Mesoscopic Structures and Coexisting Phases in Silica Films

Abstract

Silica films represent a unique two-dimensional film system, exhibiting both crystalline and vitreous forms. While much scientific work has focused on the atomic-scale features of this film system, mesoscale structures can play an important role for understanding confined space reactions and other applications of silica films. Here, we report on mesoscale structures in silica films grown under ultrahigh vacuum and examined with scanning tunneling microscopy (STM). Silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. Both holes in the film structure and atomic-scale substrate steps are observed to influence these coexisting phases. In particular, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions where the majority film structure is crystalline. At high coverages mixed zigzag and bilayer phases are observed at step edges, while at lower coverages silica phases with lower silicon densities are observed more prevalently near step edges. The STM images reveal that silica films exhibit rich structural diversity at the mesoscale.

Figure 1

Coexisting silica structures grown on Ru(0001). Side views are shown with atomic models of possible silica structures (a–d), and top-view terraces are show with scanning tunneling microscopy images (e, f). Color-coded overlays on the lower right of the STM images and as the background color for the atomic models indicate the four commonly observed structures: (a) O(2 × 2) on Ru(0001), yellow stripes; (b) monolayer silica, pink stripes; (c) zigzag phase silica, blue diamonds; and (d) bilayer silica, solid green. STM images show the coexistence of multiple phases from films with sub-bilayer coverage: (e) Holes incorporated in bilayer silica, a hole with 3O(2 × 2)-Ru (left) and a hole with monolayer silica (right); scale 11.5 nm × 11.5 nm, V_S = 0.1 V, I_T = 10 pA. (f) Zigzag patches with bilayer silica: scale 11.5 nm × 11.5 nm, V_S = 1.0 V, I_T = 30 pA.

Figure 2

Overview STM images of observed silica structures with step edges (a, b) and color-coded images (c, d). Scale: 40 nm × 40 nm. (a, c) V_S = 1.5 V, I_T = 20 pA; (b, d) V_S = 2.0 V, I_T = 50 pA.

Figure 3

Histograms showing the distributions of hole areas for three different silica film coverages. Silica films represent three distinct sample preparations, all prepared by using the standard procedure described in the Experimental Methods, with variations in the amount of silicon deposited. In each case, the bin height represents the fraction of the total number of holes which exhibit the binned range of areas. STM images at each coverage are shown as insets. Scale 15 nm × 15 nm; (a) V_S = 2.0 V, I_T = 10 pA; (b) V_S = 3.0 V, I_T = 10 pA; (c) V_S = 3.0 V, I_T = 10 pA.

Figure 4

STM images showing ring-size distributions around (a) a hole exposing O(2 × 2)-Ru located within a predominantly crystalline region of a silica bilayer film, (b) a hole exposing O(2 × 2)-Ru located within a vitreous bilayer silica region, and (c) a hole filled with ML silica. Ring sizes, defined by the number of neighboring rings, are indicated as an overlay with colored circles in (b) and (c). (d) Statistical distribution of ring sizes near and far from the holes in images (a–c). For all images, the region far from the hole exhibits comparable or greater crystallinity than the region near the hole. Scale (a) 10.4 nm × 17.9 nm, V_S = 2.0 V, I_T = 70 pA; (b) 11.4 nm × 7.5 nm, V_S = −1.0 V, I_T = 10 pA; (c) 11.4 nm × 7.5 nm, V_S = 1.0 V, I_T = 10 pA.