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2 Local Structure and Electronic State of Amorphous Nanomaterials
2.1 Spherical Aberration-Corrected Transmission Electron Microscopy
2.1.1 Introduction
With the recent developments in hardware attachments to achieve both probe- and image-corrected microscope geometries so as to obtain a sub-Angstrom resolution, the spherical aberration-corrected transmission electron microscopy (Cs-TEM) is nowadays recognized as a powerful tool for study condensed matter physics and materials science. These advantages are meeting the growing demand of nanosciences and nanotechnology for atomic-scale characterization of materials and also for inspiring people to explore the expected functions of nanosynthesized products and devices. The instrumentation developments have played an important role in advancing the imaging and analytical capability, bringing both opportunities and challenges for the electron microscopy community. In addition, when equipped with electron energy filters and electron energy loss spectrometers, this advanced instrument could study not only morphology of microstructures but also their elemental composition and chemical bonding. The application of atomic resolution imaging and spectroscopy has rapidly expanded into many scientific areas to investigate many different types of semiconductors, metals, oxides, ceramics, and even a single-dopant atom.
In this chapter, we will first review the history and introduce the working mechanism of Cs-TEM, where two important imaging modes, high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) will be described throughly. Next, we will discuss how electron energy loss spectroscopy (EELS) works with Cs-TEM to study the adsorption and reaction of molecules on metal oxide surface and to characterize the single-atom impurities or defects, which are important issues in heterostructure catalyst study. In the end, we will discuss the application of in situ transmission electron microscopy (TEM) that involves various stimuli to nanomaterials with high-resolution imaging and spectroscopy, for instance, in studying reaction mechanisms in metal-ion batteries, gas-phase reactions, and the phase transformation from amorphous to crystalline under electron beam irradiation.
2.1.2 Spherical Aberration-Corrected Transmission Electron Microscopy
One of the biggest challenges in increasing resolution of the electron microscopy image has always been the improvement of image blurring that is caused by lens aberration. Even today, 80 years after the invention of TEM, the point resolution is still limited by the spherical aberration of its objective lens. In addition, the solution to improve this was to insert a numerical correction for this spherical aberration that was based either on a series of object images taken under variable objective focus conditions [1] or on the application of holograhpic technologies [2]. To improve the resolution of microscope, the phase-shifting effects of spherical aberration and other imaging defects that originated from second-, third-order objective lens astigmatism and misalignment coma need to be removed. The introduction of the aberration corrector then allows the microscopes to reach their technique limit, which is determined either by envelope functions due to beam divergence (spatial coherence) or focal spread (temporal coherence) or by incoherent effects, such as mechanical vibrations or stray magnetic fields. With the achievement of an aberration correction up to fifth order, microscope information limits have been further improved.
When reviewing the history of improving the resolution of TEM, the earliest work reported by Scherzer [3] suggested that the two principal axial aberrations, chromatic and spherical, could be corrected by electrostatic or magnetic multipole elements; however, this was beyond the technology available at that time. The spatial resolution in a TEM was limited to about 2 Å at that time with an energy of 100–200 keV. Scherzer also pointed out three possible ways to correct the limiting aberration: (i) the use of non-round lenses, (ii) the use of lenses with charge on axes, and (iii) the use of a time-varying field. These ideas inspired many attempts to develop a practical aberration corrector for the TEM. In the late 1960s, Crewe et al. [4] introduced an alternative to the TEM imaging geometry, where they used a very small electron probe that was scanned in a raster over the research area, called STEM. Until then, STEM has become an important technique because the generated signals can be used to scan the area of interests. In particular, annular dark-field (ADF) imaging, using high-angle elastic scattering which occurs near individual atoms [5], has emerged as a new imaging technique. Compared with conventional TEM imaging, the advantages of ADF STEM were that (i) the spatial resolution was better than that of the TEM mode, (ii) it is sensitive to the atomic numbers of the imaged atoms, and (iii) it provided a positive definite transfer of specimen spatial frequencies, which allowed a direct interpretation of results with fewer ambiguities. Meanwhile, the STEM was also compatible with other analytical techniques, including EELS [6], which was possible to collect atomic information about atomic species, bonding environment, and local electronic structures [7, 8].
The path toward realizing the hardware suitable for direct aberration correction at TEM was long and arduous. To fulfill an atomic resolution in electron microscopy, in the 1990s, Rose [9] proposed a solution based on two electromagnetic hexapoles and four additional lenses. The correction was achieved as the primary aberrations of second order from the first hexapole which are compensated by the
