Novel Two-Dimensional Silica Monolayers with Tetrahedral and Octahedral Configurations

Freestanding and well-ordered two-dimensional (2D) silica monolayers with tetrahedral (T-silica) and octahedral (O-silica) building blocks are found to be stable by first-principles calculations. T-silica is formed by corner-sharing SiO4 tetrahedrons in a rectangular network, and O-silica consists of edge-sharing SiO6 octahedrons. Moreover, the insulating O-silica is the strongest silica monolayer, and can therefore act as a supporting substrate for nanostructures in sensing and catalytic applications. Nanoribbons of T-silica are metallic, while those of O-silica have band gaps regardless of the chirality and width. We find the interaction of O-silica with graphene to be weak, suggesting the possibility of its use as a monolayer dielectric material for graphene-based devices. Considering that the sixfold-coordinated silica exists at high pressure in the bulk phase, the prediction of a small energy difference of O-silica with the synthesized silica bilayer, together with the thermal stability at 1000 K, suggests that synthesis of O-silica can be achieved in experiments.