Philippe Sonnet

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.

Selective scanning tunnelling microscope electron-induced reactions of single biphenyl molecules on a Si(100) surface.

Selective electron-induced reactions of individual biphenyl molecules adsorbed in their weakly chemisorbed configuration on a Si(100) surface are investigated by using the tip of a low-temperature (5 K) scanning tunnelling microscope (STM) as an atomic size source of electrons. Selected types of molecular reactions are produced, depending on the polarity of the surface voltage during STM excitation. At negative surface voltages, the biphenyl molecule diffuses across the surface in its weakly chemisorbed configuration. At positive surface voltages, different types of molecular reactions are activated, which involve the change of adsorption configuration from the weakly chemisorbed to the strongly chemisorbed bistable and quadristable configurations. Calculated reaction pathways of the molecular reactions on the silicon surface, using the nudge elastic band method, provide evidence that the observed selectivity as a function of the surface voltage polarity cannot be ascribed to different activation energies. These results, together with the measured threshold surface voltages and the calculated molecular electronic structures via density functional theory, suggest that the electron-induced molecular reactions are driven by selective electron detachment (oxidation) or attachment (reduction) processes.

Surface-isomerization dynamics of trans-stilbene molecules adsorbed on Si(100)-2 x 1.

Photoinduced trans-cis isomerization studies of stilbene molecules in the gas phase have led to a precise understanding of the corresponding molecular dynamics. Yet, when such molecules are adsorbed on surfaces, these reactions are expected to be strongly modified as compared to what is know in the gas phase. In this work, a low temperature (5 K) scanning tunneling microscope (STM) is used to image the trans-stilbene molecules deposited on a Si(100)-2 x 1 surface at 12 K. trans-Stilbene undergoes conformational changes during the adsorption process such that four different stilbene conformers are observed: trans-stilbene (TS), cis-stilbene (CS), and two new conformers I(1) and I(2). Furthermore, electronic excitation of individual stilbene molecules, by means of tunnel electrons, is shown to activate specific reversible molecular surface isomerization (TS <--> I(1) and CS <--> I(2)) combined with diffusion across the surface. Calculated STM topographies, using the tight binding method, indicate that the CS and TS molecules are physisorbed. The molecular conformations of the surface isomers I(1) and I(2) are suggested to be analogous to transient states conformations of the stilbene molecule when stabilized by the silicon surface. The measurements of the molecular surface isomerization and diffusion reaction yields are used to build a qualitative potential energy surface of the various stilbene reactions. The molecular surface-isomerization dynamics is shown to be influenced by the type of dopant (n or p). This is related to surface charging, which reveals modifications in the stilbene ionization potential.

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

The (3 × 3) silicene on the (4 × 4) Ag(1u20091u20091) 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.

SiC(0001) 3 × 3 Heterochirality Revealed by Single-Molecule STM Imaging

We report a description of the SiC(0001) 3 x 3 silicon carbide reconstruction based on single-molecule scanning tunneling microscopy (STM) observations and density functional theory calculations. We show that the SiC(0001) 3 x 3 reconstruction can be described as contiguous domains of right and left chirality distributed at the nanoscale, which breaks the to date supposed translational invariance of the surface. While this surface heterochirality remains invisible in STM topographies of clean surfaces, individual metal-free phthalocyanine molecules chemisorbed on the surface act as molecular lenses to reveal the surface chirality in the STM topographies. This original method exemplifies the ability of STM to probe atomic-scale structures in detail and provides a more complete vision of a frequently studied SiC reconstruction.

STM and DFT Investigations of Isolated Porphyrin on a Silicon‐Based Semiconductor at Room Temperature

Metalloporphyrins represent a class of flexible molecules with a nearly square planar core conformation and a two dimensional conjugated p-electron delocalization. Due to their interesting physicochemical properties, metalloporphyrins adsorbed on a surface can be used in many technological applications such as molecular electronics, light-harvesting arrays for solar energy generation, catalysts, sensors, etc. The fine determination of the conformation and arrangement of adsorbed molecules on a surface are key points, since they are strongly related to the physical and chemical properties of the final organic–inorganic interfaces. They are changed by the subtle balance of internal deformation and substrate–molecule interactions, leading to a conformational adaptation of the molecule on the substrate lattice. These features are even more relevant in semiconductors than in metals because the moleculesemiconductor interactions are usually greater than molecule– metal interactions. Although scanning tunneling microscopy (STM) is a remarkable tool to investigate individual adsorbed molecules on semiconductors, experimental STM images of metalloporphyrins were achieved only on metals and only an unique very recent article investigates theoretically the adsorption of a metalloporphyrin on a Si(111)-H surface. Herein, we report the first experimental investigation at room temperature of the adsorption of Cu-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) porphyrin (Cu-TBPP) as a model of metalloporphyrin on a passivated silicon based surface (Si(111)-B) using STM and by theoretical calculations in order to fully understand the conformational adaptation of the Cu-TBPP on a Si(111)-B surface.

Electronic and atomic structure of GeSi(111)-7 × 7 in the initial stages of Ge chemisorption

Abstract We present a theoretical study of the initial stages of Ge chemisorption on Si(111)-7 × 7 in the DAS model. Three Ge adsorption mechanisms are studied: substitution, addition and substitution followed by addition of a Si atom (permutation). The threefold hollow (H3) and threefold filled (T4) sites are considered as possible adsorption sites. We compute the adsorption energies and the densities of states in the framework of the crystalline extension of the extended Huckel theory. Substitution seems to be unlikely when compared to addition and permutation. An interpretation of the surface states A, A1 and A2 observed in photoemission is proposed.

DFT‐D Studies of Single Porphyrin Molecule on Doped Boron Silicon Surfaces

We present a theoretical study in the framework of density functional calculations, taking into account the van der Waals interactions (DFT-D) of isolated Cu-5,10,15,20-tetrakis(3,5-di-tert-butyl-phenyl) porphyrin (Cu-TBPP) molecules in a C2v conformation adsorbed on a Si(111)√3x√3R30°-boron surface [denoted Si(111)-B]. With this approach, we investigate interactions between perfect or boron-defect Si(111)-B substrates and the Cu-TBPP molecule as well as the consequences of demetallation of Cu-TBPP. For each model, we determine the structural equilibrium, the spatial charge-density distribution and the electronic properties of the ground state. We conclude that there is potential for Si adatom capture by a porphyrin without strong modification of the porphyrin response, as seen from simulated scanning tunneling microscopy (STM) images.

Full DFT-D description of a nanoporous supramolecular network on a silicon surface.

We present a full density-functional-theory study taking into account the van der Waals interactions of a 2D supramolecular network adsorbed on the Si(111)√3x√3R30°-boron surface denoted SiB. We show that, contrarily to the previous calculations [B. Baris, V. Luzet, E. Duverger, Ph. Sonnet, F. Palmino, and F. Chérioux, Angew. Chem., Int. Ed. 50, 4094 (2011)] molecule-molecule interactions are attractive, thanks to van der Waals corrections which are essential to describe such systems. We confirm the importance of the substrate effect to achieve the molecular network on the boron doped silicon surface without covalent bond. Our simulated STM images, calculated in the framework of the bSKAN code, give better agreement with the experimental STM images than those obtained by the integrated LDOS calculations within the Tersoff-Hamann approximation. The tungsten tip presence is essential to retrieve three paired lobes as observed experimentally. The observed protrusions arise from the phenyl arms located above silicon adatoms.

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.