Transmission electron diffraction techniques

The ordered structure and molecule orientation in the monolayers, as suggested by the Hardy model, have been studied by various means. Electron diffraction techniques, for example, including both reflection and transmission, have been employed to examine the molecular orientation of adsorbed monolayers or surface hlms. The observations from these studies can be summarized as follows [3]. 

Convergent-Beam eleetron Diffraction and Microdiffraction) become available on analytical transmission electron microscopes. Most of the electron diffraction techniques use a stationary incident beam, but some specific methods like the precession method take advantage of a moving incident beam. 

Various electron diffraction techniques are available on modem transmission electron microscopes. Selected-Area Electron Diffraction (SAED) and Microdiffraction are performed with a parallel or nearly parallel incident beam and give spot patterns. Convergent-Beam Electron Diffraction (CBED) and Large-Angle Convergent-Beam Electron Diffraction (LACBED) are performed with a focused and defocused convergent beam... 

Recent relevant work has made use of transmission electron diffraction, the theory of which we discussed in Section 2.5. This technique has a very straightforward interpretation which, combined with improved experimental methods, has provided very precise results. Two papers are of particular importance in this context. Garoff et al. [137] studied monolayers of cadmium stearate. Their substrates were 2 nm layers of SiO coated on 10 nm layers of amorphous carbon on 200 mesh Ni elec-... 

Changes in the morphological and structural characteristics of the carbon deposit resulting from pretreatment of the iron catalyst in H2S were determined from a combination of transmission electron microscopy techniques, X-ray diffraction, surface area measurements and controlled oxidation studies in CO2. Iron powder 200 mesh) was purchased from Johnson Matthey Inc. (99 99% purity) and had a BET surface area of 0.3 m2/g at -196°C, The gases used in this work CO (99 9%), hydrogen (99.999%), ethylene (99.999%), H2S/argon mixtures and helium (99,999%) were obtained from Alphagaz company and used without further purification. 

X-ray and electron diffraction techniques have been used to obtain the data on epitaxy. X-ray diffraction methods are particularly useful for thick oxide films and have the advantage of giving diffraction patterns from both the oxide film and the metal suhstrate. For oxide films less than several hundred Angstroms thick, electron diffraction techniques are necessary in most cases, hi general, an electron diffraction pattern is not obtained from the metal substrate unless the oxide film is extremely thin, the surface is only partially covered with oxide, or the metal surface is rough. Reflection type diffraction techniques have been used with bulk specimens and transmission techniques with thin specimens and stripped oxide films from bulk metal specimens. Bach technique has its special advantages and limitations, but these will not be discussed here. 

These characteristics confer very interesting mechanical properties on microfibrils. Transmission electron-diffraction methods have made a contribution to the quantification of the degree of crystalhnity. Thus, using the technique of image reconsfruction it was shown that, in the microfibril of Valonia cellulose, which has a diameter of about 200 A, there could be more than 1000 cellulose chains, all aligned parallel in an almost perfect crystalline array. 

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

Transmission electron microscopy (tern) is used to analyze the stmcture of crystals, such as distinguishing between amorphous siUcon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain stmctures. Tern requires samples that are very thin (10—50 nm) sections, and is a destmctive as well as time-consuming method of analysis. 

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

Asbestos fiber identification can also be achieved through transmission or scanning electron microscopy (tern, sem) techniques which are especially usefiil with very short fibers, or with extremely small samples (see Microscopy). With appropriate peripheral instmmentation, these techniques can yield the elemental composition of the fibers using energy dispersive x-ray fluorescence, or the crystal stmcture from electron diffraction, selected area electron diffraction (saed). 

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of sihcon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-siUcon carbide formed by transformation from the P-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

The Fe-B nanocomposite was synthesized by the so-called pillaring technique using layered bentonite clay as the starting material. The detailed procedures were described in our previous study [4]. X-ray diffraction (XRD) analysis revealed that the Fe-B nanocomposite mainly consists of Fc203 (hematite) and Si02 (quartz). The bulk Fe concentration of the Fe-B nanocomposite measured by a JOEL X-ray Reflective Fluorescence spectrometer (Model JSX 3201Z) is 31.8%. The Fe surface atomic concentration of Fe-B nanocomposite determined by an X-ray photoelectron spectrometer (Model PHI5600) is 12.25 (at%). The BET specific surface area is 280 m /g. The particle size determined by a transmission electron microscope (JOEL 2010) is from 20 to 200 nm. 

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