The Ten-Fold Surface of The Decagonal Al 72 Ni 11 Co 17 Quasicrystal Studied by Leed, Spa-Leed, AES and STM
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THE TEN-FOLD SURFACE OF THE DECAGONAL Al72Ni11Co17 QUASICRYSTAL STUDIED BY LEED, SPA-LEED, AES AND STM. ERIK J. COX1, JULIAN LEDIEU1, RÓNÁN McGRATH1, RENEE D. DIEHL2, CYNTHIA J. JENKS3 and IAN FISHER3 1
Surface Science Research Centre, The University of Liverpool, Liverpool, L69 3BX, UK. Dept. of Physics, Pennsylvania State University, University Park, PA 16802, USA. 3 Ames Laboratory, Iowa State University, Ames, IA 50011, USA. 2
ABSTRACT The ten-fold surface of the decagonal Al72Ni11Co17 (d-Al-Ni-Co) quasicrystal has been investigated using low energy electron diffraction (LEED), spot profile analysis LEED (SPALEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). This was done as a function of both annealing temperature and annealing time. The long-range order of the surface, as indicated by LEED, increases both as a function of annealing time and temperature. STM shows the surface to be rough and cluster-like at low annealing temperatures (≤725 K), whilst annealing to temperatures in excess of 725 K results in the formation of terraces. These terraces are small (≤ 100 Å width) at lower annealing temperatures and increase in size (100 Å ≤ x ≤ 500 Å) as the annealing temperature is increased (≥ 850 K). They are characterised by the presence of three-fold protrusions which align preferentially. STM images show single height steps as expected due to the periodicity of d-Al-Ni-Co in the z direction. To date it has not been possible to obtain atomic resolution, although this work is continuing. INTRODUCTION It has been found that quasicrystals exhibit properties which lend themselves to technological applications, for example; low coefficients of sliding friction, high hardness, resistance to oxidation and the possibility for hydrogen storage [1]. To understand these properties fully a comprehensive understanding of the surface is a necessity. There are also other fundamental questions to be addressed such as how the sample can be best prepared so that the surface is quasicrystalline and how the surface structure compares to the bulk structure. Recent advances have made it possible to routinely manufacture large, high quality single grain quasicrystals suitable for analysis using surface science techniques, in particular LEED and STM [2,3]. This has led to an increase in activity by surface scientists to provide answers to the questions outlined above. d-Al-Ni-Co is one of the most studied quasicrystals to date and the bulk structure is relatively well understood. The structure is periodic in the direction normal to the 10-fold surface but quasiperiodic in the planes orthogonal to this direction. The atomic structure of quasicrystals have been described as 3-D extensions of the 2-D planar Penrose tiling©. However, Steinhardt et al [4] have proposed an alternative model for the quasicrystalline planes of d-Al-Ni-Co – a single
K11.3.1
repeating ‘quasi-unit-cell’. This picture utilises identical decagons as the tiles, but unlike conventional periodic unit cells these quasi-unit-cells are
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