Quasicrystalline Surface Research Articles

Evolution of Topological Order in Xe Films on a Quasicrystal Surface

We report results of the first computer simulation studies of a physically adsorbed gas on a quasicrystalline surface Xe on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using a semiempirical gas-surface interaction, based on conventional combining rules, and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. The evolution of the bulk film begins in the second layer, while the low coverage behavior is epitaxial. This transition from epitaxial fivefold to bulklike sixfold ordering is temperature dependent, occurring earlier (at lower coverage) for the higher temperatures.

Oxidation and adhesion on the quasicrystalline AlPdMn surface studied by nanolithography

We present the novel application of a relatively new technique (nanolithography) to the study of quasicrystalline surface oxidation. The 5-fold surface of an AlPdMn alloy was oxidized using a metallized AFM tip. The electrochemical nature of this process was confirmed by investigating the influence of humidity and polarity of the applied voltage on the quasicrystalline oxide. Oxides of different thickness and adhesive properties were created by altering the applied voltage and the humidity during the lithographic process. The technique can be used in an exhaustive study of properties of the various types of oxides that form on the AlPdMn surface and the preliminary results of one such study are reported.

Diffraction from one- and two-dimensional quasicrystalline gratings

The diffraction from one- and two-dimensional aperiodic structures is studied by using Fibonacci and other aperiodic gratings produced by several methods. By examining the laser diffraction patterns obtained from these gratings, the effects of aperiodic order on the diffraction pattern was observed and compared to the diffraction from real quasicrystalline surfaces. The correspondence between diffraction patterns from two-dimensional gratings and from real surfaces is demonstrated.

Absorption kinetics of thin Al films on quasicrystalline AlPdMn

In order to study the thermodynamical stability of a crystalline overlayer on a quasicrystalline surface, thin Al films are deposited on the five- and three-fold-symmetry surface of the Al 70Pd 20Mn 10 quasicrystal. Subsequently, the decrease of the film thickness is monitored as a function of time at different substrate temperatures using Auger electron spectroscopy. We find that diffusion of Al into the substrate is negligible below roughly 280 K and 300 K for the two investigated surfaces, respectively. For higher temperatures, Arrhenius parameters are extracted from the diffusion data which suggest that Al absorption is mediated by a fast vacancy-diffusion process.

Initial oxidation of quasicrystals – an investigation by high-resolution RBS and ERDA

Surfaces of icosahedral AlPdMn with 5-fold orientation can be prepared either to be fully icosahedral or to form a cubic overlayer. When decagonal AlNiCo is cut perpendicular to the 10-fold axis, a fully quasicrystalline surface is obtained, however a ‘mixed-type’ (quasicrystalline-periodic) surface is obtained, when cut parallel to this axis. A comparison of the oxidation behavior of these different surfaces should therefore shed light on the suggestion by Rivier [N. Rivier, New Horizons in Quasicrystals: Research and Applications, in: A.I. Goldman, D.J. Sordelet, P.A. Thiel, and J.M. Dubois (Eds.), World Scientific, Singapore, 1997, p. 188] that quasicrystals are more corrosion resistant than their crystalline counterparts. The initial oxidation of the surfaces was studied by measuring oxygen depth profiles with high-resolution ERDA. The paper also reports about the preparation of the surfaces and the measurement of highly resolved depth profiles of the metallic components by RBS during the preparation. In both cases a high-resolution electrostatic spectrometer was used for the energy analysis. The results are in favor of an enhanced oxidation resistance of quasicrystalline surfaces.

Friction and fretting on quasicrystals under vacuum

In order to investigate friction and fretting on quasicrystals at room temperature in a reliable manner, we have subjected sintered samples to tests in a vacuum chamber evacuated to a residual pressure lower than 10 −5 mbar. Pin-on-disc tests and fretting tests were ran using hard steel spherical counterparts. Experiments in vacuum reveal that the variation of the coefficient of friction with cumulated sliding distance is very different when one considers Cr-hard steel riding on alumina, on a (true) quasicrystal or on a high-carbon steel. Sharp minima in friction coefficients and adhesive forces occur when the quasicrystalline surface is loaded in contact with a metallic surface. The characteristic friction values measured in high vacuum are smaller than in ambient atmosphere by at least a factor 2, thus indicating that the oxide layer is involved to a significant extent in determining the tribological mechanisms. The discussion focuses on an interpretation of the differences observed between samples, based on their chemical composition and possible bonding to steel. For the first time, an estimate of the true surface energy of a quasicrystal is derived from measurements.

Electronic excitations at the threefold-symmetric surface of the quasicrystal Al–Pd–Mn

In the search for the electronic origin of the peculiar magnetic behavior of the Al 70Pd 20Mn 10 quasicrystal, we have performed electron energy-loss spectroscopy measurements on a surface, oriented perpendicular to one of the threefold-symmetry directions of this icosahedral alloy. Two different surface structures, a cubic and a quasicrystalline surface phase, are induced by Ar + ion bombardment and annealing on the same bulk quasicrystal. Comparative measurements on these surfaces indicate that the 3p–3d transition in the quasicrystal is so weak that it is effectively masked by plasmon excitations. This transition would disappear for a fully occupied 3d band and, for this case, local magnetic moments at the Mn site would be quenched.

Debye temperature of the pentagonal surface of the quasicrystal Al–Pd–Mn

The temperature dependence of the specular-beam intensity in low-energy electron diffraction is determined at the fivefold-symmetry surface of the icosahedral quasicrystal Al–Pd–Mn. The logarithm of the relative intensity is linear in temperature up to 570 K, suggesting the applicability of the standard Debye theory also to quasicrystalline surfaces. First, the temperature dependence of the Bragg-peak energy is measured by recording I– V spectra at different temperatures. The resulting shift to lower energies is accounted for mainly by the lattice expansion perpendicular to the surface. A value of about 0.8 eV is found in the temperature range considered here and included in the analysis. As a result of several measurements, a Debye temperature of 298±7 K is found at 43 eV beam energy. This quantity is a measure for the atomic mean-square vibrational amplitude 〈 u 2〉 normal to the surface. The value associated with the pentagonal surface is very close to the bulk value of 312 K, determined in X-ray diffraction.

Pseudomorphic starfish: nucleation of extrinsic metal atoms on a quasicrystalline substrate

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.

Nanostructures formations, electrical transitions and implanted impurity migration in α-alumina modified by Ti ion beam assisted W deposition and subsequent thermal annealing

Concentration depth profiles of implanted impurities and surface microstructure of modified polycrystalline α-Al 2O 3 were studied in the vicinity of the post-implantation annealing temperature of 1380 °C where a transition to low-resistance state occurs. The modification procedure was Ti ion implantation with simultaneous deposition of W (IBAD) and subsequent isochronal annealing in vacuum at temperatures ranging from 900 to 1500 °C. Direct metal vapor vacuum arc pulsed implantation of Ti ions (50–150 keV, 10 17 ions per cm 2, RT) was used. This treatment of ceramics is of interest for both basic and applied research. On the scientific side, it is intended to elucidate the correlation between the physical properties of ceramics and their nanostructure. From the point of view of the applications, this treatment is promising for production of low-cost, high ohmic thin film resistors and advanced gradient thermally stable metal/ceramic interfaces. Rutherford backscattering spectrometry (RBS) of 4He + ions with initial energy of 1.8 MeV was used to determine the concentration depth profiles of the implanted impurity in the Al 2O 3 matrix. Scanning electron microscopy was applied to surface microstructure studies of the ceramics. RBS spectra were measured from the alumina samples annealed at temperatures corresponding to the maximum of the sheet resistivity–annealing temperature curve. The effective diffusion coefficient of implanted Ti in Al 2O 3 was found to be 2.43×10 −14 cm 2 s −1 for a temperature range of 1150–1230 °C. Nanocrystalline grains with a typical diameter of approximately 40–50 nm were formed at all post-implantation annealing temperatures. These nanocrystallites agglomerate into larger particles, the latter forming a quasi-crystalline surface structure at approximately 1150 °C. The transition to the low resistance electric state in Al 2O 3 at the post-implantation annealing temperature of approximately 1380 °C occurs simultaneously with a redistribution of the implanted Ti ions towards the near-surface layer and with a formation of a 2D honeycomb (average structure unit is 330 nm in length) nanostructure like an infinite cluster. The grain growth depression observed at temperatures near or above 1150 °C is consistent with the assumption that a diffusion barrier of the W and Ti constituents is formed on the nanocrystallite surfaces.

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.

Epitaxial film growth on quasiperiodic surfaces

We performed depositions of noble metals, such as Pt and Ag, onto quasicrystalline surfaces in order to investigate the nature of film growth on quasiperiodic surfaces. Pt depositions onto the ten-fold quasiperiodic surface of decagonal Al–Ni–Co quasicrystal yielded an epitaxial layer of multiply twinned PtAl 2 crystals with the aid of predeposited In, which acted as a surfactant. This is quite similar to the case of Au depositions on the ten-fold surface, where an epitaxial layer of multiply twinned AuAl 2 crystals was formed. In the case of Ag deposition, no epitaxial films were formed by similar procedures even under the existence of an In layer.

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.

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.

High resolution photoemission spectroscopy study near the Fermi edge for different surfaces prepared on the icosahedral AlPdMn phase

High resolution photoemission measurements performed at low temperatures on a single-grained sample of the AlPdMn icosahedral phase show that the density of states N(E) strongly depends on the nature of the surface. For an ordered quasicrystalline surface, prepared by Ar etching and ultra high vacuum annealing, a dip feature is observed in N(E) near the Fermi level, which energy dependence can be analyzed with a simple square-root power law. By contrast, N(E) varies only little with energy both for a disordered surface and a crystalline surface of the same sample. A sharp Fermi edge is then clearly observed. This shows that the metallic character of the surface of a quasicrystal is strongly reduced when the surface presents a quasicrystalline ordering.

C 60 adsorption on the quasicrystalline surface of Al 70Pd 21Mn 9

Room temperature adsorption of C 60 on the flat quasicrystalline surface of Al 70Pd 21Mn 9 has been investigated using scanning tunnelling microscopy. A dispersed overlayer is formed at low coverage, with avoidance of step-edges. There is no evidence of island formation or clustering. As the coverage is increased, a higher density layer is formed with no evidence of the formation of hexagonal ordered adsorbate structures seen on other substrates. This is followed by the onset of second layer formation. A range of bonding sites for C 60 molecules is implied from measurements of apparent molecular heights and from thermal effects. Detailed analysis of the surface at a low coverage (∼0.065 ML) provides evidence of adsorbate local order, with Fibonacci ( τ-scaling) relationships between the C 60 molecules. Where this occurs, the preferred adsorption site is tentatively identified as the pentagonal hollow. These local correlations however are not found to extend over larger regions of the surface.

High resolution experiment on the electronic density of states in icosahedral-Al–Pd–Mn

We present experiments of photoemission and tunneling spectroscopy which indicate the presence of a dip in the density of states around the Fermi level EF in the icosahedral-Al–Pd–Mn (i-Al–Pd–Mn) phase. From high resolution photoemission experiments on a single grain of i-Al–Pd–Mn, we have got clear evidence that the surface state (quasicrystalline or disordered) strongly affects the measured density of states. For a quasicrystalline surface we observe a pseudo-gap feature at EF. The remaining density of states at EF is compatible with the γ term of specific heat. The band edge shows a square root dependence in the vicinity of EF. This is similar to the single square root anomaly found by tunneling spectroscopy around EF in the range ±300meV. The photoemission data show a filling of this feature for the disordered surface where a Fermi edge clearly develops.

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.

Electronic structure of quasicrystalline surfaces: Effects of surface preparation and bulk structure

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 ${2p}_{3/2}$ 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 ${2p}_{3/2}$ 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 than the surface obtained by fracturing at room temperature.