The Research On Low-dimensional Ferroelectric Transition Metal Oxide Dihalide

With the rapid development of science and technology,electronic technology has been widely used in all aspects of human society,electronic devices also become indispensable in our daily life.And the evolution towards multifunctionality,high integration and miniaturization in electronic devices,put forward more requirements on related materials.At the same time,low-dimensional materials can just the demands for electronic device applications,due to their rich and excellent characteristics.Especially,the low-dimensional ferroelectric materials with switchable spontaneous polarizations,have great promise to be applied as senors and non-volatile memory devices.The macroscopic ferroelectricity in lowdimensional ferroelectric materials originates from the highly ordered arrangement of local dipoles,which still can survive at the ultimate nanoscale regime.Therefore,low dimensional ferroelectric materials meet all demands as the minimized electronic devices.More importantly,low-dimensional ferroelectric materials can always exhibit various properties in strong coupling with each other giving rise to new and valuable physical phenomena,as well as new functionalities for device applications.In this doctoral dissertation,our research focuses on low dimensional ferroelectric transition metal oxyhalide,including two dimensional(2D)ferroelectric MOX2(M=Zr,Hf;X=Cl,Br,I)monolayers and one-dimensional(1D)ferroelectric NbOI3.2D ferroelectric material MOX2 is obtained through screening of the 2D layered material database combined with first principle calculations based on density functional theory.Their intrinsic ferroelectricity and other physical properties coupled to ferroelectric polarization are under investigations.In research of NbOI3 1D ferroelectric material,beside its spontaneous ferroelectricity,we also design 1D electronic device based on NbOI3 and simulate its transport properties.The main contents of this paper include the following three aspects:1、The crystal structure and ferroelectric properties of 2D MOX2 monolayer.Through screening of 2D materials from vdW layered materials database combined with first principles calculations,we predict a new class of 2D ferroelectric materials-MOX2 monolayers,which have not been synthesized experimentally.Through phonon spectrum calculations,all possible structural phases for MOX2 monolayers have been determined,and their structural stabilities were also analyzed.It is confirmed that ferroelectric phase can be assigned the ground state stable structural phase for MOX2 monolayers.And the basic physical parameters including lattice parameters,ferroelectric polarizations and energy band gaps for ferroelectric MOX2 monolayers are calculated.In the end,Monte Carlo(MC)and ab initio molecular dynamics(MD)simulations are performed to simulate the ferroelectric to paraelectric phase transition for MOX2 monolayers.Based on the simulated Curie temperature,except HfOI2,all other MOX2 monolayers are able to maintain stable spontaneous polarization at room temperature.2、The research on various physical properties coupled with ferroelectricity in 2D ferroelectric MOX2 monolayers.In this part,other physical properties coupled with ferroelectricity in 2D FE MOX2 monolayers are under investigations.The mechanical properties of MOX2 monolayers are investigated by simulating the elastic coefficients.The Young’s modulus and Poisson’s ratio of 2D MOX2 monolayer show significant in-plane anisotropy.Based on the elastic coefficient and piezoelectric strain coefficient,the piezoelectric coefficient of 2D MOX2 monolayers are obtained.2D FE-MOX2 monolayers exhibit different piezoelectric responses from the most known piezoelectric materials.The transverse piezoelectric coefficient d32 dominates the piezoelectric responses of MOX2 monolayers,leading to the larger piezoelectric response when applying stress along the in-plane non-polar axis than the polar axis.Specially,for ferroelectric ZrOI2 monolayer with semiconducting electronic properties and room temperature stable ferroelectricity,we investigate its light absorption spectrum and spin-orbit coupling effects induced by heavy I element.Ferroelectric ZrOI2 monolayer exhibits anisotropic light absorption properties and significant linear dichroism effect due to the unique orbital hybridization characters.For the incident monochromatic linearly polarized light with photon energy of 3.23 eV,optical absorption along two planar directions with a nearly 100%optical selectivity can be obtained.In the study of spin-orbit coupling,it is found that the unusual band spin splitting occurs at the top of valence band near Γ point of Brillouin Zone.Under the protection of special symmetry operations in ZrOI2 monolayer,the electron spin orientations are restricted along the out-of-plane direction,leading to the persistent spin helix(PSH)state with the extraordinarily long spin carrier lifetime.More importantly,the coupling between ferroelectric polarization and electron spin can also occur in ZrOI2 monolayer.Based on the simulated electronic band structure and spin texture for ferroelectric ZrOI2 monolayer,the manipulation of electron spin orientation via the electric field induced polarization reversal can be realized in ZrOI2 monolayers,demonstrating the effective control spin degree of freedom by the external electric field.3、The ferroelectric properties of one-dimensional NbOI3 and simulation of electronic device based on NbOI3.In this part,we study another low dimensional ferroelectric transition metal oxyhalide,i.e.,one-dimensional ferroelectric material NbOI3.Based on the similar structural simulation and analysis method,we predict all possible ferroelectric,antiferroelectric,paraelectric and antiparallell ferroelectric NbOI3 structural phases.And ferroelectric phase is also determined to the ground-stable structural phase for NbOI3.The ferroelectric to paraelectric phase transition at finite temperature is also simulated by Monte Carlo method.Curie temperature of 400 K is predicted,demonstrating NbOI3 as 1D ferroelectric material with room temperature stable ferroelectricity.Then the interplay between ferroelectric polarization and depolarization field in ultra-thin NbOI3 material is also under investigation.It is found that stable ferroelectricity can persist in NbOI3 ultra-thin slab,which only two unit cell along the polar axis.We propose 1D electronic device based on NbOI3,and demonstrate rectifying effect for such a device using the transport properties simulations.