Surface oxidation and thin film preparation of AlCuFe quasicrystals

This paper reviews the work carried out over the last decade by the Nancy team on surface oxidation and thin film preparation of AlCuFe icosahedral quasicrystals. After discussing the problems linked with the surfaces of these quasicrystals, this review addresses the issue of the preparation of quasicrystalline surfaces and the first steps of oxidation under very low pressure of oxygen. This paper compares the nucleation and growth of oxide on i-AlCuFe quasicrystal and on a classical crystalline phase of this alloy: ω-AlCuFe. Aluminium diffusion is studied through the aluminium segregation on the surface that occurs during exposure to oxygen. Some surface properties of quasicrystals are reviewed with regard to oxidation. The evolution of physical parameters such as surface energy, friction coefficient and optical emissivity is described. This review also deals with the preparation of i-AlCuFe thin films. Two protocols to make i-AlCuFe thin films with free surfaces are described and discussed. The mechanical resistance and the tribological behaviour of these thin films, the oxygen and carbon influence on the final crystalline structure, and the quasicrystallization kinetics are presented.     

Oxidative and tribological properties of amorphous and quasicrystalline approximant Al-Cu-Fe thin films

The origins of the tribological properties and corrosion resistance of amorphous and quasicrystalline approximant alloys have been studied by comparing their properties in thin Al-Cu-Fe alloy films with compositions lying near the quasicrystalline region of the ternary compositional phase diagram. Six sputtered thin films of an Al-Cu-Fe alloy were studied using X-ray diffraction, X-ray photoemission spectroscopy (XPS), and an in situ ultrahigh vacuum (UHV) tribometer. The films were annealed in UHV to induce the formation of orthorhombic, rhombohedral, and amorphous bulk structures. The properties of these thin films were then determined in the same UHV apparatus without exposing the films to air. The rates of surface oxidation by H2O and O2 were measured using XPS. Although the oxidation rates and oxide thicknesses were dependent on the oxidant, they were not sensitive to the structures of the films. Friction was measured between identical samples in sliding contact. The friction coefficients (micros = 0.36 +/- 0.11 to 0.56 +/- 0.08) were comparable to those observed in other experiments using quasicrystals and approximants in UHV; however, there was no strong correlation between the friction coefficients and either the film structure or the degree of surface oxidation. These results suggest that the tribological and corrosion resistance properties of these quasicrystalline approximant alloys are not directly connected to crystalline structure.     

Toward the discovery of new soft quasicrystals: From a numerical study viewpoint

This is a progress review of an emerging research front: soft quasicrystals including liquid crystalline dendrons, nanoparticles, mesoporous silica, colloids, ABC star and linear terpolymers, and even water and silicon. As an aid to readers, we explain the basics of quasicrystals developed in solid‐state physics: orders in quasicrystals, higher dimensional crystallography, approximants, phason randomness, and the origin of quasicrystal formation. Then we review some numerical studies from early to recent ones. Our main purpose is to elucidate how to construct quasicrystalline structures: The introduction of additional components or a new length‐scale is the key to discover new quasicrystals. As a case study, we describe our recent studies on ABC star terpolymer systems and present the results of simulations of dodecagonal polymeric quasicrystals. In the case of dodecagonal quasicrystals, one easily finds that the key is to search square‐triangle tiling structures with changing components. Application to photonic quasicrystals is reviewed as well. Our hope is that this review will contribute furthering quasicrystal chemistry. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Structure and oxidation at quasicrystal surfaces

We have investigated the atomic and electronic structure, chemical composition, and oxidation characteristics of the surfaces of icosahedral, Al-rich quasicrystals, using a variety of surface-sensitive techniques (LEED, XPS, STM, AES). We have systematically investigated the way that these traits vary with preparation conditions (e.g. sputtering and then annealing to various temperatures, vs. fracture), with surface symmetry (e.g. 2f vs. 3f vs. 5f surfaces), and with bulk composition (e.g. i-Al–Pd–Mn vs. i-Al–Cu–Fe). We have also compared our results for the quasicrystals with results for crystalline approximants and other related crystalline phases. Our main conclusions are that, under specific conditions of sputter-annealing, the bulk atomic and electronic structures of the clean quasicrystal propagate to the surface. Also, the oxidation chemistry is dominated by that of the primary constituent, aluminum.     

Growth modes of organic semiconductor thin films using organic molecular beam deposition: epitaxy,van der Waals epitaxy,and quasi-epirtaxy

We compare the ultrahigh vacuum growth of several different archetype organic molecular compounds on a variety of substrates, and find that the resulting thin films can form epitaxial, van der Waals epitaxial (vdWE) or quasi-epitaxial (QE) structures. Epitaxial and vdWE films form lattices commensurate with that of the substrate, where epitaxy refers to chemisorbed films while vdWE refers to purely vdW-bonded, physisorbed films. In contrast, QE films are incommensurate with the substrate lattice. In spite of this incommensurability, QE films can exhibit long range order, and have a unique orientational relationship with the substrate lattice. Quasi-epitaxial growth can result in strained structures, where the strain energy is relieved by variations in the internal lattice degrees of freedom. It has been found that QE growth can thus result in stable, quasi-equilibrium film structures which are different than the bulk structure of that same material. Organic thin films such as the phthalocyanines, 3,4,9,10-perylenetetracar☐ylic dianhydride, and the fullerenes grown on graphite, alkali halide crystals, Au, semiconductors and other substrates are all discussed.     

Vitrification and structural differences between metal glass,quasicrystal, and Frank-Kasper phases

The concept of icosahedral short-range order is extended to metallic glass, quasicrystal, and Frank-Kasper phases. The cluster model, together with the theory of local structural fluctuations, explains the static elasticity of glass, which distinguishes glass from liquid. An elastic peak of the dynamic structural factor indicates the possibility of transverse mode propagation in glass. As opposed to dislocations and disclinations in crystals, those in glass are artificially introduced defects, which serve as easily perceptible structural models. Thermodynamic relaxation theory may only be used for limited groups of vitrifying compounds the same applies to representation of vitrification as the second-order phase transition. The structure of real quasicrystals may not be adequately represented by Penrose tiling even after its decoration. This is associated with packing defects, inclusions of other phases, and chemical inhomogeneities. Quasicrystals have specific defects in an icosahedrally coordinated network of bonds, which distinguish them from Frank-Kasper phases. Criteria for isolating physically realizable Penrose tiling from all possible mosaics of this type are suggested. Structural distortions that transfer the diffraction rings of quasicrystalline samples into ellipses are explicable even in a linear approximation for the stress field created by a phason. The term “long-range order” seems to be wrong even for ordinary crystals. For quasicrystals, the notion of “rotational” order is more pertinent. Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 138–158, January–February, 1996. Translated by L. Smolina     

DEDUCTION OF QUASILATTICE WITH FIVE-FOLD SYMMETRY AND PARTICLE FRACTAL STRUCTURE MODEL

In the structure of quasicrystal, the coordination icosahedron has long ordering but no translation ordering. The author dealt with the building principle ofquasicrystal and thought that two principles played a certain role in the quasicrystal structure, i.e. the icosahedron principle and the golden mean principle. We obtained the most simple.structure model of quasicrystals, and could explain all details of the high-resolution electron microscopic image of the A1-Mn quasicrystal based on the two principles. The author's model has the characteristic of fractal structure, therefore, we call it the particle fractal structure madeh The author has made a systematic deduction of quasicrystal point group, forms, possible type of quasicrystal lattice.     

Importance of bulk properties in the structure and evolution of cleavage surfaces of quasicrystals

The present work reviews the recent achievements in probing bulk properties of quasicrystals by using cleavage surfaces and surface sensitive techniques. In particular, it is shown that the cluster–subcluster-based structure of the cleavage surface of icosahedral Al–Pd–Mn quasicrystals can be related to the presence of stable atom clusters in the bulk, which force the crack front to circumvent them. Furthermore, by subjecting cleavage surfaces of differently pre-annealed Al–Pd–Mn quasicrystals to a post-cleavage heat treatment, we demonstrate that bulk vacancies migrate toward the surface, where they initiate structure and composition changes. These studies allow us to characterize Al–Pd–Mn quasicrystals with respect to their bulk vacancy concentration. As-grown Al–Pd–Mn quasicrystals are found to contain a supersaturation of all chemical species of vacancies in near stoichiometric composition, whereas long term pre-annealed material has a much lower, and predominantly Al, vacancy concentration. Analogous experiments for decagonal Al–Ni–Co quasicrystals show that as-grown Al–Ni–Co has a lower vacancy concentration than as-grown Al–Pd–Mn.     

Liquid quasicrystals

Recent discovery of Liquid crystals with quasicrystalline symmetry (liquid quasicrystals), is reviewed. These liquid quasicrystals were created by self-organised packing of supramolecular micelles, analogous to the way by which normal quasicrystals are formed from atoms in metal alloys. This opens the way to obtaining self-assembled photonic band gap quasicrystals, and has important implications for the study of both quasicrystals and supramolecular self-assembly.     

Dimension of the Gibbs function topological manifold: 3. Configuration entropy determined by the isotopic composition of binary quasicrystals

Even though the graph representation of thermodynamic states rules out the existence of stable binary quasicrystals, Tsai’s research team was able to discover two such systems in 2000. Our previous article showed that in terms of isotopic composition, Tsai’s two quasicrystals, as well as several other ones that have been discovered since 2000, are pseudobinary. Such systems owe their stability to high configuration entropy, which is determined by their isotopic composition. In this article, we shall use the basic methods of statistical thermodynamics to show that configuration entropy for systems without symmetry (including translational symmetry) is always higher than the entropy of their symmetrical counterparts. Thermodynamic calculations for a model of a 1-D binary Fibonacci quasicrystal, \(\hbox {AB}_{\uptau }\), indicate that the internal energy of this system is the same as the internal energy of its approximant, \(\hbox {AB}_{\mathrm{t}}(t\approx \uptau \)), i.e., the crystalline counterpart of the quasicrystal. We shall show that assuming Element A in both systems is composed of two isotopes, the configuration entropy corresponding to a single atom of the quasicrystal is about 20 % higher than the entropy of its crystalline approximant. This article also includes calculations of the configuration entropies of the few pseudobinary quasicrystals that have been synthesised so far. These entropies are exceptionally high, most often amounting to about 1.5 \(k_{\mathrm{B}}\), due to a large number of isotopic components of these quasicrystals. It in unknown whether such a high value is sufficient to ensure the stability of the quasicrystalline phase compared to the crystalline phase, as the number of configurations of the approximant required for calculations is astronomical.     

Electrical and thermal transport properties of icosahedral and decagonal quasicrystals

Electrical and thermal transport properties of quasicrystals are reviewed on the examples of i-Ag-In-Yb and i-Al-Cu-Fe icosahedral phases and d-Al-Co-Ni decagonal phase. Using samples of single-grain morphology and high structural quality, and performing the measurements along well-defined crystallographic directions, the following basic questions in the context of physical properties of quasicrystals are addressed, both experimentally and theoretically: (1) are the unusual transport properties of quasicrystals introduced by the quasiperiodicity of the structure or are they a consequence of complex local atomic order with no direct relationship to the quasiperiodicity; (2) what is the role of the electronic structure of quasicrystals in their electronic transport properties, especially the pseudogap in the electronic density of states in the vicinity of the Fermi energy; (3) what is the anisotropy of the transport coefficients along different crystallographic directions for icosahedral and decagonal quasicrystals and (4) what are the true intrinsic properties of quasicrystalline phases?     

Vanadium oxide surface studies

The vanadium oxides can exist in a range of single and mixed valencies with a large variety of structures. The large diversity of physical and chemical properties that they can thus possess make them technologically important and a rich ground for basic research. Here we assess the present status of the microscopic understanding of the physico-chemical properties of vanadium oxide surfaces. The discussion is restricted to atomically well-defined systems as probed by surface techniques. Following a brief review of the properties of the bulk oxides the electronic and geometric structure of their clean single crystal surfaces and adsorption studies, probing their chemical reactivity, are considered. The review then focuses on the growth and the surface properties of vanadium oxide thin films. This is partitioned into films grown on oxide substrates and those on metal substrates. The interest in the former derives from their importance as supported metal oxide catalysts and the need to understand the two-dimensional overlayer of the so-called “monolayer” catalyst. On the single crystal metal substrates thin oxide layers with high structural order and interesting properties can be prepared. Particular attention is given to ultrathin vanadium oxide layers, so-called nano-layers, where novel phases, stabilised by the substrate, form.     

Origins for epitaxial order of sexiphenyl crystals on muscovite(001)

The epitaxial order of sexiphenyl crystals on muscovite(001) is investigated by x‐ray diffraction, lattice misfit calculations and atomic force microscopy. Depending on the substrate temperature during the thin film growth process, different epitaxial orientations are formed. Sexiphenyl thin films prepared at 370 K preferentially form crystals with the crystallographic (11‐1) planes parallel to the substrate surface while at 434 K a strong fraction of crystals with (11‐2) orientations is grown. The epitaxial orders of sexiphenyl crystals are compared with lattice misfit calculations. The in‐plane order of the {11‐1} crystals can be explained by a point‐on‐line coincidence I, which reveals that the interface is formed by undisturbed crystal surfaces. The epitaxial order of the {11‐2} oriented crystals is characterised by the experimental observation that low indexed crystal directions in the sexiphenyl(11‐2) plane and the muscovite(001) surface coincide with each other, forming a near‐coincidence case. Corrugations of the substrate surface are responsible for this second type of epitaxial order. Characteristic features in the thin film morphology could be correlated to the two observed epitaxial orientations of the sexiphenyl crystals. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of the cubic to hexagonal structure transition in the metastable system TiN-AlN

Structural transitions of metastable Ti_1–xAl_xN coatings on technically relevant substrates were determined as a function of the Ti/Al ratio. Ti_1–xAl_xN films with different Ti/Al ratios were deposited on high speed steel (HSS) substrates at substrate temperatures of 300?° and 500?°C by means of reactive magnetron sputtering ion plating (MSIP). A Ti/Al compound target was used as well as a cluster arrangement of one Ti and one Al target for comparison. The composition of the films was determined by electron probe microanalysis (EPMA), the crystallographic structure by thin film X-ray diffraction (XRD). The analyses revealed that films deposited with Ti/Al ratios of 44/56 and 36/64 had grown in cubic NaCl structure, a film with a Ti/Al ratio of 32/68 was two-phase, and a Ti/Al ratio of 25/75 led to a hexagonal film in wurtzite structure. Only small differences of the lattice parameters could be observed in dependence of temperature: At 300?°C the lattice parameters of the cubic structure corresponded exactly to Vegard‘s law, whereas they slightly decreased in the films deposited at 500?°C. The application of a cluster arrangement instead of a compound target resulted in nearly the same lattice parameters and peak shapes.     

Thin metal films on quasicrystalline surfaces

Thin metal films with a thickness of one or over one monolayer formed on quasicrystalline surfaces were studied using reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, X-ray photoelectron diffraction and scanning tunneling microscopy. The substrates were the 10f surface of d-Al–Ni–Co and the 5f surface of i-Al–Pd–Mn. The metals deposited were Au, Pt, Ag and In. None of these metals forms any ordered layer by deposition onto clean quasicrystalline surfaces. However, if a submonolayer of In is present atop the 10f surface, an epitaxial layer of multiply-twinned AuAl₂ crystals is formed by Au deposition and subsequent annealing. This is also the case for Pt deposition, but not for Ag deposition. Although the surfactant effect of In is also observed in the case of Au deposition on the 5f surface of i-Al–Pd–Mn, the ordered layer formed is a film of Au–Al alloy with icosahedral symmetry. No ordered films are formed by Pt or Ag deposition onto the 5f surface, regardless of the presence of an In-precovered layer. A Sn film monolayer induced by surface diffusion was also studied.     

Characterization of the epoxy-metal interphase: FTIR-ERAS and spectra calculation for ultra-thin films

The interphase between polymers and metal substrates can be elucidated by studying ultra-thin films. In this paper, spin coating films (20 nm - 2 μm) are prepared from a two-part epoxy adhesive on Au, Al and Cu. After room temperature cure, the chemical structure of the epoxy films is studied by FTIR external reflection absorption spectroscopy (FTIR-ERAS) and the results are compared with the state in the polymer bulk as measured by attenuated total reflection (FTIR-ATR). Inevitably, this evaluation of the thin film spectra has to account for the optical situation which is significantly different from bulk measurements. Spectra calculation provides the essential tool for a reasonable comparison between bulk and thin film spectra, thus allowing for a detailed quantitative analysis. Thickness and substrate effects on the interphase can then be separated from the optical situation of the measurement. The results reveal very specific features caused by adhesive interactions and different cure behaviour in the interphase on different metal surfaces.     

Structure and electronic structure of quasicrystal and approximant surfaces: a photoemission study

The structure and electronic structure of different high-symmetry surfaces of either quasicrystalline or approximant Al–Pd–Mn were studied by means of photoemission-based techniques such as X-ray photoelectron diffraction (XPD) and ultraviolet photoelectron spectroscopy. We find that the twofold (2f), 3f and 5f surfaces of icosahedral Al–Pd–Mn exhibit all the symmetry elements of the icosahedral non-crystallographic group. These XPD experiments can be modeled by single-scattering cluster calculations.

The bulk-terminated icosahedral or approximant surfaces are recovered after ion sputtering followed by annealing at T≈500–600 °C. A wealth of ordered surface phases (with different compositions) are found after sputtering and depending on the annealing temperature as, for example, a crystalline bcc multitwinned phase (for T<400 °C) or a stable decagonal quasicrystalline surface (for T>650 °C).

The icosahedral surfaces are characterised by a lowering of the density of states close to the Fermi edge, compatible with the opening of a pseudogap, as expected for a quasicrystal. The crystalline overlayers are characterised by a sharp Fermi edge, while the approximant and decagonal quasicrystalline surfaces also have a lowered density of states.     

准晶的晶体学特征

10年前,具有五次旋转对称而无平移周期序的准晶问世,在与晶体及晶体学有关的学科领域里引起强烈反响。10年来的系统研究澄清了准晶晶体学中的一些问题,如(1)准晶还是五重孪晶? (2)为什么要有五次旋转对称? (3)为什么会有准周期性? (4)稳定的准晶存在吗? (5)准晶与近似晶体相有什么异同?     

Structural characterization of a pentacene monolayer on an amorphous SiO2 substrate with grazing incidence x-ray diffraction

Grazing incidence X-ray diffraction reveals that a pentacene monolayer, grown on an amorphous SiO2 substrate that is commonly used as a dielectric layer in organic thin film transistors (OTFTs), is crystalline. A preliminary energy-minimized model of the monolayer, based on the GIXD data, reveals that the pentacene molecules adopt a herringbone arrangement with their long axes tilted slightly from the substrate normal. Although this arrangement resembles the general packing features of the (001) layer in single crystals of bulk pentacene, the monolayer lattice parameters and crystal structure differ from those of the bulk. Because carrier transport in pentacene OTFTs is presumed to occur in the semiconductor layers near the dielectric interface, the discovery of a crystalline monolayer structure on amorphous SiO2 has important implications for transport in OTFTs.