Stable Single-Layer Honeycomblike Structure of Silica

Silica or ${\mathrm{SiO}}_2$, the main constituent of Earth’s rocks has several 3D complex crystalline and amorphous phases, but it does not have a graphitelike layered structure in 3D. Our theoretical analysis and numerical calculations from the first principles predict a single-layer honeycomblike allotrope, $h\alpha$ silica, which can be viewed to be derived from the oxidation of silicene and it has intriguing atomic structure with reentrant bond angles in hexagons. It is a wide band gap semiconductor, which attains remarkable electromechanical properties showing geometrical changes under an external electric field. In particular, it is an auxetic metamaterial with a negative Poisson’s ratio and has a high piezoelectric coefficient. While it can form stable bilayer and multilayer structures, its nanoribbons can show metallic or semiconducting behavior depending on their chirality. Coverage of dangling Si orbitals by foreign adatoms can attribute new functionalities to $h\alpha$ silica. In particular, ${\mathrm{Si}}_2{\mathrm{O}}_5$, where Si atoms are saturated by oxygen atoms from top and bottom sides alternatingly can undergo a structural transformation to make silicatene, another stable, single layer structure of silica.

Figure 1

(a) Side and top views of $h\beta$ silica. Large blue and small red balls stand for Si and O atoms. (b) Optimized structure of $h\alpha$ silica with its hexagonal and rectangular unit cells. Two types of Si atoms, i.e., $s{p}^3$ bonded and $s{p}^2$ bonded are indicated. (c) Calculated phonon bands and tilted view of the MD simulation in $4\times 4$ unit cell. (d) Free energy versus temperature.

Figure 2

(a) The electronic band structure of $h\alpha$ silica. (b) Isosurface charge densities of the lowest conduction(CB) and highest valence (VB) band, and contour plots of the total charge density in the planes of O-Si-O for the $s{p}^2$- and the $s{p}^3$-bonded Si atoms. (c) Images of the total charge density. (d) Variation of the band gap as a function of the uniaxial strains ${\epsilon }_x$ and ${\epsilon }_y$.

Figure 3

(a) Electronic band structure of zigzag nanoribbon with $N_Z=4$. (b) Same for the armchair nanoribbon with $N_A=4$. (c) Variation of the width of the armchair nanoribbon $\mathrm{\Delta }w$ and bond angle $\mathrm{\Delta }\varphi$ under an in-plane electric field $\overrightarrow{E}$ applied perpendicular to the axis.

Figure 4

(a) Silicatene derived from ${\mathrm{Si}}_2{\mathrm{O}}_5$. (b) Atomic structure of silicatene with side, top, and tilted views. Pentagons are formed from three O and two Si atoms. (c) Electronic band structure. Bands derived from O-O bonds are indicated.