Emergent Physical Properties In Two-Dimensional Layered Polar Materials By First-Principle Investigations

Symmetry plays a vital role in prompting the development of physics.And the symme-try breaking is also the major research topic in condensed matter physics,semiconductor physics and solid electronics.Polar materials usually lead to many characteristics due to their special asymmetric structure,including spontaneous polarization,anisotropic elec-tronic structure,photoelectric and electromechanical properties.With the breaking of cen-ter inversion symmetry,the coupling of charge and spin in some special polar materials will bring novel physical properties,such as electrically controllable photocurrent and spin photocurrent.Therefore,polar materials with emerging physical effects are expected to be applied as new high-performance electronic devices.Due to the demand of high integration and miniaturization of devices,exploring new two-dimensional（2D）layered polar materials with atomic thickness and excellent characteristics at room temperature are urgent.As a result of weak interlayer interactions,such as van der Waals（vd W）and hydrogen bonding interactions,layered materials can usually be mechan-ically exfoliated into multilayer or even monolayer.Moreover,the monolayer（multilayer）exfoliated from its bulk counterpart accompanied by the reduced dimension and breaking of out-of-plane periodicity,will probably exhibit distinct properties including spatial sym-metry,electronic structure and mechanical response.For example,some three-dimensional（3D）systems with central inversion symmetry may become polar materials without cen-tral inversion symmetry after exfoliation into a 2D monolayers.At the same time,vd W heterostructures formed by stacking of 2D layered materials through vd W and hydrogen bonding interactions are expected to have new properties lacking in 3D bulk.Therefore,it is of significance in scientific research and practical application to explore and study 2D lay-ered polar materials that can be applied to new miniaturized and highly integrated electronic devices.Through calculation and simulation of mechanical properties,electronic states and photo-electric response properties based on the first principles and symmetry analysis,this disserta-tion reveals the emerging novel properties,including negative piezoelectricity,intrinsic aux-eticity,electrically controllable charge or spin photocurrent,of two kinds of 2D layered polar materials,i.e.,polar group-Ⅳ dipnictide monolayer AB₂（A=Si,Ge,B=P,As）and fer-roelectric Ruddlesden-Popper（RP）layered halide perovskite monolayer（MA）₂Pb（SCN）₂I₂ due to breaking of center inversion,spontaneous polarization and anisotropic structure as well.Moreover,the applications of these two materials as miniaturized electronic devices have been explored.Our specific research include:1、The electronic structure,optical absorption properties and mechanical response along distinct planar axes in family of group-Ⅳ pnictides AB and AB₂（A=Si,Ge,B=P,As）monolayers are systematically simulated,analysed and compared.As recently synthesized 2D layered compounds,the group-Ⅳ monopnictide AB and dipnictide AB₂,show in-plane anisotropy due to low-symmetry crystal structures and different bonding and coordination aspects along the two planar axes,giving rise to the obvious difference in elastic modulus and Poisson’s ratio,effective carrier mass and light absorption coefficient along corresponding direction.In particular,the exfoliation of AB₂monolayer from its bulk counterpart lead to the centrosymmetric P bam space group of bulk converting to non-centrosymmetric P mc2₁ space group of AB₂monolayer,which means a new polar 2D material appears.The breaking of spatial inversion symmetry endows with AB₂ monolayer some novel properties.On the one hand,the unique crystal structure with corner-sharing of AB₄tetrahedrons give rise to the intrinsic auxeticity with negative Poisson’s ratio（NPR）in AB₂ monolayer.Among AB₂ monolayers,the NPR of SiP₂ is extraordinarily large up to-0.32,which is of the largest magnitude among all synthetic 2D auxetic materials.Therefore,the auxetic material is expected to be applied to biomedicine,sensors,fasteners and protective equip-ments.The tetrahedral rotation and dilation deformation mechanism is responsible for the NPR and can be applied to uncover more 2D auxetic materials composed of corner-sharing tetrahedrons.On the other hand,owing to the in-plane spontaneous polarization of the sys-tem and the special structural form of corner-sharing of AB₄ tetrahedrons,AB₂ monolayers hold negative piezoelectricity.In particular,the magnitude of the d₃₃coefficient up to-11.84p C/N for Si As₂ monolayer is close to the experimentally synthetic 2D ferroelectric material Cu In P₂S₆.At last,an analytical relation including Bader effective charge and ion polariz-ability has also been developed to quantitatively describe and accurately predict the negative piezoelectric responses of AB₂ monolayers,which can be extended to uncover other new 2D polar materials with negative longitudinal piezoelectricity.2、We calculate and analyze the structural symmetry,ferroelectric polarization,electronic structure and photocurrent of（MA）₂PbI₂（SCN）₂bulk and monolayer.Unlike AB and AB₂ bulk with vd W interactions studied in the previous chapter,hydrogen bonding inter-layer interactions between the organic MA cations and inorganic anion SCN are indispens-able for stabilizing the layered structure of the ferroelectric RP layered halide perovskites（MA）₂PbI₂（SCN）₂ bulk.As a half number of hydrogen bonds are along interlayered direc-tion,when down to monolayer limit,the structure of（MA）₂PbI₂（SCN）₂monolayer is no longer the original monolayer of bulk.When（MA）₂PbI₂（SCN）₂ monolayer is exfoliated from its bulk,the intra-layer hydrogen bonding will be spontaneously re-arranged accom-panied by the new distortion modes of PbI₆octahedrons,which convert the space group from P mn2₁of bulk to P mc2₁of monolayer.Secondly,considering the structural charac-teristics,the polarization of（MA）₂PbI₂（SCN）₂ bulk and monolayer are mainly contributed by Pboff-center displacement.The switching of its in-plane polarization can be realized through simultaneous reversal of MA molecular dipoles and Pbpolar displacement.And the moderate polarization switching barrier is predicted for the（MA）₂PbI₂（SCN）₂mono-layer,comparable with ferroelectric perovskite oxide PbTiO₃.Therefore,（MA）₂PbI₂（SCN）₂ bulk and monolayer have spontaneous ferroelectric polarization,which is lack in polar AB₂monolayer.More importantly,under the synergistic effects of the strong spin-orbit coupling（SOC）aris-ing from Pband I heavy elements and non-centrosymmetric spatial asymmetry,spin splitting of energy bands occur in proximity to the VBM and CBM of（MA）₂PbI₂（SCN）₂monolayer.Under the symmetry constraint of C_2vlocal symmetry near the center of Brillouin zoneΓ,the band edges of（MA）₂PbI₂（SCN）₂monolayer are characterized by persistent spin texture-PST,with persistent spin helix-PSH in real space.Consequently,the spin orientation of spin carriers remain unchanged during scattering with non-magnetic impurities and defects,ex-traordinarily extending the spin lifetime of carriers.In addition,the spin degree of freedom within（MA）₂PbI₂（SCN）₂monolayer can be reversed in response to the switching of polar-ization,which can naturally meet the requirement of electrically controllable spin in spin-tronics.In consideration that,we adopt the nonlinear optical response method to simulate the spin photocurrent based on bulk spin photovoltaic effect-BSPV of（MA）₂PbI₂（SCN）₂monolayer.Our simulation indicates that photocurrent or spin photocurrent can be both obtained in（MA）₂PbI₂（SCN）₂monolayer under either above-gap LPL or CPL illumination.Spatially separated pure spin and charge photocurrent can be obtained along two planar axes,respectively.Moreover,the switching of polarizations can reverse the photocurrent di-rections.Therefore,spintronic device based on（MA）₂PbI₂（SCN）₂monolayer can perform“optical-generation,electric-writing and magnetic-reading”nonvolatile logical operations.