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International audience; On-surface synthesis has provided exciting concepts for the building of covalently bonded molecular nanostructures as well as the exploration of new synthetic pathway alternatives to chemical synthesis in solution. The surface-supported reaction of precursor molecules can result in the formation of 2D molecular networks as well as novel 0D molecular structures. In this study, we investigate in ultrahigh vacuum by low-temperature scanning tunneling microscopy the adsorption of the chiral molecule (R)-6-bromo-3-phenyl-2,3-dihydro-1H-inden-1-one (BrPhINDO) on the Cu(111) surface. Annealing an as-deposited layer of molecules allows to activate two kinds of homocoupling reactions, the Ullmann-like coupling and the Knoevenagel reaction, resulting in the formation of various low-dimensional structures. By studying their prochirality and comparing with the products obtained with the enantiomer precursor, we demonstrate adsorption-induced chiral inversion inducing partial racemization.

International audience; Germanene, as an artificial graphene-like near room temperature topological insulator, compatible with ubiquitous silicon technology, is potentially the most promising artificial Xene for ultra-scale nanoelectronics. Here, we follow its emergence and development when prepared in situ under ultra-high vacuum in clean and controlled conditions by dry epitaxy on prominent metal surfaces (e.g., aluminum, silver, gold). We describe its predicted electronic properties and its birth in 2014, even if it was just a renaissance, as it was only understood after 51 years after an undeciphered birth certificate lost in oblivion. We emphasize the lifting of germanene flakes from an aluminum template with the tip of a scanning tunneling microscope, and their repositioning to form bilayer germanene with Bernal stacking. Finally, we discuss the growth of monolayer germanene in a single phase harboring Dirac fermions, following a bottom-up synthesis strategy by segregation on a gold thin film in epitaxy on a germanium support.

Surface and near-surface morphology evolution of helium-irradiated tungsten due to thermal cycling up to the ITER-relevant temperature of 1350 K was studied using electron microscopy and positron annihilation spectroscopy techniques. Holes at the surface and bubbles in the near-surface of recrystallized polycrystalline tungsten samples were created by 75 eV helium plasma irradiation with the fluence of 3 × 1023 He m−2 at the surface temperature of 1073 K. Subsequent annealing experiments were combined with a detailed electron microscopy analysis to investigate the shape and density changes of holes and helium bubbles with respect to grain orientation. We show that the initially circular holes and round bubbles became faceted upon heating with 1 K s−1 ramp up to 870 K. Annealing cycles up to 1350 K induced resulted in bubbles removal in the first 5 nm below the surface and surface smoothing. Electron energy loss spectroscopy measurements allowed estimation of helium gas density and pressure inside bubbles. Positron annihilation spectroscopy allowed to investigate the nature of defects and their evolution in the helium-irradiated tungsten with thermal cycling.

Tungsten (W) is the material selected for the divertor exhaust of the international nuclear fusion experiment ITER. In this harsh environment, the interactions of heat loads and ion fluxes with W can induce temporary or permanent evolution in the optical properties. Poor knowledge of such evolution during a plasma operation can lead to errors in temperature measurements performed by optical diagnostics. Therefore, it is of fundamental importance to characterize possible changes in W optical properties. In this work, we studied the role of morphology and temperature on the optical response of W. The reflectivities of five W samples with different roughness values (20–100 nm) were measured during laser annealing (25–800 °C) in the visible and near-infrared domains (500–1100 nm). We observed an increase in reflectivity after annealing and we demonstrated that it was due to a change in the chemical composition of the surface, in particular a reduction in the amount of native oxide. Moreover, we show that roughness does not sensibly vary in the investigated temperature range. By highlighting the role played by roughness and surface impurities (e.g., oxide), we provide insight in how W optical properties can evolve in tokamaks where high ion fluxes, heat loads, and impurities can induce the evolution of both the morphology and surface composition of W.

International audience; We present measurements of the sticking probability of ammonia on two metals, tungsten and 316L stainless steel, covered with natural surface impurities as they will be used for the international nuclear fusion experimental reactor ITER. Using a collimated supersonic molecular beam at two different kinetic energies (55 and 255 meV), varying the sample temperature in the 130–425 K range, and characterizing the surface composition with Auger electron spectroscopy, we observe similar sticking features on both surfaces, consistent with a nondissociative adsorption mediated by two precursors having different trapping probabilities. First, the initial sticking probability decreases with increase in the surface temperature. Second, the sticking probability increases with the surface coverage up to near-saturation coverage, where it declines. Both features cannot be described together with the Kisliuk model (intrinsic + extrinsic precursors with identical trapping probabilities) or the modified Kisliuk model (direct adsorption + extrinsic precursor). Thus, we derive a generalized and separable Kisliuk (GSK) model that is able to reproduce quantitatively these two experimental observations thanks to intrinsic and extrinsic precursors having different trapping probabilities. The GSK model assumes a negligible transfer from the intrinsic precursor to the extrinsic precursor, which allows one to extract precursors kinetic parameters in a two-step analysis. The GSK analysis indicates that the ammonia trapping probability is lower on the bare surface (intrinsic precursor) than on the NH3-covered surface (extrinsic precursor). Furthermore, the barriers between the two precursor wells and the deep adsorption well are found below the vacuum level. Finally, we measure that the sticking probability does not decline to zero, i.e., steady-state sticking is observed with a probability up to 0.15 at a beam energy of 55 meV and a surface temperature of 220 K. This observation is consistent with NH3 multilayer adsorption. These experimental results and their fitting with the GSK model offer the starting point to a predictive determination of the fusion fuel inventory related to ammonia in the international experimental reactor ITER.

International audience; This work describes the adsorption of pyrene molecules on a Ag(111) surface. We first demonstrate that despite its high vapor pressure, pyrene molecules can form ordered films under ultrahigh vacuum conditions, presenting a well-contrasted diffraction pattern. Studies using high-resolution electron energy loss spectroscopy and ultraviolet photoelectron spectroscopy provide compelling evidence of a physisorbed system where the molecules only weakly interact with the substrate underneath. Comparisons with theoretical calculations, as well as with data obtained from optical spectroscopies, clearly demonstrate that the vibrational and electronic properties of the adsorbed molecules are similar to the expected ones for pristine pyrene. Finally, we used temperature-programmed X-rays photoelectron spectroscopy to study the desorption process of pyrene on the Ag(111) surface and estimate its activation energy to desorption.

Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome lattice of silicene on aluminum (111) substrates. We provide evidence of such an exotic 2D Si allotrope through scanning tunneling microscopy (STM) observations, high-resolution core-level (CL) and angle-resolved photoelectron spectroscopy (ARPES) measurements, along with Density Functional Theory calculations., 13 pages, 6 figures

We introduce silicene, the Si-based analog of graphene, an emerging two-dimensional topological insulator, and its realization by epitaxial growth on the favored silver (111) substrate. We describe the structural, electronic and vibronic properties of monolayer silicene, and the growth and electronic properties of multilayer silicene. We next address the hydrogenation and oxidation of these new silicon allotropes. We further report the fabrication of the first field-effect transistors made with a silicene channel. Finally, we discuss exotic forms of silicon at zero and one dimension, namely benzene-like nanodots and pentasilicene-like nanoribbons, before drawing enticing perspectives.

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International audience; The time evolution of fuel retention in materials relevant for future fusion reactors is compared for two different tungsten microstructures: single-crystal versus recrystallized poly-crystals. The initial retention of both types of sample is similar. It decays exponentially with a time constant of ~18 hours at 300 K (the so-called short-term retention). After 48 hours at room temperature, a constant deuterium retention is measured (long-term retention) with the single-crystal containing systematically less deuterium than poly-crystals. Macroscopic rate equations models are built with density functional theory inputs to reproduce deuterium desorption observables with the MHIMS-R code. We found that the native oxide layer could explain the desorption peak located at ~450 K as well as most of the short-term retention in the single-crystal. The native oxide together with, dislocations for single-crystal and grain boundaries for poly-crystals, are responsible for the long-term retention. Dislocations should explain the desorption peak located at ~815 K for mechanically polished samples. The dual role of most of tungsten defects is related to their multi-trapping properties with filling-level-dependent detrapping energies. Finally, the use of an effective diffusivity of deuterium through the native oxide layer, i.e. its diffusion barrier character, is evaluated.

Original reaction pathways can be explored in the on-surface synthesis approach where small aromatic precursors are confined to the surface of single crystal metals. The bis-indanedione molecule reacted with itself on silver surfaces in different ways, through a Knoevenagel reaction or an oxidative coupling, leading to the formation of a variety of new molecular compounds and covalently-linked 1D or 2D networks. Noteworthy, original reaction products were obtained that cannot be synthesized in traditional solvent-based chemistry. The lowest activation temperature for the homo-coupling reactions was found on the Ag(111) surface. The Ag(110) was highly selective in terms of coupling reaction type, while on Ag(100) the temperature could finely control the selectivity. The on-surface synthesis approach is shown here to be particularly efficient to produce original compounds in mild conditions, using activation temperatures as low as 200 °C. The different structures were characterized by scanning tunnelling microscopy (STM) together with X-ray photoelectron emission spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS).

We report a systematic study that quantifies nitrogen retention and ammonia production on tungsten and that sheds light on the mechanism for ammonia formation on ITER’s divertor material. Saturation of the nitrogen-implanted layer in polycrystalline tungsten is observed at room temperature for a nitrogen ion fluence in the low 1021 N+ m−2 range. Nitrogen desorption from this N-implanted layer occurs in the 800–1100 K temperature range and exhibits a zero-order kinetics with an activation energy of 1.45 eV and a prefactor of 5 × 1024 m−2 s−1. Following nitrogen and deuterium co-implantation, deuterated ammonia production is observed during temperature programmed desorption between 350 K and 650 K in conjunction with deuterium desorption. In contrast, nitrogen desorption still occurs above 800 K. Significant production of ammonia is obtained only when the nitrogen layer created by ion implantation is approaching saturation and the amount of nitrogen lost to ammonia production is only in the percent range. This result is understood by repeating cycles of deuterium implantation and thermo-desorption below the desorption temperature of the nitrogen layer. The exponential decay of the amount of produced ammonia with cycle number demonstrates that nitrogen diffusion to the surface is negligible in the ammonia production temperature range and that ammonia formation occurs at the outermost surface layer. The maximum quantity of ammonia produced from the present N implanted layer is below 2 × 1018 ND3 m−2, which is limited by the nitrogen atom surface density. Surface vibrational spectroscopy demonstrates the presence of ammonia precursors on the nitrogen-implanted tungsten surface upon deuterium implantation. These ammonia precursors can be created also at room temperature through the dissociative chemisorption of thermal D2 catalysed by nitrogen present at the tungsten surface and, more efficiently, by adsorption of deuterium atoms.

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.

This article deals with surface loss on quartz of atomic hydrogen (H) and its isotope deuterium (D) in a low-pressure (10 Pa) pulsed inductively coupled plasma. The atomic temporal decay in the post discharge is measured by two-photon absorption laser-induced fluorescence (TALIF). From the loss rate, the atomic surface loss probability is determined. In pure hydrogen or pure deuterium gas, no isotopic effect on surface kinetics has been observed and the surface loss probabilities of H and D were found to be almost identical and equal to ∼1.8%. However, despite the lack of difference in surface loss probability, a net isotopic effect on surface loss rate due to the mass difference between the isotopes is measured. Hydrogen atoms diffuse faster and have higher flux to the plasma chamber walls than deuterium atoms. Hydrogen atoms are therefore lost at higher rate than deuterium atoms. Based on the observed isotopic difference and on the comparison between H and D TALIF signals, the isotopic effects on H and D atom production are discussed.

International audience; Chemical functionalization of graphene is one method pursued to engineer new properties into a graphene sheet. Graphene oxide is the most commonly used chemical derivative of graphene. Here we present experimental evidence for the formation of enolate moieties when oxygen atoms are added to the graphene basal plane. The exotic functional groups are stabilized by simultaneous bond formation between the graphene sheet and the underlying Ir(111) substrate. Scanning tunneling microscopy images demonstrate the patterned nature of CO bond formation and x-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy are used to characterize the enolate moiety. The results present a new mechanism for the formation of patterned graphene oxide and provide evidence of a functional group rarely considered for graphene oxide materials. Graphene is a 2D semi-metal in which all carbon atoms are sp 2 hybridized. 1 Chemical functionalization of graphene is driven by the desire to engineer the physical and chemical properties of pristine graphene sheets, and to introduce new means of incorporating graphene interfaces with other materials. 2-4 The addition of simple atomic species, such as hydrogen, 5 oxygen 6,7 or fluorine, 8 has been shown to change the electronic band structure of graphene from that of a semi-metal to that of a semiconductor or insulator. Graphene oxide is a term used to describe graphene materials that have been subjected to oxidation reactions. Supported graphene oxide materials have been studied as sensor materials, for their potential magnetic properties and as clusters to form graphene oxide quantum dots. 9-12 The growth of metallic nanoparticles on supported graphene sheets is also facilitated by the presence of CO bonds at the graphene basal plane, 4 with attachment or cleavage of particle growth initiators dependent on the nature of the CO bond. 13 This may be a viable method for atomic layer deposition on graphene films, allowing for integration of graphene sheets in more complex electronic devices. 14 Consequently, the nature of the CO bond formed during synthesis of graphene oxide is of general interest and importance. It is widely assumed that an oxygen atom bonding to the basal plane of a graphene sheet is initially covalently bound through the formation of an epoxy group. 15-18 The stability of different oxygen moieties on metal-supported graphene sheets was, however, recently explored for graphene islands on Ru(0001). 19 Those authors, building on theoretical results from Jung et al., 20 provided the first experimental evidence for the existence of the enolate moiety when oxygen atoms bind to a graphene sheet supported by a metal substrate. Their calculations indicate that interaction with an underlying metallic substrate can guide the formation of enolate groups when an oxygen atom binds to one carbon atom, and the adjacent carbon atom is suitably positioned to bind down to an underlying metal atom. A 370 meV gain in energy was reported for the formation of an enolate group, versus an epoxy group, in certain areas of the moiré pattern formed between the graphene lattice and atoms from the Ru(0001) surface. 19 Graphene on Ru(0001), however, is already a strongly coupled system with good evidence for the preexistence of a carbon-metal bond even before oxygen atoms are introduced. 21 Hence, it is timely to consider the nature of the CO bond for other, less strongly coupled systems. Graphene on Ir(111) (Gr/Ir(111)) demonstrates weak van der Waals interaction between the graphene sheet and the Ir(111) surface atoms. 22 Moreover, single crystal graphene sheets with a low concentration of defects and highly oriented domains are routinely prepared on Ir(111) substrates following chemical vapour deposition. 23 The attachment of oxygen atoms to Gr/Ir(111) has been reported previously but the possibility of enolate formation was not considered. 15,16 Here, we revisit this system adding new high-resolution electron energy loss spectroscopy (HREELS) data to demonstrate that at a low flux of oxygen atoms, enolate formation dominates and occurs only at select sites on the Gr/Ir(111) surface, generating a long-range pattern of graphene oxide nano-dots. The slight lattice mis-match between the carbon atoms in the graphene sheet and the Ir(111) surface atoms leads to a long-range repeating moiré pattern. 23 The moiré unit cell contains: TOP regions where Ir surface atoms are positioned directly beneath the center of a hexagonal ring in the graphene sheet; and HCP and FCC regions, where every second C atom is positioned directly above an Ir atom. HCP and FCC regions differ in the arrangement of Ir atoms in the second and third Ir layer. The remaining carbon atoms, in between

Hydrogen functionalization of graphene by exposure to vibrationally excited H2 molecules is investigated by combined scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy measurements, and density functional theory calculations. The measurements reveal that vibrationally excited H2 molecules dissociatively adsorb on graphene on Ir(111) resulting in nanopatterned hydrogen functionalization structures. Calculations demonstrate that the presence of the Ir surface below the graphene lowers the H2 dissociative adsorption barrier and allows for the adsorption reaction at energies well below the dissociation threshold of the H–H bond. The first reacting H2 molecule must contain considerable vibrational energy to overcome the dissociative adsorption barrier. However, this initial adsorption further activates the surface resulting in reduced barriers for dissociative adsorption of subsequent H2 molecules. This enables functionalization by H2 molecules with lower vibrational energy, yielding an avalanche effect for the hydrogenation reaction. These results provide an example of a catalytically active graphene-coated surface and additionally set the stage for a re-interpretation of previous experimental work involving elevated H2 background gas pressures in the presence of hot filaments.

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., On-surface synthesis, in which molecular units assemble and couple on a defined surface, can access rare reaction pathways and products. Here, the authors synthesize functionalized organic nanoribbons on the Ag(110) surface, and monitor the evolution of the covalent reactions by an unorthodox vibrational spectroscopy approach.

International audience; A 13.56 MHz RF discharge in hydrogen was studied within the pressure range of 1-17 10 Pa, and at power range of 400-1000 W. The electron energy distribution function and 18 electron density were measured by a Langmuir probe. The gas temperature was determined by 19 the Fulcher-α system in pure H2, and by the second positive system of nitrogen using N2 as the 20 probing gas. The gas temperature was constant and equal to 450 ± 50 K in the Capacitively 21 Coupled Plasma mode (CCP), and it was increasing with pressure and power in the Inductively 22 Coupled Plasma mode (ICP). Also the vibrational temperature of ground state of hydrogen 23 molecules was determined to be around 3100 and 2000 ± 500 K in ICP and CCP mode, 24 respectively. The concentration of atomic hydrogen was determined by means of actinometry, 25 either by using Ar (5 %) as the probing gas, or by using H2 as the actinometer in pure hydrogen 26 (Q1 rotational line of Fulcher-α system) The concentration of hydrogen density was increasing 27 with pressure in both modes, but with a dissociation degree slightly higher in the ICP mode (a 28 factor 2). 29 30 Submitted to Physica Scripta 31

International audience; Density functional theory (DFT) studies show that in tungsten a mono vacancy can contain up to six hydrogen isotopes (HIs) at 300 K with detrapping energies varying with the number of HIs inthe vacancy. Using these predictions, a multi trapping rate equation model has been built and used to model thermal desorption spectrometry (TDS) experiments performed on single crystaltungsten after deuterium ions implantation. Detrapping energies obtained from the model to adjust temperature of TDS spectrum observed experimentally are in good agreement with DFTvalues within a deviation below 10%. The desorption spectrum as well as the diffusion of deuterium in the bulk are rationalized in light of the model results.

International audience; Hydrogen atom adsorption on high-quality graphene on Ir(111) [gr/Ir(111)] is investigated using high-resolution electron energy loss spectroscopy. The evolution of the vibrational spectrum, up to 400 meV, of gr/Ir(111) upon increasing hydrogen atom exposures is measured. The two dominant binding configurations of atomic hydrogen are identified as (1) graphanelike hydrogen clusters on the parts of the graphene more strongly interacting with the Ir(111) surface and (2) dimers bound more weakly to the freestanding parts of the graphene. The graphanelike surface structures lead to increased corrugation of the graphene sheet, yielding graphane-related phonon components. Additionally, a recent theoretical prediction of the existence of a bending character for a LO/TO graphane chair phonon mode is experimentally verified. No clear evidence was found for hydrogen bound on both sides of a high-quality graphene sheet and phonon features strongly suggest interactions between graphanelike hydrogen clusters and Ir atoms in the substrate.

An experimental investigation of the epitaxial growth of iron-phthalocyanines on Ag(111) has been conducted in the monolayer range by low-energy electron diffraction and scanning tunneling microscopy. During the growth, the molecular overlayer undergoes few structural phase transitions, and various superlattices with oblique or rectangular unit cells have been identified. We show that the morphologies of the most stable molecular superstructures formed for different molecular coverage are adopted by various phthalocyanines with different central metal atom and phthalocyanine derivatives.

In this work, we study the abstraction of deuterium (D) by hydrogen (H) atoms (respectively H by D) on both perfectly crystalline and ion bombarded graphite surfaces using high resolution electron energy loss spectroscopy. On an initially defect free graphite surface, abstraction of D by H (reciprocally and reversibly H by D) is a quite efficient process at room temperature. Measurements demonstrate that the abstraction occurs almost essentially with the adsorbed atoms on “weakly” reactive sites. Reciprocally, atoms adsorbed on defects with sp3 configuration participate in a much lower extent to the abstraction mechanism. Obviously, this phenomenon always occurs when exposing a surface to an atomic hydrogen beam and illustrates the competition between sticking and molecules formation by abstraction.

We modeled damaged-irradiated carbon plasma facing component PFCs by graphite surfaces with vacancies. We combined theoretical and experimental investigations with a view to building tools to make a better assignment of the observed defects from the scanning tunneling microscope (STM) images. The theoretical part of this work is based on periodic density functional theory calculations. The electronic structure is calculated and the density of state along with the electronic density near the Fermi level are plotted. STM images are further simulated and their features are interpreted. On the experimental side, we performed scanning tunneling microscopy (STM) imaging at low bias voltage in order to probe the electronic modifications of the graphite surface induced either by H + (or D+) ion bombardment and/or atomic H (respectively D) adsorption. A connection between STM images and the reactivity of defective graphite surfaces towards H is also proposed.

We have studied the adsorption of metal-free phthalocyanine on 4 x 2/c(8 x 2) surface reconstruction of both InSb(001) and InAs(001) surfaces, by means of low-energy electron diffraction and high-resolution electron energy loss spectroscopy. We show that the molecule-surface interaction is stronger in the case of the InAs substrate as compared to the InSb one. Indeed, we observed that at full monolayer coverage, when the molecules are adsorbed on the InAs, and only in that case, there are substantial energy shifts of both the substrate surface plasmon and the first valence exciton of the molecules. These shifts are evidence of charge transfer from the molecules toward the substrate responsible for a strong molecule-substrate interaction such as chemisorption. In spite of the difference between the two substrates, at monolayer completion, H 2 Pc on InAs and InSb adopts a common (4n x 3) surface arrangement. The same that was already reported for the nonplanar lead and tin phthalocyanines (respectively, PbPc and SnPc) adsorbed on InSb(001).

International audience; Retention of deuterium ion implanted in polycrystalline tungsten samples is studied in situ in an ultra-high vacuum apparatus equipped with a low-flux ion source and a high sensitivity thermo-desorption setup. Retention as a function of ion fluence was measured in the 10^17 -10^21 D+/m^2 range. By combining this new fluence range with the literature in situ experimental data, we evidence the existence of a retention = fluence^ 0.645±0.025 relationship which describes deuterium retention behavior on polycrystalline tungsten on 8 orders of magnitude of fluence. Evolution of deuterium retention as a function of the sample storage time in vacuum at room temperature was followed. A loss of 50% of the retained deuterium is observed when the storage time is increased from 2 h to 135 h. The role of the surface and of natural bulk defects on the deuterium retention/release in polycrystalline tungsten is discussed in light of the behavior of the single desorption peak obtained with Temperature Programmed Desorption.

International audience; Code development to solve numerically the model equations of diffusion and trapping of hydrogen in metals. Parametrization of the model trapping parameters (detrapping energies and density): fitting of experimental TDS spectrum. Confrontation model/experiment: evolution of retention with fluence and implantation temperature. Investigation of period of rest between implantation and TDS on retention and depth profile. a b s t r a c t Relevant parameters for trapping of Hydrogen Isotopes (HIs) in polycrystalline tungsten are determined with the MHIMS code (Migration of Hydrogen Isotopes in MaterialS) which is used to reproduce Thermal Desorption Spectrometry experiments. Three types of traps are found: two intrinsic traps (detrapping energy of 0.87 eV and 1.00 eV) and one extrinsic trap created by ion irradiation (detrapping energy of 1.50 eV). Then MHIMS is used to simulate HIs retention at different fluences and different implantation temperatures. Simulation results agree well with experimental data. It is shown that at 300 K the retention is limited by diffusion in the bulk. For implantation temperatures above 500 K, the retention is limited by trap creation processes. Above 600 K, the retention drops by two orders of magnitude as compared to the retention at 300 K. With the determined detrapping energies, HIs outgassing at room temperature is predicted. After ions implantation at 300 K, 45% of the initial retention is lost to vacuum in 300 000 s while during this time the remaining trapped HIs diffuse twice as deep into the bulk.

The realization of the first Field Effect Transistors operating at room temperature, based on a single layer silicene channel, open up highly promising perspectives, e.g., typically, for applications in digital electronics. Here, we describe recent results on the growth, characterization and electronic properties of novel synthetic two-dimensional materials beyond graphene, namely silicene and germanene, its silicon and germanium counterparts., Conference paper presented at the SPIE DEFENSE + SECURITY, held on April 20-24th, 2015, in Baltimore, Maryland, United States.

Atomically thin 2D materials are emerging as a new frontier of materials science. They offer ultimate thickness scaling for nanoelectronic devices that could substantially improve electrostatic control and enable a number of low-power versatile applications, such as ultra-thin-channel field-effect transistors, optoelectronic devices, and solar cells.1 Graphene, the archetypal 2D material, cannot be used in conventional transistor technology because of the inherent difficulty of opening an electronic bandgap in this gapless material. Typically, the chalcogenides (that is, sulfides, selenides, and telluride), with bandgaps comparable to those of conventional group IV or group III–V semiconductors, may have the potential of scaling down to a truly atomic scale. However, in practice these materials are challenging to integrate with existing silicon-based electronics.

In this paper we investigate the local adsorption geometry of CoPc films adsorbed on a Ag(100) surface and its effect on the molecule-substrate interactions. Using high-resolution electron energy loss spectroscopy we demonstrate that the charge-transfer mechanism at the organic-metal interface depends on the structural properties of the CoPc film. We discuss these findings in terms of the molecular adsorption sites, as determined by X-ray photoelectron diffraction measurements and multiple scattering simulations. We show that the distance between the central Co atom and the topmost Ag layer is of the order of 0.3 nm. In addition, we demonstrate that the Co Pc molecules adsorb on different atomic sites depending on the superlattice symmetry, as varied by changing the coverage. This is further confirmed by low-energy electron diffraction and scanning tunneling microscopy measurements, emphasizing that, depending on the molecular superstructure adopted by the molecules, there are one or two molecules per unit cell.

We have herein studied the adsorption of tin-phthalocyanine on the indium rich 4 × 2/c(8 × 2) reconstruction of the InSb(0 0 1) surface, using scanning tunneling microscopy, low-energy electron diffraction and high-resolution electron energy loss spectroscopy. The tin-phthalocyanine molecules form a stable monolayer with a characteristic long-range one-dimensional order, imposed by the twofold symmetry of the substrate surface reconstruction. High-resolution electron energy loss studies of both the vibrational and the electronic eigenmodes of the molecules show that phthalocyanines lie flat on the surface. In that, they behave the same way as lead phthalocyanines, which leads us to suggest that a common adsorption configuration may be adopted by non-planar phthalocyanines on the InSb(0 0 1)-4 × 2/c(8 × 2) surface.

The first steps of structural and electronic modifications of a graphite surface bombarded with argon, hydrogen and deuterium ions were investigated using high resolution electron energy loss spectroscopy (HREELS). The energy and the damping of the low energy plasmon mode of graphite (E//C mode) were studied with respect to the bombardment settings. We show that argon bombardment affects the energy of the plasmon mode, while no similar change is observed after hydrogen (deuterium) bombardments. This can be related to the variation of inter-planar distance between two graphene layers. Moreover, the damping of the plasmon mode can be correlated with the interstitial defect concentration. Concerning the reactivity of the bombarded surfaces, we demonstrate that deuterium bombardment produce a non-deuterated surface. This last is very reactive to a further atomic deuterium exposure, as it is shown by the formation of C–D bondings. The deuterated sites can be removed after thermal annealings between 473 and 783 K. The occurrence of a chemical erosion mechanism accompanying this deuteration is discussed.

Sub-monolayer and monolayer of lead phthalocyanine deposited on InSb(1 0 0) (4 × 2)/c(8 × 2) surface have been investigated by scanning tunneling microscopy and low energy electron diffraction. Molecules first adsorb on the indium rows of the (4 × 2)/c(8 × 2) structure in the [1 1 0] direction and diffuse at the surface in order to form two-dimensional islands. The molecule–substrate interaction stabilizes the PbPc molecules on the In rows. It weakens the interaction between molecules located in adjacent rows resulting in numerous gliding planes between the molecular chains, in the direction parallel to the rows. At monolayer completion, a long-range one-dimensional order is adopted by the molecules in the [1 1 0] direction.

This paper focuses on the structural arrangements, as seen in scanning tunneling microscopy, chemical and electric properties, as studied especially by high resolution synchrotron radiation photoelectron spectroscopy and energy loss spectroscopy of sub-monolayer up to few monolayer thin films of phthalocyanines adsorbed on narrow band gap III–V semiconductors where they form ordered superstructures, as well as on silicon surfaces. Special emphasis will be put on non-planar metal phthalocyanines where the central atom is either lead or tin. These molecules have been studied in detail from the experimental point of view as well as theoretically by ab initio quantum calculations. Comparison with metal free and metal substituted planar phthalocyanines (H 2 Pc, CuPc, CoPc, …) adsorbed on semiconductors or noble metal surfaces is included. Perspectives and novel potential applications are finally outlined.

International audience; 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 (root 3 x root 3) R30 degrees 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 (root 3 x root 3) R30 degrees form, but with different domain orientations resulting from the structural properties of the initial wetting monolayer.

International audience; Following the publication in 2012 of the first compelling evidence of the synthesis of silicene, the silicon based counterpart of graphene, the last two years have seen a surge of articles on elemental, novel two-dimensional materials beyond graphene. Here, research in this burgeoning field is highlighted

International audience; 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.

A series of isochronal annealing experiments have been performed using high resolution electron energy loss spectroscopy in order to probe the thermal decomposition of monohydride and dihydride species from Ge-covered Si(001) surfaces. The Ge coverage was varied from submonolayer until completely covering the silicon surface. The films were exposed to atomic hydrogen at room temperature and the intensities of the hydrogen vibrational features were monitored as a function of annealing temperature. Marked steps in the stretching mode intensities are observed, directly connected to monohydride decomposition. The existence of a desorption state specific to surface alloying is thus clearly identified in the Ge-H stretching intensity temperature dependence curve, interpreted as involving the recombinative desorption of two hydrogen atoms from mixed Ge-Si dimers. From the scissors mode intensity variation, it is deduced that both Ge and Si dihydride species decompose simultaneously at all coverages, and that it always occurs before monohydride desorption takes place. Furthermore, we observe the ${\mathrm{G}\mathrm{e}\ensuremath{-}\mathrm{H}}_{2}$ formation enhancement due to the presence of the neighboring silicon atoms. Finally, the desorption of all hydride species is shown to be boosted when increasing the Ge contents at the Si(001) surface. To summarize this study, the complete experimental desorption diagram of the hydrogen population, as functions of both stoichiometry and temperature, is presented.

High Resolution Electron Energy Loss Spectroscopy (HREELS) is used to study the free charge carrier gas in epitaxial two-dimensional erbium silicide on Si(111). A broad loss feature is observed on the HREELS spectra that we clearly establish as a two-dimensional plasmon, and a simple procedure is proposed to extract its width and energy dispersion. In addition, we performed an analysis of the specular data in the frame of the dielectric theory using a three layer model. The plasmon is identified as one of the thin film modes with the lowest energy that corresponds in the limit of small parallel momentum transfer to charge fluctuations parallel to the surface. From this analysis, the carrier concentration in the silicide is extracted and found to be in good agreement with electronic band structure deduced from previous photoemission measurements. Further experimental results, concerning the evolution of the plasmon dispersion and damping brought about by changes in the mean island size and temperature down to 110 K are presented.

Synchrotron x-ray scattering has been used to investigate the structure and growth of perhaps the simplest of all films: xenon physisorbed on the Ag(111) surface. High-resolution x-ray scans of the in-plane structure and lower-resolution scans (specular and nonspecular) of the out-of-plane order were performed. The Xe films were prepared under both quasiequilibrium and kinetic growth conditions, and have fewer structural defects than those investigated previously by others on graphite substrates. Under quasiequilibrium conditions, the bulk Xe-Xe spacing is reached at monolayer completion, and the monolayer and bilayer lattice constants at coexistence are inferred equal to within 0.005 \AA{}, consistent with theoretical calculations. The Xe/vacuum interface profile for a complete monolayer and bilayer grown at quasiequilibrium is found to be sharper than for kinetically grown films. At coverages above two layers, diffraction scans along the $\mathrm{Xe}(01l)$ rod for quasiequilibrated films are consistent with the presence of two domains having predominantly an ABC stacking sequence and rotated 60\ifmmode^\circ\else\textdegree\fi{} with respect to each other about the surface normal. Annealing of these films alters neither the population of the two domains nor the fraction of ABA stacking faults. The thickest film grown under quasiequilibrium conditions exceeds 220 \AA{} (resolution limited). Under kinetic growth conditions, x-ray intensity oscillations at the Xe anti-Bragg position of the specular rod are observed as a function of time, indicating nearly layer-by-layer growth. Up to four complete oscillations corresponding to a film of eight layers have been observed before the intensity is damped out; the number of oscillations is found to depend on the substrate temperature, the growth rate, and the quality of the Ag(111) substrate. The specular reflectivity from kinetically grown films at nominal coverages of three and four layers has been analyzed using a Gaussian model which gives a film thickness standard deviation of 0.5 and 1.0 layers, respectively. Diffraction scans along the $\mathrm{Xe}(01l)$ rod of these films indicate a larger fraction of ABA stacking faults than found for thicker films. These results demonstrate the difficulty of kinetically growing Xe films thicker than two layers which have an ideal slab geometry.

Hydrogen desorption from two-dimensional (2D) and bulk-like (3D) erbium silicides on Si(111) has been investigated by means of thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (HREELS). By TDS, hydrogen is found to desorb as a single feature with a maximum in temperature located at 620 K on 2D silicide, whereas two desorption peaks at 460 K and 615 K, respectively, are measured on 3D silicide. These spectra are analyzed in the frame of the formulation developed by Redhead, and compared with hydrogen desorption from a monohydride Si(111) surface. On erbium silicide films, first-order kinetics is always observed in sharp contrast with the second-order kinetics measured for hydrogen desorption from the Si(111) surface. This remarkable behavior can be understood in terms of a pairing mechanism in which, for stability reasons, the H chemisorption sites appear in the form of pairs that contain either 0 (empty pair) or 2 (filled pair) H atoms. Using HREELS, two kinds of hydrogen chemisorption sites were identified prior to desorption. It is deduced that the features at 460 K on 3D and at 620 K on 2D erbium silicides are clearly related to concomitant hydrogen chemisorbed on the top layer silicon dangling bonds and in an interstitial site of the Er layer beneath. In contrast, the desorption peak at 615 K on bulk-like silicide is assigned to hydrogen absorption in bulk sites of the subsurface region.

Low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) have been used to characterize the film formed upon deposition of an Er monolayer on a Si(111) (5×5)–Ge surface and annealing at 500°C. The properties of the clean surface, as well as its reactivity towards H adsorption, are found to be essentially the same as for the well-documented two-dimensional ErSi2 silicide surface grown epitaxially on clean Si(111). From that it is deduced that the structure of the film must be similar to ErSi2, i.e. a hexagonal erbium monolayer located underneath a buckled layer of mixed SiGe composition. The main effect of Ge substitution is reflected in a change of the dielectric properties of the film, as is clearly observed with HREELS.

We examine here the effect of adsorbed atomic hydrogen (H) in the earliest stages of Ge film growth on a Si(001) substrate by solid source molecular beam epitaxy (MBE). Contrary to the gas source MBE or CVD methods, the use of a H flux, generated by hot filament dissociation, allows us to separate H and Ge supplies. In a low substrate temperature range between 100 and 350°C, we clearly observe a growth mode evolution by in-situ reflection high-energy electron diffraction (RHEED). Especially at 300°C, a change from the well-known Stranski–Krastanov (SK) mode without H to a layer-by-layer growth mode with H is evidenced. The presence of H at the growing surface indeed extends the RHEED intensity oscillations at Ge coverages well above the SK critical thickness and prevents island formation. Rather thick epitaxial Ge layers (up to 55 ML) present a persisting 2D growth with weak roughness and a reduced strain relief as revealed by RHEED in-plane lattice parameter measurements.

We have measured, in situ, the growth rate of low pressure chemical vapour deposition silicon or germanium films under hot-wire conditions using a quartz microbalance. The decomposed gases are disilane and germane at a pressure in the 10−4 to 10−5 mbar range. The gas source decomposition was achieved by a hot tungsten filament. The effects of filament temperature and gas pressure on the deposition rate are investigated and the role of primary radicals discussed. Efficient decomposition of feed gases without tungsten film contamination was obtained for a filament temperature of 1800 K. We show that films deposited at room temperature on Si(001) substrate are amorphous. Films prepared at 620 K are well ordered and exhibit good crystalline qualities.