van der Waals forces control ferroelectric–antiferroelectric ordering in CuInP2S6 andCuBiP2Se6 laminar materials

摘要

We show how van der Waals (vdW) forces outcompete covalent and ionic forces to control ferroelectricordering in CuInP2S6 nanoflakes as well as in CuInP2S6 and CuBiP2Se6 crystals. While the self-assemblyof these 2D layered materials is clearly controlled by vdW effects, this result indicates that the internallayer structure is also similarly controlled. Using up to 14 first-principles computational methods, wepredict that the bilayers of both materials should be antiferroelectric. However, antiferroelectricnanoflakes and bulk materials are shown to embody two fundamentally different types of inter-layerinteractions, with vdW forces strongly favouring one and strongly disfavouring the other compared toferroelectric ordering. Strong specific vdW interactions involving the Cu atoms control this effect.Thickness-dependent significant cancellation of these two large opposing vdW contributions results ina small net effect that interacts with weak ionic contributions to control ferroelectric ordering.