Optical constants of Cu measured with reflection electron energy loss spectroscopy (REELS)

Two energy loss spectra of 1000 and 3000 eV electrons reflected from a Cu surface are analysed to give the normalized distribution of energy losses in a single surface and volume inelastic scattering process. These single scattering loss distributions are subsequently fitted to theoretical expressions for the differential inverse inelastic mean free path (DIIMFP) and differential surface excitation probability (DSEP) providing the real and imaginary part of the dielectric function in terms of a set of Drude-Lorentz oscillators. The optical constants obtained in this way are subjected to several sum rule checks and compared with other experimental data and with density-functional-theory (DFT) calculations. The present optical data agree excellently with the DFT-results, while the earlier optical data deviate significantly from these two data sets for energies below 30 eV. The mean free path for inelastic electron scattering for energies below 2000 eV is derived from the dielectric data and is found to agree satisfactorily with values reported earlier.     

Mesoscopic conductance fluctuations in graphene

We study fluctuations of the conductance of micron-sized graphene devices as a function of the Fermi energy and magnetic field. The fluctuations are studied in combination with analysis of weak localization which is determined by the same scattering mechanisms. It is shown that the variance of conductance fluctuations depends not only on inelastic scattering that controls dephasing but also on elastic scattering. In particular, contrary to its effect on weak localization, strong intervalley scattering suppresses conductance fluctuations in graphene. The correlation energy, however, is independent of the details of elastic scattering and can be used to determine the electron temperature of graphene structures.     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.     

Surface excitation probabilities in surface electron spectroscopies

The phenomenon of surface excitation is competitive in nature for elastic and other inelastic scattering processes in surface electron spectroscopies; the knowledge of influence of surface excitations in electron energy loss spectra is then essential for quantitative surface analysis with these spectroscopies. The inelastic scattering of an electron moving in the vicinity of a surface is considered in a self-energy formalism to estimate the contribution of surface excitation in electron-solid interactions via the total surface excitation probability. The formulation uses the optical bulk dielectric function and provides the spatial and angular dependence of the differential and total inelastic cross-sections. The kinetic energy range of probing electrons considered is 100-5000 eV and the numerical evaluation of total surface excitation probabilities are performed for several metals, Au, Ag, Cu, Ni, Fe and Ti; empirical formulae for the surface excitation probability are given for each of these materials and compared with experimental results for the surface excitation parameter. The total surface excitation probability is higher in Ag as compared to other metals under consideration, for identical conditions of electron-solid interactions.     

Electron spectroscopy of iron disilicide

We have reported on the results of a complex investigation of iron disilicide FeSi₂ using characteristic electron energy loss spectroscopy, inelastic electron scattering cross section spectroscopy, and X-ray photoelectron spectroscopy. It has been shown that the main peak in the spectra of inelastic electron scattering for FeSi₂ is a superposition of two unresolved peaks, viz., surface and bulk plasmons. An analysis of the fine structure of the spectra of inelastic electron scattering cross section by their decomposition into Lorentzlike Tougaard peaks has made it possible to quantitatively estimate the contributions of individual energy loss processes to the resulting spectrum and determine their origin and energy.     

石墨烯纳米片电子结构和量子输运特性第一原理研究

用基于密度泛函理论的原子紧束缚方法计算研究单层石墨烯纳米圆片和纳米带的电子结构,并结合第一原理和非平衡函数法计算量子输运特性.通过电子能态和轨道密度分布研究纳米碳原子层的电子成键状态,结合电子透射谱、电导和电子势分布分析电子散射与输运机制.石墨烯纳米带和纳米圆片分别呈现金属和半导体的能带特征,片层边缘上电极化分别沿垂直和切向方向,电子电导出现较大的差异,来源于石墨烯纳米圆片边缘的突出碳原子环对电子的强散射.石墨烯纳米带的电子透射谱表现为近似台阶式变化并在费米能级处存在弹道电导峰,而石墨烯纳米圆片的电子能带和透射谱在费米能级处开口并且因量子限制作用呈现更加离散的多条高态密度窄能带和尖锐谱峰.     

Quantitative interpretation of EELS and REELS spectra

A critical analysis of the present day Electron Energy Loss Spectroscopy (EELS) data interpretation methods has been done. The necessity for the consideration of a target as a multilayered structure with different inelastic energy loss cross sections in the surface and the bulk layers has been shown to be a reality both for the transmission EELS and the reflection EELS (REELS). A method to reconstruct inelastic energy loss cross sections in various target layers from the experimental data has been presented. Essential qualitative and quantitative dependence of the path length distribution function for reflected electrons as a function of scattering angle has been revealed. The tested method for extraction of the information from REELS experiments with angular resolution has been presented.Received: 9 October 2003, Published online: 19 February 2004PACS: 34.80.-i Electron scattering - 34.50.Bw Energy loss and stopping power - 25.30.Fj Inelastic electron scattering to continuum     

Buffer layer free large area bi-layer graphene on SiC(0 0 0 1)

The influence of hydrogen exposures on monolayer graphene grown on the silicon terminated SiC(0 0 0 1) surface is investigated using photoelectron spectroscopy (PES), low-energy electron microscopy (LEEM) and micro low-energy electron diffraction (μ-LEED). Exposures to ionized hydrogen are shown to have a pronounced effect on the carbon buffer (interface) layer. Exposures to atomic hydrogen are shown to actually convert/transform the monolayer graphene plus carbon buffer layer to bi-layer graphene, i.e. to produce carbon buffer layer free bi-layer graphene on SiC(0 0 0 1). This process is shown to be reversible, so the initial monolayer graphene plus carbon buffer layer situation is recreated after heating to a temperature of about 950 °C. A tentative model of hydrogen intercalation is suggested to explain this single to bi-layer graphene transformation mechanism. Our findings are of relevance and importance for various potential applications based on graphene-SiC structures and hydrogen storage.     

Neutron multiplicities in inelastic collisions of132Xe with197Au

Neutrons have been detected in coincidence with charged reaction products in inelastic scattering of 7.5 MeV/u¹³²Xe ions from¹⁹⁷Au. The deduced neutron multiplicities associated with the heavy and light fragment, respectively, are roughly proportional to the total kinetic energy loss, and their ratios are close to the Au-Xe mass ratio for allQ-values. These results and the measured neutron energy spectra are consistent with the assumptions of thermal equilibrium between the fragments at scission, and of neutron emission from fully accelerated fragments. For deep inelastic events, the measured absolute multiplicities are smaller than expected from statistical model calculations, but an effect due to pre-equilibrium emission of particles — as suggested by an earlier analysis of the present data [1] — cannot be definitely established.     

电介质/半导体结构样品电子束感生电流瞬态特性

电子束照射下电介质/半导体样品的电子束感生电流(electron beam induced current,EBIC)是其电子显微检测的重要手段.结合数值模拟和实验测量,研究了高能电子束辐照下SiO2/Si薄膜的瞬态EBIC特性.基于Rutherford模型和快二次电子模型研究电子的散射过程,基于电流连续性方程计算电荷的输运、俘获和复合过程,获得了电荷分布、EBIC和透射电流瞬态特性以及束能和束流对它们的影响.结果表明,由于电子散射效应,自由电子密度沿入射方向逐渐减小.由于二次电子出射,净电荷密度呈现近表面为正、内部为负的特性,空间电场在表面附近为正而在样品内部为负,导致一些电子输运到基底以及一些出射二次电子返回表面.SiO2与Si界面处俘获电子导致界面附近负电荷密度高于周围区域.随电子束照射样品内部净电荷密度逐渐降低,带电强度减弱.同时,负电荷逐渐向基底输运,EBIC和样品电流逐渐增大,电场强度逐渐减小.由于样品带电强度较弱,表面出射电流和透射电流随照射基本保持恒定.EBIC、透射电流及表面出射电流均随束流呈现近似正比例关系.对于本文SiO2/Si薄膜,透射电流随束能的升高逐渐增大并接近于束流值,EBIC在束能约15 keV时呈现极大值.     

Nonmonotonic bias voltage dependence of the magnetocurrent in GaAs-based magnetic tunnel transistors

Magnetic tunnel transistors are used to study spin-dependent hot electron transport in thin CoFe films and across CoFe/GaAs interfaces. The magnetocurrent observed when the orientation of a CoFe base layer moment is reversed relative to that of a CoFe emitter, is found to exhibit a pronounced nonmonotonic variation with electron energy. A model based on spin-dependent inelastic scattering in the CoFe base layer and strong electron scattering at the CoFe/GaAs interface, resulting in a broad electron angular distribution, can well account for the variation of the magnetocurrent in magnetic tunnel transistors with GaAs(001) and GaAs(111) collectors.     

Interference effect in combined elastic and inelastic scattering of an energetic electron reflected from a disordered medium with generation of a surface plasmon

Quantum interference in combined elastic and inelastic scattering of an energetic electron with excitation of a surface plasmon leads to a change in the shape of the corresponding peak in the electron-energyloss spectrum. The plasmon generation is suppressed near the frequency \({{\omega _p } \mathord{\left/ {\vphantom {{\omega _p } {\sqrt 2 }}} \right. \kern-\nulldelimiterspace} {\sqrt 2 }}\). The suppression increases with increasing surface-plasmon wave length, because the interference of the energetic-electron scattering processes differing in the sequential order of elastic and inelastic scattering becomes progressively more destructive. The decrease in the height of the surface-plasmon peak in the electron-energy-loss spectrum leads to a non-dissipative broadening in this peak. Quantum interference also causes a specific feature to occur in the azimuthangle dependence of the spectral intensity as the electron energy loss increases in the immediate vicinity of the surface-plasmon peak.     

Observation of interband transitions in polarized electron energy loss spectra of Gd(0001)

We have detected the 4f ⁷(5d6s)³→4f ⁸(5d6s)² interband transition in an angular-resolved, inelastic scattering experiment with spin-polarized, low-energy electrons from ferromagnetic Gd(0001). The spectrum of the inelastic scattering asymmetry clearly reveals the dominant spin-dependent energy loss mechanism involved. Furthermore its comparison with elastic scattering data allows a characterization of the combined role of diffraction and energy loss processes in inelastic electron scattering.     

Studies of Li intercalation of hydrogenated graphene on SiC(0001)

The effects of Li deposition on hydrogenated bilayer graphene on SiC(0001) samples, i.e. on quasi-freestanding bilayer graphene samples, are studied using low energy electron microscopy, micro-low-energy electron diffraction and photoelectron spectroscopy. After deposition, some Li atoms form islands on the surface creating defects that are observed to disappear after annealing. Some other Li atoms are found to penetrate through the bilayer graphene sample and into the interface where H already resides. This is revealed by the existence of shifted components, related to H–SiC and Li–SiC bonding, in recorded core level spectra. The Dirac point is found to exhibit a rigid shift to about 1.25 eV below the Fermi level, indicating strong electron doping of the graphene by the deposited Li. After annealing the sample at 300–400 °C formation of LiH at the interface is suggested from the observed change of the dipole layer at the interface. Annealing at 600 °C or higher removes both Li and H from the sample and a monolayer graphene sample is re-established. The Li thus promotes the removal of H from the interface at a considerably lower temperature than after pure H intercalation.     

The scattering of He atoms from an ordered layer of Xe atoms on Cu(001) with energy analysis

The scattering of He atoms from an ordered layer of Xe atoms deposited on Cu(001) is described. Energy analysis was carried out using a LiF crystal analyser. Energy gain and loss events were observed, the values of which were independent of the parallel momentum transfer. Results are shown for varying degrees of coverage where at large coverages a change in the energy spectrum of the scattered atoms was observed. Adsorption of Xe on a Cu surface damaged by ion bombardment yielded no inelastic information — only an elastic component was detected.     

Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.     

Scattering times in AlGaN/GaN two-dimensional electron gas from magnetotransport measurements

The various scattering times of two-dimensional electron gas were investigated in modulation-doped Al_0.22Ga_0.78N/GaN quantum wells by means of magnetotransport measurements. The ratio of transport and quantum scattering times, τ_t/τ_q∼1, shows that the dominant mobility-limiting mechanisms are short-range scattering potentials. The low-field magnetoresistance shows the weak antilocalization and localization phenomenon from which the spin-orbit scattering and inelastic scattering times are obtained. The inelastic scattering time is found to follow the T⁻¹ law, indicating that electron-electron scattering with small energy transfer is the dominant inelastic process.     

Extraction of cross-sections of inelastic scattering from energy spectra of reflected atomic particles

REELS spectra of the electrons reflected off niobium are measured with energy resolution <0.5 eV within the 5–40 eV energy range of the probing beam. The measurements were performed for the scattering angles θ = 45° and θ = 120° by means of two electron guns. The process of energy losses is described within the framework of a model with three different energy loss laws: surface, intermediate, and bulk layers are considered. Differential cross-sections of inelastic scattering are represented in the form of simple equations.     

Deposition of ZnO multilayer on LiNbO3 single crystals by DC-magnetron sputtering

Zinc oxide (ZnO) thin films were deposited on LiNbO₃ (LN) single crystals with 200 nm thicknesses by three different ways, where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. Sample, which was produced at 4-step of deposition and oxidation of Zn layer, showed high transmittance and low structural defect due to a lower photoluminescence intensity and Urbach energy. Average grain size in X-ray diffraction (XRD), scanning electron microscopy (SEM) micrograph, and atomic force microscopy (AFM) images for multilayer of ZnO was lower than monolayer of ZnO thin films. Applying multilayer coating technique leads to decrease of surface roughness and scattering on light on surface and fabrication of LiNbO₃ waveguides with lower optical loss.