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EELS is an analytical technique that measures the change in kinetic energy of electrons after they have interacted with a specimen and lost energy due to inelastic scattering. The time-varying electric field pulses of incident beam in TEM can transfer energy to sample over a range of frequencies, from the infrared to the X-ray regime as they pass near atoms, which provides spectroscopic information about the excited atom and its bonding states from the core-level excitation of the target atom. After interacting with the specimen, the inelastic scattering is strongly peaked in the vertical direction and easily passes through the hole in the center of the ADF detector. A spectrometer can then be placed on the axis that detects electron energy loss signal without interfering that signal of ADF, making the EELS compatible with ADF geometry since the birth of STEM. The strong ADF signal is often used to image and locate the of interest areas for EELS measurements [42]. A powerful feature of EELS is that the compositional and bonding information can be visualized at high spatial resolution, where the incident beams excite a core electron to empty states above the Fermi level (E_Fermi). The core-level binding energy that marks the EELS edge onset allows the specific elemental identification, as the shape of the edge reflects the underlying local partial density of states modified by the presence of a core hole [43]. The function of the core hole is to interpret the ground-state local density of states [44]. A good example for EELS analysis is that it provided useful information to study the transistor miniaturization, where the local electronic structure of gate oxide with roughly 5–6 atoms thickness can be shown by the EELS spectrum. These atoms form a thin dielectric layer, which is expected to have very different electrical and optical properties from the desired bulk SiO₂ [43]. More importantly, this technique is suitable for mapping the spatial distribution of formal charges at interfaces for 3d transition metal edges, which has been used to analyze Ti-L edge to provide spatial distribution of conduction and their screening lengths in LaTiO₃/SrTiO₃ multilayers [45].

For a typical system of EELS spectrometer, a field emission electron gun and strong electromagnetic lenses are used to form a small probe. After interacting with the specimen, the inelastic scattering electrons enter a single-prism spectrometer to produce an energy loss spectrum for a probe detecting zone [46]. A narrow slit is then inserted at the spectrum plane to provide obstacle to scattering electrons with higher angles, giving an energy-filtered transmission electron microscope (EFTEM) image on the charge coupled device (CCD) camera. By recording a sequence of EFTEM images, the electron energy loss spectrum imaging data can be read out at each pixel. Meanwhile, the post-column magnetic prism can also produce EFTEM images, with the imaging aberrations that are corrected by quadrupole and sextuple lenses. If the incident electrons have a kinetic energy of several hundred electron volts and are reflected from the surface of the sample afterward, this is called high-resolution electron energy loss spectroscopy (HREELS). By 1986, 0.4 nm resolution composition profiles were demonstrated [47]. For comparison, X-ray absorption spectroscopy (XAS) has a resolution of approximately 30 nm if using synchrotron radiation focused by a zone plate. After 2000, TEM-based energy loss spectroscopy has undergone great development; the oxidation state of an element can be studied by the near-edge fine structure of EELS, such as the Cr within the inorganic compounds with an oxidation states between 2 and 6 [48]. The near-edge fine structure can give useful information on interatomic bonding, which produced a map showing chemical and bonding information. Further study suggested that this technique can also reduce the image noise without sacrificing spatial resolution [49]. Further improvements include the gun monochromators, which are now commercially available, making the accuracy of TEM–EELS resolution close to that of XAS (∼0.1 eV). More attention has been drawn to the low-loss region of the spectrum, which is driven by the demands of the semiconductor industry and nanotechnology initiatives. With a suitable monochromator, the resolution is possible to achieve ∼10 meV to investigate the chemical bonding and phonon modes in nanostructures, even reaching a 30 keV resolution after correction of lens aberrations [50].

To achieve an atomic resolution with EELS, several requirements must be well considered: (i) Using a high-brightness electron source or a spherical aberration corrector to make the incident beam suitable for a small intense probe and (ii) avoiding the degradation in spatial resolution (dechanneling), which is caused by the transfer of the electron probe to the adjacent atomic columns. This can be resolved with multislice simulation software to preserve the intensity of the original atomic column with a convergence semi-angle of 15 mrad and a specimen of thickness less than 50 nm. (iii) The localization in inelastic scattering, which is the major factor for EELS, has been well discussed [51, 52]. This is related to the degree of coherence in inelastic scattering, while the non-locality is considered as the uncertain region with inelastic scattering partially coherent. One solution was to conduct a small convergence angle and a large collection angle, which correspond to the experimental configuration in favor of the local approximation (as shown in Figure 2.3) [53]. This can take advantage of the small convergence angle to regulate the reciprocal area of mixed dynamic because the factor suppresses non-dipole transitions, while the large collection angle is effective in reducing the interference fringes of inelastic electrons. As obtaining the core loss images, the observation of this kind of local inhomogeneity becomes important, especially for layered perovskite manganite, La_1.2Sr_1.8Mn₂O₇ [50], because this method opens up a new way to discuss the local structure and material properties, giving a better understanding of phase transition or magnetic domain wall pinning in strongly correlated materials. (iv) The stability of probe position, where the mechanical vibration of the floor from any adjacent disturbance should be reduced, and the specimen drift during the acquisition should be measured under the ADF imaging mode, the collection of semi-angle for EELS was as large as 31 mrad to reduce the extent of delocalization. In terms of the ultimate goal to realize the quantitative chemical analysis of each atomic column, more improvement needs to overcome the dechanneling and delocalization, and the absorption in electron scattering should be evaluated for each atomic column.

One of the most practical applications for EELS is to study the adsorption and reaction of molecules on metal oxide surfaces, and it is also possible to characterize the activation of adsorbed species on defects sites, particularly for O vacancies. For instance, the nature of hydrogen adsorption on TiO₂ (110) can be studied by EELS [54]. After exposing the TiO₂ (110) surface to atomic hydrogen at high temperatures, the vibration mode of O–H disappears, while no H₂O or H₂ molecules were found to desorb from the surface, which demonstrates that the H atoms adsorbed on O-bridge diffused into the bulk rather than desorption. These findings have important consequences for chemical processes involving H atoms absorbed on the TiO₂ surfaces. Besides, CO oxidation on RuO₂ (110) has also been evaluated by EELS, where CO was bonded weakly to Ru sites while undergone either desorption or reaction with neighboring O upon heating [55]. Notably, the EELS data further reveal that oxygen-depleted at the surface after CO₂ desorption. This can be restored at the O₂ atmosphere and establishes a remarkable surface redox system. This study can help to understand the mechanism of two types of Ru atom sites, where one is twofold coordinated oxygen atoms (O-bridge) and the other is fivefold coordinated Ru atoms. Another discovery was that (0001) of ZnO, with the oxygen-terminated polar surface, can be the most active surface for methanol synthesis [56]. It is expected that EELS can provide more detailed information about the growth, the chemical reactivity, and the electronic structure of metal oxide surfaces. Especially for heterogeneous catalysis, this technique can better elucidate the microscopic reaction mechanisms under industrial conditions, by bridging the material to pressure gap thereby promoting more study in surface science.

Figure 2.3 (a) Crystal structure of layered perovskite manganite La_1.2Sr_1.8Mn₂O₇. Yellow–green spheres corresponding to A site (La and Sr), blue spheres to B sites (Mn), and red spheres to oxygen. There are two different crystallographic A sites in the perovskite block (yellow arrow p) and the rock salt layer (white arrow r). (b) ADF image of the specimen observed along the [010] direction. The areas for two-dimensional EELS and drift measurement are shown by rectangles. (c) EELS spectrum acquired from the rectangular area for the two-dimensional EELS. Source: Reproduced with permission from Kimoto et al. [53]. Copyright 2007, Nature Publishing Group.

Another study carried out by EELS, particularly with an aberration corrector, is to characterize single-atom impurities or defects. The concerns include, where those atomic species are located, how exactly these atoms are bonded to another, how structural difference influences their electronic configuration, and which one exhibits unique properties, at edges or point defects as a single atom, depending on their local bonding differences [57, 58]. Also, the fine structure of electron energy loss spectra can provide the answer to the valence state of single-dopant atoms [59]. Obviously, the developments of the EELS technique have been particularly beneficial for 2D materials such as graphene [60]. To produce very high signal-to-noise data, EELS spectra were recorded with optimized acquisition conditions, such as loading graphene with Si, 1 s of acquisition time per spectrum, and calibrating the spectrometer with acceptance semi-angle of 35 mrad. Then, two types of single Si atom substitutional defects can be observed, with threefold or fourfold coordinated Si defects (as shown in Figure 2.4) [61]. The result of EELS spectra suggested that the bonding network is quite disrupted around the fourfold Si atoms, which depicted the spatial distribution of all orbitals contributing to the electron density, while the threefold showed largely undisturbed, suggesting that the threefold Si can act as a good chemical replacement for a single carbon atom, which would then integrate by distorting the sp₂ structure to make the Si atom and its neighbors pucker slightly out of the graphene plane to accommodate the longer Si–C bonds. It should be noticed that more EELS were conducted under a low voltage of 60 keV to avoid the irradiation damage to the specimen. Another example is the detection of K (Z = 19) and Ca (Z = 20), which is of prime importance in the field of biological molecules [62, 63]. While in bright-field or ADF imaging, the elastic scattering power of a Ca atom is very weak to be detected. For single-atom characterization to give better EELS counts, a small diameter is not always beneficial; instead, an appropriately larger probe might be more efficient, which is capable of covering the overall space occupied by the fullerene and avoiding the escape of those encapsulated atoms during the timescale of the spectrum acquisition.

To further gain insights into the spectral imaging of EELS, many spectra are acquired as the electron probe is rastered across the specimen, forming a 2D spectral map [64]. The number of scanned points and the signal-to-noise ratio (SNR) of EELS imaging quality are limited by the amount of signal, instrument stability, and user’s time. This is greatly improved with the advent of aberration-corrected electron microscopes, which allow a larger probe-forming apertures and improve collection optics [65, 66]. Most often, the interested area contained in the core loss energy edges, where they appear in the EELS spectrum with a shape and energy onset uniquely defined by a specimen’s excitation of core-level electrons to the available density of states in the conduction band and modified by the core–hole interaction, it has been shown that the background often follows an inverse power law [67]. The signal is usually obtained after the background has been modeled and substracted over the edge of interest. However, atomic resolution EELS maps are often oversampled with pixel dimensions smaller than the probes’ transfer limit. This can be well solved with local background averaging (LBA) to estimate the background signal. This approach provides an improved background modeling, where its position can be averaged with those from neighboring spectra to obtain an accurate background signal at every position, and the reduced noise could enable a more reliable background fit and extrapolation, showing a dramatic improvement in the image contrast and SNR. Meanwhile, if the spectrum is taken in very large energy windows, the error in each channel is not equal, especially for backgrounds in the first few hundred electron volts of a spectrum. This can be overcome by iterative weighted least-squares approaches to incorporate the change in variance over the background to combine with LBA [68]. In general, the detection limits and SNR of images extracted from spectroscopic mapping highly depend on the signal processing methods, where the pre-edge power law background modeling can greatly affect the accuracy and range of the extrapolated background. In addition, LBA works well when the background has been spatially oversampled, avoiding the distortions of EELS fine structure.

Figure 2.4 High signal-to-noise EEL spectrum acquired by the accumulating 1 s exposures while scanning repeatedly. The insets present a 50 frame average in false color from the stacks of images created during the acquisitions, showing clear threefold coordination (a) or fourfold coordination (b) of the Si atom. Source: Reproduced with permission from Ramasse et al. [61]. Copyright 2013, American Chemical Society.

In summary, the EELS acquired by Cs-TEM can yield precise electronic structure information at the sub-nano level. Optimizing acquisition procedures would result in very high SNR data. The foreign species on a substrate and even bonding differences between dopant species would be distinguishable with electron energy loss spectroscopy. Another concept that should be emphasized in this part is that experimental configuration for atomic column imaging is not only to make a small incident probe but also to optimize the acquisition condition associated with the delocalization in elastic and inelastic scattering. The incoherent EELS imaging allows to interpret the core loss images, which are informative in material science. The observation of local inhomogeneity would endow the discussion of local crystal distortions and its unique material properties beyond the stoichiometric understanding of the average crystal structure. The development of Cs-TEM has proved its powerful function of elemental and chemical analyses of site occupancy in material microstructure characterization.