Structural characterization of the d-A1-Ni-Co quasicrystal surface and xenon adsorption

by Ferralis, Nicola.

iii Quantitative low-energy electron diffraction (LEED) was used to characterize the structure of the tenfold surface of decagonal Al-Ni-Co quasicrystal. A quasicrystal slab model was used in the LEED calculation with approximations that averaged over composition and local geometries. The results show that the surface region is a relaxed, bulk terminated structure, with the same composition of the bulk. The outermost layer is slightly contracted relative to the bulk interlayer spacing, while the next layer is expanded Periodic models that approximate the actual local quasicrystal structure (approximants) were used in a more exact atomistic calculation and compared to the approximated calculation. The results obtained with both methods were consistent with each other and suggests that the use approximant structure models provide a simpler method for the determination of local geometries in quasicrystal structures. LEED isobar measurements were used to characterize the adsorption and film growth of Xe onto the tenfold surface of d-Al-Ni-Co. Xe grows layer-by-layer at temperatures between 50 K and 80 K. The structure of the first layer appears to retain the symmetry of the substrate but at the onset of second-layer adsorption, the film reorders to a 6-fold structure. This 6-fold structure has domains that are aligned along the 10-fold directions of the quasicrystal, leading to a diffraction pattern having 30-spot rings. The momentum transfer of the first-order diffraction beams from Xe coincides with one of the principle momentum transfers from the quasicrystal surface, indicating that the Xe interrow spacing is related to a principal distance on the surface. The Xe multilayer is consistent with bulk fcc(111) Xe. iv Preliminary elastic and inelastic helium scattering experiments were performed on the clean graphite to characterize its surface structure and dynamics and Xe adsorption. These experiments are used as a testing ground for similar HAS experiments which will be carried out in the near future with the same apparatus on the tenfold d-Al-Ni-Co quasicrystal surface.