We have studied the reconstruction of Pt(111) theoretically using a two-dimensional FrenkelKontorova model for which all parameters have been obtained from ab initio calculations. We find that the unreconstructed surface lies right at the stability boundary, and thus it is relatively easy to induce the surface to reconstruct into a pattern of FCC and HCP domains, as has been shown experimentally. The top layer is very slightly rotated relative to the substrate, resulting in the formation of "rotors" at intersections of domain walls. The size and shape of domains is very sensitive to the density in the top layer, the chemical potential, and the angle of rotation, with a smooth and continuous transition from the honeycomb pattern to a Moiré pattern, via interlocking triangles and bright stars. Our results show clearly that the domain patterns found on several close-packed metal surfaces are related and topologically equivalent.PACS numbers: 68.35. Bs, 61.72.bb, 68.35.Md, Due to its enormous importance as a catalyst, Pt (111) is one of the most widely studied surfaces. Under "normal" conditions, it has the flat topography expected of a bulk-truncated face-centered-cubic (FCC) (111) surface. However, experiments have shown that one can induce the surface to reconstruct into either a honeycomb structure or a pattern of interlocking triangles -for example, by heating the surface above 1330 K [1], or by placing it in a supersaturated Pt vapor [2]. The reconstructed structure is comprised of domains where the bulk FCC stacking sequence is retained, alternating with domains where the surface atoms instead occupy hexagonal-closepacked (HCP) sites, sitting directly above atoms two layers below. This is one of a family of similar reconstructions, formed by a tessellation of FCC and HCP domains, seen on Au(111) as well as various heteroepitaxial systems on the (111) faces of FCC metals, and the structurally similar (0001) faces of HCP metals [3,4,5,6]. These structures have attracted a great deal of interest, especially because they can be used as templates for growing ordered arrays of nanoparticles [7,8,9]. Possible applications for such nanostructures include nanoelectronics, information storage, and nanoscale chemical reactors. To design such nanostructures, it is desirable to understand the factors controlling the geometry and spacing of the reconstruction patterns.In this paper, we study the structure of the Pt(111) surface theoretically. We show that the unreconstructed Pt(111) surface is in fact teetering right at the brink of a domain of stability. Thus, slight changes in the environment can trigger a reconstruction, whose periodicity and geometry are very sensitive to various extrinsic and intrinsic parameters. In addition to obtaining excellent agreement with the structures reported experimentally for Pt(111), we also observe most of the structures reported experimentally for other systems.The presence or absence of such reconstructions involves a very delicate balance between various contributions to the total en...

We have studied the adsorption of NO on small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111); however, it also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the adsorption energy, and also washes out the clear size-dependent trend observed in the non-magnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.arXiv:0709.4365v1 [cond-mat.mtrl-sci]

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Inspired by recent experimental fabrication of mono-halogenated versions of Metal-Organic Framework MOF-5 (i.e., X-MOF-5, X = F to I) and some experimentally known fully halogenated MOF compounds, we systematically studied frameworks incorporating full halogenation of the BDC linkers of the prototypical Iso-Reticular Metal-Organic Framework (IRMOF) series, exemplified by MOF-5. Using quantum chemistry calculations, we find that halogenation leads to a 90° rotation of the aryl group, which is mainly ascribed to overcrowding between halogen atoms and the carboxyl and benzene ring and strong repulsion among in-plane atoms/groups. The 90° configuration decreases the repulsion, and maximizes the stabilization energy, and is therefore more stable than 0° configuration. We find that the band gap can be tuned from 4.1 to 1.5 eV as we go from F, Cl, Br, to I. This extends the optical response of these experimentally accessible materials through the visible and infrared region. We have also considered a broader range of new materials that substitute various metals for Zn. Totally, 70 materials were systematically examined computationally including (MO)(BDC-Z) (M = Zn, Cd, Be, Mg, Ca, Sr, Ba; Z = H, F, Cl, Br, I). For the full range of materials, we calculate band gaps of 4.2 to 1.0 eV, corresponding to a threshold of absorption of 290-1240 nm. Four selected materials were tested for stability using short 5 ps molecular dynamics simulations up to 600 K. The new materials with the smallest band gaps could potentially be used in near-infrared (NIR) light-emitting devices. Other properties, e.g., bulk moduli, formation energy, chemical bonding, and optical properties, were also investigated. The present results may provide new materials for use as novel photocatalysts, photoactive materials for photovoltaic cells, or functional devices in nanoelectronics and optoelectronics.