Preparation And Properties Of Organic-inorganic Hybrid Ferroelectric And Dielectric Materials And Their Self-assembled Thin Films

Organic-inorganic hybrid ferroelectric and dielectric materials have great application potential in information storage,dielectric switching,optoelectronic sensing,energy conversion,and other fields due to their simple preparation methods,easy structure tuning,lightweight and environmental friendliness.In recent years,a large number of hybrid ferroelectric and dielectric materials with good performance have been discovered,but three challenges need to be addressed to realize their industrial application:(1)the dipole ordering of molecular ferroelectrics,to achieve macroscopic ferroelectricity;(2)the lead-free substitution of lead-containing perovskites,to reduce toxicity;(3)the multi functionalization of single-functional materials,to expand their application scope.In this thesis,This paper is based on the“quasi-spherical”theory of ferroelectric chemistry,a series of lead-containing,lead-free,and rare earth perovskites were designed and synthesized,and highly oriented composite films were fabricated.The structure and properties of these materials were investigated.The main contents are as follows:First,using"quasi-spherical"(2-bromomethyl)trimethylammonium bromide(BETAB),(2-chloroethyl)trimethylammonium chloride(CETAC),and 3-chloro-2-hydroxy Propyltrimethylammonium chloride(2-3-Cl)was reacted with lead bromide to prepare BEPbBr,CEPbBr,and PbBr2-3-Cl,which belong to ABX3-type leaded perovskites.Among them,BEPbBrand CEPbBrare uniaxial ferroelectrics,while PbBr2-3-Clhas no ferroelectricity.BEPbBrhas an mmm Fm reversible ferroelectric phase transition at 302/286 K.The structure of CEPbBris similar to BEPbBr,but its phase transition temperature is slightly higher,and the phase transition mechanism is an order-disorder phase transition.BEPbBrand CEPbBrwere dispersed into the PVDF matrix to form a composite film,and the strong interaction between ferroelectric nanocrystals and PVDF molecules induced the preferred orientation of the in-situ grown nanocrystals,which could enhance the ferroelectric properties of the films.the addition of about 5%can increase theα-phase toβ-phase transition in the composite membrane from about 50%to 85%.The inorganic anion chain in PbBr2-3-Cladopts a tetragonal pyramidal five-coordinate configuration,which is a rare quadruple cross-linked helical structure.PbBr2-3-Clhas a rare 435 K high-temperature phase transition and also has semiconductor properties with an Eg of 3.25 e V.Secondly,BETABCuBrand BETABCdBrwere prepared by combining BETAB with CuBr2 and CdBr2;and a series of lead-free perovskites such as CETACCoCl,CETACCoBr,CCP,and CCFB were prepared by combining CETAC with CoCl2,CoBr2,HClO4,and HBF4.Among them,BETABCdBr,CCP,and CCFB belong to the polar space group at low temperatures and have ferroelectricity;the other three belong to the non-polar space group,have good dielectric switching characteristics and good repeatability,and are good dielectric switching materials.BETABCuBrundergoes a reversible phase transition at about 356 K,belongs to the P21/c space group before and after the phase transition,and belongs to the displacement phase transition with the space group unchanged.BETABCdBrundergoes two consecutive reversible phase transitions around 342 K and 390 K.The mmm Fmm2 paraelectric ferroelectric transition occurred around the first phase transition point.The space groups of CETACCoCland CETACCoBrbefore and after phase transition are P21/c and I41/a,respectively.Both CCP and CCFB belong to metal-free molecular ferroelectrics,and also have high theoretical spontaneous polarization values.The difference is that CCP has two continuous phase transitions at 332 and 365 K,while CCFB only undergoes a phase transition around 352 K.Finally,three novel rare-earth perovskites:(HQ)4KEu(NO3)8、(HQ)4Rb Eu(NO3)8 and(HQ)4NH4Eu(NO3)8,were prepared by reacting quinuclidine with compounds such as europium nitrate and potassium nitrate.they all belong to layered two-dimensional hybrid rare earth double perovskites.Among them,(HQ)4KEu(NO3)8 undergoes two consecutive reversible phase transitions.The first phase transition is driven by the fan-shaped rotational motion of NO3~-around Eu atoms,and the second phase transition is caused by the fan-shaped rotational motion of NO3~-around K atoms.The dielectric response of(HQ)4KEu(NO3)8 exhibits a strong frequency-dependent behavior.Its crystal emits orange-red light under the irradiation of an ultraviolet lamp,the solid-state fluorescence lifetime is 4.78 ms,and the absolute quantum yield(QY)is 42.89%.Replace K with Rb to get(HQ)4Rb Eu(NO3)8 with a similar structure,its two phase transitions are connected,and the phase transition temperature is higher.(HQ)4Rb Eu(NO3)8 also emits orange-red fluorescence under ultraviolet light,the solid-state fluorescence lifetime is 5.232ms,and the absolute quantum yield(QY)is 45.04%.Replace K with NH4~+to get(HQ)4NH4Eu(NO3)8.In the structure,the two axial NO3~-do not directly coordinate with NH4~+,but the NO3~-acting as a bridging group connects NH4~+and the quinuclidine cations are linked by hydrogen bonds.The dielectric dependence of(HQ)4NH4Eu(NO3)8 on frequency is stronger than the former two,and its solid-state fluorescence lifetime is 4.71 ms,which is shorter than the former two.Based on the“quasi-spherical”theory,this thesis successfully synthesized a series of lead-containing,lead-free,and rare earth organic-inorganic hybrid materials with ferroelectric and dielectric properties,expanding the application scope of this theory in molecular ferroelectrics and molecular dielectrics.This thesis also incorporated a small amount of molecular ferroelectrics into the PVDF matrix,and fabricated composite films with highβ-phase content and strong ferroelectricity,achieving molecular-level dipole ordering,and providing a new approach for the application of molecular ferroelectrics.In addition,this thesis designed a series of lead-free perovskite materials by changing the types of metal ions,halogens,and non-metal anions,exhibiting excellent ferroelectric and dielectric properties.At the same time,this thesis introduced rare earth elements into the perovskite structure and obtained rare earth double perovskite materials with different phase transition temperatures and photoluminescence characteristics by adjusting the types of bridging atoms or ions,offering a new avenue for developing multifunctional organic-inorganic hybrid materials.