direction of the spontaneous polarization, which can be directly inferred from the local atom displacements. The shifts of the oxygen atoms out of the Ti/Zr‐ato row can be seen directly, as well as the change of the Ti/Zr‐to‐Pb separation"/>
Figure 2.1 Transversal inversion polarization domain wall in ferroelectric PZT. Arrows give the direction of the spontaneous polarization, which can be directly inferred from the local atom displacements. The shifts of the oxygen atoms (blue circles) out of the Ti/Zr-ato row (red circles) can be seen directly, as well as the change of the Ti/Zr-to-Pb (yellow circles) separation. Source: Reproduced with permission from Jia et al. [22]. Copyright 2008, Nature Publishing Group.
The studies of aberration-corrected electron microscopy are more frequently reported in scientific literature. Indeed, people are now able to see the complexities of structure and chemistry at the atomic scale never before, enabling a better understanding of reaction and transformation pathways that fabricated desirable materials and making new devices with enhanced properties. The improvement in multiple corrector systems allows aberration control of both probe size and detector field of view and also makes it possible to give precise control over amplitude and phase of the incident and scattered electrons. When applying HRTEM to the very thin specimen under negative Cs imaging conditions, even the projected atomic structure of complex crystals can be revealed because of its strong suppression of image imaging conditions. However, conventional HRTEM completely fails in obtaining directly interpretable images [41]. More studies for analysis of imperfections of complex layer compounds, such as stacking faults and layer undulations, should be carried out. As for the STEM mode, its ability to record compositional and bonding information in ultrathin materials would open up the study of inhomogeneities, including those symmetry-breaking and spatial variations in superconductors and charge-ordered materials, and also the interdiffusion and dead layer in ferroelectrics at the sub-nanolevel. Truly, the aberration corrector on TEM has brought great progress in the way of materials science, creating materials with desirable structures and properties. The journey to fabricate new devices attached to the electron microscopy is exciting and rewarding.
Figure 2.2 Controllable nanofabrication of MoSe nanowire network from a MoSe2 monolayer by electron beam nanofabrication. Source: Reproduced with permission from Lin et al. [37]. Copyright 2014, Nature Publishing Group.
2.1.3 Electron Energy Loss Spectroscopy in TEM
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 (EFermi). 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, La1.2Sr1.8Mn2O7
