Scanning transmission dedicated

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

The dedicated scanning transmission electron microscope (STEM) is an integral tool for characterizing catalysts because of its unique ability to image and analyze nano-sized volumes. This information is valuable in optimizing catalyst formulations and determining causes for reduced catalyst performance. For many commercial catalysts direct correlations between structural features of metal crystallites and catalytic performance are not attainable. When these instances occur, determination of elemental distribution may be the only information available. In this paper we will discuss some of the techniques employed and limitations associated with characterizing commercial catalysts. 

A dedicated STEM is an instrument that has no postspecimen lenses and images only in the scanning transmission mode. It is generally equipped with a field emission electron source, which is very bright and coherent allowing for maximum resolution (10). 

Fig. 3.1. A schematic diagram of electron-optics in a dedicated Scanning Transmission Electron Microscope (VG HB501 STEM).

Scanning transmission electron microscopy gives essentially the same type of results and has the same type of difficulties as the conventional TEM. There are two types of instruments, the dedicated STEMs, which generally have a UHV column, and the TEM based instruments mostly known as AEMs (analytical electron microscopes). A detailed comparison of STEM and TEM was given in Section 2.4.1.3. There are some advantages in using the STEM on polymer samples in particular it seems that thicker samples can be used. However, the added complexity and cost, combined with lower resolution in the AEM STEM mode, make it unlikely that either kind of instrument would be purchased for polymer studies. 

Scanning transmission electron microscopy (STEM) gives essentially the same type of results and has the same type of difficulties as the conventional TEM. There are two types of instruments, the dedicated STEMs, which generally have an ultrahigh vacuum (UHV) column, and the TEM based instruments mostly known as AEMs (analytical electron micro-... 

In a few cases, analyses may be done on the process floor using a conventional scanning spectrophotometer and the usual laboratory cuvette or a specially made vial that fits into the sample cavity of the instrument. Cuvettes may be made from quartz (various grades depending upon UV transmission requirements), or from plastic (disposable) when UV wavelengths are not required for the measurements. In most cases these at-line analyzers have lower performance characteristics than dedicated laboratory instruments, but may be quite adequate for the task at hand. However, their use still requires manual sampling... 

Dedicated PET systems are usually based on lull ring detector systems with an axial field of view exceeding 15 cm, and can be operated in septa extended (2D mode) or septa retracted mode (3D) for patient examinations. Some systems only provide 3D-data acquisition modes. Most systems allow the simultaneous acquisition of 36 transversal slices and more with a theoretical slice thickness of 2-5 mm. Transmission scans for a total of 10 min are obtained prior to the radionuclide application, for the attenuation correction of the acquired emission tomographic images. All PET images are attenuation corrected and iteratively reconstructed. 

Resolution in the STEM is limited by the probe diameter, which is about 1 nm in equipment dedicated to this operating mode, at the cost of using a cold field emission gun requiring an ultravacuum. Because of the high-precision optics and the point-by-point image formation principle, the STEM combines the advantages of scanning electron microscope analysis with resolution performance levels similar to the transmission electron microscope. 

Because of this wide range of applications, much effort was dedicated to the design and synthesis of new molecules with optimized TPA efficiency in this context, the characteristics of the designed molecules (linear absorption, solubility, substituents...) will depend on the targeted application. The TPA response of moleciUes can be imderstood in the context of its TPA cross-section ajpA. which can be measmed using different techniques, such as nonlinear transmission, two-photon induced fluorescence and the Z-scan method although in a pure TPA process ajpA does not depend on the laser pulse duration, the nonlinear absorption can be more efficient in the nanosecond regime than in the femtosecond one, due to excited state reabsorption phenomena [34]. 

An important advantage of a CT transmission scan in the PET/CT system is that the scan data can be used for attenuation correction of PET emission images, obviating the need for a separate lengthy transmission scan in the dedicated PET system. The use of CT scans for attenuation correction reduces the whole-body scan time significantly. CT attenuation correction and fusion of CT and PET images are discussed in detail in Chap. 3. 

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