Tunable Dirac cones in two-dimensional covalent organic materials: C2N6S3 and its analogs

Abstract

Two-dimensional covalent organic frameworks (2D-COFs) are drawing increasing interest due to the unique configurations and exotic properties. Here, using density-functional theory calculations, we prove the stability of C₂N₆S₃ monolayer by an imagery-frequency-free phonon spectrum, and demonstrate a new ternary 2D-COF: C₂N₆O₃, C₂N₆Se₃ and C₂N₆Te₃ monolayers. The sawtooth-like linkages make the C₂N₆S₃ is soft, and sustain a biaxial tensile strain up to 24% which is as much as graphene. The electronic band structure exhibits linear dispersion near the Fermi level with a flat band right above the Dirac bands, which is unlike the other hexagonal organic monolayers with Dirac cone. The Fermi velocity is comparable to that in graphene and can be tuned by applying biaxial tensile strain. Similar results are also found in its analogs, such as C₂N₆O₃, C₂N₆Se₃ and C₂N₆Te₃ monolayers. This opens an avenue for the design of 2D Dirac materials.