Silver films grown on a rhenium(0001) surface: a combined TDS, XPS, and Δ Φ study

Abstract

The energetics and kinetics of Ag thin film growth on Re(0001) were studied by means of temperature-programmed thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and work function change (Δ Φ) measurements. The formation of three individual Ag layers shows up in TDS as three distinct desorption maxima β 1– β 3 appearing between 950 and 1010 K ( β 3), between 900 and 960 K( β 2), and between 870 and 970 K ( β 1). Except in the very low coverage ( Θ) range, in which the desorption is a first-order process, the Ag desorption follows zero-order kinetics. For the first two layers, activation energy of desorption ( E ∗ des ) is strongly Θ dependent: within the first layer, E ∗ des increases almost linearly with Θ from ≈250 kJ mol −1 at Θ=0.05 to about 290 kJ mol −1, reflecting attractive Ag–Ag interactions. From Θ=0.5 to 0.9, E ∗ des rises by only some 10 kJ mol −1. A similar (but much less pronounced) Θ dependence appears for the second monolayer. A detailed shape analysis of the submonolayer TD spectra reveals a phase equilibrium between Ag condensed in islands and individual, mobile Ag atoms (2D gas phase). In XPS, the absence of any energy shift of the Ag and Re core levels underlines the weakness of chemical Ag–Re interactions. Two Ag layers lower the work function of the Re(0001) surface by about 750 meV, with a shallow minimum near the second monolayer. We discuss our data in conjunction with previous STM and LEED results for the same system and compare this system with other Ag-on-metal systems.