Transmission electron microscope image formation

Figure 9. Simplified ray diagram (Abbe diagram) that shows simultaneous formation of the diffraction pattern and the corresponding real space image in a transmission electron microscope (TEM).

Image formation in a transmission electron microscope can be considered as a two-step process. In the first step, the electron beam is interacting with the specimen. This interaction is very strong compared to X-ray or neutron scattering and causes multiple scattering events. In order to understand this process, the classical particle description of the electron is not adequate, and the quantum mechanical wave formalism has to be used. Thus, assuming the... 

CMK-1 carbon was the first carbon material reported to exhibit well-resolved XRD lines characteristic of ordered arrays of carbon mesopores [3]. The synthesis of the carbon was achieved by carbonization of sucrose inside the MCM-48 mesoporous silica. As shown in Fig. 2, the XRD pattern exhibits a new diffraction line around 1.4 compared with its MCM-48 template. This change can be explained by the formation of two separate carbon networks in the bicontinuously mesoporous MCM-48 template. After the separating silica frameworks are removed, the two carbon networks join together. The joining of the two carbon networks attributes to the syirunetry change from cubic Io3d to either 74,/a or lower [12]. The new ordered mesoporous structure is indicated by the XRD pattern and transmission electron microscopic image shown in Fig. 2. 

This chapter describes briefly the basic construction and characteristics of the modern transmission electron microscope and discusses its principal modes of operation. Because the electron microscope is an analogue of the optical (or light) microscope, we also consider briefly the basic features of the optical microscope this will also provide a link with our earlier discussion of the optical principles of image formation by a lens. 

Figure 14.1. Schematic diagram showing the principle of image formation and diffraction in the transmission electron microscope. The incident beam/o illuminates the specimen. Scattered and unscattered electrons are collected by the objective lens and foeused back to form first an electron diffraction pattern and then an image. For a 2D or 3D crystal, the electron-diffraetion pattern would show a lattice of spots, eaeh of whose intensity is a small fraetion of that of the incident beam. In praetiee, an in-focus image has no eontrast, so images are recorded with the objeetive lens slightly defocused to take advantage of the out-of-focus phase-contrast mechanism.

When crossing a sample, an electron beam may be partially adsorbed and partially deflected. Via the use of electromagnetic lenses, a certain fraction of these electrons, and of those that have not been deflected, can be recombined to form an image. The use of transmission electron microscopy is based on controlling the electrons involved in image formation. The transmission electron microscope thus offers an image of the sample that depends on the electron-matter interactions. 

The principle of the scanning transmission electron microscope (STEM) is, at first glance, very different from that of the transmission electron microscope the electrons are focused on a probe scanned on a sample and the transmitted electrons are detected on a scintillator via a collection aperture. There is, however, a so-called reciprocity relationship between transmission electron microscopy and the STEM that can be used to describe image formation using the same formalism and facilitates the understanding of contrast. 

Resolution in the STEM is limited by the probe diameter, which is about 1 nm in equipment dedicated to this operating mode, at the cost of using a cold field emission gun requiring an ultravacuum. Because of the high-precision optics and the point-by-point image formation principle, the STEM combines the advantages of scanning electron microscope analysis with resolution performance levels similar to the transmission electron microscope. 

Figure 22. Transmission electron microscope image of an single anatase crystal formed by oriented aggregation. Crystal margins are marked with arrows tips a subset of the particles that formed the aggregate are numbered. Dislocation positions are indicated by arrows (view at low angle). The diagram on the right illustrates formation of an edge dislocation at an interface where one particle (black) has a surface step (Perm and Banfield, impublished see Perm and Banfield 1998).

The proposal that the nitrogen effect involved formation of nitrides which prevented the alumina from becoming continuous has been verified by elemental spectroscopic imaging (ESI) in a transmission electron microscope [70]. An ESI map for the elements II, Al, O, and N from a cross-section of the scale/alloy interface of Ti-50 at% Al oxidized in air for one hour at 900°C is presented in Figure 19. These maps indicate that the alumina is broken up by islands of TiN. 

FIGURE 8.61 Transmission electron microscope (TEM) images of PPy/carbon nanotube nanoscale composites with different film thicknesses due to different PPy film-formation charges (a) 86.1 mC/cm (b) 207.9 mC/cm (c) 681.9 mC/cm (d) 1308.6 mC/cm (e) Transmission electron microscope (TEM) image of a long PPy-coated carbon nanotube (681.9 mC/crtd). (From Chen, J.H., et al., Appl. Phys. A, Ti, 129, 2001. With permission.)... 

We shall use Eqs. (73) and (75) to study image formation in two types of optical instruments, the transmission electron microscope (TEM) and its scanning transmission counterpart, the STEM. This is the subject of the next section. 

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