Glancing angle diffraction techniques

Here we will focus on the bulk static structures of the phases formed by low molar mass calamitic (rod-like) mesogens. The principles involved in structural studies of columnar phases, and of polymeric and lyotropic systems, are very similar. The study of liquid crystal surfaces requires reflectivity and/or glancing angle diffraction techniques [4-6]. 

GIAB studies of sputtered thin films of different composition for tribological applications have been reported [4.157-4.159]. The technique has been used to study the structure of very thin GdS layers (deposited by chemical bath deposition) for photovoltaic applications in combination with 6-26 diffraction it enabled identification of their polytype structure [4.160]. Glancing angle diffraction in the GIAB geometry... 

Electrons are much less penetrating than x-rays. They are easily absorbed by air, which means that the specimen and the photographic film on which the diffraction pattern is recorded must both be enclosed within the evacuated tube in which the electron beam is produced. Transmission patterns can be made only of specimens so thin as to be classified as foils or films. Reflection patterns from thick specimens are recorded by a glancing-angle technique, somewhat like that shown in Fig. 6-13, on a film placed behind the specimen such a pattern will be representative only of a thin surface layer, because diffraction occurs over a depth of only a few hundred angstroms or less. Electron diffraction is therefore well suited to the study of thin surface layers. 

Surfaces structures especially will be of future interest. They can be investigated by glancing angle X-ray diffraction, since with this technique the X-rays penetrate only through a few atomic layers at the sample surface. Therefore, the measured reflections can give information about two-dimensionally ordered surface structures. 

Another method used for evaluating the resistance to oxidation of steels is Cu K X-ray diffraction with a glancing angle technique. This method aims to determine the area (A) of peaks at 29 = 35.5 (Fc304 + a-Fe203) and 29 = 44.6 (a-Fe) for uncoated (Ra) and coated (Re) samples. The ratio ilu/-R-c determines the degree of oxidation, which is expressed as the protection factor (PF) (Gugliemi, 1992). 

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