Search

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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), lowenergy 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 x 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 x 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 bee-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 x 1) LEED pattern is recovered. [ABSTRACT FROM AUTHOR]

We report the adsorption of lead adatoms on the pseudo-10-fold Al 13 Fe 4 (0 1 0) surface using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). For the submonolayer regime, Pb adatoms remain highly mobile across the surface at 300 K. STM analysis indicates the formation of irregularly shaped islands of monoatomic height. The latter do not coalesce with increasing coverage. At 0.95 MLE coverage, the LEED patterns are consistent with a pseudomorphic growth of the adatoms. This is confirmed by STM measurements which reveal local motifs qualitatively similar to those observed on the clean Al 13 Fe 4 (0 1 0) surface, i.e. prior to dosing. Apart from the absence of plasmons, the XPS measurements of Pb 4 f and Al 2 s core levels are comparable to those observed for the Pb/Al(1 1 1) system. [ABSTRACT FROM AUTHOR]

Combined X-ray photoemission spectroscopy and low energy electron diffraction investigations have been performed to study the influence of temperature on the first stages of oxidation of the (1 1 0) surface of γ-Al 4 Cu 9 complex metallic alloy in the pressure range 2–6 × 10 −7 mbar. The oxidation mechanism is driven by aluminium segregation. At 115 K, a pure aluminium oxide film is formed, which does not present any long range order. Cu O bonds are never observed. The lower oxidation kinetics compared to that previously observed at room temperature is due to blocking of the oxygen dissociation and/or a decrease in the aluminium segregation. However, aluminium segregation is still observed, even for this very low temperature, due to the low energy value of the activation energy for aluminium diffusion in γ-Al 4 Cu 9 (0.65 ± 0.12 eV) which is related to the presence of two vacancies in the crystal structure. At 925 K, fast aluminium segregation and possible oxygen diffusion into the bulk leads to the formation of domains or clusters of γ-like alumina either on top of or, more likely, below the surface. [ABSTRACT FROM AUTHOR]

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 C60 with the sample held between 573-673 K leads to a [4, -2ι 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 Å2 . 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 AI4C3 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. [ABSTRACT FROM AUTHOR]

Quantitative adsorption structure determinations on quasicrystals are scarce because most techniques for measuring surface structures are not well suited to the complex and infinite unit cells of quasicrystals. The normal incidence standing x-ray wave field technique presents a solution to these problems because it can be made inherently surface sensitive and does not involve extensive computational effort. We describe a method for applying this technique to adsorbates on quasicrystals, with specific application to a submonolayer of Si atoms on a decagonal Al-Co-Ni surface. We demonstrate the sensitivity of the technique to both adsorption site and geometry, leading to the conclusion that the Si atoms, which form six-atom pentagonal clusters, have an average height of 1.77 ± 0.05 Å above pentagonal hollow sites, with a significant height variation among the Si atoms in the cluster. In particular, the central Si atom sits more deeply than the five surrounding Si atoms, which are, on average, 2.7 Å away from the central Si atom. Although this study was performed on a decagonal quasicrystal that is periodic perpendicular to the surface, we describe how the technique can be applied to cases with no periodicity. [ABSTRACT FROM AUTHOR]

he Al5Co2 crystal is a complex intermetallic compound, whose structure can be described by a stacking of chemically bonded atomic motifs. It is a potentially new catalytic material for heterogeneous hydrogenation. A single crystal of this phase has been grown by the Czochralski technique in order to study the influence of the three-dimensional bulk substructure on the two-dimensional surface using both experimental ultrahigh vacuum surface techniques and ab initio methods based on the density functional theory. Some bulk properties are first presented, focusing on chemical bond strengths, the determination of the Al and Co chemical potentials in Al5Co2, the vibrational properties, and the specific heat. Then, the combination of experimental and computational approaches allows the identification of the surface structure, which was found to depend on the surface preparation conditions. In all cases, the surface terminates at specific bulk layers (Al-rich puckered layers) where various fractions of specific sets of Al atoms are missing, identified as Al3 atoms left at the surface resulting from cluster truncation. Finally, electron density of states calculations and spectroscopic measurements were compared and indicate a strong sp-d hybridization of the topmost pure Al layer with subsurface Co atoms. This could influence the surface reactivity and the catalytic performances of this material. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We compare the growth of a Bi thin film on the ten-fold surface of the decagonal Al-Ni-Co quasicrystal at different substrate temperatures and fluxes using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The growth at 29 K leads to the formation of a quasicrystalline monolayer, consistent with the results obtained at 300 K. Further deposition at low temperature produces pseudocubic islands with {100} surface orientation. The islands have two or four atomic layer heights. Both STM and LEED measurements reveal that there is a transition from the pseudocubic {100} film to the hexagonal {0001} surface orientation at coverage (θ) 4 < θ < 6 ML. The hexagonal domains are oriented along the high symmetry directions of the substrate. Growth at room temperature with different fluxes results in different island-height distributions. High flux predominantly produces islands of uniform four-layer high islands, whereas low flux yields islands of varying heights, yielding a rough film morphology. The two-layer high islands are found to be more stable at low temperature than at room temperature. [ABSTRACT FROM AUTHOR]

Abstract: The deposition of Si at room temperature on the ten-fold quasicrystalline surface of d-Al–Ni–Co has been investigated by scanning tunnelling microscopy. At a coverage of 0.30ML, Si pentagons in two orientations related by inversion symmetry are observed on the same substrate terrace. The side-length of the pentagons is 4.2±0.2Å. At this coverage, the Si adlayer displays quasiperiodic order. Depressions related to pentagonal features observed in STM images of the clean d-Al–Ni–Co substrate are proposed as plausible adsorption sites for the Si adatoms. As the Si coverage is increased, the well-defined structures observed are no longer distinguishable. At coverages above the monolayer, the Si overlayer follows a rough three-dimensional growth mode. [Copyright &y& Elsevier]

The adsorption behaviour of Co on the tenfold surface of d -Al 72 Ni 11 Co 17 and on the fivefold surface of i -Al 70 Pd 21 Mn 9 has been studied using scanning tunnelling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). The analysis of a distinctive quasiperiodic LEED pattern for coverages from 3–30?ML of Co on d -Al 72 Ni 11 Co 17 suggests that the Co forms in a pseudomorphic row structure composed of domains of Fibonacci spaced rows having a periodic lattice parameter along the rows of 2.5?±?0.1?Å. The same structure, though less well-ordered and with a larger lattice parameter, is formed on the fivefold surface of i -Al 70 Pd 21 Mn 9 . [ABSTRACT FROM AUTHOR]

Abstract: We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. We present evidence that the large-scale step structure observed on Al-rich quasicrystals after quenching to room temperature reflects equilibrium structure at an elevated temperature. These steps are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. For the fivefold quasicrystal surface, step diffusivity increases as step height decreases, but this trend is not obeyed in a broader comparison between quasicrystals and crystals. On a shorter scale, the steps on Al-rich alloys tend to exhibit local facets (short linear segments), with different facet lengths, a feature which could develop during quenching to room temperature. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface. [Copyright &y& Elsevier]

The adsorption of C60 on the ten-fold surface of the decagonal Al–Ni–Co quasicrystal at room temperature has been investigated using scanning tunnelling microscopy (STM) and soft X-ray photoemission spectroscopy (SXPS). STM indicates disordered adsorption of intact C60 molecules on the surface up to a coverage of 1 ML, followed by the formation of a C60 multilayer. No step decoration is observed indicating that the molecules are immobile at room temperature. SXPS data of the Al 2p core level indicate a strong interaction of the C60 molecules with the Al atoms of the substrate in the sub-monolayer regime. Valence band data indicate that the C60 multilayer desorbs after annealing to 700 K leaving only the strongly bonded layer on the surface. [Copyright &y& Elsevier]

This paper presents a brief overview of recent progress in the investigation of the potential of clean quasicrystal surfaces for use as templates for the formation of ‘artificial’ aperiodic systems. The clean fivefold surfaces of i-Al–Pd–Mn and i-Al–Cu–Fe and the 10-fold surface of d-Al–Ni–Co are briefly discussed, and their use as templates for aperiodic ‘0’-dimensional (nanostructure) systems, of 1-d (quantum wire) systems, of 2-d (monolayer) systems, and of 3-d (epilayer) structures is reviewed. [Copyright &y& Elsevier]

Direct observation of extrinsic Al atoms on the fivefold surface of icosahedral Al–Cu–Fe indicates that they form pseudomorphic islands resembling starfish. The starfish occupy specific types of sites on the laterally-bulk-terminated quasicrystalline surface. We postulate that the nucleation event consists of a diffusing Al atom dropping into an empty site at the centre of a substrate pentagon. Growth consists of the addition of five Al atoms (nearly) in lattice sites. These 6-atom starfish do not grow laterally as coverage increases, leading to islands of uniform size, and to early roughening. [Copyright &y& Elsevier]

We report on investigations of the potential for using quasicrystal surfaces as templates for molecular adsorption. The quasicrystalline surfaces of the i-Al–Pd–Mn and the d-Al–Ni–Co quasicrystals have been dosed with various molecules and the surface reactivity characterised by Fourier transform infrared spectroscopy (FTIR). The reflected signal intensities obtained are much weaker than those obtained from other crystals such as metals. Although both quasicrystals are Al-based their surface reactivities are seen to differ. For the Al–Pd–Mn surface, NO and HCOOH both dissociate at the surface and CO does not adsorb. CD4O adsorption leads to multilayers as in other crystals. C6H6 molecules stick at low coverages and at low temperatures whilst not affecting the LEED pattern. For the Al–Ni–Co surface, CO molecules stick via activated adsorption to atop sites with vibrational frequencies very similar to CO adsorption on Ni. HCOOH forms multilayers at low temperatures which is reduced to a monolayer upon heating to 173 K; changes in the symmetry of the bonding are observed, which is also seen for HCOOH on Ni surfaces. NO and CD4O showed no characteristic spectra, which may be due to dissociation or a very low sticking probability. We summarise these observations and draw some conclusions as to the most favourable route for molecular overlayer formation. [Copyright &y& Elsevier]

The adsorption of sulphur on the fivefold flat-terraced surface of the icosahedral Al–Pd–Mn quasicrystal (i-Al–Pd–Mn) has been studied using extended X-ray absorption fine structure (EXAFS), Auger electron spectroscopy and low energy electron diffraction (LEED). Surface EXAFS data collected by Auger electron yield from the S saturated surface give average bond lengths for S–Al and S–Pd of 2.10±0.06 and 2.40±0.06 A˚ respectively. The data indicate that single-site adsorption on the unreconstructed surface may be ruled out; this is consistent with near-edge EXAFS data and the disappearance of the LEED pattern upon adsorption of sub-monolayer amounts of S. The implication of this finding is that adsorbate-induced reconstruction and/or multiple-site adsorption occurs in this system. [Copyright &y& Elsevier]

[Display omitted] The (110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) techniques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved X-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface co-segregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffusion to the surface. Regarding the structures investigated by low energy electron diffraction and scanning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a (1 × 1) termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagating along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications. [ABSTRACT FROM AUTHOR]

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 planewave 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 . 4 4 eV) compared to In-In ( - 0 . 2 9 eV) and Pd-Pd ( - 0 . 3 1 eV). [ABSTRACT FROM AUTHOR]

Structural investigations of the (210) and (100) surfaces of Al5Co2 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 (210) 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. [ABSTRACT FROM AUTHOR]

Oxidation of the Al 9 Co 2 (0 0 1) and Al 13 Co 4 (1 0 0) surfaces has been performed in a wide range of temperatures at 2 × 10 −8 or 1 × 10 −7 mbar oxygen pressure. Only Al O bonding is observed. The oxidation kinetics are controlled by the quantity of oxygen sticking on the surface. Oxidation results from a competition between several effects: formation of an oxide film, dissolution of the film, oxygen desorption and oxygen dissolution into the bulk. For temperatures lower than 710 K for the Al 9 Co 2 (0 0 1) surface and 925 K for the Al 13 Co 4 (1 0 0) surface, a ∼5 Å thick oxide film is formed which does not show any long-range order and desorbs upon annealing. When oxidation is performed at higher temperatures, oxygen diffusion into the bulk is observed. A poorly ordered oxide film having a sixton structure is formed on the Al 9 Co 2 (0 0 1) surface when oxidation is performed at 775 K, which is dissolved when annealing at higher temperatures. On the Al 13 Co 4 (1 0 0) surface, only a weak streaky polar circle is observed following annealing at 925 K the film formed at room temperature, which corresponds to an hexagonal network of O atoms into small ultrathin oxide layers domains. The oxidation behaviour of the Al 9 Co 2 (0 0 1) and Al 13 Co 4 (1 0 0) surfaces has been ascribed to the strong covalent character of bonds present in these Al Co phases, which prevents aluminium diffusion. C 2 H n molecules ( n = 2, 4, 6) do not adsorb on the Al 13 Co 4 (1 0 0) surface in the experimental conditions used in this study, thus suggesting that this surface might not be the active one in the semi-hydrogenation of acetylene. [ABSTRACT FROM AUTHOR]

Abstract: The two-fold surface of the icosahedral quasicrystal Al70.2Pd21.5Mn8.3 prepared in UHV has been studied with LEED and STM. Two phases are observed: a quasiperiodic phase, in agreement with previous work, and a periodic phase tentatively identified as orthorhombic. Copper deposition on the two-fold surface leads to a multilayer exhibiting a quasiperiodic LEED pattern. [Copyright &y& Elsevier]

Abstract: Thin films of the rare-earth metal Dy were grown on W(112) at room temperature and 570K. Then the resultant film morphologies were characterised by LEED and STM. A series of novel film morphologies, including c(4×6), c(5×12) and (5×8) structures, were found that are unique among the rare-earth metals. High thermal stability was found for the Dy films (up to 1650K, close to the melting point at 1685K) such that the Dy atoms preserved an ordered structure and the Dy volatility was less than would be expected from its vapour pressure. This is an indication of strong electronic interactions between the Dy atoms and the W(112) substrate. [Copyright &y& Elsevier]

Growth of Dy on W(1 0 0) at room temperature produced a rough surface covered by rhombic-shaped islands. The islands were strained perpendicular to the direction of substrate terraces. Annealing to 820 K produced strained, but well-ordered islands of Dy with a c(8 × 2) structure, with the principal direction of strain the same as for the as-grown islands. Further investigation showed that Dy islands do not follow Gd in producing inverted-step islands on W(1 0 0). [Copyright &y& Elsevier]

The use of quasicrystals as precursors to catalysts for the steam reforming of methanol is potentially one of the most important applications of these new materials. To develop application as a technology requires a detailed understanding of the microscopic behavior of the catalyst. Here, we report the effect of leaching treatments on the surface microstructure, chemical composition, and valence band of the icosahedral (i-) Al-Cu-Fe quasicrystal in an attempt to prepare a model catalyst. The high symmetry fivefold surface of a single grain i-Al-Cu-Fe quasicrystal was leached with NaOH solution for varying times, and the resulting surface was characterized by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The leaching treatments preferentially remove Al producing a capping layer consisting of Fe and Cu oxides. The subsurface layer contains elemental Fe and Cu in addition to the oxides. The quasicrystalline bulk structure beneath remains unchanged. The subsurface gradually becomes Fe3O4 rich with increasing leaching time. The surface after leaching exhibits micron sized dodecahedral cavities due to preferential leaching along the fivefold axis. Nanoparticles of the transition metals and their oxides are precipitated on the surface after leaching. The size of the nanoparticles is estimated by high resolution transmission microscopy to be 5-20 nm, which is in agreement with the AFM results. Selected area electron diffraction (SAED) confirms the crystalline nature of the nanoparticles. SAED further reveals the formation of an interface between the high atomic density lattice planes of nanoparticles and the quasicrystal. These results provide an important insight into the preparation of model catalysts of nanoparticles for steam reforming of methanol. [ABSTRACT FROM AUTHOR]

We report the formation of large and highly twinned dendrites of the Al 13 Fe 4 approximant phase embedded in an fcc Al-rich matrix. Using a rapid cooling technique, the approximant appears as a 10-fold dendrite. Within each arm, all grains share a common [010] direction. The grain distributions within the arm are quite complex and a single dendrite arm can contain up to four different orientations. Using several techniques, including Electron Back-Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM), it has been possible to identify three types of twins, namely {100}, {001} and {20 1 ̄ } twins. From these observations and previous reports, we propose a growth mechanism to explain the formation of such specific 10-fold motifs. It corresponds to the heteroepitaxial growth of the Al 13 Fe 4 approximant from a well-facetted decagonal Al–Fe quasicrystalline seed. [Display omitted] • Growth of Al 13 Fe 4 approximant precipitates in an Al fcc matrix with 10-fold dendritic structure. • Repetitive occurrence of three types of twins to maintain rapid dendrite growth. • Evidence for grain orientation relationships among symmetrically opposite dendrite. • Heteroepitaxial growth of a decagonal approximant on a quasicrystalline template. [ABSTRACT FROM AUTHOR]

We report the formation of several complex intermetallics as surface alloys upon the adsorption of Fe on Al 9 Co 2 (001) for different dosing conditions. Up to 4 monolayers equivalent (MLE) of Fe deposited on the substrate held at 593 K, the low energy electron diffraction pattern consists of a ( 2 × 2 )R45° phase with two additional domain types rotated by ± 8 ∘ from it. The scanning tunneling microscopy (STM) measurements show that these three types of domains have the same crystallographic structure. The lattice parameters and structural motifs point towards the formation of the high-temperature Al 8 Fe 5 phase (γ -brass of Zn 8 Cu 5 -type structure). For Fe deposition between 593 K and 873 K, we have identified the formation of two phases, tentatively assigned to a ternary disordered Al 9 (Co,Fe) 2 overlayer and to the monoclinic Al 13 Fe 4 (100). The stoichiometric evolution of the grown structures have been characterized by angle-resolved x-ray photoelectron spectroscopy measurements. Density functional theory based calculations have been performed to model the Al 8 Fe 5 (100)/Al 9 Co 2 (001) interface, its structural stability and to simulate the corresponding STM images. [Display omitted] • Epitaxial growth of simple binary to complex metallic alloy phases. • Growth of the high-temperature Al 8 Fe 5 phase as thin film on Al 9 Co 2 (001). • Identification of the Al 8 Fe 5 (100) surface termination by DFT calculations. • Substrate symmetry operations driving the alternating orientations of domains. • Formation of surface and interface structures of tuneable complexity. [ABSTRACT FROM AUTHOR]

The quantitative structure determination of adsorbed species on quasicrystal surfaces has sofar appeared to present insurmountable problems. The normal incidence standing x-ray wave field technique offers a simple solution, without extensive data sets or large computations. Its application to quasicrystals raises several conceptual difficulties that are related to the phase problem in x-ray diffraction. We demonstrate their solution for the case of Si atoms adsorbed on the decagonal Co-rich modification of the Al-Co-Ni quasicrystal to determine the local structure, comprising 6-atom clusters in particular hollow sites. [ABSTRACT FROM AUTHOR]

C60 monolayers grown on Ag( 111) at room temperature form incommensurate lattices that convert into a commensurate (2√3 x 2√3)R30° phase upon annealing. The C60 molecules in the commensurate phase have been observed to exist in three different states on Ag(111), namely bright, dim, and superbright (SB). All three species are in dynamical equilibrium at 280 < T <440 K. The bright and dim species were the subject of an earlier low-energy electron diffraction study that determined their geometries on the surface and the dynamics of the switching between those two states. The study presented here takes a closer look at the SB molecules, which appear and disappear at temperature-dependent rates with a measured activation barrier of 1.5 eV. The SB molecules in the commensurate phase comprise about 0.5% of the molecules and have a spatially random distribution. The evidence suggests that the formation of the three different states of C60 on Ag( 111) is a result of stress imposed by the substrate as the C60 adopts the commensurate spacing that is slightly smaller than its natural spacing. In the incommensurate phases, there is no bright-dim contrast, but SB C60 molecules form and organize into ordered arrays that appear to correspond to the moire patterns that are produced by the mutually incommensurate lattices. This suggests that the substrate responds to the nonuniform forces imposed by the C60 molecules by producing raised islands of Ag atoms at the vertices of the moire structure. A similar island structure may account for the SB molecules in the commensurate phase. [ABSTRACT FROM AUTHOR]

By investigating the magnetism of spins on a quasiperiodic lattice, we present an experimental study of static and dynamic magnetic properties, specific heat, and magnetic entropy of the Gd3Au13Sn4 quasicrystalline approximant. The magnetic sublattice of Gd3Au13Sn4 is a periodic arrangement of nonoverlapping spin clusters of almost perfect icosahedral symmetry, where gadolinium localized f magnetic moments are distributed on equilateral triangles. The absence of disorder on the magnetic sublattice and the antiferromagnetic (AFM) interactions between the nearest-neighbor spins distributed on triangles result in geometrical frustration of spin-spin interactions. Thus, the Gd3Au13Sn4 phase can be viewed as a prototype site-ordered, geometrically frustrated spin system on icosahedral clusters. The zero-field-cooled and field-cooled magnetic susceptibilities, the alternating current susceptibility, the thermoremanent magnetization, the memory effect, the magnetic specific heat, and the magnetic entropy all show that the spin system undergoes at low temperatures a transition to a nonergodic state at the spin freezing temperature Tf ≈ 2.8 K. Below this, the ergodicity is broken on the experimental timescale, because the thermally activated correlation times for the spin reorientations become macroscopically long. The magnetic state achieved at low temperatures by continuous cooling in low magnetic fields is likely a superposition of (1) metastable states with randomly frozen spins that have no long- range order yet undergo gradual spin-freezing dynamics and (2) an AFM-like magnetically ordered state with critical slowing dynamics. The magnetic properties of the site-ordered, geometrically frustrated Gd3Au13Sn4 system are discussed in comparison to site-disordered spin glasses that contain both randomness and frustration. [ABSTRACT FROM AUTHOR]

Abstract: We present a study of Pb adsorption on the (010) surface of the orthorhombic T-Al3(Mn,Pd) crystal at 300K and 573K substrate temperatures. This study has been performed using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). For depositions at both temperatures, we observed a pseudomorphic growth up to one monolayer coverage. A high mobility of Pb adatoms at 573K yields an improvement in the structural quality of the film. Our results show that the sticking coefficient of Pb film is either considerably reduced above one monolayer or the formation of large 3-D Pb clusters occur on the surface. Similarities with previous studies will be discussed as both effects, pseudomorphy and the impossibility to grow a second Pb layer, have already been encountered on other approximant and quasiperiodic surfaces. [Copyright &y& Elsevier]

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 (HO)-oriented islands correspond to bilayer stacking (either two or four monolayers in height) while the (11 l)-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. [ABSTRACT FROM AUTHOR]

We have used scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) to elucidate the structure and thermodynamics of the (2√3 x 2√3)R30° phase of C60 on Ag(111), which consists of a mixture of molecules in two different site/orientation states. The structure analysis identifies the two types of molecules as (1) sitting on a vacancy with a hexagon face down and (2) sitting on a top site with a C-C bond down. The molecules flip between the two states at a temperature-dependent rate. We show using a thermodynamic analysis that the two states differ by 0.07 eV and are separated by an energy barrier of 0.84 eV. Their dynamical equilibrium involves the diffusion of surface vacancies between C60 molecules, producing spatially and temporally correlated flipping events. [ABSTRACT FROM AUTHOR]

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 63 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. [ABSTRACT FROM AUTHOR]

We have investigated the adsorption of Pb atoms on the (100) surface of an orthorhombic Al13Co4 crystal at 300 and 573K 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: We report the formation of a new structurally complex intermetallic phase in the Ce–Au–Sn system. This phase has been observed in an ingot with nominal composition Ce16.7Au72.2Sn11.1. In addition, we have identified by transmission electron microscopy operating in two-beam conditions the occurrence of planar defects, characterized as grains boundaries located between the cubic 1/1 CeAuSn approximant and the new phase. Tilt series of electron diffraction patterns of this new structure have been recorded along two main directions. These measurements have been used for the reconstruction of the reciprocal space. The resulting cell is hexagonal, with a h ≈1.53nm and c h ≈1.52nm. It is structurally related to the Gd13Cd58 structure type (P63/mmc), where some Cd atoms are icosahedrally coordinated. The orientation relationships between the two complex structures across planar boundaries have been deduced from the lattice fringes transmission electron microscopy images. [Copyright &y& Elsevier]

Abstract: Lead has been deposited on the 10-fold surface of decagonal Al72Ni11Co17 at room temperature to form an epitaxial quasicrystalline single-element monolayer. The overlayer grows through nucleation of nanometer-sized irregular-shaped islands and the coverage saturates at 1MLE. The overlayer is well-ordered quasiperiodically as evidenced by LEED and Fourier transforms of STM images. LEED measurements also indicate that annealing the film to 600K improves the structural quality, but STM shows that this causes the film to develop pores. Adsorption of C60 molecules on this surface showed that the pores are Pb-containing. Electronic structure measurements using X-ray photoemission spectroscopy indicate that the chemical interaction of the Pb atoms with the substrate is weak, and that Pb does not desorb from the surface upon annealing. [Copyright &y& Elsevier]

We present an overview of experimental studies of the structure of adsorbed atomic and molecular species on quasicrystal surfaces with a particular focus on those systems where quasiperiodic ordering of the adsorbate structure occurs. Atomic adsorption is illustrated with examples from studies of Fe and Gd adsorption on the five-fold surface of i-Al-Pd-Mn. Overlayer structure is correlated with several physical parameters to identify trends. It is concluded that the chemical properties of the adsorbate (metallic radius, electronegativity, melting temperature) are important indicators of quasiperiodic structuring of the adsorbate system. These correlations allow predictions of those elements likely to yield pseudomorphic overlayers. Structural effects upon molecular adsorption on clean quasicrystal surfaces have been less successful. A new approach to ordered molecular adsorption is described and illustrated for C60 adsorption on a Pb monolayer film on the ten-fold surface of d-Al-Ni-Co. [ABSTRACT FROM AUTHOR]

At RT a laminar growth has been found for Dy on W(1 1 2). For coverages more than 1 ML, epitaxial relations of Dy(0 0 0 1)&par;W(1 1 2) with Dy[1 1 2¯ 0]&par;W[1 1 1¯] were found. The high background intensity and the limited energy range over which the Dy LEED spots were visible suggests that the film was not well ordered. STM studies showed that the development of the second monolayer involved chain-like atomic rows, with irregular lengths and distances, on top of the first monolayer. For five monolayer films, flat terraces of Dy were observed by STM. Growth at 470 and 570 K was also found to be laminar, but LEED produced patterns with streaks. [Copyright &y& Elsevier]

Quasicrystals are a family of complex bi- and tri- metallic alloys having non-crystallographic symmetries and novel, physical and electronic properties. In this article we introduce these materials, describing their discovery and unusual structure. We discuss the advantages of using scanning tunnelling microscopy (STM) to study their surfaces, and we demonstrate the application of various image analysis techniques to extract quantitative information on their surface structure. Lastly we point Out some current directions in research on these fascinating materials. [ABSTRACT FROM AUTHOR]

Abstract: The five-fold surface of an Al70Pd21Mn9 quasicrystal has been studied under a variety of surface preparation conditions using medium energy ion scattering. Argon ion sputtering of the surface was seen to give rise to Mn depletion and the formation of a thick five-domain AlPd(110) layer with 〈100〉 azimuths oriented close to, but not exactly on the five-fold symmetric directions of the substrate. Annealing to just 450°C was sufficient to restore the Mn content and the icosahedral structure in the surface and near-surface region. Annealing to increasingly higher temperatures was seen to cause Mn enrichment in the surface region above the ideal composition for icosahedral structure and this was accompanied by increased amounts of disorder in the sample. For the highest temperature anneals the surface showed low apparent roughness and exhibited blocking patterns consistent with a contraction of the outermost layer of atoms in line with previous studies of this material. [Copyright &y& Elsevier]

The adsorption and reaction of trans-1,2-dichloroethene on Cu nanoclusters supported on a thin Al2O3 film grown on NiAl(110) have been studied as a function of temperature and Cu cluster size using molecular beams and temperature-programmed desorption. At 300 K, dechlorination is observed on clusters of all sizes; subsequent desorption yields benzene and acetylene as the main reaction products. The ratio of these is strongly dependent on cluster size; for small clusters only acetylene evolution is observed. With increasing cluster size the probability of trimerisation of acetylene to produce benzene increases, although the desorption of acetylene remains dominant. The adsorption and reaction are compared to previous work on a Cu(110) single crystal where benzene was the main desorption product after adsorption at 300 K. The cluster size-dependent yield of desorption products is attributed to the inability of small clusters to accommodate the dissociation products prior to the trimerisation reaction. [Copyright &y& Elsevier]

The growth of gold and copper nanoparticles on a γ-Al[sub 2]O[sub 3] single crystal surface has been investigated using LEED and STM. The clusters were grown on the sample by exposing the substrate to a metal vapour. The particle size distributions are determined as a function of the metal vapour dosage and, furthermore, the Volmer-Weber growth mechanism and a semi-log scaling law are deduced from these data. [ABSTRACT FROM AUTHOR]

• Magnetic state of a CoCrFeMnNi high-entropy alloy was studied. • CoCrFeMnNi is a spin glass below the spin freezing temperature of 20 K. • The spins exhibit out-of-equilibrium dynamics of a nonergodic system. • Memory effect was observed. The CoCrFeMnNi high-entropy alloy (HEA) is a magnetically concentrated crystalline system with all lattice sites magnetic, containing randomness (five different types of spins are randomly positioned on the lattice) and frustration (a consequence of mixed ferromagnetic and antiferromagnetic interactions). The sample material was prepared as a non-equiatomic, fully random solid solution of the five magnetic elements and we have studied experimentally the nature of the magnetic state. Upon cooling, no long-range magnetic ordering takes place, but the spin system undergoes a kinetic freezing transition to a spin glass phase, where below the spin freezing temperature T f ≈ 20 K, ergodicity of the system is broken. The observed broken-ergodicity phenomena include zero-field-cooled – field-cooled magnetization splitting in low magnetic fields, a frequency-dependent cusp in the ac susceptibility, an ultraslow time-decay of the thermoremanent magnetization and the memory effect, where the state of the spin system reached upon isothermal aging at a certain temperature can be retrieved after a reverse temperature cycle. All these phenomena are associated with the out-of-equilibrium dynamics of a nonergodic, frustrated system of coupled spins that approach thermal equilibrium, but can never reach it on a finite experimental time scale, so that we are observing only transient effects of partial equilibration within localized spin domains. [ABSTRACT FROM AUTHOR]

• A single crystal of the Al 5 Fe 2 compound is grown and characterized. • Its formation enthalpy and electronic structure are determined by DFT. • The (1 0 0) and (0 0 1) surfaces are investigated at the atomic scale. • The (0 0 1) surface exhibits a reconstruction due to the presence of Fe missing rows. • The (1 0 0) surface exhibits an incommensurate modulated structure. The bulk structure of the η -Al 5 Fe 2 intermetallic compound consists in an ordered framework of pentagonal antiprims with composition Al 2 Fe 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 Al 5 Fe 2 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 Al 5 Fe 2 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. [ABSTRACT FROM AUTHOR]

• Fe adsorption on Al(1 0 0) is characterised by a multi-technique approach. • The growth of a metastable Al 9 Fe 2 near surface alloy is observed above 593 K. • A structural coincidence is identified between substrate and intermetallic bulk layers. • The growth of the metastable phase is stabilised by an epitaxial relationship. 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 Al 9 Fe 2 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 Al 9 Fe 2 (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 Al 9 Fe 2 -Al matrix interface is sharp as revealed by transmission electron microscopy measurements. [ABSTRACT FROM AUTHOR]

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 1.09J/m². 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. [ABSTRACT FROM AUTHOR]