Surface Properties of Quasicrystals
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or not quasicrystalline—impact directly on our understanding of their surface thermodynamic, electronic, and chemical properties.
Clean Surfaces The structure of clean crystalline surfaces is commonly discussed in terms of the terrace-step-kink (TSK) model, in which atomically flat, low-Miller-index terraces are separated by steps that may themselves be rough (kinked). Scanning tunneling microscopy (STM), a tech-
nique capable of atomic-scale spatial resolution, is well-suited to probing such features. (Typically STM is used in a mode that maps the electron-density contours at a surface. These contours may reflect the positions of atomic nuclei.) Many images from STM have revealed TSK features on crystalline surfaces. Kortan et al. carried out the first STM characterization of a surface of a quasicrystalline sample, decagonal Al-Co-Cu. 1 Later Schaub et al. performed similar work on icosahedral i(Al-Pd-Mn). 2 Both studies revealed TSK-type features. This was perhaps less surprising for the decagonal quasicrystal (which is quasicrystalline in two dimensions and periodic in the third) than in the icosahedral material (which is quasicrystalline in all three dimensions). Also certain aspects of the STM images (quasiperiodic features on terraces, step heights, and sequence of step heights) were consistent with known aspects of the quasiperiodic bulk structure. In short both studies suggested that quasiperiodicity was retained at the surfaces and that the surfaces consisted of rather flat
Figure 1. Scanning tunneling micrographs at two different magnifications of a fivefold i(AI-Pd-Mn) surface. The surface was prepared by sputtering and then heating close to the melting point, in ultrahigh vacuum (UHV).24 Panel (a) shows the terrace-step-kink morphology while panel (b) shows the fine structure present on the terraces with fivefold features highlighted by circles. A 4-A step cuts across the upper middle of (b). © Springer-Verlag 1995.
MRS BULLETIN/NOVEMBER 1997
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Surface Properties of Quasicrystals
terraces separated by crooked steps. Scanning-tunneling- microscopy images obtained on an icoshahedral material appear in Figure 1. However surface structure may depend strongly on the way in which the surface is prepared. The STM images in Figure 1 were obtained from samples sputtered and annealed in ultrahigh vacuum. These standard treatments can perturb the surface composition via preferential sputtering and evaporative loss. The alternative (and less chemically disruptive) route of cleaving has also been investigated. Scanning-tunnelingmicroscopy images of a cleaved surface are shown and discussed in the article by Urban et al. in this issue. The cleaved surface is significantly rougher than the sputter-annealed surface and does not show TSK features.3 This roughness persists on different length scales but has a minimum amplitude of about 10 A. The roughness may reveal the hierarchy of clusters that comprise the bulk material, as proposed by Janot and de Boissieu.4-5 In their model, the basic building block is a pseu
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