Amorphous local structure

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

On considering more amorphous systems, having only short-range order, X-ray diffraction is not a suitable probe. Instead, a form of X-ray absorption can provide invaluable information on the local structure around the atoms in a surface layer. 

Extended X-ray absorption fine Amorphous film structure and local... 

K. Shimoda, Y. Tobu, K. Kanehashi, T. Nemoto and K. Saito, Total understanding of the local structures of an amorphous slag perspective from multi-nuclear ( Si, Al, Mg, and Ca) solid-state NMR. /. Non Cryst. Solids, 2008, 354,1036-1043. 

In order to discuss an essential feature of the laser-induced structural change, we mnst first consider the structure of amorphous films. Almost all available structural data indicate that the main constituent of a-Sbj Sei j (x < 0.05) is the chain mol-ecnle, althongh molecules with Sb branching sites may be contained in minority. That is, we assume that the local structure of a-Se containing Sb additives resembles the hexagonal (trigonal) Se structure. Accordingly, in atomic structural terms, a-Sbj Sci withx < 5at.% may be characterized as a quasi-one-dimensional chain structnre. 

Local structural features have been postulated for amorphous polymer systems, based on the asymmetry of chain-like molecules. Flory (56) has shown that molecular asymmetry in itself is no barrier to a dense random packing of the chains are sufficiently flexible. Robertson (57) suggests, however, that some degree of local alignment is required simply to accomodate linearly connected sequences in the rather limited space available. Unfortunately, Calculations of local cooperative effects are extremely difficult and sensitive to specific assumptions about available packing arrangements. 

As shown in Figure 2, the infrared vibration bands of sample A (1236, 1090, 965, 800, 564 and 465 cm-1) are close to those of sample B (1234, 1090, 964, 799, 568 and 465 cm-1). They show that the internal local structures of both hexagonal silica frameworks are almost identical. With the transformation of regular hexagonal structure into amorphous phase (amorphous-1), all of network vibration bands are shifted to... 

Two theoretical approaches for calculating NMR chemical shift of polymers and its application to structural characterization have been described. One is that model molecules such as dimer, trimer, etc., as a local structure of polymer chains, are in the calculation by combining quantum chemistry and statistical mechanics. This approach has been applied to polymer systems in the solution, amorphous and solid states. Another approach is to employ the tight-binding molecular orbital theory to describe the NMR chemical shift and electronic structure of infinite polymer chains with periodic structure. This approach has been applied to polymer systems in the solid state. These approaches have been successfully applied to structural characterization of polymers... 

The results presented in this section further illustrate that there is a considerable dependence of the band alignment at the CdS/ZnO interface on the details of its preparation. An important factor is the local structure of the ZnO film. There is considerable local disorder when the films are deposited at room temperature in pure Ar, deposition conditions that are often used in thin film solar cells. It is recalled that the disorder is only on a local scale and does not affect the long range order of the films, as obvious from clear X-ray diffraction patterns recorded from such films (see discussion in Sect. 4.2.3.3). Growth of sputter deposited ZnO on CdS always results in an amorphous nucleation layer at the interface. The amorphous nucleation layer affects the valence band offset. 

Abstract The discrete variational (DV)-Xa method was used to analyze the high-resolution soft X-ray absorption spectra (XAS) in the C /Cregion of sputtered amorphous carbon films and carbon black to elucidate their local structures. The measured XAS of amorphous carbon and carbon black were compared with those of reference compounds, and the fine structure in the XAS can be assigned by the calculated density of states of the reference compounds. Such a comparative analysis in the measured XAS and the calculated density of states of these carbon materials with reference compounds, which have been known their local or molecular structures, is a valid approach for elucidating the complex local structures of carbon materials. 

IR, Raman, NMR, ESR, UPS, XPS, AES, EELS, SIMS) [1]. However, some industrial carbon materials such as amorphous carbon films and carbon black cannot be easily characterized from the local-structure point of view by these methods, because these materials usually take amorphous and complex structures. Recently, soft X-ray emission and absorption spectroscopy using highly brilliant synchrotron radiation [2] has been utilized to characterize various carbon materials, because information on both the occupied and unoccupied orbitals, which directly reflect the local structure and chemical states, can be provided from the high-resolution soft X-ray measurements. We have applied the soft X-ray spectroscopy to elucidate the local structure and chemical states of various carbon materials [3]. Additionally, we have successfully used the discrete variational (DV)-Xa method [4] for the soft X-ray spectroscopic analysis of the carbon materials, because the DV-Xa method can easily treat complex carbon cluster models, which should be considered for the structural analysis of amorphous carbon materials. 

We have recently found that some fine structures can be commonly observed in the C K edge X-ray absorption spectra (XAS) of various sputtered amorphous carbon (a-C) films [5,6] and carbon black (CB) [7], To clarify the spectroscopic fine structures from the local-structure and chemical-state point of view, we have compared the XAS of a-C and CB with those of reference compounds, and estimated the local structures of a-C and CB from the spectroscopic analysis using the DV-Xa method. The present paper describes one of the approaches in determining the local structure of carbon materials using the soft X-ray spectroscopy as well as the DV-Xa method. 

Bianconi et al. have devoted XANES studies to the determination of the local structure of amorphous A1203 (15,29) and Si02 (29,31). In a review dedicated to problems in mineralogy and geochemistry, Waychunas and Brown (306) have considered crystalline and amorphous aluminosilicates. 

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