Quasicrystal Surfaces Research Articles

Orientation relationship between metallic thin films and quasicrystalline substrates.

ABSTRACTWe 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.

Processing of Nickel-Quasicrystal Composite Coating Produced by Pulse Reverse Current Plating

SUMMARYA new composite coating, nickel-quasicrystal, was recently produced by pulse current electrodeposition. However, the surface finish was not sufficiently smooth. In this paper, pulse reverse current, as an alternative to the use of levelling additives, was employed in the electrochemical deposition of nickel-quasicrystal composite coating. The effect of anodic time on volume percentage of embedded quasicrystal particles, surface finish, and microhardness of the composite coating was examined. With a rise in anodic time, an improved surface finish was obtained, the volume percentage reduced, and microhardness decreased.

Structure and composition of cleaved and heat-treated tenfold surfaces of decagonal Al–Ni–Co quasicrystals

We investigated cleavage surfaces perpendicular to the tenfold direction of as-grown decagonal Al–Ni–Co quasicrystals by scanning tunneling microscopy, Auger electron spectroscopy, and scanning electron microscopy. The cleavage surface is determined by a cluster–subcluster structure. The image contrast of the smallest features, 1–2 nm in diameter, is related to the columnar atom arrangements extending perpendicular to the cleavage plane, which are predicted by current models of the decagonal quasicrystal structure. No voltage dependence of the STM images is observed. The presence of surface states and an enhanced density of states are discussed. Heat-treatments of the cleaved Al–Ni–Co quasicrystal surfaces show nearly no changes in chemical composition and structure up to about 750 °C. This is correlated with a much lower concentration of vacancies in as-grown decagonal Al–Ni–Co quasicrystals as compared to that in as-grown icosahedral Al–Pd–Mn quasicrystals.

Quasicrystalline epitaxial single element monolayers on icosahedral Al-pd-mn and decagonal Al-ni-co quasicrystal surfaces.

Single element quasicrystalline monolayers were prepared by deposition of antimony and bismuth on the fivefold surface of icosahedral Al71.5Pd21Mn8.5 and the tenfold surface of decagonal Al71.8Ni14.8Co13.4. Elastic helium atom scattering and low energy electron diffraction of the monolayers show Bragg peaks at the bulk derived positions of the clean surfaces, revealing highly ordered quasicrystalline epitaxial films. Their adatom densities of (0.9+/-0.2)x10(15) cm(-2) and (0.8+/-0.2)x10(15) cm(-2) on Al-Pd-Mn and Al-Ni-Co, respectively, correspond to roughly one adatom per Al atom of the quasicrystalline substrate surfaces.

Morphological evolution of the fivefold surface ofi−AlPdMnquasicrystals

Morphology of the fivefold symmetric quasicrystal surface of AlPdMn was investigated by x-ray reflectivity and by x-ray diffraction. X-ray experiments revealed two different morphologies depending on the surface preparation. Sputtering and annealing up to 900 K, under UHV conditions, produced a rough and facetted quasicrystal surface. These features were confirmed by atomic force microscopy and scanning tunnel microscopy measurements. We also observed that an annealing above 900 K induces a rapid and irreversible transition toward a flat surface.

Low-energy ion scattering measurements of quasicrystal surfaces

The technique of low-energy ion scattering spectroscopy is useful for determining the composition and structure of ordered surfaces. It can identify atoms in the topmost layer of a solid and provide real-space data about surface atom arrangement. This technique and its application to the study of quasicrystal surfaces is described using examples of the types of information that can be obtained from the ternary icosahedral alloy Al–Pd–Mn. These include surface layer and subsurface layer compositions, surface layer symmetry, interlayer spacings, and neighboring atom distances. In the case of annealed quasicrystalline Al–Pd–Mn, the surface is found to be composed primarily of Al atoms, a Pd-rich layer appears to exist just below the Al layer, and the top layers are deficient in Mn. The Al atoms at the surface exhibit local 5-fold symmetry.

Surface structure induced by Ar +-bombardment of decagonal AlNiCo

We have investigated the surfaces of decagonal AlNiCo single quasicrystals in UHV by Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM) and Auger Electron Spectroscopy (AES). The surfaces were prepared by Ar +-sputtering and annealing. We observe well-defined RHEED patterns directly after sputtering at room temperature, revealing a ten-fold symmetry with respect to the azimuthal angle of the incident beam. Two patterns occur, alternating every 18°. This symmetry behaviour is probably caused by a bcc overlayer consisting of multiple domains in different orientations. The RHEED patterns show a strong temperature dependence. A spontaneous phase transition into a quasiperiodic structure is observed upon annealing to about 350 °C. RHEED patterns of a flat, quasicrystalline surface of good structural quality are observed after sputtering at 400 °C. As indicated by AES, the influence of oxygen on the structural behaviour seems to be important.

Quasicrystal surfaces: potential as templates for molecular adsorption

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

Model simulations of momentum-resolved photoemission from quasicrystals

Recent experiments have demonstrated that valence band dispersion may be observed on suitably prepared surfaces of quasicrystals. We present model calculations for optical transitions for one-dimensional aperiodic lattices, and show that these calculations are consistent with the experimental photoemission results. We estimate the total number of bands which can be observed in such an experiment is reduced from infinite, to just two to three bands in a typical experiment.

Faceted etch pits and light figures of icosahedral Zn-Mg-Y quasicrystals

Faceted etch pits have been observed on chemically etched single-grain icosahedral Zn-Mg-Y quasicrystal samples with C 2, C 3 and C 5 symmetry rotation axes perpendicular to their surfaces. The anisotropic etching was produced by 2.5-5%HCl solution in ethylene. The etch pits, analysed by scanning electron microscopy (SEM), follow the icosahedral symmetry of a rhombic triacontahedron. Light figures were observed on a screen placed in between a 10mW He-Ne laser and the reflecting quasicrystal surface. Both the SEM images and the light figures can be used to determine the twofold, threefold and fivefold symmetry axes of the quasilattice as well as for investigations of intrinsic and growth defects in the quasicrystals.

Identification of the atomic structure of the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal: helium diffraction and scanning tunneling microscopy studies.

High resolution He diffraction and scanning tunneling microscopy images of the fivefold surface of a single-grain i-AlPdMn quasicrystal are obtained showing an almost perfect quasicrystal order. Observed configurations can be identified within the framework of polyhedral models. The terrace terminations are found to be Al-rich planes and successions of step heights agree with distances between dense Al planes in the model. This shows the ability of recent 6D polyhedral models to describe real quasicrystalline atomic configurations.

Oxidation of Al–Pd–Mn quasicrystal surfaces

The oxidation of both, quasicrystalline and crystalline surfaces of an icosahedral Al–Pd–Mn quasicrystal cut perpendicular to its five-fold axis and of Al(1 1 1) has been studied by means of X-ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy, low-energy electron diffraction and X-ray photoelectron diffraction. The oxidation of the elements of the alloy is quantified and compared for the different phases and Al(1 1 1). The quasicrystalline surface is found to react less with oxygen than the crystalline surface and more rapidly compared to Al(1 1 1). The observed behaviour is discussed in terms of the termination of the corresponding surfaces.

Scanning probe microscopy studies of the surface of decagonal quasicrystals in ambient conditions

The surface preparation of decagonal Al–Ni–Fe and Al–Ni–Co quasicrystals using chemical polishing and mechanical fracture was investigated with regard to the requirements of scanning probe microscopy. Scanning tunnelling microscopy and atomic force microscopy studies of the morphology of fracture surfaces were carried out in ambient conditions, showing a typical dimension of 60nm for surface corrugations. Oxidic corrosion of the quasicrystal surfaces was observed. On the oxide layer, the friction between the surface and the tip is reduced.

Electron energy loss spectroscopy investigation of volume and surface plasmons at the Al–Pd–Mn fivefold surface

We have measured the electron energy losses on different Al–Pd–Mn quasicrystal surface states established by low energy electron diffraction, X-ray photoelectron spectroscopy and Auger electron spectroscopy. On the well characterized and ordered Al–Pd–Mn quasicrystalline surfaces, the energy loss signal with an energy of incident electrons at 2keV shows a strong peak at 16eV and a satellite at about 33eV. As the electron primary energy is decreased to 260eV, an energy loss at 10.5eV is clearly observed. A short ion bombardment (1μA at 500eV during 1min) erases this loss at 10.5eV which can be recovered by annealing at 800K. All these observations are simply attributed to the existence of a bulk plasmon at about 16eV and a surface plasmon at 10.5eV. The ratio between these two energies is 1.52. This is quite close to the expected 2 from the classical plasmon model. On the crystalline partially annealed surface, a bulk plasmon is observed at about 16.5eV. Oxygen-contaminated surface presents a bulk plasmon shifted towards higher energy of about 20eV.

Influence of grown-in voids on the structure of cleaved icosahedral Al–Pd–Mn quasicrystal surfaces

We investigated the changes in structure induced by heat treatment of cleaved fivefold icosahedral Al–Pd–Mn quasicrystal surfaces close to, and far away from, grown-in voids. Areas far away from grown-in voids exhibit a roughening connected above 620K with a decrease of the Al concentration, and above 820K a recovery of the surface composition. No change of the surface structure can be detected close to grown-in voids in the same temperature range. The results indicate that the surface structure is influenced by the presence of vacancies in the quasicrystals and that vacancies enhance the surface roughening during heat treatment.

X-ray diffraction study of the 5-fold plane surface of a Al 70.4Pd 21.4Mn 8.2 single-grain

The structure of a 5-fold symmetry quasicrystal surface of a thermodynamically stable icosahedral Al 70.4Pd 21.4Mn 8.2 single-grain has been studied by means of the surface X-ray diffraction technique. The model used reproduces rather well both the Bragg peaks and the surface signal. The parameters obtained from the fit of the measured structure factor show a contraction of the outermost atomic layer. The surface also tends to be richer in A1 and Pd atoms forming a very dense outer-layer.

Comparative Study of the Tribological and Oxidative Properties of AlPdMn Quasicrystals and Their Cubic Approximants

AbstractAn experimental comparison has been made between the properties of the surfaces of an Al70Pd21Mn9 quasicrystal and its Al48Pd42Mn10 approximant. The Al70Pd21Mn9 sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al48Pd42Mn10, which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation.

Low-Energy Ion Scattering Measurements from an Al-Pd-Mn Quasicrystal

AbstractEnergy-angle distributions of low-energy inert-gas ions scattered from surfaces provide information about surface composition and structure. We have measured energy spectra of He+ scattered from an Al71Pd20Mn9 quasicrystal, which was oriented perpendicular to the 5-fold axis, along various azimuthal directions. Strong scattering signals are seen from Al and Pd, but only a weak Mn signal is observed. From measurements made of He+ at an oblique angle of incidence scattered in the forward direction, we observe a 72° periodicity in the azimuthal dependence of the scattering signal intensity from Al surface atoms. The effect arises from shadowing effects involving neighboring surface atoms and provides direct evidence that Al surface atoms exist in a local environment with 5-fold symmetry. In addition, measuring the variation of the signal intensity with incidence angle provides information about neighboring atom distances, which compare favorably with a model of the quasicrystal surface derived from the bulk structure.

Quasicrystalline nature of quasicrystal surfaces: A photoemission study

Differently prepared surfaces of quasicrystalline i-Al-Pd-Mn are analyzed using angle-resolved photoemission in the X-ray and ultraviolet range of photon energies. Depending on the preparation, we find both surfaces with crystalline structure and metallic character, and surfaces with quasicrystalline structural fingerprints and a suppressed density of states at the Fermi level, compatible with a pseudogap.

Overtone atomic force microscopy studies of decagonal quasicrystal surfaces

The surface of a decagonal Al-Ni-Fe quasicrystal was investigated by overtone AFM, a special type of force modulation microscopy (FMM), where the cantilever is driven at a frequency higher than the fundamental flexural mode of the cantilever in contact with the surface. The resonance behaviour of the cantilever sample system was studied with finite element analysis (FEA). Using the excitation frequency of a torsional vibration of the cantilever, an image contrast could be obtained, which is dominated by the shear stiffness of the material. Differently orientated decagonal grains, as well as lamellae structures of different phases could be resolved on the quasicrystalline specimen. It is shown that the elastic anisotropy of the material gives rise to the contrast formation in overtone microscopy.