Solid α-B12 rhombohedral and γ-B28 orthorhombic boron as well as boron nanostructures in the form of spheres, sheets, and multirings beside a ring consisting of icosahedral B12 units were investigated using ab initio quantum chemical and density functional methods. The structure of the B100 fullerene exhibits unusual stability among all noninteracting free-standing clusters, and is more stable than the B120 cluster fragment of the γ-B28 solid, recently predicted and observed by Oganov et al. (Nature 2009, 457, 863). In addition, we compared the stability of the multirings and reported the structural transition from double-ring to triple-ring systems. This structural transition occurs between B52 and B54 clusters. We confirm that the noninteracting free-standing triangular buckled-sheet is more stable than the γ-sheet, assembled in this work, and than the α-sheet, proposed by Tang and Ismail-Beigi (Phys. Rev. Lett. 2007, 99, 115501). In contrast, however, when these sheets are considered as infinite periodic systems, then the α-sheet remains the most stable one.

ABSTRACT:The electronic and geometric structures, total and binding energies, first and second energy differences, harmonic frequencies, point symmetries, and highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of small and neutral B n (n ϭ 2-12) clusters have been investigated using density functional theory (DFT), B3LYP with 6-311ϩϩG(d,p) basis set. Linear, planar, convex, quasi-planar, three-dimensional (3D) cage, and open-cage structures have been found. None of the lowest energy structures and their isomers has an inner atom; i.e., all the atoms are positioned at the surface. Within this size range, the planar and quasi-planar (convex) structures have the lowest energies. The first and the second energy differences are used to obtain the most stable sizes. A simple growth path is also discussed with the studied sizes and isomers. The results have been compared with previously available theoretical and experimental works.

No abstract

A simple kinetic model is used to describe the interaction of H and D atomic beams with H- and D-covered metal surfaces. The atoms incident from the gas phase can have a direct Eley–Rideal reaction with an adsorbate, reflect, penetrate into the bulk, knock an adsorbate out of its binding site, or trap to form a hot atom. These hot mobile atoms can go on to react with other adsorbates, or eventually relax and stick. A coarse-graining approach, which takes advantage of the large difference between the time scales for the kinetics experiments and the reaction dynamics, allows us to derive relatively simple kinetic equations for reaction rates and coverages. The approach is similar to a kinetic random walk model developed by Küppers and co-workers [J. Phys. Chem. 109, 4071 (1998)] except that our equations can be used to derive analytical expressions for saturation coverages, rates, and yields. The model is applied to the case of H atom reactions on a Ni(100) surface, and a detailed comparison is made with both experimental and quasiclassical studies.

The reactions of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Ni(100) surface are studied. Electronic structure calculations based on density functional theory are used to examine the interaction of H atoms with the Ni(100) surface, as well as the interactions between two H atoms near the metal surface. A model potential-energy surface based on ideas from effective medium theory is fit to the results of these electronic structure calculations. Quasiclassical trajectory methods are used to simulate the interaction of low energy H and D atom beams with H and D-covered Ni(100) surfaces. It is found that hot-atom processes dominate the formation of molecular hydrogen. The distribution of energy in the product molecules is examined with regard to the various pathways available for reaction. The initial adsorbate coverage is varied and is shown to control the relative amounts of reflection, reaction, sticking, and subsurface penetration. Our results are compared with those from similar studies on Cu(111) and available experimental data for Ni(100).

No abstract

Quasiclassical methods are used to simulate the interactions of H or D atom beams with D- or H-covered Ni(100) surfaces. The Ni substrate is treated as a multilayer slab, and the Ni atoms are allowed to move. The model potential energy surface is fit to the results of detailed total-energy calculations based on density functional theory. Most of the incident atoms trap to form hot atoms, which can eventually react with an adsorbate, or dissipate their energy and stick. The incident atom is found to lose several tenths of an eV of energy into the metal, upon initially colliding with the surface. This limits reflection to a few percent, at all coverages, and secondary reactions between adsorbates are significantly lowered. Long time hot atom reactions are also found to be damped out by the inclusion of lattice motion, leading to increased sticking, even at high coverages where dissipation into the adsorbates should be the primary energy loss mechanism. Overall, the inclusion of lattice motion is found to improve agreement with experiment.

Using the basin-hopping Monte Carlo minimization approach we report the global minima for aluminium, gold and platinum metal clusters modelled by the Voter-Chen version of the embeddedatom model potential containing up to 80 atoms. The virtue of the Voter-Chen potentials is that they are derived by fitting to experimental data of both diatomic molecules and bulk metals simultaneously. Therefore, it may be more appropriate for a wide range of the size of the clusters. This is important since almost all properties of the small clusters are size dependent. The results show that the global minima of the Al, Au and Pt clusters have structures based on either octahedral, decahedral, icosahedral or a mixture of decahedral and icosahedral packing. The 54-atom icosahedron without a central atom is found to be more stable than the 55-atom complete icosahedron for all of the elements considered in this work. The most of the Al global minima are identified as some fcc structures and many of the Au global minima are found to be some low symmetric structures, which are both in agreement with the previous experimental studies.