A comprehensive analysis of the (√13 × âˆš13)R13.9° type II structure of silicene on Ag(1 1 1).

In this paper, using the same geometrical approach as for the (2 √ 3 × 2 √ 3)R30° structure (Jamgotchian et al 2015 J. Phys.: Condens. Matter 27 395002), for the (√13 × âˆš13)R13.9° type II structure, we propose an atomic model of the silicene layer based on a periodic relaxation of the strain epitaxy. This relaxation creates periodic arrangements of perfect areas of (√13 × âˆš13)R13.9° type II structure surrounded by defect areas. A detailed analysis of the main published experimental results, obtained by scanning tunneling microscopy and by low energy electron diffraction, shows a good agreement with the geometrical model.

A comprehensive study of the (2√3 × 2√3)R30° structure of silicene on Ag(1 1 1).

The deposition of one silicon monolayer on Ag(1 1 1) gives rise to a set of superstructures depending on growth conditions. These superstructures are correlated to the epitaxy between the honeycomb structure of silicon (so called silicene) and the silver substrate. In this paper, from a detailed re-analysis of experimental results, obtained by scanning tunneling microscopy and by low energy electron diffraction on the (2√3 × 2√3)R30° structure, we propose a new atomic model of the silicene layer based on periodic arrangements of perfect areas of (2√3 × 2√3)R30° surrounded by defect areas. A generalization of this model explains the main experimental observations: deviation of the average direction, Moiré patterns and apparent global disorder. In the frame of the proposed model, the apparent disorders observed on the STM images, would be topological effects, i.e. the silicene would keep a quasi-perfect honeycomb structure.

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 × âˆš13)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 × âˆš7)R19.1° superstructure which has not been observed so far.

Germanium adsorption on Ag(111): an AES-LEED and STM study.

The adsorption of germanium on Ag(111) has been investigated using Scanning Tunneling Microscopy, Auger Electron Spectroscopy and Low Energy Electron Diffraction. From the shape of the Auger peak-to-peak versus time curves, we deduce that at room temperature the growth mode is nearly layer-by-layer at least for the first two layers. In the sub-monolayer range, the growth starts by the formation of a (mean square root of 3 x mean square root of 3)R30 degrees surface superstructure which is complete at 1/3 monolayer coverage. Beyond this coverage a rectangular c(mean square root of 3 x 7) superstructure is observed. STM images reveal that this last reconstruction is formed by an ordered arrangement of self-assembled Ge tetramers giving rise to a surprising undulation of the surface.

Water on extended and point defects at MgO surfaces.

The interaction of water with extended defects such as mono- and diatomic steps at the MgO(100) surface is investigated through first-principles simulations, as a function of water coverage. At variance with flat MgO(100) terraces, water adsorption is always dissociative on mono- and diatomic steps, as well as on MgO(110) surfaces. In most of the equilibrium configurations, the oxygen of the hydroxyl groups is two- or fourfold coordinated, but single-coordinated OH groups can be stabilized at diatomic step edges. The structural properties of the hydroxyl groups are discussed as a function of their coordination numbers and mutual interactions, as well as the surface defect morphology. It is shown that characteristics of water adsorption are primarily driven by the coordination number of the surface acid-base pair where the dissociation occurs. However, the OH groups resulting from water dissociation are also considerably stabilized by the electrostatic interaction with coadsorbed protons. At low coverage such an interaction, considerably stronger than hydrogen bonding, practically hinders any proton diffusion away from its neighboring hydroxyl. The computed adsorption energies allow us to discuss the onset of water desorption from flat MgO(100) terraces, diatomic and monoatomic steps, and from Mg-O divacancy.