B. Aufray

Epitaxial growth of a silicene sheet

Using atomic resolved scanning tunneling microscopy, we present here the experimental evidence of a silicene sheet (graphenelike structure) epitaxially grown on a close-packed silver surface [Ag(111)]. This has been achieved via direct condensation of a silicon atomic flux onto the single-crystal substrate in ultrahigh vacuum conditions. A highly ordered silicon structure, arranged within a honeycomb lattice, is synthesized and present two silicon sublattices occupying positions at different heights (0.02 nm) indicating possible sp2-sp3 hybridizations.

Graphene-like silicon nanoribbons on Ag(110) : a possible formation of silicene

Scanning tunneling microscopy (STM) and ab initio calculations based on density functional theory (DFT) were used to study the self-aligned silicon nanoribbons on Ag(110) with honeycomb, graphene-like structure. The silicon honeycombs structure on top of the silver substrate is clearly observed by STM, while the DFT calculations confirm that the Si atoms adopt spontaneously this new silicon structure.

Evidence of graphene-like electronic signature in silicene nanoribbons

We report on the electronic properties of straight, 1.6 nm wide, silicene nanoribbons on Ag(110), arranged in a one-dimensional grating with a pitch of 2 nm, whose high-resolution scanning tunneling microscopy images reveal a honeycomb geometry. Angle-resolved photoemission shows quantum confined electronic states of one-dimensional character. The silicon band dispersion along the direction of the nanoribbons suggests a behavior analogous to the Dirac cones of graphene on different substrates.

Growth of silicene layers on Ag(111): unexpected effect of the substrate temperature

The deposition of one silicon monolayer on the silver (111) substrate in the temperature range 150-300 °C gives rise to a mix of (4 × 4), (2√3 × 2√3)R30° and (√13 × √13)R13.9° superstructures which strongly depend on the substrate temperature. We deduced from a detailed analysis of the LEED patterns and the STM images that all these superstructures are given by a quasi-identical silicon single layer with a honeycomb structure (i.e. a silicene-like layer) with different rotations relative to the silver substrate. The morphologies of the STM images are explained from the position of the silicon atoms relative to the silver atoms. A complete analysis of all possible rotations of the silicene layer predicts also a (√7 × √7)R19.1° superstructure which has not been observed so far.

Equilibrium of segregation in Ag/Cu(111): kinetics and isotherms

Abstract In this study, dissolution and segregation kinetics were used to deduce isotherms with which equilibria of segregation were investigated. By using this method on a model system Cu(Ag) we observed a phase transition of Ag on the Cu surface at 450°C. This phase transition has a critical point at half-coverage: below this point the two isotherms obtained from dissolution and segregation kinetics are similar to each other but above 50% coverage the two isotherms diverge. This can be interpreted either by a “percolation transition” or by a decrease of relaxation energy related to increases of silver surface concentration.

Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2 × 1)

We report results on the self-assembly of silicon nanoribbons (NRs) on the (2 × 1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon, the (2 × 1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage, a new superstructure is revealed by low energy electron diffraction (LEED), which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [1¯10] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition, the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

Silicon nano-ribbons on Ag(110): a?computational investigation

We report results of a computational investigation, based on density functional theory, of silicon self-assembled nano-ribbons (Si NRs) on Ag(110). These NRs present a honeycomb-like structure arched on the substrate and forming a closed-packed structure. The calculated STM images match the experimental ones, hinting to a possible new Si structure, mediated by the Ag substrate. The observed new electronic states near the Fermi level were reproduced by the calculations and attributed to a confinement/hybridization tandem.

Equilibrium of segregation in Ag/Cu: influence of the orientation and temperature

Abstract We have used dissolution and segregation kinetics to investigate the equilibrium superficial segregation of silver on copper. The effect of surface orientation was deduced from dissolution kinetics at 450°C. A first-order phase transition occurs on a (111) surface; there is no evidence of such a transition on a (110) surface and, on a (100) surface, it is likely that a phase transition takes place at high coverage only. The effect of temperature was studied from segregation kinetics of silver on a (111)-oriented 0.45 at% Cu(Ag) solid solution at 450, 416 and 390°C. For θ −1 was evaluated. For θ > 0.6 we found a slow-down in kinetics which is ascribed to the diminution of the relaxation energy with the silver superficial concentration.

Formation of a one-dimensional grating at the molecular scale by self-assembly of straight silicon nanowires

Upon submonolayer deposition of silicon onto the anisotropic silver (110) surface flat lying individual Si nanowires, all oriented along the [−110] direction, can be grown at room temperature with a high aspect ratio. Upon deposition at ∼200°C, these one-dimensional nanostructures self-assemble by lateral compaction to form a regular array of essentially identical nanowires, ∼1.6nm in width, covering uniformly the entire substrate surface. They realize, at macroscopic sizes, a highly perfect one-dimensional grating with a molecular-scale pitch of just 2nm.Upon submonolayer deposition of silicon onto the anisotropic silver (110) surface flat lying individual Si nanowires, all oriented along the [−110] direction, can be grown at room temperature with a high aspect ratio. Upon deposition at ∼200°C, these one-dimensional nanostructures self-assemble by lateral compaction to form a regular array of essentially identical nanowires, ∼1.6nm in width, covering uniformly the entire substrate surface. They realize, at macroscopic sizes, a highly perfect one-dimensional grating with a molecular-scale pitch of just 2nm.

Burning match oxidation process of silicon nanowires screened at the atomic scale

Silicon oxide nanowires hold great promise for functional nanoscale electronics. Here, we investigate the oxidation of straight, massively parallel, metallic Si nanowires. We show that the oxidation process starts at the Si NW terminations and develops like a burning match. While the spectroscopic signatures on the virgin, metallic part, are unaltered we identify four new oxidation states on the oxidized part, which show a gap opening, thus revealing the formation of a transverse internal nanojunction.