[en] We have investigated the effect of hydrogen interaction with silicene monolayer and multilayer films grown on Ag(111) by means of scanning tunneling microscopy, low energy diffraction and high-resolution electron energy loss spectroscopy. On monolayer silicene, we demonstrate that the recently reported hydrogenated γ-(3 × 3) is indeed observed, but is found often in coexistence with the usual “flower” pattern of the clean silicene-(3 × 3), also labeled α-(3 × 3). In the vibrational spectra, features related to Si−H vibrational modes show shifts towards lower energies as compared to hydrogenated Si surfaces. This substantial bond softening is indicative of a partial charge filling of the bonding or anti-bonding orbitals of the Si−H bonds as compared to Si(111)−H. Indirectly, this is an evidence of the hybridization of monolayer silicene with the Ag(111) substrate. At multilayer coverage, the situation is clearly different. Indeed, hydrogen strongly perturbs the √3 × √3 reconstruction of the multilayer film, leading to its local dissolution and to the formation of protrusions due to clustering of atoms. The vibrational spectrum of the √3 × √3 silicene closely resembles that of a highly hydrogenated Si surfaces, with the simultaneous presence of Si−H, Si−H₂ and Si−H₃ bonds.

[en] Recent results obtained on prototypical metal-semiconductor systems, that is Ag and Pb overlayers on Si(111) as examples of abrupt interfaces and Au films on Si(100) as example of a reactive interface, mostly derived from synchrotron radiation studies, are reviewed in this paper

[en] In this article, we review some of the most recent progress and understanding in the low temperature surface phase transitions at prototypical metal-semiconductor interfaces. We essentially focus on quantitative surface structural information obtained by using a significant variety of specialised techniques for the individual phases of a model system, namely, tin on Ge(1 1 1) substrates. The strengths and limitations of the structural results obtained by using scanning tunnelling microscopy, photoelectron diffraction and surface X-ray diffraction are discussed in relation to their support with respect to possible mechanisms recently invoked in the literature as being at the origin of the phase transition. These investigations show that a large progress has been made in this field, taking into account the very valuable experimental and theoretical contributions provided by different groups. There remain, however, essential unresolved problems, which will be analysed in the light of the limitations of these structural methods and the difficulty presented by the complex adsorbate systems studied

[en] The results of a photoemission study of icosahedral single-grain ZnMg(Y, Ho) quasicrystals and their crystalline approximant Zn₂Mg are presented. Synchrotron radiation photoemission measurements have been performed on in situ cleaved samples at low temperature of ∼90 K and 10^-10 mbar pressure. The spectra obtained indicate simple metal-type valence bands (VB) with distinct Fermi edge cutoff. Analysis of the Fermi edge spectra reveals the presence of a pseudogap in i-ZnMg(Y, Ho) quasicrystals. The pseudogap corresponds to a rather sharp DOS dip at the Fermi level with the ∼50% (of the normal DOS) depth and the ∼0.8 eV width. The values of i-ZnMg(Y, Ho) pseudogap parameters are comparable with those of other quasicrystals

[en] Here, we present a few examples, largely reflecting our own contribution, which reveal novel interesting phenomena uncovered at well characterized semiconductor surfaces, essentially thanks to detailed synchrotron radiation studies. Clean surfaces as well as surfaces covered with sub-monolayer films of unreactive metal adsorbates are selected to highlight peculiar structural features, such as surface and/or sub-surface dimerization at InAs(100) surfaces, subtle electronic states, i.e. adsorbate-induced quantized two-dimensional electron gas at InAs surfaces and ultra-fast dynamical behaviours, such as the flip-flop motion of buckled dimers on the (100) face of group IV semiconductors. (author)

[en] Both synchrotron monochromatic X-ray topography, in the reflection mode, and synchrotron white-beam X-ray topography, in the transmission mode, were used for the investigation of defects in α-HgI₂ crystals. The combination of these two techniques appeared to be very fruitful for the characterization of these defects either in relation to the growth conditions or with the deformation introduced during the processing of lamellae for the detector fabrication. These capabilities were particularly enhanced by using the ESRF which is a 3rd generation synchrotron source. (orig.)

[en] The possibility to synthesize honeycomb silicene has recently been demonstrated upon providing compelling evidence through the combination of complementary experimental results and density functional theory calculations (Vogt et al 2012 Phys. Rev. Lett. 108 155501). In this case silicene is grown on Ag(111) substrates and shows a nearest neighbour distance of two Si atoms of ∼0.23 nm in agreement with theoretical results for free-standing silicene. In another publication from another group of authors a different silicene arrangement has been claimed previously, where the silicene sheet is strongly compressed with a Si-Si distance amounting to only 0.19 nm (Lalmi et al 2010 Appl. Phys. Lett. 97 223109). This has led to the fundamental question whether silicene could support such a large compressive strain. We will show that the apparent contradictions in the literature can be explained based on a thorough analysis, which reveals that the pure Ag(111) surface can mimic a honeycomb structure, which could easily be misinterpreted as a strained silicene layer. Our discussion will show that there is no evidence for the existence of such strong compressively strained silicene layers.

[en] The deposition of 1/3 Pb monolayer at room temperature onto Ag(111) leads to a (√(3)x√(3))R30 deg. superstructure. We present here a detailed structural study of this surface structure by synchrotron radiation surface x ray diffraction (SR-SXRD) and scanning tunnel microscopy (STM). We show that Pb atoms are embedded into the silver top surface layer forming an ordered Ag₂Pb surface alloy despite the strong tendency of the system to phase separate in the bulk. Quenched molecular dynamics simulations allow us to interpret this ordering reversal, in terms of size-mismatch induced surface alloying

[en] Two-dimensional elemental topological insulators including silicene, germanene and stanene are currently the hottest topics in condensed matter physics. We first review the recent progress on electronic and topological properties of their monolayers from a fundamental viewpoint. Next, we describe their experimental realizations by epitaxial growth and their actual physical properties. We start with the description of the topological nature of generic Dirac systems and then apply it to silicene by introducing the spin and valley degrees of freedom. Based on them, we classify all topological insulators in the general honeycomb system. We discuss topological electronics based on honeycomb systems. We introduce the topological Kirchhoff law, which is a conservation law of topological edge states. A field effect topological transistor is proposed based on the topological edge states. We show that the conductance is quantized even in the presence of random distributed impurities. Monolayer topological insulators will be a key for future topological electronics and spin-valleytronics. The outstanding example of the realization of such monolayer Si, Ge and Sn novel artificial allotropes is the canonical 3 × 3 reconstructed epitaxial silicene phase grown in situ under ultra-high vacuum on the silver (111) surface. Its realization in 2012 has preceded the synthesis of germanene, followed by that of stanene, respectively on Au(111) and Bi₂Te₃ substrates. Further growth of Si and Ge over monolayer epitaxial silicene and germanene leads to layered thin films displaying Dirac fermion characteristics. Amazingly, Si deposition onto Ag(110) templates yields massively parallel, pentasilicene-like nanoribbons, a novel form of one-dimensional silicon. Field Effect Transistors have been already fabricated both with single and multi-layer silicene channels, clearly demonstrating potential applications in electronics of silicene and such related materials, which are directly compatible with the current, ubiquitous, Si-based technology. Finally, enticing prospects are outlined.

[en] We have grown an atom-thin, ordered, two-dimensional multi-phase film in situ through germanium molecular beam epitaxy using a gold (111) surface as a substrate. Its growth is similar to the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in scanning tunneling microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced density functional theory calculations we can identify it as a √3 × √3 R(30°) germanene layer in conjunction with a √7 × √7 R(19.1°) Au(111) supercell, presenting compelling evidence of the synthesis of the germanium-based cousin of graphene on gold. (paper)