T. Angot

24 h stability of thick multilayer silicene in air

Thick epitaxial multilayer silicene films with a root 3 x root 3R(30 degrees) surface structure show only mild surface oxidation after 24 h in air, as measured by Auger electron spectroscopy. X-ray diffraction and Raman spectroscopy measurements performed in air without any protective capping, as well as, for comparison, with a thin Al2O3 cap, showed the (002) reflection and the G, D and 2D Raman structures, which are unique fingerprints of thick multilayer silicene.

Hydrogen adsorption on graphite (0001) surface: A combined spectroscopy–density-functional-theory study

The adsorption of H/D atoms on the graphite (0001) surface is investigated by means of both high-resolution electron-energy loss spectroscopy (HREELS) and periodic first-principle density-functional theory. The two methods converge towards two modes of adsorption: adsorption in clusters of about four hydrogen atoms and adsorption in pairs of atoms on contiguous carbon sites. The desorption energies estimated from the calculated dissociation energies range from 8 to 185 kJ mol(-1) leading to an estimated surface coverage at saturations of 30-44 at. %. These results are compared with previous thermal desorption spectroscopy results. New HREEL signal assignments are proposed based on quantum calculations.

Hydrogen-graphite interaction: Experimental evidences of an adsorption barrier.

The interaction of H atoms having relatively low average kinetic energy (∼0.025 eV) with both perfectly clean and D-covered HOPG surfaces is investigated using high resolution electron energy loss spectroscopy. From this study we confirm, in a controlled fashion, the presence of the theoretically predicted adsorption barrier since no adsorption is detected for such H atoms on HOPG. Moreover, we demonstrate that the exposure of a D saturated HOPG surface to these H atoms results in the complete removal of adatoms, with no further adsorption despite the prediction of the adsorption barrier to vanish for H dimers in para configuration. Therefore, the recombinative abstraction mechanism which competes with the adsorption process is more efficient.

Growth and structural properties of silicene at multilayer coverage

At monolayer coverage, silicene on Ag(1 1 1) may present different structural phases depending on the growth conditions. At multilayer coverage, only one structural phase has been reported: the [Formula: see text] phase. However, no link between the structural arrangement of the monolayer and that of the multilayer has been addressed. In this paper, reporting experimental work based on low-energy electron diffraction and scanning tunneling microscopy, we focus on the structural aspects of a multilayer film of silicene. We demonstrate that it exhibits one structural arrangement, namely the [Formula: see text] form, but with different domain orientations resulting from the structural properties of the initial wetting monolayer.

Graphitization of the 6H-SiC(0001) surface studied by HREELS

By using high-resolution electron energy loss spectroscopy (HREELS), we have studied the vibrational properties of the various 6H–SiC(0 0 0 1) reconstructions, from the Si-rich to the graphitized surface.The 6H–SiC(0 0 0 1)-(3 � 3) exhibits the Fuchs–Kliewer (FK) optical phonons commonly observed on strongly polar materials.The lowering of the energy width of the elastically reflected electrons with increasing primary energies reveals the coupling of FK with the plasmon that derives from the bulk doping level.No particular modification in the HREELS spectra is observed after preparation of the 6H–SiC(0 0 0 1)-( p 3 � p 3)R30 surface.On the (6 p 3 � 6 p 3)R30 reconstructed surface, the FK phonon modes display both a blue shift and an increased damping factor.In the ultra-violet energy region we observed a loss structure at � 6 eV whose dispersion relation allows to readily conclude on the presence of a pure graphite layer: it almost perfectly match the dispersion relation measured on highly oriented pyrolitic graphite for the so-called ‘p-plas

Multilayer silicene: clear evidence

One year after the publication of the seminal paper on monolayer 3 by 3 reconstructed silicene grown on a silver (111) substrate, evidence of the synthesis of epitaxial root3 by root3 reconstructed multilayer silicene hosting Dirac fermions was presented. Although a general consensus was immediately reached in the former case, in the latter one, the mere existence of multilayer silicene was questioned and strongly debated. Here, we demonstrate by means of a comprehensive x-ray crystallographic study, that multilayer silicene is effectively realized upon growth at rather low growth temperatures (~200{\deg}C), while, instead, 3D growth of silicon crystallites takes place at higher temperatures, (~300{\deg}C). This transition to bulk like silicon perfectly explains the various data presented and discussed in the literature and solves their conflicting interpretations.

Ordered phthalocyanine superstructures on Ag(110)

Organic-metal interfaces, in particular, self-assembling systems, are interesting in the field of molecular electronics. In this study, we have investigated the formation of the Ag(110)-iron phthalocyanine (FePc) interface in a coverage range of less than 1 and up to 2 ML using synchrotron based photoelectron spectroscopy and low energy electron diffraction. As-deposited FePc forms a densely packed first layer exhibiting a 3 x 2c(6 x 2) symmetry. Upon thermal treatment the order at the interface is modified depending on the initial FePc coverage, resulting in less densely packed but still ordered superstructures. The first monolayer is relatively strongly bound to the substrate, leading to the formation of an interface state just below the Fermi level. The highest occupied molecular orbital of FePc in the second layer is found at 1 eV higher binding energy compared to the interface state.

Growth of epitaxial SiGe nanostructures at low temperature on Si(100) using hot-wire assisted gas source molecular beam epitaxy

Abstract Ge/Si and Si 1− x Ge x nanostructures were grown on Si(100) at 350 °C substrate temperature with a significant growth rate of about 10 A min −1 using hot-wire decomposition of disilane and germane in an ultrahigh vacuum environment. A quartz crystal microbalance (QCM) was used to monitor growth rates and to study the influence of feed gas pressures. In-situ X-ray photoelectron spectroscopy measurements allowed us to calibrate alloy stoichiometry monitored by QCM. Ex-situ high-resolution transmission electron microscopy observations demonstrated the 2D epitaxial growth of partly relaxed nanolayers. These promising results may be closely related to a surfactant-like role of atomic hydrogen, which appears as a beneficial sub-product of the hot-wire decomposition process of hydride sources.

High resolution electron energy loss spectroscopy study of the Si(001) 3 × 1 hydrogenated surface

Abstract Hydrogenation of the Si(001) 2 × 1 surface held at 400 K has been performed in a new experimental set-up for a characterization by high resolution electron energy loss spectroscopy (HREELS). Angular scan along the [110] direction recorded at 55 eV primary energy confirms the 3 × 1 surface reconstruction. HREELS performed at 6 eV demonstrates the existence of a dihydride phase, in particular through the observation of the scissor mode around 904 cm −1 . The good quality of this surface, as shown by a high count rate on the elastic peak, allowed to obtain a resolution as good as 45 cm −1 at room temperature. Lineshape studies of the vibrational peaks related to hydrogen reveal an abnormal width of the 639 cm −1 mode. In fact, the resolution was improved by cooling down to ∼ 110 K, and a double peak was clearly resolved, separated by 35 cm −1 . In view of previous HREELS studies of the Si(001) 1 × 1 and 2 × 1 hydrogenated surfaces, as well as of theoretical calculations of the vibrational modes of SiH 2 species, the assignment of these modes is given. For the first time, a clear monohydride signature within a dihydride phase is reported by HREELS.

On-surface synthesis of aligned functional nanoribbons monitored by scanning tunnelling microscopy and vibrational spectroscopy

In the blooming field of on-surface synthesis, molecular building blocks are designed to self-assemble and covalently couple directly on a well-defined surface, thus allowing the exploration of unusual reaction pathways and the production of specific compounds in mild conditions. Here we report on the creation of functionalized organic nanoribbons on the Ag(110) surface. C–H bond activation and homo-coupling of the precursors is achieved upon thermal activation. The anisotropic substrate acts as an efficient template fostering the alignment of the nanoribbons, up to the full monolayer regime. The length of the nanoribbons can be sequentially increased by controlling the annealing temperature, from dimers to a maximum length of about 10 nm, limited by epitaxial stress. The different structures are characterized by room-temperature scanning tunnelling microscopy. Distinct signatures of the covalent coupling are measured with high-resolution electron energy loss spectroscopy, as supported by density functional theory calculations.