Adsorption, surface restructuring and alloy formation in the [formula omitted] system

Abstract

The adsorption of Na on the reconstructed Au(111) surface has been studied by scanning tunneling microscopy, low-energy electron diffraction (LEED), thermal desorption spectroscopy and Auger electron spectroscopy. Upon deposition at 85 K, adsorbed Na atoms order spontaneously at coverages between 0.2 and 0.6 monolayers (ML) on the reconstructed substrate. A sequence of hexagonal overlayer structures is observed by LEED. Annealing the Na-covered sample to 300 K results in irreversible changes of the surface structures: Intermixed NaAu surface layers are formed for θ Na > 0.25 ML. A well-ordered c(4 × 2) substitutional structure develops at θ Na ≈ 0.50 ML, where every fourth Au surface atom is replaced a Na atom. The structures formed at 300 K are metastable, and upon annealing to 600 K the equilibrium surface configurations evolve: For small Na coverages the periodicity of the Au(111) Chevron phase is reduced. At θ Na ≈ 0.23 ML the Na atoms induce a surface phase transformation of the Au(111) Chevron phase into a poorly ordered domain structure with an isotropically contracted surface layer and hexagonal symmetry. At even higher coverages a mixed NaAu 2 structure is formed in the topmost layer which covers the entire surface at θ Na ≈ 0.50 ML. The NaAu 2 structure is arranged in a hexagonal lattice with a (1.08√3 × 1.08√3)R30° geometry where the Au atoms form a honeycomb structure whose central sites are occupied by Na atoms. The lattice misfit between the surface and the second layer leads to a regular hexagonal Moiré pattern of ≈ 38 A ̊ periodicity and ≈ 0.5 A ̊ corrugation. This phase is regarded as a two-dimensional alloy. With adsorbed Na multilayers, Na atoms penetrate into deeper substrate layers and a thin NaAu 2 alloy film is formed.