Continuous network structure of two-dimensional silica across a supporting metal step edge: An atomic scale study

The network structure of a silica bilayer film at a monolayer-bilayer transition and across a supporting metal step edge was studied at the atomic scale by scanning tunneling microscopy. The ring size distribution, ring-ring distances, and height profiles are analyzed across the step edge region. Density functional theory proposes two models to explain the observed network structure: a pinning of the lower layer to the substrate and a carpetlike mode. The results indicate a continuous coverage of the silica bilayer film across the step edge.

Figure 1

(a) Ring resolved STM image of a ML-BL silica film transition region ($V_S=1$ V, $I_T=10$ pA, $T=4.2$ K, scan area = 15.3 nm $\times$ 8.1 nm; applied filters: align rows, equalize). The STM image is superimposed by the color-coded ring-ring (b) and Si-Si ring network model (c). The color codes for distances and ring sizes are provided at the right.

Figure 2

(a) Ring resolved STM image of the silica film across a single Ru(0001) step edge ($V_S=2$ V, $I_T=400$ pA, $T=295$ K, scan area = 15.3 nm $\times$ 8.1 nm). (b), (c) The STM image, superimposed by the ring networks in analogy to Fig. 1.

Figure 3

DFT models of the crystalline silica bilayer across a Ru step edge. The topmost oxygen atom positions are plotted in comparison with STM line profiles. (a) Case I and (b) case II: DFT model on a $\mathrm{Ru}\left(10\overline{1}20\right)$ surface. (c) Case III: DFT model on a $\mathrm{Ru}\left(10\overline{1}31\right)$ surface. (d) STM line profiles of a Ru step edge covered with ${\mathrm{SiO}}_2$ ($V_S=2$ V, $I_T=400$ pA, $T=295$ K). For comparison, the STM line profile of a bare Ru(0001) is plotted ($V_S=2$ V, $I_T=300$ pA, $T=295$ K). The vertical dotted lines mark the position of the substrate step edges. The inclined dotted lines mark the unit cell used for DFT calculations.