Phase-contrast TEM

Fig. 8.3 Phase-contrast TEM image of SiC whiskers in an alumina matrix after initial stage of oxidation in which graphitic carbon (C) and silicate glass (G) are formed at the interface.

Fig. 8.4 Phase-contrast TEM image of SiC whisker in alumina matrix that has undergone oxidation. The silicate glass (G) has begun to react with alumina to form a mullite nucleus (M), and graphitic carbon (C) surrounds the crystallite.14...

Figure Bl.17.5. Examples of CTFs for a typical TEM (spherical aberration = 2.7 mm, 120 keV electron energy). In (a) and (b) the idealistic case of no signal decreasing envelope fimctions [77] are shown, (a) Pure phase contrast object, i.e. no amplitude contrast two different defocus values are shown (Scherzer focus of 120 mn imderfocus (solid curve), 500 mn underfocus (dashed curve)) (b) pure amplitude object (Scherzer focus of 120 mn underfocus) (c) realistic case mcluding envelope fimctions and a mixed weak...

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. 

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)...

Fig. 3 TEM (top) and AFM phase contrast images (bottom) of aqueous micelles formed by a PS200-P2VP140-PEO590 ABC triblock copolymer at pH > 5 (left) and pH< 5 (right). For TEM pictures, the PS and P2VP blocks have been stained by Ru04. AFM images have been recorded with tapping mode (contrast scale black 0°, white 45°). Adapted from [47]...

Based on TEM studies of supported metal catalysts, several workers have concluded that their catalysts were made of two-dimensional discs or rafts , where virtually all atoms are at the particle surface. However, sample tilting experiments in TEM have shown that great care should be exercised in the interpretation of TEM images of small particles (<2 nm in size), since phase contrast effects may dominate and variations in the particle contrast with specimen orientation can occur as a result of amplitude contrast effects (Treacy and Howie 1980). Sample tilting is therefore necessary to ensure correct interpretations of TEM images of metal-particle catalysts. This will be discussed further in the following sections. 

Transmission electron microscopy (TEM) has been an underutilized yet valuable too in particle size characterization of MC particles in LB films. Monolayer films of trioctylphosphine oxide-capped CdSe (18), spread as a monolayer on an aqueous subphase, were transferred to a TEM grid. A close-packed hexagonal arrangement of 5.3-nm (cr —4%) crystallites was found. TEM images were also obtained for HMP-stabilized CdS incorporated in BeH/octadecylamine films (79) and for CdS formed under an amine-based surfactant monolayer and transferred to a TEM grid (14). In one study, direct viewing of CdS and CdSe particles made from Cd2+-FA films on TEM grids was not possible due to poor phase contrast between the particles and the film (30). Diffraction patterns were observed, however, that were consistent with crystalline (3-CdS or CdSe. Approximately spherical particles of CdSe could... 

There are three main formation mechanisms of TEM image contrast massthickness contrast, diffraction contrast and phase contrast. When electrons pass... 

GM = geometric mean GSD geometric standard deviation PCM = phase contrast microscopy TEM = transmission electron microscopy... 

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