STEM techniques

Disadvantages of STEM relate to image resolution, image quality and the cost of dedicated instruments. The loss of resolution and image contrast compared to TEM must be considered for each application as the images tend to be softer in STEM. The better resolution in TEM also is a major factor in imaging if the structures of interest require ultimate resolution. On the other hand, there are specimens which cannot be 

Known technique typically applied to solve this problem  

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The cost-effective deployment of large areas of solar collectors will most probably be polycrystalline materials, with all index planes emerging at the surface. Therefore, it is not expected that the structural determination of solar materials surfaces will be applied except for a few special cases. However, determining S/S inter facial structures is important, as shown by Figs. 6-9 in Ref. 2. The challenge in solar interface research will be to understand the changes in surface activity of heterogeneous real surfaces and interfaces. Here, SEM and possibly STEM techniques should be used extensively. 

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). 

In nanoclay composites, particularly from montmoril-lonite, the shape and dimensions of platelets after extrusion process were observed using TEM and wet-STEM. The differences in the size observed between platelets using TEM and STEM techniques were attributed to the preparation by ultramicrotomy and the effect of projection [72]. Thus, in this case, the combination of microscopy techniques revealed the effect of each step of the process. 

Finally, signiflcant contrast increase of stained samples was reported when the HAADF-STEM technique was used for several systems, such as cellulose microfibers and whiskers within poly(lactic acid) [85],... 

The unique power of the scanning transmission electron microscopy (STEM) technique is its ability to examine isolated unstained structures in projection by mapping boundaries, internal mass distribution, and site-specific cluster labels, as well as measuring total mass. The digital STEM image can be compared directly to a computed projection of a model structure assembled from known or postulated components. If any feature of the proposed model is incorrect, the STEM image will provide direct statistical evidence as to the extent and significance of the discrepancy. This objective approach permits inclusion of a priori information from biochemistry and/or other structural studies to form a self-... 

In 1939 Kossel and Mollenstedt demonstrated the possibility of obtaining CBED patterns from crystals with more information than obtained from the more conventional focused patterns for special beam directions. Later, in 1940, it was demonstrated that the intensity of a reflection is a function of the excitation error, and the thickness, t, of a plane parallel crystal slab [3]. Lately, more advances in CBED became possible with the introduction of field emission guns and scanning TEM (STEM) techniques. Thus, the STEM instrument has electron optics similar to CBED for obtaining diffraction results from areas as small... 

High-iesolution scanning and transmission electron microscopy (HRSEM, HRTEM) can provide very specific information about surface films on any kind of particles. A comparison between pristine particles and particles scraped from cycled electrodes can provide very comprehensive information. Using element analysis, STEM techniques, and selected area electron diffraction (SAED), it is possible to map surface species in a nanometric scale [30]. 

The FEG-STEM technique was also applied to measuring the effect of irradiation on the levels of matrix Cu in an A533B-type steel with a bulk Cu value of 0.33 wt% (JPA). It appears that some homogeneous fine-scale (intra-granular) precipitation of Cu occurred during fabrication, with 0.08 wt% Cu in sub-microscopic clusters in the as-irradiated material. ... 

Currently, about 90% of is produced via steam reforming of natural gas or light oil fraction [72]. The reformate contains 1-10% of CO, which is poisonous to the noble electrocatalysts in PEMFCs. The WGS reaction is a key step to produce and upgrade H. Moreover, for mobile and residential fuel cell applications, the WGS catalysts must be nonpyrophoric, active over a broad temperature range, and stable during frequent shutdown/start-up cycles. Zhai et al. demonstrated that the xPt-yNa-SiOj catalyst was highly active in low-temperature WGS reaction [73]. Using the HAADF-STEM technique, they identified that on the Na-modified... 

Dispersious of IF-WS2 can also be smdied by the wet scaiming transmission electron microscopy (STEM) technique, which allows the observatiou of nano-objects dispersed in a... 

Figure 2.31 Characterization of the IF-WS2 dispersion in oU by the wet STEM technique. This...

XRE gives the total amount without differentiating between the amounts in exsolved regions and in solid solution in alite. The microscopy techniques permit the selection of regions without clusters. With SEM techniques, because of the large interaction volume, subsurface clusters not visible from the surface may be included, but the relatively large distance between exsolved regions makes this unlikely. The STEM technique allows a localised analysis of the composition, since the interaction volume is small because of the small thickness of the sample. Table 8.1 compares the results obtained with the different techniques to the calculated amount of each element from the synthesis recipe. 

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