Designing New Two-dimensional Ferroelectrics Based On Van Der Waals Stackings,Functionalizations Or Mixed-valence Systems

The rapid development of technology has promoted the miniaturization and integration of electronic devices.However,the Moore's law which has been effective over the past few decades is drawing to the end because of various challenges.The current prevailing dynamic random access memory(DRAM)is volatile where the stored data will be lost when the power is cut off.The ferroelectric-based nonvolatile memory is a promising candidate for the next-generation random access memory which possesses many advantages,such as higher writing performance,longer data retention times,lower power usage,etc.Nevertheless,for the conventional ferroelectrics,their poor compatibility with the silicon wafer and the finite size effect have impeded the miniaturization of the ferroelectric memories.Two-dimensional(2D)materials with atomic scale thickness,abundant species and the absence of dangling bonds can be integrated into semiconductor circuit due to the van der Waals interface.Hence,it would be promising to explore 2D ferroelectrics for the developing of ferroelectric nonvolatile memories.Density functional theory-based first-principles calculations can be used to quantitatively predict various properties of materials,which were successful in previous studies.By using this method,we predicted three 2D ferroelectric families following completely different strategies.Our calculations can facilitate the developing of the 2D ferroelectrics and pave the way for future experiments.First,we try to search 2D intrinsic ferroelectrics in various layered binary compounds with honeycomb lattice.Our first-principles calculations show that AB stacked bilayers with broken inversion symmetry possess vertical spontaneous polarization.We also propose a hitherto unreported interlayer sliding mechanism of ferroelectric switching.This switching mechanism is still effective in multilayers and even bulks with the same stacked forms.The bilayer of 2D ferromagnets can even be multiferroics with coupled electric polarization and magnetism.Our research reveals a vast family of 2D ferroelectrics whose interlayer sliding ferroelectricity has been verified by following experiments.Next,we explore the existence of ferroelectricity in 2D functionalized materials.We propose a facile and efficient functionalization method for 2D porous carbon nitride(C₃N₄,C₂N and CN).When a domain of carbon nitride is immersed in a solution of metal halides with mobile cations/anions,each cavity can be functionalized by a unit of metal halide.Functional groups can induce ferroelectricity in these 2D systems,and the direction of the polarization is coupled with the valleys in Brillouin zone.Moreover,our calculations show that their bandgaps can be tuned by controlling the metal-halide functionalization density,which makes them promising in photovoltaics.Finally,we construct 2D ferroelectrics in mixed-valence systems.Our calculations show that 2D mixed-valence monolayer can be synthesized via epitaxial growth of Sn S on Sn S₂monolayer.This mixed-valence system possesses a high vertical polarization and a moderate switching energy barrier.Due to the small energy difference between polar and nonpolar states with distinct thicknesses,akin to morphotropic phase boundary,an ultra-high piezoelectric coefficient can be obtained in the phase transformation under pressure.The strain gradient via mechanical bending can also switch the polarization which makes this system play a vital role in nanogenerator with high input voltage.Additionally,it possesses multiple metastable phases with distinct electronic properties.The transformations between them can be controlled by external strain,electric field and low frequency linearly polarized light.