Topographic and other methods

We turn now to methods designed mainly for the examination of nearly perfect crystals. Here the interest is not so much in measuring the angular disorientation of one part of the crystal with respect to another, but rather in disclosing the presence of individual imperfections, such as dislocations. All of these methods involve the reflection of characteristic Kct radiation. 

Most of the methods described below are called topographic, in the sense that the diffraction spot has a fine structure that is an image or map of the crystal examined. To be topographic, a one-to-one correspondence must exist between points on the diffraction spot and points on the crystal. 

The phenomenon of extinction plays a major role in image formation in x-ray topography. As we saw in Sec. 4-12, extinction has its maximum effect in the 

Not only does this important experiment demonstrate quite directly the relation between nonuniform lattice strain and x-ray reflecting power, but it also shows that extinction can be reversibly varied in an almost perfect crystal. 

This method involves the reflection of Ka. radiation from the face of the specimen crystal [8.16-8.18]. There is therefore no restriction on specimen thickness. 

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Heights determined by these methods should be regarded as a guide rather than as a mathematically precise decision. The conclusions may need to be modified in the light of particular local circumstances such as valleys, hills and other topographical features. 

The use of top-down lithographic techniques to topographically pattern substrates and thereby control the film thickness has been used to create submicron patterns that contain oriented microdomains. This approach is generally described as the graphoepitaxy method and will be discussed in further detail in Sect. 4.1, with other methods which use top-down approaches to control the bottom-up block copolymer patterns. 

The essence of the topographic methods is that they map the interrsity of the diffracted beam over the surface of the crystal. Defects affect the diffracted intensity, so give contrast in the image. The methods are quite sertsitive enough to reveal individual dislocatiorrs, precipitates, magnetic domains and other long-range strain fields but cannot reveal point defects except in dense clusters. 

If we consider now the emission values, at the point where man is exposed, we notice that they are not only function of the emissions of PAH but also of a lot of other parameters meteorological and topographic conditions, sampling methods and analytical techniques (1), choice of the reference substance(s) submitted to the analyses. 

Laser beam deflection offers a convenient and sensitive method of measuring cantilever deflection. In the non-contact mode, the AFM derives topographic images from measurements of attractive forces the tip does not touch the sample. On the other hand, in the contact mode, repulsion forces between the tip and the sample produce topographic images. 

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