High-resolution STEM

As shown above, the size and distribution of minute particles are conveniently investigated by high-resolution STEM with a HAADF detector (60,63). The intensity in HAADF images is a monotonic function of the sample thickness and atomic number, a pre-requisite for the electron tomography experiments described below. 

Figure 3.17 TEM image for a cryomicrotomed PFSgo-h-PDMSgoo film cast from toluene. Inset High-resolution STEM image of this sample obtained in the dark-field mode. (From Raez et al.63 Reproduced with permission.)...

The composition of these nanozones can be easily determined in STEM mode by placing the electron probe exactly on a particular nanozone for EELS analysis. The advantage of using EELS in spot analysis is that the high resolution STEM probe (2 angstrom diameter) may be used. This provides a very precise spacial analysis and also allows us to analyze extremely narrow nanozones as in this case for the thin dark layer. An example of this work is provided in Figure 5 where the STEM-derived nanozone model is illustrated... 

Fig. 19.7 LHS) (a and b) High-resolution STEM images of Pt hollow spheres and (c and d) line scans parallel and perpendicular to the lattice planes (RHS) comparison of the ESA of solid and hollow Pt spheres based on hydrogen desorption charges and ORR-specific and Pt mass activities measured at 0.9 V in RDE measurements [47]...

FIGURE 13.10 (a) High-resolution STEM image of a Pt hoUow particle, (b) Pt mass activity as a function of voltage cycling time (profile shown in figure) for hollow and sohd Pt nanoparticles. The average particle sizes were 6.5 and 3.2nm in diameter, respectively. Reprinted with permission from Ref. [90]. American Chemical Society. 

The smallest sources, the field emission sources, are most useful for high resolution STEM (Section 3.2.1) and analytical microscopy. Because of the extremely severe radiation damage under these conditions and the need for ultrahigh vacuum, they are almost never used for polymers. 

These impressive results are underlaid by a thorough characterization by means of high resolution STEM and XPS, showing that the photocatalysts do indeed consist of size-selected clusters and are surprisingly stable in aqueous solution. 

Application High-resolution signal (TEM, STEM) Back-scattering of electrons (BSE signal in SEM) Analytical signal (TEM, STEM, SEM) Emission of secondary electrons (SE signal in SEM)... 

The uniqueness and desirability of EELS is realized when it is combined with the power of a TEM or STEM to form an Analytical Electron Microscope (AEM). This combination allows the analyst to perform spatially resolved nondestructive analysis with high-resolution imaging (< 3 A). Thus, not oiJy can the analyst observe the microstructure of interest (see the TEM article) but, by virtue of the focusing ability of the incident beam in the electron microscope, he or she can simultaneously analyze a specific region of interest. Lateral spatial resolutions of regions as small as 10 A in diameter are achievable with appropriate specimens and probe-forming optics in the electron microscope. 

The STEM is unrivaled in its ability to obtain high-resolution imaging combined with microanalysis from specimens that can be fashioned from almost any solid. Major applications include the analysis of metals, ceramics, electronic devices... 

The annular dark-field detector of the field-emission STEM (see Figure 2) provides a powerful high-resolution imaging mode that is not available in the conventional TEM or TEM/STEM. In this mode, images of individual atoms may be obtained, as shown in Figure 4 (see Isaacson, Ohtsuki, and Utlaut ). Some annular dark-field... 

C. E. Lyman, H. G. Stenger, and J. R. Michael. Ultramicroscopy 22, 129, 1987. This paper demonstrates high-resolution compositional imaging with the field-emission STEM. 

Figure 7. High resolution TEM image of a single Au nanoparticle observed inside a stem of alfalfa seedlings grown in gold emiched medium. The inset corresponds to the fast Fourier transform of the crystalline particle. (Reprinted from Ref. [28], 2002, with permission from American Chemical Society)...

---