Obtaining quantitative information on surface excitations from reflection electron energy‐loss spectroscopy (REELS)

A new analysis of reflection electron energy‐loss spectroscopy (REELS) spectra is presented. Assuming inelastic scattering in the bulk to be quantitatively understood, this method provides the distribution of energy losses in a single surface excitation in absolute units without the use of any fitting parameters. For this purpose, REELS spectra are decomposed into contributions corresponding to surface and volume excitations in two steps: first the contribution of multiple volume excitations is eliminated from the spectra and subsequently the distribution of energy losses in a single surface scattering event is retrieved. This decomposition is possible if surface and bulk excitations are uncorrelated, a condition that is fulfilled for medium‐energy electrons because the thickness of the surface scattering layer is small compared with the electron elastic mean free path. The developed method is successfully applied to REELS spectra of several materials. The resulting distributions of energy losses in an individual surface excitation are in good agreement with theory. In particular, the so‐called begrenzungs effect, i.e. the reduction of the intensity of bulk losses due to coupling with surface excitations near the boundary of a solid‐state plasma, becomes clearly observable in this way. Copyright © 2003 John Wiley & Sons, Ltd.     

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 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.     

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.     

Optical constants of organic insulators in the UV range extracted from reflection electron energy loss spectra

Reflection electron energy loss spectroscopy (REELS) spectra were measured for seven insulating organic compounds (DNA, Irganox 1010, Kapton, polyethylene [PE], poly(methyl methacrylate) [PMMA], polystyrene [PS] and polytetrafluoroethylene [PTFE]). Optical constants and energy band gaps were extracted from the measured REELS spectra after elimination of multiple electron scattering via a deconvolution and fitting the normalised single scattering energy loss spectra to Drude and Drude–Lindhard model dielectric functions, constrained by the Kramers–Kronig sum and f-sum rules. Satisfactory agreement is found for those optical constants for which literature data exists. For PTFE, the observed features in the optical data correspond to its electronic structure.     

Determination of the parameters of atomic pair correlations in nickel oxide films by the electron energy loss fine structure method

The electron energy loss extended fine structure (EELFS) spectra were obtained from the pure nickel surface (M _2,3 EELFS) of a stoichiometric NiO film (NiM _2,3 and OK EELFS spectra) and the “nonhomogeneous” oxide film on the surface of nickel Ni-O (NiM _2,3 and OK EELFS spectra). The amplitudes and intensities of electron transitions for the core levels of atoms were calculated with regard for the multiplicity of electron impact excitation of the corresponding core levels of atoms. The corresponding normalized oscillating terms were isolated using the results of calculations based on the experimental EELFS spectra. Agreement between the experimental and calculated (on Ni and NiO test objects) data showed that the theoretical approaches used and the calculated data for describing the EELFS spectra are good approximations. Using the results of calculations and the parameters of secondary electron elastic scattering (FEEF-8 data) we obtained the atomic pair correlation functions from the experimental normalized oscillating parts of the EELFS spectra by Tikhonov’s regularization method.     

New Tools for Investigating Electromagnetic Hot Spots in Single‐Molecule Surface‐Enhanced Raman Scattering

Surface‐enhanced Raman scattering (SERS) is quickly growing as an analytical technique, because it offers both molecular specificity and excellent sensitivity. For select substrates, SERS can even be observed from single molecules, which is the ultimate limit of detection. This review describes recent developments in the field of single‐molecule SERS (SM‐SERS) with a focus on new tools for characterizing SM‐SERS‐active substrates and how they interact with single molecules on their surface. In particular, techniques that combine optical spectroscopy and microscopy with electron microscopy are described, including correlated optical and transmission electron microscopy, correlated super‐resolution imaging and scanning electron microscopy, and correlated optical microscopy and electron energy loss spectroscopy.     

Evaluation of poly(4‐methyl‐1‐pentene) as a dielectric capacitor film for high‐temperature energy storage applications

Poly(4‐methyl‐1‐pentene) (P4MP) was characterized to evaluate its viability as a high‐temperature dielectric film for capacitors. Detailed investigation of thermal, mechanical, rheological, and dielectric properties was carried out to assess its high‐temperature performance and processability. P4MP was melt‐processable below 270 °C without degradation and application temperatures as high as 160–190 °C can be achieved. The dielectric constant and loss of melt‐processed P4MP films was comparable to biaxially oriented polypropylene (BOPP) capacitor films, although the dielectric strength was lower. Enhancements in dielectric strength up to 250–300% were achieved via solution‐processing P4MP films, which could be easily scaled up on a roll‐to‐roll platform to yield isotropic, free‐standing films as thin as 3–5 μm. The influence of crystal structure, crystallinity, and surface morphology of these films on the dielectric properties was examined. The dielectric strength was further increased by 450% through biaxial stretching of solution‐cast films, and a Weibull breakdown field of 514 V/μm was obtained. The dielectric constant was very stable as a function of frequency and temperature and the dielectric loss was restricted to <1–2%. Overall, these results suggest that BOP4MP is a promising candidate to obtain similar energy density as a BOPP capacitor film but at much higher operating temperatures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1497–1515     

Inelastic mean free path of low‐energy electrons in condensed media: beyond the standard models

The most established approach for ‘practical’ calculations of the inelastic mean free path (IMFP) of low‐energy electrons (~10 eV to ~10 keV) is based on optical‐data models of the dielectric function. Despite nearly four decades of efforts, the IMFP of low‐energy electrons is often not known with the desired accuracy. A universal conclusion is that the predictions of the most popular models are in rather fair agreement above a few hundred electron volts but exhibit considerable differences at lower energies. However, this is the energy range where their two main approximations, namely, the random‐phase approximation (RPA) and the Born approximation, may be invalid. After a short overview of the most popular optical‐data models, we present an approach to include exchange and correlation (XC) effects in IMFP calculations, thus going beyond the RPA and Born approximation. The key element is the so‐called many‐body local‐field correction (LFC). XC effects among the screening electrons are included using a time‐dependent local‐density approximation for the LFC. Additional XC effects related to the incident and struck electrons are included through the vertex correction calculated using a screened‐Hubbard formula for the LFC. The results presented for liquid water reveal that XC may increase the IMFP by 15–45% from its Born–RPA value, yielding much better agreement with available experimental data. The present work provides a manageable, yet rigorous, approach to improve upon the standard models for IMFP calculations, through the inclusion of XC effects at both the level of screening and the level of interaction. Copyright © 2015 John Wiley & Sons, Ltd.     

PVDF/PMMA dielectric films with notably decreased dielectric loss and enhanced high‐temperature tolerance

Polymer dielectrics generally have comparatively low dielectric constant, operating temperatures, and/or high dielectric loss, which limits their uses especially in harsh environment. In this article, a novel trilayered nanocomposite film (TNF) was constructed via solution‐casting and, subsequently, hot‐pressing process, which was composed of two outer layers of polyvinylidene fluoride (PVDF, high dielectric constant) and a middle layer of polymethyl methacrylate (PMMA, high glass transition temperature, T_g). The two outer layers of TNF were filled with barium strontium titanate nanoparticles to further increase the dielectric constant of PVDF. The PMMA in the middle layer was used to largely suppress the dielectric loss and simultaneously improve the temperature tolerance of TNF. Results show that the introduction of PMMA induced oriented crystal formation in the interface regions between PVDF and PMMA components. Moreover, most of the impurity ions were dramatically immobilized by partly oriented α crystals and high T_g PMMA layer until the temperature exceeded 120 °C. Therefore, the TNFs showed a high‐temperature tolerance and notably decreased loss, which are promising for widespread energy storage applications where harsh working conditions are present. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1043–1052     

Irreversible Structural Changes of Copper Hexacyanoferrate Used as a Cathode in Zn-Ion Batteries

The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn-ion battery, are investigated using electron microscopy techniques. The evolution of Zn_xCu_1−xHCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration.     

Ab initio ground‐state potential energy function and vibration‐rotation energy levels of imidogen,NH

The accurate ground‐state potential energy function of imidogen, NH, has been determined from ab initio calculations using the multireference averaged coupled‐pair functional (MR‐ACPF) method in conjunction with the correlation‐consistent core‐valence basis sets up to octuple‐zeta quality. The importance of several effects, including electron correlation beyond the MR‐ACPF level of approximation, the scalar relativistic, adiabatic, and nonadiabatic corrections were discussed. Along with the large one‐particle basis set, all of these effects were found to be crucial to attain “spectroscopic” accuracy of the theoretical predictions of vibration‐rotation energy levels of NH. © 2015 Wiley Periodicals, Inc.     

Local A‐Site Layering in Rare‐Earth Orthochromite Perovskites by Solution Synthesis

Cation size effects were examined in the mixed A‐site perovskites La_0.5Sm_0.5CrO₃ and La_0.5Tb_0.5CrO₃ prepared through both hydrothermal and solid‐state methods. Atomically resolved electron energy loss spectroscopy (EELS) in the transmission electron microscope shows that while the La and Sm cations are randomly distributed, increased cation‐radius variance in La_0.5Tb_0.5CrO₃ results in regions of localised La and Tb layers, an atomic arrangement exclusive to the hydrothermally prepared material. Solid‐state preparation gives lower homogeneity resulting in separate nanoscale regions rich in La³⁺ and Tb³⁺. The A‐site layering in hydrothermal La_0.5Tb_0.5CrO₃ is randomised upon annealing at high temperature, resulting in magnetic behaviour that is dependent on synthesis route.     

Surface excitations in electron spectroscopy. Part I: dielectric formalism and Monte Carlo algorithm

The theory describing energy losses of charged non‐relativistic projectiles crossing a planar interface is derived on the basis of the Maxwell equations, outlining the physical assumptions of the model in great detail. The employed approach is very general in that various common models for surface excitations (such as the specular reflection model) can be obtained by an appropriate choice of parameter values. The dynamics of charged projectiles near surfaces is examined by calculations of the induced surface charge and the depth‐ and direction‐dependent differential inelastic inverse mean free path (DIIMFP) and stopping power. The effect of several simplifications frequently encountered in the literature is investigated: differences of up to 100% are found in heights, widths, and positions of peaks in the DIIMFP. The presented model is implemented in a Monte Carlo algorithm for the simulation of the electron transport relevant for surface electron spectroscopy. Simulated reflection electron energy loss spectra are in good agreement with experiment on an absolute scale. Copyright © 2012 John Wiley & Sons, Ltd.     

Interphase/interface modification on the dielectric properties of polycarbonate/poly(vinylidene fluoride‐co‐hexafluoropropylene) multilayer films for high‐energy density capacitors

Unique three‐component multilayer films with ATBTATBTA configuration were fabricated using forced assembly multilayer coextrusion for novel dielectric systems. The dielectric breakdown strength, displacement–electric field hysteresis, and dielectric spectroscopy of 65‐layer polycarbonate (PC)/tie/poly(vinylidene fluoride‐co‐hexafluoropropylene) (P(VDF‐HFP)) were investigated with various tie materials. Three different tie materials, poly(methyl methacrylate) (PMMA), styrene‐co‐acrylonitrile copolymer with 30% acrylonitrile content (SAN30), and poly(ethylene terephthalate‐co‐1,4‐cycohexanedimethylene terephthalate) (PETG) were chosen owing to their various degrees of interaction with either P(VDF‐HFP) or PC. The 65‐layer PC/PMMA/P(VDF‐HFP) films exhibited a 25% enhancement in breakdown properties, 50% higher energy density, 40% smaller hysteresis loop areas, and orders of magnitude slower ion migration relative to the 33‐layer PC/P(VDF‐HFP) control. These property improvements are mainly attributed to the localized interactions at PMMA/P(VDF‐HFP) and PMMA/PC interfaces, forming interphase regions. The modified PMMA/P(VDF‐HFP) interphase region can effectively hinder the migration of impurity ions in P(VDF‐HFP), reducing their mobility within the layer. Additionally, a small fraction of PMMA can lead to slightly increased dielectric constant of the composite films owing to strong interaction between PMMA and P(VDF‐HFP). The other two systems with PETG and SAN30 as tie layers exhibited marginal improvements in dielectric properties owing to their weaker interactions with the P(VDF‐HFP) layers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 978–991     

Structural characterization of modern and fossilized charcoal produced in natural fires as determined by using electron energy loss spectroscopy

Charcoal produced in natural fires is widespread, but surprisingly little is known about its structure and stability. TEM and electron energy loss spectroscopy (EELS) were used to characterize the organized graphite-like microcrystallites and amorphous nonorganized phases of modern charcoal that had been produced in natural fires. In addition, a semiordered structure was identified in two modern charcoal samples. Fossilized charcoal contains fewer graphite-like microcrystallites than modern samples. EELS spectra confirmed that the dominant structure in fossilized charcoal is amorphous carbon. EELS measurements also revealed that only the nonorganized phase contains oxygen, which indicates that the degradation of the fossilized charcoal structure occurs mainly through oxidation processes. The few graphite-like microcrystallites found in fossilized charcoal were composed of onion-like structures that are probably less prone to oxidation owing to their rounded structures.     

Real‐time lock‐in energy loss imaging with a novel high‐speed image processing CCD video camera

We have developed a high‐speed image processing CCD video camera for real‐time energy‐loss imaging using a conventional electron microscope with an energy‐loss imaging facility. As an initial demonstration of real‐time lock‐in energy‐loss imaging, a background‐subtracted energy‐loss image was observed by attaching the high‐speed image processing CCD video camera to an analytical electron microscope equipped with a floating‐type energy‐loss imaging analyser. Copyright © 2003 John Wiley & Sons, Ltd.