Title: Double feature seminar on Life at the edge: Graphene nanoribbons & Ambient and super-critical water via ab initio molecular dynamics: Schemes involving van der Waals forces Speaker: Dr. Ari P. Seitsonen, University of Zurich, Switzerland Date/Time: Thursday, August 5, 2010, 10:30 am Location: CSRI Building/Room 90 (Sandia NM) Brief Abstract: Graphene has many excellent and unique properties, and it has become very intensively studied a material. Whereas the pure graphene is a semi-metal, for some applications, such as electronic devices, a band gap is required. This can be achieved by restricting the spatial extent of the graphene sheet into one or zero dimensions. We first determine, by means of density functional theory, the stability and the geometry of one- dimensional graphene nanoribbons (GNRs) edges in the environment composed of common gases such as hydrogen [1,2], oxygen, water, ammonia and carbon dioxide [3]. From thermodynamical analysis we find that the most stable structures with either armchair or zigzag edge reconstruction are characterized by a non-metallic/non-magnetic nature, and are compatible with Clar’s sextet rules, well known in organic chemistry. These results are particularly relevant to both synthesis of GNRs where the most recent experimental routes are either via water- and/or ammonia-containing solutions, and preparation of nanoribbons by "opening" carbon nanotubes in atmosphere of gaseous oxygen. We further analyse the electronic structure by simulat- ing the scanning tunnelling microscope images. They are found to faithfully reflect the expected electronic configuration. We also study the confinement of the electrons of graphene in the vicinity of the Fermi energy by hybridising nanoribbons or zero-dimensional dots in a plane with an insulating material, hexagonal boron nitride. Such structures have recently been demonstrated in experiments [4,5]. The electronic states appear as very similar to those in saturated GNRs. In the second part of the presentation we shall discuss a practical way of producing those graphene nanoribbons; a recent bottom-up approach is shown to produce mono-disperse ribbons, up to a length of 50 nm [6]. In collaboration with Tobias Wassmann, A Marco Saitta, Michele Lazzeri and Francesco Mauri [1] "Structure, stability, edge states and aromaticity of graphene ribbons", [2] "Clar’s theory, π-electron distribution and geometry of graphene [3] Ari P Seitsonen, A Marco Saitta, Tobias Wassmann, Michele Lazzeri and [4] "Atom-by-atom structural and chemical analysis by annular dark-field [5] "Atomic layers of hybridised boron nitride and graphene domains", Lijie [6] "Site-specific Atomically precise bottom-up fabrication of graphene Abstract 2: The understanding of the properties of pure water, and solvation of simple ions in it, is an important prerequisite for example in many chemical reactions, electrochemistry and biological systems. Despite its importance relatively little is still known about the characteristics like the coordination in the solvation shell and diffusion, or mobility. Furthermore the data is often controversial. We use ab initio molecular dynamics to investigate the solvation of ions in water. To extend the range of studied pressure and temperature range we simulate the solvation both in the ambient and super-critical state. Also, since van der Waals/weak interactions have been found [1,2] to play a role in pure liquid water we have tested the effect of including an empirical van der Waals term into the dynamics. We shall also discuss other recent simulations of water, for example using hybrid functionals [3,4]. In collaboration with Guillaume Ferlat, Rodolphe Vuilleumier, I-Chun Lin, Mauricio Coutinho-Neto, Ivano Tavernelli and Ursula Röthlisberger [1] "Importance of van der Waals Interactions in Liquid Water", I-Chun Lin, [2] "Isobaric-isothermal molecular dynamics simulations utilising density [3] "Molecular Dynamics Simulation of Liquid Water: Hybrid Density [4] "Auxiliary Density Matrix Methods for Hartree−Fock Exchange Calculations", CSRI POC: Anatole von Lilienfeld, 505-284-8829 |