Atomic Structure of a Thin Silica Film on a Mo(112) Substrate: A Two-Dimensional Network of ${\mathrm{SiO}}_4$ Tetrahedra

The structure of a thin single crystalline ${\mathrm{SiO}}_2$ film grown on Mo(112) has been studied by scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. In excellent agreement with the experimental results, density functional theory calculations show that the film consists of a two-dimensional network of corner sharing $[{\mathrm{SiO}}_4]$ tetrahedra, with one oxygen of each tetrahedron binding to the protruding Mo atoms of the Mo(112) surface.

Figure 1

STM images of the ${\mathrm{SiO}}_2/\mathrm{Mo}(112)$ film. Size and tunneling parameters: (a) $100\times 100{\mathrm{nm}}^2$, $V_s=2\mathrm{V}$, $I=0.2\mathrm{nA}$, (b) $8\times 8{\mathrm{nm}}^2$, $V_s=1.2\mathrm{V}$, $I=0.35\mathrm{nA}$, (c) $8\times 8{\mathrm{nm}}^2$, $V_s=0.65\mathrm{V}$, $I=0.75\mathrm{nA}$. The arrow indicates the antiphase domain boundary running along the $[\overline{1}10]$ direction. The insets in (b) and (c) show the close up of the atomically resolved STM images (left) and simulated images (right) [the tunneling gap is set 6 Å at 1.2 V in (b) and 4 Å at 0.65 V in (c)].

Figure 2

IRAS-spectra of silica films prepared with ${}^{18}\mathrm{O}$ and ${}^{16}\mathrm{O}$. Calculated frequencies of the three IR active vibrations are represented by bars for three different models shown in Fig. 3, with the height proportional to the intensity normal to the surface. Vibrational modes (view along $[\overline{1}\overline{1}1]$ direction) of the most stable structure $A$ are schematically shown. O and Si are represented by black and gray spheres, respectively. For clarity the Mo(112) surface is drawn as sticks.

Figure 3

Three models of the single-layer ${\mathrm{SiO}}_2/\mathrm{Mo}(112)$ film. The ${\mathrm{Si}}_4{\mathrm{O}}_{10}$ surface unit cell is indicated.