New Spiral Form of Carbon Nitride with Ultrasoftness and Tunable Electronic Structures.

The structural diversity and multifunctionality of carbon nitride materials distinct from pure carbon materials are drawing increasing interest. Using first-principles calculations, we proposed a stable spiral structure of carbon nitride, namely spiral-CN, which is composed of sp-hybridized carbon and pyridine nitrogen with a 60° helical symmetry along the -direction. The stability was verified from the cohesive energy, phonon spectrum, and elastic constants. Despite the strong covalent bonds of the spiral framework, the spiral-CN exhibits a hardness lower than 12.00 GPa, in sharp contrast to the superhardness of cubic carbon nitrides reported in previous literature, which can be attributed to the unique porous configuration. The softness of the spiral-CN was also confirmed by the small ideal strengths, which are, respectively, 33.00 GPa at a tensile strain of 0.22 along the [1̅21̅0] direction and 18.00 GPa at a shear strain of 0.52 in the (0001)[1̅21̅0] direction. Electronic band structure of spiral-CN exhibits metallic features. A metal-semiconductor transition can be triggered by hydrogenation of the pyridine nitrogen atoms of spiral-CN. Such a new three-dimensional spiral framework of sp-hyperdized carbon and nitrogen atoms not only enriches the family of carbon nitride materials but also finds application in energy conversion and storage.