target="_blank" rel="nofollow" href="#ulink_00042f6a-c13d-521e-b54c-3b522393a8cc">Figure 2.7 Nanoparticle-mediated crystal nucleation and growth in amorphous Bi to crystal-phase transformation. (a)–(d) HRTEM images showing the nanoparticle coalescence-mediated nucleation of nanocrystal. (e) and (f) HRTEM images showing the nanoparticle coalescence-induced growth of nanocrystal and the formation of grain boundaries. Inset images are the corresponding FFT patterns. The electron dose is 18000 eA−2 s−1. Source: Reproduced with permission from Li et al. [100]. Copyright 2018, WILEY-VCH Verlag GMbH& Co. KGaA, Weinheim.
Overall, in-situ and ex-situ study with spherical corrected TEM has shown its potential in direct visualization and spectroscopy of electrochemical processes at the atomic level. It is no doubt to foresee that in situ TEM would hold a promising future for the nanoscale electrochemistry.
2.1.5 Summary and Outlook
The progress of Cs-TEM has fulfilled the old dream of materials science: a direct link between atomic-level structural information and macroscopic properties. 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 creating new devices with enhanced properties. The use of aberration-corrected TEM is capable of imaging and analyzing materials at the sub-nano resolution in an easily managed way. Moreover, the acquisition of EELS by Cs-TEM can provide distinguishable information about bonding differences between dopant species, which can work with atomic imaging to carry out elemental and chemical analyses of site occupancy. This further paves the way for the study toward physical chemistry at the sub-nanolevel. In addition, based on in-situ TEM equipped with spherical aberration corrector, researchers are good at capturing localized information and inhomogeneity, which opens up the ways for investigating defects in the crystal lattice, charge transport, and phase boundary migration kinetics.
For the future study of Cs-TEM, several improvements are necessary. One is the irreversible structural changes especially at interfaces and surfaces when exposing to the irradiation of highly intense electron beam at higher magnification. This may possibly be alleviated by operating the microscope at a lower voltage or carried out experiments with cryo-TEM. The second is the long-term stability of the aberration corrector; this would require further improvement in corrector hardware to allow aberration control over probe size for longer periods. The third is the sample preparation, which has become more demanding, aiming to eliminate the surface oxide or contamination layers to avoid the background noise to the image signal. For EELS, the aim is to overcome the dechanneling, delocalization, and absorption in electron scattering of each atomic column. In addition, for in situ TEM, more effort should be devoted to high-speed computers for real-time recording to obtain aberration parameters, and then combining faster data acquisition with STEM, which would then open up new chances for three-dimensional tomographic chemical imaging and real-time imaging of STEM-based environmental microscopy in different atmospheres or liquid cells.
2.2 X-ray Absorption Fine Structure Spectrum
2.2.1 Introduction
X-ray absorption fine structure spectroscopy (XAFS) is a unique tool for studying the local structure around the selected elements that are contained within a material, at the atomic and molecular scale. Owing to its element-specific and short-range nature, the core-level XAFS is now routinely used to elucidate the local structural, vibrational, and other physical properties of complex, aperiodic materials. The XAFS encompasses both the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge spectra (XANES). The XANES refers to the structure in the X-ray absorption spectrum at high and low energies relative to the absorption edge with the crossover typically at about 20–30 eV above the edge. However, EXAFS refers to the oscillations at energies above the XANES region. The extraordinary capabilities of these spectroscopies come at a price: they depend on the comparisons with quantitative simulations of the spectra. However, theories of X-ray spectra have become increasingly accurate and sophisticated, generally overcoming this limitation.
In this chapter, we will start with a brief introduction of XAFS, including the conceptions and classifications of XAFS. Then, two branches of XAFS, EXAFS and XANES, and their corresponding theories, mechanisms, and scope of applications will be discussed in detail. Finally, attention will be focused on the application of XAFS for amorphous material characterization. XAFS can be applied not only to crystals but also to the materials that possess little or no long-range translational order: amorphous systems, glasses, quasicrystals, disordered films, membranes, solutions, liquids, metalloproteins, and even molecular gases. In the last section, a series of researches on amorphous materials based on the data gathered and analyzed from XAFS will be reviewed.
2.2.2 Extended X-ray Absorption Fine Structure
To clearly elucidate with the term XAFS, we can start with XAS. When X-ray passes through a sample thickness of d, its intensity I0 will decay to I because of the absorption of the sample, so the X-ray absorption coefficient of the sample, μE, can be defined as:
(2.1)
The X-ray absorption spectrum is to measure the change curve of X-ray absorption coefficient with X-ray energy. After the absorption edge, there will be a series of oscillations. This kind of small structure is generally a few percent of the absorption cross section, that is, XAFS.
The XAS encompasses both the EXAFS and XANES, where the terms refer, respectively, to the structure in the X-ray absorption spectrum at high and low energies relative to the absorption edge with the crossover typically at about 20–30 eV above the edge [104].
Formally, the base theory of the XAFS is Fermi’s golden rule (Figure 2.8) [105]:
(2.2)
However, it requires a summation over the exact many-body final states |F>| and its energy EF. The normalized fine structure in the XAS in terms of the oscillatory contributions from near-neighbor atoms can be described as:
(2.3)
where μ0(E) is the absorption coefficient in the case of isolated atoms, without considering the smooth absorption background caused by scattering, and μ(E) is the experimentally measured absorption coefficient with neighboring atoms.
Figure 2.8 The spectra and origin of XANES and EXAFS.
Incorporating many-body effects can follow the two-step approach. The first step is the production of the photoelectron, by photoexcitation from a certain core state, with one-body absorption μ(1)(E). The second is the effect of the inelastic losses and secondary excitations, which can be represented by an energy-dependent “spectral function” A(E, E′), which subsequently broadens and shifts the spectrum. Incorporation yields an exact representation of the many-body XAS in terms of a convolution
(2.4)
