SGTools1.The interaction between X-rays and matter
Figure 1 The interaction between X-rays and matter
According to X-ray theory, the incident X-rays mainly interact with electrons of the atoms in a material. The main forms of the interaction between X-ray and matter include absorption (producing fluorescence, which mainly occurs near the absorption edge), transmission, and scattering, as shown in Figure 1. The scattering is divided into inelastic scattering (wavelength change, outer electron interaction) and elastic scattering (the wavelength does not change, the inner layer binds electrons). The elastic X-rays scattered by different electrons have a coherent effect owing to the spatial density distribution. Therefore, the electron density distribution in the materials can be analyzed according to the spatial distribution of the elastic X-rays[1].
2.The experimental methods of SAXS
Figure 2 Schematic diagram of the experimental method of SAXS
Small-angle X-ray scattering (Figure 2) refers to the scattering of X-rays at small angles as the X-rays interact with the matter which is sensitive to the electron density difference within the materials. In the recent years, the SAXS technique has become an important tool for structural characterization of materials on the scale of nanometers. According to the X-ray scattering angle, the experimental method can be divided into small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Small-angle scattering corresponds to large-scale structures in material (1–100 nm), wide-angle scattering corresponds to small-sized structures(less than 1 nm, atomic-scale structures), and the grazing incidence method is used to obtain nanoscale structural information in the film system[2, 3].
Table 1 The main experimental methods of SAXS
| Experimental method | 2θ range | q (nm-1) @10 keV | Size range | Application system |
| TSAXS | 0.1–8º | 0.00628–0.628 | 1–100 nm | Colloidal solution, polymer, liquid crystal, fiber, mesoporous |
| WAXS | 8–120º | 7.06–87.72 | 0.01–1 nm | Analysis of crystal structure of powder, block, and fiber |
| GISAXS | 0.1–8º | 0.00628–0.628 | 1–100 nm | Quantum dot film, block copolymer film, liquid crystal film, mesoporous film |
Another important experimental method is anomalous small-angle X-ray scattering, which can obtain contrast variation in a multicomponent system[4]. It is based on the fact that for X-rays whose energy is close to the absorption edge of an element, the atomic scattering factor of that element is reduced by a few electrons from its value far from the edge; then, a contrast variation can be obtained by simply varying the energy of the incident X-ray photon. In recent years, the simultaneous SAXS-WAXS has been widely used to realize multiscale structure characterization from the atom to the nanoscale. Table 1 shows the detection scale range and typical application system of the main SAXS experimental methods when the energy of the incident X-ray is 10 keV. The samples of TSAXS can be in various forms, such as solutions, powders, and blocks. WAXS is mainly used for the analysis of crystal structures such as powders, blocks, and fibers. GISAXS is mainly used for nanoscale structure analysis of thin films.
References
1. Guinier, A., G. Fournet, and K.L. Yudowitch, Small-angle scattering of X-rays. 1955.
2. Porod, G., O. Glatter, and O. Kratky, Small angle X-ray scattering. by O. Glatter and O. Kratky, Academic Press, London, 1982: p. 17.
3. Renaud, G., R. Lazzari, and F. Leroy, Probing surface and interface morphology with grazing incidence small angle X-ray scattering. Surface Science Reports, 2009. 64(8): p. 255-380.
4. Naudon, A., Anomalous small-angle x-ray scattering (ASAXS), in Modern Aspects of Small-Angle Scattering. 1995, Springer. p. 203-220.