High resolution microanalysis in materials science using electron energy loss measurements

The physical basis of microanalysis using measurements of electron energy losses associated with atom ionization or plasmon excitation in thin electron microscope specimens is explained in a simple manner. In addition the equipment used to resolve both the high and low energy regions of the loss electron spectrum is described. It is shown that ionization loss analysis is still in its infancy, but plasmon loss analysis has now been providing quantitative microanalytical data on light metal alloys for 8 years. The results obtained from both techniques and their application to specific metallurgical problems are reviewed. Conclusions are drawn concerning the future use of these techniques in high resolution microanalysis.     

Validity of the dipole approximation in TEM‐EELS studies

Nondipole effects in electron energy‐loss spectroscopy are evaluated in terms of deviation of the inelastic scattering from a Lorentzian angular distribution, which is assumed in established procedures for plural‐scattering deconvolution, thickness measurement, and Kramers‐Kronig analysis. The deviation appears to be small and may be outweighed by the effect of plural (elastic + inelastic) scattering, which is not removed by conventional deconvolution methods. In the core‐loss region of the spectrum, non‐Lorentzian behaviour stems from a reduction of the generalized oscillator strength from its optical value and (for energies far above an ionization threshold) formation of a Bethe‐ridge angular distribution. At incident energies above 200 keV, retardation effects further distort the angular dependence, even for core losses just above threshold. With an on‐axis collection aperture, non‐dipole effects are masked by the rapid falloff of intensity with scattering angle, but they may become important for off‐axis measurements. Near‐edge fine structure is sensitive to nondipole effects but these can be minimized by use of an angle‐limiting collection aperture. Microsc. Res. Tech. 77:773–778, 2014. © 2014 Wiley Periodicals, Inc.     

A proposal for dichroic experiments in the electron microscope

Building upon the similarities between inelastic electron scattering and X-ray absorption we show that dichroism can be observed in electron energy loss spectrometry (EELS) in the transmission electron microscope (TEM). Natural or magnetic linear dichroism can be studied in electron scattering experiment with definite wave vector transfer in the interaction.The detection of circular dichroism in the TEM relies on interferometric EELS in a particular scattering geometry that allows extraction of the mixed dynamic form factor from energy loss spectra. Similarities between dichroic signals in energy loss near edge structures and X-ray absorption near edge structures are discussed, and a new experimental setup for dichroic measurements in the TEM is proposed.     

On-line recording and processing of CTEM images

The direction dependence of electron energy losses was measured with a Möllenstedt energy analyzer which was attached to a convergent beam electron diffraction camera. High energy losses could be observed by adjusting the potential of the middle electrode. The investigations were performed with perfect MgO single crystal platelets of about 400 Å thickness. K-losses of oxygen and magnesium were analyzed. The greatest effect of the direction dependence was observed for the K-loss of Mg. By these results the validity of the reciprocity theorem was confirmed even for higher energy loss electrons up to at least 1.3 keV for 40 keV primary electrons.     

Image-EELS: Simultaneous recording of multiple electron energy-loss spectra from series of electron spectroscopic images

A new approach for element microanalysis with energy-filtering transmission electron microscopy (EFTEM) is presented which was accomplished with the CEM 902 electron microscope (Zeiss, Germany). This method is called Image-EELS, because it is a synthesis of electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Series of energy-filtered images at increasing energy losses are recorded from one area with a TV camera. In a second step the intensity of selected regions in the image stack is measured with an image analysis system and plotted as a function of the energy loss. Thus many spectra from different objects can be calculated from one image series and compared with each other. The spatial resolution of EELS is considerably enhanced, the noise is decreased because many pixels from irregular objects are integrated, and the information from ESI can be analysed as a function of the energy loss.     

Treating retardation effects in valence EELS spectra for Kramers-Kronig analysis

Retardation effects such as Cerenkov losses and waveguide modes alter the valence electron energy-loss spectrum of semiconductors and insulators as soon as the speed of the probing electron exceeds the speed of light inside the probed medium. This leads to the dilemma, that optical properties from these media cannot be determined correctly using electron energy-loss spectrometry (EELS) if no corrections are applied. In this work we present two ways out of this dilemma: a reduction of the beam energy and the application of an off-line correction. We demonstrate the accuracy of these two methods by using two similar layers of Si(x):H having slightly different refractive indices and discuss the impact of the normalization parameter during Kramers-Kronig analysis (KKA) on the obtained dielectric properties. We further demonstrate that KKA can be applied without the use of standard specimens, if thickness determination using transmission electron microscopy and EELS is accurate enough.     

Mapping inelastic intensities in diffraction patterns of magnetic samples using the energy spectrum imaging technique

We present the quantitative measurement of inelastic intensity distributions in diffraction patterns with the aim of studying magnetic materials. The relevant theory based on the mixed dynamic form factor (MDFF) is outlined. Experimentally, the challenge is to obtain sufficient signal for core losses of 3d magnetic materials (in the 700-900eV energy-loss range). We compare two experimental settings in diffraction mode, i.e. the parallel diffraction and the large-angle convergent-beam electron diffraction configurations, and demonstrate the interest of using a spherical aberration corrector. We show how the energy spectrum imaging (ESI) technique can be used to map the inelastic signal in a data cube of scattering angle and energy loss. The magnetic chiral dichroic signal is measured for a magnetite sample and compared with theory.     

An electron energy loss spectrometer designed for studies of electronic energy losses and spin waves in the large momentum regime

Based on 143° electrostatic deflectors we have realized a new spectrometer for electron energy loss spectroscopy which is particularly suitable for studies on surface spin waves and other low energy electronic energy losses. Contrary to previous designs high resolution is maintained even for diffuse inelastic scattering due to a specific management of the angular aberrations in combination with an angle aperture. The performance of the instrument is demonstrated with high resolution energy loss spectra of surface spin waves on a cobalt film deposited on the Cu(100) surface.     

Enhancement of resolution in core-loss and low-loss spectroscopy in a monochromated microscope

The significant enhancement of the energy resolution in the new generation of commercially available monochromated transmission electron microscopes presents new challenges in term of selecting the correct experimental conditions and understanding the various effects that can potentially influence the quality of the EELS data. In this respect we investigated the effect of point spread function of the detector and spectrum-diffraction mixing on the energy resolution and the intensity of the zero loss peak tails. Alternative approaches to improve the energy resolution by mathematical methods have been tested. By using a simple and commonly available test case (Si L(2,3) edges) we assessed the efficiency of the deconvolution algorithms to improve the resolution. The results show that the deconvolution is not always successful in improving the resolution of the core loss EELS data and the results may not always be reliable. Contrary to this, the application of the Richardson-Lucy deconvolution algorithm on some bandgap measurements data appears to be very effective. The procedure proved successful in removing the contribution of the zero-loss peak tails and allows an easier access to spectroscopic information starting at energy losses as low as of 0.5 eV with monochromated spectra and 1 eV with the non-monochromated spectra.     

High spatial resolution spectroscopy in the elemental microanalysis and imaging of biological systems

The application of analytical electron microscopy to the high spatial resolution study of biological systems is reviewed. Specimen preparation, quantitative analysis, capabilities and limitations are all discussed, principally in the context of energy-dispersive X-ray analysis. Results are presented using both current techniques and the developing quantitative image analysis. Finally the role of new instrumental approaches, including electron energy loss spectrometry, is discussed.     

Coherence and incoherence of inelastically scattered electron waves

Whether the elastically and inelastically scattered electron waves are mutually coherent is a key question for the quantitative evaluation of the contrast of a high resolution transmission electron microscope image and for confirming the energy filtering ability of electron holography. Using a simplified object model composed of only two scattering centers the elastically and inelastically scattered electron waves and their detection are discussed. The only used proof for the main results of this paper is the orthogonality of the object states. The reasoning can be extended straightforward to a general case, e.g. a complicated object as well as complicated interaction Hamiltonians as long as they remain time independent. The results of this paper are: (1) The elastically and inelastically scattered waves which have different energy losses are mutually incoherent. The reason for that is simply that the corresponding excited object states are mutually orthogonal; (2) the inelastically scattered waves which have the same energy loss and are scattered by the same object state are coherent, whereas they are incoherent if they are scattered by different object states though they have the same energy; (3) the coherence degree of the scattered electron waves is proportional to the modulus of the scalar product of the corresponding object states.     

Contribution of electron energy loss spectroscopy to the development of analytical electron microscopy.

The combined use of an electron energy loss spectrometer and an electron microscope provides some chemical information at the nanometer scale. The physics of the interaction processes between the incident electron beam and the thin sample foil is reviewed in terms of energy and momentum transfer. This analysis of the content of an electron energy loss spectrum allows us to establish rules for a satisfactory use of the information and to discuss the detection limits of this newly developed microanalytical technique.     

TEM-EELS study of low-friction superlattice TiAlN/VN coating: the wear mechanisms

A 20–50 nm thick tribofilm was generated on the worn surface of a multilayer coating TiAlN/VN after dry sliding test against an alumina counterpart. The tribofilm was characterized by applying analytical transmission electron microscopy techniques with emphasis on detailed electron energy loss spectrometry and energy loss near edge structure analysis. Pronounced oxygen in the tribofilm indicated a predominant tribo-oxidation wear. Structural changes in the inner-shell ionization edges of N, Ti and V suggested decomposition of nitride fragments.     

Important aspects of investigating optical excitations in semiconductors using a scanning transmission electron microscope

Since semiconductor structures are becoming smaller and smaller, the examination methods must also take this development into account. Optical methods have long reached their limits here, but small dimensions are also a challenge for electron beam techniques, especially when it comes to determining optical properties. In this paper, electron microscopic methods of investigating optical properties are discussed. Special attention is given to the physical limits and how to deal with them. We will cover electron energy loss spectrometry as well as cathodoluminescence spectrometry. We pay special attention to inelastic delocalisation, radiation damage, the Čerenkov effect, interference effects of optical excitations and higher diffraction orders on a grating analyser for the cathodoluminescence signal.     

Energy Loss in the Impact of Elastic Spheres on a Rigid Half-Space in Presence of Adhesion

Adhesion can cause energy losses in asperities or particles coming into dynamic contact resulting in frictional dissipation, even if the deformation occurring is purely elastic. Such losses are of special significance in impact of nanoparticles and friction between surfaces under low contact pressure to hardness ratio. The objective of this work is to study the effect of adhesion during the normal impact of elastic spheres on a rigid half-space, with an emphasis on understanding the mechanism of energy loss. We use finite element method for modeling the impact phenomenon, with the adhesion due to van der Waals force and the short-range repulsion included as body forces distributed over the volume of the sphere. This approach, in contrast with commonly used surface force approximation, helps to model the interactions in a more precise way. We find that the energy loss in impact of elastic spheres is negligible unless there are adhesion-induced instabilities. Significant energy loss through elastic stress waves occurs due to jump-to-contact and jump-out-of-contact instabilities and can even result in capture of the elastic sphere on the half-space.     

The influence of relativistic energy losses on bandgap determination using valence EELS

Since monochromated transmission electron microscopes have become available, the determination of bandgaps and optical properties using electron energy loss spectrometry (EELS) has again attracted interest. The underlying idea is very simple: below the bandgap energy no transitions can contribute to the valence EELS signal. However, the bandgap cannot be directly read out from the recorded data. Therefore the optical properties cannot be determined correctly from the low loss using the Kramers-Kronig relations. We will discuss under which conditions relativistic effects may be suppressed. It is demonstrated that scanning TEM (STEM) geometry is not applicable for most bandgap measurements.     

超磁致伸缩致动器能量损耗特性分析

提出了超磁致伸缩棒内部平均磁场计算方法,结合动态J-A模型以及线圈阻抗公式得到了超磁致伸缩致动器的能耗特性。分析了超磁致伸缩棒能耗、线圈能耗以及超磁致伸缩致动器总能耗随频率的变化趋势。分析结果表明,超磁致伸缩棒以及线圈能耗均随着频率的增大而增大,而且超磁致伸缩棒能耗占超磁致伸缩致动器总能耗的比例会随着频率的增大而增大。计算了油冷条件下超磁致伸缩棒的表面温度,计算结果与实验结果较为吻合,证明了能耗模型的正确性。分析过程及方法为超磁致伸缩致动器的设计和控制提供了有益的参考。     

数控机床主动力系统载荷能量损耗系数的计算获取方法

机床量大面广,能量消耗巨大并且能量效率低、能量效率规律复杂,因此机床能量效率的研究和应用正在全球迅速兴起。机床主动力系统载荷能量损耗,是机床最复杂的部分,一般占到机床切削能量的10%~20%,其具体多少决定于载荷损耗系数。针对机床主动力系统载荷损耗系数在线获取非常困难,以及现有获取方法存在切削仪器昂贵、测量过程复杂以及许多机床无法安装等不足,提出一种根据数控机床主动力系统载荷能量损耗数学模型而建立的载荷损耗系数计算获取方法。上述数学模型由三部分组成:基于主电动机的变频器额定功率、额定功率损耗和散热器额定功率等基础参数的变频器功率损耗近似计算模型;基于主电动机额定功率、基频、空载功率和基频下的效率等基础参数的主电动机功率损耗模型;基于机床机械传动系统中每种传动副的数量和载荷效率等基础参数的机械传动系统载荷损耗模型。实际计算时只需获取基础参数和测量机床主动力系统空载功率,便可计算出机床主动力系统的载荷能量损耗系数。基于上述计算方法,计算获取了数控加工中心PL700主动力系统的载荷能量损耗系数。实验结果证实了该方法的准确性和实用性。该方法可广泛用于数控机床的切削功率获取、数控机床能量效率评价和工件能量效率预测及优化等研究和应用中,具有多方面的应用前景。