Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 213

Analytical (scanning) transmission electron microscopy [(S)TEM], currently is the best method for both imaging and determining chemical compositions [18] whenever the relatively large excitation volume of SEM becomes problematic. The price to be paid is the necessity to prepare samples as thin as a few 10 nm, without preparation artifacts, to make them transparent to electrons. The task is difficult, but over the years a wide range of techniques have been developed for this purpose, including mechanical grinding, ion beam etching, preparation of replica, ultramicrotomy, and FIB machining of lift‐out TEM lamellae [19]. In this respect one should distinguish between samples taken from the bulk and cross‐sectional samples cut out from specifically targeted areas (e.g. if a specific source of devitrification in a glass has to be assessed for further analysis) for which FIB machining is particularly well suited.

In its principle, a transmission electron microscope is amazingly similar to a light microscope: the electron source corresponds to the lamp; beams are formed by electromagnetic lenses (which are true zoom lenses as one can finely change their focal length by altering the lens current); image and object planes are defined; and after converting the electron distribution into light with a scintillator, an image is formed and registered by a CCD or CMOS camera. Of course, however, TEM offers a number of tremendous advantages over light microscopy that result from the much lower wavelengths of electrons compared with those of visible light. First and foremost, it has a much higher spatial resolution, as the best commercially available instruments can now resolve lattice planes as close as 50 pm apart [20]. Second, unlike for optical lenses, the focal lengths of electron lenses can be changed during operation, allowing for switching between parallel illumination and formation of a focus spot of less than 100 pm in diameter for dedicated analyses of very small sample regions. Third, one can switch between imaging and diffraction modes, which makes it possible to identify crystalline phases from the recorded electron diffraction patterns. Fourth, the higher energy of electrons (as compared with visible light) allows electronic transitions to be excited, laying the ground not only for analyzing chemical compositions (EDXS) but also for assessing the coordination and valence of appropriate elements (electron energy loss spectroscopy [EELS] [21]).

In conventional TEM, the sample is illuminated with a parallel beam. Bright‐ or dark‐field micrographs are recorded depending on whether or not one includes in the image formed the primary (central) beam in the diffraction plane, which is the back focal plane (Figure 3). Contrast, i.e. the visibility of objects, in bright‐field imaging is simply explained: if a certain area scatters (inelastically) or diffracts (elastically) more strongly than its surroundings, the electrons passing through this area will be deflected to higher angles than the non‐scattered or less scattered/diffracted electrons. By introducing a contrast aperture (the objective aperture in Figure 3), one blocks electrons traveling far from the optical axis. As a consequence, the corresponding areas in the micrograph appear less bright.

For glasses and glass ceramics, microstructures typically generate various kinds of contrasts. In liquid–liquid phase separation, droplets containing elements heavier than those of the surrounding matrix appear as less bright regions on micrographs (although atomic number differences can be so small as to make phase‐separated droplets hardly discernable). Crystalline precipitates always appear much darker than their glassy surroundings for two reasons: First, crystal lattices induce strong diffraction up to high angles, particularly when they are well aligned along a low‐index crystallographic axis. Second, when these precipitates preferentially incorporate heavy elements, their cross sections for inelastic scattering are also larger than those of their glass surroundings. The effect is seen in Figure 4a for the bright‐field TEM micrograph of a MAS glass ceramics similar to that discussed above for SEM (both from the same green glass, with just a different annealing such that only zirconia precipitated in that case).

Figure 3 Electron beam paths in the parallel sample illumination mode in TEM. Left: direct beam used for imaging (bright‐field imaging technique). Right: diffracted beam used for imaging (dark‐field imaging technique). In both cases one selects the appropriate beam by moving the objective aperture in the back focal plane (diffraction plane).

As already mentioned, one can readily tune electron lenses to the desired focal length to image also the back focal (diffraction) plane. A well‐oriented zirconia is revealed in this way in the MAS glass‐ceramic sample (Figure 4a). Information on circular areas of less than 200 nm in diameter can even be obtained with the so‐called selected area electron diffraction (SAED) aperture as illustrated in Figure 4b for the MAS sample. Indexing of the pattern further shows that the ZrO₂ crystal is aligned along the [100] direction, which unambiguously indicates its tetragonal symmetry.

Dark‐field and bright‐field imaging are again closely related (Figures 3 and 4c) because a small contrast‐enhancing aperture can also be positioned in the diffraction plane of the objective lens in such a way that the direct (undiffracted) beam is not allowed to pass through it. Originating either from crystal diffraction spots or from amorphous diffraction rings, some of the diffracted intensity is allowed instead to travel further down the microscope column and form an image. Because the relatively small apertures (>5 μm) are circular holes and not annular rings, dark‐field imaging always catches only a fraction of the diffracted intensity, resulting in much longer exposure times and associated risk of running into drift issues. Dark‐field imaging is nevertheless often useful since its sensitivity to small differences in diffracting/scattering power is clearly superior to that of bright‐field imaging. In addition, it often makes a “quick survey” possible for crystalline areas in the sample. As an example, the zirconia crystals embedded in the glassy MAS matrix appear as the bright area in the dark‐field image of Figure 4c, where only significantly diffracted or scattered electrons contribute to the contrast with the dark surrounding glassy matrix.

Figure 4 Transmission electron micrographs of the MAS glass ceramics. (a) Bright‐field image where crystalline ZrO₂ features appear dark in a glass matrix of bright contrast. (b) Electron diffraction pattern taken from a sample area circled in (a). (c) Dark‐field TEM micrograph of the same sample area. Crystal appearing bright because the objective aperture is moved away from the direct beam to allow only diffracted beams to pass by. (d) Aberration‐corrected high‐resolution TEM micrograph of a part of the sample where a ZrO₂ crystal has a boundary with the surrounding glassy matrix.

With high‐resolution TEM (HR‐TEM), one can image directly rows of atoms in the crystalline parts of the specimen provided that the sample is thin enough and that crystallographic directions are well aligned with respect to the optical axis, which is accomplished by phase‐contrast imaging. The physics involved will not be expounded here because it is beyond the scope of this chapter, but one can think of a crystalline specimen as a weak phase object analogous to a phase grating for visible light [18]. Interatomic potentials within the crystals then affect the wave functions of the incoming electrons so that one can extract from these modifications a picture of the atomic lattice of the crystal that is visualized on a fluorescent screen or captured with a CCD or CMOS camera. The resolution that can be obtained is well below the nanometer range, as shown in Figure 4d where a sharp boundary separates the regular crystal lattice of zirconia from the disordered atomic arrangement of the glass matrix.