CdTe and PbTe nanostructures on the oxidized pentagonal surface of an icosahedral AlPdMn quasicrystal

By means of congruent evaporation, we have deposited CdTe and PbTe onto the oxidized fivefold-symmetry surface of an icosahedral AlPdMn quasicrystal. This procedure results in the formation of nanocrystals in both cases. While the azimuthal orientations of the crystallites are random, the polar orientations are well defined. The crystalline CdTe and PbTe domains expose their (1 1 1) and (0 0 1) faces, respectively, which are aligned parallel to the pentagonal surface of the quasicrystal. The nanometric size of the domains is not a result of the lattice mismatch between the growing film and the substrate as usually observed in molecular-beam epitaxy, but of the limited size of the oxide domains of the substrate surface.     

Stabilization of the pentagonal surface of the icosahedral AlPdMn quasicrystal by controlled Si absorption

The Debye-temperature of the pentagonal surface of the icosahedral AlPdMn quasicrystal (QC) is measured by means of low-energy electron diffraction after the absorption of different amounts of Si. We observe an increase of the surface Debye-temperature from 300±7 K for the freshly prepared surface to 330±7 K after the absorption of 60-Å Si. Because the quasicrystalline order persists at the surface in spite of the diffusion of Si into the substrate, we suggest that the diffusion is dominated by a vacancy-mediated process.     

Charge density determination in icosahedral AlPdMn quasicrystal using quantitative convergent beam electron diffraction

Charge density distribution in icosahedral AlPdMn quasicrystal has been studied on a single-crystal specimen by using quantitative convergent beam electron diffraction (QCBED) technique. The QCBED systematic row method was used in the refinement of structure factors. To refine the low-order structure factors, the wave-mechanical formulation of electron diffraction dynamical theory was used in the calculation of electron diffraction intensities for the quasicrystal in fitting the experimental intensity line scan profiles. The shapes of atomic surfaces (occupation domains) were described with symmetry-adapted series of surface harmonics. An iterative procedure was used in determination of structure factors of the quasicrystal. The structure factors of nine strongest symmetry inequivalent reflections according to X-ray diffraction experiment were refined with QCBED technique. The average of refinement results for a given reflection performed on several CBED patterns, which were slightly different in orientation and sample thickness, and on different line scans, was taken as the value of structure factor for the reflection. The obtained structure factors for electrons were transformed into X-ray structure factors with Mott formula. The bonding charge density map for the quasicrystal was constructed with the obtained nine structure factors. Assuming that the atoms are spheres, the gain or loss of electrons for different atoms were calculated. It shows that identical atoms can have different valences at different kinds of positions. The bonding charge is localized along certain directions.     

Ni deposition on the pentagonal surface of an icosahedral Al-Pd-Mn quasicrystal

We have evaporated Ni on the pentagonal surface of an icosahedral Al-Pd-Mn quasicrystal kept at room temperature. At the initial stage of growth, Ni intermixes with the substrate surface. Subsequently, Al from the quasicrystal matrix migrates to growing layers. The modified chemical composition in an initially icosahedral region near the surface induces a structural transformation. An Al-Pd-Mn alloy is formed which consists of five cubic domains with dimensions in the nm-range exposing their (1 1 0) faces parallel to the surface. These domains are azimuthally rotated by 2π/5 with respect to each other and aligned with symmetry directions of the icosahedral substrate. Al-Mn-Ni, Al-Ni, and Ni overlayers adopt both structure and orientation of these domains which stabilises Ni in a novel body-centred cubic phase. Ni-rich overlayers exhibit out-of-plane magnetic ordering.     

Planar and cluster structure of icosahedral quasicrystals

Twofold-, threefold-, and fivefold-symmetry elements are observed in the near-surface region of the quasicrystal Al₇₀Pd₂₀Mn₁₀ using a real-space imaging technique based on secondary-electron emission. The observed icosahedral point-group symmetry implies the presence of atomic clusters within the analyzed region of the solid. The same surface produces perfect low-energy electron diffraction patterns typical for well-defined crystallographic planes. We present a model which reconciles this dual structural nature of quasicrystals. Our model is constructed from an icosahedral seed followed by concentric symmetry-preserving “growth” to form the macroscopic solid. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported in part by the Schweizerischer Nationalfonds.     

Electron spectrum and wave functions of icosahedral quasicrystals

Electron spectra and wave functions of icosahedral quasicrystals have been investigated in the tight-binding approximation using the two-fragment structural model (the Amman-MacKay network) with “central” decoration. A quasicrystal has been considered as a limiting structure in a set of optimal cubic approximants with increasing lattice constants. The method of level statistics indicates that the energy spectrum of an icosahedral quasicrystal contains a singular (nonsmooth) component. The density of electron states has been calculated for the first four optimal cubic approximants of the icosahedral quasicrystal, and the respective Lebesgue measures of energy spectra of these approximants have been obtained. Unlike the case of a one-dimensional quasiperiodic structure, the energy spectrum of an icosahedral quasicrystal does not contain a hierarchical gap structure typical of the Cantor set of measure zero in a one-dimensional quasicrystal. Localization of wave functions in an icosahedral quasicrystal has been studied, and their “critical” behavior has been detected. The effect of disorder due to substitutional impurities on electron properties of icosahedral quasicrystals has been investigated. This disorder makes the electron spectrum “smoother” and leads to a tendency to localization of wave functions. Zh. éksp. Teor. Fiz. 113, 1009–1025 (March 1998)     

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. Received 19 February 2000 and Received in final form 6 November 2000     

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.     

High-temperature surface oxidation of the decagonal AlCoNi quasicrystal

The interaction of oxygen with the 10-fold-symmetry surface of the decagonal Al_72.9Co_16.7Ni_10.4 quasicrystal at high temperatures was investigated by low-energy-electron diffraction and Auger electron spectroscopy. The results are consistent with a well-ordered aluminum-oxide layer possessing a hexagonal antiphase domain structure with a limited lateral size of about 35 Å. We claim that the separation distances of the domain boundaries, separating domains of equal orientation, are primarily a consequence of the preferential cluster nucleation on decagonal Al-Co-Ni. The domains are azimuthally oriented along one direction of the two sets of five twofold-symmetry axes lying on the decagonal surface in accordance with the local symmetry of the quasicrystal surface, while the size of the domains can be explained in terms of self-size-selecting arguments.     

Structure of the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal: experimental evidence of bulk truncations at larger interlayer spacings

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 are separated by larger interlayer spacings (gaps). Both step-height distribution and high resolution scanning tunneling microscopy images on terraces reveal bulk truncations at larger gaps.     

Structure of dislocations and stacking faults in the complex intermetallic ξ′-(Al–Pd–Mn) phase

We present the results of an electron microscopy study of defects in plastically deformed single crystals of the intermetallic ξ^′-(Al–Pd–Mn) phase. Pure edge dislocations with two different Burgers vector directions and four different Burgers vector magnitudes were found. All Burgers vector magnitudes observed can be described in terms of irrational fractions of the unit-cell parameters, and we have observed Burgers vector directions that can be indexed using irrational indices. The stacking faults observed have displacement vectors whose magnitudes and directions are incompatible with the unit cell of the ξ^′ phase. A comparison of the Burgers vectors observed in this study with those reported for the corresponding icosahedral quasicrystal shows that they are equivalent with respect to their directions and lengths. This leads to the conclusion that local order rather than long-range periodic (or quasiperiodic) order governs the structure of defects in these materials.     

Step structure on the fivefold Al-Pd-Mn quasicrystal surface, and on related surfaces

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 quasicrystal-crystal interface between icosahedral Al-Pd-Mn and deposited Co: Evidence for the affinity of the quasicrystal structure to the CsCl structure

We have evaporated Co on the pentagonal surface of an icosahedral Al-Pd-Mn quasicrystal kept at room temperature. For submonolayer Co deposits, five AlCo domains are formed exposing their (1 1 0) faces parallel to the surface and rotated by 72° with respect to each other. The orientational relationship between these domains and the substrate is determined by the optimum matching of the average quasicrystal structure and the CsCl structure with a lattice constant of about 2.88 Å. For further deposition, body-centred cubic Co domains grow epitaxially on the AlCo domains.     

An electronmicroscopic study of hexagonal phases related to quasicrystals in Zn-Mg-RE alloys

A number of hexagonal phases with inflated/deflated a lattice parameters but equal c parameters are found to exist with compositions close to that of the icosahedral phase. Surprisingly, no cubic or orthorhombic approximants are found. The closeness of the hexagonal phases to the icosahedral phase is shown through the electron diffraction features. Through stereographic analysis of the hexagonal phases it is shown that they consist of three icosahedral units rotated by 120 degrees about a common twofold axis, forming the sixfold axis. It is suggested that this phase falls into a class of crystals closely related to both the icosahedral and decagonal quasicrystals, but are not approximants to them. These phases can instead be considered to be approximant to the hexagonal quasicrystal.     

Anisotropic nonlinear elastic properties of an icosahedral quasicrystal

We show that the nonlinear behavior of transverse acoustic waves can reveal the anisotropic structure of an icosahedral quasicrystal, at the macroscopic level. We report experiments performed in i-Al-Pd-Mn. We observe that a primary transverse acoustic wave can generate a second harmonic transverse acoustic wave. We also observe a specific relation between the polarization directions of those waves. These observations are manifestations on a macroscopic scale of the long-range order in quasicrystals.     

Magnetic properties of a monocrystalline quasicrystal in the Al-Pd-Mn system

Magnetic susceptibility of a monocrystalline icosahedral Al_70.2Pd_21.3Mn_8.5 quasicrystal was measured over the temperature range from 4 to 1100 K. The susceptibility was found to include the temperature-independent diamagnetic contribution, the temperature-dependent Curie’s contribution, and the contribution from the Pauli paramagnetism of an electron system with energy gap. An analysis of the low-temperature susceptibility revealed the presence of about 0.008% of ions with magnetic moment 4µ_B in the quasicrystal at 4 K. It is assumed that the ions with uncompensated magnetic moments appear near the structural vacancies in the quasicrystal lattice. The energy gap between the valence and conduction bands is estimated at Δ=0.64 eV, and the effective mass of charge carriers is equal to approximately 70 electron masses.     

Transition temperature to crystal phase of Al86Mn14 quasicrystal ultrafine particles determined by direct observation and characterization of surface oxide layer

Al–Mn quasicrystal ultrafine particles can be produced by the advanced gas evaporation method (AGEM), which is a method of preparing ultrafine alloy particles by coalescence growth among the particles near the evaporation sources. We investigated the phase transition temperature from a quasicrystal to a stable crystal, by examining successive electron diffraction patterns of an ultrafine particle in an in situ experiment using a transmission electron microscope. In spite of the report that the Al₈₆Mn₁₄ quasicrystal transforms into the crystal phase at around 400–670 °C on thin film specimens, the quasicrystal ultrafine particle transformed at 800 °C, i.e., the quasicrystal ultrafine particle is more stable. Since the cross-sectional view of the surface oxide layer of the quasicrystal ultrafine particles can be easily observed, the surface oxides of η-Al₂O₃ and MnO were characterized as a result of the oxidation of residual atoms on the surface of the produced alloy particles including the quasicrystals. The conditions required for Al–Mn quasicrystal ultrafine particle formation by the AGEM can be estimated under the cooling rate of 10⁵ K/s.     

Microstructure of a composite with a quasicrystalline phase fraction obtained by directional solidification of Al61Cu27Fe12 alloy

The microstructure of a composite containing a quasicrystal phase, i.e. so-called crystal–quasicrystal (CQ) composite, was studied. The CQ composite was obtained by the Bridgman method via solidification of Al₆₁Cu₂₇Fe₁₂ alloy (numbers indicate at%). The process was conducted at a pull out rate of v = 0.07 mm/min. The average temperature gradient in the heating zone was 43 K/cm. The composite matrix consisted of cubic β phase Al(Fe, Cu), with reinforcement of λ-phase rod-shaped fibres surrounded by a quasicrystal icosahedral ψ phase, which also existed in the fibre core. The fibres were rhomboidal in cross-section. The composite was studied using X-ray and electron diffraction, light-optical and scanning electron microscopy (SEM), X-ray topography and Laue diffraction.     

The surface structure of icosahedral Al68Pd23Mn9 measured by STM and LEED

For the first time the surface structure of an icosahedral quasicrystal has been successfully investigated in Ultra-High Vacuum (UHV) by Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED). After cleaning an i-Al₆₈Pd₂₃Mn₉ sample in UHV by cycles of ion-sputtering and annealing at temperatures close to the melting point atomically flat terraces are observed by STM. Successive step heights show quasiperiodic order according to the Fibonacci chain. The normals of these terraces are parallel to a five-fold axis as revealed by highly resolved STM images. On the terraces five-fold stars and pentagonal holes are observed. Their orientation is the same on all terraces investigated. Additionally to this long-range orientational order, a high degree of quasiperiodic order is found for the pentagonal holes. This indicates that the quasiperiodic order of this highly ordered and thermodynamically stable quasicrystal extends even up to the surface. Both the step heights and the distances between the pentagonal holes well agree with the values derived from a structural model of this material. Five-fold symmetric LEED patterns can be analysed by means of the Fourier transform of a Fibonacci pentagrid as suggested by the STM data. The analysis yields the same line separations within the pentagrid as deduced from the STM experiments.     

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.