Manganese Oxide Thin Films on Au(111): Growth Competition between MnO and Mn3O4

The growth of manganese oxide on a Au(111) support has been examined over a wide range of preparation conditions and film thicknesses by means of scanning tunneling microscopy (STM), electron diffraction, and photoelectron spectroscopy. Two oxide polymorphs were found to coexist on the gold surface. Whereas MnO-type structures prevail at oxygen lean preparation conditions, annealing in oxygen gives Hausmannite Mn3O4 as the dominant phase. Both polymorphs adopt square structures that only permit row-matched growth on the hexagonal gold, explaining the high tendency for oxide dewetting at elevated temperature. The manganese oxide films are thus polycrystalline and consist of a large number of submicrometer grains with different orientations and surface terminations. A variety of square and line patterns were identified on top of the oxide crystallites, all of them being compatible with either the primitive 5.8 X 5.8 angstrom(2) cell of Mn3O4(001) or an ordered MnO(100) vacancy structure. STM conductance spectroscopy provides insight into the electronic properties of the Mn-O islands and yields the approximate size of the band gap as well as the energy position of localized Mn 3d levels in the gap region. Our work illuminates the structural and electronic properties of manganese oxide films grown on Au(111) and therefore complements earlier studies performed on Pt(111), Pd(100), and Ag(100) supports.