Structure and morphology of the tenfold surface of decagonalAl71.8Ni14.8Co13.4in its low-temperature random tiling type-I modification

Abstract

We have investigated the structure and morphology of the tenfold surface of decagonal Al 71.8 Ni 14.8 Co 13.4 by highly surface sensitive He atom scattering (HAS), high resolution low energy electron diffraction (SPA-LEED), and low temperature scanning tunneling microscopy (STM). The SPA-LEED patterns reveal more than 500 individual diffraction spots in the k-vector range of ͉k ʈ ͉ Ͻ 3 Å −1 . The positions of all observed diffraction spots agree with the surface projections of the reciprocal lattice structure of the type-I bulk phase. HAS shows identical spot positions as SPA-LEED, thus demonstrating a top surface layer with long range quasicrystalline order and a reciprocal lattice structure consistent with that of a bulk truncated surface. SPA-LEED peak widths are found to vary between different diffraction orders. Based on an analysis of a randomized Fibonacci sequence, this is linked to the random nature of the tiling of the type-I structure. STM measurements reveal a surface morphology characterized by rough single-height steps separating terraces with widths on the order of 100 Å. Two different surface terminations are observed, a coarse and a fine one, frequently coexisting on single terraces. The fine structure termination directly reflects the atomic structure of a bulk truncated surface, allowing a random rhombic tiling to be identified. In order to compare diffraction, real-space data, and atomic structure models, the Patterson function and autocorrelation of the surface structure, respectively, are studied. This allows an understanding of the coarse structure termination as consisting of subunits of a few atoms each arranged statistically on sites defined by the atomic tiling of the bulk tenfold planes.

The surface science of quasicrystals

“…STM shows wide terraces separated by steps of 2Å height [35,37,39,40]. High resolution images show pentagonal features, which have the same orientation in a single terrace, but are rotated by 36…”

“…Helium atom scattering (HAS) shows a specular (reflected peak) intensity comparable to that of high-quality cleaved surfaces of periodic crystals, e.g., GaAs(110) [45]. LEED patterns from the surface show a dense pattern of spots distributed with symmetry expected from the bulk [35]. An example of a spot-profile-analysis-LEED image from the tenfold surface of the type-I superstructure is given in figure 3.…”

“…Al-Ni-Co exhibits a number of different decagonal phases depending on the exact composition and temperature ( [9] and references therein). Surface studies have been performed on two of these phases, the type-I superstructure phase [35] and the Co-rich phase [36][37][38][39][40]. These phases share common structural features.…”

Atomic scale coexistence of periodic and quasiperiodic order in a 2-fold Al-Ni-Co decagonal quasicrystal surface

“…The 10-fold surface of the Al-Ni-Co decagonal quasicrystal-the surface that is quasicrystalline in both dimensions-has been studied by low-energy electron diffraction ͑LEED͒, ion scattering spectroscopy, and scanning tunneling microscopy 6-10 ͑STM͒, but there are still important issues, such as atomic structure and composition, that remain controversial. 8,9 Some authors have attributed sites of high tunneling probability in the STM images to transition metal atoms, 8 while others have identified these sites as Al atoms. 7,9 A further point of discussion is whether the surface termination is enriched in Al relative to the bulk composition.…”

“…8,9 Some authors have attributed sites of high tunneling probability in the STM images to transition metal atoms, 8 while others have identified these sites as Al atoms. 7,9 A further point of discussion is whether the surface termination is enriched in Al relative to the bulk composition. 6,7 In contrast to the 10-fold surface however, only a few studies of the 2-fold surface 6,11,12 have been reported.…”