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Changes in the visual characteristics of uranium oxide surfaces and morphology following storage under different conditions of temperature and relative humidity may provide insight into the history of an unknown sample. Sub-samples of three α-U3O8 materials—one that was phase-pure and two that were phase-impure—were stored under controlled conditions for two years. Scanning electron microscopy was used to image the oxides before and after storage, and a morphology lexicon was used to characterize the images. Temporal changes in morphology were observed in some sub-samples, and changes were greatest following exposure to high relative humidity.

An understanding of the relationships between morphologic signatures of uranium oxide materials and precursor materials used to prepare them would be valuable for determining process history of intercepted materials. Here, three different high purity uranium oxide materials were prepared from a single source of uranyl peroxide hydrate (UO2(O2)·xH2O) to determine variability of texture between the precipitate precursor and calcination products. Scanning electron microscopy images of the materials were characterized using a lexicon of descriptors. Analyses revealed that morphologic textures of calcination products were indeed equivalent to that of the UO2(O2)·xH2O precipitate, revealing a morphologic signature for forensic analyses of process history.

We have developed a lexicon to consistently and objectively describe morphological features observed in scanning electron microscopy (SEM) images. Here we provide the lexicon flowsheet, define the terminology, and detail step-by-step characterization of SEM images collected from a set of actinide oxides. We conclude that this lexicon can be used to characterize texture and surface features, particle structure and size, and grain boundaries in an image of a material. The lexicon should be applicable to characterization of images collected from other techniques for measuring morphology, as well. LA-UR-15-26746

We have employed X-ray photoelectron spectroscopy to characterise the growth and thermal stability of a Bi thin film on the five-fold icosahedral Al-Pd-Mn surface. The growth can be characterised as Stranski–Krastanov, in agreement with previous STM studies. As a function of annealing temperature, the multilayer desorbs first and thereafter coverages of 1 ML and 0.5 ML are stable for significant temperature ranges.

International audience; Thin films of copper grown on 5-fold i-AlPdMn at room temperature consist of domains that are rotationally aligned with the five primary symmetry directions of the substrate and which have one-dimensional aperiodic order. This aperiodic order is evident in scanning tunneling microscopy (STM) images as wide and narrow rows that are spaced according to a Fibonacci sequence. A low-energy electron diffraction (LEED) study of this film indicates that the structure within the domains is periodic along the rows, with a repeat distance equal to the nearest-neighbor separation in bulk Cu. To determine the complete structure, a dynamical LEED experiment was performed for a 5-layer Cu film at a sample temperature of 85 K. The analysis was performed using two different computational methods, one based on quasicrystalline slabs and the other on periodic approximants. Of the model structures tested, the film is found to be most consistent with a structure based on the Cu{100} surface structure, but having aperiodic displacements, both in-plane and out-of-plane, along a direction.

The room temperature adsorption behaviour of Fe on the five-fold surface of i-Al–Pd–Mn has been studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). A complex growth scenario for Fe adsorption on this substrate is observed with STM. At coverages up to about 3 MLE (monolayer equivalent), layer-by-layer growth is observed whereby small clusters and islands are formed which eventually coalesce into almost complete monolayers. No LEED pattern is observed, indicating that the layers are disordered. The AES results rule out intermixing. Above this coverage, there is a transition to a multilayer island growth mode. The islands are rotated by 72° and have the bcc(1 1 0) Fe structure. The results are compared with previous work on Fe adsorption on this substrate and on Al and Fe single crystal substrates.

Nucleation of Ag islands on the five-fold surface of icosahedral Al–Pd–Mn is influenced strongly by trap sites. Submonolayers of Ag prepared by deposition at 365 K and with a flux of 1 × 10−3 monolayers/s exhibit a variation in Ag island densities across different terraces. Comparisons with previous work and with rate equation analysis indicate that trap sites are not saturated under these experimental conditions and that the difference in island densities is not necessarily due to variation in trap densities. While it could have a number of different origins, our results point to a terrace-dependent value of the effective diffusion barrier for Ag adatoms.

In material science and bio-medical domains the quantity and quality of microscopy images is rapidly increasing and there is a great need to automatically detect, delineate and quantify particles, grains, cells, neurons and other functional "objects" within these images. These are challenging problems for image processing because of the variability in object appearance that inevitably arises in real world image acquisition and analysis. One of the most promising (and practical) ways to address these challenges is interactive image segmentation. These algorithms are designed to incorporate input from a human operator to tailor the segmentation method to the image at hand. Interactive image segmentation is now a key tool in a wide range of applications in microscopy and elsewhere. Historically, interactive image segmentation algorithms have tailored segmentation on an image-by-image basis, and information derived from operator input is not transferred between images. But recently there has been increasing interest to use machine learning in segmentation to provide interactive tools that accumulate and learn from the operator input over longer periods of time. These new learning algorithms reduce the need for operator input over time, and can potentially provide a more dynamic balance between customization and automation for different applications. This paper reviews the state of the art in this area, provides a unified view of these algorithms, and compares the segmentation performance of various design choices.

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 0.30 ML, 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 ± 0.2 A. 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.

The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface of icosahedral Al–Pd–Mn leads to the formation of a several monolayers thick Au–Al alloy film. An AlAu film with 1:1 stoichiometry is formed, which shows no evidence of ordered structure, being either amorphous or polycrystalline. Annealing to 325 °C causes more Al to diffuse into the film, producing Al2Au but still with no indication of structure. Experiments using 0.5 ML of pre-deposited In demonstrated a surfactant effect as the In 'floated' on the surface during growth and produced a reduction in film roughness. However, contrary to previous findings the film was still either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodic structure. Experiments were also conducted using smaller doses of Au to look for the formation of an epitaxial layer and, if formed, determine the registry with the substrate. However, no change in the Pd blocking curves for the surface could be seen, suggesting that the Au does not adsorb in well defined sites. This result is not surprising when considering that even for these low doses Al is drawn into the film, changing the composition and probably the structure of the topmost layers of the substrate, so that the potential adsorption sites on the clean surface may no longer exist.

We present an analysis of Sn film growth on the fivefold surface of the icosahedral Al–Cu–Fe quasicrystal at multilayer coverage. Scanning tunnelling microscopy of the Sn-deposited surface revealed that film morphology differed from terrace to terrace. Most terraces exhibited flat-topped clusters with preferential growth at the step edges, while a minority of terraces showed a smooth film. The height, lateral size and density of clusters also varied between terraces. Clusters have a preference for specific heights.

Our scanning tunneling microscope studies on clean surfaces of fivefold icosahedral Al70.2Pd20.7Mn9.1 reveal that, after annealing at 900 K, two types of flat depressions are visible on terraces: voids (small, shallow, flat-bottomed depressions bordered by 2.5 A steps) and pits (deeper, and often larger, depressions). They are different not only in their dimensions but in their behavior: voids preferentially nucleate on some of the terraces and, after annealing at 925 K, their coalescence and growth leads a new type of termination to be exposed. Pits, however, do not destroy the terraces on which they exist, under the experimental conditions. At the bottom of 4.1 A-deep pits, 2.5 A-deep voids nucleate, so that the total depth of the pit is 6.6 A. We propose that 2.5 A void-rich terraces are metastable terminations, and that these metastable terminations are also exposed at the bottom of the 4.1 A-deep pits.

The stability of i-Cd–Yb was studied between ambient temperature and 90 K using single-crystal diffraction. To study the influence of mechanical stress at low temperatures, a modified grinding-jar housing was constructed and ball-milling experiments were carried out at 77 K. In neither case structural changes were observed. This indicates the thermodynamic stability of this binary quasicrystal.

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(1 1 0) layer with 〈1 0 0〉 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.

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.

The growth of Cu on the five-fold surface of the icosahedral Al–Cu–Fe is investigated by scanning tunneling microscopy. The clean surface exhibits atomically flat terraces separated by steps of different heights. The steps are found to be frequently bunched. For a coverage close to a monolayer, Cu forms a nearly closed film of a height corresponding to a monolayer of Cu. The film, however, does not exhibit long-range order.

A 5-fold surface of the icosahedral AlPdMn (i-AlPdMn) quasicrystal (QC) was studied by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM) under ambient conditions. We show that a very complicated inhomogeneous surface morphology may develop on the 5-fold surface prepared by ion sputtering and annealing cycles in UHV. Sputtering and sublimation both contribute to material removal on the surface. The sublimation takes place at the annealing temperature at sites of surface defects induced by mechanical polishing and sputtering in UHV. The surface is characterized by nano-sized clusters of about 40 nm in size which form terraces. Results give further evidence for the cluster-based structure of the i-AlPdMn QC.

Scanning thermal microscopy is used to examine the thermal behavior of specific surfaces of a ∼0.8 cm3 single-grain Al74Ni10Co16 decagonal quasicrystal. The response of a micro-thermal probe in contact with aperiodic and periodic surfaces in air reveals the anisotropic heat flow of the decagonal structure. Heat dissipation with the probe on the aperiodic surface is higher than when the probe is on the periodic surface. While the SThM technique is generally considered to be constrained to the surface region below the nominally 2–5 μm probe tip radius, the heat flow data obtained are clearly comprised of contributions from both the lateral surface around the probe tip and a volume normal to the surface in contact. Heat flow in the decagonal Al74Ni10Co16 quasicrystal can be modeled by an elliptical distribution of thermal diffusion. Parameters for the model used in this study were obtained by making bulk thermal diffusivity measurements using the laser flash method on specimens along the 2- and 10-fold directions. The model was applied to a surface oriented 45° to the major axes and verified from bulk measurements obtained from a sample cut along this orientation.

Because of the aperiodic nature of quasicrystals, lattice planes, in the traditional sense, do not exist for quasicrystals. For decagonal quasicrystals, it has been proposed, however, that one can link the aperiodic and periodic directions using what are termed net planes [Acta Crystallogr. A 57 (2001) 333]. These net planes are thought to play a critical role in the stability and growth of decagonal quasicrystals. To explore their potential role during growth and to shed light on their structural stability, we have studied single-grain surfaces of decagonal Al–Ni–Co by low energy electron diffraction and scanning tunneling microscopy. Our results suggest that these planes do indeed have special stability.

The growth and properties of a Pd thin film deposited on a structurally complex surface are studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission spectroscopy. The substrate is the pseudo-10-fold surface of the ξ ′ -Al 77.5 Pd 19 Mn 3.5 crystal, an approximant of the AlPdMn quasicrystalline phase. Spectroscopic data are consistent with a low density of electronic states around the Fermi level for the clean surface, in agreement with the poorly conducting nature of this intermetallic alloy. Deposited Pd atoms readily form small metal particles with relatively homogeneous size on the substrate, for coverages ranging from the submonolayer regime up to 7 ML. The Pd particles do not coalesce with time and are relatively stable upon annealing up to 920 K. Their average size increases slightly with film thickness. The local electronic structure of the Pd thin film has been studied by STS and appears to be particle-size dependent, with a transition from non-metal to metal, deduced from I ( V ) characteristics with increasing particle size. A shift of the Pd-3d core level is also measured by photoemission, and it can be correlated with a shift of the Pd-d band toward the Fermi level. The growth mode of the Pd film on the ξ ′ approximant surface and its electronic structure are discussed in relation to Pd thin films grown on metal-oxides that are used as model catalysts.

The nucleation and growth of Ag thin films deposited on structurally complex alloy substrates is studied by scanning tunneling microscopy and low-energy electron diffraction. The substrates are high-symmetry surfaces of either icosahedral or decagonal quasicrystals or of related periodic crystals called approximants. At low coverage, completely different film morphologies are observed depending on the surface free energy of the substrate (relative to Ag). At high coverage, the formation of hexagonal nanocrystals is observed in all cases. These have fcc symmetry and pyramid-like multilayer stacking along the 〈1 1 1〉 direction. The constituent hexagonal islands have one, two or five different orientations depending on the rotational symmetry of the substrate on which the growth occurred. The films can be artificially smoothened by O 2 induced coarsening, leading to a patchwork of Ag nanocrystals with a perfect texture, which are separated by grain boundaries.

It has been speculated that the large increase in magnetostriction in Fe–Ga alloys results from local short-range ordering of the Ga atoms along specific crystallographic directions in the disordered Fe structure. The structural transitions associated with different cooling rates from the high temperature disordered state were investigated with X-ray diffraction of oriented single crystals of Fe–19 at% Ga. Results are presented for long-range ordering during slow cooling and indirect evidence of local short-range ordering of Ga atoms in the disordered state when the alloys are quenched is also presented. In the latter case, the short-range ordering of Ga atoms leads to a tetragonal distortion of the lattice. The dependence of the magnetostrictive response of Fe–Ga alloys on thermal history has been found to be directly related to these structural transformations in Fe–19 at% Ga alloys and experimental support for the proposed magnetostriction model based on Ga–Ga pairing along [100] crystallographic directions is presented.

We present angle-resolved low-energy ion scattering data that provide information about the composition of the topmost surface layers and atomic arrangement for a clean, 5-fold surface of a single-grain of icosahedral Al–Pd–Mn. The results show that the surface of this material has 5-fold periodicity and has a surface composition with a higher Al content than the bulk material (Al-86, Pd-13, Mn-1 versus Al-71, Pd-20, Mn-9 atom%). A predominant neighbor atom distance of 7.6±0.5 A is calculated from the results, consistent with scanning probe measurements and bulk models of Al–Pd–Mn quasicrystals. We calculate a nearest neighbor distance of 3.0±0.2 A. We also find that a Pd-rich layer exists just below an Al-rich outer layer, in agreement with dynamical low-energy electron diffraction calculations.

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. CD 4 O adsorption leads to multilayers as in other crystals. C 6 H 6 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 CD 4 O 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.

We discuss and compare approaches to using Auger data for semi-quantitative determination of surface compositions of Al-based quasicrystals. We have examined two quasicrystalline phases: icosahedral Al–Cu–Fe and Al–Pd–Mn. We find that sensitivity factors obtained from pure elemental standards lead to underestimation of concentrations by as much as a factor of 2.5 for the minor constituents, Fe and Mn, and to overestimation by as much as a factor of 1.2 for the major constituent, Al. Surprisingly, calibration with a crystalline phase close in composition to the quasicrystal does not necessarily lead to more accurate results. Auger analyses of fractured surfaces, which are then compared to bulk analyses, is the best option for sensitivity factor calibration. Even without absolute calibration of sensitivity factors, however, surface analysis is extremely useful for detection of secondary phases, and is superior to common bulk analysis methods.

Flux-growth techniques have been successfully applied to the preparation of several families of quasicrystals and related approximant phases. Large (up to 1 cm 3 ) single-grain samples have been obtained for icosahedral R–Mg–Zn (R=Y, Gd, Tb, Dy, Ho and Er), decagonal Al–Ni–Co, icosahedral Al–Pd–Mn, icosahedral Al–Ga–Pd–Mn, and the ξ′ Al–Pd–Mn approximant phase. The samples are obtained by slowly cooling ternary (or quaternary) melts of a composition that intersects the primary solidification surface of the desired phase in the equilibrium ternary alloy phase diagram (i.e. this is a self-flux technique). The technique clearly reveals the growth habit, and results in remarkably strain-free quasicrystals.

We elucidate the nature of the surface electronic properties of quasicrystalline Al-Pd-Mn. We do this by using photoelectron and Auger electron spectroscopies, and by making a variety of comparisons -- across types of bulk samples, and across methods of surface preparation. The main conclusions are these: (i) The narrow Mn 2p{sub 3/2} core-level line observed in the icosahedral phase is a fingerprint of a suppression in the density of states (a pseudogap) at the Fermi level and is not unique to the quasicrystalline phase. It is also independent of the symmetry of the quasicrystalline surface. The Auger line shape is also affected and may be used as a fingerprint of a pseudogap. (ii) A similarly narrow Fe 2p{sub 3/2} core-level line characterizes the icosahedral Al-Cu-Fe quasicrystal, consistent with the expectation that the electronic structure is of general importance in the stabilization of icosahedral phases. (iii) In icosahedral Al-Pd-Mn, the pseudogap of the bulk is not retained up to the surface immediately after fracture, but can be restored by annealing, or by sputter annealing to sufficiently high temperatures. Assuming that the pseudogap reflects an electronic stabilization of the atomic structure, these results suggest that the heat-treated surfaces are more stable thanmore » the surface obtained by fracturing at room temperature.« less

Directional solidification and quenching experiments have been conducted to examine the formation and origin of the faceted porosity commonly observed in icosahedral Al–Pd–Mn alloys. Faceted porosity has been previously observed in both Czochralski grown and Bridgman grown single crystals during single-phase solidification of the icosahedral phase. Detailed observations in Bridgman grown single crystals indicate that the distribution and quantity of pores are non-uniformly distributed along the length of the as-grown ingot. This non-uniform distribution suggests that their formation is influenced by both chemical segregation during solidification as well as convection in the liquid. Controlled solidification of this alloy followed by quenching of the solid–liquid interface provides for a time–temperature history of the porosity formation and morphological development as a function of processing conditions.

Icosahedral Al71Pd21,Mn8 quasicrystals have been grown from the ternary melt via a ‘self-flux’ technique. The nucleation is exceptionally well controlled, and individual grains can have masses of up to almost 30 g (limited only by the dimensions of the growth crucible). The quasicrystals grown by this technique are readily separated from the melt by decanting and clearly manifest a dodecahedral morphology with pentagonal facets. X-ray and electron-diffraction data indicate a remarkably well-ordered structure. with a correlation length of over 1000 A. The magnetic susceptibility has a temperature-independent diamagnetic component and a temperature-dependent paramagnetic term. For temperatures greater than approximately 60 K, the paramagnetic term follows a Curie-Weiss temperature dependence characterized by an average effective moment of 0.62 ± 0.02μB/Mn and a Weiss temperature of −12.8 ± 0.1 K. Deviations from the Curie-Weiss temperature dependence for temperatures less than approximately 60 K ma...

High-quality, single-Frain samples of decagonal Al-Ni-Co quasicrystals with volumes of up to 0.8 cm3 and an approximate composition of Al72Ni11 Co17 have been grown from the ternary melt. The precise growth methods and conditions are discussed in detail. X-ray diffraction patterns of crushed single grains and of single-grain samples are exceptionally sharp indicating a high degree of structural order with no evidence of second phases. Transmission electron micrographs also reveal sharp diffraction patterns in the even n layers but diffuse scattering in the odd n layers, indicative of a structure intermediate between the Al73Co27 and Al69Ni31 end members. The electrical resistivity measured directly from cut bars for currents flowing perpendicular to the crystalline direction (i.e. perpendicular to the c axis) is non-metallic, with a broad maximum around 180 K, and a value of approximately 310 μω cm at 300 K. The resistivity for currents flowing parallel to the c axis is metallic and is almost an ...

Reciprocal lattice mapping has been performed on Fe0.81Ga0.19 crystals by omega-omega/2theta, Psi-phi, and omega-phi scans. A strong elongation of the (c) peak was found along the (c) direction. omega scans revealed short lateral correlation lengths xi along (c) and strong diffuse scattering along the (c). Multiple domains with monoclinic symmetry (angle similar to190degrees) were observed by Psi-phi and omega-phi scans on the (001)(c) face, and were also tilted with respect to each other. The results show an average cubic structure with orthorhombic structural modulations, and two structural domain states that result in a limiting monoclinic symmetry. (C) 2002 American Institute of Physics.

The growth of Bi on a pseudomorphic Bi monolayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal has been investigated using low energy electron diffraction and scanning tunnelling microscopy. Initially randomly oriented pseudocubic islands are formed with a preference for an even number of layers. Subsequently a morphological transformation takes place to hexagonal Bi islands, which align along high symmetry directions of the substrate. The Bi flux is found to have a strong effect on which island structure is preferred. When C(60) is adsorbed on the three different allotropes of Bi present in this system, hexagonal C(60) islands are formed in each case. On the pseudocubic and hexagonal islands, the C(60) islands are aligned with the substrate. We discuss the energetic, kinetic and geometrical factors which influence the morphological transformation referred to above.

Recent work on the growth of thin metal films on quasicrystalline substrates has indicated the formation of so-called “magic height” islands with multiples of 4 atomic layers (AL) arising as a result of quantum size effects, which lead to enhanced stability. Here the results of a study are reported of Bi deposition on i-Al-Pd-Mn using medium-energy ion scattering to characterize the island thickness and the structural arrangement of Bi atoms within the islands. In addition, data were taken from annealed surfaces after Bi cluster desorption to leave a single aperiodic monolayer of Bi at the surface. Scattered-ion energy spectra from the Bi islands are consistent with a single Bi monolayer covered with mainly 4 AL islands for both 1.8 and 3.2 monolayer equivalent coverages but with some occupation of 2 and 8 Al islands as well. The angular dependence of the scattered-ion intensity (“blocking curve”) from Bi has been compared with simulations for various models of both rhombohedral Bi and a distorted “black-phosphorus”-like structure. The data demonstrate bilayer formation within the Bi islands. In the case of the aperiodic Bi monolayer, the blocking curves from substrate scattering are found to be inconsistent with two high-symmetry sites on the quasicrystalline surface that theory indicates are energetically favorable but do not exclude the formation of pentagonal arrangements of Bi atoms as seen in other recent experimental work.

We present a comprehensive study of Bi thin-film growth on quasicrystal surfaces. The substrates used for the growth are the fivefold surface of icosahedral $(i)$-Al-Cu-Fe and $i$-Al-Pd-Mn and the tenfold surface of decagonal $(d)$-Al-Ni-Co quasicrystals. The growth is investigated at 300 and 525 K substrate temperatures and at different coverage $(\ensuremath{\theta})$ ranging from submonolayer to ten monolayers. The film is characterized by scanning tunneling microscopy, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy. At 300 K, the deposited Bi yields a quasicrystalline film for $\ensuremath{\theta}\ensuremath{\le}1$. For $1l\ensuremath{\theta}l5$, it forms nanocrystallites with (100) surface orientation. The islands have magic heights, which correspond to the stacking of four atomic layers (predominantly). The selection of magic heights is interpreted in terms of quantum size effects arising from the electron confinement within the film thickness. The islands establish rotational epitaxial relationship with the substrate. For higher coverage, the film grows with monatomic height, not with magic heights, and reflects the symmetry of the bulk Bi. When deposition is performed at 525 K, terrace diffusion is more effective, resulting in the aggregation of Bi adatoms developing into a smooth monolayer with quasiperiodic order. At this temperature, multilayers do not adsorb.

Scanning tunneling microscopy has been used to study the formation of a Bi monolayer deposited on the fivefold surface of $i{\text{-Al}}_{70}{\text{Pd}}_{21}{\text{Mn}}_{9}$. Upon deposition of low submonolayer coverages, the nucleation of pentagonal clusters of Bi adatoms of edge length $4.9\text{ }\text{\AA{}}$ is observed. The clusters have a common orientation leading to a film with fivefold symmetry. By inspection of images where both the underlying surface and the Bi atoms are resolved, the pentagonal clusters are proposed to nucleate on pseudo-Mackay clusters truncated such that a Mn atom lies centrally in the surface plane. The density of these sites is sufficient to form a quasiperiodic framework, and subsequent adsorption of Bi atoms ultimately leads to the formation of a quasicrystalline monolayer. The suggested nucleation site is different from that proposed on the basis of recent density-functional theory calculations.

Scanning tunneling microscopy (STM) has been used to investigate the nucleation and growth of Ag islands on the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal. Analysis of the data as a function of deposition temperature, from $127\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, reveals that island density is constant, while at higher temperature it decreases. To model this behavior, we first show that the potential energy surface describing bonding of Ag at various locations on the surface is complex, with a few sites acting as traps for clusters of adatoms. We then develop a rate equation model which incorporates enhanced nucleation at trap sites relative to nucleation at regular sites on terraces. It recovers the temperature dependence of the island density, plus previous flux-scaling data. Our model suggests that the critical size for both types of nucleation sites is large---corresponding to stable clusters of at least 6 Ag atoms---and that binding between atoms at trap sites is significantly stronger than at free terrace sites. The data and the model, combined, provide guidance about the conditions of temperature and flux under which saturation of trap sites can be expected. This, in turn, provides a general indicator of the conditions that may favor localized pseudomorphic growth at low coverage, here and in other systems.

The elucidation of the local atomic structure of a pseudomorphic film of Cu deposited on the five-fold surface of $i\text{\ensuremath{-}}{\mathrm{Al}}_{70}{\mathrm{Pd}}_{21}{\mathrm{Mn}}_{9}$ using medium-energy ion scattering spectroscopy is reported. Monte Carlo calculations, using the VEGAS code, have been utilized to simulate the blocking of $100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{He}}^{+}$ ions scattered from the overlayer. The coordinates of the Cu atoms in the overlayer derived from this procedure are consistent with a structure occurring in five rotational domains. Each domain consists of nanoscale strips of fcc $\mathrm{Cu}(100)$ with the $⟨110⟩$ azimuth aligned along the five-fold directions of the quasicrystalline substrate. The strips are arranged according to a one-dimensional Fibonacci sequence with long and short widths related by the golden mean $\ensuremath{\tau}$. Upon annealing the film transforms to an alloyed structure composed of five orientational domains of fcc material with the (110) axis perpendicular to the surface.

International audience; The adsorption behaviour of benzene (C6H6) on the five-fold surface of the icosahedral Al70Pd21Mn9 quasicrystal has been investigated. Reflection absorption infrared spectroscopy (RAIRS) data taken at a temperature of 92 K indicate molecular adsorption. The adsorption geometry of the molecules is found to change from parallel to the surface to oblique geometries as the dose is increased from monolayer to multilayer coverages. Scanning tunnelling microscopy (STM) images show that the molecules adsorb in a disordered fashion. The adsorbate layer is found to completely desorb at 153 K leaving the substrate unchanged.

International audience; The adsorption behaviour of Co on the ten-fold surface of d-Al72Ni11Co17 and on the five-fold surface of i-Al70Pd21Mn9 has been studied using scanning tunneling 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-Al72Ni11Co17 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 five-fold surface of i-Al70Pd21Mn9.

Silicon atoms are found to form an ordered overlayer on the fivefold surface of the icosahedral $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Mn}$ quasicrystal. Using scanning tunneling microscopy, the adsorption site is identified as the center of truncated clusters which are building blocks of the bulk structure. Comparison with theoretical models suggests that this site is favored because of bonding to Mn atoms.

We have investigated by scanning tunneling microscopy the growth of Bi and Ag thin films on the fivefold surface of ${\mathrm{Al}}_{63}{\mathrm{Cu}}_{24}{\mathrm{Fe}}_{13}$ and ${\mathrm{Al}}_{72}{\mathrm{Pd}}_{19.5}{\mathrm{Mn}}_{8.5}$ quasicrystal, respectively. For both systems, we observe the formation of islands with magic height, corresponding to the stacking of a specific number of atomic layers. We interpret this unusual growth morphology in terms of quantum size effects, arising from the confinement of the electron within the film. The magic island heights are thus a direct manifestation of the electronic structure of the quasicrystalline substrates.

We investigate a thin Sn film grown at elevated temperatures on the fivefold surface of an icosahedral $\mathrm{Al}\mathrm{Cu}\mathrm{Fe}$ quasicrystal by scanning tunneling microscopy (STM). At about one monolayer coverage, the deposited Sn is found to form a smooth film of height consistent with one-half of the lattice constant of the bulk Sn. Analysis based on the Fourier transform and autocorrelation function derived from high-resolution STM images reveals that Sn grows pseudomorphically and hence exhibits a quasicrystalline structure.

Recently we reported the formation of a quasiperiodic Cu thin film on the fivefold icosahedral Al-Pd-Mn quasicrystal using scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. Here we provide details pertaining to the growth, stability, and structure of this film. Structural information has been gained by LEED measurements carried out at 85 K. Cu atoms are organized periodically with a nearest-neighbor distance of 2.5{+-}0.1 A along the aperiodically spaced rows. Above 8 ML spontaneous mass transport resulting in island formation has been observed by STM. These observations point to ascending adatoms being responsible for the formation of 3D features. Finally, flashing the multilayer film to 570 K results in the desorption or diffusion of Cu into the bulk and the formation of five domains of a periodic structure.

The pseudo-tenfold surface of the ${\ensuremath{\xi}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Al}}_{77.5}{\mathrm{Pd}}_{19}{\mathrm{Mn}}_{3.5}$ crystal, an approximant of the icosahedral $\mathrm{Al}--\mathrm{Pd}--\mathrm{Mn}$ quasicrystal, is investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy. The observed RHEED patterns of the surface after sputtering are found to be consistent with those of a simple cubic lattice with $(1\overline{1}0)$ surface plane. The [001] and [110] axes of the surface plane are oriented along the principal low-index axes of the bulk. The RHEED patterns of the sputter-annealed surface consist of diffraction streaks with periodic spacings expected for the bulk truncated surface. The surface prepared under different preparation methods is found to exhibit different step-height distribution and terrace morphology. A longer annealing yields a high density of shallow pentagonal pits on terraces, separated predominantly by $0.80\text{\ensuremath{-}}\mathrm{nm}$ high steps and occasionally by double steps. In contrast, the surface prepared with shorter annealing time exhibits highly perfect terraces with $0.80\text{\ensuremath{-}}\mathrm{nm}$-high steps and additional unusual steps of heights close to $0.40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. All step heights observed for both preparation methods are consistent with interlayer spacings of the bulk model.

We investigate the equilibration of a fivefold surface of the icosahedral $\mathrm{Al}\penalty1000-\hskip0pt\mathrm{Pd}\penalty1000-\hskip0pt\mathrm{Mn}$ quasicrystal at 900--915 and $925--950\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, using scanning tunneling microscopy. After annealing at the lower temperatures, there is a high density of shallow voids on some terraces but not on others; at $925--950\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the void-rich terraces are much rarer. The terminations that are consumed by voids exhibit a distinctive local atomic configuration, called a ``ring'' by previous authors. Apparently, through growth and coalescence of the voids, a different termination becomes exposed on the host terraces, which also leads to a change in step heights at the edges of the terraces. We suggest that the shallow steps associated with the voids, and the ring configuration, signal a surface that is in an intermediate stage of structural equilibration.

International audience; Based on scanning tunneling microscopy of the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal and the refined structure model of the isostructural i-Al-Pd-Mn, we present evidence that the surface corresponds to bulk truncations at the positions where blocks of atomic layers areseparated by larger interlayer spacings (gaps). Both step-height distribution and high resolution scanning tunneling microscopy images on terraces reveal bulk truncations at larger gaps.

Thermal expansivity ($\ensuremath{\alpha}$, $1--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), heat capacity (${C}_{P}$, $1--108\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), electrical conductivity ($\ensuremath{\sigma}$, $1--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) and magnetic susceptibility ($\ensuremath{\chi}$, $1--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) data have been obtained for a Bridgman-grown single grain $i{\text{\ensuremath{-}}\mathrm{Al}}_{68.9}{\mathrm{Pd}}_{21.6}{\mathrm{Mn}}_{9.5}$ quasicrystal (BR) for direct comparison with data previously published for a flux-grown single grain sample [Phys. Rev. B 65, 184206 (2002); PRB], and present $\ensuremath{\sigma},\phantom{\rule{0.3em}{0ex}}\ensuremath{\chi}$ and ${C}_{P}$ data for a second flux-grown sample described in an earlier publication [Philos. Mag. B 79, 1673 (1999); PM]. Fortuitously, comparative analyses show these samples to have essentially the same composition. At all temperatures, $\ensuremath{\sigma}$ and $\ensuremath{\chi}$ for BR are, respectively, approximately one-third and one-quarter those for PRB. The ${C}_{P}$'s are the same $(\ifmmode\pm\else\textpm\fi{}1%)$ down to $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, below which the BR ${C}_{P}$ decreases more rapidly to one-half that for PRB at $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $\ensuremath{\alpha}$'s agree to $\ifmmode\pm\else\textpm\fi{}2%$ from 300 to $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with a more rapid decrease for BR below $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, eventually to 0.6 ${\ensuremath{\alpha}}_{\mathrm{PRB}}$ below $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The total Gr\"uneisen parameters are similar at all temperatures. The two methods for sample growth differ primarily in a quenching of the flux-grown sample to room temperature after growth, while the Bridgman-grown sample cools very slowly, resulting in slightly different phases, and magnetic properties which are associated with lattice defects. An attempt to convert the single grain flux-grown sample to the phase of the Bridgman sample using an $800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ anneal and a slow cool to room temperature was not successful, with the appearance of second phase inclusions. These inclusions are ascribed to slightly different compositions for the two phases [see Boissieu et al., Philos. Mag. A 78, 305 (1998)]. This suggests that a single grain flux-grown sample with an $\mathrm{Mn}$ composition near 9% cannot be converted by annealing and slow cooling into a single grain LT phase, and vice versa. These considerations may not apply to samples with $\mathrm{Mn}$ compositions closer to 8%.

An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on thefivefold surface of the icosahedral Al70 Pd21 Mn9 quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of S = 4.5 �0.2 �A and L = 7.3 0.3 A, whose ratio equals � =1.618... within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.

The elastic constants C′, C44 and CL of a single crystal of the Ni40Ti50Cu10 (numbers indicate at.%) alloy have been measured in the premartensitic region (320–350 K) and have been found to exhibit softening on approaching the B2→B19 transition from high temperature. This means that these elastic constants decrease with decreasing T. The anisotropy factor A=C44/C′ is relatively small (≅2.2) over all the temperature range and slightly increases with decreasing temperature down to the transition start-temperature Ms. Thus, its temperature coefficient ∂A/∂T has opposite sign of that exhibited by the binary Ni50.8Ti49.2 alloy prior to the B2→B19′ transition. The present results support the view recently expressed by some authors that the B2→B19′ transition occurring in binary Ni–Ti alloys is a consequence of concomitant lattice instabilities against {1 1 0}〈1 1 0〉 B 2 and {0 0 1}〈 1 1 0〉 B 2 shear strains respectively corresponding to the C′ and C44 ultrasonic propagation modes.

We present experimental results on the structure of Ag thin films grown on high-symmetry surfaces of both quasicrystals and approximants. For coverages above ten monolayers, Ag form fcc nanocrystals with (111) plane parallel to the surface plane. Depending on the substrate surface symmetry, the Ag nanocrystals exist in one, two or five different orientations, rotated by a multiple of 2π/30. The orientation relationship between crystalline films and substrates appears to be determined by the following principles: high atomic density rows of the adsorbate are aligned along high atomic density rows of the substrate.