Marie-Christine Hanf

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.

Magnetic properties of epitaxial Fe(Si1-xFex) films grown on Si(111)

Abstract Iron silicide thin films (200xa0A Fe(Si 1− x Fe x ) with 0⩽ x ⩽1 and local cubic CsCl structure) have been grown by coevaporation at room temperature (RT) on Si(1xa01xa01). X-ray magnetic circular dichroism (XMCD) and magneto-optic Kerr effect (MOKE) measurements indicate that the films are ferromagnetic at RT for x ranging from 1 (pure Fe) to 0.15 (Fe 1.35 Si). The magnetization is parallel to the film surface, and the magnetic anisotropy is uniaxial, with the easy axis lying along a [1xa00xa01] Si crystallographic direction and the hard axis along a [1xa02xa01] Si direction of the substrate. MOKE measurements show that the magnitude of the saturation field increases with increasing Si concentration, while XMCD data indicate that the average local magnetic moment carried by the Fe atoms decreases with decreasing Fe concentration. Models which involve the diminution of the number of Fe nearest neighbors are proposed for the description of the behavior of the Fe moments.

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.

Magnetic properties in epitaxial binary iron and ternary iron-cobalt silicide thin films grown on Si(1 1 1)

Abstract Binary Fe(Si 1− x Fe x ) iron and ternary Fe 3− y Co y Si iron cobalt silicide thin films (thicknessxa0:xa0200xa0A) with local CsCl structure epitaxially grown at room temperature on Si(1xa01xa01) have been studied through X-ray magnetic circular dichroism (XMCD) and magneto-optic Kerr effect (MOKE) vector magnetometry. The binary Fe(Si 1− x Fe x ) films have ferromagnetic properties at room temperature in the range 0.09⩽ x 3− y Co y Si films are ferromagnetic as well in the whole range 0 y 3 Si) favored by the absence of orientation dependence of the fourth-order cubic term of the magnetocrystalline anisotropy energy in (1xa01xa01) crystallographic plane. The easy axis corresponds invariably to the direction normal to the incidence plane of the atomic flux during evaporation. The substitution of iron by silicon in Fe(Si 1− x Fe x ) films increases the coercive field but does not strongly affect the uniaxial anisotropy constant. In Fe 3− y Co y Si alloys, a similar trend is observed and the increase of coercive force may be related to defects generated by alloying. The change of the magnetic moments of Fe and Co versus y , as obtained by XMCD, can be understood in terms of simple models.

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.

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.

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.