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A method for subsurface visualization and characterization of hidden subsurface nano-structures based on Scanning Tuneling Microscopy/Spectroscopy (STM/STS) has been developed. The nano-objects buried under a metal surface up to several tens of nanometers can be visualized through the metal surface and characterized with STM without destroying the sample. This non-destructive method exploits quantum well (QW) states formed by partial electron confinement between the surface and buried nano-objects. The specificity of STM allows for nano-objects to be singled out and easily accessed. Then, their shape, size and burial depth can be determined by analyzing the spatial distribution and oscillatory behavior of the electron density at the surface of the sample. The proof of concept was demonstrated by fabricating argon nanoclusters embedded into a single-crystalline Cu matrix. Taking advantage of the specific electronic band structure Cu and inner electron focusing, we experimentally demonstrated that noble-gas nanoclusters of several nanometers large buried as deep as 80 nm can be detected, characterized and imaged. The ultime depth of this ability is estimated as 110 nm. This approach using QW states paves the way for an enhanced 3D characterization of nanostructures hidden well below a metallic surface., Accepted in Nanoscale Horizons Nanoscale Horiz., 2023, Accepted Manuscript https://doi-org.bases-doc.univ-lorraine.fr/10.1039/D3NH00052D

International audience; Recently, the discovery of the quasiperiodic order in ultra-thin perovskite films reinvigorated the field of 2-dimensional oxides on metals, and raised the question of the reasons behind the emergence of the quasiperiodic order in these systems. The effect of size-mismatch between the two separate systems has been widely reported as a key factor governing the formation of new oxide structures on metals. Herein, we show that electronic effects can play an important role as well. To this end, the structural, thermodynamic, electronic and magnetic properties of freestanding two-dimensional oxide quasicrystalline approximants and their characteristics when deposited over metallic substrates are systematically investigated to unveil the structure–property relationships within the series. Our thermodynamic approach suggests that the formation of these aperiodic systems is likely for a wide range of compositions. In addition, the magnetic properties and work functions of the thin films can be controlled by tuning their chemical composition. This work provides well-founded general insights into the driving forces behind the emergence of the quasiperiodic order in ternary oxides grown on elemental metals and offers guidelines for the discovery of new oxide quasicrystalline ultra-thin films with interesting physical properties.

International audience; Replacing noble metal catalysts with inexpensive, environmentally harmless, active, selective and stable substitutes, is a great challenge for the chemical industry. In this paper, the noble metal-free Al5Co2(210) complex intermetallic surface is experimentally identified as active and selective for the semi-hydrogenation of butadiene. The catalyst surface structure and chemical composition are determined by experimental techniques-surface X-ray diffraction (SXRD), scanning tunneling microscopy (STM)-combined with ab initio calculations. Theoretical investigations of the adsorption properties under reaction conditions demonstrate that the surface Co atomic density drastically impacts the thermodynamic feasibility of the hydrogenation reaction, and they provide information on the reaction mechanism. This work offers insights for the rational design of Al-based catalysts for hydrocarbon hydrogenation reactions.

A new ternary phase with a composition Al1+xV2Sn2–x (x = 0.19) has been found during investigation of the Al–V–Sn ternary system. Single-crystal X-ray diffraction measurements reveal that this ternary phase crystallizes with an orthorhombic structure with a = 5.5931(1) A, b = 18.8017(5) A, and c = 6.7005(2) A (space group Cmce). This compound is thus isostructural to the GaV2Sn2 structure type, showing a layered structure composed of vanadium cluster bands formed with pentagonal faces intercalated by Sn atom layers. High-resolution transmission electron microscopy measurements confirm the orthorhombic structure. Regarding lattice perfection, no dislocation could be identified within the probed Al1.19V2Sn1.81 single-crystal lamella. Ab initio calculations reveal a reduction of the density of states at the Fermi level, which could be attributed to both a Hume–Rothery effect combined with strong spd hybridization.

The physical significance of clusters in Al13TM4 compounds is invesitgated through ab initio methods based on density functional theory., Clusters, i.e. polyhedral geometric entities, are widely used to describe the structure of complex intermetallic compounds. However, little is generally known about their physical significance. The atomic and electronic structures of the Al13TM4 complex intermetallic compounds (TM = Fe, Co, Ru, Rh) have been investigated using a wide range of ab initio tools in order to examine the influence of the chemical composition on the pertinence of the bulk structure description based on 3D clusters. In addition, since surface studies were found to be a relevant approach to address the question of cluster stability in complex phases, the interplay of the cluster substructure with the 2D surface is addressed in the case of the Al13Co4(100) and Al13Fe4(010) surfaces.

We report the adsorption of Fe for several dosing conditions on Al(1 0 0) surface held at different temperatures. For 2 monolayer equivalent of Fe deposited on the substrate held at 593 K, we observe two rotational domains of a ( 5 × 5 )R26.6° structure by low energy electron diffraction and scanning tunneling microscopy. We identify this structure as the formation of the metastable Al9Fe2 intermetallic phase. The segregation energy and the structural stability of this intermetallic has been analysed using density functional theory based calculations. We propose that the formation of the Al9Fe2(0 0 1) metastable phase at the expense of other thermodynamically stable Al-Fe compounds, relies on a specific structural similarity between pure Al (0 0 1) planes of the intermetallic and Al(1 0 0) layers. These ‘common’ planes lead to the epitaxial relationship (1 0 0)Al [0 0 1]Al ∥ ( 0 0 1 ) Al 9 Fe 2 [ 1 3 ¯ 0 ] Al 9 Fe 2 . The Al9Fe2-Al matrix interface is sharp as revealed by transmission electron microscopy measurements.

International audience; We present an experimental and theoretical study of Pauli paramagnetism and martensite stabilization in a near equiatomic NiTi shape memory alloy. We demonstrate a direct correlation between strain-induced shear of the B19' NiTi lattice and its electronic and thermodynamical features. An increase of the monoclinic angle β from 97.4 to 98 degrees induces a 7 % decrease of the magnetic susceptibility because of a shift and deepening of a dip in B19' density of state at the Fermi level. It also produces a decrease of the B19' enthalpy, which translates into an increase of the martensite-to-austenite transition temperature by 60 K. Near equiatomic NiTi alloys carry remarkable properties such as shape memory effect and pseudoelasticity. Both effects are born of first-order reversible phase transformation between low-symmetry monoclinic B19' martensite phase and high-symmetry cubic B2 austenite phase. Tuning the transition temperatures and martensite transformation hysteresis has been a constant objective. It has yet been achieved by adjusting NixTi1-x concentration [1] or by substituting element in ternary NiTiX alloys (with X=Hf, Pd, Cu,…)[2,3]. Geometrical compatibility of B19' and B2 phases [4], change of valence electron number [5] and thermodynamics [1,6] have been investigated to explain the influence of theses chemical changes on NiTi-based alloys characteristics. Interestingly, no direct correlation between electronic features, transition temperatures and NiTi lattice distortion for a fixed concentration has yet been demonstrated, whereas the complex microstructure of NiTi under strain has been heavily studied [7-9]. Moreover, magnetism, which is known as a good probe of the electronic properties, has been rarely studied in NiTi alloys [10-13], although its impact on the magnetic resonance imaging of NiTi-based stents and biomedical implants has often been highlighted [14,15]. Here, we perform systematic X-ray diffraction (XRD) and magnetization measurements in order to characterize the influence of the tensile strain on both the martensite-to-austenite transition temperature and magnetic susceptibility of a NiTi sheet. Ab-initio calculations correlate the measured strain-induced shear of the martensite lattice to particular features of the B19' energy and density of states at the Fermi level. Figure 1. Magnetic susceptibility χ of NiTi versus temperature for as grown sample (black curve), 5% strained sample (red curve), 16.5% strained sample (blue curve). We use NiTi (50.6 at% Ti; 49.4 at% Ni ± 0.5%) free-standing polycrystalline 20µm thick sheets, grown by DC magnetron sputtering as described in details in Ref. [16]. Fig.1 shows the DC Magnetic susceptibility χ measured by SQUID magnetometer as a function of temperature under a constant magnetic field of 2T. Magnetic field is applied in the plane of the NiTi sheet. Note that χ does not depend on the measurement direction in the film plane because of the polycrystalline nature of the NiTi sheet. As-grown NiTi sample overcomes a hysteretic transition between a value M= 23 mJ.T-2 .kg-1 in the martensite phase and A=32.5 mJ.T-2 .kg-1 in the austenite phase, in agreement with [10]. A magnetic transition temperature of around 360 K (330K) matches the martensite-to-austenite (respectively austenite-to-martensite) transition in XRD curves, in agreement with [8,17] for similar concentrations. Tensile tests are performed using a Deformation Device System

International audience; A few low order approximants to decagonal quasicrystals have been shown to provide excellent activity and selectivity for the hydrogenation of alkenes and alkynes. It is the case for the Al13Co4 compound, for which the catalytic properties of the pseudo-twofold orientation have been revealed to be among the best. A combination of surface science studies, including Surface X-Ray Diffraction, and calculations based on Density Functional Theory, is used here to derive an atomistic model for the pseudo-twofold o-Al13Co4 surface, whose faceted and columnar structure is found very similar to the one of the twofold surface of the d-Al-Ni-Co quasicrystal. Facets substantially stabilize the system, with energies in the range 1.19-1.31 J/m 2 , i.e. much smaller than the ones of the pseudo-tenfold (1.49-1.68 J/m 2) and pseudo-twofold (1.66 J/m 2) surfaces. Faceting is also a main factor at the origin of the Al13Co4 catalytic performances, as illustrated by the comparison of the pseudo-tenfold, pseudotwofold and facet potential energy maps for hydrogen adsorption. This work gives insights towards the design of complex intermetallics catalysts through surface nanostructuration for optimized catalytic performances.

International audience; The unique electronic and crystallographic structure of intermetallics is known to result in excellent catalytic performances for selected chemical reactions. Moreover, owing to the specific bonding network of these compounds, a high structural stability of their surfaces is generally assumed, even under reaction conditions. Transition metal (TM = Fe, Co) aluminides of the Al 13 TM 4 stoi-chiometry have previously demonstrated high activities and selectivities in partial hydrogenation of alkynes and alkadienes. Focusing on the Al 13 Co 4 (100) surface as a model catalyst for butadiene hydrogenation, the hydrogen-rich reaction conditions are predicted-based on DFT calculations and atomistic thermodynamics-to modify the relatively flat surface structure identified under ultra-high vacuum, in the form of highly cohesive clusters emerging from the bulk lattice. Unlike the flat one, this termination presents favorable adsorption properties, able to make it catalytically active and fully selective to butenes. In addition, its contrasted catalytic behavior as compared to that of the reference Al 13 Fe 4 (010) surface-which is more active but less selective-is rationalized in terms of butadiene, butene and hydrogen co-adsorption properties. This work demonstrates that a realistic description of the surface structures under reaction conditions is mandatory to the design of new-generation catalysts based on the complex topology of intermetallic surfaces.

International audience; Complex metallic alloys (CMAs) may be defined as those intermetallic compounds having large (>~nm) unit cell dimensions. This includes quasi crystals as a special case, the unit cell being infinite. The discovery of quasi crystals motivated the study of CMAs, and the surface science community became active in the field once stable samples of sufficient size were produced. While the initial surface science activity centred on clean surface preparation, increasingly the formation of thin films, both metallic and molecular grew in importance. In this chapter, we give a brief introduction to this topic and then focus on several current areas of interest. These include the growth and characterization of ultra-thin metallic films of diverse architectures, the formation through deposition of novel molecular overlayers and thin films, complex intermetallics such as surface alloys and the potential use of intermetallic surfaces for catalytic reactions.

A single crystal of the Al45Cr7 crystalline approximant has been grown using the Czochralski method. The structure of this phase has been resolved in the C2/m space group. Chromium atoms are systematically surrounded by Al atoms forming a distorted icosahedron. The icosahedra are linked either by a vertex or a triangular face or overlap along a five-fold axis. The (010) surface of this single crystal has been investigated under ultrahigh vacuum and shows a (1x1) termination at specific bulk planes. The terraces have a nanostructured appearance which is ascribed to the preservation of the CrAl12 clusters at the surface. Ab initio calculations shows the presence of a deep pseudogap at the Fermi level, due to both a Hume-Rothery effect combined with strong spd hybridization. The isoelectronic charge densities highlight the fact that the charges are mainly confined within the CrAl12 icosahedron providing support to a certain stability of the clusters which could explain the nanostructured surface morphology encountered. First results on Al thin film deposition on the (010) surface are also reported.

The tunability offered by alloying different elements is useful to design catalysts with greater activity, selectivity, and stability than single metals. By comparing the Pd(111) and PdZn(111) mode...

International audience; Good wetting is generally observed for liquid metals on metallic substrates, while poor wetting usually occurs for metals on insulating oxides. In this work, we report unexpected large contact angles for lead on two metallic approximants to decagonal quasicrystals, namely, Al5Co2 and Al13Co4. Intrinsic surface wettability is predicted from first principles, using a thermodynamic model based on the Young equation, and validated by the good agreement with experimental measurements performed under ultra-high vacuum by scanning electron microscopy. The atomistic details of the atomic and electronic structures at the Pb-substrate interface, and the comparison with Pb(111)/Al(111), underline the influence of the specific electronic structures of quasicrystalline approximants on wetting. Our work suggests a possible correlation of the contact angles with the density of states at the Fermi energy and paves the way for a better fundamental understanding of wettability on intermetallic substrates, which has potential consequences in several applications such as supported catalysts, protective coatings, or crystal growth.

Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al13Co4(100), m-Al13Fe4(010) and m-Al13Ru4(010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al13Co4(100) and o-Al13Co4(010). As a result, the following room-temperature activity order is determined: Al13Co4(100) < Al13Co4(010) < Al13Ru4(010) < Al13Fe4(010). Moreover, Al13Co4(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al13Fe4(010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al13TM4 surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al13TM4 compounds., Graphical abstract

International audience; Intermetallic cage compounds have crystal structures characterized by three-dimensional (3D) frameworks of host atoms forming cages that may be filled with guest atoms. They can be considered as a class of complex metallic alloys (CMAs), a broad family of crystalline materials having large or giant unit cells in which a cluster substructure exists. Their bulk physical properties have attracted interest in recent years, particularly as several of them are considered as promising thermoelectric materials. As a different type of CMAs, they may exhibit interesting surface properties which have not been explored until now. Here, we report a detailed investigation of the (100) surface of the Ce3Pd20Si6 compound and describe its relationship with the 3D cage framework. The study has been carried out using both experimental surface science methods under ultrahigh vacuum conditions and theoretical calculations based on the density functional theory (DFT). After annealing at moderate temperatures, the surface exhibits two different types of terminations, one of them being a metastable (2 × 2) reconstruction. Annealing at higher temperatures leads to a single termination with a (1 × 1) structure. Twelve different surface models have been considered for DFT calculations. The surface phase diagram deduced from first-principles calculations with the chemical potentials as variables provides insight into the relative stability of the different surface models. Combining experimental and theoretical results leads to the conclusion that the surface forms at corrugated planes of the bulk structure that preserve intact the Pd12Si6 cages, with additional Pd surface atoms. It is concluded that despite this Pd-based cage compound being clearly metallic, covalent-like interactions between Pd and Si atoms on the host cage structure significantly influence its surface structure.

The energetic stability and oxidation behaviour of quaternary chromium silicides mainly depends on their chromium content. In this work, the pseudolamellar crystal structures of six representatives of the X3T2(T1-xCrx)2Si6 compounds (X = Nb or Ti and T = Fe, Co or Ni) are determined by resonant X-ray diffraction using the synchrotron facility, combined with calculations based on the density functional theory. Experimentally, the transition metal positions are determined using the anomalous scattering effect close to the energy absorption threshold. First principles calculations carried out on the as-determined structures show that the preferential site occupations are explained by thermodynamic arguments. Theoretical calculations highlight the influence of Cr atoms on the lattice parameters, and reveal that the experimental Cr content corresponds to the amount required to reach the equilibrium between X3T2(T1-xCrx)2Si6 and X4T4Si7.

The bulk structure of the η -Al5Fe2 intermetallic compound consists in an ordered framework of pentagonal antiprims with composition Al2Fe inside which nearly continuous chains of fractionally occupied Al sites exist. At low temperature, ordering of these Al channel atoms can occur, leading to various superstructures. Although the Al5Fe2 intermetallic phase is of technological importance being present in protective coating for steel parts, a detailed surface investigation of this compound has not been reported yet. Here we describe the growth of a single crystal of the Al5Fe2 compound by the Czochralski method. Its bulk structure is identified as the low temperature polymorph η ″ . Density Functional Theory calculations were performed to determine its formation enthalpy and its electronic structure. A deep pseudogap is noticeable at the Fermi energy, and this compound is found to be magnetic. Two samples have been extracted, presenting a surface oriented either perpendicular to the channel’s direction ((0 0 1) surface) or parallel to them ((1 0 0) surface). The two surfaces have been investigated by X-ray photoemission spectroscopy, low-energy electron diffraction and scanning tunneling microscopy. Both surfaces exhibit some superstructures of various complexities, whose origin can be explained in the light of density functional theory calculations performed on model surfaces.

The adsorption of Ir adatoms on Al(100) has been investigated under various exposures and temperature conditions. The experimental and theoretical results reveal a diffusion of Ir adatoms within the Al(100) surface selvedge already at 300 K. Above 593 K, two domains of a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.{6}^{\ensuremath{\circ}}$ phase are identified by low energy electron diffraction (LEED) and scanning tunneling microscopy measurements. This phase corresponds to the initial growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound at the Al(100) surface. The ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic domains are terminated by bulklike pure Al layers. The structural stability of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) grown on Al(100) has been analyzed by density functional theory based calculations. Dynamical LEED analysis is consistent with an Ir adsorption leading to the growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic compound. We propose that the epitaxial relationship ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}(001)\ensuremath{\parallel}\mathrm{Al}$(100) and ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}[100]\ensuremath{\parallel}\mathrm{Al}[031]/[013]$ originates from a matching of Al atomic arrangements present both on Al(100) and on pure Al(001) layers present in the ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound. Finally, the interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ precipitates and the Al matrix has been characterized by transmission electron microscopy measurements. The cross-sectional observations are consistent with the formation of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) compounds. These measurements indicate an important Ir diffusion within Al(100) near the surface region. The coherent interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ and the Al matrix is sharp.

International audience; The purpose of this study is to find new ternary intermetallic compounds within the Al-Cr-Sc system, which is typical of a push-pull system in which two constituents (Cr and Sc) are immiscible, whilst they form compounds, respectively, with the third constituent (Al). By arc-melting of the three components aluminium (Al), chromium (Cr) and scandium (Sc) under inert atmosphere, a range of different alloys was produced. The microstructure was observed using scanning electron microscopy (SEM) and the phase compositions were analysed by electron dispersive X-ray spectroscopy (EDS). The samples were measured with X-ray diffraction (XRD) and transmission electron microscopy. The results were confirmed by Rietveld refinement. Three different phases were found in the microstructure of the samples, the two known binary phases Al 16 Cr 10 and Al 3 Sc and a new ternary phase coined f-Al 8 Cr 4 Sc. f-Al 8 Cr 4 Sc showed great similarities to other Al 8 Cr 4 RE compounds with tetragonal I4/mmm crystal structure. Starting from the structure of already known Al 8 Cr 4 RE crystals, ab initio calculations were performed to determine the crystallographic parameters of f-Al 8 Cr 4 Sc and also to investigate its electronic structure, which identified the energy band factor as the key factor that determines the stability of this compound. The lattice parameters and atomic positions were found in good agreement with the ones obtained by Rietveld refinement. Experimental atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image analysis of the f-Al 8 Cr 4 Sc phase confirmed the predicted atomic model.

International audience; The formation of long-range ordered aperiodic molecular films on quasicrystalline substrates is a new challenge that provides an opportunity for further surface functionalization. This aim can be realized through the smart selection of molecular building blocks, based on symmetry-matching between the underlying quasicrystal and individual molecules. It was previously found that the geometric registry between the C 60 molecules and the 5-and 10-fold surfaces was key to the growth of quasiperiodic organic layers. However, an attempt to form a quasiperiodic C 60 network on i-Ag-In-Yb substrates was unsuccessful, resulting in disordered molecular films. Here we report the growth of 5-fold symmetric corannulene C 20 H 10 molecules on the 5-fold surfaces of i-Ag-In-Yb quasicrystals. Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) revealed long-range quasiperiodic order and 5-fold rotational symmetry in self-assembled corannulene films. Recurrent decagonal molecular rings were seen, resulting from the decoration of specific adsorption sites with local pentagonal symmetry by corannulenes, adsorbed with their bowl-openings pointing away from the surface. They were identified as (Ag, In)-containing rhombic triacontahedral (RTH) cluster centers and pentagonal Yb motifs, which cannot be occupied simultaneously due to steric hindrance. It is proposed that symmetry-matching between the molecule and specific substrate sites drives this organization. Alteration of the molecular rim by the introduction of CH substituents appeared to increase molecule mobility on the potential energy surface and facilitate trapping at these specific sites. This finding suggests that rational selection of molecular moiety enables the templated self-assembly of molecules leading to an ordered aperiodic corannulene layer.

The structure of the quasicrystalline approximant Al13Co4(100) has been determined by surface x-ray diffraction (SXRD) and complementary density-functional-theory (DFT) calculations. Thanks to the use of a two-dimensional pixel detector, which speeds up the data acquisition enormously, an exceptionally large set of experimental data, consisting of 124 crystal truncation rods, has been collected and used to refine this complex structure of large unit cell and low symmetry. Various models were considered for the SXRD analysis. The best fit is consistent with a surface termination at the puckered type of planes but with a depletion of the protruding Co atoms. The surface energy of the determined surface model was calculated using DFT, and it takes a rather low value of 1.09 J/m 2. The results for the atomic relaxation of surface planes found by SXRD or DFT were in excellent agreement. This work opens up additional perspectives for the comprehension of related quasicrystalline surfaces.

International audience; The growth of C60 films on the pseudo-tenfold (100) surface of the orthorhombic Al13Co4 qua-sicrystalline approximant was studied experimentally by scanning tunneling microscopy, low-energy electron diffraction and photoemission spectroscopy. The (100) surface terminates at bulk-planes presenting local atomic configurations with five-fold symmetry-similar to quasicrystalline surfaces. While the films deposited at room temperature were found disordered, high-temperature growth (up to 693 K) led to quasi-ordered molecular films templated on the substrate rectangular unit mesh. The most probable adsorption sites and geometries were investigated by density functional theory (DFT) calculations. A large range of adsorption energies was determined, influenced by both symmetry and size matching at the molecule-substrate interface. The quasi-ordered structure of the film can be explained by C60 adsorption at the strongest adsorption sites which are too far apart compared to the distance minimizing the intermolecular interactions, resulting in some disorder in the film structure at a local scale. Valence band photoemission indicates a broadening of the molecular orbitals resulting from hybridization between the substrate and overlayer electronic states. Dosing the film at temperature above 693 K led to molecular damage and formation of carbide thin films possessing no azimuthal order with respect to the substrate.

Structural investigations of the ($2\overline{1}0$) and (100) surfaces of ${\mathrm{Al}}_{5}{\mathrm{Co}}_{2}$ using experimental ultrahigh vacuum techniques and ab initio computational methods are presented in this work. Both surfaces are identified and show bulk terminations where only specific atoms remain. These atoms can be seen as truncated parts of chemically bonded atomic motifs which have been identified in the bulk in a previous work [M. Meier et al., Phys. Rev. B 91, 085414 (2015)]. Whereas the ($2\overline{1}0$) surface presents a single termination, the structure of the (100) topmost layers is found to be highly dependent on the preparation conditions, especially on the annealing temperature. This behavior, also observed for the (001) surface studied previously, can be partly explained when considering the bonding strength of the truncated motif parts with the subsurface.

Point island models (PIMs) are presented for the formation of supported nanoclusters (or islands) during deposition on flat crystalline substrates at lower submonolayer coverages. These models treat islands as occupying a single adsorption site, although carrying a label to track their size (i.e., they suppress island structure). However, they are particularly effective in describing the island size and spatial distributions. In fact, these PIMs provide fundamental insight into the key features for homogeneous nucleation and growth processes on surfaces. PIMs are also versatile being readily adapted to treat both diffusion-limited and attachment-limited growth, and also a variety of other nucleation processes with modified mechanisms. Their behavior is readily and precisely assessed by kinetic Monte Carlo simulation., 27 pages, 13 figures

International audience; The atomic and electronic structures of the (100) and (001) surfaces of the Al5Co2 complex metallic alloy are studied by ab initio calculations. The relative stability of the possible surface planes built from bulk truncation is calculated and the influence of the atomic surface density, the interlayer spacing and the surface chemical composition on the plane selection is discussed. In addition, we show that the simulated images of scanning tunneling microscopy for each possible termination present a specific signature that appears to be sufficient to experimentally identify the surface plane.

Complex metallic alloys having isolated transition-metal elements in the surface layer have been reported to work well as selective hydrogenation catalysts. We report an experimental determination of the surface structure of one such compound $\mathrm{A}{\mathrm{l}}_{13}\mathrm{F}{\mathrm{e}}_{4}(010)$. The structure was determined using low-energy electron diffraction. The best-fit structure terminates in a layer similar to the puckered bulk layer but lacking some of the Al and Fe atoms. Protruding Fe atoms are located in the middle of adjacent pentagonal Al formations, connected to each other by Al ``glue'' atoms. The top interlayer spacing is compressed relative to the bulk with oscillating relaxations observed for subsequent layers at the surface.

We report the formation of an ordered C60 monolayer on the Al9Co2(001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of C 60 with the sample held between 573–673 K leads to a [4, −2|1,3] phase. The growth of C60 proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91 A^2. The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each C60 in the observed superstructure. Several defects (vacancies, protrusions,. . .) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging C60 molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of Al4C3 domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the Al9Co2(001) surface dictates the carbide domain orientations.

International audience; The intermetallic compound InPd (CsCl type of crystal structure with a broad compositional range) is considered as a candidate catalyst for the steam reforming of methanol. Single crystals of this phase have been grown to study the structure of its three low-index surfaces under ultra-high vacuum conditions, using low energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). During surface preparation, preferential sputtering leads to a depletion of In within the top few layers for all three surfaces. The near-surface regions remain slightly Pd-rich until annealing to ∼580 K. A transition occurs between 580 and 660 K where In segregates towards the surface and the near-surface regions become slightly In-rich above ∼660 K. This transition is accompanied by a sharpening of LEED patterns and formation of flat step-terrace morphology, as observed by STM. Several superstructures have been identified for the different surfaces associated with this process. Annealing to higher temperatures (≥750 K) leads to faceting via thermal etching as shown for the (110) surface, with a bulk In composition close to the In-rich limit of the existence domain of the cubic phase. The Pd-rich InPd(111) is found to be consistent with a Pd-terminated bulk truncation model as shown by dynamical LEED analysis while, after annealing at higher temperature, the In-rich InPd(111) is consistent with an In-terminated bulk truncation, in agreement with density functional theory (DFT) calculations of the relative surface energies. More complex surface structures are observed for the (100) surface. Additionally, individual grains of a polycrystalline sample are characterized by micro-spot XPS and LEED as well as low-energy electron microscopy. Results from both individual grains and “global” measurements are interpreted based on comparison to our single crystals findings, DFT calculations and previous literature.

International audience; Earlier studies of C 60 adsorption on Au(111) reported many interesting and complex features. We have performed coordinated low-energy electron diffraction, scanning tunneling microscopy (STM), and density functional theory studies to elucidate some of the details of the monolayer commensurate (2Ý3 × 2Ý3)R30° phase. We have identified the adsorption geometries of the two states that image as dim and bright in STM. These consist of a C 60 molecule with a hexagon side down in a vacancy (hex-vac) and a C 60 molecule with a carbon-carbon 6:6 bond down on a top site (6:6-top), respectively. We have studied the detailed geometries of these states and find that there is little distortion of the C 60 molecules, but there is a rearrangement of the substrate near the C 60 molecules. The two types of molecules differ in height, by about 0. A, which accounts for most of the difference in their contrast in the STM images. The monolayer displays dynamical behavior, in which the molecules flip from bright to dim, and vice versa. We interpret this flipping as the result of the diffusion of vacancies in the surface layers of the substrate. Our measurements of the dynamics of this flipping from one state to the other indicate that the activation energy is 0.66 ± 0.03 eV for flips that involve nearest-neighbor C 60 molecules, and 0.93 ± 0.03 for more distant flips. Based on calculated activation energies for vacancies diffusing in Au, we interpret these to be a result of surface vacancy diffusion and bulk vacancy diffusion. These results are compared to the similar system of Ag(111)-(2Ý3 × 2Ý3)R30°-C 60 . In both systems, the formation of the commensurate C 60 monolayer produces a large number of vacancies in the top substrate layer that are highly mobile, effectively melting the interfacial metal layer at temperatures well below their normal melting temperatures.

International audience; A combination of experimental methods was used to study the structure of In thin films deposited on the Pd(111) surface and the alloying behavior. X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy results indicate that surface alloying takes place at room temperature. Below 2 monolayer equivalents (MLEs), the LEED patterns show the formation of three rotational domains of InPd(110) of poor structural quality on top of the Pd(111) substrate. Both core-levels and valence band XPS spectra show that the surface alloy does not yet exhibit the electronic structure characteristic of the 1:1 intermetallic compound under these conditions. Annealing the 1 MLE thin film up to 690 K yields to a transition from a multilayer InPd near-surface intermetallic phase to a monolayer-like surface alloy exhibiting a well ordered ( 3‾‾√×3‾‾√ ) R30∘ superstructure and an estimated composition close to In2Pd3. Annealing above 690 K leads to further In depletion and a (1 × 1) pattern is recovered. The ( 3‾‾√×3‾‾√ ) R30∘ superstructure is not observed for thicker films. Successive annealing of the 2 MLE thin film leads the progressive disappearance of the InPd diffraction spots till a sharp (1 × 1) pattern is recovered above 690 K. In the high coverage regime (from 4 to 35 MLE), the formation of three rotational domains of a bcc-In7Pd3 compound with (110) orientation is observed. This In-rich phase probably grows on top of interfacial InPd(110) domains and is metastable. It transforms into a pure InPd(110) near-surface intermetallic phase in a temperature range between 500 and 600 K depending on the initial coverage. At this stage, the surface alloy exhibits core-level chemical shifts and valence band (VB) spectra identical to those of the 1:1 InPd intermetallic compound and resembling Cu-like density of states. Annealing at higher temperatures yields to a decrease of the In concentration in the near-surface region to about 20 at.% and a (1 × 1) LEED pattern is recovered.

International audience; We present a review on recent work performed on periodic complex metallic alloy (CMA) surfaces. The electronic and crystallographic structures of clean pseudo-tenfold, pseudo-twofold, sixfold surfaces will be presented along with the recent findings on CMA of lower structural complexity, i.e. with a smaller unit cell. The use of CMA surfaces as templates for thin film growth and the formation of surface alloy will also be introduced. The reactivity of these complex surfaces and their impact in the field of heterogeneous catalysis will be discussed. Finally, common trends among these systems will be highlighted when possible and future challenges will be examined.

International audience; The (110) surface of the InPd intermetallic compound and the In–Pd surface alloy properties are investigated in the framework of the density functional theory, within the projector augmented plane-wave method. Surface segregation is calculated to be energetically unfavorable at stoichiometric InPd(110) surfaces, while indium antisites are shown to segregate to the surface in off-stoichiometric InPd(110) systems. Concerning surface alloys obtained by burying In-doped Pd layers in Pd(111), we demonstrated that the most stable ones are those presenting atomic indium concentrations below 50 at. % (11 at. %, 25 at. %, 33 at. %). According to our calculations, the In-doped Pd layers with concentration above or equal to 50% lead to In-doped Pd multilayers, each presenting an atomic indium concentration below 50 at. %. Alloying and segregation effects in InPd intermetallic compound and In–Pd surface alloys clearly agree with the larger bonding strength of In–Pd (−0.44 eV) compared to In–In (−0.29 eV) and Pd–Pd (−0.31 eV).

We have investigated the structure of the Al13Fe4(010) surface using both experimental and ab initio computational methods. The results indicate that the topmost surface layers correspond to incomplete puckered (P) planes present in the bulk crystal structure. The main building block of the corrugated termination consists of two adjacent pentagons of Al atoms, each centered by a protruding Fe atom. These motifs are interconnected via additional Al atoms referred to as "glue" atoms which partially desorb above 873 K. The surface structure of lower atomic density compared to the bulk P plane is explained by a strong Fe-Al-Fe covalent polar interaction that preserves intact clusters at the surface. The proposed surface model with identified Fe-containing atomic ensembles could explain the Al13Fe4 catalytic properties recently reported in line with the site-isolation concept [M. Armbruster et al., Nat. Mater. 11, 690 (2012)].

International audience; The rich literature about the intermetallic compound ZnPd as well as several ZnPd near-surface intermetallic phases is reviewed. ZnPd is frequently observed in different catalytic reactions triggering this review in order to collect the knowledge about the compound. The review addresses the chemical and physical properties of the compound and relates these comprehensively to the catalytic properties of ZnPd in methanol steam reforming--an interesting reaction to release hydrogen for a future hydrogen-based energy infrastructure from water/methanol mixtures. The broad scope of the review covers experimental work as well as quantum chemical calculations on a variety of Pd-Zn materials, aiming at covering all relevant literature to derive a sound state-of-the-art picture of the understanding gained so far.

International audience; Atomic oxygen adsorption on the pure aluminium terminated Al9Co2 (001) surface is studied by first principle calculations coupled with STM measurements. Relative adsorption energy of oxygen atoms have been calculated on different surface sites along with the associated STM images. The local electronic structure of the most favourable adsorption site is described. The preferential adsorption site is identified as a "bridge" type site between the cluster entities exposed at the (001) surface termination. The Al-O bonding between the adsorbate and the substrate presents a covalent character, with s − p hybridization occuring between the states of the adsorbed oxygen atom and the aluminium atoms of the surface. The simulated STM image of the preferential adsorption site is in agreement with experimental observations. This work shows that oxygen adsorption generates important atomic relaxations of the topmost surface layer and that sub-surface cobalt atoms strongly influence the values of the adsorption energies. The calculated Al-O distances are in agreement with those reported in Al2O and Al2O3 oxides and for oxygen adsorption on Al(111).

The growth and stability of Bi thin films on the Al13Co4(100) surface has been investigated from the submonolayer to high-coverage regime by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) for temperatures ranging from 57 to 633 K. Initially, Bi adsorption leads to the formation of a pseudomorphic monolayer, followed by the growth of islands of different heights with increasing coverage. The in-plane structure, island height, and island morphology indicate that these islands adopt either a pseudocubic (110) or hexagonal (111) orientation normal to the surface. The (110)-oriented islands correspond to bilayer stacking (either two or four monolayers in height) while the (111)-oriented islands correspond to either three- or four-layer stacking. The in-plane orientation of (110) islands with respect to the substrate is random, while (111) islands adopt one of four possible orientations. In addition, the (111) islands show a moire structure. The fact that Bi islands grow with either (110) or (111) orientation simultaneously on the same substrate relates to a subtle energy balance between both orientations according to ab initio calculations, allowing both structures to coexist. The island density dependence versus both deposition temperature and flux, their most frequent structure type, reshaping effects, and chemical reactivity of the different allotropes are also discussed in this paper.

International audience; A detailed study of the (001) surface of the Al2Cu crystal using both experimental and ab initio computational methods is presented in this work. The combination of both approaches gives many arguments to match the surface plane with a bulk truncated surface model terminated by incomplete Al planes. The missing rows of Al atoms lead to a 2√2×√2R 45° surface reconstruction with two domains rotated by 90° from each other. Ab initio calculations demonstrate that the energetic cost associated with the removal of pairs of Al atoms is the lowest for the two nearest surface Al atoms (covalentlike interaction). They reveal that the remaining atomic rows of various widths are oriented according to the graphitelike Al 6_3 nets used to describe the Al2Cu bulk structure. The surface dynamics observed at 300 K at the Al2Cu surface is also presented. Finally, configurational and vibrational entropies are introduced to discuss the reduced surface plane density.

In a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study of the surface of Al13Co4(100), all techniques have found that after annealing to 1165 K, the surface structure is consistent with a dense Al-rich plane with surface Co atom depletion. Various structure models were considered, and in the LEED study, the best agreement was found with a model that consists of Al-rich terminating planes with no Co atoms, and otherwise a structure similar to the bulk puckered layers. This structure was also found to be stable in the DFT study. The best-fit structural parameters are presented for the two domains of this structure, which contain bipentagons that can be related to the pentagonal bipyramidal structures in the bulk, plus additional glue atoms between them. These domains are not strictly related to each other by symmetry, as they have different surface relaxations. The STM study found significant differences in the surfaces of samples grown by different methods and is able to explain a different interpretation made in an earlier study.

International audience; We have investigated the adsorption of Pb atoms on the (100) surface of an orthorhombic Al13Co4 crystal at 300 and 573 K substrate temperatures. This complex metallic alloy is an approximant to the decagonal Al-Ni-Co quasicrystal. At submonolayer coverage and at 300 K, Pb adatoms remain highly mobile and adsorb preferentially within the hollow site situated in between adjacent Al pentagonal clusters present at the surface. These experimental findings are supported by ab initio calculations based on density functional theory (DFT). For both temperature regimes, Pb adsorption leads to the formation of pseudomorphic monolayers above which the high adsorbate mobility prohibits the growth of additional layers. For the high-temperature deposition, we propose a structural model for the Pb film and discuss its relationship with the underneath substrate.

We present a detailed study of the (110) surface of the γ-Al4Cu9 crystal using both experimental methods and first-principles calculations based on density-functional theory. Our experimental approach, using low-energy electron diffraction, scanning tunneling microscopy (STM) images, and x-ray photoelectron spectroscopy highlights the presence of two types of surface terminations. Combining experimental results and simulations provides many arguments to match the two observed surfaces with the two puckered terminations built from bulk truncation: (i) calculations show that these two puckered terminations present lower surface energies compared to another conceivable flat termination obtained also from bulk truncation, (ii) step height measurements are consistent with calculated interlayer spacings and (iii) simulated STM images are in reasonable agreement with the experimental ones and mirror the experimental voltage dependence.

International audience; We first present an extended introduction of the various methods used to extract electronic and structural information from the K pre-edge X-ray absorption spectra of 3d transition metal ions. The K pre-edge structure is then modelled for a selection of 3d transition metal compounds and analyzed using first-principles calculations based on the density functional theory (DFT) in the local density approximation (LDA). The selected compounds under study are presented in an ascending order of electronic structure complexity, starting with the Ti K-edge of rutile and anatase, and finishing with the Fe K-edge of the cyanomet-myoglobin. In most cases, the calculations are compared to polarized experimental spectra. It is shown that DFT-LDA methods enable us to reproduce satisfactorily the experimental features and to understand the nature of the electronic transitions involved in the pre-edge region. The limiting aspects of such methods in modelling the core-hole electron interaction and the 3d electron-electron repulsion are also pointed out.

We present a detailed study of the (100) surface of the orthorhombic Al13Co4 crystal using both experimental and ab initio computational methods. This complex metallic alloy is an approximant of the decagonal Al-Ni-Co quasicrystalline phase. After sputter-annealing preparation of the surface at 1073 K, the low-energy electron diffraction pattern recorded exhibits a pseudotenfold symmetry with lattice parameters consistent with those of the bulk model. At this stage, scanning tunneling microscopy (STM) measurements reveal two different types of surface terminations. A comparison between these two surface structures and the bulk planes indicate that the terminations correspond either to an incomplete puckered layer (P) or to an incomplete flat layer (F). At 1173 K, the majority of the surface consists of P layer terminations. STM images calculated from our proposed surface model are in good agreement with experimental images. X-ray photoelectron diffraction patterns and single scattering cluster calculations further confirm that the local atomic arrangements present in the bulk model are preserved within the near-surface region.

International audience; The growth of complex intermetallics as surface alloys is investigated by annealing Al thin films deposited on Cu(111) substrate in ultrahigh vacuum. Already at room temperature, the large lattice mismatch between Al and Cu results in interfacial intermixing. Upon annealing, various phases are formed by diffusion depending on the thickness of the Al films and the annealing temperature. The surface structures are characterized by scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy. Three different superlattice phases are identified as well as the complex Hume-Rothery γ-Al4Cu9 phase. The epitaxial relationships between the surface phases and the Cu(111) substrate are determined. We further investigate the electronic structure of the γ phase by density functional calculations. Experimental valence bands are compared to calculated density of states and simulated STM images are used to identify possible bulk planes appearing as surface termination.