Fivefold Surface Research Articles

Sulphur adsorption on the fivefold surface of the i-Al–Pd–Mn quasicrystal

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 Å 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.

STM study of the atomic structure of the icosahedral Al-Cu-Fe fivefold surface

We use scanning tunneling microscopy (STM) to investigate the atomic structure of the icosahedral (i-) Al-Cu-Fe fivefold surface in ultra high vacuum (UHV). Studies show that large, atomically flat terraces feature many ten-petal ``flowers'' with internal structure. The observed flower patterns can be associated with features on Al rich dense atomic planes generated from two-dimensional cuts of bulk models based on x-ray and neutron diffraction experiments. The results confirm that the fivefold surface of i-Al-Cu-Fe corresponds to a bulk-terminated plane.

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.

Tiling of the fivefold surface of Al 70Pd 21Mn 9

The nature of the fivefold surface of Al 70Pd 21Mn 9 has been investigated using scanning tunnelling microscopy. From high resolution images of the terraces, a tiling of the surface has been constructed using pentagonal prototiles. This tiling matches the bulk model of Boudard et al. [J. Phys.: Cond. Matter 4 (1992) 10149], which allows us to elucidate the atomic nature of the surface. Furthermore, it is consistent with a Penrose tiling T *((P1)r) obtained from the geometric model based on the three-dimensional tiling T *(2F) . The results provide direct confirmation that the fivefold surface of i-Al–Pd–Mn is a termination of the bulk structure.

Structural aspects of the fivefold quasicrystalline Al–Cu–Fe surface from STM and dynamical LEED studies

We investigate the atomic structure of the fivefold surface of an icosahedral Al–Cu–Fe alloy, using scanning tunneling microscopy (STM) imaging and a special dynamical low energy-electron diffraction (LEED) method. STM indicates that the step heights adopt (primarily) two values in the ratio of τ, but the spatial distribution of these two values does not follow a Fibonacci sequence, thus breaking the ideal bulk-like quasicrystalline layer stacking order perpendicular to the surface. The appearance of screw dislocations in the STM images is another indication of imperfect quasicrystallinity. On the other hand, the LEED analysis, which was successfully applied to Al–Pd–Mn in a previous study, is equally successful for Al–Cu–Fe. Similar structural features are found for both materials, in particular for interlayer relaxations and surface terminations. Although there is no structural periodicity, there are clear atomic planes in the bulk of the quasicrystal, some of which can be grouped in recurring patterns. The surface tends to form between these grouped layers in both alloys. For Al–Cu–Fe, the step heights measured by STM are consistent with the thicknesses of the grouped layers favored in LEED. These results suggest that the fivefold Al–Cu–Fe surface exhibits a quasicrystalline layering structure, but with stacking defects.

Characterization of surface morphologies at the Al–Pd–Mn fivefold surface

Fivefold surfaces of Al–Pd–Mn quasicrystal have been prepared under ultra-high vacuum (UHV) by sputtering and annealing above 700K and characterized by electron diffraction, Auger spectroscopy and X-ray photoemission spectroscopy (LEED, AES, XPS). X-ray reflectivity at grazing incidence reveals that an in situ temperature annealing at 900K is necessary to irreversibly produce the signature of a flat and bulk terminated surface. Air-AFM investigations of surfaces prepared below or in the vicinity of this transformation temperature show a large scale structure in the micrometer range with high steps and parallel terraces. Terrace morphology is dominated by clusters with a typical size of about 10nm. Nanometer steps can coexist with this cluster structure. With UHV-STM, terrace roughness based on cluster assembly is found to extend down to the nanometer scale.

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.

Decomposition of the five-fold surface of Al70Pd21Mn9 at elevated temperature

Several surface phases have previously been identified on the five-fold surface of the Al70Pd21Mn9 quasicrystal. Two of these, the clustered and terraced phases, have been discovered by scanning tunnelling microscopy (STM) studies. In this paper, we report STM images showing the existence of a new phase on this surface. This new topography was obtained by annealing the sample close to its melting point for several minutes. A model is presented which exhibits the main features of the structure. The unit-cell dimensions match those of the Al3Mn orthorhombic phase. Mechanical polishing followed by sputtering and annealing restored a pentagonal quasicrystalline surface indicating that the decomposition is a near-surface phenomenon.

Investigation of the fivefold icosahedral AlPdMn surface by photoemission and Auger electron spectroscopy

Investigation of the fivefold icosahedral AlPdMn surface by photoemission and Auger electron spectroscopy

Structural and electronic properties of Au onTiO2(110)

The structure and electronic properties of 1-ML Au supported on ${\mathrm{TiO}}_{2}(110)$ have been theoretically investigated using the full potential linearized augmented plane-wave method. Based on total-energy and atomic-force calculations, the preferential adsorption site for the Au adatom is predicted to be the atop site above the fivefold surface Ti atom, with a bond length of ${d}_{\mathrm{A}\mathrm{u}\ensuremath{-}\mathrm{T}\mathrm{i}}=2.66\AA{}$ and an adsorption energy of -1.49 eV per adatom. The adsorption of Au enhances the binding energies of the valence bands and core levels of the surface Ti and O atoms, the Au d bands are destabilized with metallic gap states appearing near the Fermi level. This Au-substrate interaction leads to a more active Au site for the ${\mathrm{A}\mathrm{u}/\mathrm{T}\mathrm{i}\mathrm{O}}_{2}(110)$ system compared to the Au(001) surface, in agreement with recent experimental observations.

A LEED comparison of structural stabilities of the three high-symmetry surfaces of Al–Pd–Mn bulk quasicrystals

It is shown that low-energy electron diffraction (LEED) patterns of the three high-symmetry surfaces (fivefold, threefold and twofold) of icosahedral Al–Pd–Mn are all compatible with quasicrystallinity, under specific conditions of preparation. This conclusion results from comparing symmetries of experimental surface LEED patterns with bulk X-ray diffraction data which are converted to the conditions of the LEED experiment. This conclusion is also based upon an analysis of relative diffraction spot spacings in LEED. Hence, none of the three surfaces exhibits a massive lateral reconstruction, i.e. massive deviation from quasicrystallinity. The LEED pattern of the fivefold surface is distinct from the LEED pattern of the pseudo-tenfold surface of an orthorhombic approximant. We believe that this rules out the possibility that the fivefold surface of the icosahedral quasicrystal reconstructs to an approximant with tenfold or pseudo-10-fold symmetry. The twofold and threefold surfaces facet more readily, indicating qualitatively that they are less stable than the fivefold surface.

Unique Shapes of Micro-Pits Formed in an Al–Pd–Mn Icosahedral Quasicrystal by Anodic Etching

The shapes of micro-pits formed in an Al-Pd-Mn icosahedral quasicrystal have been examined by alternating sequence of electrochemical etching and scanning electron microscopy (SEM) observations. Anodic etching was executed on fivefold and twofold surfaces of the single-quasicrystal. Prolonged anodic etching in a solution of CH 3 OH + HNO 3 (volume ratio 3 : 1) followed by SEM observations has revealed that two different types of micro-pits, i.e. pre-existing micro-voids and electrochemical etch-pits, develop into the same regular shape. The micro-pits develop into a pentagonal pyramid on the fivefold surface, and a flat-bottomed rhombic pyramid on the twofold surface. It is shown that these two polyhedral etch-figures correspond to two different sections of a regular rhombic triacontahedron.

Investigation of the fivefold icosahedral AlPdMn surface by photoemission and Auger electron spectroscopy

Surface composition and electronic structure of AlPdMn quasicrystalline alloy have been studied for temperature ranging from 300 K to 873 K. The quasicrystalline nature of the in-situ prepared surface was identified by the unusual behaviour of the core level photoemission peaks of Mn as already demonstrated by Thiel et al. Quantitative estimation of surface concentration by measuring the peak on background ratio in Auger electron spectroscopy (AES) do not allow to concluded to surface segregation of one element whatever the substrate temperature. Auger transitions implying valence band electrons in the Auger relaxation reveal interesting shapes and energies modifications that can be correlated with the typical electronic structure of this alloy. The appearance of strong satellite structure in the Auger and photoemission spectrum can be attributed to bulk plasmon of the quasicrystalline phase.

Surfactant-Mediated Growth of Epitaxial Films on Quasiperiodic Surfaces

AbstractWe have succeeded for the first time in inducing epitaxial growth of metal films on quasiperiodic surfaces. Depositions of Au onto the tenfold surface of a decagonal Al-Ni-Co quasicrystal were originally attempted to create a quasicrystal of a single element as an epitaxial film. Although the desired film was not obtained on the clean surface, a multiply twinned AuAl2 crystalline layer with a (110)-oriented surface was formed with the aid of an In pre-deposition layer. The presence of pre-deposited In also worked as a surfactant for the case of Au-deposition onto the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal and promoted the formation of an ordered film of Au-Al alloy. X-ray photoelectron diffraction images from this ordered film show that the alloy has an icosahedral structure as a long-range order, suggesting the formation of a quasicrystalline film.

Study of the response of the fivefold i-AlPdMn surface to different temperature treatments

Chemical and structural surface evolution of an icosahedral AlPdMn sample was systematically studied after ultrahigh vacuum surface preparations based on ion sputtering followed by annealing. Depending on the annealing temperature we observed different surface phases. For annealing temperatures below 700K a thin crystalline surface was formed on the quasi-crystal bulk. For annealing temperatures between 700 and 900K we found a quasi-crystalline surface that can be interpreted as a bulk-terminated one. For high temperature annealing (>950K) the surface observed at high temperature was different to that obtained after cooling to room temperature. The cooled sample exhibited again a crystalline thick layer.In order to study the eventual influence of the original quasi-periodic structure on the evaporation, we further investigated the high temperature annealing regime from 950 to 1050K, where we observed the initial stage of a preferential evaporation of manganese atoms. Finally, we obtained direct experimental evidence for a spontaneous deoxidation of the quasi-crystal.

Fine structure on flat surfaces of quasicrystalline Al-Pd-Mn

We have analyzed the fine structure revealed by scanning tunneling microscopy for a flat (within 0.8 \AA{}) fivefold surface of i-Al-Pd-Mn. Even though features in the image appear to be arranged randomly, self-similar features are separated by distinct distances. The distribution of such distances is compatible with the separations between pseudo-Mackay icosahedra tangent to the topmost layer, and with separations between other cluster-based units. We propose that the fine structure is due to electronic structure imposed by the clusters.

Temperature-induced phase transition observed at a fivefold icosahedral quasicrystalline AlPdMn surface

A switch in symmetry from fivefold to tenfold has been observed at the surface of a monograin icosahedral AlPdMn quasicrystal cut perpendicularly to a fivefold axis as a function of annealing temperature and/or annealing time at a constant temperature. A continuous change in stoichiometry was simultaneously monitored by X-ray photoelectron spectroscopy. The surface structure study was performed using full-hemispherical X-ray photoelectron diffraction and low-energy electron diffraction.

Surface oxidation of a quasicrystalline Al–Cu–Fe alloy:No effect of surface orientation and grain boundaries onthe final state

We have used x-ray photoelectron spectroscopy and Auger electron spectroscopy to examine the characteristics of oxides on two types of quasicrystalline Al–Cu–Fe samples. One type was formed by consolidation of powders, resulting in multiple grains with random surface orientations. The other was a single grain, oriented to expose a fivefold surface. Both were oxidized to saturation in a variety of environments at room temperature. We measured the elemental constituents that oxidized, the extent of oxygen-induced Al segregation, and the depth of the oxide. Under the conditions of our experiments, there was little, if any, significant difference between the two types of samples. Hence, surface orientation and bulk microstructure played little or no role on the final state of the oxide under these conditions.

Structural study of the five-fold surface of the Al 70Pd 21Mn 9 quasicrystal

We report an investigation of the five-fold surface of the Al 70Pd 21Mn 9 quasicrystal using STM, LEED and AES. Surfaces sputtered and annealed to 875 K showed excellent five-fold symmetric LEED patterns, and STM revealed cluster-like protrusions. Surfaces sputtered and annealed to 1050 K also showed sharp LEED patterns but here STM showed flat surfaces which could be imaged with atomic resolution. AES indicated these annealed surfaces were Mn deficient. Threshold analysis of images of the flat surfaces shows that the surface can be generated by tiling with pentagons. Large protrusions on the flat surfaces were found where the pentagonal tiling breaks down. Angles between these protrusions were found to match angles present in the stereographic projection of the icosahedral group m 35.

Characterization of the five-fold plane surface of an Al 70.4Pd 21.4Mn 8.2 quasi-crystal by means of surface X-ray diffraction

The surface of a five-fold symmetry quasi-crystal of a thermodynamically stable icosahedral Al 70.4Pd 21.4Mn 8.2 single grain has been studied by means of X-ray diffraction. The calculated intensity reproduces rather well both the Bragg peaks and surface signal. The fit of the measured structure factor shows a contraction of the outermost atomic layer. The system also tends to be richer in Al and Pd, the atoms forming a very dense outer layer.