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 second hexapole element. This long hexapole arrangement generates a combined aberration that compensates the positive spherical aberration of the objective lens through suitable hexapole excitation. Moreover, in the early 2000s, several indirect aberration compensation methods had been developed for experimental determination of the axial aberration coefficient, which mainly relied on measurements taken from image wave functions [10] and were applicable to crystalline specimens as well as thin amorphous materials. Initially, a phase correlation function (PCF) [11] was used to determine the defocus difference between neighboring images at high accuracy. The absolute focus and astigmatism are subsequently measured from the restored image wave function of a reference image using a phase contrast index (PCI) function. This provided estimates of the coefficients of the wave aberration function, which can then recover both phase and modulus of the specimen exit wave function under either linear or nonlinear imaging, also enhancing resolution to complement direct aberration correction. After that, indirect and direct approaches have been combined [12], for a focal series data set, and the elimination of tilt-induced axial coma relaxes the requirement of using parallel illumination and enables the illumination to be converged onto the specimen area of interest, which reduced delocalization of image components in the electron optically corrected image. Also, localized compensation of higher order aberrations up to the fifth order was possible. Notably, for modern electron microscope, the Cs-TEM is equipped with double correctors. Taking JEM-2200FS as an example, when only HRTEM data are required, the upper corrector is switched off and the condenser system of the microscope can be used as normal. A small voltage is applied to one of the hexapoles in the upper corrector to compensate for a residue threefold astigmatism arising from the gun lens or any residue field from the first hexapole. When switching to STEM imaging, a small adjustment to transfer lenses in the upper corrector is possible which allows broad parallel illumination to be achieved satisfactorily with both upper hexapoles strongly excited. The correction of both pre- and post-field allows the use of a large condenser and objective aperture sizes [13].
Nowadays, as the aberration-corrected probe is much sharper and much more intense, atomic-scale microanalysis can be realized. However, for small negative-Cs HRTEM imaging, it is applicable only to very thin samples with a thickness of typically a few nanometers. On the other hand, the Cs-corrected high-angle ADF STEM imaging, using incoherent thermal diffuse scattering (TDS) electrons as the main signal sources, caused the signal of light atomic columns easily drowned out when they are located adjacent to the heavy atomic columns. This is attributed to the large difference in scattering power for specific TDS in different elements. Then, the annular bright-field (ABF) STEM realized a visualization of both light and heavy atomic column. This is due to the reduced effect of spatial coherence, which is Cs-dependent. This means that much larger bright-filed (BF) collection angles are allowed without loss of resolution. Thus, the bright-filed scanning transmission electron microscopy (BF-STEM) with aberration-corrected technique of high signal quality can be recorded, with the atomic columns identified by dark spots, which are independent of probe forming lens defocus and sample thickness [14]. Also, ABF can collect electrons with small angles relative to the direction of the incident electron beam to resolve atomic columns (Z) with high contrast [15]. The BF-STEM image is somewhat similar to an HRTEM image when a circular BF detector adopting a small collection angle that is close to an on-axis point detector, making it sensitive to light atomic columns. Nevertheless, the concern for this ABF-STEM is that the signal of light atomic columns is still weak, even though it is a breakthrough toward visualizing light atomic columns compared to those in the dark field. In the case of TEM, image delocalization at surfaces and interfaces is greatly reduced because of the highly coherent electron source. For this corrected TEM imaging, an additional benefit is the visibility of low-Z elements such as oxygen,