The Growth And Properties Study Of Molecular Ferroelectrics With Diisopropylammonium Cation

Since the first found of ferroelectricity in Rochelle salt, thousands of ferroelectrics have been discovered including inorganic and organic ferroelectrics. Recent years, some organic ferroelectrics have been reported to have perfect ferroelectric properties which may exeed some inorganic ones. For example, the spontaneous polarization of croconic acid and diisopropylammonium bromide are more than 20 μC cm-2, which is close to that of BaTiO3. The found of these two molecular ferroelectrics causes a hot spot of studies of molecular ferroelectrics around the world. Comparing to inorganic ferrroelectrics, molecular ferroelectrics are cheap and non-polluting, which can be got easily.In this work, molecular ferroelectrics containing diisopropylamminium cations will be studied. The study would be divived into three parts.The first parts:A solution method using 12-crown-4 as an additive in solvent is found to easily obtain ferroelectric DIPAB single crystals. It indicates that the addition of 12-crown-4 facilitates the crystallization of a ferroelectric DIPAB crystal. Ferroelectric DIPAB’s pyroelectric coefficient measured by dynamic pyroelectric current is about 65-325 μC m-2 K-1 at room temperature, which reveals that DIPAB is a good candidate for infrared detectors. Therefore, this modified method is very useful to grow ferroelectric DIPAB and its derivative. A ferroelectric thin film of DIPAB was successfully fabricated on a Si substrate using a spin coating method from aqueous solution via 12-crown-4 addition at room temperature. The ferroelectric DIPAB film with a thickness of hundreds of nanometers is distributed discontinuously on the substrate in narrow strips. The direction of polarization is along the narrow strip. Piezoresponse force microscopy (PFM) shows that the ferroelectric films have two kinds of domain structures:non-charged antiparallel stripe domains and charged head-to-head (H-H)/tail-to-tail (T-T) type domains.12-crown-4 has been proved to play important roles in forming the H-H/T-T type domains. The Chynoweth method shows that the DIPAB films synthesized in this way show a better pyroelectric effect than DIPAB crystals.The second part:A series of chlorine doped Diisopropylammonium Bromide (DIPAB-C) single crystals were grown successfully. The DIPAB-C crystals have two phase transitions (from P212121 to P21 at T1, and from P21 to P21/m at T2) in warming process while only one (P21/m to P21) in cooling process. The ferroelectric-to-paraelectric transition temperature can be modulated from 423 K to 429 K by chlorine concentration. The spontaneous polarization decreased from 23 μC/cm2 for undoped DIPAB crystal to 13 μC/cm2 for chlorine concentration of x=0.08, then it has a stable value of 14 μC/cm2 for x>0.1. It indicates that the ferroelectric properties, including spontaneous polarization, the phase transition temperatures, and the lattice parameters, etc., can be modulated by doping with congeners.The third part:The transition temperatures of molecular ferroelectrics are usually lower than those of inorganic ferroelectrics due to weaker electron-electron and electron-phonon interactions in molecular ferroelectrics. Therefore, above-room-temperature molecular ferroelectrics are very rare which hinders their application in modern electronic fields. Here we report a new kind of above-room-temperature molecular ferroelectric, diisopropylammonium perchlorate (DIPAP), which combines fast switchable molecular dielectrics. The ferroelectric to paraelectric phase transition occurs at about 338 K in the heating process, which was confirmed by structure analysis (from space group of P1 to P21/c), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectric permittivity, pyroelectric as well as Raman spectra. The spontaneous polarization measured from pyroelectric current and ferroelectric hysteresis loop is about 0.1 μC cm"2, while the calculated value by first principles is as high as 20 μC cm-2. This large theoretical spontaneous polarization value can be achieved through designing and modulating a molecular structure to a fully polarized one.