Scanning Transmission Electron Microscopy of Nanomaterials

The following sections are included:

• Preface
• Acknowledgments
• Contents
• List of Contributors
• List of Abbreviations
• List of Symbols
• For Students and Beginners
• Softwares for Simulation
• Table of Values of Related Physical Constants
• Table of Electron Wavelength

The following sections are included:

• Need for electron nanoprobe imaging
• Comparison of TEM, SEM, and STEM
• Advantages of STEM
• Application possibilities of STEM
• Brief introduction to the chapters in this book
• References

In this chapter, we present a historical survey of the development of STEM instruments. Readers should be able to answer the question, “What is STEM?” after reading this chapter and Chapter 3, which explains the physics of STEM. These chapters are a good introduction to STEM.

In this chapter we explain the probe-forming system, the basic design of STEM, the reciprocity theorem between TEM and STEM, and the imaging theory of STEM. We also give brief descriptions of advanced techniques in STEM, such as electron-energy loss spectroscopy (EELS) mapping, depth-sectioning imaging, secondary electron (SE) imaging, confocal STEM, and convergent-beam electron diffraction (CBED).

In this chapter, applications of atomic-resolution STEM to nanomaterials and biological specimens are described. Annular dark field (ADF) imaging is particularly attractive in that it produces an image contrast scaling strongly with atomic number (Z) and which, not being overly sensitive to either specimen thickness or to lens aberrations, is often amenable to direct and visual interpretation. Moreover, STEM allows for the recording of multiple types of signal simultaneously. The range of fields to which STEM is being applied continues to expand, and we cannot possibly do justice to the many applications of STEM to materials analysis in a single chapter. We will, however, endeavor to give a flavor of what is possible by reviewing some highlights in the field of nanomaterials and biological science.

Atomic-resolved high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is widely recognized as a powerful technique for direct structural and compositional analysis in nanoscience, as shown in Chapter 4. Projected atomic columns are identified by bright spots that are insensitive to the defocus value of a probe-forming lens and specimen thickness under all conditions. Using an annular darkfield (ADF) detector to collect thermal diffuse scattering (TDS) electrons at a high angle, it is found that the bright spot intensity is strongly dependent on not only the atomic number (Z) but also the number of atoms in the atomic column. It has, however, been recently reported that spherical aberration (C_s)-uncorrected HAADF-STEM images of some materials cannot be intuitively interpreted on the basis of the atomic number. In this chapter, first the theory of HAADF-STEM based on the Bloch wave method and second the various kinds of simulation results are described for understanding the image contrast.

In this chapter, a basic theory of annular bright field (ABF) STEM is described using the so-called “s-state model” based on the channeling theory for dynamical electron diffraction. The effectiveness of this method for observing complex structures and dopant atoms in crystals was already explained in Sections 3.9.5 and 4.1.3.

Electron energy-loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) is one of the standard tools of analytical electron microscopy. Owing to a small incident probe of sub-nanometer diameter, it realizes atomic resolution analysis, which is a major advantage in STEM-based techniques. In this chapter we briefly review EELS in STEM, particularly for high spatial-resolution imaging using inelastic scattering.

In this chapter, density functional theory (DFT) is described for calculation of electron energy-loss near-edge structure (ELNES) obtained in STEM-EELS. Fine structure in ELNES originates from an electron transition from a core-orbital to unoccupied bands. The spectral profile of ELNES reflects the partial density of states (PDOS) of the unoccupied bands, and can provide information about electronic structures and chemical bonding around objective atoms in materials.

A lens for electron microscopy has optical imperfections; the coefficients of spherical aberration (C_s) and chromatic aberration (C_c) are always positive in a magnetic and electrostatic round lens. In conventional instruments, the dominant geometrical aberration that limits the resolution is the third-order spherical aberration. Aberration correctors with multipoles have been practically developed since the 1990s in TEM and STEM. By using spherical aberration correctors, the geometrical limitation of resolution has been overcome. Nowadays, spherical aberration correctors are widely used in electron microscopes. Their advantages for high-resolution imaging and analytical capabilities using STEM have been demonstrated in materials science (see Chapter 4). In this chapter, aberration correctors using multipoles will be reviewed, and hexapole-type correctors are described in some detail.

The scanning electron microscope (SEM) is widely used in industry and materials sciences for observation and characterization ranging from micro- to nanometers (μm to nm) in size. The most popular operation mode of the SEM is secondary electron (SE) imaging, which is traditionally employed to explore surface morphology. While the resolution limit of SEM has been considered to be approximately 1 nm, recent SEM with a cold field emission gun (FEG) achieved an ultimate resolution of better than 0.4 nm at 30 kV. The recent incorporation of a spherical aberration (C_s) corrector to obtain sub-nanometer electron probes also contributed greatly to the improvement of atomic resolution. In this chapter, we first describe the history of SEM, the factors governing SE image resolution, and previous high-resolution work. In the last section, we detail atomically resolved SE imaging by using a STEM with a spherical aberration corrector.

Scanning confocal electron microscopy (SCEM) is an extension of scanning transmission electron microscopy (STEM) and the electron analog of scanning confocal optical microscopy (SCOM). The purpose of this method is to obtain images of a thin layer located at a certain depth in a specimen — a procedure called “depth sectioning.” Although the capability of SCEM has been hindered by aberrations of lenses used in electron microscopes, the situation is being rapidly improved by ongoing research on aberration correction. In this chapter we describe SCEM from the basic imaging theories up to recent experimental achievements.

One of the most useful observation methods using electron microscopes is electron tomography, which gives three-dimensional (3D) structural information on nanomaterials. In this chapter the basic principles of electron tomography and its application to inorganic materials are described.

In the preceding chapters we have explained image formation in bright field (BF), annular dark field (ADF), including HAADF, and annular bright field (ABF)-STEM, and their application to materials and biological science, using elastically and inelastically scattered electrons falling onto STEM detectors. There are other imaging modes in STEM, such as electron holography and Lorentz electron microscopy. These methods use the phase of electron waves. In this chapter, we give an overview of these methods, although not quite as many papers on these topics have been published.

The present book is intended to share our knowledge of scanning transmission electron microscopy (STEM), from basic to advanced level, for graduate students and researchers, in a textbook style. In order for readers to gain a good comprehension of STEM imaging, the authors have tried to explain the physics of STEM, from the basics and without omitting the mathematical formulae, as clearly as possible. On the other hand, the reference list may not fully cover all the relevant papers of scientific achievements. For that purpose, readers are recommended the book “Scanning Transmission Electron Microscopy: Imaging and Analysis” by Pennycook and Nellist (2011). This final chapter reviews the current situation of STEM, and future prospects are briefly discussed from the viewpoint of the present author. Comments and prospective ideas from readers are welcome (e-mail: n-tanaka@esi.nagoya-u.ac.jp).

The following sections are included:

• A1 Fourier Transforms for TEM and STEM
• A2 Inelastic Scattering Coefficient and Mixed Dynamical Form Factor
• A3 Ronchigram and Geometrical Aberrations in STEM
• A4 Coherent Convergent-Beam Electron Diffraction in STEM
• A5 Double Resolution in STEM and Ptychography
• A6 Multislice Method for STEM Image Simulation
• A7 Introduction to Bethe’s Dynamical Diffraction Theory
• A8 Dynamical Diffraction Theory by Van Dyck
• A9 Frozen Phonon Method for TDS Calculation in STEM Image Simulation
• A10 Relativistic Correction in Electron Microscopy

The following sections are included:

• Author Index
• Subject Index