Springer Handbook of Microscopy

Book Preface

his chapter provides an overview of the essential theory and instrumentation relevant to high-resolution imaging in the transmission electron microscope together with selected application examples. It begins with a brief historical overview of the field. Subsequently, the theory of image formation and resolution limits are discussed. We then discuss the effects of the objective lens through the wave aberration function and coherence of the electron source. In the third section, the key instrument components important for HRTEM imaging are discussed; namely, the objective lens, electron sources and monochromators, energy filters and detectors. The theory and experimental implementation of exit wavefunction reconstruction from HRTEM images is detailed in the fourth section, including examples taken from studies of complex oxides. The final section treats the simulation of HRTEM images with particular reference to the widely adopted multislice method.

High-resolution transmission electron microscopy (HRTEM) uses a self-supporting thin sample (typically tens of nanometers in thickness) illuminated by a highly collimated electron beam at energies of between 30 keV and 1 MeV. A series of magnetic electron lenses image the electron wavefield at the exit face of the sample onto a detector at high magnification. HRTEM has evolved from initial instrumentation constructed by Knoll and Ruska [1.1–3] to its current state where individual atom columns in a wide range of materials and orientations can be routinely imaged using sophisticated computer-controlled microscopes (Fig. 1.1). For this reason HRTEM is integral to char-acterization of materials and modern instrumentation now occupies a central place in many laboratories worldwide and has made a substantial contribution to key areas of materials science, physics and chemistry, and instrument development for HRTEM also supports a substantial commercial industry of manufacturers [1.4–7]. HRTEM has also made substantial contributions to structural biology for which the 2017 Nobel prize in Chemistry was awarded (see [1.8–13] for reviews of selected representative examples from this field; see also Chap. 4 in this volume). However, because of limitations of space we will not consider this aspect further herein.
Numerous HRTEM studies of bulk semiconductors [1.14–18], defects (Fig. 1.2) [1.19, 20] and interface structures (Fig. 1.3) [1.21, 22] in these materials, of metals and alloys [1.23–26], and of ceramics, particularly oxides [1.27–30], have been reported in a vast literature spanning several decades (for additional gen-Fig. 1.1 A modern 30
300 kV HRTEM fitted with a cold field-emission gun, probe- and image-forming aberration correctors, and a range of digital detectors for HRTEM and STEM (scanning transmission electron microscopy) and configured for full remote operation
eral reviews see [1.31–38]). An excellent collection of representative HRTEM images can be found in [1.31, 39, 40]. More recently HRTEM has become an essential tool in the characterization and discovery of nanoscale (Fig. 1.4)[1.41, 42] and most recently low dimensional materials, particularly graphene [1.43–45] (Fig. 1.5) largely facilitated by the availability of low-voltage high-resolution instruments.
Finally, we note that HRTEM and in particular the development of in situ capabilities (see later) has made a substantial contribution to the study of heterogeneous catalysts [1.46, 47].
Of crucial importance to its success in all of these areas is the ability of HRTEM to provide real-space images of the atomic configuration at localized structural irregularities and defects in materials, that are inaccessible to broad-beam bulk diffraction methods and which largely control their properties.
Advances in instrumentation for HRTEM over the same timescale have enabled this information to be recorded with increasing resolution and precision (see Chap. 12 in this volume) leading to improvements in the quantification of the data obtained.

This chapter concentrates on HRTEM at atomic resolution. Following a brief historical overview of the development of HRTEM (for more detailed articles outlining some of the key events in the broad history of electron microscopy, see [1.5, 7, 48, 49]) we begin by outlining some of the theory pertinent to image formation at high resolution and the effects on recorded images of the aberrations introduced by imperfect objective optics. We also provide various definitions of resolution.
The second section surveys the key instrumental components affecting HRTEM and provides an outline of currently available solutions. The final section describes computational approaches to both the recovery of the specimen exit-plane wavefunction (coherent detection) from a series of images and methods available for HRTEM image simulation.