We suggest that the herringbone reconstruction of the Au(lll) surface results from the spontaneous formation of "stress domains." The surface is described theoretically by a 2D Frenkel-Kontorova model. Upon including long-range elastic interactions, the stress-domain pattern is energetically favored. The surface topography and structure factor obtained by relaxing atomic coordinates using molecular dynamics bear a strong resemblance to scanning tunneling microscopy pictures and x-ray scattering data, respectively. Our numerical estimate for the separation between domain walls is consistent with experimental results.

Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because of their ability to switch between high-spin and low-spin states with external stimuli such as voltage and light. In actual devices, the molecules are deposited on a substrate, which can modify their properties. However, surprisingly little is known about such molecule−substrate effects. Here we show for the first time, by grazing incidence X-ray diffraction, that an Fe II spin-crossover molecular layer displays a well-defined epitaxial relationship with a metal substrate. Then we show, by both density functional calculations and a mechanoelastic model, that the resulting epitaxial strain and the related internal pressure can induce a partial spin conversion at low temperatures, which has indeed been observed experimentally. Our results emphasize the importance of substrate-induced spin state transitions and raise the possibility of exploiting them.

The morphology of small metal clusters can have a big impact on their electronic, magnetic, and chemical properties. This has been shown earlier, for example, for Au(20) clusters on MgO(001), where planar and tetrahedral geometries are possible for the gold atoms. While the planar geometry is more desirable for catalytic applications, it is disfavored in the usual situation. While earlier suggestions that have been made for tilting this balance in favor of the planar isomer are of considerable fundamental interest, they do not easily lend themselves to practical applications. Here, we suggest a conceptually simple but practicable way of achieving the same goal: viz., by doping the MgO substrate with Al atoms. We show, by performing density functional theory calculations, that this stabilizes the planar over the tetrahedral arrangement by an energy difference that is linearly proportional to the dopant concentration and is insensitive to the position of the dopant atom. The charge transferred to the Au cluster also depends monotonically on the doping concentration. This work is of interest for possible applications in the field of gold nanocatalysis.

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO(2) and CH(4) to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH(2), H(2)PO(3), NO(2), and CH(3). Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO(2) to exceed that of CH(4) by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH(2), H(2)PO(3), NO(2), and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO(2)/CH(4) selectivity are discussed.

The thermal properties of bulk copper are investigated by performing ab initio DFT and DFPT calculations and using the quasiharmonic approximation for the free energy. Using both the LDA and the GGA for the exchangecorrelation potential, we compute the temperature dependence of the lattice constant, coefficient of thermal expansion, bulk modulus, pressure derivative of the bulk modulus, phonon frequencies, Grüneisen parameters, and the electronic and phonon contributions to the specific heats at constant volume and constant pressure. We obtain answers in closer agreement with experiment than those obtained from more approximate earlier treatments.The LDA/GGA errors in computing anharmonic quantities are significantly smaller than those in harmonic quantities. We argue that this should be a general feature, and also argue that LDA/GGA errors should increase with temperature.

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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...

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