Electronic structure of supported clusters

Using the DV-Xa method, Johnson and Pepper (1982) have calculated the electronic structure of one iron, nickel or copper atom adsorbed on a sapphire A10g cluster. They find evidence of a hybridization between the metal d states and the substrate oxygen orbitals. In the series, the electrons fill all bonding states and an increasing number of anti-bonding states, which yields a weakening of the interfacial bond. Metal-cation bonds are not taken into account in this approach. 

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In summary, our study shows the power and the necessity of computational methods in unravelling the structural chemistry of silica-supported metal clusters. It has been shown that the stmcture of such clusters is highly sensitive to the processes by which the cluster-support interaction occurs and not just dependent on the strength of the binding interaction. Further studies are aimed at also investigating the electronic structure of supported clusters and the effects of ligands on cluster structure. 

The most widely used technique to get information on the electronic structure of clean surfaces, nanostructures on surfaces, or even molecules adsorbed on surfaces is ultraviolet photoelectron spectroscopy (UPS). The difficulty of this method, when applying it to clusters on surfaces, is to obtain sufficient spectral contrast between the low number of adsorbed clusters and the substrate [45]. Thus, electron energy loss spectroscopy (EELS) is more successfully used as a tool for the investigation of the electronic structure of supported clusters. An interesting test case for its suitability is the characterization of supported monomers, i.e., single Cu atoms on an MgO support material [200], as this system has been studied in detail before with various surface science techniques [201-204]. The adsorption site of Cu on MgO(lOO) is predicted... 

Theoretical studies have been devoted to various aspects of the first stages of deposition, such as the electronic structure of supported clusters, the electronic structure of epitaxial monolayers and the modelling of growth modes. 

As was mentioned previously, photoemission has proved to be a valuable tool for measurement of the electronic structure of metal cluster particles. The information measured includes mapping the cluster DOS, ionization threshold, core-level positions, and adsorbate structure. These studies have been directed mainly toward elucidation of the convergence of these electronic properties towards their bulk analogues. Although we will explore several studies in detail, we can say that studies from different laboratories support the view that particles of 150 atoms or more are required to attain nearly bulk-like photoemission properties of transition and noble metal clusters. This result is probably one of the most firmly established findings in the area of small particles. 

M. G. Mason, Electronic Structure of Supported Small Metal Clusters, Phys. Rev. B 27, 749-762 (1983). 

The magnetization on the Ni and Co clusters is largely unchanged also in the supported species. In some cases, however, there is a partial quenching of the magnetic moment which is generally restricted to the metal atoms in direct contact with the oxide anions [203]. Thus, despite the relatively strong MgO/M4 bonds (C04 is bound on MgO by 2.0eV, Ni4 by 2.4eV), the electronic structure of supported transition metal moieties is only moderately perturbed. These conclusions are valid only for an ideal defect-free surface ... 

For small clusters with up to 10-20 atoms, the interaction with the support can play a direct role on the geometrical or electronic structure of the cluster, thus notably affecting its reactivity [212]. In this respect, point defects or morphological irregularities on the oxide surface can influence the chemical properties of the particle, hence its catalytic activity [16,213,214]. 

There are no known examples of supported clusters dispersed in crystallo-graphically equivalent positions on a crystalline support. Thus, no structures have been determined by X-ray diffraction crystallography, and the best available methods for structure determination are various spectroscopies (with interpretations based on comparisons with spectra of known compoimds) and microscopy. The more nearly uniform the clusters and their bonding to a support, the more nearly definitive are the spectroscopic methods however, the uniformities of these samples are not easy to assess, and the best microscopic methods are limited by the smallness of the clusters and their tendency to be affected by the electron beam in a transmission electron microscope furthermore, most supported metal clusters are highly reactive and... 

Molecular-dynamics simulations also showed that spherical gold clusters is stable in the form of FCC crystal structure in a size range of = 13-555 [191]. This is more likely a key factor in developing extremely high catalytic activity on reducible Ti02 as a support material. Thus, it controls the electronic structure of Au nanoparticles (e.g. band gap and BE shift of Au 4f7/2 band) and thereby the catalytic activity. 

The basic theories of physics - classical mechanics and electromagnetism, relativity theory, quantum mechanics, statistical mechanics, quantum electrodynamics - support the theoretical apparatus which is used in molecular sciences. Quantum mechanics plays a particular role in theoretical chemistry, providing the basis for the valence theories which allow to interpret the structure of molecules and for the spectroscopic models employed in the determination of structural information from spectral patterns. Indeed, Quantum Chemistry often appears synonymous with Theoretical Chemistry it will, therefore, constitute a major part of this book series. However, the scope of the series will also include other areas of theoretical chemistry, such as mathematical chemistry (which involves the use of algebra and topology in the analysis of molecular structures and reactions) molecular mechanics, molecular dynamics and chemical thermodynamics, which play an important role in rationalizing the geometric and electronic structures of molecular assemblies and polymers, clusters and crystals surface, interface, solvent and solid-state effects excited-state dynamics, reactive collisions, and chemical reactions. 

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