Origin of the half-metallic properties of graphitic carbon nitride in bulk and confined forms

The electronic and magnetic properties of graphitic carbon nitride (g-C₄N₃) in both its bulk and confined form are investigated by using density functional theory. Our calculations show that the 2p_z orbital of the carbon atoms connecting the sym-triazine structure units plays a decisive role in the half-metallic character of these materials. In the bulk two-dimensional (2D) case, the 2p_z orbital of the connecting carbon atom couples weakly with the frontier molecular orbitals of the sym-triazine ring due to symmetry mismatch and a large energy separation. As a consequence a narrow energy band is formed near the Fermi level. This spin splits in virtue of the Stoner criterion and thereby gives rise to spin polarization in the (1 × 1) primitive cell. Such a magnetic state is not destroyed by a (2 × 2) reconstruction, which further enhances the half-metallic character of the 2D g-C₄N₃ sheet. When the dimension is reduced, size and edge effects become dominant. Both zigzag and armchair one-dimensional g-C₄N₃ nanoribbons with H-terminated edges preserve the half-metallicity, at least down to a critical ribbon width. Furthermore, the electronic and magnetic properties of these g-C₄N₃ nanoribbons can be tuned by controlling the π-type coupling between the passivating functional groups and the 2p_z orbital of the connecting carbon atoms at the ribbon edges, demonstrating that g-C₄N₃ may be an interesting material platform for applications in future spintronic devices.