Epitaxial growth of lead and bismuth on silver and silver rich alloys

• Main Author: McLeod, Ian
• Other Authors: Dhanak, Vin; Sharma, Hem
• Published: University of Liverpool 2014
• Subjects: 530; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.617505

The growth of Bi on Ag(111) induces different surface structures, including(√3×√3)R30°surface alloy, Bi-(p×) overlayer and Bi(110) thin film as a function of increasing Bi coverage. These structures have been studied using low-temperature scanning tunneling microscopy, low-energy electron diffraction, ab initio calculations and photoemission spectroscopy from core-levels and valence bands at room temperature. At 1/3 of a monolayer of Bi on Ag(111) leads to the formation of BiAg2 surface alloy with a long range ordered (√3×√3)R30° superstructure. A detailed analysis of this structure using LEED I-V measurements together with DFT calculations is presented. The Bi atom is found to replace one top layer Ag atom in each unit cell, forming a substitutional BiAg2 surface alloy. This mode of accommodation of Bi was found to be energetically favourable based on ab initio total-energy calculations. Based on photoemission spectroscopy we find the sp-derived Shockley surface state on Ag(111) is rapidly quenched upon deposition of Bi, due to the strong variation of inplane surface potential in the Ag2Bi surface alloy. The core levels of Bi 4f of the (√3×√3)R30° Ag2Bi alloy and Bi(110) thin film are shifted to lower binding energy by ~0.6 eV and ~0.3 eV compared with the Bi bulk value, respectively. Mechanisms inducing the core level shifts are discussed as due to a complex superposition of several factors. At coverage above a critical value of 0.55 monolayers, the Ag2Bi alloy phase gradually converts into an ordered Bi (p × √3) overlayer structure supported on Ag(111). We postulate that the dealloying transition is likely driven by compressive strain induced by incorporation of large-size Bi atoms into Ag at a high coverage and the subsequent lack of miscibility of Ag and Bi bulk phases. After completion of the dealloying process, Bi(110) thin films can be grown epitaxially on top of Ag(111) with a chemically abrupt interface. The deposition of 1/3 of a monolayer of Pb on Ag(111) leads to the formation of Ag2Pb surface alloy with a long range ordered (√3x√3)R30° superstructure. A detailed analysis of this structure using LEED I-V and MEIS measurements together with DFT calculations is presented. Quasicrystals, materials with aperiodic long-range order, are usually formed by at least two elements with a specific chemical composition. In this study, we demonstrate that, a single element multilayer quasicrystal can be formed on a template. Using a hitherto unexplored quasicrystalline substrate, the icosahedral Ag-In-Yb quasicrystal, and various experimental techniques including scanning tunneling microscopy combined with theoretical calculations of adsorption energies, we identify the initial adsorption sites for Pb atoms on the substrate and hence deduce the multilayer structure of the Pb overlayer. We propose a mechanism for the growth of epitaxial materials on the quasicrystalline substrate.