Effect of vacancy defect and strain on the structural, electronic and magnetic properties of carbon nitride 2D monolayers by DFTB method.

We investigate the electronic and magnetic properties of CnNm(C 6 N 6 , C 2 N, C 3 N and C 3 N 4 ) using density functional tight-binding (DFTB) method. We find that these compounds are dynamically stable and their electronic band gaps are in the range of 0.59-3.28 eV. We show that the electronic structure is modulated by strain and the semiconducting behavior is well preserved except for C 3 N at +5% biaxial strain, where a transition from semiconductor to metal was observed. Under +3% biaxial strain, C 3 N 4 undergoes a transition from an indirect (K-Γ) to a direct (Γ-Γ) band gap. Moreover, band gap of C 2 N transforms from direct (Γ-Γ) to indirect (M-Γ) under +4% biaxial strain. However, no change in the nature of the band gap of C 6 N 6 . Further, when the studied materials under uniaxial tensile strain, their bandgaps reduce. Our theoretical study showed that an indirect-to-direct nature transition may occur for C 6 N 6 and for C 3 N 4 , which broadens their applications. On the other hand, magnetism is observed in all N-vacancy defected CnNm, which encourages its application in spintronic. Moreover, calculations of formation energies indicate that N-vacancy is energetically more favorable than C-vacancy in both C 2 N and C 3 N 4 . Opposite behavior found for C 6 N 6 and C 3 N.