Transmission electron microscopic feature

Transmission electron microscopes (TEM) with their variants (scanning transmission microscopes, analytical microscopes, high-resolution microscopes, high-voltage microscopes) are now crucial tools in the study of materials crystal defects of all kinds, radiation damage, ofif-stoichiometric compounds, features of atomic order, polyphase microstructures, stages in phase transformations, orientation relationships between phases, recrystallisation, local textures, compositions of phases... there is no end to the features that are today studied by TEM. Newbury and Williams (2000) have surveyed the place of the electron microscope as the materials characterisation tool of the millennium . 

By use of a scanning transmission electron microscope, with the incident beam grazing the crystal surface, the structural features on surfaces have also been revealed with a resolution of I08 or better... 

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 scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. 

The hybridization of carbon atoms is the major structural parameter controlling DLC film properties. Electron energy loss spectroscopy (EELS) has been extensively used to probe this structural feature [5. 6]. In a transmission electron microscope, a monoenergetic electron beam is impinged in a very thin sample, being the transmitted electrons analyzed in energy. Figure 27 shows a typical... 

Fig. 2. Principal features of (a) a transmission electron microscope and (b) a scanning electron microscope.

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. 

This chapter describes briefly the basic construction and characteristics of the modern transmission electron microscope and discusses its principal modes of operation. Because the electron microscope is an analogue of the optical (or light) microscope, we also consider briefly the basic features of the optical microscope this will also provide a link with our earlier discussion of the optical principles of image formation by a lens. 

Rapid alloying (RA) is a fast diffusion process that was experimentally discovered by Yasuda, Mori, and co-workers (YM) in binary microclusters. By using an evaporator, they deposited individual solute atoms (Cu) on the surface of host nano-sized clusters that are supported by amorphous carbon him at room temperature. YM observed the alloying behavior with a transmission electron microscope in in situ condition as schematically described in Fig. 1. In Ref. 7 it is demonstrated that Au clusters promptly changed into highly concentrated, homogeneously mixed (Au-Cu) alloy clusters. RA is similarly observed in various nano-sized binary clusters, such as (Au-Ni), (In-Sb), (Au-Zn), and (Au-Al) [7]. They examined the presence and absence of RA for clusters of different sizes. Consequently, YM summarize the unusual features of RA as follows ... 

The structural features of the solid carbon deposit were established from examinations carried out in a JEOL 2000EXII transmission electron microscope. This instrument has a lattice fringe resolution of 0.14 nm. Suitable transmission specimens were prepared by ultrasonic dispersion of a small quantity of the carbonaceous deposit in isobutanol and then application of a drop of the supemate to a holey carbon film. Inspection of many areas of such specimens revealed that in dl cases the major type of material generated in these reactions consisted of filamentous carbon structures. 

One study in our laboratories investigated the direct link between PPy film morphology and mechanical properties.126 Observation of the fracture surfaces of the films showed a roughened surface with cone-shaped features (Figure 3.9), similar to those observed in transmission electron microscope (TEM) micrographs of the film cross sections (described earlier). It was concluded from this study that the cone boundaries are points of weakness within the film that allow easier crack propagation. Thus, films with less prominent boundaries should show improved fracture resistance. 

In the transmission electron microscopic (TEM) studies, we found that it was exceedingly difficult to obtain ultramicrotomed sections of iPS-iPP blends, whereas the iPS-fo-iPP diblock copolymer could be cut with relative ease. This result exhibits one major difference between the diblock copolymer and the corresponding homopolymer blend. Unfortunately, owing to the difficulty of finding a selective staining technique, the sample of diblock copolymer did not display visible contrast or obvious structural features in the TEM studies. However, the results of SEM studies do reveal a clear difference between the blend and the diblock copolymer the macrophase separation is revealed on the etched surface of the blend and is not present in the copolymer (Figure 3). The diblock copolymer exhibits only a finely dispersed and continuous submicron structure throughout the field of view, as expected. 

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