Growth and Characterization of Crystalline Silica Films on Pd(100)

Silica films grown on Pd(100) were characterized by Auger electron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). While no evidence of long-range order could be detected for films grown below 600 K, STM images of these films nevertheless revealed flat surfaces through which the step-terrace structure of the substrate could be seen. Annealing the films in 10(-6) Torr of O-2 above 975 K resulted in crystalline bilayers that produced hexagonal LEED patterns with a periodicity twice that of the substrate and with one of the overlayer close-packed directions paralleling Pd[011]. The extent of the crystalline domains was limited to typically five repeat units along two of the three close-packed directions of the film but was tens of repeat units long along the third. The lattice matching to the substrate expands the spacing in the bilayer on Pd(100) compared to bulk crystalline SiO2 and bilayers observed on other substrates; as a consequence, it is suggested that the regular domain boundaries that form help relieve stress. The dominant features in high-resolution STM images were dark pores surrounded by six other pores; consistent with prior studies, these features are assigned to six-membered rings of corner-sharing SiO4 tetrahedra. Elongation of the pores at the domain boundaries is attributed to insertion of edge-sharing tetrahedra into the rings. Ab initio calculations on freestanding bilayers were performed to understand the effect of the substantial strain on the growth and structure of the film. The results indicate that relaxation orthogonal to the commensurate direction can greatly reduce the strain energy; as a consequence, the square substrate promotes epitaxial growth of crystalline SiO2 by providing an incommensurate direction along which the film can relax.