First-principles Design Of Two-dimensional Ferroelectrics/Multiferroics And Multifunctional Applications

Multiferroics refer to the material that has two or more kinds of ferroic orders(ferromagnetic(antiferromagnetic),ferroelectric,ferroelastic)with order parameters switchable by external stimuli.Such functional materials have great applications in various fields(new magnetoelectric devices,non-volatile storage,and spintronic devices).In the past,research on multiferroics was mainly in perovskite materials.Since then,thin-film perovskite materials have attracted much attention.Since graphene was successfully stripped in 2004,a large number of two-dimensional materials have been discovered.Two-dimensional materials with atomic thickness provide new possibilities for ferroelectrics.For application consideration,atomically thin ferroelectrics,if any,would make ultra-high-density data storage possible.The van der Waals interaction at the interface between 2D materials and 3D semiconductors also allows lattice mismatch for epitaxial growth.The scheme of‘magnetic reading+electrical writing'is easier to be realized in 2D multiferroics.In this article,we have carried out a series of theoretical research around multiferroics,not only designed a series of low-dimensional multiferroics with magnetoelectric coupling,but also tried to combine ferroelectric and photovoltaic materials to find ferroelectric and excellent photovoltaic materials.(1)We showed that halogen-modified phosphene systems can obtain vertical ferroelectric polarization and a"movable"magnetic two-dimensional multiferroic metal-free material by first-principles calculations.Its“mobile”magnetism could be controlled by ferroelectric switching upon application of an external electric field.These materials have great advantages for non-volatile storage because of their high switching ratio when designed as a multiferroic tunnel junction.Because covalently bonded ferroelectrics and ionic bonded ferroelectrics have different characteristics,they still have robust vertical polarization in the depolarized field.This excellent feature can be extended to other van der Waals bilayer structures for non-volatile storage.(2)We have proposed a new type of single-molecule ferroelectric and multiferroic designs.In these systems,each molecule can store one bit of data,which can be used as an ultra-high density memory.The storage density breaks through the limitation of the traditional memory with a storage density of up to 10⁶GB/in.²(3)We had revealed the origin of vertical polarization in the WTe₂ bilayer by first-principles calculations:vertical polarization is caused by vertical charge transfer between layers due to translation in the plane,and translation between layers can be achieved by sliding between layers.Similar interlayer slip ferroelectricity may exist in a series of van der Waals bilayers and even bulk materials.(4)We designed a series of functional hybrid organic-inorganic perovskites(HOIPs)with oxygen-containing cations based on first-principles calculations.We also found that the formation of these HOIPs releases more energy than MAPbI₃,illustrating the possibility of simple fabrication of these materials.When a proton is bound to a molecule,it is ferroelectric because the cations have spontaneous dipole moments.Some of these HOIPs not only have direct band-gaps in the optimal range of 0.9-1.6 e V,but also have a more ideal light absorption spectrum than MAPbI₃.