Scanning Transmission Electron Microscopy

The development of STEM is outlined from the first developments by Baron Manfred von Ardenne, through the first successful field emission gun STEM by Albert Crewe and his collaborators, to its widespread application today in the era of aberration correction. The review focuses on the development and understanding of incoherent imaging and electron energy loss spectroscopy at atomic resolution and will not include details on microanalysis, low loss imaging, or specialized modes such as cathodoluminescence. Although it attempts to cover all the major advances in approximately chronological order, undoubtedly there are omissions and an overemphasis on developments that the author is most familiar with from his own history.

The principles underlying imaging in the scanning transmission electron microscope are described. Particular focus is made on bright-field and annular dark-field imaging modes to illustrate the difference between coherent and incoherent imaging. In the case of annular dark-field imaging, the effects of dynamical diffraction and thermal diffuse scattering are discussed. The extension to three-dimensional imaging by optical sectioning is included, with particular reference to resolution limits and the bounds of transfer.

The electron Ronchigram is a form of inline hologram that offers a convenient way to directly see and measure electron optical aberrations. Any user of an aberration-corrected STEM is likely to benefit from a basic understanding of how such an image is formed and used. This chapter will review the formation of the electron Ronchigram with a particular emphasis on the effects and measurement of aberrations. This review will be largely based on our own approach and previously published work.

In this chapter, we present the basis of spatially resolved electron energy-loss spectroscopy (EELS). We mainly focus on the spectrum-imaging (SPIM) technique. After summarising the information found in an EELS spectrum, the instrumentation and analysis techniques relevant to the SPIM are thoroughly discussed. Finally, applications involving a broad range of energy losses, typically 1–1000 eV, are discussed, in order to illustrate the whole field of scientific domains which is thus opened.

One of the main advantages of scanning transmission electron microscopy (STEM) is the capability of recording a number of signals at the location of the electron beam, including characteristic X-rays and the measurement of the distribution of energy lost by the primary electron beam. Due to their importance in materials research, the use of these two techniques, known in general as “analytical electron microscopy,” has been the topic of extended reviews and monographs (Botton 2008, Joy et al. 1986, Sigle 2005, Williams and Carter 1996). In general these techniques are used, primarily, to extract local information on the composition of the sample with a resolution limited in part by the delocalization of the signal due to the long-range interaction discussed in Chapter 6 of this book and in part by the broadening of the beam due to the sample thickness. In this chapter, we will focus on the particular subset of analytical signals that allow the extraction of information on the chemical environment of the atoms probed by the fast primary electron beam. Such information is mainly provided in STEM by Energy Loss Near Edge Structures.

Recently developed aberration correctors have brought significant improvements in materials characterization. In aberration-corrected scanning transmission electron microscopy (STEM), the incident probe dimensions can be refined significantly, and image resolution has already reached sub-angstrom levels in high-angle annular dark-field (HAADF) STEM imaging (Batson et al. 2002, Nellist et al. 2004). In addition, materials characterization at the atomic level can routinely be performed by electron energy-loss spectrometry (EELS) in aberration-corrected STEM (e.g. Varela et al. 2005). The aberration correction of the incident beam is also very useful for X-ray energy-dispersive spectrometry (XEDS) because the spatial resolution can be dramatically improved with the refined probe (Watanabe et al. 2006).

We discuss how electron tomography can be implemented using the STEM imaging mode and how 3D reconstructions can be obtained from nanoscale objects. The theory of tomographic reconstruction and how the STEM HAADF signal is ideal for tomographic acquisition are described. Practical issues are highlighted including the acquisition of tilt series, image alignment and choice of reconstruction algorithm. The visualisation and analysis of the reconstruction is also discussed. Problems and advantages specific to STEM tomography are highlighted and a number of examples of STEM tomography across the field of materials science are presented.

This chapter describes electron nanodiffraction and its use for structural imaging and analysis. The chapter is organized into five sections. The first section contains an introduction to electron nanodiffraction and diffraction imaging and the motivations for developing these techniques. Section 9.2 covers different electron nanodiffraction techniques and their implementation. It is followed by a discussion on the information that can be obtained from electron nanodiffraction and the theory behind it. In outlining the theory for electron nanodiffraction in Section 9.3, we first rely on the kinematical theory to give a conceptual foundation for electron nanodiffraction. The treatment of the kinematical theory is then followed by the well-developed electron multiple scattering theory for CBED of medium to thick crystals. The kinematical theory can be used for both crystalline and noncrystalline atomic structures, while the theory for CBED is largely limited to crystals or crystals with certain types of defects. For the treatment of electron multiple scattering of arbitrary structures, we included a description of the multi-slice method, which is a numerical method often used for electron diffraction simulations. The description of the electron nanodiffraction techniques in Section 9.2 and theory in Section 9.3 are followed by a few application examples in Section 9.4. We focus on the applications of SEND for imaging strain, nanostructures, and defects. The application examples given here serve to highlight the potential of SEND rather than a complete review of electron nanodiffraction applications. We also provide the experimental details in Section 9.4 to give some ideas of what it takes to do SEND. The chapter is finished with a conclusion.

The success of aberration correction in the scanning transmission electron microscope (STEM) is revolutionizing the study of complex oxide materials, especially transition metal oxides. These are fascinating systems that exhibit the most disparate physical behaviors such as colossal magnetoresistance, orbital and/or charge ordering, magnetoelectronic phase separation or high Tc superconductivity to just name a few. Thanks to their relatively large lattice parameters and the fact that both O and transition metals exhibit absorption edges well within the reach of modern electron energy loss spectrometer (EELS) optics, they are ideal systems for such types of electron microscopy studies. Since many of the aforementioned phenomena exhibit characteristic length scales in the nanometer regime, they are affected by reduced dimensionality (e.g., thin films or heterostructures), proximity to other materials, or depend on nanometric active regions (e.g., defects, interfaces, etc.). Understanding such phenomena must therefore rely heavily on probes capable of studying simultaneously the structure, chemistry and electronic properties with atomic resolution in real space. In this chapter we will review a number of applications of aberration corrected STEM-EELS to transition metal oxides, mainly those with the perovskite structure. We will go over the current state-of-the-art of the techniques, capabilities, sensitivity and interpretation of measurements and apply this knowledge to the study of bulk and nanoscale systems, thin films and interfaces based on materials such as cuprates, titanates, manganites and cobaltites.

Grain boundaries and interfaces of ceramics have peculiar atomic and electronic structures, caused by the disorder in periodicity, providing the functional properties, which cannot be observed in a perfect crystal. These structures are also influenced by the grain boundary and interface characters such as misorientation angle, grain boundary plane, lattice misfit and so on. In the vicinity of the grain boundaries and interfaces around the order of 1 nanometer, dopants or impurities are often segregated, and they play a crucial role in the material properties. STEM utilizing the Cs corrector enables us to identify the atomic columns and the location of the dopants on the grain boundaries and interfaces. In this chapter, after reviewing the general concept for grain boundary and interface, the latest results obtained by STEM are shown for low angle grain boundaries (dislocation boundary), coincidence site lattice (CSL) grain boundaries, grain boundary segregation, amorphous grain boundary, coherent and incoherent hetero-interface structure in various ceramics. In addition, new STEM techniques including annular bright field (ABF) is demonstrated for directly observing light elements which are main constituent components in ceramics. Several STEM images are analyzed to understand the structure-property relationship by the first principles calculation based on the observation results.

In this chapter, we discuss the application of scanning transmission electron microscopy in the field of semiconductor research, using specific examples from the literature.

Heterogeneous catalysts are an important set of materials that increase the rate of chemical reactions. The increase in the reaction rate can be many orders of magnitude and depends on the degree to which the activation energy of the reaction can be lowered. Heterogeneous catalysts have traditionally played major roles in fields such as fuel processing, chemical synthesis and polymer production (van Santen et al. 1999). More recently they have played increasing roles in the environmental technology, energy production and materials synthesis. The field is undergoing a significant expansion and it is widely recognized that, for substantial progress, it is necessary to develop an atomic-level understanding of the interaction between the catalyst and the reactants/product in order to design novel catalytic materials that can address the problems of the 21 st century.

Quasicrystals represent aperiodically ordered form of solids with symmetries long thought forbidden in nature. Since their discovery, the fundamental key question has been “Where are the atoms?” For the last decade major strides have been made in determining atomic structure of quasicrystals, largely by direct imaging using scanning transmission electron microscopy (STEM). In this chapter, we describe effective use of annular dark-field STEM imaging for structural analysis of quasicrystals.

The main goal of this chapter is to introduce the concept of “low-loss” or “valence loss” electron energy loss spectroscopy (VEELS) in the STEM. Much of the discussion will assume that the microscope is aligned to form the optimum probe size (as described in other chapters in this book) with only special attention being drawn to the monochromator and how its use modifies the electron optics of the microscope (i.e., how the probe is formed). VEELS is traditionally described by energy loss processes that are seen in the 0–50 eV region of the spectrum (Figure 16–1) and processes that are typically characterized as collective excitations. These collective oscillations can provide key insights into optical and electronic properties that are fundamentally different from the composition and structure information that is typically extracted from core-loss spectra. Here we will provide a basic physical model for these collective excitations that allows materials properties to be interpreted from experimental spectra acquired in the STEM.

Atomic column-resolved electron energy-loss spectroscopy (EELS) in combination with Z-contrast imaging in a scanning transmission electron microscope (STEM) has become a very popular approach for characterizing the atomic and electronic structures of interfaces and defects in a wide range of solid-state materials. However, while the development of aberration correction in electron microscopes now allows for sub-Å spatial resolution to be achieved, many of these high-resolution experiments are currently limited to the ambient environment inside the microscope column. Yet, there is an increased interest in the areas of catalyst and functional oxide research to utilize in situ heating and cooling experiments with high spatial resolution. In this chapter, we will describe recent advances in atomic-resolution variable temperature EELS and discuss the required setup and techniques for achieving high-resolution variable temperature Z-contrast imaging and EELS. In particular, we will concentrate on three examples where in situ heating and cooling experiments in the temperature range between 10 and 700 K have been crucial to understanding the magnetic and electronic transport properties of functional oxide materials. We will also discuss some of the limitation of current heating/cooling sample holder technologies for high-resolution Z-contrast imaging and EELS.

Fluctuation electron microscopy is a technique for measuring nanoscale order in amorphous materials. Implementing fluctuation microscopy using electron nanodiffraction in a STEM has significant advantages, including improved coherence and a high degree of flexibility in the probe forming optics. Here we review the fluctuation microscopy technique in the STEM, including theory, practice, and example applications.