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Title: Comparison between experimental surface data and bulk structure models for quasicrystalline A1PdMn: Average atomic densities and chemical compositions

Authors: B. Unal, C. J. Jenks and P. A. Thiel

Contact: Pat Thiel

Journal: Physical Review B (2008) 77, 195419.

Abstract:

We have examined bulk structure models for icosahedral A1PdMn in terms of the densities, compositions, and interplanar spacings for the fivefold planes that might represent physical surface terminations. We focus on four models that contain no partial or mixed occupancies, but some comparison is made to a fifth model containing such sites. Each of the four models contains paired planes (layers) that can be separated into two main families on the basis of three features: the relative densities of the two planes, the gap separating the layer from the nearest atomic plane, and the Pd content in the topmost plane. The experimental data and other arguments lead to the conclusion that the family with no Pd in the top plane is favored. Finally, all models show that correlations should be expected between the heights of steps that delineate terraces and average compositional and/or structural features of the terraces.

Title: Iron deposition on the tenfold surface of the Al72Ni11Co17 decagonal quasicrystal

Authors: L. H. Wearing, J. A. Smerdon, L. Leung, S. S. Dhesi, J. Ledieu, P. Bencok, I. Fisher, C. J. Jenks and R. McGrath

Contact: Cynthia Jenks

Journal: Journal of Physics-Condensed Matter (2008) 20, 015005.

Abstract:

The adsorption behaviour of Fe on the tenfold surface of the decagonal quasicrystal Al72.6Ni10.5Co16.9 has been studied using scanning tunnelling microscopy, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and x-ray magnetic circular dichroism (XMCD). The results show the growth of a disordered film up to a coverage of 9 MLE (monolayer equivalent) after which polycrystalline island growth is observed. These islands are interconnected, and the LEED pattern indicates that they preferentially align along five directions. The AES results indicate that the film is composed of Fe intermixed with the substrate elements. The XMCD results point to an induced magnetic moment for Co and Ni in the intermixed layer/interface region.

Title: Quasicrystal surfaces

Authors: P. A. Thiel

Contact: Pat Thiel

Journal: Annual Review of Physical Chemistry (2008) 59, 129-152.

Abstract:

Studies of ordered, yet nonperiodic, metallic alloys known as quasicrystals have generated exciting questions and fundamental insights about the relationship between surface atomic structure and surface properties. In this review, I give examples from oxidation, friction, heterogeneous catalysis, and solid film growth, in which the Al-rich quasicrystals exhibit unusual properties, relative to crystalline materials of similar composition.

Title: Atomic scale friction and adhesion properties of quasicrystal surfaces

Authors: J. Y. Park and P. A. Thiel

Contact: Pat Thiel

Journal: Journal of Physics-Condensed Matter (2008) 20, 314012.

Abstract:

In this paper, we highlight recent studies of the atomic scale friction and adhesion properties of quasicrystals. We review tribological studies carried out in different mechanical regimes (elastic and inelastic) and at different length scales (macroscale and nanoscale). We address the role of the surface oxide and the nature of mechanical contact in determining friction and adhesion properties. We discuss the relationship between the aperiodic atomic structure of quasicrystals and their low friction, for both elastic and inelastic regimes.

Title: Friction anisotropy: A unique and intrinsic property of decagonal quasicrystals

Authors: J. Y. Park, D. F. Ogletree, M. Salmeron, C. J. Jenks, P. A. Thiel, J. Brenner and J. M. Dubois

Contact: Pat Thiel

Journal: Journal of Materials Research (2008) 23, 1488-1493.

Abstract:

We show that friction anisotropy is an intrinsic property of the atomic structure of Al-Ni-Co decagonal quasicrystals and not only of clean and well-ordered surfaces that can be prepared in vacuum [J.Y. Park et al., Science 309, 1354 (2005)]. Friction anisotropy is manifested in both nanometer-size contacts obtained with sharp atomic force microscope tips and macroscopic contacts produced in pin-on-disk tribometers. We show that the friction anisotropy, which is not observed when an amorphous oxide film covers the surface, is recovered when the film is removed due to wear. Equally important is the loss of the friction anisotropy when the quasicrystalline order is destroyed due to cumulative wear. These results reveal the intimate connection between the mechanical properties of these materials and their peculiar atomic structure.

Title: Self-assembly, structure, and electronic properties of a quasiperiodic lead monolayer

Authors: J. Ledieu, L. Leung, L. H. Wearing, R. McGrath, T. A. Lograsso, D. Wu and V. Fournee

Contact: Tom Lograsso

Journal: Physical Review B (2008) 77, 073409.

Abstract:

A quasiperiodic Pb monolayer has been formed on the fivefold surface of the Al-Pd-Mn quasicrystal. Growth of the monolayer proceeds via self-assembly of an interconnected network of pentagonal Pb stars, which are shown to be tau inflated compared to similar structural elements of the quasiperiodic substrate. Measurements of the electronic structure of the system using scanning tunneling spectroscopy and ultraviolet photoemission spectroscopy reveal that the Pb monolayer displays a pseudogap at the Fermi level which is directly related to its quasiperiodic structure.

Title: Quasicrystals at interfaces

Authors: V. Fournee, J. Ledieu and P. Thiel

Contact: Pat Thiel

Journal: Journal of Physics-Condensed Matter (2008) 20, 310301.

 

Title: Measurements of contact angles of water on Al-based intermetallic surfaces

Authors: J. M. Dubois, V. Fournee, P. A. Thiel and E. Belin-Ferre

Contact: Pat Thiel

Journal: Journal of Physics-Condensed Matter (2008) 20, 314011.

Abstract:

A systematic study was performed of the contact angle of minute droplets of water deposited on various aluminium-based intermetallics in ambient conditions. The dataset was complemented by measurements of electronic partial densities of states and oxide layer thicknesses. We studied a broad variety of specimens, ranging from simple cubic, CsCl-type phases to quasicrystals and high-order approximants. The thickness of the oxide layer, that is always present in air, was varied between 2 and more than 10 nm. Within these limits, the contact angle varies considerably and, surprisingly, decreases with increasing thickness of the oxide dielectric. Furthermore, it is clearly the highest on samples that show the largest crystal complexity, i.e. quasicrystals.
It follows that the reversible adhesion energy directly deduced from contact angle is essentially controlled by the (squared) density of states at Fermi energy in the bulk of the sample and the inverse of the (squared) thickness of the native oxide film at the surface of the material. We interpret these results in terms of electrostatic image forces developed in the conduction cloud by the dipoles of the water molecules.

 


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