Surface profile imaging

The (113), (110) and (112) crystal planes on the surface are indicated, (b) HRTEM image of HgBa2Cu04+6 viewed down the [010] direction, showing a disordered coating on the (100) surface. The insets are EDS spectra from the surface area (top left) and the interior area (top right). 

Structural information as well as subterranean structures of solids. The surface-related properties of materials can therefore be better understood [46], There are several other advantages of surface investigation by HRTEM. For example, specimen preparation is simple. Normally, small particles with any size and any morphology can be directly used. Multiple scattering can normally be ignored, since the surface areas are often very thin and can legitimately be treated as weak phase objects, where the image intensity indicates the projected electrostatic potential. 

By exposing a clean crystal surface to the electron beam, a series of H RTEM surface profile images can be recorded and a movement of surface atoms from a high-energy site to a low-energy site may be observed [47]. 

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In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Briscoe et al 1984). Figure 2.1 b) shows a surface profile image (Datye et al 1992). 

Surface properties of mesoporous materials are sometimes important and TEM surface profile imaging is often used to investigate the surface structures of these materials. The advantages of this method are that it can be used to study the surfaces of small crystallites of almost any morphology, that the specimen preparation is as simple as that for the studies of bulk structures without requiring any special treatment and that, unlike scanning tunnelling... 

Figure 3. TEM surface profile images of MCM-41 specimens overheated at 165 °C for (a) 96 h and (b) 48 h. The view directions are along (a) the [100] direction and (b) the pore axis.

Figure 10.11b shows a typical HRTEM surface profile image of La2Cu04. It was found that the image contrast pattern in the top surface layers is different from that in the bulk area. Image simulation revealed that the (001) surface of La2Cu04 crystals was frequently coated by several atomic layers of C-La203 [47, 48]. 

When HRTEM is used for examining nanoparticles of oxides, in which the proportion of surface area greatly increases, most structural information concerns the surface. For example, HRTEM images of core-shell quantum dots can show the shell structure and its thickness directly. HRTEM images of metal oxide nanotubes can also be regarded as surface profile images. The appHcation of TEM in nanomaterials will be further discussed below. 

D. White, S. Ramdas, J. L. Hutchinson, and P. D. BiUyard [1989] Surface Profile Imaging of a Bismuth Uranium Oxide, Bi2UOe, Ultramicroscopy i, 124—131. 

Figure 3-30. Surface profile image of a growing gold crystal recorded along [110]. The left arrow in a) shows a column of only 2-3 atoms, b) 3s later, two additional columns have been formed. Figures c) and d) were recorded within O.Ss and show how a new missing row reconstruction is accomplished. During the crystal growth columns are frequently removed.

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