Reconstructive adsorption of Na on Al(111) studied by scanning tunneling microscopy

We present a scanning-tunneling-microscopy (STM) study of the adsorption behavior of Na on Al(111) at room temperature. For this system, a reconstruction of the close-packed metal surface is realized for both ordered structures, the (√3 × √3 )R30° structure with Θ=0.33 and the (2×2) phase with Θ=0.50. For the first one, this leads to properties that are quite uncommon for alkali metals on close-packed metal surfaces and that are detectable by STM: island formation already for low coverages, structure-selective nucleation at Al step edges, and a low adsorbate mobility corresponding to a diffusion coefficient of D=${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$ at T=300 K. The mobile species is identified as a Na atom residing in a substrate vacancy (substitutional Na). For the diffusion mechanism, a concerted motion of Na and the underlying vacancy is proposed. The adsorption site of Na was found to be in registry with the Al lattice. These findings confirm the reconstruction model recently proposed for Na in the (√3 × √3 )R30° structure, in which Na atoms are adsorbed above vacancies in the Al(111) surface, i.e., on substitutional sites. Upon further uptake of Na, the domain walls of the (√3 × √3 )R30° structure change from light to heavy walls. The latter are the nuclei for the (2×2) phase.

This phase has two Na atoms per unit cell. Nevertheless, it is characterized by a single protrusion per unit cell in the STM images, and a sixfold rotational symmetry. On large terraces the mass transport involved in the phase transition (√3 × √3 )R30°→(2×2) is analyzed, from which a number of three Al surface atoms per unit cell of the (2×2) phase was derived. This leads to a structure model for the (2×2) phase where one Na atom is adsorbed on a substitutional site as in the lower coverage (√3 × √3 )R30° structure and the second Na atom resides on the hollow sites of the mixed Na-Al layer. This bilayer model explains the thermal-desorption characteristics and exhibits only a single Na-Al bond length of 3.3 Å, in agreement with recent surface-extended x-ray-absorption fine-structure results. A similar model with one additional Al atom recently proposed is discussed in context with the present STM data.