M12X-type metal clusters of icosahedral symmetry are discussed in terms of the giant atom concept on the basis of the electronic shell structures in free-electron-like metal clusters. The first-principles calculations for Au12,13, Ag12,13, Al12,13, Au12Cr, and Al12Si clusters have been done by the self-consistent local density functional scheme using the norm-conserving pseudopotential in the linear combination of atomic orbitals method. Electronic shell structures in icosahedral clusters are formed by 6s, 5s and 3p orbitals or peripheral atoms in Au, Ag, and Al clusters, respectively. By doping Cr into Au12and Si into Al12clusters M13clusters are stabilized by about 0.7 eV atom-1(Au12Cr) and 0.2 eV atom-1(Al12Si) because of the closed shell formation. Interactions between clusters, cluster and surface, and cluster and metal tip used in the atomic force microscope are discussed based on the Harris functional scheme. Dimers of Au12Cr and Al12Si are weakly bound, and the Au13cluster on Si(111) has two kinds of binding, but Al13does not. The repulsive or attractive nature of the force between the cluster and Pt tip is considerably dependent on the cluster-tip distance.
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