Scanning tunneling microscopy of MnOx ultrathin films on Au(111)

Abstract

Two structurally distinct phases of manganese oxide ultrathin films were grown on Au(111) substrates and imaged at atomic resolution by scanning tunneling microscopy (STM). The so-called MnOx fishbone phase is only a few monolayers thick and nucleates epitaxially on the bare Au(111) substrate. The surface of this phase exhibits two parallelogram unit cells with sizes and included angles of (14.6 ± 0.2) × (5.6 ± 0.2) Å2, 88 ± 1° and (13.6 ± 0.1) × (5.6 ± 0.2) Å2, 80 ± 1°. The other thicker phase is called the square phase. It is only observed growing on top of the fishbone phase and has a surface unit cell of edge dimension 5.8 ± 0.1 Å. The square phase is thought to be a (001) termination of hausmannite Mn3O4. In addition, less common intermediary surface structures are also observed. This study demonstrates the transition of the crystal structure of an oxide film from a unique ultrathin film structure that is epitaxially constrained by the interaction with the Au(111) substrate to that of a thicker film with the structure of a bulk crystal.