Scattering angle dependence of signal/background ratio of inner-shell electron excitation loss in eels

Scattering angle dependence of the signal/background ratio of Si K-shell and A1 K-shell electron excitation losses has been measured for single crystals and evaporated films. The ratio changed periodically with the scattering angle, and maxima were found to be located between Bragg reflections including the center beam. Thus the ratio is improved between the Bragg reflections and just outside the incident beam, which is very important as a practical technique for elemental analysis in the higher energy loss region in EELS.     

Thickness measurements with electron energy loss spectroscopy

Measurements of thickness using electron energy loss spectroscopy (EELS) are revised. Absolute thickness values can be quickly and accurately determined with the Kramers-Kronig sum method. The EELS data analysis is even much easier with the log-ratio method, however, absolute calibration of this method requires knowledge of the mean free path of inelastic electron scattering lambda. The latter has been measured here in a wide range of solids and a scaling law lambda approximately rho(-0.3) versus mass density rho has been revealed. EELS measurements critically depend on the excitation and collection angles. This dependence has been studied experimentally and theoretically and an efficient model has been formulated.     

Tilt dependence of the secondary electron emission at low excitation energy

Influence of the specimen's slope on the secondary electron emission has been experimentally studied. Strong deviations from the inverse cos law have been observed and corresponding phenomenological equation (taking into account this deviation) is suggested. The consequences of the dependence on the topography contrast of low- and high-voltage scanning electron microscopy (SEM) image, especially for three-dimensional (3-D) reconstruction, are considered.     

Signal/background ratio of ionization edges in EELS

We confirm earlier reports that the signal/background ratio of the aluminum K-edge can sometimes be improved by using thicker specimens and an off-axis collection aperture. However, we interpret this behaviour as an artifact arising from the presence of a high instrumental background, which can be avoided by appropriate design of the electron detector.     

Physical background of electron backscattering

The parameters governing electron backscattering from thin films and from bulk solids are reviewed: Atomic scattering cross-section, atomic number, single/multiple scattering, film thickness, scattering angular distribution, angle of incidence, diffraction effects. Their influence on some important contrast mechanisms due to backscattered electrons in scanning electron microscopy (thickness contrast, Z/material contrast, tilting/topography contrast, crystal orientation contrast) is discussed. The most frequently used backscattering electron detection systems are briefly described.     

一种多频率同步信号激励电流源设计

生物电阻抗频谱(BIS)多频率同步快速测量系统中的激励电流源必须满足频谱宽、谱能量均衡、相位同步、输出阻抗高等特殊要求.基于Walsh函数设计了一种周期、二值的九频率同步信号f(9,t),它在1、2、4、8、16、32、64、128和256次等9个主谐波上相位同步,能量分布均衡,功率总和占到了平均总功率的65.52％.介绍了f(9,t)的FPGA实现方法和电压控制电流源(VCCS)驱动电路设计.负载实验表明,VCCS的负载电压理论仿真波形与实测波形高度一致,且具有较高的输出阻抗.确立了一种比较理想的多频率同步激励电流源,为BIS的多频率同步快速测量奠定基础.     

Thickness and grain size dependence of limit strains in sheet metal stretching

A theoretical analysis is presented of thickness and grain size dependence of the limit strains in isotropic sheet metals under biaxial tension. Sheet surfaces are assumed to consist of asperities which increase proportionally with the equivalent strain and the grain diameter of the material. The limit strains are given as the sum of the strain up to Swift instability modified by taking into consideration the effect of surface asperities and the subsequent strain up to a localized neck which is based on the Marciniak analysis. The thickness ratio of the material in the Marciniak model is determined making use of the magnitude of the surface roughness at the modified Swift instability.Numerical results show that the limit strains decrease in all cases as thickness to grain size ratio decreases. The increase in surface roughness with plastic strain varies from material to material and this is taken into account in predicting limit strains.     

Quantitative electron energy loss spectroscopy in biology

The potential for applying electron energy loss spectroscopy (EELS) in biology is assessed. Some recent developments in instrumentation, spectrometer design, parallel detection and elemental mapping are discussed. Quantitation is demonstrated by means of the spectrum from DNA which gives an elemental ratio for N:P close to the expected value. A range of biologically important elements that can be usefully analyzed by EELS is tabulated and some possible applications for each are indicated. Detection limits and the effects of radiation damage are illustrated by spectra from the protein, insulin, and from the fluorinated amino-acid, histidine. Calcium detectability under optimum conditions may be as low as 1 mmol/kg dry weight. The application of EELS to analysis of cryosectioned adrenomedullary (chromaffin) cells is described in order to help determine the composition of the secretory granule. Water content can be determined from the amount of inelastic scattering as measured by the low-loss spectrum. The nitrogen/phosphorus ratio can be measured to provide information about the relative concentrations of ATP, chromogranin, and catecholamines. Quantitative EELS elemental maps are obtained in the STEM mode from chromaffin cells in order to measure the distribution of light elements.     

Dynamics of inner-shell electrons in ion-atom collisions

Excitation and ionization are treated in time-dependent variational theory. We show how the variational method is related to and can be combined with perturbation theory. The quality of variational wave functions is estimated and a practicable error estimate is found. We use the method for a study of dynamical binding corrections in asymmetric but otherwise arbitrary collisions. The effect of recoil is analysed and its dependence on impact parameter and velocity is explained in analogy to classical mechanics.     

Wear mechanisms description in nanoscale by SEM/TEM of multilayer Zr/ZrN coatings in dependence on phases ratio

As a result of loading with an external force during the wear process, coating deforms uniformly. After a certain limit load is exceeded, coating deformation is localised through the formation of the so-called shear bands. It has been showed experimentally the process of shear bands formation. The microstructural characterisation before and after the mechanical tests was performed using scanning and transmission electron microscopy (SEM and TEM) on cross-sections of the samples. The analysis indicated that in the case of multilayer coatings where the ratio of the metallic to the ceramic phase is 1:1, the shear bands are formed at an angle of 45°. With a greater proportion of the ceramic phase to metallic (ratio 1:2), the shear band changed the shear angle from ∼45° to ∼90°. Mechanical in situ tests were carried out in the chambers of SEM and TEM. The scratch tests in the SEM were done with the simultaneous observation of the phenomena occurring on the surface of the tested materials showed that at a scratch force of 0.04 N, the additional outer a-C:H layer was damaged, which was shown in the form of a fault in the force–displacement diagram, and in the form of splits visible in the SEM image. However, the application of this additional layer had a positive effect on the wear mechanism of the entire coating structure. The test also indicated that in the case of coatings with phases ratio 1:2 and 1:4 (metallic to ceramic), the characteristics of the brittle material were demonstrated, unlike the coating with a 1:1 phase ratio, where plastic properties predominated. However, for the 1:2 phase ratio coating, the chip was more ductile than for the chip formed when testing a 1:4 phase ratio coating. For in situ mechanical testing in the TEM, a straining holder was used. The test showed that the shear band angle for a 1:1 ratio coating has changed from 45° to 90° due to the different direction of force interaction.     

New developments in electron energy loss spectroscopy

In the electron microscope, spectroscopic signals such as the characteristic X-rays or the energy loss of the incident beam can provide an analysis of the local composition or electronic structure. Recent improvements in the energy resolution and sensitivity of electron spectrometers have improved the quality of spectra that can be obtained. Concurrently, the calculations used to simulate and interpret spectra have made major advances. These developments will be briefly reviewed. In recent years, the focus of analytical electron microscopy has moved away from single spectrum acquisition to mapping and imaging. In particular, the use of spectrum imaging (SI), where a full spectrum is acquired and stored at each pixel in the image is becoming widespread. A challenge for the application of spectrum imaging is the processing of such large datasets in order to extract the significant information. When we go beyond the mapping of composition and look to map bonding and electronic structure this becomes both more important and more difficult. Approaches to processing spectrum imaging data sets acquired using electron energy loss spectroscopy (EELS) will be explored in this paper.     

Some effects of electron channeling on electron energy loss spectroscopy

As an electron beam (of order 100 keV) travels through a crystalline solid it can be channeled down a zone axis of the crystal to form a channeling peak centered on the atomic columns. The channeling peak can be similar in size to the outer atomic orbitals. Electron energy loss spectroscopy (EELS) measures the losses that the electron experiences as it passes through the solid yielding information about the unoccupied density of states in the solid. The interaction matrix element for this process typically produces dipole selection rules for small angle scattering. In this paper, a theoretical calculation of the EELS cross section in the presence of strong channeling is performed for the silicon L23 edge. The presence of channeling is found to alter both the intensity and selection rules for this EELS signal as a function of depth in the solid. At some depths in the specimen small but significant non-dipole transition components can be produced, which may influence measurements of the density of states in solids.     

Parallel recording of electron energy loss spectra

Two silicon photo diode array devices were tested as parallel recording detectors for electron energy loss spectrometry (EELS). The direct bombardment of a Reticon photodiode array detector with high energy electrons (80 keV) causes an irreversible increase in diode dark current. The dark current saturates the detector amplifier after a dose of 10^?6 C/diode making it unsuitable for EELS. A scintillator coupled SIT vidicon is sensitive enough to count two high energy electrons with a spatial resolution of 100 μm, corresponding to 5 eV energy resolution with the electron optical system described. The large pixel-to-pixel gain variation inherent in the scintillator and vidicon can be reduced by averaging the spectrum over a large area of the target perpendicular to the dispersion direction. The L-edge of calcium for a 4 × 10^?3 weight fraction concentration biological specimen is observable in a 40 s parallel recorded spectrum. The minimum detectable concentration of calcium is estimated tobe ten times better for EELS than EDS X-ray analysis.     

Current applications of electron energy loss spectroscopy

It is undoubtedly true that the advent of efficient energy loss spectrometers for transmission microscopes over the last few years has been of considerable assistance, at least qualitatively, for the analysis of light elements and, to a more limited extent, in structure interpretation. Rather frustratingly, given the potentially better spatial resolution of EELS over EDX, realistic quantitative analysis remains difficult, and similarly - while fascinating effects are seen in, for example, the crystallographic orientation dependence of the signal - these are currently only broadly interpretable in relation to those observed in EDX. The reasons for this are discussed, as are the relative advantages of large and small collection angles for different types of experiment.     

Deconvolution of core electron energy loss spectra

Different deconvolution methods for removing multiple scattering and instrumental broadening from core loss electron energy loss spectra are compared with special attention to the artefacts they introduce. The Gaussian modifier method, Wiener filter, maximum entropy, and model based methods are described. Their performance is compared on virtual spectra where the true single scattering distribution is known. A test on experimental spectra confirms the good performance of model based deconvolution in comparison to maximum entropy methods and shows the advantage of knowing the estimated error bars from a single spectrum acquisition.