Image, dark-field

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]. 

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. 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.

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])... 

Overall platelet dimensions of mineral aurichalcite did not appear to change during calcination, but became polycrystalline and porous. By dark field Imaging in the TEM, the ZnO particles were observed to be uniformly and highly dispersed. The porosity can be accounted for by the approximately threefold increase in density of Zn atoms upon decomposition of aurichalcite to ZnO. For this density change to occur with a constant overall platelet volume, pores must form. 

Characterization methods. The 100 kV Vacuum Generator HB-5 STEM was used to mlcroanalyze samples. The HB-5 has a KEVEX SI(LI) energy dispersive X-ray spectrometer (EDS) and micro area electron diffraction (MAED) capabilities In conjunction with simultaneous bright and dark field Imaging capabilities. A more detailed explanation of the Instrumental operation can be obtained In a publication by C. Lyman(12). 

Figure 4. Electron diffraction pattern (bottom left) of a typical disordered wol-lastonite specimen. Evidence of the disorder comes from the streaked (Tc odd) diffraction spots. The fringes shown in the dark-field image (a) are 7 A apart. Dark-field image b, taken from the streaked diffraction spots, shows a one-dimensional image of the disordered wollastonite. Dark-field image c shows a two-dimensional image which shows the haphazard stacking of the triclinic and monoclinic...

Electrons are diffracted by particles if these are favorably oriented towards the beam, enabling one to obtain dark field images as well as crystallographic information. 

Figure 7.5 shows images of a Pd/AI2CF catalyst in bright and dark field mode. In the latter, the geometry was chosen such to selectively detect the (111) and (200) diffraction rings of Pd. Features indicated by arrows clearly demonstrate the better contrast and resolution of the dark field image [14],... 

Dark field images can be used to determine the shape of a small particle as shown by Yacaman et al. (6-7). When an electron beam enters on a small particle the presence of wedges will produce a splitting on the diffracted spots as shown by Gomez et al (8). A... 

It is sometimes argued that - by reciprocity - STEM and HRTEM can perform similar tasks since a STEM can be thought of as a HRTEM with detector and electron source being exchanged if the detector is a point detector on the optical axis . Indeed, bright field -and annular dark field imaging can be executed in either a STEM or a HRTEM as shown for the latter in Figure 4. ... 

Figure 4. Bright - and annular dark field image of a CdSe tetrapod and an adjacent gold particle recorded in a TEM. The line profiles demonstrate Z-contrast in the dark field image.

---