And Experimental Results

It has already been pointed out that for the calculation of EM images the projected potentials of the samples must be known. Obviously, to be able to compare experimental images with calculated images the atomic positions in the sample must be known. Generally, certain structures are assumed. We will discuss these structures, which have recently been observed in the case of metals such like Au, Ag, Pd, Cu, etc. Examples are only given for Cu and Ag samples. We will also discuss structures of compounds. For each typical model, examples of EM images as computer simulations and experimental images will be given. The experiments are performed with a Philips CM 2oo FEG microscope equipped with a field emission gun (2oo kV, Cs — 1.35 mm). The calculations are performed with the data of this microscope. In the following, fcc structures in the form of cuboctahedra and hexagonal structures are investigated. Examples of multiply twinned particles with 5-fold symmetry (MTP), that is, icosahedra and decahedra, are also presented. Small clusters consisting of only a few of atoms can be explained by means of the electronic shell model introduced by Knight et al. [33] and Ekardt [34]. However, clusters under the present investigation have the most stable form with closed atomic shells, which are caused entirely by their geometrical arrangement [35]. The stable clusters consist of defined numbers of atoms, that is, magic numbers that depend on the structures.

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