Directing the Structure of Two-Dimensional Silica and Silicates

Density functional theory was used to assess the viability of approaches to controlling the structure of a recently discovered two-dimensional form of SiO₂. In accord with prior work, a hexagonal bilayer of mirror image planes of corner-sharing SiO₄ tetrahedra in six-membered rings yielded only a slightly higher energy than α-quartz. Structures including four- through eight-membered rings were evaluated and in certain cases found to be as little as 17 meV/Si higher in energy than the hexagonal bilayer. When either biaxial or uniaxial tensile strain was applied, combinations of eight-, six-, and four-membered rings became favored due to the lower density of structures with larger rings. These findings, together with experiments that reveal expansion of silica bilayers to match the lattice of metal substrates, suggest that epitaxial strain may be used to control the bilayer structure. Replacement of Si with Ge and Al as prototypical tetravalent and trivalent dopants was also investigated. Substituting Ge for Si was energetically unfavorable and offered no obvious advantage for structural control over pure SiO₂ bilayers. In contrast, Al substitution was energetically favorable and only minimally distorted the bilayer. It was found that while the hexagonal bilayer remained favored, the extra-framework electron donors K and H that accompany each Al preferred to occupy larger rings when possible, thus forcing Al to reside in large rings as well. This suggests that the bilayer structure may be controlled through substitution of Si for trivalent dopants and selection of extra-framework electron donors that favor larger rings