Electron Energy-Loss Spectroscopy in the Electron Microscope. Third Edition

Size: px

Start display at page:

Download "Electron Energy-Loss Spectroscopy in the Electron Microscope. Third Edition"

Dora Bishop
5 years ago
Views:

1 Electron Energy-Loss Spectroscopy in the Electron Microscope Third Edition

3 R.F. Egerton Electron Energy-Loss Spectroscopy in the Electron Microscope Third Edition 123

4 R.F. Egerton Department of Physics Avadh Bhatia Physics Laboratory University of Alberta Edmonton, AB, Canada ISBN e-isbn DOI / Springer New York Dordrecht Heidelberg London Library of Congress Control Number: st edition: Plenum Press nd edition: Plenum Press 1996 Springer Science+Business Media, LLC 2011 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper Springer is part of Springer Science+Business Media (

5 Preface to the Third Edition The development of electron energy-loss spectroscopy within the last 15 years has been remarkable. This progress is partly due to improvements in instrumentation, such as the successful correction of spherical (and more recently chromatic) aberration of electron lenses, allowing sub-angstrom spatial resolution in TEM and STEM images and (in combination with Schottky and field-emission sources) much higher current in a focused probe. The incorporation of monochromators in commercial TEMs has improved the energy resolution to 0.1 ev, with further improvements promised. These advances have required close attention to the mechanical and electrical stability of the TEM, including thermal, vibrational, and acoustical isolation. The energy-loss spectrometer has been improved with a fast electrostatic shutter, allowing millisecond acquisition of an entire spectrum and almost simultaneous recording of the low-loss and core-loss regions. Advances in computer software have made routine such processes as spectral and spatial deconvolution, spectrum-imaging, and multivariate statistical analysis. Programs for implementing density functional and multiple-scattering calculations to predict spectral fine structure have become more widely available. Taken together, these improvements have helped to ensure that EELS can be applied to real materials problems as well as model systems; the technique is no longer mainly a playground for physicists. Another consequence is that radiation damage is seen to be a limiting factor in electron beam microanalysis. One response has the development of TEMs that can achieve atomic resolution at lower accelerating voltage, in an attempt to maximize the information/damage ratio. There is also considerable interest in the use of lasers in combination with TEM-EELS, the aim being picosecond or femtosecond time resolution, in order to study excited states and perhaps even to conquer radiation damage. For the third edition of this textbook, I have kept the previous structure intact. However, the reading list and historical section of Chapter 1 have been updated. In Chapter 2, I have retained but shortened the discussion of serial recording, making room for more information on monochromator designs and new electron detectors. In Chapter 3, I have added material on energy losses due to elastic scattering, retardation and Cerenkov effects, core excitation in anisotropic materials, and the delocalization of inelastic scattering. Chapter 4 now includes a discussion v

6 vi Preface to the Third Edition of Bayesian deconvolution, multivariate statistical analysis, and the ELNES simulation. As previously, Chapter 5 deals with practical applications of EELS in a TEM, together with a discussion of factors that limit the spatial resolution of analysis, including radiation damage and examples of applications to selected materials systems. The final section gives examples of TEM-EELS study of electronic, ceramic, and carbon-based materials (including graphene, carbon nanotubes, and polymers) and the measurement of radiation damage. In Appendix A, the discussion to relativistic effects is extended to include recent theory relating to anisotropic materials and magic-angle measurements. Appendix B contains a brief description of over 20 freeware programs written in MATLAB. They include programs for first-order prism focusing, atomic-displacement cross sections, Richardson Lucy deconvolution, the Kröger formula for retardation and surface losses, and translations of the FORTRAN and BASIC codes given in the second edition. The table of plasmon energies in Appendix C has been extended to a larger number of materials and now also contains inelastic mean free paths. I have added an Appendix F that summarizes some of the choices involved in acquiring energy-loss data, with references to earlier sections of the book where these choices are discussed in greater detail. Throughout the text, I have tried to give appropriate references to topics that I considered outside the scope of the book or beyond my expertise. The reference list now contains about 1200 entries, each with an article title and page range. They are listed alphabetically by first author surname, but with multiauthor entries (et al. references in the text) arranged in chronological order. I am grateful to many colleagues for comment and discussion, including Les Allen, Phil Batson, Gianluigi Botton, Peter Crozier, Adam Hitchcock, Ferdinand Hofer, Archie Howie, Gerald Kothleitner, Ondrej Krivanek, Richard Leapman, Matt Libera, Charlie Lyman, Marek Malac, Sergio Moreno, David Muller, Steve Pennycook, Peter Rez, Peter Schattschneider, Guillaume Radtke, Harald Rose, John Spence, Mike Walls, Masashi Watanabe, and Yimei Zhu. I thank Michael Bergen for help with the MATLAB computer code and the National Science and Engineering Council of Canada for continuing financial support over the past 35 years. Most of all, I thank my wife Maia and my son Robin for their steadfast support and encouragement. Edmonton, AB, Canada Ray Egerton

7 Contents 1 An Introduction to EELS InteractionofFastElectronswithaSolid The Electron Energy-Loss Spectrum The Development of Experimental Techniques Energy-Selecting (Energy-Filtering) Electron Microscopes Spectrometers as Attachments to a TEM Alternative Analytical Methods Ion Beam Methods Incident Photons Electron Beam Techniques Comparison of EELS and EDX Spectroscopy DetectionLimitsandSpatialResolution Specimen Requirements Accuracy of Quantification EaseofUseandInformationContent Further Reading Energy-Loss Instrumentation Energy-AnalyzingandEnergy-SelectingSystems The Magnetic Prism Spectrometer Energy-Filtering Magnetic Prism Systems The Wien Filter Electron Monochromators Optics of a Magnetic Prism Spectrometer First-Order Properties Higher Order Focusing Spectrometer Designs PracticalConsiderations Spectrometer Alignment The Use of Prespectrometer Lenses TEM Imaging and Diffraction Modes EffectofLensAberrationsonSpatialResolution vii

8 viii Contents Effect of Lens Aberrations on Collection Efficiency EffectofTEMLensesonEnergyResolution STEMOptics Recording the Energy-Loss Spectrum Spectrum Shift and Scanning Spectrometer Background Coincidence Counting Serial Recording of the Energy-Loss Spectrum DQE of a Single-Channel System Serial-Mode Signal Processing Parallel Recording of Energy-Loss Data Types of Self-Scanning Diode Array Indirect Exposure Systems Direct Exposure Systems DQE of a Parallel-Recording System DealingwithDiodeArrayArtifacts Energy-SelectedImaging(ESI) Post-column Energy Filter In-Column Filters Energy Filtering in STEM Mode Spectrum Imaging Comparison of Energy-Filtered TEM and STEM Z-ContrastandZ-RatioImaging Physics of Electron Scattering ElasticScattering General Formulas Atomic Models DiffractionEffects Electron Channeling Phonon Scattering EnergyTransferinElasticScattering InelasticScattering Atomic Models Bethe Theory DielectricFormulation Solid-StateEffects Excitation of Outer-Shell Electrons VolumePlasmons Single-ElectronExcitation Excitons RadiationLosses SurfacePlasmons Surface-Reflection Spectra Plasmon Modes in Small Particles

9 Contents ix 3.4 Single, Plural, and Multiple Scattering Poisson slaw Angular Distribution of Plural Inelastic Scattering Influence of Elastic Scattering Multiple Scattering Coherent Double-Plasmon Excitation The Spectral Background to Inner-Shell Edges Valence-Electron Scattering Tails of Core-Loss Edges BremsstrahlungEnergyLosses Plural-Scattering Contributions to the Background Atomic Theory of Inner-Shell Excitation Generalized Oscillator Strength RelativisticKinematicsofScattering IonizationCrossSections The Form of Inner-Shell Edges Basic Edge Shapes DipoleSelectionRule EffectofPluralScattering ChemicalShiftsinThresholdEnergy Near-EdgeFineStructure(ELNES) Densities-of-States Interpretation Multiple-Scattering Interpretation Molecular-Orbital Theory Multiplet and Crystal-Field Effects Extended Energy-Loss Fine Structure (EXELFS) CoreExcitationinAnisotropicMaterials DelocalizationofInelasticScattering Quantitative Analysis of Energy-Loss Data Deconvolution of Low-Loss Spectra FourierLogMethod FourierRatioMethod Bayesian Deconvolution Other Methods Kramers KronigAnalysis Angular Corrections Extrapolation and Normalization Derivation of the Dielectric Function CorrectionforSurfaceLosses Checks on the Data Deconvolution of Core-Loss Data FourierLogMethod FourierRatioMethod

10 x Contents Bayesian Deconvolution Other Methods Separation of Spectral Components Least-Squares Fitting Two-Area Fitting Background-Fitting Errors Multiple Least-Squares Fitting MultivariateStatisticalAnalysis Energy- and Spatial-Difference Techniques ElementalQuantification IntegrationMethod CalculationofPartialCrossSections Correction for Incident Beam Convergence Quantification from MLS Fitting Analysis of Extended Energy-Loss Fine Structure FourierTransformMethod Curve-Fitting Procedure Simulation of Energy-Loss Near-Edge Structure (ELNES) Multiple Scattering Calculations BandStructureCalculations TEM Applications of EELS Measurement of Specimen Thickness Log-RatioMethod Absolute Thickness from the K K Sum Rule Mass Thickness from the Bethe Sum Rule Low-Loss Spectroscopy IdentificationfromLow-LossFineStructure Measurement of Plasmon Energy and Alloy Composition CharacterizationofSmallParticles Energy-Filtered Images and Diffraction Patterns Zero-Loss Images Zero-LossDiffractionPatterns Low-Loss Images Z-Ratio Images ContrastTuningandMPLImaging Core-Loss Images and Elemental Mapping Elemental Analysis from Core-Loss Spectroscopy Measurement of Hydrogen and Helium Measurement of Lithium, Beryllium, and Boron Measurement of Carbon, Nitrogen, and Oxygen MeasurementofFluorineandHeavierElements SpatialResolutionandDetectionLimits Electron-OpticalConsiderations

11 Contents xi LossofResolutionDuetoElasticScattering DelocalizationofInelasticScattering StatisticalLimitationsandRadiationDamage Structural Information from EELS Orientation Dependence of Ionization Edges Core-LossDiffractionPatterns ELNES Fingerprinting Valency and Magnetic Measurements fromwhite-lineratios UseofChemicalShifts Use of Extended Fine Structure Electron Compton (ECOSS) Measurements Application to Specific Materials Semiconductors and Electronic Devices Ceramics and High-Temperature Superconductors Carbon-Based Materials Polymers and Biological Specimens Radiation Damage and Hole Drilling Appendix A Bethe Theory for High Incident Energies and Anisotropic Materials A.1 Anisotropic Specimens Appendix B Computer Programs B.1 First-Order Spectrometer Focusing B.2 Cross Sections for Atomic Displacement andhigh-angleelasticscattering B.3 Lenz-Model Elastic and Inelastic Cross Sections B.4 SimulationofaPlural-ScatteringDistribution B.5 Fourier-Log Deconvolution B.6 Maximum-Likelihood Deconvolution B.7 Drude Simulation of a Low-Loss Spectrum B.8 Kramers KronigAnalysis B.9 Kröger Simulation of a Low-Loss Spectrum B.10 Core-LossSimulation B.11 Fourier Ratio Deconvolution B.12 Incident-Convergence Correction B.13 Hydrogenic K-Shell Cross Sections B.14 Modified-Hydrogenic L-Shell Cross Sections B.15 Parameterized K-, L-, M-, N-, and O-Shell CrossSections B.16 Measurement of Absolute Specimen Thickness B.17 TotalInelasticandPlasmonMeanFreePaths B.18 Constrained Power-Law Background Fitting Appendix C Plasmon Energies and Inelastic Mean Free Paths

12 xii Contents Appendix D Inner-Shell Energies and Edge Shapes Appendix E Electron Wavelengths, Relativistic Factors, and Physical Constants Appendix F Options for Energy-Loss Data Acquisition References Index

Transmission Electron Microscopy

L. Reimer H. Kohl Transmission Electron Microscopy Physics of Image Formation Fifth Edition el Springer Contents 1 Introduction... 1 1.1 Transmission Electron Microscopy... 1 1.1.1 Conventional Transmission