Superior stiffness and vibrational spectroscopic signature of two-dimensional diamond-like carbon nitrides

Abstract

Abstract Carbon nitride materials are promising for applications in electronics, clean energy production, and heat dissipation. Two-dimensional (2D) diamond-like carbon nitrides α -C 2 N 2 , β -C 2 N 2 , and γ -C 4 N 4 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 -C 2 N 2 is the strongest among them, with a 2D Young’s modulus E 2D equal to 616(6) N/m, followed by the γ -C 4 N 4 with an E 2D equal to 632(6) N/m and 581(7) N/m along its zigzag and armchair directions, respectively, and the β -C 2 N 2 with an E 2D 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 -C 4 N 4 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 A 1 ′ and E ′ modes of the α -C 2 N 2 , the A 1 g and E g modes of the β -C 2 N 2 , and of the A g and B 1 g modes of the γ -C 4 N 4 , 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.