In recent years, nanoparticles have become the focus of increasing interest. A variety of different techniques are in use to prepare small clusters. The main aim of this chapter is to describe two different techniques for the preparation of clusters in the size range between 1 and 10 nm. However, examples of long needles or rods diameters of a few nanometers and lengths in the micrometer range are presented. The outstanding characteristic behavior in comparison with bulk material depends entirely on the size and geometrical structures of the clusters. These characteristic properties are very important, for instance, in the case of heterogeneous catalysis, to name only one of their important features. High-resolution transmission electron microscopy (HRTEM) is one of the most powerful tools for the structural characterization of particles, even though its interpretation sometimes turns out to be difficult. Computer simulations and the comparison between calculated images and experimental pictures serve as an important technique for overcoming this handicap. Following the description of some experimental designs for preparing these nanoparticles, techniques for performing these simulations will be discussed in detail. It will also be explained how structural and symmetry information can be obtained with the use of the Fourier transform of the images. From the images in the reciprocal space, much valuable information, such as lattice parameters and lattice angles, can be obtained, which are also described in this chapter. Examples of experimental results will be given. The main focus, however, is on Cu clusters prepared by the inert gas aggregation technique, although materials like Ag and CdS are also discussed. Furthermore, some examples of chemical reaction techniques that result in ligand stabilized clusters are described. The procedure of the chapter is the following: description of the preparation of clusters, computer simulation using the mul-tislice technique, and, finally, comparison between examples of different materials with computer simulations and presentation of the results. Finally, the chemical reactivity of Cu clusters for the oxidation process is discussed.

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