The Ground State of Epitaxial Germanene on Ag(111).

Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, appear very promising due to their outstanding electronic properties, such as the quantum spin Hall effects. While there have been many claims of germanene monolayers up to now, no experimental evidence of a honeycomb structure has been provided up to now for these grown monolayers. Using scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and density functional theory, we have elucidated the Ge-induced (109×109)R±24.5° reconstruction on Ag(111). We demonstrate that a powerful algorithm combining SXRD with STM allows us to solve a giant surface reconstruction with more than a hundred atoms per unit cell. Its extensive unit cell indeed consists of 98 2-fold or 3-fold coordinated Ge atoms, forming a periodic arrangement of pentagons, hexagons, and heptagons, with the inclusion of six dispersed Ag atoms. By analogy, we show that the (77×77)R±19.1° reconstruction obtained by segregation of Ge through an epitaxial Ag/Ge(111) film possesses a similar structure, i.e., Ge pentagons/hexagons/heptagons with a few Ag atoms. Such an organization is more stable than that of pure Ge monolayers and can be assigned to the ground state of epitaxial germanene.

Two-Dimensional Functionalized Ultrathin Semi-Insulating CaF2 Layer on the Si(100) Surface at a Low Temperature for Molecular Electronic Decoupling.

The ability to precisely control the electronic coupling/decoupling of adsorbates from surfaces is an essential goal. It is important for fundamental studies not only in surface science but also in several applied domains including, for example, miniaturized molecular electronic or for the development of various devices such as nanoscale biosensors or photovoltaic cells. Here, we provide atomic-scale experimental and theoretical investigations of a semi-insulating layer grown on a silicon surface via its epitaxy with CaF2. We show that, following the formation of a wetting layer, the ensuing organized unit cells are coupled to additional physisorbed CaF2 molecules, periodically located in their surroundings. This configuration shapes the formation of ribbons of stripes that functionalize the semiconductor surface. The obtained assembly, having a monolayer thickness, reveals a surface gap energy of ∼3.2 eV. The adsorption of iron tetraphenylporphyrin molecules on the ribbons of stripes is used to estimate the electronic insulating properties of this structure via differential conductance measurements. Density functional theory (DFT) including several levels of complexity (annealing, DFT + U, and nonlocal van der Waals functionals) is employed to reproduce our experimental observations. Our findings offer a unique and robust template that brings an alternative solution to electronic semi-insulating layers on metal surfaces such as NaCl. Hence, CaF2/Si(100) ribbon of stripe structures, whose lengths can reach more than 100 nm, can be used as a versatile surface platform for various atomic-scale studies of molecular devices.

Tip-Induced Switch of Germanene Atomic Structure.

A new germanene crystallographic structure is investigated by scanning tunnelling microscopy and density functional theory calculations. We found that germanene can crystallize in two stable but different structures when grown on Al(111) at the same temperature. These structures are evidenced in scanning tunnelling images by a honeycomb contrast and by a hexagonal contrast. These contrasts are relevant of a Ge network with one (hexagonal) or two (honeycomb) Ge atoms per unit cell shifted upward with respect to the other Ge atoms. These structures appear alternatively and can be turned on and off by a tip-induced process.

Using strain to control molecule chemisorption on silicene.

The strain dependence of benzene chemisorption on a silicene freestanding layer has been studied by means of density functional theory calculations. It appears that the molecule, which is adsorbed via a [4+2] pseudo-cycloaddition on the substrate, is more stable when adsorbed on strained than on unstrained silicene since the adsorption energy increases (in absolute value) with tensile or compressive strain. These results, which were not easily predictable, are interpreted in terms of strain-induced reinforcement of the Si-C bonds, formation of a pz-like atomic orbital at the silicene atoms, which augments the silicene reactivity and, for compressive or large tensile strains, increasing of the sp3 character of the Si-Si bonds.

Tailoring the germanene-substrate interactions by means of hydrogenation.

Thanks to density functional calculations, the influence of hydrogenation on the interactions between a (2 × 2) germanene layer and a (3 × 3) Al(111) substrate has been investigated. It appears that the Ge-Al inter-atomic distance increases with hydrogen coverage, while the interaction energy and charge transfer between the Ge layer and the Al topmost plane drastically diminish, thus reducing the electrostatic interactions. Moreover, hydrogenation also lowers the electron density at the interface, weakening the chemical interaction between the Ge layer and the Al surface, and opening the door to a possible decoupling of the germanene layer from the Al substrate.

Opening the way to molecular cycloaddition of large molecules on supported silicene.

Within density functional theory, the adsorption of the H2Pc molecule on the (3 × 3) silicene/(4 × 4) Ag(111) surface has been investigated. We observe an electronic redistribution in the central macrocycle of the H2Pc molecule and the formation of two Si - N covalent bonds between the molecule and the silicene, in agreement with a cycloaddition reaction. However, while on SiC(0001)(3 × 3) or Si(111)(√3×√3)R30°-boron, the H2Pc molecule remains planar, and the H2Pc molecule takes a butterfly conformation on the silicene/Ag substrate due to an electrostatic or a polarization repulsion between the molecule and the silicene. Our study opens a way to the experimental adsorption of large organic molecules on supported silicene.

Molecular functionalization of silicene/Ag(111) by covalent bonds: a DFT study.

Among the 2D crystals, silicene, which forms sp(2)-sp(3) bonds, is expected to present a higher reactivity than graphene, characterized by sp(2) bonds only. However, silicene functionalization, in particular with organic molecules, remains an open question. By means of density functional theory, we study the adsorption of benzene, a model organic molecule, on (3 × 3) silicene on the (4 × 4) Ag(111) surface. Our calculations show that the dispersion interactions must be taken into account in order to describe this system properly. The adsorption energy is calculated by means of the semi-empirical dispersion-corrected density functional theory (DFT-D2) and the optB86b-vdW density functional. The charge density and electron localization function maps indicate that the molecule is chemisorbed on the silicene surface by means of two Si-C covalent bonds. In agreement with charge density difference calculations, two C-C double bonds are formed in the benzene molecule, which adopts a butterfly configuration. The silicene lattice is slightly deformed upon benzene adsorption, but the Si-Si distance remains the same as in bare silicene/Ag(111). Bader analysis shows a charge transfer from top Si atoms to both molecules and Ag substrates. Finally, we show that the covalent functionalization of silicene is possible.

Continuous germanene layer on Al(111).

Germanene, a 2D honeycomb structure similar to silicene, has been fabricated on Al(111). The 2D germanene layer covers uniformly the substrate with a large coherence over the Al(111) surface atomic plane. It is characterized by a (3 × 3) superstructure with respect to the substrate lattice, shown by low energy electron diffraction and scanning tunnelling microscopy. First-principles calculations indicate that the Ge atoms accommodate in a very regular atomic configuration with a buckled conformation.

Spatial analysis of interactions at the silicene/Ag interface: first principles study.

The (3 × 3) silicene on the (4 × 4) Ag(1 1 1) surface is investigated by means of density functional theory calculations. We focus on the nature of the interactions between the silicene and the Ag surface, in particular in terms of spatial charge localisation. No true covalent bonds are formed between the silicene and the Ag surface, but there is an overlap between the charge densities of the bottom Si atoms and the nearest Ag atoms. Charge difference calculations show that a clear charge reorganisation takes place when bringing together the silicene and the Ag substrate. According to Bader charge calculations, the top Si atoms are slightly positively charged, while the Ag surface plane carries a negative charge. This indicates that an electrostatic interaction exists between the top Si atoms and the below-lying Ag atoms, resulting in the first possible explanation of the Ag buckling.

C60 molecules grown on a Si-supported nanoporous supramolecular network: a DFT study.

C60 fullerene assemblies on surfaces have attracted considerable attention because of their remarkable electronic properties. Now because of the competition between the molecules-substrate and the molecule-molecule interactions, an ordered C60 array is rather difficult to obtain on silicon surfaces. Here we present density functional theory simulations on C60 molecules deposited on a TBB (1,3,5-tri(1'-bromophenyl)benzene) monolayer lying on the Si(111)-boron surface (denoted SiB). The C60 molecules are located in the nanopores formed by the TBB network. Adsorption energy calculations show that the SiB surface governs the C60 vertical position, whereas the TBB network imposes the C60 lateral position, and stabilizes the molecule as well. The low charge density between the C60 and the SiB substrate on one hand, and on the other hand between the C60 and the TBB molecules, indicates that no covalent bond is formed between the C60 and its environment. However, according to charge density differences, a drastic charge reorganisation takes place between the Si adatoms and the C60 molecule, but also between the C60 and the surrounding TBB molecules. Finally, calculations show that a C60 array sandwiched between two TBB molecular layers is stable, which opens up the way to the growth of 3D supramolecular networks.