Hiroyuki Ikemoto

The encapsulation of sulfur (S) within single-walled carbon nanotubes (S@SWCNTs) provides a unique opportunity to stabilize and study the structure and bonding of one-dimensional sulfur chains, which do not exist in a free state. In this work, we present a detailed structural investigation of S@SWCNTs using temperature-dependent x-ray absorption fine structure spectroscopy. The S–S covalent bond length was 2.057 ± 0.004 Å , which is identical to that of bulk α -S within the experimental uncertainty. Crucially, both the bond length and its mean-square relative displacement exhibited no discernible temperature dependence up to 300 K, indicating an exceptionally robust covalent bond. Comparison between the experimental and theoretical x-ray absorption near edge structure spectra suggested that the possible chain conformations are zigzag and linear, rather than three-turn helical. Photoemission spectroscopy measurements revealed that the C 1s core level binding energy did not change upon sulfur encapsulation, confirming the absence of interaction between the sulfur chains and the host SWCNTs.

Tin nanoparticles (n-Sn) are expected to undergo a phase transition between α -Sn (a semiconductor with a diamond structure) and β -Sn (a metal with a tetragonal structure) as a function of particle size, similar to the temperature-induced transition that occurs near room temperature. X-ray absorption fine structure measurements were conducted to identify the local structure and extract detailed structural parameters of n-Sn with a diameter of 24.7 Å. The nearest-neighbor atomic distance suggests that the local configuration of n-Sn resembles the diamond-like structure of α -Sn. However, several experimental observations have indicated that n-Sn does not adopt a perfect diamond structure like that of α -Sn, but rather exhibits an amorphous character. These observations include a decrease in the coordination number, an increase in the static component of the mean-squared relative displacement of the first atomic correlation, and the disappearance of all atomic correlations beyond the nearest-neighbor. Furthermore, the covalent bonds in n-Sn elongate with increasing temperature, in contrast to those in α -Sn which exhibits no temperature dependence. The covalent bond strength of n-Sn was also found to be weaker than that of α -Sn.

This study performs extended x-ray absorption fine structure (EXAFS) measurements for the S-K edge in the temperature range of 10 and 300 K in the transmission mode using a photodiode to detect the transmitted x-rays. It provides the first report of temperature variations in the structural parameters of α-S. As the temperature increases from 10 to 300 K in the Fourier transform of k χ ( k ) the first peak corresponding to the covalent bond of the eight-membered ring becomes slightly low anomalously despite thermal disturbances. However, as in normal materials, the second peak at 300 K decreases to approximately half of that at 10 K, which contains several intra- and inter-ring correlations. All structural parameters of the covalent bond obtained by nonlinear least squares fitting exhibit missing temperature variations. A value of zero for the asymmetric parameter in the EXAFS (C ₃) implies that the potential of the covalent bond is symmetric, and the constant value of the mean square relative displacement (MSRD) with temperature implies that the potential is extremely high. The Einstein model fitting for the temperature variation in the MSRD yields an Einstein temperature of 942 K and force constant (K) of 405 N m⁻¹. The value of K is the largest among those of chalcogen elements.

In this review, we make a survey of the structure studies for the chalcogen elements and several chalcogenides in liquid, amorphous and nanosized state by using X-ray absorption fine structure (XAFS). The chalcogen elements have hierarchic structures; the chain structure constructed with the strong covalent bond as a primary structure, and the weaker interaction between chains as a secondary one. Existence of these two kinds of interactions induces exotic behaviors in the liquid, amorphous and nanosized state of the chalcogen and chalcogenides. XAFS is a powerful structure analysis technique for multi-element systems and the disordered materials, so it is suitable for the study of such as liquid, amorphous and nanosized mixtures. In section 2, the structures for the liquid state are discussed, which show the interesting semiconductor-metal transition depending on their temperatures and components. In section 3, the structure for the amorphous states are discussed. Especially, some of chalcogens and chalcogenides present the photostructural change, which is important industrial application. In section 4, the structures of nanosized state, nanoparticles and isolated chain confined into the narrow channel, are discussed. The studies of the nanoparticle and the isolated chain reveal the alternative role between the intrachain covalent bonds and the interchain interaction.

Grazing-incidence small-angle X-ray scattering (GISAXS) patterns have multiple superimposed contributions from the shape of the nanoscale structure, the coupling between the particles, the partial pair correlation, and the layer geometry. Therefore, it is not easy to identify the model manually from the huge amounts of combinations. The convolutional neural network (CNN), which is one of the artificial neural networks, can find regularities to classify patterns from large amounts of combinations. CNN was applied to classify GISAXS patterns, focusing on the shape of the nanoparticles. The network found regularities from the GISAXS patterns and showed a success rate of about 90% for the classification. This method can efficiently classify a large amount of experimental GISAXS patterns according to a set of model shapes and their combinations.

A new program calledmiXAFSfor the analysis of X-ray absorption fine-structure (XAFS) data is presented.miXAFScan analyze the XAFS functions simultaneously for all measured X-ray absorption edges of the constituent elements in a sample under the constraints for the structural parameters over the edges. The program provides a surface plot of theR-factor as a function of two structural parameters, which is useful to validate the optimized structural parameters. The structural parameters can be obtained from the XAFS data in a few steps using the setting file and batch process. The program, which is coded in MATLAB and freely available, runs on Macintosh and Windows operating systems. It has a graphical user interface and loads experimental data and XAFS functions in a variety of ASCII data formats.