Strongly-interacting nanoparticle systems models

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

This nanoparticle sample exhibits strong anisotropy, due to the uniaxial anisotropy of the individual particles and the anisotropic dipolar interaction. The relative timescales (f/xm) of the experiments on nanoparticle systems are shorter than for conventional spin glasses, due to the larger microscopic flip time. The nonequilibrium phenomena observed here are indeed rather similar to those observed in numerical simulations on the Ising EA model [125,126], which are made on much shorter time (length) scales than experiments on ordinary spin glasses [127]. 

Figure 4.4.1 Schematic representation of the model systems discussed within the chapter (A) nanoparticle growth influenced by dopants in the support, (B) nanoparticle deposition from solution, (C) strong metal support interaction, and (D) photochemistry at supported nanoparticles as a function of size.

Differences in the activity of Nb O and a rather inert support such as Al O were evident after growing Pd nanoparticles (mean size 3.5 nm) on both oxide surfaces. In contrast to Pd-Al Og, the Pd-Nb O model system underwent strong structural changes when annealed above 300 K. Although this observation applies to metal-support interaction rather than to oxide activity, it clearly demonstrates the potential of SFG for detecting such interactions, and is thus briefly presented here. 

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