Abstract:

Carbon nitride materials are promising for applications in electronics, clean energy production, and heat dissipation. Two-dimensional (2D) diamond-like carbon nitrides α-C2N2, β-C2N2, and γ-C4N4 rise as beyond graphene semiconductors. Here, we apply first-principles calculations and group theory to study their structure, mechanical properties, and vibrational signature. The α-C2N2 is the strongest among them, with a 2D Young’s modulus E2D equal to 616(6) N/m, followed by the γ-C4N4 with an E2D equal to 632(6) N/m and 581(7) N/m along its zigzag and armchair directions, respectively, and the β-C2N2 with an E2D equal to 582(9) N/m. These materials are about 2 times stiffer than graphene, and are the stiffest among 2D networks of carbon and nitrogen atoms. The zigzag direction of 2D γ-C4N4 is approximately 8% stronger than its armchair direction, unusual for in-plane anisotropic 2D materials, where the armchair direction is considerably weaker than the zigzag direction. These findings from stress–strain analysis are consistent with the high sound speed and elastic constants values we found by using 2D density-functional perturbation theory framework, suggesting them for mechanical reinforcement. We report the phonon wavenumber, atomic vibrational pattern, and Raman and infrared spectra for all polytypes. The longitudinal and transverse optical modes of the in-plane isotropic polytypes display the breakdown of LO–TO splitting, characteristic of 2D polar crystals. We found that the difference between their phonon wavenumbers can be probed in their unpolarized Raman and infrared spectra. The simulated angular dependency of the Raman intensity under backscattering parallel and cross polarizations show how to assign the A1′ and E′ modes of the α-C2N2, the A1g and Eg modes of the β-C2N2, and of the Ag and B1g modes of the γ-C4N4, being key for polytype identification. These results provide comprehensive information on the emerging 2D diamond-like carbon nitrides, necessary for further developments on their synthesis, characterization, and future device fabrication.