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International audience; The observation of atomic ordering and signatures of defects in self-assembled Ge islands using x-ray diffraction techniques have been previously treated as unrelated subjects. However, mutual understanding can be achieved when both subjects are studied in a common frame. Here, we report on measurements and analysis of both defects and atomic ordering in Si1-xGex islands epitaxially grown on Si(001) substrates as a function of growth temperature. By using x-ray diffraction and mapping around a bulk forbidden reflection, defect sizes, and in-plane spacing between nearby dislocations are extracted and related to the composition of the islands. The results fit well with an independent determination using selective wet chemical etching and atomic force microscopy measurements. Moreover, the temperature dependence of the ordered domain size is discussed. Although both atomic ordering and defect formation take place independently in the system, it is found that the relaxation provided by the onset of defects does not affect the formation of ordered domains, recently pointed out to be stabilized by strain and surface equilibrium on islands facets. (C) 2016 AIP Publishing LLC.

We have investigated the atomic structure of graphene/Ir(111) supported platinum clusters with on average fewer than 40 atoms by means of surface x-ray diffraction (SXRD), grazing incidence small angle x-ray scattering (GISAXS), and normal incidence x-ray standing waves (NIXSW) measurements, in comparison with density functional theory calculations (DFT). GISAXS revealed that the clusters with 1.3 nm diameter form a regular array with domain sizes of 90 nm. SXRD shows that the 1--2 monolayer high, (111) oriented Pt nanoparticles grow epitaxially on the graphene support. From the combined analysis of the SXRD and NIXSW data, a three-dimensional (3D) structural model of the clusters and the graphene support can be deduced which is in line with the DFT results. For the clusters grown in ultrahigh vacuum the lattice parameter is reduced by $(4.6\ifmmode\pm\else\textpm\fi{}0.1)%$ compared to bulk platinum. The graphene layer undergoes a strong Pt adsorption induced buckling, caused by a rehybridization of the carbon atoms below the cluster. In situ observation of the Pt clusters in CO and ${\mathrm{O}}_{2}$ environments revealed a reversible change of the clusters' strain state while successively dosing CO at room temperature and ${\mathrm{O}}_{2}$ at 575 K, pointing to a CO oxidation activity of the Pt clusters.

We have studied the nano-patterning of CoO film induced by misfit dislocation network at the interface with the Ag(001) substrate. Grazing incidence diffraction (GIXD), X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED) have been used to characterize chemistry and structure of the CoO layers. The XPS spectrum of the Co 2p core level permitted to establish the stoichiometric growth of CoO. The structure of the CoO film together with the absorption sites of cobalt and oxygen atoms was determined, thanks to GIXD measurements. Moreover we have followed the evolution of the in-plane lattice constant of the CoO as a function of the film thickness. It turns out that the CoO film growth starts with the same in-plane lattice constant of the Ag(001) substrate up to 3-4 ML; afterwards the in-plane parameter of CoO steadily increases before reaching a stable value of 2.98 angstrom at 23 ML. During the relaxation process, at about 8 ML of film thickness, we observe the formation of a buried misfit dislocation network. These dislocations, that have a period of 9.2 nm for a film thickness of 23 ML, induce mosaicity in the CoO film which then appears as a regular distribution of tilted domains. (C) 2006 Elsevier B.V. All rights reserved.

Two-dimensional hexagonal arrays of Pt nanoparticles (1.5 nm diameter) have been obtained by deposition of preformed and size selected Pt nanoparticles on graphene. This original self-organization is induced, at room temperature, by the 2D periodic undulation (the moiré pattern) of graphene epitaxially grown on the Ir(111) surface. By means of complementary techniques (scanning tunneling microscopy, grazing incidence X ray scattering), the Pt clusters shapes and organization are characterized and the structural evolution during annealing is investigated. The soft-landed clusters remain quasi-spherical and a large proportion appears to be pinned on specific moiré sites. The quantitative determination of the proportion of organized clusters reveals that the obtained hexagonal array of the almost spherical nanoparticles is stable up to 650 K, which is an indication of a strong cluster-surface interaction.

International audience; The structure of a graphene monolayer on Ir(111) has been investigated {\it in situ} in the growth chamber by surface x-ray diffraction including the specular rod, which allows disentangling the effect of the sample roughness from that of the nanorippling of graphene and iridium along the moiré-like pattern between graphene and Ir(111). Accordingly we are able to provide precise estimates of the undulation associated with this nanorippling, which is small in this weakly interacting graphene/metal system and thus proved difficult to assess in the past. The nanoripplings of graphene and iridium are found in phase, i.e. the in-plane position of their height maxima coincide, but the amplitude of the height modulation is much larger for graphene (\(0.379 \pm 0.044\) \AA) than, {\it e.g.}, for the topmost Ir layer (\(0.017 \pm 0.002\) \AA). The average graphene-Ir distance is found to be \(3.38 \pm 0.04\) \AA.

International audience; The In situ growth of Nanostructures on Surfaces (INS) endstation of the ESRF BM32 beamline: a combined UHV-CVD and MBE reactor for in situ X-ray scattering investigations of growing nanoparticles and semiconductor nanowires This paper presents the upgraded 'In situ growth of Nanoscructures on Surfaces' (INS) endstation of the InterFace beamline IF-BM32 at the European Synchrotron Radiation Facility (ESRF). This instrument, originally designed to investigate the structure of clean surfaces/interfaces/thin-films by surface X-ray diffraction, has been further developed to investigate the formation and evolution of nanostructures by combining small-and wide-angle X-ray scattering methodologies, i.e. grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXD). It consists of a UHV chamber mounted on a z-axis type goniometer, equipped with residual gas analysis, reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES) to complete the X-ray scattering investigations. The chamber has been developed so as up to eight sources of molecular beam epitaxy (MBE) can be simultaneously mounted to elaborate the nanostructures. A chemical vapor deposition (CVD) setup has been added to expand the range of growing possibilities, in particular to investigate in situ the growth of semiconductor nanowires. This setup is presented in some detail, as well as the first in situ X-ray scattering measurements during the growth of silicon nanowires.

The conditions of formation of the Au-induced Si-(6×6) reconstruction, its stability, as well as its atomic structure are studied experimentally using grazing incidence x-ray diffraction techniques. This reconstruction is found to form at 680 K when cooling down eutectic droplets previously obtained by the dewetting of thin gold films deposited at room temperature on a Si(111) substrate. The quality of the reconstruction is found to depend on the annealing temperature. The formation of the (6×6) reconstruction at low temperature after it has been destroyed by ion bombardment, together with the recovery of the Si surface, give evidence of its high stability and highlight the surfactant properties of gold atoms. The determination of the Si(111)-(6 × 6)Au atomic structure is performed using quantitative surface x-ray diffraction with an existing complex model proposed in the literature as a starting structure. The resulting gold structure is found to be 1 ML thick and consists in two domains related by a mirror. They are composed of trimer and pentagonal units with special sites presenting a partial occupancy of 0.5. Our experimental data set does not provide enough accuracy to determine the positions of the Si atoms of the substrate, but ab initio calculations tend to confirm that they are only slightly displaced from the bulk position.

The growth mode, strain state and shape of Ge islands were analyzed in situ, during their growth on Si(001), by combining Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence X-ray Diffraction (GIXD) measurements. GISAXS measurements provide the detailed evolution of the shape of the grown quantum dots (QDs). GIXD allows the monitoring of the island nucleation and the strain state of the QDs. The shape, size, strain and interdiffusion are found to strongly depend on the growth temperature between 773 K and 923 K. At low T, the 3D islands are small, nearly fully relaxed and do not strain the substrate. At higher T, larger islands with significant Si content are grown, and induce deformation in the silicon substrate.

5 pages, 2 figures, 1 table.-- PACS 61.10.Eq, 61.10.Ht, 61.10.Nz, 61.46.+ w, 68.65.Cd, 68.65.Hb, We report on a general method that takes advantage of the full capability of anomalous diffraction and can be applied to the challenging case of small size embedded nanostructures. We study in situ and ex situ GaN Quantum Dots (QDs) grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), and encapsulated by an AlN epilayer. We investigate the QD strain and composition that are related to size, morphology, and cap layer thickness by means of anomalous diffraction in grazing incidence. The X-ray energy is tuned across the Ga K-edge where the Ga atoms scattering power is strongly modified and diffraction becomes chemically selective, giving direct information on composition. Quantitative analysis of the oscillatory extended region above the edge gives information on composition and out-of-plane strain of the dots.

The growth mode, strain state and shape of Ge islands were analyzed in situ, during their growth on Si(0 0 1), by combining grazing incidence small angle X-ray scattering (GISAXS) and X-ray diffraction (GIXD) measurements. GISAXS measurements provide the detailed evolution of the shape of the grown quantum dots (QDs) and allow characterizing the degree of ordering. GIXD allows monitoring the island nucleation, the evolution of the in-plane size and the epitaxial orientation of the QDs, as well as the diffusion of the wetting layer into the islands during growth. It is found that for a deposition temperature of 500 °C, an amount of about one atomic layer is transported from the four monolayers (ML) thick wetting layer into the 3D islands.

The structure, thermal stability, interfacial chemical composition, and growth parameters, derived from in situ grazing incidence x-ray diffraction (GIXD) and grazing incidence small angle x-ray scattering, ex situ atomic force microscopy, and transmission electron microscopy, of three ferromagnetic/antiferromagnetic metal/oxide interfaces [Co, Ni81Fe19 (permalloy) and Co70Fe30 on single crystalline NiO(111)] are reported. The samples were prepared in the 300–770 K temperature and 0–200 A thickness ranges. The important role of Fe in the composition and creation of the interfacial compounds as well as in the film flatness is discussed. The strong influence of the NiO(111) single crystal in determining the crystalline structure of the metal layers is evidenced. The limitations of GIXD in the description of the growth morphology were investigated in detail and are discussed with respect to the substrate crystalline quality.

International audience; Compositional atomic ordering is a crucial issue in the epitaxial growth of nanoparticles and thin films. Here, we report on a method based on x-ray diffuse scattering close to basis forbidden Bragg reflections to infer the lateral location, the symmetry, and the thermal stability of ordered domains in GeSi dome-shaped islands on Si(001) after growth and during annealing. We observe that atomic ordering does not disappear after annealing, demonstrating that it is a resilient metastable phenomenon. (C) 2015 AIP Publishing LLC.

The structure of ultrathin silicon layers obtained by molecular hydrophobic bonding is investigated. The twist and tilt angles between the two crystals are accurately controlled. The buried Si|Si interface is observed by transmission electron microscopy and by grazing incidence X-ray techniques. For low twist angle values (/spl psi/

Grazing-incidence X-ray diffraction data are combined with energy-minimization calculations to analyse the atomic structure of the Al-rich (3sqrt{3} x 3sqrt{3})R 30 deg reconstructed surface of sapphire alpha-Al_2O_3(0001). The experiments on the BM32 beamline of the ESRF provide the non-integer-order diffraction intensities and, after Fourier transform, an incomplete Patterson map. The computer simulations are implemented to obtain structural information from this map. In the simulations, the interactions between the Al overlayer atoms were described with the Sutton-Chen potential and the interactions between the overlayer and the sapphire substrate with a laterally modulated Lennard-Jones potential. We have shown that the hexagonal reconstructed unit cell is composed of triangles where the two layers of Al adatoms are FCC(111) ordered whereas between the triangles the stacking is FCC(001).

We report various experiments that cast some light on the growth process of self-organized dots and vertical Co pillars on Au(1 1 1). We first report grazing incidence small angle X-ray scattering (GISAXS) on Co dots and films, that allowed us to perform a crystallographic study of the order in the self-organized array, and also yielded a statistical view of the coalescence process. We then report scanning tunneling microscopy (STM) and spectroscopy (STS) images performed during the course of vertical growth. Details about the atomic growth process and the possible influence of the growth technique used are reported.

International audience

The structural properties of Ge islands grown by molecular beam epitaxy on SrTiO 3 (001) substrates are investigated. We report on the effect of the temperature on the epitaxial relationship and morphology of the Ge islands. By combining X-ray diffraction measurements and atomic force microscopy we evidence a correlation between the island size and shape, the structural properties of the interface and the sample texture. In particular, we show that the growth temperature affects the nature of the interface between the semiconductor and the perovskite oxide and thus the island orientation, that evolves from fully (001) oriented to a mixture of (111) and (001) oriented islands.

A detailed structural analysis of the $\mathrm{NiO}(111)\ensuremath{-}p(2\ifmmode\times\else\texttimes\fi{}2)$ surface by surface x-ray diffraction is presented and previous interpretations are revisited. The surface adopts the octopolar reconstruction after air annealing and a spinel-like configuration after UHV annealing under low oxygen pressure corresponding to a reduced state. The transformation rate is determined and discussed. Both configurations and their transformation allow drawing a coherent scheme of the behavior of this polar oxide surface.

We present a structural investigation of the polar (111) surface of CoO single crystals by grazing incidence X-ray diffraction. The surface is found to be always covered by a ∼50 A-thick spinel Co3O4 layer that was quantitatively characterized. Attempts to remove this layer by UHV annealings or Ar+ bombardments lead to the formation of metallic Co clusters on the surface that transform again in Co3O4 by annealing under oxygen.

The experimental results obtained by X-ray photoelectron spectroscopy (XPS), in-situ grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) on the growth of silver thin films on MgO(001) surfaces have been interpreted in terms of a general model which reproduce all the data for a wide thickness range. Silver films grow along a three-dimensional Volmer–Weber growth mode. Clusters are pictured as truncated pyramids whose shape is described through parameters that vary continuously during growth, thus evidencing an evolution, which cannot be taken into account by simpler models. The corresponding growth laws for the clusters were quantitatively determined.

Using grazing-incidence x-ray diffraction, the p(2x2) surface structures of the single crystal NiO(111) and a 5 monolayer thick NiO(111) film on Au(111) were both shown to exhibit locally the theoretically predicted octopolar reconstruction, with some important differences. The single crystal exhibits a single Ni termination with double steps. The thin film exhibits both possible terminations (O and Ni) and single steps. These surfaces were found to be nonreactive with respect to hydroxylation.

The growth at room temperature of Pd on an atomically flat MgO(001) surface has been investigated by grazing incidence x-ray scattering. The structure and morphology of deposits in the range 0.2 to 183 monolayers (ML) were analyzed in situ, in ultrahigh vacuum (UHV). The growth proceeds by nucleation, growth, and coalescence of islands. Channeling between islands starts around 18 ML, and above 35 ML, the film is continuous and fully covers the substrate. Pd is in cube-on-cube epitaxy on MgO(001), with an average lattice parameter between those of bulk Pd and MgO. Neither stacking faults nor twins were found. As growth proceeds, Pd is increasingly relaxed, but most of the Pd in the first one or two atomic planes is fully lattice matched with the substrate. The islands are coherent with the substrate up to \ensuremath{\sim}4\char21{}5 ML of Pd deposited. Above 5 ML, interfacial misfit dislocations are introduced at their edges. These dislocations reorder to form a square network above 35 ML. We have determined the epitaxial site, above oxygen ions of the substrate, and the interfacial distance $(2.22\ifmmode\pm\else\textpm\fi{}0.03\AA{}).$ The evolution of the interfacial distance as a function of the amount of Pd deposited is deduced by a quantitative analysis of the substrate crystal truncation rods. Similarities and differences with the Ag/MgO and Ni/MgO interfaces are discussed, and general trends on the interfacial structure and morphology are deduced.

The structure and morphology of Ag deposits grown at room temperature on high-quality MgO(001) surfaces have been investigated in situ, from 0.2 to 300 equivalent monolayers (ML) of Ag deposited. Surface x-ray diffraction and grazing incidence small angle x-ray scattering parallel and perpendicular to the surface were combined. Nucleation, growth, and coalescence of islands are found from 0.2 ML. The average in-plane width, height, and in-plane separation of growing islands are deduced and are found to reproduce well the Mg $1s$ x-ray photoemission spectroscopy spectrum previously reported by other authors. The height over width ratio of the islands is $\ensuremath{\sim}0.37\ifmmode\pm\else\textpm\fi{}0.05$ at all stages of the deposit. Ag grows in cube-on-cube epitaxy with respect to the MgO(001) substrate. A very unusual evolution of the state of strain in Ag with increasing amount of Ag deposited is observed. Below 4--6 ML (island width smaller than 90 \AA{}), the small Ag islands are coherent with the MgO. Below 1 ML (island width smaller than 35 \AA{}), they have their bulk lattice parameter, and between 1 and 4 ML they become more and more strained by the MgO substrate, with an average lattice parameter intermediate between those of Ag and of MgO. Around 4--6 ML, the islands reach a critical size and misfit dislocations are introduced at the edges. Above 30 ML, the film is almost continuous, and the interfacial misfit dislocations reorder to form a square network, oriented along 〈110〉 directions. Stacking faults appear in Ag at this stage. A small amount of twinned Ag also starts to grow around 4 ML. This unusual evolution of the strain in the Ag islands and the following introduction of misfit dislocations are interpreted on the basis of a one-dimensional Frenkel-Kontorova model involving a very weak Ag-MgO interaction and a weak corrugation of the interatomic potential. Quantitative measurements and analysis of the MgO crystal truncation rods (CTR's) during growth were shown to provide different structural parameters of the interface that are important for theoretical calculation, especially the epitaxial site, above oxygen atoms of the substrate, and the interfacial distance $(2.52\ifmmode\pm\else\textpm\fi{}0.1\AA{}).$ The origin of the interference along the CTR's is discussed according to the strain state of the epitaxial Ag.

AlN layers were grown by molecular beam epitaxy on GaN single crystals (grown at high-hydrostatic pressure). The layers were examined in situ using RHEED, high-resolution X-ray reflectivity and X-ray diffraction at grazing incidence conditions. It was found that layers grown on single-crystal GaN had very smooth surfaces (3–5 A RMS roughness). A 100 A thick AlN layer was almost strained, while a 1500 A thick one was partially relaxed. The obtained results, which differ from previously published for AlN epitaxy on GaN on sapphire, are the first concerning lattice relaxation of AlN grown on GaN single crystals. The measurements performed for the strained AlN layer enabled us to determine the Poisson ratio for this compound ν =0.20±0.02.

The behaviour of the NiO(111) surface has been investigated by grazing incidence X-ray scattering (GIXS). We show on single crystals with an improved crystalline quality that NiO(111) is always p(2×2) reconstructed. After air annealing the structure is close to the predicted octopolar reconstruction; after in situ annealing and oxidation it is very different. The transformation of the surface is continuous with temperature but seems to be only based on two possible atomic configurations.

The growth of Pd on MgO(001) has been investigated by grazing incidence X-ray scattering. Deposits in the range of 0.2 to 183 monolayers (ML) were analyzed in situ, in UHV. The growth proceeds by nucleation and growth of islands from 0.2 ML. The coalescence starts around 18 ML and is complete at 35 ML. Pd is in cube-on-cube epitaxy with respect to MgO(001), and has a lattice parameter close to that of bulk Pd. The epitaxial site, above oxygen ions of the substrate, and the interfacial distance (2.22±0.02 A for 1 ML of Pd deposited) were determined for the first time as a function of the amount of Pd deposited, by a quantitative analysis of the substrate crystal truncation rods.

The growth of Pd on MgO(001) has been investigated by grazing incidence X-ray scattering. Deposits in the range 0.2 to 183 monolayers (ML) were analyzed in situ, in UHV. The growth proceeds by nucleation and islands growth from 0.2 ML. The coalescence starts at 18 ML and is complete at 35 ML. Pd is in cube on cube epitaxy with respect to the MgO(001) substrate, and has a lattice parameter close to that of bulk Pd. The islands are coherent with the substrate up to ∼5 ML of Pd deposited. Above 5 ML, interfacial misfit dislocations are introduced at the edges of the islands. Above 35 ML, the interfacial misfit edge dislocations reorder to form a square network. The dislocation lines are oriented along 〈110〉 directions, and the Burger's vectors are of the 1/2〈110〉 type.

We describe a new surface X-ray diffractometer, which is optimized to combine surface X-ray diffraction (SXRD), absorption spectroscopy (SEXAFS) and grazing incidence small angle scattering (GISAXS). This instrument is particularly well suited for real time studies of material elaborated in-situ with molecular beam deposition (MBD) techniques. The goniometer allows for large in-plane and out-of-plane momentum transfer with high accuracy. Owing to the flipping mechanism of the sample holder, it can perform absorption experiments with polarisation directions normal and parallel to the sample surface, while keeping a grazing incidence. Several MBD sources as well as complementary surface sensitive electron techniques (RHEED and Auger spectroscopy) can be used simultaneously with X-rays. Finally, the samples can easily be inserted in the X-ray chamber via an UHV transport system and a fast entry load-lock module.

We report an in situ grazing incidence x-ray scattering study of the Ni/MgO (001) interface during its formation at room temperature. In-plane measurements reveal that the interface is sharp and that the epitaxial relationship is complex. In the early stages of growth, up to one monolayer of “on site” Ni, in-plane strained to the lattice parameter of MgO, is observed. An oxygen epitaxial site is found. For the on-site fraction the interfacial and Ni interlayer distances were determined. For coverages larger than one monolayer two distinct lattices exist: expanded Ni (001) and Ni (110). The latter exhibits several orientations with respect to the substrate depending on the thickness. The Ni (110) orientations disappear by annealing at high temperature, leaving only the Ni cube/cube orientation. The layer was also almost fully transformed into NiO (001) by high temperature oxidation.

Apres une breve introduction portant sur l'etude de surfaces et d'interfaces par rayons X nous presentons dans cet article les bases theoriques elementaires de cette technique ainsi qu'une breve introduction a la problematique liee aux surfaces d'oxydes et aux interfaces metal/oxydes. Nous montrons que cette technique est bien adaptee a ce type de materiaux et qu'elle permet de repondre avec precision a des questions aussi diverses que la relaxation de surface, la reconstruction de surface et la croissance de couches minces. Le propos est illustre par des experiences recentes realisees sur diverses lignes de l'ESRF (European Synchrotron Radiation Facility).

The misfit dislocation network at the Ag/MgO(001) interface has been investigated by grazing incidence x-ray diffraction on Ag films of different thicknesses, between 100 and 1500 \AA{} deposited at room temperature by molecular beam epitaxy on MgO(001) surfaces with different step densities. They were analyzed after growth and after annealing at increasing temperatures. A square network of edge dislocations is found to release the 3% lattice misfit between Ag and MgO. The dislocation lines are found oriented along 〈110〉 directions, with $a/2[110]$ Burgers vector. Quantitative analysis shows that the substrate is deformed according to the elasticity theory. The as-grown Ag films contain stacking faults and twins, in amounts that increase with increasing substrate surface step density. Annealing the film at low temperature 300 \ifmmode^\circ\else\textdegree\fi{}C eliminates the stacking faults. Annealing at higher temperatures induces a recrystallization of the Ag epilayers, as well as an improved ordering of the dislocation network. For increasing temperatures, the period of the dislocation network instantaneously adapts to the actual lattice parameter misfit. After cooling, a large residual deformation is found in Ag, which is smaller when the substrate step density is larger. This is interpreted in terms of an energetic barrier for the nucleation of misfit dislocations.

Nous avons applique la technique de diffraction de surface des rayons X a l'etude de la croissance de l'interface Ni/MgO(001), A partir de mesures effectuees dans le plan de diffraction nous montrons que l'interface est abrupte et que deux reseaux coexistent. En plus d'une partie en epitaxie cube sur cube (CC) dilatee, une fraction du Ni est orientee (110). L'orientation de la partie (110) par rapport au MgO(001) sous-jacent ainsi que la dilatation du reseau (001) evoluent en fonction de l'epaisseur. Ces deux aspects structuraux semblent donc intimement lies. Dans les stades initiaux le Ni s'epitaxie sur les sites du MgO. Lorsque l'epaisseur augmente les differentes orientations de type (110) apparaissent, chacune dans une gamme d'epaisseur bien definie.

The behaviour of the NiO(111) surface has been investigated by grazing incident X-ray scattering. After air annealing and prior to any in-situ treatment, the surface is unreconstructed. We discuss the stability of this polar surface and show that NiO(111) surfaces of good crytalline quality can be prepared and investigated by grazing incident X-ray scattering. The (1×1) surface is shown to be stabilized by the adsorption of light elements.

The structure of MgO(001) single crystal surfaces that are of very high crystalline quality, extremely flat, clean and stoichiometric, was investigated by grazing incidence X-ray scattering. A new procedure for preparing these surfaces is described, which includes high-temperature annealing in air followed by high-temperature ion bombardment, and annealing under partial oxygen pressure. The relaxation, −0.56±0.35%, and rumpling, 1.07±0.5%, were determined with a much better accuracy than in any previous experimental study. These values are compared with those given in previous experimental and theoretical studies. The atomic structure of the Ca-segregated MgO(001) surface was also investigated. The most likely structure is the theoretically predicted ( 2 × 2 ) R45° reconstruction. Atomic displacements within this unit cell were determined.

We have investigated the unreconstructed (0001) surface structure of sapphire (α-Al2O3) by Grazing Incidence X-ray Scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surface. The most likely model yields a single Al layer termination with relaxations of the first four planes of -51%, +16%, -29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.

The growth of Ag on MgO(001) was investigated from sub monolayer up to 600 Å coverage. A new method for the preparation of very flat, clean and stoichiometric MgO(001) surfaces is described. The growth is found to be intrinsic, with negligible influence of surface defects. Twins appear at the very first stages of deposition while stacking faults form during the coalescence. A detailed mechanism is proposed for the elastic and plastic misfit relaxation processes. The introduction of a new dislocation within an island is, whatever its size, preceded by a strong deformation of its edges. Plastic relaxation is shown to be a continuous process that starts well before coalescence and the appearance of an interfacial dislocation network.

Experimental determinations of the atomic structure of insulating oxide surfaces and metal/oxide interfaces are scarce, because surface science techniques are often limited by the insulating character of the substrate. Grazing incidence X-ray scattering (GIXS), which is not subject to charge effects, can provide very precise information on the atomic structure of oxide surfaces: roughness, relaxation and reconstruction. It is also well adapted to analyze the atomic structure, the registry, the misfit relaxation, elastic or plastic, the growth mode and the morphology of metal/oxide interfaces during their growth, performed in situ. GIXS also allows the analysis of thin films and buried interfaces, in a non-destructive way, yielding the epitaxial relationships, and, by variation of the grazing incidence angle, the lattice parameter relaxation along the growth direction. On semi-coherent interfaces, the existence of an ordered network of interfacial misfit dislocations can be demonstrated, its Burger's vector determined, its ordering during in situ annealing cycles followed, and sometimes even its atomic structure can be addressed. Careful analysis during growth allows the modeling of the dislocation nucleation process. This review emphasizes the new information that GIXS can bring to oxide surfaces and metal/oxide interfaces by comparison with other surface science techniques. The principles of X-ray diffraction by surfaces and interfaces are recalled, together with the advantages and properties of grazing angles. The specific experimental requirements are discussed. Recent results are presented on the determination of the atomic structure of relaxed or reconstructed oxide surfaces. A description of results obtained during the in situ growth of metal on oxide surfaces is also given, as well as investigations of thick metal films on oxide surfaces, with lattice parameter misfit relaxed by an array of dislocations. Recent work performed on oxide thin films having important physical properties such as superconductivity or magnetism is also briefly reviewed. The strengths and limitations of the technique, such as the need for single crystals and surfaces of high crystalline quality are discussed. Finally, an outlook of future prospects in the field is given, such as the study of more complex oxide surfaces, vicinal surfaces, reactive metal/oxide interfaces, metal oxidation processes, the use of surfactants to promote wetting of a metal deposited on an oxide surface or the study of oxide/liquid interfaces in a non-UHV environment.

The story of some not yet solved problems is presented, putting emphasis on the respective role of instrumentation, ab initio theories, phenomenology and simulations.

We report an in situ grazing incidence X-ray scattering study of the NiO(111) single crystal surface. It evidences a relatively high stability of this surface. After an air anneal at 1300 K it is flat and almost not reconstructed. Annealing at 860 K in ultra-high vacuum leads to two domains: one covered by epitaxial and relaxed metallic Ni clusters associated with a highly ordered interfacial supercell on the coincidence site lattice, presumably a misfit dislocation network and one exhibiting a p(2 × 2) reconstruction. After oxidation the metallic Ni transforms again into NiO(111) and only a p(2 × 2) reconstructed flat surface remains. In these conditions macroscopic (100) facetting was never observed. The general behaviour of the NiO(111) surface is discussed and compared with other polar and non-polar oxide surfaces, with emphasis on the specific properties which could explain its stability.

The objective of this study was to assess if drinking water meeting currently accepted microbiological standards is the source of gastrointestinal illnesses and to attempt to identify the source(s) of these illnesses. A randomized prospective study was conducted over a period of 16 months (September 1993-December 1994) in a middle class suburban community served by a single water filtration plant. A representative sample of 1400 families were selected and randomly allocated in four groups of 350, to the following regimens: (1) tap water; (2) tap water from a continuously purged tap; (3) bottled plant water; (4) purified bottled water (tap water treated by reverse osmosis or spring water). The water treatment plant produced wather that met or exceeded current North American regulations for drinking water quality. The distribution system was found to be in compliance for both coliforms and chlorine. Using the purified water group as the baseline, the excess of gastrointestinal illness associated with tap wa...

International audience; The effects of the temperature on the structure of a single layer of graphene on Ir(111) have been investigated in situ in the growth chamber by grazing incidence x-ray diffraction between 10 and 1300 K. In addition, the effect of two growth temperatures has been studied. The graphene lattice parameter of the main, nonrotated phase displays a characteristic hysteresis whose lower and upper branches correspond to phases in which integer numbers (m,n) Ir of the Ir lattice parameters match integer numbers (p,q) Gr of the graphene lattice parameters, i.e., commensurate phases, (19,0) Ir × (21,0) Gr and (9,0) Ir × (10,0) Gr , respectively. With a higher growth temperature, graphene presents, in addition to a main nonrotated, incommensurate phase, domains with a small rotation (2.37 •) relative to the substrate, corresponding to a (7,2) Ir × (8,2) Gr commensurate phase. Despite the weak interaction between graphene and iridium, the thermal expansion coefficient of graphene is positive at all temperatures, even at liquid helium temperature contrary to free-standing graphene. It is also close to the iridium thermal expansion because of the tendency to form commensurate phases, which creates a strong link between the graphene and substrate lattices and decreases the elastic energy.

International audience; Strains strongly affect the properties of low-dimensional materials, such as graphene. By combining in situ, in operando, reflection high energy electron diffraction experiments with first-principles calculations, we show that large strains, above 2%, are present in graphene during its growth by chemical vapor deposition on Ir(111) and when it is subjected to oxygen etching and ion bombardment. Our results unravel the microscopic relationship between point defects and strains in epitaxial graphene and suggest new avenues for graphene nanostructuring and engineering its properties through introduction of defects and intercalation of atoms and molecules between graphene and its metal substrate.

We report here an interface accommodation mechanism observed by using in situ grazing incidence X-ray diffraction in the very early stages of Ge epitaxial growth on SrTiO3. This mechanism leads to interface-localized misfit accommodation and involves two regimes: very early dislocation emergence followed by a damped collective oscillatory lattice parameter evolution. We show that this behavior is compatible with the simplest nonlinear Frenkel-Kontorova model assuming the weak elastic-chain/substrate interaction.

The structure and morphology of the Ag/MgO(001) interface have been investigated in situ by grazing incidence X-ray diffraction during the first stages of Ag deposition by molecular beam epitaxy. At the very beginning, Ag is perfectly registered, consisting of wide, one monolayer high platelets that cover half of the surface. The growth then proceeds in the form of islands, with Ag relaxation on surfaces. Finally, plastic relaxation occurs through a misfit dislocation network. The epitaxial site (on top of oxygen atoms) and interfacial distance are experimentally determined for the first time. Comparison with previous theoretical results on this model metal/insulator interface shows that the most recent ab initio calculations predict the correct interfacial structure.

This chapter supplies an introduction to the use of grazing incidence x-ray scattering as a surface sensitive tool for the study of near-surface nanostructures as well as a presentation of more recent developments using focused x-rays. An instrumental part is presenting the technical challenges that have to be met in surface diffraction and when using focusing techniques. Particular information is supplied on the potential of diffraction applicable in situ during the growth of semiconductor nanostructures. The interpretation of intensity distributions recorded in grazing incidence diffraction (GID) and grazing incidence small angle x-ray scattering (GISAXS) is explained quantitatively and in detail. The reader is then guided through examples all based on the investigation of the growth of Ge islands on Si(001), were all steps of the growth are presented, from a change in the surface reconstruction up to multifaceted islands. In the last section, x-ray diffraction with focused coherent beams is presented as a new tool to retrieve spatially resolved information from individual nanostructures.