Transmission electron microscopy advantage

Transmission electron microscopy is very widely used by biologists as well as materials scientists. The advantage of being able to resolve 0.2 nm outweighs the disadvantages of TEM. The disadvantages include the inabiUty of the common 100-kV electron beam to penetrate more than a few tenths of a micrometer (a 1000-kV beam, rarely used, penetrates specimens about 10 times thicker). Specimen preparation for the TEM is difficult because of the... 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

The most significant advantage of these more quantitative methods is that, in a binary system, only one sample is needed to determine the position of both phase boimdaries in a two-phase field. Further, if the alloy lies in the two-phase field over a wide range of temperatures, it is feasible that only one alloy need be used to fix the phase boundaries over this range of temperature. In a ternary system the analogous position is found with three-phase fields and, as these also define the limiting tie-lines of the three sets of two-phase fields, substantial information can be gained from the accurate determination of only one alloy. More recently transmission electron microscopy (TEM) has been used which is particularly valuable when microstructures are very fine as, for example, found in yTiAl alloys (Chen et al. 1994). 

In this report, the advantages of applying transmission electron microscopy (TEM) in this field are demonstrated. For example, it allows us to observe directly the mesopore systems, to detect the local structures such as surface structures, local defects and the morphologies of the particles, to image directly ordered and partially ordered metal nanoparticles loaded inside the mesopores and to identify possible new phases in a multiphasic specimen. 

Scanning Transmission Electron Microscopy (STEM) [22]. STEM represents a merger of the concepts of TEM and SEM. Modes of operation and mechanisms of contrast and of imaging are essentially the same as in CTEM but the main advantage of STEM is the ability to carry out microanalysis at very high resolution (see Section H). 

The advantage of this reaction lies on the creation of nitrogen, which makes an inert ambience to prevent nanoparticels from further oxidation. So it is convenient route to make metal nanoparticles. The formation of the assembled iron particles can be directly seen by transmission electron microscopy (TEM) image (in Figure la), the average size of assembled spheres is about 210 10 nm. The X ray diffraction pattern of the products (Figure lb) revealed the formation of cubic Fe [21]. 

Transmission electron microscopy is valuable because it allows imaging of even the smallest supported clusters of a heavy metal such as platinum. Dark field microscopy has been used to advantage for platinum clusters consisting of < 20 atoms each in zeolite KLTL [30]. Even single platinum atoms can be detected on zeolite supports thinner than about 20 nm, but the precision with which clusters can be pinpointed in the structure is limited by beam damage-induced distortion of the zeolite framework [30]. 

This advantage can be used for growing nanowires (wires with nanometric diameter). Nanoporous membranes that can be fabricated by the anodic oxidation of aluminum are appropriate templates. This process leads to the formation of an alumina layer with parallel nanopores, as shown in Fig. 15A, which can then be filled by electrodeposition. Fig. 15B shows a schematic view of a multilayer nanowire and Fig.l5C a transmission electron microscopy image of a Cu/ CuCoNi layered nanowire grown in the nanopores. 

High-resolution transmission electron microscopy can be understood as a general information-transfer process. The incident electron wave, which for HRTEM is ideally a plane wave with its wave vector parallel to a zone axis of the crystal, is diffracted by the crystal and transferred to the exit plane of the specimen. The electron wave at the exit plane contains the structure information of the illuminated specimen area in both the phase and the amplitude.. This exit-plane wave is transferred, however affected by the objective lens, to the recording device. To describe this information transfer in the microscope, it is advantageous to work in Fourier space with the spatial frequency of the electron wave as the relevant variable. For a crystal, the frequency spectrum of the exit-plane wave is dominated by a few discrete values, which are given by the most strongly excited Bloch states, respectively, by the Bragg-diffracted beams. 

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