  Free-standing 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. 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 six-fold 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 suggest that synthesis of O-silica can be achieved in
experiments.
