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

Abstract

Ultrathin crystalline silica layers grown on a Mo͑112͒ 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 SiO 2 /Mo(112). The long-range order of the silica epilayer which grows commensurate with a c(2ϫ2) surface unit mesh on the Mo͑112͒ substrate is studied by low-energy electron diffraction ͑LEED͒. The defect structure of the silica epilayer is characterized in a spot profile analysis ͑SPA͒-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 ͑STM͒ 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 Å 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 Mo͑112͒ surface. These various surface science studies allow us to propose models for the growth and structure of the silica epilayer on the Mo͑112͒ surface.

On the geometrical and electronic structure of an ultra-thin crystalline silica film grown on Mo(112)

“…In this model, the forth ("apical") oxygen atom of each tetrahedra (which remains uncoordinated in the case of a monolayer silica film) must be stabilized either by hydrogen, thus forming silanol (Si-OH) groups, or shared with further silica layers. Since neither silanol nor hydroxyl species were observed on these films [18], this implied that the silica film was of several layers in thickness.…”

“…2a). In addition, well-resolved images of antiphase domain boundaries (APDB) running along the 10] 1 [ direction as predicted by LEED studies [18] were obtained as indicated in Fig. 2a.…”

“…In particular, formation of two-membered rings were invoked to explain a weak vibration band around 795 cm -1 experimentally observed by Schroeder et al [18].…”

“…Although the first attempts to synthesize crystalline silica films can be traced back to the early 90's [13,14], the preparation of well-ordered thin silica films was reported only recently by Schroeder et al [15][16][17][18] using a Mo(112) substrate, who basically modified a recipe originally applied to the Mo(100) and Mo(110) substrates by Goodman and coworkers [13,14]. The film exhibited a c(2x2) diffraction pattern with respect to Mo(112), and showed at least two oxygen species in X-ray photoelectron spectra (XPS) and a strong phonon absorption band centered at 1048 cm -1 observed by infrared reflection absorption spectroscopy (IRAS).…”

“…In the first structural model proposed by Schroeder et al [18], the film was bonded to the Mo substrate through three oxygen atoms sitting in pseudo three-fold hollow sites formed between protruding and trough Mo atoms, forming rows on the Mo(112) 4 ] tetrahedron is the building unit for the silica films, the proposed structure involved a maximum coordination of oxygen atoms to the Mo surface, i.e. three O atoms of each tetrahedra were bonded to the Mo atoms.…”