Report on the 42nd IUVSTA workshop ‘Electron scattering in solids: from fundamental concepts to practical applications’

A summary is given of the workshop entitled ‘Electron Scattering in Solids: from fundamental concepts to practical applications,’ which was held in Debrecen, Hungary, on July 4–8, 2004, under the sponsorship of the International Union of Vacuum Science, Technique, and Applications (IUVSTA). This workshop was held to review the present status and level of understanding of electron‐scattering processes in solids, to identify issues of key importance (hot topics) in the light of the most recent scientific results, and to stimulate discussions leading to a deeper understanding and new solutions of current problems. This report contains summaries of presentations and discussions in sessions on elastic scattering of electrons by atoms and solids, inelastic scattering of electrons in solids, modeling of electron transport in solids and applications, and software. The principal areas of application include Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy, elastic‐peak electron spectroscopy (EPES), reflection electron energy‐loss spectroscopy (REELS), secondary‐electron microscopy, electron‐probe microanalysis (EPMA), and the use of coincidence techniques in electron‐scattering experiments. A major focus of the workshop was determination of the inelastic mean free path of electrons for various surface spectroscopies, particularly corrections for surface and core‐hole effects. Published in 2005 by John Wiley & Sons, Ltd.     

Fundamental properties characterizing low-pressure microwave-induced plasmas as excitation sources for spectroanalytical chemistry

Experimental measurements of the spectroscopic temperature and the electron temperature in low-pressure rare gas plasmas sustained by a microwave generator operating at 2450 MHz have revealed divergent values. These measurements have been interpreted on the basis of a radiative recombination model originally proposed by Schlüter. The importance of Penning ionization by metastable rare gas atoms in the excitation of foreign atoms has been discussed in terms of this model.On the basis of the radiative recombination model for these plasmas, the parameters of analytical importance are the concentration and energy of electrons in a high energy electron group, the concentration and energy of electrons in a low energy electron group, and the concentration of metastable rare gas atoms. Measurements of the spectroscopic temperature of an argon plasma have revealed that the energy of electrons in the low energy electron group is not greatly affected by applied microwave power and pressure over the range from 1–25 torr. The energy of electrons in the high energy electron group is not greatly affected by pressure and applied microwave power over the range studied, but has been shown to depend on the ionization potential of the plasma gas. The total electron concentration is not greatly affected by gas pressure for low applied powers, but varies with applied power, particularly at low pressures. The concentration of metastable argon atoms has been shown to depend on both the applied power and pressure. Studies of the excitation of mercury by these plasmas have led to results which are consistent with the radiative recombination model.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

Measurements and calculations of absorbed dose in electron beam radiation processing

An electron beam current densimeter has been described and used for dose measurement in EB radiation processing. An improved bipartition model of electron transport is applied to calculate the reflection coefficients and energy deposition distributions produced by 0.2–3 MeV electrons in semi-infinite media of Al, C, MAR and nylon, and the energy deposition produced by 2 MeV electrons in the two-layer medium consisting of copper and polystyrene. In addition, the depth dose distribution of 300 keV electrons in Ti-air-nylon composite-layer has been evaluated. The calculation results are in good agreement with the measurement data.     

Experimental check of the high-frequency behavior of the electrons in molecular and atomic gas plasmas

The high-frequency behavior of the electron component in collision-dominated nitrogen plasmas of dc glow discharges, acted upon by an additional microwave field, has been studied on an adequate kinetic basis for field frequencies exceeding the characteristic frequency for energy dissipation in electron collisions with nitrogen molecules. In particular, the phase delay of the electron current density with respect to the driving microwave field has been calculated. To check the validity of the results obtained by the electron kinetic approach, the phase delay has been experimentally determined adapting an appropriate microwave resonator method to the dc plasma. The comparison of the theoretically and experimentally determined phase delay of the ac electron current in the nitrogen plasma leads to a good agreement in the entire range of high-field frequencies and confirms the conclusions on the high-frequency behavior of the electrons deduced from the electron kinetic approach. Using previous results for a neon plasma, the remarkable impact of the atomic data of the collision processes in different gases on the high-frequency behavior of the electron component in these gas plasmas is additionally evaluated.     

Design of electron energy analyzers for electron impact studies

The design and construction of an electron energy analyzer for the study of electron impact processes in atoms, molecules and solids is described. The analyzer incorporates a 180° hemispherical deflector and five-element entrance optics. Focusing characteristics and angular behavior of the analyzer have been investigated by using the electron-ray tracing simulation program, SIMION. The entrance lens system to the hemispherical deflector has been designed to have high collection efficiency for low-energy electrons. The fringing field correction has been done by tilting the input beam angle outward for real aperture configuration.     

Reductive electron transfer and transport of excess electrons in DNA

In principle, DNA-mediated charge transfer processes can be categorized as either oxidative hole transfer or reductive electron transfer. In research on DNA damage, major efforts have focused on the investigation of oxidative hole transfer or transport, resulting in insights on the mechanisms. On the other hand, the transport or transfer of excess electrons has a large potential for biomedical applications, mainly for DNA chip technology. Yet the mechanistic details of this type of charge transfer chemistry were unclear. In the last two years this mechanism has been addressed in gamma-pulse radiolysis studies with randomly DNA-bound electron acceptors or traps. The major disadvantage of this experimental setup is that the electron injection and trapping is not site-selective. More recently, new photochemical assays for the chemical and spectroscopic investigation of reductive electron transfer and electron migration in DNA have been published which give new insights into these processes. Based on these results, an electron-hopping mechanism is proposed which involves pyrimidine radical anions as intermediate electron carriers.     

Measuring the electron affinity of organic solids: an indispensable new tool for organic electronics

Electron affinity is a fundamental energy parameter of materials. In organic semiconductors, the electron affinity is closely related to electron conduction. It is not only important to understand fundamental electronic processes in organic solids, but it is also indispensable for research and development of organic semiconductor devices such as organic light-emitting diodes and organic photovoltaic cells. However, there has been no experimental technique for examining the electron affinity of organic materials that meets the requirements of such research. Recently, a new method, called low-energy inverse-photoemission spectroscopy, has been developed. A beam of low-energy electrons is focused onto the sample surface, and photons emitted owing to the radiative transition to unoccupied states are then detected. From the onset of the spectral intensity, the electron affinity is determined within an uncertainty of 0.1 eV. Unlike in conventional inverse-photoemission spectroscopy, sample damage is negligible and the resolution is improved by a factor of 2. The principle of the method and several applications are reported.

石墨烯膜质子传输巨大光效应的微观机理

Graphene monolayers are permeable to thermal protons and impermeable to other atoms and molecules, exhibiting their potential applications in fuel cell technologies and hydrogen isotope separation. Furthermore, the giant photoeffect in proton transport through catalytically activated graphene membranes was reported by Geim et al. Their experiment showed that the synergy between illumination and the catalytically active metal plays a key role in this photoeffect. Geim et al. suggested that the local photovoltage created between metal nanoparticles and graphene could funnel protons and electrons toward the metal nanoparticles for the production of hydrogen, while repelling holes away from them, causing the giant photoeffect. However, based on static electric field theory, this explanation is not convincing and the work lacks an analysis on the microscopic mechanism of this effect. Herein, we provide the exact microscopic mechanism behind this phenomenon. In semi-metal pristine graphene, most photon excited hot electrons relax to lower energy states within a timescale of 10⁻¹² s, while the typical timescale of a chemical reaction is 10⁻⁶ s. Thus, hot electrons excited by incident photons relax to lower energy states before reacting with protons through the graphene. When graphene is decorated with metal, electron transfer between the graphene and the metal, induced by different work functions, would result in the formation of interface dipoles. When using metals such as Pt, Pd, Ni, etc., which can strongly interact with graphene, local dipoles form. Protons are trapped around the negative poles of the local dipoles, while electrons are around the positive poles. Upon illumination, the electrons are excited to metastable excited states with higher energy levels. Due to the energy barriers around them, the free electrons in the metastable excited states will have a relatively longer lifetime, which facilitates the production of hydrogen through their effective reaction with protons that permeated through the graphene. The concentration of high-energy electrons under illumination was estimated, and the results showed that more electrons are energized to the excited state with strong illumination. According to the analysis, the giant photoeffect in proton transport through the catalytically activated graphene membrane is attributed to long-lived hot electrons and a fast proton transport rate. Since there is no change in the activation energy of the reaction, the metal catalyst increases the rate of the reaction by increasing the number of successful collisions between the reactants to produce the significant photoeffect. This might lead to a new microscopic mechanism that clarifies the role of the catalyst in improving the efficiency of photo(electro)catalytic reactions.     

Spatial relaxation of electrons in nonisothermal plasmas

The spatial relaxation of electrons to homogeneous states under the action of space-independent electric fields is investigated in helium, krypton, and N₂ plasmas for various electric field strengths. These investigations are based on a new method recently developed for solving the one-dimensional inhomogeneous electron Boltzmann equation in weakly ionized, collision-dominated plasmas. Elastic as well as conservative inelastic collisions of electrons with gas atoms have been included in the kinetic treatment. The spatial relaxation is caused by an imposed direct disturbance in the velocity distribution of the electrons on a spatial boundary. A pronounced dependence of the relaxation structure and the resultant relaxation length on the atomic data of the electron collision processes in different gases has been found. Furthermore the relaxation process sensitively depends on the electric field strength in the region of medium field values.     

Progress in electron–ion collisions studies

Some recent measurements of excitation of ions by electrons studied in beam–beam experiments are highlighted. The emphasis is on the study of the regularities of electron–ion scattering dynamics related to the excitation and decay of atomic autoionizing states both in electron and radiative channels. Some results are presented in comparison with the data obtained by other scientific groups and with existing theoretic calculations. The significant role of relativistic and correlation effects on dynamics of excitation process has been established.     

Monte-Carlo simulation of microwave noise in n-type InSb

Numerical Monte Carlo calculations of the electron noise temperature dependence on the electric field strength in n-type InSb are presented. It is established that hot electron noise temperature in strongly compensated InSb increases with the increase of electron density due to more intensive electron–electron scattering stimulating delocalization of electrons from the bottom of the conduction band. For low electron density, when the electron–electron scattering is negligibly small, the electron noise temperature is found to become close to the lattice temperature in a wide range of electric field strength in which the electron gas cooling effect takes place. Satisfactory agreement between calculations of the electron noise temperature and available experimental data has been obtained.     

Wigner crystallization of quadratically dispersing electrons in graphene

The graphene surface with the unpaired π electrons presents an ideal two‐dimensional electron system. Although the effective massless Dirac fermions are important, they are not the only carriers that describe the quantum transport in graphene. Above zero energy, the current carrying carriers in graphene are the usual electrons. This electron density may vary depending on the surface defects and π–σ interaction, and this may lead to a possible Wigner crystallization on the surface of graphene. Calculations for nonmagnetic, ferromagnetic, and antiferromagnetic Wigner crystals are carried out based on the Koster–Kohn variational principle for direct calculation of Wannier functions. The effect of positive background due to the carbon ions is suitably treated. From our results, we find that Wigner crystallization is possible in grapheme, if we consider the electrons on the surface, which obey quadratic dispersion relation. The electron crystal with ferromagnetic phase and face centered square lattice structure has the lowest energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Born-Floquet theory of electron-atom collisions in the presence of a laser field

We present a theory of fast electron-atom collisions in the presence of a strong laser field, which treats the interaction of the laser field with both the projectile electron and the target atom in a fully non-perturbative way. The theory is illustrated by considering the laser-assisted “elastic” scattering of fast electrons by atomic hydrogen, for non-resonant as well as resonant cases.     

Formation of Temporary Negative Ions and Their Subsequent Fragmentation upon Electron Attachment to CoQ0 and CoQ0H2

Ubiquinone molecules have a high biological relevance due to their action as electron carriers in the mitochondrial electron transport chain. Here, we studied the dissociative interaction of free electrons with CoQ₀, the smallest ubiquinone derivative with no isoprenyl units, and its fully reduced form, 2,3-dimethoxy-5-methylhydroquinone (CoQ₀H₂), an ubiquinol derivative. The anionic products produced upon dissociative electron attachment (DEA) were detected by quadrupole mass spectrometry and studied theoretically through quantum chemical and electron scattering calculations. Despite the structural similarity of the two studied molecules, remarkably only a few DEA reactions are present for both compounds, such as abstraction of a neutral hydrogen atom or the release of a negatively charged methyl group. While the loss of a neutral methyl group represents the most abundant reaction observed in DEA to CoQ₀, this pathway is not observed for CoQ₀H₂. Instead, the loss of a neutral OH radical from the CoQ₀H₂ temporary negative ion is observed as the most abundant reaction channel. Overall, this study gives insights into electron attachment properties of simple derivatives of more complex molecules found in biochemical pathways.     

Monte Carlo transport of electrons and positrons through thin foils

In measurements on electrons traversing matter it is important to know the transmission through that medium, their path-lengths and their angular distribution through matter. This allows one to seek improvement in techniques which employ electrons, including medical applications and materials irradiation. This work presents a simulation of the transport of beams of electrons and positrons through thin foils using an analog Monte Carlo code that simulates in a detailed way every electron movement or interaction in matter. As those particles penetrate thin absorbers, it has been assumed that they interact with matter only through elastic scattering, with negligible energy loss. This type of interaction has been described quite precisely because its angular form influences very much the angular distribution of electrons and positrons in matter. With this code it has been calculated that the number of particles, with energies between 100 and 3000 keV, which are transmitted through different media of various thicknesses as well as their angular distributions, show good agreement with the experimental data. The discrepancies are less than 5% for thicknesses lower than about 30% of the corresponding range in the tested material. As elastic scattering is very anisotropic, its angular distribution resembles a collimated incident beam for very thin foils, becoming slowly more isotropic when absorber thickness is increased.     

Role of Nondipole Transitions in Formation of K-Edge Extended Electron Energy Loss Fine Structure

For extended electron energy loss fine structure (EELFS) in the case of ionized K-level, the effects of nondipole processes are estimated at different excitation energies and scattering angles of incident electron. A multiplet resolution converging fast for any scattering angles of incident electron is suggested, and simple analytical expressions up to the quadrupole term are derived. Using these estimates, we have calculated the Al K-edge EELFS spectrum and compared the calculated data with the experimental results. The problem of violation of the dispersion law of secondary electrons is discussed; this problem is caused by the finite lifetime of the excited electronic subsystem of the sample compared to the dispersion law of free electrons.     

钙钛矿太阳能电池电子传输层的制备及应用

目前,有机-无机杂化钙钛矿太阳能电池(PSC)的器件效率已经超过25％.电子传输层作为PSC中的重要组成部分在提取和传输光生电子,阻挡空穴,修饰界面,调节界面能级和减少电荷复合等方面起着关键作用.无机n型材料,例如TiO2、ZnO、SnO2和其他金属氧化物材料具有成本低和稳定性好的特点,经常在传统PSC中被用作电子传输...