Ferroelectricity And Its Manipulation In Two-dimensional Buckling Honeycomb Structures

Low-dimensional ferroelectrics have received remarkable attention because of their significant potential for ferroelectric integrated device applications.Traditional three-dimensional ferroelectric materials with critical thickness have depolarization fields formed by surface charge accumulation as the thickness drops to a critical dimension that inhibits the ferroelectricity of the system.Fortunately,the emergence of two-dimensional(2D)ferroelectrics has turned it around.The thickness of twodimensional materials at the atomic level is much smaller than the critical thickness of traditional perovskite ferroelectric thin film materials.If the ferroelectricity can be realized in two-dimensional materials,the limitation of the critical thickness of traditional ferroelectric materials can be overcome.In recent years,many theoretical works have successfully predicted two-dimensional materials with intrinsic ferroelectricity.Based on the direction of ferroelectric polarization,ferroelectricity can be divided into in-plane and out-of-plane polarization.However,most of the ferroelectric materials that have been discovered are in-plane polarized,and the number of two-dimensional ferroelectric materials with out-of-plane polarization is relatively small.This is attributed to the fact that the dipole perpendicular to the film surface is suppressed by the surface energy and the depolarization field,so that most of the ultrathin 2D materials have in-plane spontaneous polarization.Observing stable out-of-plane spontaneous polarization in ultra-thin two-dimensional materials remains a great challenge.However,ferroelectric materials with out-of-plane spontaneous polarization are more suitable for high-density data memory designs.A large number of 2D ferroelectric materials with out-of-plane ferroelectricity remain to be discovered.Based on the first-principles method,this thesis presents a systematic study of the spontaneous electric polarization in 2D buckled honeycomb and its manipulation.The main contents are as follows:i.The formation mechanism of the two-dimensional buckling honeycomb structure was researched by first-principles calculations.The two-dimensional honeycomb structure is polar due to buckling of the surface,but the degree of buckling and the energy barrier to switching electric polarization are determined not solely by the chemical composition.In a comprehensive way,we combined the electronegativitydifference,interatomic distance and the distribution of charge density to describe quantificationally the polarity of chemical bonds using the first principles calculation.It is shows the characteristics of long bond-length but small electronegativitydifference.It is shown that buckling is formed only under certain strength of orbital interactions.Since the two-dimensional buckling honeycomb structure has an inherent centrosymmetry breaking,which brings convenience for the next study of its ferroelectricity.ii.In comparison with the two-dimensional planar honeycomb structure,nonplanar buckling honeycomb structure with intrinsically broken centrosymmetry,which is the origin of its ferroelectricity.We explored the ferroelectric behavior of the twodimensional buckling honeycomb structure by using the ?-GaP as an example to illustrate the effect of strain on ferroelectricity.We found that the spontaneous polarization is positively correlated with the electronegativity difference within a certain range,and the compression strain can effectively manipulate spontaneous polarization and switch barrier.In addition to ferroelectricity,the two-dimensional buckling honeycomb material also has valley degrees of freedom.Two energy degenerate valleys can be split by magnetic near neighbor effect.A combination of magnetic proximity effect and the ferroelectric polarization can manipulation valley degrees of freedom.The valley polarization is driven by ferroelectric polarization,which provides a new and effective way for the manipulation of valley degrees of freedom.