V. Fournée
Iowa State University
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Featured researches published by V. Fournée.
Surface Science | 2003
V. Fournée; A. R. Ross; Thomas A. Lograsso; James W. Evans; Patricia A. Thiel
Abstract The nucleation and growth of Ag thin films deposited on structurally complex alloy substrates is studied by scanning tunneling microscopy and low-energy electron diffraction. The substrates are high-symmetry surfaces of either icosahedral or decagonal quasicrystals or of related periodic crystals called approximants. At low coverage, completely different film morphologies are observed depending on the surface free energy of the substrate (relative to Ag). At high coverage, the formation of hexagonal nanocrystals is observed in all cases. These have fcc symmetry and pyramid-like multilayer stacking along the 〈1xa01xa01〉 direction. The constituent hexagonal islands have one, two or five different orientations depending on the rotational symmetry of the substrate on which the growth occurred. The films can be artificially smoothened by O 2 induced coarsening, leading to a patchwork of Ag nanocrystals with a perfect texture, which are separated by grain boundaries.
Surface Science | 2003
T. Cai; J. Ledieu; R. McGrath; V. Fournée; Thomas A. Lograsso; A. R. Ross; Patricia A. Thiel
Abstract Direct observation of extrinsic Al atoms on the fivefold surface of icosahedral Al–Cu–Fe indicates that they form pseudomorphic islands resembling starfish. The starfish occupy specific types of sites on the laterally-bulk-terminated quasicrystalline surface. We postulate that the nucleation event consists of a diffusing Al atom dropping into an empty site at the centre of a substrate pentagon. Growth consists of the addition of five Al atoms (nearly) in lattice sites. These 6-atom starfish do not grow laterally as coverage increases, leading to islands of uniform size, and to early roughening.
Surface Science | 2000
Zhi-Xun Shen; W. Raberg; M. Heinzig; Cynthia J. Jenks; V. Fournée; M.A. Van Hove; Thomas A. Lograsso; Dwight W. Delaney; T. Cai; P. C. Canfield; I. R. Fisher; A. I. Goldman; Matthew J. Kramer; Patricia A. Thiel
Abstract It is shown that low-energy electron diffraction (LEED) patterns of the three high-symmetry surfaces (fivefold, threefold and twofold) of icosahedral Al–Pd–Mn are all compatible with quasicrystallinity, under specific conditions of preparation. This conclusion results from comparing symmetries of experimental surface LEED patterns with bulk X-ray diffraction data which are converted to the conditions of the LEED experiment. This conclusion is also based upon an analysis of relative diffraction spot spacings in LEED. Hence, none of the three surfaces exhibits a massive lateral reconstruction, i.e. massive deviation from quasicrystallinity. The LEED pattern of the fivefold surface is distinct from the LEED pattern of the pseudo-tenfold surface of an orthorhombic approximant. We believe that this rules out the possibility that the fivefold surface of the icosahedral quasicrystal reconstructs to an approximant with tenfold or pseudo-10-fold symmetry. The twofold and threefold surfaces facet more readily, indicating qualitatively that they are less stable than the fivefold surface.
Journal of Chemical Physics | 2003
Anthony R. Layson; James W. Evans; V. Fournée; Patricia A. Thiel
Preexposure to molecular oxygen gas, O2,g, can have a strong effect on the nucleation and growth of Ag islands on Ag(100) at 250 K. At this temperature, molecular oxygen dissociates efficiently at kink sites on steps. Subsequent deposition of Ag produces a far lower density of Agad islands than without oxygen. There is an associated increase in the Ag flux-scaling exponent, from 0.28 for the oxygen-free surface to 0.9 for the preexposed surface. Two-step deposition experiments show that species containing atomic oxygen diffuse freely across terraces and steps at this temperature and on the time scale of deposition. We hypothesize that the nucleating species contains both Ag and O, and that nucleation of islands is highly reversible (critical size i≫1). The diffusion of small islands, if it occurs, is not sufficient to explain the data.
Journal of Physics: Condensed Matter | 2002
V. Fournée; James W. Anderegg; A. R. Ross; Thomas A. Lograsso; Patricia A. Thiel
Specific features in the electronic structure of Al-transition metal quasicrystals are analysed by a combination of Auger and x-ray photoelectron spectroscopies. We first demonstrate that different degrees of asymmetry in the transition metals 2p core-level lineshape observed across different types of surface structure correspond to variations in the density of states at the Fermi level, DOS(EF). Using this effect, we explore the controversial issue of whether the quasicrystalline, decagonal AlNiCo system is electronically stabilized. We find strong evidence for the presence of a reduced DOS(EF) in this system, as expected for electronically stabilized compounds, and as observed in the quasicrystalline, icosahedral AlPdMn and AlCuFe alloys. Finally, qualitative information on the nature of the electronic states in quasiperiodic structures extracted from the core-valence-valence Auger lines are presented and discussed.
Physical Review Letters | 2005
V. Fournée; H. R. Sharma; Masahiko Shimoda; An Pang Tsai; Barış Ünal; A. R. Ross; Thomas A. Lograsso; Patricia A. Thiel
Physical Review B | 2003
V. Fournée; T. C. Cai; A. R. Ross; Thomas A. Lograsso; James W. Evans; Patricia A. Thiel
Physical Review B | 2000
V. Fournée; Patrick J. Pinhero; James W. Anderegg; Thomas A. Lograsso; A. R. Ross; Paul C. Canfield; I. R. Fisher; Patricia A. Thiel
Physical Review B | 2002
V. Fournée; A. R. Ross; Thomas A. Lograsso; James W. Anderegg; C. Dong; Matthew J. Kramer; I. R. Fisher; Paul C. Canfield; Patricia A. Thiel
Surface Science | 2005
J. Ledieu; Erik J. Cox; R. McGrath; Neville V. Richardson; Qiao Chen; V. Fournée; Thomas A. Lograsso; A. R. Ross; K.J. Caspersen; Barış Ünal; James W. Evans; Patricia A. Thiel