Prediction Of Ultra-flexible 2D Carbon Nitride And Tunable Molecular Sieves

Based on evolutionary algorithm and first-principles calculation,we have carried out a comprehensive structural search for two-dimensional C₃N₄ system and found three structures,i.e.,P6/m-C3N4,P2/m-C3N4 and P6?2m-C₃N₄,which are more stable than graphitic-C₃N₄.More importantly,through deformation testing,it is found that all three structures exhibit ultra-flexible characteristics.Their flexibility is much better than graphitic-C₃N₄,and the minima in the energy landscapes are remarkably broad.When the lattice constants change±5%,the energy（stress）changes of P6/m-C3N4,P2/m-C3N4 and P6?2m-C₃N₄ are just±23.52,±42.21 and±3.41 meV/atom（±0.33,±0.53and±0.07 GPa）,respectively,far less than the average value of graphitic-C₃N₄,125.24meV/atom（3.27 GPa）.In other words,the stress could be 46 times smaller than that of graphitic-C₃N₄.This remarkable behavior is attributed predominantly to the pronounced angular flexibility of the N-C-N bond linking the C₃N₃ rings.In addition,the connected C₃N₃ rings formed hexagonal（128?²）,quadrilateral（112?²）and trigonal（108?²）pores,respectively.The pores are tunable.It is predicted that they have broad application prospects in tunable molecular sieves.Finally,we analyze the variation of electronic structure during the deformation of new structures.