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. ■ INTRODUCTION Silica (SiO2) is one of the most abundant materials in Earth’s crust. Bulk silica has a rich phase diagram which includes αand β-quartz, αand β-cristobalite, tridymite, and stishovite. In general, silica prefers SiO4 tetrahedral unit at ambient conditions consisting of fourfold-coordinated Si at the center and twofold-coordinated O at the corners. At high pressure, the phases formed by the SiO6 octahedral unit with sixfoldcoordinated Si at the center and threefold-coordinated O at the corners are observed. Stishovite silica with the octahedral unit is thermodynamically stable above 7 GPa, and is the densest among the allotropes of silica due to its compact structure. Silica films spontaneously form on a clean silicon surface when the surface is exposed to air. Such two-dimensional (2D) silica films have found their importance in several applications, such as dielectric layers in integrated circuits and supporting substrates for catalysis. However, their amorphous nature limits the dielectric properties, and blocks the fabrication of continuous interfaces formed by the dielectric layer with other materials. Therefore, from an application’s perspective, well-ordered and freestanding 2D silica has great advantages over amorphous films. In particular, the monolayer crystalline silica is a much sought after 2D material as an ideal dielectric for the van der Waals (vdW) heterostructures. Following the stability criterion of the bulk form, 2D silica consisting of SiO4 tetrahedrons has recently been investigated. Silicatene with the SiO2.5 stoichiometry is realized on the Mo(112) surface. It is formed by corner-sharing SiO4 tetrahedrons in a hexagonal pattern. One of the oxygen atoms in each tetrahedron is bonded strongly to the Mo surface through the Si−O−Mo link, thus limiting the transferability of silicatene. Also, a bilayer silica has been successfully deposited on Ru(0001) surface with corner-sharing tetrahedrons. It has the SiO2 stoichiometry, and is weakly anchored on the metal surface due to vdW interactions. Such ultrathin bilayers, which could possibly be peeled off from the substrate, are now being explored by experiments. On the theoretical front, structural and electronic properties of α-silica with alternating sp and sp bonds have been reported recently. In spite of the extensive experimental efforts, freestanding and transferable monolayer silica has not yet been realized and the monolayer configuration beyond the tetrahedral building blocks has seldom been reported. There also appears to be a lack of fundamental understanding about the stability, electronic structure, and suitability of silica monolayers as a noninteracting dielectric in vdW electronics. Considering the importance of freestanding 2D oxides as potential dielectric layers in nanoelectronic devices and a wide range of other applications, such as photonics and optoelectronics, we have investigated the possible freestanding silica monolayers together with their stability, electronic band structure, and mechanical properties by the use of density functional theory (DFT) and particle swarm Received: February 17, 2015 Revised: May 11, 2015 Published: May 20, 2015 Article