Surface alloy formation by adsorption of holmium on Ag/Mo(112) bimetallic surfaces

摘要

Highlights•Surface alloy is formed on the bimetallic Ag/Mo(112) surfaces, with 1 or 2 ML of Ag, upon deposition of Ho.•LEED and DFT results indicate formation of the c(2 × 2) structures at 0.5 ML of Ho.•In the c(2 × 2) structures, an incomplete ML of Ho is placed below the topmost layer of Ag.•The work function change is positive for 0.5 ML of Ho deposited on 2 ML Ag/Mo(112).•Adsorption of Ho on Ag/Mo(112) bimetallic surfaces is substantially different than on the clean Mo(112).AbstractWork function change measurements, low energy electron diffraction (LEED) and density functional theory (DFT) are used to determine the structures formed on Ag/Mo(112) bimetallic surfaces upon deposition of 0.5 monolayer (ML) of holmium. As the bimetallic surfaces, we have chosen the Mo(112) substrate covered with 1 or 2 ML of Ag. Such surfaces have the same symmetry as the Mo(112) face but different electronic properties. LEED experiment indicates that the c(2 × 2) structure is formed on (1 ML Ag)/Mo(112) bimetallic surface upon deposition of 0.5 ML of Ho. DFT calculations show that a type of Ag–Ho surface alloy is formed, with Ho atoms 0.6 Å below the distorted layer of Ag. This is neither a substitutional nor a subsurface alloy. It is found that the adsorption structure formed on the (2 ML Ag)/Mo(112) bimetallic surface depends on the annealing temperature. After deposition of 0.5 ML of Ho at 300 K, the LEED pattern of p(2 × 2) symmetry is observed. Annealing of the overlayer at 640 K irreversibly changes the p(2 × 2) pattern into a pattern of c(2 × 2) type. The results of DFT computations show that the c(2 × 2) structure of the Ag–Ho surface alloy is energetically most favorable. In this structure, 0.5 ML of Ho is between the two monolayers of Ag, and the symmetry of the topmost layer is changed. The work function change calculated for the c(2 × 2) structure is in a good agreement with the measured value (0.22 eV). The results show that adsorption of Ho on the Ag/Mo(112) bimetallic surfaces is substantially different than on the clean Mo(112).Graphical abstractDisplay Omitted