Atomic structure of an Al-Co-Ni decagonal quasicrystalline surface

We have analyzed the structure and composition of the first layer of an Al₇₂Co₁₆Ni₁₂ tenfold surface by means of scanning tunneling microscopy (STM), ion scattering spectroscopy (ISS), and Auger electron spectroscopy (AES). High-resolution STM images reveal local structures that have decagonal symmetry in addition to the usual pentagonal symmetry of the surface. This quasicrystal surface resembles a random tiling instead of an ideal quasiperiodic tiling. After annealing at 1100 K, the total surface atomic density found by ISS is (9±1) × 10¹⁴ cm^-2. The surface densities of Al and TM (transition metal, i.e., Co and Ni) are determined as (8±1) × 10¹⁴ cm^-2 and (1.0×0.2) × 10¹⁴ cm^-2, respectively from ISS, indicating a similar density of Al and much lower density of the TM atoms in the surface layer than in a truncated bulk. The Al surface atomic density agrees well with the number of corrugation maxima in the STM images. A model of the arrangement of the Al atoms in the top layer is presented. Scanning tunneling spectroscopy (STS) is performed to study the local electronic structure. The STS spectrum at the corrugation maxima is similar to that at the corrugation minima. A few ≈0.12 nm high protrusions in the STM images are attributed to local oxide clusters due to their STS spectra different from the corrugation maxima and through in situ STM observations during exposure to O₂ gas at 2 × 10^-6 Pa at RT.