Structural and mechanical properties of low-density amorphous carbon nitrides by molecular dynamics simulations

Based on ab initio molecular dynamics method, four amorphous CNx structures with different stoichiometries (CN0.47, CN0.67, CN0.92, and CN1.3) were generated within a 100-atom supercell. Characterizations of the pair correlation functions, bond length and the fraction of bond types of the amorphous carbon nitrides revealed that the N content in such structures plays a key role in determining the obtained structural networks. The calculations of the cohesive energy give direct evidence that experimentally synthesized amorphous carbon nitride films usually possess low nitrogen concentration. The competition relationship between sp2 and sp3 bonds was analyzed by electronic effect and electron density of states. The bonding states, especially the sp3, affect greatly the hardness of these a-CNx compounds, implying a possible way to enhance the hardness of films via tuning the bonding states.

► We generated four with different stoichiometries a-CNx using MD method.
► The competition between sp2–sp3 of a-CNx is interpreted by the electronic effect.
► No superhard stoichiometry a-CNx was found.
► The hardness of the a-CNx was determined by the fraction of sp3 C–C bond.