Design,Synthesis And Domain Structures Toward Molecular Ferroelectrics

Great interest has been captured in molecular ferroelectrics as viable alternatives to conventional inorganic ferroelectrics,by taking advantage of their easy and environmentally friendly processing,light weight,and mechanical flexibility.Similar to the research mode in the field of traditional materials,the studies on molecular ferroelectrics mainly consist of the discovery of new materials and the fundamental research for the application of the existing ones.On the one hand,high Curie temperature,high spontaneous polarization,and low coercive field have always been the directions of performance optimization for the molecular systems;meanwhile,the existing molecular ferroelectrics lack some unique properties.For example,the topological domain structures are widely studied in inorganic systems that have not been found in molecular systems.Therefore,it is of great significance to design and synthesize new molecular ferroelectrics with high performance.On the other hand,some reported molecular ferroelectrics have been found to have excellent performance.However,the domain structures and the domain evolution under external stimuli（such as temperature,electric field and stress）are not clear,which play a key role in the performance of ferroelectric/piezoelectric devices.The most typical example is trimethylchloromethyl ammonium trichlorocadmium（TMCM-CdCl₃）.It possesses a record piezoelectric coefficient d33 of 220 p C/N among the known mono-composition molecular ferroelectrics.However,the research on its domain structure is lacking.Moreover,the origin of such a high piezoelectric efficiency has so far been unclear.The investigation of the polarization switching under external stimulus for TMCM-CdCl₃ should provide direct insight into understanding the origin of large piezoresponse.This thesis has done the following work on the above two aspects of molecular ferroelectric materials:1)Toward molecular ferroelectrics,the interaction between the molecules has a vital impact on the crystal structure and physical properties,which determines that ferroelectricity can be achieved or optimized through rational molecular design.On the basis of this,we replaced the anions in the reported ferroelectric [（MeO-C₆H₄-NH₃）（18-crown-6）][BF4]（MeO-C₆H₄-NH₃=4-methoxyanilinium）with larger TFSA （TFSA=bis（trifluoromethanesulfonyl）ammonium）to construct a new Host-Guest Inclusion Ferroelectrics [（MeO-C₆H₄-NH₃）（18-crown-6）][TFSA].The introduction of a larger TFSA anion changes the environment inside the crystal,bringing new C-H···O-S interactions between（MeO-C₆H₄-NH₃）+ cations and TFSA anions and a high-energy barrier of molecular motions,which causes a lower symmetry at room temperature and is favorable for multiaxial ferroelectricity.As expected,[（MeO-C₆H₄-NH₃）（18-crown-6）][TFSA] is a multiaxial ferroelectric with the Aizu notation of mmm Fm,whose Curie temperature（Tc）gets successfully increased up to 415 K.2)Based on TFSA again,we successfully synthesized two hydrogen-bonded molecular ferroelectrics,[HDABCO][TFSA]（DABCO=1,4-diazabicyclo[2.2.2] octane）and its deuterated one [DDABCO][TFSA],whose ferroelectricity is triggered by the proton ordering in the one dimensional hydrogen-bonding chains.The Tc gets successfully increased from 274.3 K in [HDABCO][TFSA] to 327.4 K in [DDABCO][TFSA] by the deuterium isotope effect.Benefits from the low energy for proton transfer,they have lower coercive voltage,exhibiting great potential in low-power memory.3)Based on the molecular design strategy of H/F substitution,we synthesized a two-dimensional organic-inorganic hybrid perovskite ferroelectric（4,4-DFPD）2Pb I4（4,4-DFPD is 4,4-difluoropiperidinium）.Comprehensive characterizations reveal that（4,4-DFPD）2Pb I4 is a molecular ferroelectric with excellent properties.It has a high phase transition temperature（428.5 K）,large spontaneous polarization（10.0 μC/cm²）,low coercive field（7.1 k V/cm）and suitable direct band gap（2.24 e V）.The most attractive observation is the ubiquitous formation of exotic quadrant-like domain patterns in the as-grown thin films.The discovery of vortex domain structure in molecular systems will bring new physical phenomena and applications.4)We have studied the polarization evolution of the（110）-oriented TMCM-CdCl₃ thin films under variable temperature,electric field and tensile strain conditions.By resolving the polarization directions of the strain induced domains,we observed a metastable phase that can be stabilized by external stress in TMCM-CdCl₃.It reveals that the easy and reversible switching between the stable phase and the metastable phase is responsible for the high-performance piezoelectric property of TMCM-CdCl₃.