Equilibrium geometries and ground state spin multiplicities of neutral and anionic coinage metal fluoride XF n clusters (X ) Cu, Ag, and Au; n ) 1-7) are obtained from density functional theory-based calculations. Our results show that in the case of neutral and anionic CuF n and AgF n clusters, a maximum of 4 F atoms (n max ) 4) can be bound atomically to the metal atoms, while remaining F atoms bind to the other F atoms to form F 2 units. In contrast, a Au atom can bind up to six F atoms dissociatively. This contrasting binding scenario observed for these metal fluoride clusters is explained using the natural bond orbital analysis. The neutral XF n (X ) Cu, Ag) clusters are stable against dissociation into X and F atoms up to n ) 6, while AuF n clusters are stable up to n ) 7. Similarly, with the exception of AgF 7 and AuF 6 , all neutral clusters studied are stable against dissociation into F 2 molecules. On the other hand, XF n clusters are stable against dissociation into F atoms and F 2 molecules over the entire size range, indicating the increased stability of anionic species over their neutral counterparts. Even more striking is the fact that the electron affinities of these clusters can be as large as 8 eV, far exceeding the electron affinity of Cl that has the highest value in the periodic table. These clusters are thus classified as superhalogens. † Part of the special issue "Protected Metallic Clusters, Quantum Wells and Metallic Nanocrystal Molecules".
Unique determination of the atomic structure of technologically relevant surfaces is often limited by both a need for homogeneous crystals and ambiguity of registration between the surface and bulk. Atomically resolved secondary-electron imaging is extremely sensitive to this registration and is compatible with faceted nanomaterials, but has not been previously utilized for surface structure determination. Here we report a detailed experimental atomic-resolution secondary-electron microscopy analysis of the c(6 × 2) reconstruction on strontium titanate (001) coupled with careful simulation of secondary-electron images, density functional theory calculations and surface monolayer-sensitive aberration-corrected plan-view high-resolution transmission electron microscopy. Our work reveals several unexpected findings, including an amended registry of the surface on the bulk and strontium atoms with unusual seven-fold coordination within a typically high surface coverage of square pyramidal TiO5 units. Dielectric screening is found to play a critical role in attenuating secondary-electron generation processes from valence orbitals.
Theoretical calculations based on density functional theory have found (PbS)(32) to be the smallest cubic cluster for which its inner (PbS)(4) core enjoys bulk-like coordination. Cubic (PbS)(32) is thus a "baby crystal," i.e., the smallest cluster, exhibiting sixfold coordination, that can be replicated to obtain the bulk crystal. The calculated dimensions of the (PbS)(32) cluster further provide a rubric for understanding the pattern of aggregation when (PbS)(32) clusters are deposited on a suitable surface, i.e., the formation of square and rectangular, crystalline nano-blocks with predictable dimensions. Experiments in which mass-selected (PbS)(32) clusters were soft-landed onto a highly ordered pyrolytic graphite surface and the resulting aggregates imaged by scanning tunneling microscopy provide evidence in direct support of the computational results.
Using a combination of density functional theory and anion photoelectron spectroscopy experiment, we have studied the structure and electronic properties of CuCl(n)(-) (n = 1-5) and Cu(2)Cl(n)(-) (n = 2-5) clusters. Prominent peaks in the mass spectrum of these clusters occurring at n = 2, 3, and 4 in CuCl(n)(-) and at n = 3, 4, and 5 in Cu(2)Cl(n)(-) are shown to be associated with the large electron affinities of their neutral clusters that far exceed the value of Cl. While CuCl(n) (n ≥ 2) clusters are conventional superhalogens with a metal atom at the core surrounded by halogen atoms, Cu(2)Cl(n) (n ≥ 3) clusters are also superhalogens but with (CuCl)(2) forming the core. The good agreement between our calculated and measured electron affinities and vertical detachment energies confirm not only the calculated geometries of these superhalogens but also our interpretation of their electronic structure and relative stability.
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