing and dissipative shocks. The spin of the black hole enhances the dissipation further. We estimate the maximum energy release, which is observed close to $100{{\ \rm per\ cent}}$ in the extreme cases. This could be useful in explaining various observed phenomena namely the formation and the systematic evolution of quasi-periodic oscillations, and the time lags in between hard and soft X-ray photons (e.g. XTE J1550−564, GRO J1655−40, etc.) during their outbursts.

Size-dependent dielectric function for electron-energy-loss spectra of plasmonic nanoparticles

ABSTRACTA size-dependent complex dielectric function is proposed to describe the impact that size effects have on the dielectric response for electron energy loss spectroscopy (EELS) of plasmonic nanoparticles. Our implementation is based on experimental bulk complex refractive index and the modification of the Lorentz-Drude model. Our theoretical framework is verified and analysed by performing numerical simulation comparisons of EELS for Au spherical nanoparticles of different sizes. The results show that finite-size effects cannot be neglected for a broader size range of up to at least 200 nm for Au spherical nanoparticles. Moreover, the EELS regions in which contributions of surface or bulk energy loss are dominant are confirmed by the optical extinction spectra of Au spherical nanoparticles of different sizes, which takes into account the size-dependent dielectric function. The results provided here provide a suitable and versatile framework for the design of plasmonic elements on the nanometre scale.

Evaluation of robustness to surface conditions of the target factor analysis method for determining the dielectric function from reflection electron energy loss spectra: Application to GaAs

Target factor analysis (TFA) of a series of angle‐resolved reflection electron energy loss spectra (REELS) was recently demonstrated to be a useful method to determine bulk energy loss functions (ELFs), which by the TFA are separated from the surface‐loss structures of REELS. The dielectric function is then readily derived by Kramers–Kronig analysis of the ELF. The advantage of the method compared with other methods, which are also based on the analysis of REELS, is that the condition of the outermost surface region is unimportant because the excitations that occur there are removed by the TFA and ideally a pure bulk component is determined. Our method is thus particularly useful for determining the ELF from compound materials that are hard to clean without modifying the outermost atomic layers. In this paper, the robustness of the method was studied by applying it to three GaAs samples with different surface compositions caused by different surface cleaning methods. The results showed that when electrons of energy 3000–4500 eV were used, the resulting bulk ELFs were essentially identical except for small differences for the sample that had the largest thickness of the modified surface layer. It is concluded that this is a useful method, provided that the thickness of the modified layer is kept to a minimum by using shallow angle sputtering and by using REELS electrons at a sufficiently high energy that a major part of the electron trajectories are at a depth larger than the thickness of the modified surface layer. Copyright © 2013 John Wiley & Sons, Ltd.

Formation of density singularities in ideal hydrodynamics of freely cooling inelastic gases: A family of exact solutions

We employ granular hydrodynamics to investigate a paradigmatic problem of clustering of particles in a freely cooling dilute granular gas. We consider large-scale hydrodynamic motions where the viscosity and heat conduction can be neglected, and one arrives at the equations of ideal gas dynamics with an additional term describing bulk energy losses due to inelastic collisions. We employ Lagrangian coordinates and derive a broad family of exact nonstationary analytical solutions that depend only on one spatial coordinate. These solutions exhibit a new type of singularity, where the gas density blows up in a finite time when starting from smooth initial conditions. The density blowups signal formation of close-packed clusters of particles. As the density blow-up time tc is approached, the maximum density exhibits a power law ∼(tc−t)−2. The velocity gradient blows up as ∼−(tc−t)−1 while the velocity itself remains continuous and develops a cusp (rather than a shock discontinuity) at the singularity. The gas temperature vanishes at the singularity, and the singularity follows the isobaric scenario: the gas pressure remains finite and approximately uniform in space and constant in time close to the singularity. An additional exact solution shows that the density blowup, of the same type, may coexist with an “ordinary” shock, at which the hydrodynamic fields are discontinuous but finite. We confirm stability of the exact solutions with respect to small one-dimensional perturbations by solving the ideal hydrodynamic equations numerically. Furthermore, numerical solutions show that the local features of the density blowup hold universally, independently of details of the initial and boundary conditions.

Channelled relativistic outflows in active galactic nuclei: analytic solutions for the evolution of particle spectra

Context. Calculations of highly energetic neutrino and TeV γ-ray emission from relativistic jets of Active Galactic Nuclei (AGN) are often based on a model that involves a collimated relativistic blast wave, in which the spectral evolution of energetic particles is determined by the interplay between the particle injection by sweep up of the interstellar medium, energy losses by radiation and diffusive escape. Aims. To date such models only have been solved numerically because of the highly non-linear nature of the time-dependent equations describing particle spectra and bulk energy loss. However it is difficult to see how parameters and groupings of parameters influence the resulting numerical solutions, except through intensive investigations of many numerical simulations. Therefore analytic solutions are particularly helpful. Methods. We provide exact mathematical solutions to a very broad class of AGN type models. By selecting different functional behaviors of parameter values, we cover a large compendium of possible situations. The comparison of the exact solutions with observational information can help to improve our understanding of the evolution of individual AGN. Exact solutions can also be used to provide controls on the appropropriateness and accuracy of numerical programs used to solve the equations. Results. We provide an analytical description of the evolution of proton spectra according to the pick-up model. We analyze the behavior of the particle spectra in the plasma frame. The solutions are determined by two competing processes: the deceleration of the jet plasmoid and particle cooling via radiation.

Test of dielectric–response model for energy and angular dependence of plasmon excitations in core-level photoemission

We compare experimental measurements of the loss structure appearing in the Si 2p and Si 1s photoemission lines for a large range of emission angles (between 0° and 82°) and kinetic energies (125–3660 eV) with calculations derived within a semiclassical dielectric–response formalism [A. Cohen Simonsen, F. Yubero, S. Tougaard, Phys. Rev. B 56 (1997) 1612]. It is found that this semi-classical dielectric description of the energy-loss processes reproduces the relative intensity of surface to bulk energy losses appearing in the lower kinetic energy side of the main photoelectron peaks as well as the relative intensity of the losses with respect to the zero-loss peak. The absolute ratio of intensity of the loss structure to the zero-loss is ∼25–35% lower compared to experiment using self-consistent inelastic mean free paths in the analysis.

Electron energy loss spectroscopic investigation of Co metal, CoO, and Co 3O 4 before and after Ar + bombardment

X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (ELS) have been used to study a sputter-cleaned polycrystalline Co metal foil and CoO plus Co 3O 4 powders before and after ion bombardment. The ELS data have been collected using primary beam energies ranging from 100 to 1150 eV in order to distinguish between surface and bulk features and to facilitate determination of peak locations. It is shown that ELS spectra from these reference materials are considerably different and should, consequently, allow species identification in future catalyst characterization studies. The main surface features obtained from Co metal are observed at 3.7 and 7.0 eV, while the major bulk energy losses are located at 8.2, 9.8, 23.1, 25.7, and 28.1 eV. Despite the collected data on Co ELS and the electronic structure of Co metal, unambiguous peak assignments are difficult. Interband transitions in the energy loss region under investigation originate from the relatively localized Co 3d filled states, but the limited information about the empty states in the conduction band make specific assignments of the energy loss features in the Co ELS spectra challenging. XPS analysis of as-entered CoO and Co 3O 4 reveals that both samples consist mainly of Co 3O 4, in agreement with a Co 3O 4-terminated CoO surface. After prolonged Ar + sputtering both oxides are CoO-like according to high-resolution spectra of the Co 2p binding energy region. In agreement with the XPS data the ELS spectra obtained from as-entered CoO and Co 3O 4 are very similar. In contrast, ELS spectra obtained from CoO and Co 3O 4 after prolonged Ar + sputtering are considerably different. The ELS spectra obtained from CoO are not altered significantly with sputtering suggesting that the Co 3O 4 layer cannot be removed using Ar + bombardment. The sputtered Co 3O 4 instead gives rise to ELS spectra that appear CoO-like. The major ELS features in the spectra obtained from Co 3O 4 are located at 2.5, 6.7, 22.1, and 25.0 eV, while those obtained from Ar +-sputtered Co 3O 4 (CoO) are observed at 7.0, 8.6, 18.3, 21.9, and 36.1 eV. The relative intensities of the features at 6–8 eV and 22–25 eV are considerably different in the two sets of spectra. The intensity ratio of the peaks at 6.7 and 25.0 eV in Co 3O 4 is significantly higher than the 7.0 to 21.9 eV peak intensity ratio of CoO. Although several of the energy losses in the two spectra overlap and make identification difficult despite the large intensity difference, there are peaks that are specific for each compound, i.e. energy losses at 2.5 and 28.5 eV in Co 3O 4 and at 36.1 eV in CoO. Consequently, ELS can be used to differentiate the two cobalt oxides and information complementary to XPS data can be obtained from the ELS data.

Study of the initial stage of yttrium oxidation using characteristic electron-energy-loss spectroscopy

The characteristic electron-energy-loss (EEL) spectra of the pure surface of metallic yttrium and of this surface in the initial stages of oxidation are recorded. The energy of the primary electron beam Ep is 200–1000 eV. The spectra exhibit high-and low-frequency peaks. During oxidation, the positions of the basic peaks in the EEL spectra are significantly shifted. The peaks corresponding to the bulk energy loss shift toward higher energies upon oxidation. The peak corresponding to the low-frequency surface oscillations also shifts, but toward lower energies, and its intensity monotonically decreases with increasing oxygen dose. The differences between the spectra recorded at different Ep are explained as resulting from an increase in the electron escape depth with Ep.

Experimental determination of electron inelastic scattering cross-sections in Si, Ge and III–V semiconductors

Elastic peak EPES-REELS spectra have been measured on polySi, aGe, GaAs, GaP, InSb and InAs samples for primary energies E=0.2–5 keV, using the ESA 31 HSA spectrometer. The investigated specimens exhibit plasmon dominated loss spectra. We applied two approaches to separate surface and bulk plasmons. Firstly the normalized K( E, E L) λ i( E) inelastic scattering spectra, deduced from REELS (reflection electron energy loss spectroscopy) experiments, are fitted at E= E p1l bulk plasmon energy. They determine the λ i inelastic mean free path (IMFP) of electrons and their surface excitation parameter P s( E) as well. The bulk plasmon peak dominates at E=5 keV. It is described by the 3 parameters Lorentzian-type formula of Tougaard. Above E p11 the K( E, E L) λ i spectra are overlapping. Composing the difference between the normalized experimental spectra and the spectrum at 5 keV results in the surface plasmon contribution after renormalization. P s( E) is given by the ratio of the integrated surface plasmon with the elastic peak. Secondly we used a direct Monte Carlo simulation, assuming that surface and bulk energy losses are independent processes. In our model a shape of energy loss function based on optical data was used. Surface loss function intensities are fitted according to the measured spectra and P s( E) values are directly obtained from the simulation. The experimental REELS spectra of Si resulted in good agreement with Monte Carlo simulation. The P s( E) parameters of Si and Ge resulted in reasonable agreement with the relationship and parameter described by Werner.

Comparison of energy‐loss functions from REELS spectra with surface and bulk energy loss functions for Ag

AbstractEffective energy‐loss functions were derived from the reflection electron energy‐loss spectroscopy (REELS) spectra of Ag by an extended Landau approach. The effective energy‐loss functions obtained are close to the surface energy‐loss function in the low‐energy‐loss region, but tend to be closer to the bulk energy‐loss function in the higher energy‐loss region for higher primary energy. The REELS spectra incorporating the effective energy‐loss function are also reproduced in a Monte‐Carlo simulation model and confirm that the simulation reproduces the experimental REELS spectra with considerable success. Copyright © 2003 John Wiley & Sons, Ltd.

Nonlinear Evolution of Thermal Structures. Numerical Approach

A numerical algorithm using a two stage, two level difference scheme has been developed to solve the heat transfer equation with nonlinear heat diffusion and bulk energy losses. The algorithm is an extension of the scheme developed by Meek and Norbury (1982). The first stage calculates an intermediate value which is used in a second stage to estimate a new value. The scheme is consistent, second-order convergent in space and almost second order in time. It has been applied to the nonlinear stability and time evolution of thermal structures constituted by optically thin plasmas with solar abundances. The configuration has been assumed to be heated at a rate ∼ Tm, cooled at a rate ∼ Tn and a thermal conduction coefficient ∼ Tk. In particular, the second order analytical approximation considered in previous papers (Ibanez and Rosenzweig, 1995; Steele and Ibanez, 1997) has been worked out for arbitrary amplitude of the initial temperature disturbance. Particular cases of interest in Astrophysics are considered.

Secondary-electron yields from thin foils: A possible probe for the electronic stopping power of heavy ions.

We have measured heavy-ion-induced (${Z}_{P}$=2,10,18,36,54; 15 keV/u\ensuremath{\le}${E}_{P}$/${M}_{P}$\ensuremath{\le}600 keV/u secondary-electron (SE) yields from sputter-cleaned entrance (${\ensuremath{\gamma}}_{B}$) and exit surfaces (${\ensuremath{\gamma}}_{F}$) of thin solid foils (C, Al, Ti, Ni, and Cu; d\ensuremath{\approxeq}1000 A\r{}) in ultrahigh vacuum (p${=10}^{\mathrm{\ensuremath{-}}7}$ Pa). A pronounced increase of the forward to backward SE yield ratio R=${\ensuremath{\gamma}}_{F}$/${\ensuremath{\gamma}}_{B}$ with increasing ${Z}_{P}$ is observed. The SE yield to energy-loss ratio ${\ensuremath{\Lambda}}^{\mathrm{*}}$=\ensuremath{\gamma}/${S}_{e}$ has been found to be smaller for heavy ions (HI) than for light ions (H and He); i.e., ${\ensuremath{\Lambda}}^{\mathrm{*}}$(HI)${\ensuremath{\Lambda}}^{\mathrm{*}}$(He)${\ensuremath{\Lambda}}^{\mathrm{*}}$(H). Also, at low projectile velocities (${v}_{P}^{2}$50 keV/u), the value of ${\ensuremath{\Lambda}}^{\mathrm{*}}$ increases with decreasing ${v}_{P}$. The velocity and projectile dependence of both R and ${\ensuremath{\Lambda}}^{\mathrm{*}}$ can be described within simple extensions of Schou's SE emission transport theory and a semiempirical Sternglass-type model introduced by Koschar and co-workers as caused by nonequilibrium projectile energy losses ${S}_{e}^{\mathrm{*}}$ near the surfaces. The near-surface energy losses are reduced compared to tabulated bulk energy loss values ${S}_{e}$ both for forward and backward emission under the assumption of a proportionality between SE yields and dE/dx. The ${Z}_{P}$-dependent reduction factors, i.e., the ratios ${S}_{e}^{\mathrm{*}}$/${S}_{e}$, as well as material parameters \ensuremath{\Lambda}=\ensuremath{\gamma}/${S}_{e}^{\mathrm{*}}$, are deduced from the SE yield measurements. Nevertheless, a rough overall proportionality \ensuremath{\gamma}\ensuremath{\sim}dE/dx over four decades of both forward and backward secondary-electron yields \ensuremath{\gamma} and electronic energy losses dE/dx in a wide range of projectile velocities (15 keV/u \ensuremath{\le}${E}_{P}$/${M}_{P}$\ensuremath{\le}16 MeV/u) and projectile nuclear charges ${Z}_{P}$ (1\ensuremath{\le}${Z}_{P}$\ensuremath{\le}92) is found.

Peel Adhesion of Solid Films-The Surface and Bulk Effects

Abstract The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds. Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material. The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities. The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the...

Phonons in Metal Films and Fast-Electron Energy Loss in the Phonon Spectral Range

The phonons in a metal film and their excitation by fast-charged particles normally transmitted through the film are studied within the framework of a classical dielectric theory. The bulk phonons are described by means of the jellium model which treats the ions as a continuum plasma and the conduction electrons in the Thomas-Fermi screening approximation. The dispersion relation of the surface modes is obtained from the matching boundary conditions which must be satisfied by the solutions of Maxwell's equations. We find two kinds of modes connected with surface effects, which are distinguished by the nature of the fields which they induce inside the film. Firstly, there is a true-surface-mode branch which exists only at short wave-lengths. Secondly, we find a family of modes which carry oscillatory fields in the direction perpendicular to the film and which are called surface-bulk modes because of their mixed character. The energy loss experienced by fast electrons through the excitation of phonons is divided as usual into a bulk energy loss and various surface energy losses, all of which are studied analytically in detail. The effective bulk-loss intensity in a film remains significantly larger than the surface losses, even for very thin films. This indicates that the bulk phonons interact almost as strongly with electrons in thin films as in thick films, which contrasts with the case of plasmons where the effective bulk energy loss vanishes for sufficiently thin films. The predicted bulk energy loss due to phonons should be observable in sufficiently thick metal films with available experimental techniques. On the other hand, since the surface losses would be much enhanced for specularly reflected electrons at grazing-angle incidence they might become observable under such conditions.