Stability of silicon-doped heterofullerenes Carlo Massobrio Institut de Physique et de Chimie des Matériaux de Strasbourg, 23 Rue du Loess BP 43 F Strasbourg Cedex 2, France Why to dope fullerenes ?? Create new nanostructures from a very stable one Enhance chemical reactivity Here: substitutional doping C: yellow, Si: black C 40 Si 20 Other doping exist: endohedral (encapsulation inside) coating (addition on the cage)

Stability of silicon-doped heterofullerenes Carlo Massobrio Institut de Physique et de Chimie des Matériaux de Strasbourg, 23 Rue du Loess BP 43 F Strasbourg Cedex 2, France Why to dope fullerenes ?? Create new nanostructures from a very stable one Enhance chemical reactivity Here: substitutional doping C: yellow, Si: black Other doping exist: endohedral (encapsulation inside) coating (addition on the cage) C 40 Si 20

Research context: atomic-scale materials at IPCMS Diffusion on metallic surfaces (Au(111)) (CMD) Liquid and glasses (intermediate range order) (FPMD) GeSe 2 Lamellar hybrid organic-inorganic solids (FPMD) Tools: first-principles molecular dynamics (FPMD) (Car-Parrinello) to optimize and follow temporal evolution or (large metallic systems) Classical n-body potentials (CMD) Nanosystems: Stability of unsupported (free) clusters (FPMD) Cu 2 (OH) 3 NO 3

Si-doping of fullerenes: experimental facts C 2n-q Si q 2n=32-100, q<4 Fullerene geometry preserved Si atoms close to each other From J. Chem. Phys 110, 6927 (1999) “…for Si, a value close to 12 seems to be the upper limit” (M. Pellarin, C. Ray, J. Lermé, J. L. Vialle, M. Broyer X. Blase, P. Kéghelian, P. Mélinon, A. Perez) Concerning the upper limit of Si for doping, what else is known..?? Si 60 is not stable as a cage, It relaxes to a “puckered” ball ( M.Menon, K. R. Subbaswamy, CPL 219, 219 (1994))

Basics of structural properties and bonding in C 59 Si and C 58 Si 2 (I.M.L. Billas, C.Massobrio, M. Boero, M. Parrinello, W. Branz, F. Tast, N. Malinowski, M. Heinebrodt, T. P. Martin, JCP 111, 6787 (1999)) HOMO LUMO

C 59 Si: Localized orbitals (Wannier functions) and electron localization function (ELF) ELF= 0.8 ELF= Si Si bonds to the neighbors C in a distorted or “weak” sp 2 manner

Toward highly Si-doped fullerenes: the case of C 54 Si 6 (Masahiko Matsubara and CM, JCP 122, (2005)) Thermal behavior highlights weaker connections, likely to break at high temperatures (see Si-Si hh in isomer A) The four most stable isomers From T=0K to T=3000K in 12 ps (stepwise)

Highly silicon-doped fullerenes… Geometry optimization for C 40 Si 20, C 36 Si 24, C 30 Si 30 (Masahiko Matsubara and CM, JPhysChem 109, 4415 (2005)) Increased doping: decrease of binding energy (C 60 : 8.17 eV/atom, C 40 Si 20 : 6.81 eV/atom, C 30 Si 30 : 6.13 eV/atom)

A fully half-doped substitutional fullerene retains a cage-like structure..!! Probability isodensty surface associated with the total valence charge density Si atoms cap the fullerene via Si-Si-Si triads (angles scattered around the tetrahedral value) “forced “ sp 2 bonding

Stability of the cage vs increased doping content: issues to be addressed Threshold value for the number of Si-C replacements FPMD on structures vibrationally stable From T=0K to T=5000K 24 ps To ensure adiabaticity, Nosé-Hoover thermostat on electronic degrees of freedom at the higher temperatures Mechanism of instability with increasing Si content

Topological analysis and charge pattern Si atoms can be classified as outer (neighbors of C) and inner (not neighboring C) Ionic interaction prevails at the Si-C frontier, but not inside (Si-Si interactions) With increased doping, the number of Si outer does not exceed 12: instability due to Si inner and triggered by temperature Our proposal: threshold for Si inner = Si outer (C 40 Si 20 ): is it consistent with dynamics?