Compositional contrast

Compositional contrast refers to the variation in gray levels in an SEM image that correspond to variation in chemical composition in a specimen. An image formed by BSEs exhibits very useful compositional contrast if the specimen consists of more than one chemical element. The origin of compositional contrast arises because the capability of BSEs to escape from the specimen depends on the atomic numbers of the specimen atoms. The backscatter coefficient (rj) characterizes such capability. 

When the E-T detector is used for compositional contrast, the Faraday cage should be negatively charged (-50 V) to exclude SEs with energies lower than the bias voltage of the cage. Some SEM systems are equipped with a dedicated detector for BSE collection, which is placed above the specimen. To obtain compositional contrast, we can switch detection from the SE mode to the BSE mode by simply pushing a button on the SEM control panel. 

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Areally close compositional contrast is seen in the recent rift area of Djibouti. On the western border, the terminal Lake Abhe is fed by the sodium-carbonate-type Awash River from the Ethiopian volcanic highlands, whereas the Asal Lake near an arm of the Gulf of Aden is fed by seawater through recent fault zones and hydrothermal inputs in addition to local runoff, and precipitates halite and gypsum (Fontes et al., 1979). 

In an SEM image with composition contrast as shown in Figure 4.16b, which areas contain the heavy metallic elements bright or dark Explain. 

Compositional contrast, as well as modulus, can also be assessed by laterally resolved imaging of normal or lateral forces in the contact mode AFM. In the corresponding mapping [121] (see also Chap. 4), force-displacement curves are recorded for each pixel. Subsequently, the pull-off forces, as a measure for adhesion, and the indentation part of the loading curves, to extract/fit the elastic modulus, are evaluated for each pixel. In particular for adhesion mapping, the use of chemically functionalized AFM probe tips [122], has been shown to be a suitable approach to map chemical composition and functional group distributions down to the sub-50 nm scale [123]. The mapping of adhesion, friction, and surface mechanical properties will be treated in more detail in Chap. 4. 

Quantum dots are detectable by SEM by topographical contrast with secondary electron imaging (SEI), compositional contrast with backscattered electron imaging (BEI), and scanning transmission electron imaging. By SEI, the quantum dots appear as particles on exposed surfaces that can resemble normal biological structures. However, the CdSe core and ZnS coating impart... 

Imaging modes other than secondary electron, such as cathododlumniscence, are available on the SEM instrument, but the only technique other than secondary electron that is likely to be of very much interest in solving adhesion problems is the use of backscattered primary electrons, which give a topographical image with some compositional contrast. 

The dependence of 6 and especially of r] on atomic number Z (Fig. 82) leads to an atomic number or compositional contrast that can be used for the discrimination of phases with different mean atomic numbers, as demonstrated by a eutectic alloy shown in Figure 8.5. 

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