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2011 | Book

An Introduction to EELS Read chapter Read first chapter

Electron Energy-Loss Spectroscopy in the Electron Microscope

Author: R.F. Egerton

Publisher: Springer US

Part of: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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About this book

Within the last 30 years, electron energy-loss spectroscopy (EELS) has become a standard analytical technique used in the transmission electron microscope to extract chemical and structural information down to the atomic level. In two previous editions, Electron Energy-Loss Spectroscopy in the Electron Microscope has become the standard reference guide to the instrumentation, physics and procedures involved, and the kind of results obtainable. Within the last few years, the commercial availability of lens-aberration correctors and electron-beam monochromators has further increased the spatial and energy resolution of EELS. This thoroughly updated and revised Third Edition incorporates these new developments, as well as advances in electron-scattering theory, spectral and image processing, and recent applications in fields such as nanotechnology. The appendices now contain a listing of inelastic mean free paths and a description of more than 20 MATLAB programs for calculating EELS data.

Table of Contents

Frontmatter
Chapter 1. An Introduction to EELS
Abstract
Electron energy-loss spectroscopy (EELS) involves analyzing the energy distribution of initially monoenergetic electrons after they have interacted with a specimen. This interaction is sometimes confined to a few atomic layers, as when a beam of low-energy (100–1000 eV) electrons is “reflected” from a solid surface. Because high voltages are not involved, the apparatus is relatively compact but the low penetration depth implies the use of ultrahigh vacuum. Otherwise information is obtained mainly from the carbonaceous or oxide layers on the specimen’s surface. At these low primary energies, a monochromator can reduce the energy spread of the primary beam to a few millielectron volts (1991) and if the spectrometer has comparable resolution, the spectrum contains features characteristic of energy exchange with the vibrational modes of surface atoms, as well as valence electron excitation in these atoms. The technique is therefore referred to as high-resolution electron energy-loss spectroscopy (HREELS) and is used to study the physics and chemistry of surfaces and of adsorbed atoms or molecules. Although it is an important tool of surface science, HREELS uses concepts that are somewhat different to those involved in electron microscope studies and will not be discussed further in the present volume. The instrumentation, theory, and applications of HREELS are described by Ibach and Mills (1982) and by Kesmodel (2006).
R.F. Egerton
Chapter 2. Energy-Loss Instrumentation
Abstract
Complete characterization of a specimen in terms of its inelastic scattering would involve recording the scattered intensity J(x, y, z, θ x , θ y , E) as a function of position (coordinates x, y, z) within the specimen and as a function of scattering angle (components θ x and θ y ) and energy loss E. For an anisotropic crystalline specimen, the procedure would have to be repeated at different specimen orientations. Even if technically feasible, such measurements would involve storing a vast amount of information, so in practice the acquisition of energy-loss data is restricted to the following categories (see Fig. 2.1):
R.F. Egerton
Chapter 3. Physics of Electron Scattering
Abstract
It is convenient to divide the scattering of fast electrons into elastic and inelastic components that can be distinguished on an empirical basis, the term elastic meaning that any energy loss to the sample is not detectable experimentally. This criterion results in electron scattering by phonon excitation being classified as elastic (or quasielastic) when measurements are made using an electron microscope, where the energy resolution is rarely better than 0.1 eV. The terms nuclear (for elastic scattering) and electronic (for inelastic scattering) would be more logical but are not widely used.
R.F. Egerton
Chapter 4. Quantitative Analysis of Energy-Loss Data
Abstract
This chapter discusses some of the mathematical procedures used to extract quantitative information about the crystallographic structure and chemical composition of a TEM specimen. Although this information is expressed rather directly in the energy-loss spectrum, plural scattering complicates the data recorded from specimens of typical thickness. Therefore it is often necessary to remove the effects of plural scattering from the spectrum or at least make allowance for them in the analysis procedure.
R.F. Egerton
Chapter 5. TEM Applications of EELS
Abstract
This final chapter is designed to show how the instrumentation, theory, and methods of EELS can be combined to extract useful information from TEM specimens, with the possibility of high spatial resolution. As in previous chapters, we begin with low-loss spectroscopy and energy filtering, followed by core-loss analysis and elemental mapping, including factors that determine detection sensitivity and spatial resolution. Structural information obtained through the analysis of spectral fine structure is then discussed, and a final section shows how EELS has been applied to a few selected materials systems. Meanwhile, Table 5.1 lists the information obtainable by energy-loss spectroscopy and by alternative high-resolution methods.
R.F. Egerton
Appendix A. Bethe Theory for High Incident Energies and Anisotropic Materials
Abstract
Even for 100-keV incident electrons, it is necessary to use relativistic kinematics to calculate inelastic cross sections, as in Section 3.6.2. Above 200 keV, however, an additional effect starts to become important, representing the fact that the electrostatic interaction is “retarded” due to the finite speed of light. At high incident energies and for isotropic materials, Eq. (3.26) should be replaced by (Møller, 1932; Perez et al., 1977)
R.F. Egerton
Appendix B. Computer Programs
Abstract
The computer codes discussed in this appendix generate spectra, process spectral data, or calculate scattering cross sections or mean free paths. They are designed as a supplement to Digital Micrograph scripts (Mitchell and Schaffer, 2005) and can be downloaded from http://​tem-eels.​com or from http://​tem-eels.​ca
All are written in MATLAB script. A program to convert DigitalMicrograph data files into MATLAB format is available. As these programs may be updated from time to time, the description that follows in this appendix may not be exact.
R.F. Egerton
Appendix C. Plasmon Energies and Inelastic Mean Free Paths
Abstract
The following table lists the atomic number Z, atomic weight A, and density ρ of some common elements and compounds, together with their chemical symbol and crystal structure, using the notation: a = amorphous, b = body-centered cubic, c = cubic, f = face-centered cubic, h = hexagonal, l = liquid, o = orthorhombic, r = rhombohedral, t = tetragonal.
R.F. Egerton
Appendix D. Inner-Shell Energies and Edge Shapes
Abstract
The following table gives threshold energies E k (in eV) of the ionization edges observable by EELS, based on data of Bearden and Burr (1967), Siegbahn et al. (1967), Zaluzec (1981), Ahn and Krivanek (1983), and Colliex (1985). The most prominent edges (those most suitable for elemental analysis) are shown in italics. Where possible, an accompanying symbol is used to indicate the observed edge shape:
R.F. Egerton
Appendix E. Electron Wavelengths, Relativistic Factors, and Physical Constants
Abstract
Table E.1 lists (as a function of the kinetic energy E 0 of an electron) values of its wavelength λ, wave number k 0, velocity v, relativistic factor γ, effective kinetic energy T, and the parameter 2γT used to calculate the characteristic scattering angle θ E = E/(2γT). For values of E 0 not tabulated, these parameters can be calculated from the following equations:
R.F. Egerton
Appendix F. Options for Energy-Loss Data Acquisition
Abstract
Table F.1 summarizes some of the procedural choices involved in the recording of energy-loss data. As discussed on p. 291, there are several ways of using the information contained in inelastic scattering. An energy-loss spectrum provides much quantitative information, such as the local thickness (p. 293), chemical composition (p. 269, 324), and the crystallographic and electronic structure (Section 5.6) of a defined region of the specimen. Energy-filtered imaging is more useful for showing variations in thickness, composition or bonding, or simply for optimizing the contrast arising from structural features (Section 5.3). A spectrum image (p. 103) combines the spatial and energy-loss information and allows sophisticated procedures such as multivariate statistical analysis (p. 265) to be applied to previously acquired data. Energy-filtered diffraction can be useful for the quantitative interpretation of diffraction patterns (p. 317), for examining the directionality of chemical bonding (Fig. 3.60) or for finding out which scattering processes contribute to the energy-loss spectrum of a particular specimen (Section 3.3).
R.F. Egerton
Backmatter
Metadata
Title
Electron Energy-Loss Spectroscopy in the Electron Microscope
Author
R.F. Egerton
Copyright Year
2011
Publisher
Springer US
Electronic ISBN
978-1-4419-9583-4
Print ISBN
978-1-4419-9582-7
DOI
https://doi.org/10.1007/978-1-4419-9583-4

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