Diffraction mode

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

The former procedure is the method of choice during operation in the image mode, while the latter condition is desirable for maximizing source coherency in the diffraction mode. 

A TEM provides the means to obtain a diffraction pattern from a small specimen area. This diffraction pattern is obtained in diffraction mode, where the post-specimen lenses are set to examine the information in the transmitted signal at the back focal plane of the objective lens. 

The signal to descan the diffraction image (when in diffraction mode)... 

In the case of P Cu-phthalocyanine (space group P2i/c) only 8 different zones were found at 100 kV via SAED using a rotation-tilt holder (a=14.50 A, b= 4.70 A, c= 19.32 A, b=123°). With a rotation-double-tilt holder and in nano diffraction mode it was possible to detect 17 different zones at 300 kV (initial zone [001] and tilt about a and b ) from one crystal at room temperature (a=14.50A, b=4.84A, c=19.4lA, p=120.3°). In Fig. 5 the initial zone (middle) and two other zones obtained through different tilt series (a left hand side b right hand side) are shown exemplarily. 

The knowledge of the structure and the morphology of the metal clusters is necessary if we want to understand the reaction kinetics at the atomic level. The more versatile technique to study the structure and the morphology of supported metal cluster is TEM. It can provide directly the structure and the epitaxial relationships on a collection of clusters in the diffraction mode. By High Resolution TEM it is possible to get this information at the level of one cluster [83]. By using high-resolution profile imaging it is possible to measure the lattice distortion at the interface [84], These capabilities are very unique for TEM. Such structural information can be obtained in situ by diffraction techniques but only on a collection of clusters [14, 29]. To illustrate the structural characterization by TEM we present the case of Pd clusters on MgO(l 0 0), which will be discussed in the next sections. 

In these experiments small particles are normally made by evaporating the substance onto carbon or another support usually used in electron microscopy. Many substances form small, almost spherical particles rather than a thin homogeneous film. In the imaging mode of the electron microscope the mean radius of the particles is measured. Then the lattice constant is determined in the diffraction mode. Particles of different radii are made by changing the evap-... 

There is another diffraction mode, called convergent beam electron diffraction (CBED), in which the incident electron beam is focused to a fine spot on the specimen. If the convergence angle is appropriately chosen, the diffraction pattern consists of an array of nonoverlapping disks. For thin specimens ( SO nm) the CBED disks are featureless, but... 

Figure 3.4 Optical paths of (a) diffraction mode and (b) image mode. SAD, selected area diffraction aperture.

A diffraction pattern is formed on the back-focal plane of the objective lens when an electron beam passes through a crystalline specimen in a TEM. In the diffraction mode, a pattern of selected area diffraction (SAD) can be further enlarged on the screen or recorded by a camera as illustrated in Figure 3.16. Electron diffraction is not only useful to generate images of diffraction contrast, but also for crystal structure analysis, similar to X-ray diffraction methods. SAD in a TEM, however, shows its special characteristics compared with X-ray diffraction, as summarized in Table 3.4. More detailed SAD characteristics are introduced in the following section. 

Kikuchi lines are pairs of parallel lines consisting of one bright and one dark line in the diffraction mode as shown in Figure 3.35. Kikuchi lines are named after the Japanese scientist, Kikuchi, who discovered them in 1928. Kikuchi lines appear when the selected area for diffraction is moved to a thicker section in the specimen where the diffraction spots become weaker, or even disappear. 

In diffraction mode, the projector lens system is adjusted in order to image the electron wave located at the back focal plane of the objective lens. What is seen on the screen is the intensity of this electron wave, which for coherent elastic scattering is called an electron diffraction pattern. 

The atomic structure of the carbon deposited was studied by TEM using a JEM-IOOC electron microscope in the micro-diffraction mode. The structure of the carbyne crystals is analyzed by atomic force microscopy (AFM). The images are obtained by contact mode. The film thickness was determined from SEM observations of the film cross sections. 

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