THE GROWTH OF IRON-OXIDE FILMS ON PT(111) - A COMBINED XPD, STM, AND LEED STUDY

The three complementary surface structure probes of X-ray photoelectro n diffraction (XPD), scanning tunneling microscopy (STM), and low-ener gy electron diffraction (LEED) have been combined in a single instrume nt. This experimental system has been utilized to study the growth of iron oxide films on Pt(lll) over the thickness range from 0.75 to 3.00 monolayers (ML). Each film was formed by first depositing an overlaye r of purr Fe with a certain coverage in ML (ranging from 0.75 ML to 3. 00 ML) and then thermally oxidizing the Fe at a temperature of 980 K a nd in an oxygen pressure of 4 x 10(-6) Torr. For films up to similar t o 1 ML in thickness, a bilayer of Fe and O similar to those in bulk Fe O parallel to a(lll) plane formed. In agreement with a prior STM and L EED study by Galloway et al., we find this bilayer to be an incommensu rate oxide film forming a lateral superlattice or Moire structure with short- and long-range periodicities of similar to 3.1 and 26.0 A. Fro m the XPD data, in addition, it can be concluded that the topmost oxyg en layer is highly relaxed inward by similar to 0.6 Angstrom as compar ed to the bulk FeO (111) interplanar spacing, and that the stacking of the topmost O atoms with respect to the underlying Pt is dominated by one of two structurally very similar possibilities. It is furthermore necessary to consider interactions over at least four atomic layers ( O, Fe, and the first two Pt layers) to explain this dominance of one s tacking type. For thicker iron oxide films from 1.25 to 3.0 ML, the gr owth mode is essentially Stranski-Krastanov: iron oxide islands form o n top of the FeO(lll) bilayer mentioned above. For iron oxide films of 3.0 ML. thickness, X-ray photoelectron spectroscopy (XPS) yields an F e 2p(3/2) binding energy and an Fe:O stoichiometry consistent with the presence of Fe3O4. XPD data further prove this overlayer to be Fe3O4( 111)-magnetite in two almost equally populated domains with a 180 degr ees rotation between them. The structural parameters for this Fe3O4 ov erlayer generally agree with those of a previous LEED study, except th at we do not find a terminating partial monolayer of Fe and arrive at a significant difference in the first Fe-O interplanar spacing. Overal l, this work demonstrates the considerable benefits to be derived by u sing this particular set of complementary surface structure probes in such epitaxial growth studies. (C) 1998 Published by Elsevier Science B.V. All rights reserved.