Preparation Of Molecular Ferroelectric Single Crystals And Modulation Of Their Properties

Ferroelectrics,a class of solid(crystals or polymers)or liquid crystal materials which enables the electric switching of spontaneous polarization,have important applications in the field of electronics,such as capacitors,nonvolatile memory,field effect transistors,sensors and so on.Since the discovery of the first ferroelectric crystal,Rochelle salt,the research and application of ferroelectrics are mainly focused on inorganic ferroelectrics and a few organic polymers.One trend of modem electronics is the fabrication of pure organic electronic devices,therefore the research of organic ferroelectrics has become a hot spot recently.Their invention may not only pose a fresh challenge to materials chemistry,but may also find many new applications in organic electronics due to the advantages of lightness,flexibility and non-toxicity.A few molecular based ferroelectrics have been discovered,which show good performances comparable with those of inorgance ferroelectrics.These discoveries not only change the traditional opinions about molecular ferroelectrics,like low Curie temperature and small spontaneous polarization,but also make the practical applications of molecular ferroelectrics possible.In addition,when combined with other properties like optics and magnetism,spontaneous electric polarization can then be a new way to manipulate the optical and magnetic properties of matials.On the other hand,the electric polarization may be tuned by light and magnetic field.Such multifunctional materials,like luminescent ferroelectrics and materials with magnetoelectric coupling,are of significance both for fundamental research and applications of electro-optical devices,spintronics and magnetoelectric sensors.This thesis mainly focuses on the investigation of diisopropylammonium anion based molecular ferroelectrics,the preparation of single crystal of metal-organic complexs and tuning the properties of molecular ferroelectrics by magnetic doping.The following achievements have been reached:1.The proper condition for direct growth of diisopropylammonium bromide(DIPAB)crystal with ferroelectric phase in solution was studied.Results suggested that the water in the solvent/environment plays a key role in the phase of DIPAB single crystal during crystallization process.In addition,by vacuum thermal evaporation of the DIPAB powder,DIPAB thin films were successfully prepared on different substrates.2.The scaling behavior of the dynamic hysteresis of DIPAB crystal was investigated for the first time.The scaling relations of P-E loop area(A)against frequency(f)were divided into three different f-region,which is very different from the the single peak evolution in inorganic materials.Considering the characteristics of the molecular structure of DIPAB,this three-stage behavior was ascribed to the coexistence of order-disorder of DIPA+ and displacement of Br-in DIPAB.The scaling relations of A to the f and applied electric field(E0)were obtained by fitting data for each r-region.3.Crystal structures and ferroelectric properties of six diisopropylammonium cation(DIPA＋)based single crystals with different anions(HF2-,Cl-,BC-,I-,NO3-,ClO4-)were investigated.Experimental results illustrate that their ferroelectric properties are closely related to the electronegativity and structure of counter anions.The experimental results were supported by density-functional theory(DFT)calculations.4.A new organic-inorganic hybrid compound DIPA2MnBr4 was synthesized,which shows a structural phase transition around 420 K during the heating process.The room temperature ferroelectricity of DIPA2MnBr4 was confirmed by P-E loop measurement and using piezoresponse force microscopy(PFIM).Besides,the crystals of DIPA2MnBr4 emit strong green light while illuminated by a UV lamp,and the quantum yield measured at room temperature is up to 62.2%.Investigation of gas sensing performance of DIPA2MnBr4 shew that the fluorescence has fast and reproducible response towards alcohol vapours.5.Using Mn(Ⅱ)as the metal center,a serice of of amine-Mn(Ⅱ)-chlorine complexs with different amines and pyrrolidine-Mn(Ⅱ)-halogen complexs with different halide ions were prepared.The effects of organic ligands and halide ions on the crystal structure,magnetism and fluorescence of these complexs were investigated.6.The structure and properties of croconic acid,Co(Ⅱ)and Mn(Ⅱ)complexes of croconate were studied.Then derived from the idea of inorganic semiconductor doping,Mn(Ⅱ)and Co(Ⅱ)doped croconic acid crystals that perform near-room temperature ferroelectricity,magnetism and magnetoelectric coupling were prepared.Under magnetic field,the temperature dependence of dielectric constant of the Mn(Ⅱ)doped croconic acid crystals of(1:5)showed a significant anomaly,which shifted to higher temperature with stronger magnetic field,indicating that there may be a magnetoelectric coupling in this Mn(Ⅱ)doped croconic acid crystal.