Crystalline-Vitreous Interface in Two Dimensional Silica

The interface between a crystalline and a vitreous phase of a thin metal supported silica film was studied by low temperature scanning tunneling microscopy. The locally resolved evolution of Si-Si nearest neighbor distances and characteristic angles was evaluated across the border. Furthermore, we investigated the behavior of the ring size distribution close to the crystalline-vitreous transition. The crystalline order was found to decay gradually within about 1.6 nm into the vitreous state.

Figure 1

(a) Atomically resolved STM image of the crystalline-vitreous interface in the silica film ($V_S=2\mathrm{V}$, $I_T=100\mathrm{pA}$, $\mathrm{\text{scan area}}=12.3\mathrm{nm}\times 7.0\mathrm{nm}$). (b) The STM image, superimposed by the atomic model of the topmost layer (Si: large green balls; O: small red balls). The bar below indicates the crystalline and the vitreous areas. The black arrow shows the direction of the interface analysis.

Figure 2

Evaluation of the Si-Si distances. (a) The Si-Si NN distances are visualized by colored bars. The color scale represents the distance between two Si atoms (see the scale bar). (b) The position of ten vertical, 1.23 nm wide, and 6.95 nm high slices. (c) The average Si-Si NN distances for every slice are plotted vs the lateral coordinate of the respective slice center. Error bars represent the standard deviation of a Gauss fit to the distance distribution. The dashed line specifies the average value for the whole image ($0.303\mathrm{nm}\pm 0.025\mathrm{nm}$).

Figure 3

Evaluation of Si-Si NN DDOs. (a) The orientation of the directed Si-Si NN distances is displayed using different colors. The color scale indicates the orientation of Si-Si line segments with respect to the abscissa of the image plot (see the scale bar). Arrows show examples of rows in the crystalline phase, where the DDOs slightly alternate. (b) The position of 50 vertical, 0.25 nm wide, and 6.95 nm high slices. (c) DDO values for every slice plotted vs the slice’s lateral coordinate.

Figure 4

Analysis of the rings at the crystalline-vitreous interface of the silica film. (a) Visualization of the different ring sizes. Polygon areas range from 0.1 to $0.5{\mathrm{nm}}^2$ and ring sizes from four to nine Si atoms per ring (see the scale bar). (b) The position of 30 vertical, 0.41 nm wide, and 6.95 nm high slices. (c) The ring size distribution plotted for every slice. A logarithmic gray scale is used. (d) The number of rings per slice for every ring size plotted vs the lateral coordinate. (e) The crystallinity of every slice plotted vs lateral coordinate. The dashed horizontal line represents the overall crystallinity value of a vitreous region ($C=0.42$; see Supplemental Material [24] for a detailed analysis).