First-Principles Studies Of 2D Honeycomb Multiferroic Materials And Multiferroic Couplings

Multiferroic materials generally refer to materials with two or more basic ferroic orders?like ferroelectric order,ferromagnetic order,ferroelastic order,etc.?in one phase,which involve a rich physical mechanism and have a wide range of applications.Since the discovery of the first magneto-electric multiferroic material Cr₂O₃ in 1960,the field has been developed for more than 50 years,during the time it has experienced ups and downs.The main reason lies in the mutual repulsion of ferroelectricity and ferromagnetism.First,ferroelectric materials are usually semiconductors or insulators,while ferromagnetic materials are usually metals;secondly,the"d⁰-ness"restricts the coexistence of ferroelectricity and ferromagnetism.In the past,the study of multiferroic materials mainly focused on the bulk perovskite structure.A successful example is the perovskite structure bismuth ferrite?BiFeO₃?.However,such bulk multiferroic materials usually exhibit anti-ferromagnetic property,this limits its applications.On the other hand,the slowdown of"Moore's Law"made the industry desire for the miniaturization and integration of multi-functional devices more and more.However,due to the limitation of the finite size effect,the depolarization field and the charge shielding effect,the miniaturization of the conventional perovskite structure ferroelectrics/multiferroics has encountered serious obstacles.This led people pay more and more attention to low-dimensional materials.Compared with bulk materials,low-dimensional materials?mainly two-dimensional materials?have natural advantages in miniaturization and integration of devices.In addition,low-dimensional materials are more prone to spatial symmetry breaking due to the reduction of dimensions,which is conducive to the emergence of ferroelectricity.The way to get magnetism is also more diverse in low-dimensional materials?eg by introducing light elements such as hydrogen or fluorine or other functional groups?.It is an opportunity for us to combine ferromagnetism and ferroelectricity in two-dimensional materials.In this paper,we carry out a series of theoretical researches on two-dimensional multiferroic materials.In addition to the magnetoelectric multiferroic materials,we also present theoretical evidence for a class of ferroelastic-ferroelectric-type multiferroic materials.Specifically:?1?The design of two-dimensional metal-free organic multiferroic material for multifunctional integrated circuits.We report first-principles calculations evidence of C₆N₈H organic network is the first 2D organic multiferroic material with coexisting ferroelectricity?FE?and ferromagnetism?FM?properties.Its ferroelectricity stems from the proton-transfer within the 2D organic network,in which a longer-range proton-transfer pathway is induced by the facilitation of oxygen molecule when the system is exposed to the air.Such oxygen-assisted ferroelectricity also leads to a high Curie temperature and a coupling between FE and FM.We also find that hydrogenation and carbon doping can transform the 2D g-C₃N₄ network from an insulator to an N/P-type magnetic semiconductor with modest bandgap.Akin to the dopant induced N/P channels in silicon wafer,a variety of dopant created functional units can be integrated into the g-C₃N₄ wafer by design for nanoelectronic applications.?2?The designs of two-dimensional diluted multiferroic semiconductors.We studied the transition-metal interlayer doped bilayer MoS₂ or similar systems by carrying out first-principles calculations implemented in VASP.The results show that suitable transition-metal interlayer doping can induce ferroelectricity or multiferroicity?FE and FM?in bilayer MoS₂ or similar systems.Especially,ferroelectricity?FE?and ferromagnetism?FM?are obviously coupled,which means the polarization orientation and spin density distribution can be cross-regulated by electric field and magnetic field.The bandgap of the intercalated systems can also be tuned via controlling the doping density of ions,which may render a spatial-varying bandgap for efficient light adsorption plus the enhanced exciton separation and open-circuit voltage for ferroelectric photovoltaics.In particular,other intriguing properties like topological superconductivity may also be endowed in ferroelectric metal Cu_xBi₂Se₃.?3?The multiferroicity of honeycomb binary compounds:coupling of ferroelasticity and ferroelectricity.We present first-principles calculations evidence of honeycomb elements are generally ferroelastic?FA?.For monolayer with metallic bonds like stanene,stanane and InBi that have been synthesized,the ferroelastic mechanism can be achieved under ambient conditions,for their ferroelastic pathway renders low switching barrier and stress with the highest energy/stress point in the elastic region.Especially that the binary compounds are also ferroelectric?FE?.The FA and FE are strongly coupled,which means the orientation of polarization and ferroelasticity can be cross-regulated by electric field and stress.A general trend of linear decrease in switching barrier with uprising metallicity for the same group compounds is revealed,and similar behaviors can be extended to bulk zinc-blended or wurtzite structure that can be deemed as multilayer stacking of buckled monolayer.