Morphological and electronic properties of ultrathin crystalline silica epilayers on a Mo(112) substrate

Abstract

Ultrathin crystalline silica layers grown on a Mo(112) substrate have been shown to be a useful silica model oxide support in surface science model catalyst studies. As the oxide support material plays an important role in the catalytic process, a multitechnique surface science study is presented to characterize the morphological and electronic properties of the heteroepitaxial system ${\mathrm{SiO}}_{2}/\mathrm{Mo}(112).$ The long-range order of the silica epilayer which grows commensurate with a $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface unit mesh on the Mo(112) substrate is studied by low-energy electron diffraction (LEED). The defect structure of the silica epilayer is characterized in a spot profile analysis (SPA)-LEED study. Antiphase domain boundaries split the silica epilayer into an array of silica crystal grains whose average size and shape is determined. Aiming to prepare flat silica surfaces, the change in the surface roughness with progress in the film preparation is monitored in a combined SPA-LEED and scanning tunneling microscopy (STM) study and seen to influence also the Si-O stretching frequency in the infrared-reflection-absorption spectroscopy spectra. In STM images of the final silica film an average surface roughness of about 1 \AA{} is detected. It is possible to visualize the silica film unit cell periodicity. A combined anger electron spectroscopy and ultraviolet photoelectron spectroscopy valence band study confirms the silica film stoichiometry and the growth of a 4:2 coordinated silica polymorph on the Mo(112) surface. These various surface science studies allow us to propose models for the growth and structure of the silica epilayer on the Mo(112) surface.