Dark-field

In SEM and STEM, all detectors record the electron current signal of tire selected interacting electrons (elastic scattering, secondary electrons) in real time. Such detectors can be designed as simple metal-plate detectors, such as the elastic dark-field detector in STEM, or as electron-sensitive PMT. For a rigorous discussion of SEM detectors see [3],... 

Figure Bl.17.3. STEM detectors (a) conventional bright and dark-field detectors, electrons are detected according to their different scattering angles, all other positional infonnation is lost (b) positional detector as developed by Haider and coworkers (Haider etal 1994).

Another specialized application of EM image contrast is mass measurement. Using the elastic dark-field image in the STEM or the inelastic image in the EETEM, a direct measurement of the scattering mass can be performed. Eor reviews on this teclmique see [60.61]. 

Figure Bl.18.5. Dark-field illumination the aperture of the objeetive is smaller than the aperture of the beams allowed by the annular diaphragm.

At this point it is worth comparing the different techniques of contrast enliancements discussed so far. They represent spatial filtering teclmiques which mostly affect the zeroth order dark field microscopy, which eliminates the zeroth order, the Schlieren method (not discussed here), which suppresses the zerotii order and one side band and, finally, phase contrast microscopy, where the phase of the zeroth order is shifted by nil and its intensity is attenuated. 

Tdrdk P, Sheppard C J R and LaczikZ 1996 Dark field and differential phase contrast imaging modes in confocal microscopy using a half aperture stop Optik 103 101-6... 

In dark-field electron microscopy it is not the transmitted beam which is used to construct an image but, rather, a beam diffracted from one facet of the object under investigation. One method for doing this is to shift the aperture of the microscope so that most of the beam is blocked and only those electrons... 

Figure 4.11 Electron micrographs of polyethylene crystals, (a) Dark-field illumination shows crystals to have a hollow pyramid structure. (Reprinted with permission from P. H. Geil, Polymer Single Crystals, Interscience, New York, 1963.) (b) Transmission micrograph in which contrast is enhanced by shadow casting [Reprinted with permission from D. H. Reneker and P. H. Geil, /. Appl. Phys. 31 1916 (I960).]...

Figures 4.1 la and b, respectively, are examples of dark-field and direct transmission electron micrographs of polyethylene crystals. The ability of dark-field imaging to distinguish between features of the object which differ in orientation is apparent in Fig. 4.11a. The effect of shadowing is evident in Fig. 4.11b, where those edges of the crystal which cast the shadows display sharper contrast.

Fig. 10. A dark field (DF) transmission electron micrograph showing interface in a continuous fiber (F) a-Al202 (F)/Mg alloy (ZE41A) matrix (M) within...

Fig. 1. Darkfield illumination the image is formed by light scattered from the specimen detail against a dark field.

Fig. 3. Schematic diagram of a dark field system for measuriag the light scatteriag by a spherical red blood cell where V = volume of sample and HC = hemoglobin concentration. Optical flow cell system having double-angular interval detection at angles 9 and 02.

There are three primary image modes that are used in conventional TEM work, bright-field microscopy, dark-field microscopy, and high-resolution electron microscopy. In practice, the three image modes differ in the way in which an objective diaphragm is used as a filter in the back focal plane. 

Figure 3 Bright-field (a) and dark-field (b) STEM images of crushed ceramic particles dispersed on a "holey" carbon film supported on an electron microscope grid (shown at the right).

The annular dark-field detector of the field-emission STEM (see Figure 2) provides a powerful high-resolution imaging mode that is not available in the conventional TEM or TEM/STEM. In this mode, images of individual atoms may be obtained, as shown in Figure 4 (see Isaacson, Ohtsuki, and Utlaut ). Some annular dark-field... 

Figure 5 Images of a thin region of an epitaxial film of Ge on Si grown by oxidation of Ge-implanted Si (a) conventional TEM phase contrast image with no compositional information and b) high-angle dark-field STEM image showing atomically sharp interface between Si and Ge. (Courtesy of S.J. Pennycook)...

S. J. Pennycook. EMSA Bulletin. 19, 67, 1989. A summary of compositional imaging using a high-angle annular dark-field detector in a field emission STEM instrument published by the Electron Microscopy Society of America, Box EMSA Woods Hole, MA 02543. 

Fig. 6. PCNTs with partially deposited carbon layers (arrow indicates the bare PCNT), (a) as-grown, (b) partially exposed nanolube and (c) 002 dark-field image showing small crystallites on the tube and wall of the tube heat treated at 2500 C.

The conventional hand of a particular isochiral cluster of tubes can be deduced from dark field diffraction contrast tilting experiments [26]. 

Fig. 7. Dark-field micrographs from Ref. for the Ni-0.247 A1 alloy after the two-step quench, first to T = 903 K, and then to T = 343 K (which supposedly corresponds to areas b and d, respectively, in Fig. 1), at following times t after the second quench (in minutes) (a) 0, (b) 10, (c) 100, (d) 1000, and (e) 10000.

Fig. 3 (left) TEM bright-field and (middle) dark-field images, and (right) selected area diffraction pattern from a 20 vol% Si3N4/5052 Al composite at 548 °C. (from Ref. [8,9])... 

Figure 48-6. Dark field electron micrograph of a proteoglycan aggregate in which the proteoglycan subunits and filamentous backbone are particularly well extended. (Reproduced, with permission, from Rosenberg L, Heilman W, Kleinschmidt AK Electron microscopic studies of proteoglycan aggregates from bovine articular cartilage. J Biol Chem 1975 250 1877.)...

---