LAUSR

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.