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- Berland, Kristian, 1983
(författare)
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Bound by long-range interactions: Molecular crystals and benzene on Cu(111)
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates molecular systems bound by long-range interactions. Westudy molecular crystals held together by van der Waals forces and the physisorption of benzene (Bz) on Cu(111). We also study the self-organization of benzeneon the Cu(111) surface which is influenced both by the (semi) long-range van derWaals forces, and the very-long-ranged indirect electronic interactions mediated bya metallic surface state on the Cu(111) surface. The van der Waals bound systems are investigated from first-principle principlesusing a van der Waals density functional (vdW-DF), [Phys. Rev. Lett. 92, 246401 (2004)]. We examine the character of these interactions, by studying the asymptoticaccount of the van der Waals forces, and compare full vdW-DF with an asymptotic atom-based pair potentials extracted from vdW-DF. The results show that the charac-ter of the binding differs in the two cases, with vdW-DF being significantly enhanced at relevant binding separations. This suggests that the use of asymptotic atom-based pair potentials as an approximation for non-local correlation is not consistent. Formolecular crystals, we study the structure and cohesive energy of hexamine and the platonic hydrocarbons cubane and dodecahedrane. The calculated lattice parame-ters and cohesion energy agree well with experiments. For adsorption of benzene on Cu(111), we obtain a binding energy in good agreement with experiment. We alsostudy diffusion barriers and find that the molecules can move almost freely along a honeycomb web of “corridors” passing between fcc and hcp hollow sites via bridgesites. To account for the surface-state mediated interactions, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this in-direct interaction. This description is based on input from DFT calculations of the metallic surface state on Cu(111), in addition to a parameterization of the benzene-induced scattering properties. We combine vdW-DF calculations of benzene dimers and diffusion barriers with the estimates of the surface-state mediated interactionsand find that the more compact (hexagonal) Bz-overlayer phase, with lattice constant ̊a = 6.74 A, is due to direct Bz-Bz vdW attraction, and the stability of the sparser hexagonal Bz phase, a = 10.24 A, is due to surface-state mediated interactions. Thus, we can account well for benzene self-organization on Cu(111). These results illustrate the current state of development of quantum-physical based approaches to systems bound by long-range interactions.
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| 2. |
- Berland, Kristian, 1983, et al.
(författare)
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Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111)
- 2009
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 80:15, s. 155431-
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Tidskriftsartikel (refereegranskat)abstract
- Using a van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)], we perform ab initio calculations for the adsorption energy of benzene (Bz) on Cu(111) as a function of lateral position and height. We find that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive interactions) changes the relative stability of eight binding-position options and increases the binding energy by over an order of magnitude, achieving good agreement with experiment. The admolecules can move almost freely along a honeycomb web of “corridors” passing between fcc and hcp hollow sites via bridge sites. Our diffusion barriers (for dilute and two condensed adsorbate phases) are consistent with experimental observations. Further vdW-DF calculations suggest that the more compact (hexagonal) Bz-overlayer phase, with lattice constant a=6.74 Å, is due to direct Bz-Bz vdW attraction, which extends to ~8 Å. We attribute the second, sparser hexagonal Bz phase, with a=10.24 Å, to indirect electronic interactions mediated by the metallic surface state on Cu(111). To support this claim, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this indirect interaction. Thus, we can account well for benzene self-organization on Cu(111).
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