Atomic number contrast

Atomic number contrast can be used to estimate concentrations in binary alloys because the actual BSE signal increases somewhat predictably with the concentration of the heavier element of the pair. 

Run-of-the-mill instruments can achieve a resolution of 5-10 nm, while the best reach 1 nm. The remarkable depth of focus derives from the fact that a very small numerical aperture is used, and yet this feature does not spoil the resolution, which is not limited by dilfraction as it is in an optical microscope but rather by various forms of aberration. Scanning electron microscopes can undertake compositional analysis (but with much less accuracy than the instruments treated in the next section) and there is also a way of arranging image formation that allows atomic-number contrast, so that elements of different atomic number show up in various degrees of brightness on the image of a polished surface. 

Figure 5.11. EPMA maps of Roman shears (CP = atomic number contrast). (Courtesy Dr C. J. Salter,...

The main drawbacks of this approach are the low availability of such instruments in laboratories, and the fact that many samples are sensitive to ion beam damage, require specific preparation (95), and can induce low contrast. Moreover, the imaging between two milling periods is typically performed in the backscattered electrons mode, which is not always favorable this is the case for carbon nanotubes in a polymer matrix as the atomic number contrast is low. This is probably the reason why, even if the FIB/SEM approach is used on polymer nanocomposites, it not used in the literature for carbon nanotubes in polymer matrix. In this last application, the tomo-STEM technique is a good alternative to obtain images of relatively thick samples with high contrast and resolution (91). 

The distribution of phases in the material is described by backscattered electron images (atomic number contrast). 

Figure 9.] 2 Atomic number contrast images for Pd particles carried on alumina.

Backscattered electrons Energies up to Incident beam energy Beam electrons scattered back after collision Atomic number contrast, channeling patterns, magnetic contrast... 

Atomic number (Z) contrast is very useful in the analysis of foreign materials and catalysts, contamination detection, and metallurgical phase analysis. An example of atomic number contrast is shown in Figure 6. 

Figure 6 (A) BSE micrograph of the surface of a Ru/SiOa catalyst. The backscatter image shows mainly atomic number contrast. The Ru clusters appear bright (point 1), while the lighter silica spheres are darker (point 2) (B) EDS spectmm from points 1 and 2.

Fig. 4.38 Secondary electron image of a mineral filled polymer composite (A) does not reveal the nature of the dispersed filler particles. Atomic number contrast in the backscattered electron image (B) clearly shows the mineral filler (brightness increases with atomic number).

BSE are not much used in LVSEM, although the resolution and atomic number contrast are both good. There are many more secondaries than backscattered electrons at low voltage, and this is especially true for low atomic number samples such as polymers. It is also comparatively difficult to detect these few low energy backscattered electrons. A scintillator [40] or a multi-channel electron plate can detect them directly [41, 42]. Alternatively they can be detected indirectly, through the secondaries produced by their collision with the chamber walls (SE3) [30, 43]. 

The yield of reflected backscattered electrons (BSE) increases with increasing atomic number of the sample material. Elements with a higher atomic number offer a higher probability of backscattered electrons, giving an additional atomic number contrast material contrast) to the surface topograp y contrast. 

Polymer matrices are also commonly reinforced with mineral fillers or fibers, such as calcium carbonate, talc, wollastonite, clay and mica [306]. SE images of fracture surfaces show the wetting behavior or adhesion of the filler by the polymer matrix. Figure 4.34 shows secondary electron image (A) and backscattered electron image (B) micrographs of a mineral filler in a matrix of a commercial polymer. SEI does not reveal the nature of the filler in the matrix whereas BEI does reveal the mineral filler due to atomic number contrast. BE imaging is important in the... 

SEI, and imaging with atomic number contrast, by BEI imaging. Comparison of these two imaging modes has been shown in Chapter 5. 

Atomic number contrast Micro composition Microcomposition... 

Scanning (BEI) Atomic number contrast lmm-20nm 10-10,000x... 

---