High-resolution scanning tunneling microscopy investigation of the (12110) and (10000) two-fold symmetric d-Al-Ni-Co quasicrystalline surfaces

The d-Al-Ni-Co quasicrystal exhibits two crystallographically inequivalent twofold symmetric planes: the {10000} and {12110} surface planes. In particular the twofold symmetric surfaces are very interesting since they are spanned by perpendicular periodic and aperiodic axes. We investigated the (12110) surface of two different compositions d-Al70Ni15Co15 and d-Al72.9Ni10.4Co16.7 and the (10000) d-Al72.9Ni10.4Co16.7 surface by means of low-temperature (5 K) scanning tunneling microscopy (STM) in ultrahigh vacuum. All three surfaces are characterized with atomic resolution and display large and flat terraces with a columnar structure along the periodic axis. Both compositions of the (12110) d-Al-Ni-Co STM investigations revealed that the surface is faceted into {12110} and {10000} facets with a 1: 1 area ratio. The (12110) facets represent the bulk periodicity of 0.4 nm within the prominent columnar structure which is attributed to a stacking of Al dimers. The sequence of the columnar structure along the aperiodic axis as well as the step heights between the (12110) surfaces could be attributed to the pentagonal tiling edge length of the bulk model. Furthermore, the (12110) surface is identified as one of the densest planes in the bulk model possessing a slightly lower Al concentration as the nominal bulk composition. However, a difference in the fine structure within the columns is observed between the two investigated compositions. The (10000) facet presents an identical surface structure as the unfaceted (10000) surface of the quasicrystal. At the (10000) surface two terraces with different surface structures are identified and compared to the bulk model. One of them shows a bias voltage depending structure. In contrast to the bulk model the minimal observed periodicity on the (10000) surface is doubled to 0.8 nm. On the other hand the aperiodic step height sequence and the order of the columnar motifs along the aperiodic axis are in agreement with the bulk model. The height distribution analysis of the (10000) surfaces exhibits that the surface topmost layer consists of planes which are less dense than the (12110) surface planes.