NISXW study of Si adsorbed on an Al-Co-Ni quasicrystal

The normal incidence standing x-ray wavefield (NISXW) technique has never before been applied to the determination of adsorption structures on quasicrystals, even though it is quite clear that, under the right conditions, x-ray standing waves do exist in quasicrystals. This omission may be due to a misconception that the relationship between the phase of the standing waves and the atoms at a quasicrystal surface is arbitrary. We have performed a NISXW experiment for the adsorption of Si atoms on the nominally 10-fold surface of the decagonal Al-Co-Ni quasicrystal. NISXW spectra were obtained for a Si coverage of about 0.3, for two different angles of incidence: normal to the 10-fold surface, and at an angle of about 60${\circ}$ from the surface normal. These angles correspond to two strong x-ray reflections of the quasicrystal. The intensity of the Si 1s photoemission signal was measured in order to determine the location of the Si atoms.order to accurately model the 5-fold symmetry of the surface, our analysis employed a 200 {\AA} x 200 {\AA} x 8 {\AA} structure model for the quasicrystal. The results indicate that the Si atoms have an average height of 1.80 {\AA} above the surface, and are arranged in 6-atom pentagonal clusters centered at points of 5-fold symmetry. This study demonstrates the feasibility for using NISXW as a structural tool for adsorbed atoms or molecules on quasicrystal surfaces.