The Role of Defects in Nucleation and Growth

A key aspect of the deposition and diffusion of metal atoms and clusters on oxide surfaces is the role played by point and extended defects. There is little doubt that the nucleation and growth of clusters and small metal particles occur at defect sites [26-33]. A recent example is that of atomically resolved STM images of Au atoms on Ti02 [34] (Fig. 2.2). 

relatively little is known about the nature of the defects involved in this process. So far most (if not all) of the attempts to define the site where two metal atoms combine to form a dimer and start the nucleation of the cluster are limited to theoretical analyses [35,36]. Also, in this case data are extremely scarce and the results are not unambiguous. To make an example, there is general consensus that oxygen vacancies (F centers, see Sect. 2.3.4) at the surface of MgO act as strong traps for the metal atoms diffusing on the surface [26-33]. However, the general assumption that F centers are nucleation 

An accurate answer to this question is probably not possible without the use of a combined theoretical-experimental approach. Based on experimental signatures, one often ends up with a shortlist of possible candidates rather than an unambiguous identification of the defect. The reverse is done by theory. A specific structure is assumed for a defect, and its properties and geometry are calculated at first principle level. If the calculated properties match the experimental ones, there is a high probability that the defect is identified and understood. Some successful examples of combination of theory and experiment to identify point defects and sites where metal atoms are deposited have been reported in the last few years. In this chapter, we report some of these results related to a specific surface, MgO(lOO), which turns out to be one of the better characterized in the large family of oxide surfaces. 

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