Synchrotron X-ray Scattering

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Figure 1: © Synchrotron Soleil

Synchrotron radiation is an electromagnetic wave generated when charged particles are accelerated to very high speed of light radially. When produced, the radiation is highly polarized and contains energy spanning the whole electromagnetic spectrum. It provides very powerful tools in material characterization. Figure 1 shows the generation of synchrotron radiation in synchrotron facilities. In these facilities, synchrotron radiation is created either by bending magnets, undulators or wigglers.

 
Synchrotron X-ray scattering is a powerful modern technique which is capable of probing microscopic structures with electron density variation. It has many applications in research fields such as physics, material science, chemistry, biology, medical science and etc. as well as in industry including food, cosmology and etc. Particularly, grazing-incidence wide-angle and small-angle X-ray scattering are commonly used to study molecular and nanoscale structure of thin film optoelectronics.
 

Figure 2: GIXS measuring geometry

Grazing-incidence wide angle x-ray scattering (GIWAXS) utilizing the same principle that governs x-ray diffraction, is a powerful technique to investigate the thin film ordered structures in molecular level. Under the basic setting of a GIWAXS measurement, the surface of sample stage is tilted, usually only to a very small angle (close to the critical angle of the material) with respect to the incidence light. A 2-dimensional detector is placed after the sample stage. When incident x-ray shines on the sample, diffraction happens and the detector would collect all the Bragg peaks in a 2-dimensional manner. This technique is especially useful to thin film samples. Due to the limited probing depth defined by the small incident angle, this method reveals information only a few hundreds of nanometers from the sample surface without the disturbance from strong background signal. The 2D diffraction pattern could reveal orientational information with respect to the substrate. The grazing incidence also elongates the footprint of X-ray on the sample to increase the scattering intensity as well as the statistics. Synchrotron light is usually employed to characterize of weakly scattered organic thin films. Nowadays, state-of-art in house SAXS/WAXS systems, such as Xeuss 2.0 in our central lab, are available to produce quality X-ray scattering images. The physical scale probed by this technique is usually between ~0.01 nm to ~1 nm (~0.1 Å^-1< q < ~10 Å^-1), relating to the crystal structure of the samples.
 
Grazing-incidence small angle x-ray scattering (GISAXS) utilizes the same setting as GIWAXS, but with a much smaller q range. The detector is brought further away to collect small-angle scattering. The physical scale probed by this technique is usually between ~1 nm to ~100 nm (~0.001 Å^-1< q < ~0.1 Å^-1), relating to the aggregations and phase-separation domains in the sample.