Modeling vitreous silica bilayers

Abstract

Theoretical modeling is presented for a free-standing vitreous silica bilayer which has recently been synthesized and characterized experimentally in landmark work. While such two-dimensional continuous random covalent networks should likely occur on energetic grounds, no synthetic pathway had been discovered previously. Here the bilayer is modelled using a computer assembly procedure initiated from a single layer of a model of amorphous graphene, generated using a bond switching algorithm from an initially crystalline graphene structure. Each bond is decorated with an oxygen atom and the carbon atoms are relabeled as silicon, generating a two dimensional network of corner sharing triangles. Each triangle is transformed into a tetrahedron, by raising the silicon atom above each triangular base and adding an additional singly coordinated oxygen atom at the apex. The final step in this construction is to mirror-reflect this layer to form a second layer and attach the two layers to form the bilayer. We show that this vitreous silica bilayer has the additional macroscopic degrees of freedom to form easily a network of identical corner sharing tetrahedra if there is a symmetry plane through the center of the bilayer going through the layer of oxygen ions that join the upper and lower monolayers. This has the consequence that the upper rings lie exactly above the lower rings, which are tilted in general. The assumption of a network of perfect corner sharing tetrahedra leads to a range of possible densities that we characterize as a flexibility window; with some similarity to flexibility windows in three dimensional zeolites. Finally, using a realistic potential, we have relaxed the bilayer to determine the density and other structural characteristics such as the Si-Si pair distribution functions and the Si-O-Si bond angle distribution, which are compared with experimental results obtained by direct imaging.

Detailed analysis of plastic shear in the Raman spectra of SiO2 glass

“…1)) with an increasing number of particles. The obtained distribution is in agreement with the last results of D. Waroquiers [23], and a similar distribution was also obtained in the recent experimental results of monolayer SiO 2 [24][25][26]. For unknown reasons, the number of small 3-and 4-membered rings is always larger in our systems than in experimental measurements.…”

Structural and Electronic Heterogeneity of Two Dimensional Amorphous Silica Layers

“…A recent theoretical analysis of amorphous silica bilayers concluded that the tetrahedra must tilt about the central plane. [ 17 ] As a result, unlike the crystalline bilayer, the three oxygens forming the base of the tetrahedra at the interface will not all be the same distance from the Pd surface, thereby creating variations in the coupling between the oxygens and the metal surface. This tilting is expected to be most dramatic at high stress areas, consistent with the observation of the fading walls of larger rings.…”

Ring correlations in random networks

“…These new results on 2D glasses have opened up numerous opportunities to study the structure of glasses using actual atomic coordinates. Recent work on 2D glasses includes modeling of silica bilayers [13,14], ring distribution [15], medium-range order [16], suitable boundary conditions to recover missing constraints in the surface [17] and the refinement of experimental samples [18]. Rigidity theory has also uncovered a connection between 2D glasses and jammed disk packings [19,20].…”