X-ray standing wave imaging of metal atoms on semiconductor and oxide surfaces

The 1/3 monolayer (ML) Sn/Si(III)-(√3 x √3)R30° surface structure has been extensively studied using x-ray standing waves (XSW). The summation of several XSW measured hkl Fourier components results in a three-dimensional, model-independent direct-space image of the Sn atomic distribution. While the image demonstrates that the Sn atoms are located at Si(111) T4-adsorption sites, it alone cannot determine whether the Sn atomic distribution is flat or asymmetric. However, conventional XSW analysis can make this distinction, concluding that one-third of the Sn atoms are located 0.26 A higher than the remaining two-thirds. This "one up and two down" distribution is consistent with the vertical displacements predicted by a dynamical fluctuations model. A second sample prepared in a slightly different manner exhibits the same long-range surface symmetry, but a direct space image clearly reveals that a significant fraction of the Sn atoms in the second surface have substituted for Si atoms in the bottom of the Si surface bilayer.; In addition, the electronic and atomic-scale structure of 1/2 ML V on alpha-Fe 2O3(0001) as a function of the vanadium oxidation state has also been investigated using x-ray photoemission spectroscopy (XPS) and XSW direct-space imaging. The XSW data is used to generate a direct-space image of the vanadium distribution as the surface is oxidized and reduced using atomic oxygen and hydrogen. The direct-space atomic density profile shows the average adsorption height of the V atoms is increased by 0.39 A after the vanadium is oxidized, which suggests the oxidized vanadium is arranged in VO4 units. The direct-space image also shows a secondary V position, which can be explained by the chemical interaction between these VO4 units. These structural changes can be understood in light of recent XPS results, which suggest the exposure of the vanadium adlayer to atomic oxygen converts the vanadium into a V2O5 film and reoxidizes the hematite at the film-substrate interface. The current XSW results also suggest the oxidation of the supported vanadium is a reversible process; by exposing the submonolayer vanadia to atomic hydrogen, the oxidized vanadium is reduced and the geometry of the as-deposited surface is restored.