Kinetically Stable and Highly Ordered Two-Dimensional CN2 Crystal Structures

We present three dynamically and thermally stable polymorphs of CN2, which we identify through a systematic, multistep, and comprehensive structure search. They consist of nitrogen-rich structural motifs and modestly large pores. Kinetic stability is examined through long-term molecular dynamical simulations at an elevated temperature, where the interatomic interactions are treated using a new machine learning potential. Using density functional theory, we demonstrate their structural and electronic properties and compare them to polymorphs already available in the literature. The proposed structures are considerably more thermodynamically stable than their counterparts while desirably exhibiting energy levels suitable for photocatalytic water splitting. Also, calculations of enthalpy up to a modest value of external pressure show that the predicted polymorphs are thermodynamically more favorable than the previously proposed structures for high-pressure CN2.