Structural analysis of FeO(111)/Ag(001): undulation of hexagonal oxide monolayers due to square lattice metal substrates

Abstract

Iron oxide monolayers are grown on Ag(0 0 1) via reactive molecular beam epitaxy (metal deposition in oxygen atmosphere). The monolayer shows FeO stoichiometry as concluded from x-ray photoemission spectra. Both low energy electron diffraction as well as scanning tunneling microscopy demonstrate that the FeO layer has a quasi-hexagonal (1 1 1) structure although deposited on a surface with square symmetry. Compared to bulk values, the FeO(1 1 1) monolayer is unidirectionally expanded by 3.4% in < 11 (2) over bar > directions while bulk values are maintained in < 1 (1) over bar0 > directions. In < 11 (2) over bar > directions, this lattice mismatch between FeO(1 1 1) monolayer and Ag(0 0 1) causes a commensurate undulation of the FeO monolayer where 18 atomic rows of the FeO(1 1 1) monolayer match 17 atomic rows of the Ag(0 0 1) substrate. In < 1 (1) over bar0 > directions, however, the FeO(1 1 1) monolayer has an incommensurate structure.