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The intermetallic alloy Al5Co2 structure was studied by X-ray photoelectron diffraction and applying the standard software MSCD that shows a (1 × 1) termination. The reliability factor indicated the best termination belongs to the Al-rich surface layer.

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

A binary phase with Al

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.

Controlling material surface properties is crucial for many applications. While coatings and functional layers are widely used to tailor surface properties, another route to control topography and electronic surface charge relies on the design of new material surfaces. Here, we present a detailed study of the (100) surface structure of a BaAuGe intermetallic clathrate, a class of covalently bonded cage compounds with tunable electronic properties. Using a combination of experimental and computational methods, we demonstrate that the surface structure preserves the cages of the bulk structure up to the surface and contains an ordered arrangement of Ba surface atoms. Ab initio calculations show that the surface structure is stabilized through electron charge transfer from protruding Ba to surface Ge and Au atoms, saturating the dangling bonds. As a consequence, surface atoms recover an electronic environment similar to that of the bulk phase. Such a charge balanced stabilization mechanism may be general at ...

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.

Nanostructured alloy surfaces present unique physical properties and chemical reactivities that are quite different from those of the close-packed low-index surfaces. This can be beneficial for the design of new catalysts and electronic and data-storage devices. However, the growth of such surface nanostructures is not straightforward at the atomic scale. The cluster-based bulk structure of intermetallic compounds presents an original alternative to build surfaces with specific morphologies, in comparison to more traditional methods based on mechanical, chemical, or plasma treatments. It relies on their specific electronic structures-built from a network of bonds with a combination of ionic, covalent-like, and metallic characters, and also depends on the experimental conditions. In this paper, a few surface structures of cluster-based intermetallics are reviewed, with a special emphasis on quasicrystals and clathrates. We show how the intrinsic electronic properties of such compounds, as well as the surface preparation conditions, impact their surface morphologies, which can further influence the growth of atomic and molecular thin films at their surface.

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.

Designing surfaces with specific atomic and electronic structures is a main goal in nanoscience. One possible approach is based on complex intermetallic compounds, a class of materials possessing a large variety of atomic arrangements and properties. Among them, the covalently bonded Ba-Au-Ge intermetallic clathrates present tunable electronic properties. Using a combination of experimental and computational methods, we show that the Ba8Au5.25Ge40.75 (110) surface preserves the cages of the bulk structure. The surface is stabilized by electron charge transfer from protruding Ba to surface Ge and Au atoms, ensuring the saturation of the dangling bonds. The surface orientation controls the surface nanostructuration, by the selection of the types of protruding cages, while the surface electronic structure is impacted by the Au surface concentration. Finally, the surface electronic structure of our model for Ba8Au5.25Ge40.75(110) presents a metallic character and a rather low work function (3.58 eV), useful f...

The atomic structure of the Al13Ru4(010) approximant surface, investigated for annealing temperatures ranging from 873 K to 1143 K, can be described by pentagonal motifs and vacancies. It exhibits an atypical surface reconstruction which manifests itself in real space by well-separated stripes running about 10○ off the [001] direction. Their mutual ordering is drastically improving at high annealing temperature. This is simultaneously accompanied by a filling of the pentagonal hollow sites present in the stripes with atoms or groups of atoms. We provide evidence that these unidirectional features are of true surface origin. The bulk electronic density of states calculated within the density functional theory framework matches the experimental valence band spectra. In addition to step edges, molecular adsorption reveals that stripes are initially the most reactive sites, leading to anisotropic island growth.

International audience; Density functional theory calculations are used to investigate the catalytic properties of the o-Al13Co4(100) surface toward the semihydrogenation of acetylene. The dissociation of the H-2 molecule is the key process in this case, since the surface termination does not contain protruding Co atoms, according to a combination of surface science studies and ab initio calculations. However, compared to the pure Al(100) and Al(111) surfaces, H-2 dissociation on o-Al13Co4(100) proceeds much easier, due to the presence of surface Co atoms slightly below the mean position of the termination plane. A possible mechanism for the hydrogenation is presented, highlighting the selectivity of the reaction on this Al-rich surface.

Compared to elemental crystals (Al, Cu, Ag, etc.), the local atomic arrangement within Al-based complex intermetallics is usually best described by highly symmetric clusters decorating the unit cell. With the latter containing tens to several thousand atoms (or an infinite number for the case of quasicrystals), this translates to structurally complex surfaces exhibiting unique potential energy landscapes. This review will focus on the different studies reporting the adsorption of C60 molecules on such complex metallic alloy surfaces, aiming to benefit from this complexity to create exotic molecular nanostructures. First, we will recall the main adsorption mechanisms and surface phases that have been identified when fullerene adsorption is carried out on single crystal surfaces. Second, we will discuss how surfaces of increasing structural complexity impact the film properties. The presence of five-fold symmetric adsorption sites is another intrinsic property of these complex intermetallic surfaces. As will be presented in this review, this leads to specific molecular orientations to maximize substrate–adsorbate symmetry matching, hence introducing another degree of freedom to create new 2-D molecular architectures. The local electronic interactions at the adsorption site interface will also be introduced. Furthermore, the different fullerene structures formed upon adsorption on aperiodic surfaces of varying chemical composition and on Bi allotropes will be discussed. Finally, suggestions will be given for future work along with the foreseen area of interests.

International audience; Density functional theory calculations are used to investigate the catalytic properties of the low-index surfaces of Al5Co2 toward the semihydrogenation of acetylene. Adsorption energies of species involved in the reaction (H, C2H2, C2H3, and C2H4) are calculated on the (001), (100), and (2-10) surfaces. Hydrogen adsorption sites are found to be stabilized by subsurface cobalt atoms, showing an electron donor character. Scaling relations between the adsorption energies of C2Hx are established. Surface activity has been investigated on the (2-10) surface. A possible reaction path for the semihydrogenation of acetylene is proposed using nudged elastic band calculations, starting from a 3-fold adsorption site made of three aluminum atoms. The corresponding activation energy of the rate-controlling step is calculated to be 60 kJ/mol. The activation energy of the rate-controlling step toward total hydrogenation is evaluated to be 106 kJ/mol, thus suggesting a selective surface for catalysis.

SSCI-VIDE+ECI2D+LPI; International audience; Non-noble intermetallic compounds have recently shown promising catalytic performances for the partial hydrogenation of alkynes and alkenes. In this work, the properties of alpha-Al4Cu9(110) toward the gas-phase hydrogenation of butadiene have been investigated at total pressures of 2-20 mbar and temperatures of 110-180 °C. The model catalyst is active and 100% selective to butenes. Moreover, although less active than Al13Fe4(010), which had been previously evaluated for the same reaction, it is more selective and more stable. The combination of catalytic tests with pre- and postreactionAuger electron spectroscopy measurements, and comparative tests with Cu(110), shows that copper governs the reaction on alpha-Al4Cu9(110). However, the lower activity of the (Curicher) sputtered Al4Cu9 surface with respect to the annealed one, and the differences between Al4Cu9 and Cu surfaces in terms of butene isomer distribution, butene conversion kinetics and sensitivity to poisons, demonstrate the unique character of the intermetallic compound.

The chemical bonding in the intermetallic compounds ZnPd and ZnPt with the tetragonal CuTi type of structure, and in the chemically related intermetallic compounds ZnNi, ZnCu, ZnAg, and ZnAu with the cubic CsCl type of structure, is examined applying the electron localizability approach. The analysis of the chemical bonding reveals charge transfer from zinc to the transition metal (TM), in line with the respective electronegativities. While in ZnNi, ZnCu, and ZnAg as well as ZnPd and ZnPt the Zn–TM interactions are described by multi-center bonding and ionic interactions, mainly ionic interactions are found in the intermetallic compound ZnAu. Going from ZnNi over ZnPd to ZnPt, a gradual change from isotropic to more and more anisotropic distribution of the atomic interactions is detected, resulting in a more pronounced tetragonal distortion.

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

A first principle study of lead adsorption on the pseudo-10-fold surface of the orthorhombic Al13Co4 intermetallic compound has been performed using four possible structural models as substrate. Coverage between atomic adsorption to near 0.08 atm./A2 has been considered. Relative adsorption energies of Pb atoms on different surface sites in addition with simulated scanning tunneling microscopy images were obtained for each model. Our ab initio calculations demonstrate that the experimental results published recently, highlighting (i) the presence of a preferential adsorption site and (ii) the formation of Pb rows extending along the [010] direction [N. J. Phys. 2011, 13, 10301114] can be only reproduced using a dense Al-rich termination as the substrate structural model. This result confirms the previously established structural model for the (100) surface of orthorhombic Al13Co4 [Phys. Rev. B 2011, 84, 085411]. This study then provides an understanding of the experimental observations for the different lead coverages. © 2012 Wiley Periodicals, Inc.

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.

International audience; We present a study of the atomic surface diffusion at low temperature of Cr adatoms deposited on Cu(100), Cu(111), and Au(111) surfaces. Time-dependent X-ray magnetic circular dichroism (XMCD), Monte Carlo simulations, and the Kimball−Shortley iterative method are used to evidence a mechanism of quantum tunneling involved in the Cr adatom surface diffusion at temperatures below 40 K. We show that the XMCD line shape is correlated to the atomic configuration of Cr deposited on a metallic surface. A diffusion rate ranging from 0.03 Hz for Cu(100) to 30 Hz for Au(111) at 10 K is reported from the time evolution of the XMCD line shape. A quantum equation of motion for the Cr adatom is derived and solved by using a Kimball−Shortley iterative method. Good agreement between the calculated Bohr periods and the experimental diffusion rates is reported.

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; A new ternary phase with a composition of Al3AuIr has been found in the Al-rich area of the Al–Au–Ir system. Differential thermal analysis indicates a melting point of 990 °C, and single-crystal X-ray diffraction measurements reveal that this ternary phase adopts a Ni2Al3 structure type (space group P3̅m1) with a = 4.2584(5) Å and c = 5.1991(7) Å. This compound is isostructural to the Al3Cu1.5Co0.5 phase also found in the Al-rich part of the Al–Cu–Co ternary diagram. Experimental evidence combined with ab initio calculations point toward an Al3AuIr phase stabilized by a Hume–Rothery mechanism. Quantum chemical calculations indicate two-center and multicenter interactions in the Al3AuIr phase. Layered distribution of two-center interactions separated by regions with four- and five-center bonds suggests a preferential cleavage of the material at puckered planes perpendicular to the [001] direction.

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.