title: Surface structure and epitaxy on Ag-In-Rare earth quasicrystals and related approximants
Creators: name: Hars, Sanger
Publisher: name: University of Liverpool Repository
Year: 2016

description: This thesis reports on work carried out in the Surface Science Research Centre of the University of Liverpool and in the Jean Lamour Institute, Université de Lorraine, Nancy, France. The research described in this thesis is the experimental investigation of epitaxy on quasicrystal surfaces and the determination of the surface structure of related approximants. Quasicrystals, which are new phases of solids conceptualised in 1982, show aperiodic long range order with lack of translational symmetry. Moreover, quasicrystals possess forbidden rotational symmetries such as five-fold and ten-fold. Quasicrystal surfaces have extensively been studied, however their studies were previously limited to Al-based quasicrystals. In this thesis surface studies are extended to the new ternary Ag-In-Yb quasicrystal and related approximants. Approximants are periodic metallic compounds whose chemical composition and atomic structure are close to their related quasicrystals, but which possess large unit cells. The specific goals of this thesis are: to understand the surface structure of Ag-In-rare earth (rare earth (RE) = Tb, Gd and Yb) approximants whose structure is similar to their parent quasicrystals; secondly, to achieve artificial single element quasicrystals through the epitaxy on the five-fold surface of icosahedral Ag-In-Yb quasicrystal. Two main surface techniques have been used throughout the work of this thesis, Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED). The study of the surface structure of Ag-In-RE 1/1 approximants is presented. The bulk structure of these approximants is formed by the same rhombic triacontahedral (RTH) cluster as their parent icosahedral Ag-In-Yb quasicrystal. The RTH cluster consists of five successive atomic shells. The innermost shell is a tetrahedron of four Cd atoms. The second shell is a decagon of twenty Cd atoms. Twelve Yb atoms make an icosahedra in the third shell. The fourth shell is an icosadodecahedron of thirty Cd atoms. Finally, the outer-most shell is the rhombic triacontahedron (RTH) of ninety two Cd atoms. The RTH clusters are packed periodically in approximants. The aim of this work is to understand the surface of approximants and find out how they relate to the surface of the parent quasicrystal. It is found that the surface of Ag-In-Tb and Ag-In-Gd develops facets along different crystallographic directions. The comparison of observed step heights and high resolution STM images of facets with the bulk structure model showed that the surface of approximants similar to the surface of parent quasicrystal, are bulk terminated. That means the surface corresponds to bulk truncation. The five-fold surface of the icosahedral Ag-In-Yb quasicrystal was used as a template and two different elements, Bi and In were deposited on the surface. In this work single element quasicrystalline structure was successfully achieved through the deposition of both Bi and In on the surface. Both elements formed quasicrystalline layers on the surface. By comparing the STM images of quasicrystalline layers with the atomic structure of the RTH clusters above the surface, it was found that the adsorbate atoms occupy atomic sites of the RTH clusters. The Bi and In adsorbates were found to occupy different atomic sites. In addition to the symmetry, the structure and chemistry of the substrate seem to play a role in achieving a quasicrystalline thin film.