Amorphous Nanomaterials. Lin Guo

to much heavier metal atoms in semiconductors, with a slightly negative Cs value [16]. Moreover, the study about the impact of beam–specimen interactions still continues, especially in case that the sample is damaged by an intense electron beam while the atoms or point defects conveniently remain in place. This is resolved by using an aberration-corrected STEM operated at 60 keV, which allows atom-by-atom structural and chemical analysis, and identification of individual atoms of low-Z elements with a negligible electron–beam damage [17]. The operation of the microscope at lower electron energies offers opportunities to characterize some important classes of materials that are ultrathin, such as single graphene sheets.

Using Cs-TEM to characterize nanomaterials with atomic resolution was an achievement in the past three decades. A contribution was to understand high-temperature superconductivity by attempting to image oxygen in YBa₂Cu₃O₇. It solved one key problem: how the occupation of specific atomic sites with oxygen influences electronic properties [18, 19]. In 1992, oxygen was accessible by an imaging technique, as shown that it was visible at atomic resolution in the electron wave function at the exit plane of the specimen when reconstructed by computational techniques [20]. With the development of aberration-corrected technique, oxygen concentration measurements were carried out for the first time in studying lattice defects in BaTiO₃ in 2004 [21]. The trend of modern aberration-corrected instruments is to produce images with quality that can virtually put finger on individual atomic positions and even the individual lateral atomic shift. One example for precise characterization of individual atoms is shown in Figure 2.1, where a ferroelectric domain boundary in the microelectronic storage material PbZr_0.2Ti_0.8O₃ of the order of 40 pm of the oxygen, zirconium, and titanium atoms out of their symmetry positions was clearly revealed [22].

This powerful technique fulfilled the old dream of materials science: a direct link between atomic-level information and macroscopic properties. Specifically, the realization of atomic resolution by aberration-corrected TEM can greatly influence the future development of semiconductor devices because their continued miniaturization relies on critical components, including those 5–10-atom-thick gate oxides in transistors [23, 24], magnetoresistive read heads with a thickness of 1–2 nm [25, 26], and tunnel junctions in magnetic memories with comparable thickness [27, 28]. The success or failure of the semiconductors mentioned above is decided by the determination of the thickness and composition of these ultrathin layers. It also gains insights into the chemistry, interdiffusion, and electronic structures of interlayers. Notably, STEM has also proved very effective in measuring the changes in compositions, electronic structure, and bonding of interfaces of those semiconductors [29, 30]. Detection of single-dopant atoms by STEM is regarded as a powerful tool to understand materials for transistor scaling [31], to detect the spatial distribution of single vacancies [32], or to study their electronic fingerprints on the local densities of states [33]. Besides, the typical examples of using Cs-corrected BF-STEM imaging can be found with the resolve of hydrogen atomic columns in a crystalline sample [34]. Also, studies on obtaining atomic resolution BF-STEM images using a medium collection angle have been carried out, where the detection of both light and heavy atomic columns with a medium collection angle for a [001]-oriented SrTiO₃ single crystal was realized [35]. This middle-angle BF-STEM imaging is particularly robust to against variations in the probe-forming lens defocus and sample thickness, which laid a good foundation to analyze realistic materials. After 2010, the ultra-STEM represents a new trend in atomic resolution imaging, which used lower acceleration voltages, the so-called “gentle STEM” [36]. The operating voltage is only 60 keV, which is well below the knock-on damage voltage of graphene, making it easy to study the intrinsic defects. Even so, the edge atoms and defects are more easily to be knocked into metastable configurations because they are weakly bonded. However, this can be taken advantages to investigate atomic dynamics of nanostructure and even created nanostructures by electron beams (Figure 2.2) [37, 38]. The ultra-STEM is well matched with the recent increasing interests in two-dimensional (2D) materials, as its simultaneous efficient ADF and EELS imaging can achieve insights into vacancy and defect configurations [39]. Meanwhile, discoveries such as ordered arrays of oxygen vacancies with dramatic effects on nanosheet properties were presented by this ultra-STEM [40].

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