R.F. Egerton

Electron Energy‐Loss Spectroscopy in the Electron Microscope

Chapter 1. An Introduction to EELS 1.1. Interaction of Fast Electrons with a Solid 1.2. The Electron Energy-Loss Spectrum 1.3. The Development of Experimental Techniques 1.3.1. Energy-Selecting (Energy-Filtering) Electron Microscopes 1.3.2. Spectrometers as Attachments to Electron Microscopes 1.4. Alternative Analytical Methods 1.4.1. Ion-Beam Methods 1.4.2. Incident Photons 1.4.3. Electron-Beam Techniques 1.5. Comparison of EELS and EDX Spectroscopy 1.5.1. Detection Limits and Spatial Resolution 1.5.2. Specimen Requirements 1.5.3. Accuracy of Quantification 1.5.4. Ease of Use and Information Content 1.6. Further Reading Chapter 2. Energy-Loss Instrumentation 2.1. Energy-Analyzing and Energy-Selecting Systems 2.1.1. The Magnetic-Prism Spectrometer 2.1.2. Energy-Filtering Magnetic-Prism Systems 2.1.3. The Wien Filter 2.1.4. Electron Monochromators 2.2. Optics of a Magnetic-Prism Spectrometer 2.2.1. First-Order Properties 2.2.2. Higher-Order Focusing 2.2.3. Spectrometer Sesigns 2.2.4. Practical Considerations 2.2.5. Spectrometer Alignment 2.3. The Use of Prespectrometer Lenses 2.3.1. TEM Imaging and Diffraction Modes 2.3.2. Effect of Lens Aberrations on Spatial Resolution 2.3.3. Effect of Lens Aberrations on Collection Efficiency 2.3.4. Effect of TEM Lenses on Energy Resolution 2.3.5. STEM Optics 2.4. Recording the Energy-Loss Spectrum 2.4.1. Spectrum Shift and Scanning 2.4.2. Spectrometer Background 2.4.3. Coincidence Counting 2.4.4. Serial Recording of the Energy-Loss Spectrum 2.4.5. DQE of a Single-Channel System 2.4.6. Serial-Mode Signal Processing 2.5. Parallel Recording of Energy-Loss Data 2.5.1. Types of Self-Scanning Diode Array 2.5.2. Indirect Exposure Systems 2.5.3. Direct Exposure Systems 2.5.4. DQE of a Parallel-Recording System 2.5.5. Dealing with Diode Array Artifacts 2.6. Energy-Selected Imaging (ESI) 2.6.1. Post-Column Energy Filter 2.6.2. In-Column Filters 2.6.3. Energy Filtering in STEM Mode 2.6.4. Spectrum-Imaging 2.6.5. Elemental Mapping 2.6.6. Comparison of Energy-Filtered TEM and STEM 2.6.7. Z-Contrast and Z-Ratio Imaging Chapter 3. Physics of Electron Scattering 3.1. Elastic Scattering 3.1.1. General Formulas 3.1.2. Atomic Models 3.1.3. Diffraction Effects 3.1.4. Electron Channeling 3.1.5. Phonon Scattering 3.1.6. Energy Transfer in Elastic Scattering 3.2. Inelastic Scattering 3.2.1. Atomic Models 3.2.2. Bethe Theory 3.2.3. Dielectric Formulation 3.2.4. Solid-State Effects 3.3. Excitation of Outer-Shell Electrons 3.3.1. Volume Plasmons 3.3.2. Single-Electron Excitation 3.3.3. Excitons 3.3.4. Radiation Losses 3.3.5. Surface Plasmons 3.3.6. Surface-Reflection Spectra 3.3.7. Plasmon Modes in Small Particles 3.4. Single, Plural, and Multiple Scattering 3.4.1. Poissons Law 3.4.2. Angular Distribution of Plural Inelastic Scattering 3.4.3. Influence of Elastic Scattering 3.4.4. Multiple Scattering 3.4.5. Coherent Double-Plasmon Excitation 3.5. The Spectral Background to Inner-Shell Edges 3.5.1. Valence-Electron Scattering 3.5.2. Tails of Core-Loss Edges 3.5.3. Bremsstrahlung Energy Losses 3.5.4. Plural Scattering Contributions to the Background 3.6. Atomic Theory of Inner-Shell Excitation 3.6.1. Generalized Oscillator Strength 3.6.2. Relativistic Kinematics of Scattering 3.6.3. Ionization Cross Sections 3.7. The Form of Inner-Shell Edges 3.7.1. Basic Edge Shapes 3.7.2. Dipole Selection Rule 3.7.3. Effect of Plural Scattering 3.7.4. Chemical Shifts in Threshold Energy 3.8. Near-Edge Fine Structure (ELNES) 3.8.1. Densities-of-States Interpretation 3.8.2. Multiple-Scattering Interpretation 3.8.3. Molecular-Orbital Theory 3.8.4. Multiplet and Crystal-Field Effects 3.9. Extended Energy-Loss Fine Structure (EXELFS) 3.10. Core Excitation in Anisotropic Materials 3.11. Delocalization of inelastic Scattering Chapter 4. Quantitative Analysis of Energy-Loss Data 4.1. Deconvolution of Low-Loss Spectra 4.1.1. Fourier-Log Method 4.1.2. Fourier-Ratio Method 4.1.3. Bayesian Deconvolution 4.1.4. Other Methods 4.2. Kramers-Kronig Analysis 4.3. Deconvolution of Core-Loss Data 4.3.1. Fourier-Log Method 4.3.2. Fourier-Ratio Method 4.3.3. Bayesian Deconvolution 4.3.4. Other Methods 4.4. Separation of Spectral Components 4.4.1. Least-Squares Fitting 4.4.2. Two-Area Fitting 4.4.3. Background-Fitting Errors 4.4.4. Multiple Least-Squares Fitting 4.4.5. Multivariate Statistical Analysis 4.4.6. Energy- and Spatial-Difference Techniques 4.5. Elemental Quantification 4.5.1. Integration Method 4.5.2. Calculation of Partial Cross Sections 4.5.3. Correction for Incident-Beam Convergence 4.5.4. Quantification from MLS Fitting 4.6. Analysis of Extended Energy-Loss Fine Structure 4.6.1. Fourier-Transform Method 4.6.2. Curve-Fitting Procedure 4.7. Simulation of Energy-Loss Near-Edge Structure (ELNES) 4.7.1. Multiple-Scattering Calculations 4.7.2. Band-Structure Calculations Chapter 5. TEM Applications of EELS 5.1. Measurement of Specimen Thickness 5.1.1. Log-Ratio Method 5.1.2. Absolute Thickness from the K-K Sum Rule 5.1.3. Mass-Thickness from the Bethe Sum Rule 5.2. Low-Loss Spectroscopy 5.2.1. Identification from Low-Loss Fine Structure 5.2.2. Measurement of Plasmon Energy and Alloy Composition 5.2.3. Characterization of Small Particles 5.3. Energy-Filtered Images and Diffraction Patterns 5.3.1. Zero-Loss Images 5.3.2. Zero-Loss Diffraction Patterns 5.3.3. Low-Loss Images 5.3.4. Z-Ratio Images 5.3.5. Contrast Tuning and MPL Imaging 5.3.6. Core-Loss Images and Elemental Mapping 5.4. Elemental Analysis from Core-Loss Spectroscopy 5.4.1. Measurement of Hydrogen and Helium 5.4.2. Measurement of Lithium, Beryllium, and Boron 5.4.3. Measurement of Carbon, Nitrogen, and Oxygen 5.4.4. Measurement of Fluorine and Heavier Elements 5.5. Spatial Resolution and Detection Limits 5.5.1. Electron-Optical Considerations 5.5.2. Loss of Resolution due to Elastic Scattering 5.5.3. Delocalization of Inelastic Scattering 5.5.4. Statistical Limitations and Radiation Damage 5.6. Structural Information from EELS 5.6.1. Orientation Dependence of Ionization Edges 5.6.2. Core-Loss Diffraction Patterns 5.6.3. ELNES Fingerprinting 5.6.4. Valency and Magnetic Measurements from White-Line Ratios 5.6.5. Use of Chemical Shifts 5.6.6. Use of Extended Fine Structure 5.6.7. Electron-Compton (ECOSS) Measurements 5.7. Application to Specific Materials 5.7.1. Semiconductors and Electronic Devices 5.7.2. Ceramics and High-Temperature Superconductors 5.7.3. Carbon-Based Materials 5.7.4. Polymers and Biological Specimens 5.7.5. Radiation Damage and Hole Drilling Appendix A. Bethe Theory forHigh Incident Energies and Anisotropic Materials Appendix B. Computer Programs B.1. First-Order Spectrometer Focusing B.2. Cross Sections for Atomic Displacement and High-Angle Elastic Scattering B.3. Lenz-Model Elastic and Inelastic Cross Sections B.4. Simulation of a Plural-Scattering Distribution B.5. Fourier-Log Deconvolution B.6. Maximum-Likelihood Deconvolution B.7. Drude Simulation of a Low-Loss Spectrum B.8. Kramers-Kronig Analysis B.9. Kroger Simulation of a Low-Loss Spectrum B.10. Core-Loss Simulation 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-, N-, N- and O-shell Cross Sections B.16. Measurement of Absolute Specimen Thickness B.17. Total-Inelastic and Plasmon Mean Free Paths B.18. Constrained Power-Law Background Fitting Appendix C. Plasmon Energies and Inelastic Mean Free Paths Appendix D. Inner-Shell Energies and Edge Shapes Appendix E. Electron Wavelengths and Relativistic Factors Physical Constants Appendix F. Options for Energy-Loss Data Acquisition References Index

Electron energy-loss spectroscopy

Solid-state scientists have many analytical techniques to choose from. But electron energy-loss spectroscopy does some things that no other technique can quite match. Electron energy-loss spectroscopy can, for example, form chemical maps of nanometre-sized regions in solid samples. It can be used to characterize interfaces buried deep in samples and to identify trace elements in biological specimens. It can even look at an individual row of atoms in a crystal and identify the type of atoms and their bonding states.

Vibrational spectroscopy in the electron microscope

Vibrational spectroscopies using infrared radiation, Raman scattering, neutrons, low-energy electrons and inelastic electron tunnelling are powerful techniques that can analyse bonding arrangements, identify chemical compounds and probe many other important properties of materials. The spatial resolution of these spectroscopies is typically one micrometre or more, although it can reach a few tens of nanometres or even a few ångströms when enhanced by the presence of a sharp metallic tip. If vibrational spectroscopy could be combined with the spatial resolution and flexibility of the transmission electron microscope, it would open up the study of vibrational modes in many different types of nanostructures. Unfortunately, the energy resolution of electron energy loss spectroscopy performed in the electron microscope has until now been too poor to allow such a combination. Recent developments that have improved the attainable energy resolution of electron energy loss spectroscopy in a scanning transmission electron microscope to around ten millielectronvolts now allow vibrational spectroscopy to be carried out in the electron microscope. Here we describe the innovations responsible for the progress, and present examples of applications in inorganic and organic materials, including the detection of hydrogen. We also demonstrate that the vibrational signal has both high- and low-spatial-resolution components, that the first component can be used to map vibrational features at nanometre-level resolution, and that the second component can be used for analysis carried out with the beam positioned just outside the sample—that is, for ‘aloof’ spectroscopy that largely avoids radiation damage.

K-shell ionization cross-sections for use in microanalysis

A hydrogenic model with Zener screening constant is used to calculate generalized oscillator strengths of K-shell ionization, the optical values showing good agreement with X-ray absorption data. Energy-differential, partial, integral and total K-shell cross-sections are computed using the Born approximation for elements of low atomic number hombarded by electrons of energy >30 keV. The results appear to be consitent with Auger, X-ray and energy-loss measurements. The calculations can be performed rapidly using a short computer program, enabling quantitative results to be obtained from electron energy loss spectrometry.

Formulae for light-element micro analysis by electron energy-loss spectrometry

Simple expressions are derived for elemental concentration in terms of quantities measurable from the energy-loss spectrum and suitable cross sections for inner-shell excitation. The derivation takes into account the predominant scattering processes occurring in a thin sample: elastic and quasi-elastic scattering and valence-electron excitation. Consideration is given to the choice of specimen orientation and thickness, the detection angle and energy window, from the point of view of accuracy of the simple formulae and other factors. The requirements for displaying chemical information in an energy-filtered image are discussed briefly.

Fabrication of submicrometer regular arrays of pillars and helices

We use a glancing-angle deposition technique to produce regular lattices of submicrometer pillars and helices with a two-dimensional lattice constant below 1 μm and structure heights of 0.5–10 μm. Possible applications of such structures include photonic crystals and magnetic-storage media.

Control of radiation damage in the TEM

The problem of electron-beam damage in the transmission electron microscope is reviewed, with an emphasis on radiolysis processes in soft materials and organic specimens. Factors that determine the dose-limited resolution are identified for three different operational modes: bright-field scattering-contrast, phase-contrast and dark-field microscopy. Methods of reducing radiation damage are discussed, including low-dose techniques, cooling or encapsulating the specimen, and the choice of imaging mode, incident-beam diameter and incident-electron energy. Further experiments are suggested as a means of obtaining a better understanding and control of electron-beam damage.

Physical Principles of Electron Microscopy

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Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures (as well as from solution) yielded significant variations in morphology, resulting in different optical-absorption spectra and tran...

Observations of the microscopic growth mechanism of pillars and helices formed by glancing-angle thin-film deposition

We have studied the mechanisms influencing growth of thin films onto an oblique rotating substrate by cross-sectional transmission electron microscopy and scanning electron microscopy. We have analyzed the growth of pillars and helices in random and regular arrays, and have examined the influence of introducing a line of missing nuclei on the growth of regular array of pillars and helices.