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.     

Construction of database of effective energy‐loss functions

Effective energy‐loss functions for Al, Cu, Ag and Au were derived from the reflection electron energy‐loss spectroscopy (REELS) spectra for 1 keV electrons using extended Landau theory. Features of the obtained effective energy‐loss functions are close to those of optical surface energy‐loss functions, revealing the significant contribution of the low energy loss below a few tens of electron‐volts in the REELS spectrum for Cu, Ag and Au. The REELS spectra were reproduced using the newly derived effective energy‐loss functions, leading to the confirmation that this type of database of the effective energy‐loss function is very useful not only for more comprehensive understanding of the measured spectrum of surface electron spectroscopies but also for practical background subtraction in surface electron spectroscopy. Copyright © 2003 John Wiley & Sons, Ltd.     

Calculations of Energy-Loss Function for 26 Materials

We present a fitting calculation of energy-loss function for 26 bulk materials, including 18 pure elements (Ag, Al, Au, C, Co, Cs, Cu, Er, Fe, Ge, Mg, Mo, Nb, Ni, Pd, Pt, Si, Te) and 8 compounds (AgCl, Al₂O₃, AlAs, CdS, SiO₂, ZnS, ZnSe, ZnTe) for application to surface electron spectroscopy analysis. The experimental energy-loss function, which is derived from measured optical data, is fitted into a finite sum of formula based on the Drude-Lindhard dielectric model. By checking the oscillator strength-sum and perfectscreening-sum rules, we have validated the high accuracy of the fitting results. Furthermore, based on the fitted parameters, the simulated reflection electron energy-loss spectroscopy (REELS) spectrum shows a good agreement with experiment. The calculated fitting parameters of energy loss function are stored in an open and online database at http://micro.ustc.edu.cn/ELF/ELF.html.     

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

Effective 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.     

Construction of the Active Site of Metalloenzyme on Au NC Micelles

For developing an efficient nanoenzyme system with self-assembly strategy, gold nanocrystal micelles (Au NC micelles) with inserted catalytic Zn(II) centers were constructed by self-assembly of a catalytic ligand [N,N-bis(2-aminoethyl)-N’-dodecylethylenediamine] Zn(II) complexes (Zn(II)L) on the surface of Au NC via hydrophobic interaction. The functionalized Au NC micelles acted as an excellent nanoenzyme model for imitating ribonuclease. The catalytic capability of the Au NC micelles was evaluated by accelerating the cleavage of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP). These functionalized Au NC micelles exhibited considerable ribonuclease-like activities by a factor of 4.9×104 (kcat/kuncat) for the cleavage of HPNP in comparison to the spontaneous cleavage of HPNP at 37 ℃. The catalytic capability of the functionalized Au NC micelles can be considerably compared to other models reported previously as nanoenzymes under the comparable conditions.     

Studies of AuNi alloys by electron spectroscopies with the aid of the line shape analysis by the pattern recognition method

The spectrum of electrons elastically backscattered from the surface and within its vicinity reflects the probability of electron elastic backscattering on the surface atoms, quasi‐elastic scattering and the inelastic scattering visible in the low energy side of the elastic peak. The method for investigating the processes of electron elastic backscattering on the surface atom is called the elastic peak electron spectroscopy (EPES). In the present work, AuNi alloys of different compositions are investigated using X‐ray photoelectron spectroscopy (XPS) and the EPES method with the aid of the line shape analysis called the fuzzy k‐nearest neighbour (fkNN) rule. The line shape analysis was found to be applicable for EPES spectroscopy. It allows distinguishing the surfaces exhibiting various surface roughness, texture and grain size, and quantifying the selected information depths. The quantitative results obtained from the XPS analysis and the EPES spectra line shape analysis indicated Au surface segregation with Au surface enriched profile. Quantitative discrepancies are discussed within the non‐uniform concentration profiles of constituents due to sputter cleaning and annealing, different diffusion coefficients for Au and Ni, differences in the information depths sampled by XPS and EPES methods and differences in electron elastic backscattering cross‐sections for Ni and Au. Copyright © 2006 John Wiley & Sons, Ltd.     

On the Hopping Efficiency of Nanoparticles in the Electron Transfer across Self‐Assembled Monolayers

Redox reactions of solvated molecular species at gold‐electrode surfaces modified by electrochemically inactive self‐assembled molecular monolayers (SAMs) are found to be activated by introducing Au nanoparticles (NPs) covalently bound to the SAM to form a reactive Au–alkanedithiol–NP–molecule hybrid entity. The NP appears to relay long‐range electron transfer (ET) so that the rate of the redox reaction may be as efficient as directly on a bare Au electrode, even though the ET distance is increased by several nanometers. In this study, we have employed a fast redox reaction of surface‐confined 6‐(ferrocenyl) hexanethiol molecules and NPs of Au, Pt and Pd to address the dependence of the rate of ET through the hybrid on the particular NP metal. Cyclic voltammograms show an increasing difference in the peak‐to‐peak separation for NPs in the order Au<Pt<Pd, especially when the length of the alkanedithiol increases from octanedithiol to decanedithiol. The corresponding apparent rate constants, k_app, for decanedithiol are 1170, 360 and 14 s^?1 for NPs of Au, Pt and Pd, respectively, indicating that the efficiency of NP mediation of the ET clearly depends on the nature of the NP. Based on a preliminary analysis rooted in interfacial electrochemical ET theory, combined with a simplified two‐step view of the NP coupling to the electrode and the molecule, this observation is referred to the density of electronic states of the NPs, reflected in a broadening of the molecular electron/NP bridge group levels and energy‐gap differences between the Fermi levels of the different metals.     

Graphene oxide sheet–polyaniline nanohybrids for enhanced photovoltaic performance of dye‐sensitized solar cells

In this study, photovoltaic (PV) properties of dye‐sensitized solar cells (DSSCs) incorporated with graphene oxide nanosheet‐polyaniline (GOS‐PANI) nanohybrid/poly(ethylene oxide) (PEO) blend gel electrolytes were investigated. Chemical structure and composition of GOS‐PANI nanohybrids were characterized by Raman spectroscopy and X‐ray photoelectron spectroscopy. The images of transmission electron microscopy revealed that PANI nanorods were anchored to the single‐layered GOS for the GOS‐PANI nanohybrids. Ionic conductivities of the GOS‐PANI/PEO–based gel electrolytes were measured using a conductivity meter. The electrochemical catalytic activities of the GOS‐PANI nanohybrids were determined through cyclic voltammetry. These GOS‐PANI nanohybrids were served as the extended electron transfer materials and catalyst for the electrochemical reduction of I₃^?. Due to the enhancement of the ionic conductivity and electrochemical catalytic activity of the gel electrolyte, better PV performance was observed for the DSSCs based on the GOS‐PANI containing electrolytes as compared to the pristine PEO electrolyte‐based DSSC sample. Moreover, PV performances of the GOS‐PANI/PEO–based DSSCs were closely related to the PANI content of GOS‐PANI nanohybrids. The highest photo‐energy conversion efficiency (5.63%) was obtained for an optimized GOS‐PANI/PEO (5:95, w/w) blend gel electrolyte‐based DSSC sample. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 321–332     

Thermal characterization and kinetic analysis of nano‐ and micro‐Al/NiO thermites: Combined experimental and molecular dynamics simulation study

In the experimental part of this study, thermal properties of the Al and NiO composites in micro‐ and nano‐sized Al are investigated. Differential scanning calorimetry (DSC) analysis of the onset temperatures of ignition, activation energy (E_a), frequency factor (A), rate constant (k), critical ignition temperature of thermal explosion (T_b), and self‐accelerating decomposition temperature (T_SADT), as well as the thermodynamic parameters (ΔS^≠ , ΔH^≠ , and ΔG^≠ ) are used to explore the thermal behavior and analyze the kinetics. Thermal analysis suggests that the mechanism is based on solid–solid diffusion and liquid–gas for the nano‐ and micro‐Al/NiO composite, respectively. Our results indicate that the incorporation of nano‐Al particles can significantly reduce the ignition temperature, E_a, A, k, T_b, and T_SADT. In the second part of this work, molecular dynamics (MD) simulation is used to investigate the behavior of Al/NiO thermite reaction using the Reaxff force field to evaluate the experimental results. Theoretically, MD results show 1,154 K as the reaction ignition temperature, which is in reasonably good agreement with experimental temperature of 893°C (1,166 K). The radial distribution function (RDF) shows that no reaction occurs at 500 K but it is complete at 1,200 K.     

Calculations of electron inelastic mean free paths. XI. Data for liquid water for energies from 50 eV to 30 keV

We calculated electron inelastic mean free paths (IMFPs) for liquid water from its optical energy‐loss function (ELF) for electron energies from 50 eV to 30 keV. These calculations were made with the relativistic full Penn algorithm that has been used for previous IMFP and electron stopping‐power calculations for many elemental solids. We also calculated IMFPs of water with three additional algorithms: the relativistic single‐pole approximation, the relativistic simplified single‐pole approximation, and the relativistic extended Mermin method. These calculations were made by using the same optical ELF in order to assess any differences of the IMFPs arising from choice of the algorithm. We found good agreement among the IMFPs from the four algorithms for energies over 300 eV. For energies less than 100 eV, however, large differences became apparent. IMFPs from the relativistic TPP‐2M equation for predicting IMFPs were in good agreement with IMFPs from the four algorithms for energies between 300 eV and 30 keV, but there was poorer agreement for lower energies. We calculated values of the static structure factor as a function of momentum transfer from the full Penn algorithm. The resulting values were in good agreement with results from first‐principle calculations and with inelastic X‐ray scattering spectroscopy experiments. We made comparisons of our IMFPs with earlier calculations from authors who had used different algorithms and different ELF data sets. IMFP differences could then be analyzed in terms of the algorithms and the data sets. Finally, we compared our IMFPs with measurements of IMFPs and of a related quantity, the effective attenuation length. There were large variations in the measured IMFPs and effective attenuation lengths (as well as their dependence on electron energy). Further measurements are therefore required to establish consistent data sets and for more detailed comparisons with calculated IMFPs. Copyright © 2016 John Wiley & Sons, Ltd.     

XPS study of early stages of Al/Au interface formation

The interface formation between Au polycrystalline substrate kept at temperature of liquid nitrogen and Al deposit and its evolution during sample thermal treatments have been investigated by means of XPS. The intermixing of the Al and Au atoms at the interface occurred readily already at temperature of liquid nitrogen and resulted in the appearance of well‐resolved components on the high‐binding‐energy side of the Au(4f) spectra assignable to AlAu intermetallic phase. The sample annealing resulted in the growth of the amount of intermetallic compound formed at liquid nitrogen temperature. The formation of the initial AlAu phase at the Al/Au interface in our experimental conditions can be explained on the basis of its thermodynamic stability. Copyright © 2009 John Wiley & Sons, Ltd.     

激光诱导铝纳米膜熔化机理的分子动力学模拟

采用耦合一维双温模型的分子动力学方法研究了纳米级的铝膜在飞秒激光辐照下的熔化机制.这种方法不仅能够在原子水平上展现金属膜的各种微观行为,还能有效地描述金属膜的激光能量吸收、传递和金属电子热传导等过程.模拟结果表明,与其它金属相比,铝膜在飞秒激光辐照下的电子温度、晶格温度以及内部压力等呈现出不同的变化.铝膜在较高强度激光辐照下会很快发生全局一致的熔化,这与镍膜上下非均匀的熔化不同.并且由于铝的电子-声子耦合强度较高导致铝膜较镍膜和金膜熔化得更快.模拟结果显示,铝膜的熔化时间与实验测量的超快激光诱导的铝膜熔化时间一致.进而从理论上支持激光诱导的铝膜熔化是一个热力学熔化过程.     

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.     

Inelastic scattering and energy loss of swift electron beams in biologically relevant materials

The inelastic mean free path and the stopping power of swift electrons in relevant biomaterials, such as liquid water, DNA, protein, lipid, carotene, sugar, and ice are calculated in the framework of the dielectric formalism. The Mermin Energy Loss Function – Generalized Oscillator Strength model is used to determine the energy loss function of these materials for arbitrary energy and momentum transfer using electron energy‐loss spectroscopy data as input. To ensure the consistency of the model, efforts are made so that both the Kramers–Kronig and f‐sum rules are fulfilled to better than 2%. Our findings indicate sizeable differences in the inelastic mean free path and stopping power among these biomaterials for low‐energy electrons. For example, at 100‐eV electron energy, the inelastic mean free path in protein is 25% smaller than for water and around 10% smaller than for the other biomaterials. The stopping power values of protein, DNA, and sugar are rather similar but 20% larger than for water. Taking into account these results, we conclude that electron interactions with living tissues at the nanometric scale cannot be reliably described using only liquid water as the surrogate of the biological target. Copyright © 2016 John Wiley & Sons, Ltd.     

A numerical solution of the pair equation of a model two‐electron diatomic system

It has been well‐documented that about 90% of the total correlation energy of atomic systems can be obtained by solving so‐called pair equations. For atoms, this approach requires solving partial differential equations (PDE) in two variables. In case of a diatomic molecule, we face devising a method for treating PDEs in five variables. This article shows how a well‐established finite difference method used to solve Hartree–Fock equations for diatomic molecules can be extended to solve numerically a model two‐electron Schrödinger equation for such systems. We show that using less than 100 grid points in each variable, it is possible to obtain the total energy of the helium atom and hydrogen molecule with a chemical accuracy and the S energy of the helium atom and hydride ion as accurately as the best results available. © 2015 Wiley Periodicals, Inc.     

Effective depths for surface excitation derived by reflection electron energy‐loss spectroscopy analysis

A method of estimation is proposed for determining the effective depth of surface excitation. For this, the effective differential inverse inelastic mean free path (DIIMFP) is presumed to be represented as a linear combination of theoretical DIIMFPs for surface and bulk excitation, which are derived by the use of optical dielectric constants. The effective DIIMFP in the approach is derived by a reflected electron energy‐loss spectroscopy analysis based on the extended Landau approach. The present analysis for 1 kV electrons has led to a simple estimation of the effective depth for surface excitations (～14.5 Å for Al and ～21 Å for Ag), agreeing well with an estimation given by υ/ω _s, where υ and ω _s are the velocity of the primary electrons and the average surface plasmon frequency, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal‐oxide‐supported gold nanoparticles (NPs) have emerged as a new type of visible‐light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible‐light irradiation (λ>430 nm) of TiO₂‐supported Au NPs with a bimodal size distribution (BM‐Au/TiO₂) gives rise to the long‐range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO₂. As a result of the enhancement of charge separation, BM‐Au/TiO₂ exhibits a high level of visible‐light activity for the one‐step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO₂ (UM‐Au/TiO₂) is photocatalytically inactive.     

Synthesis of Au/Al2O3 Nanocatalyst and its Application in the Reduction of p‐Nitrophenol

Gold (Au) nanoparticles supported on alumina (Al₂O₃) were prepared at several pH levels via the deposition‐precipitation (DP) method. The effects of pH at below and above the isoelectric point (IEP) of Al₂O₃ as well as the pH adjustment before and after the addition of the support into the gold chloride solution were investigated. The results revealed the formation of cationic, clusters and metallic Au on alumina. The catalytic activity of these species was tested in the reduction of p‐nitrophenol (p‐NP) using hydrazine as a reductant. The catalytic reaction was monitored spectrophotometerically and the highest rate constant (k‐) achieved based on pseudo first order kinetic model was 12.7 × 10^‐3 s^‐1. Structural and elemental characterizations of the supported gold nanoparticles were carried out using X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy‐dispersive X‐rays (EDX), atomic absorption spectrometry (AAS), and ultraviolet‐visible spectroscopy (UV‐Vis).