No abstract

Ferroelectrics are used in a wide range of applications including memory elements, capacitors, and sensors. Recently, molecular ferroelectric crystals have attracted interest as viable alternatives to conventional ceramic ferroelectrics, due to their solution processability and lack of toxicity. Here we show that a class of molecular compounds-known as plastic crystals-can exhibit ferroelectricity if the constituent molecules are judiciously chosen from polar ionic molecules. The intrinsic features of plastic crystals, for example the rotational motion of molecules and phase transitions with lattice symmetry changes, provide the crystals with unique ferroelectric properties relative to conventional molecular crystals. This allows flexible alteration of the polarization axis direction in a grown crystal by applying an electric field. Owing to the tunable nature of the crystal orientation, together with mechanical deformability, this type of molecular crystal represents an attractive functional material which could find use in a diverse range of applications. Table of contents summaryA major drawback of molecular ferroelectric crystals, low dimensionality, has now been overcome by ionic plastic crystals. Molecular rotation and phase transitions intrinsic to the crystals make the materials unique molecular ferroelectrics, where the polarization axis direction in a grown crystal can be flexibly altered by applying an electric field.

Preparation of single-layer manganese oxide nanosheets (monosheets) comprised of edge-shared MnO(6) octahedra has relied on multistep processing involving a high-temperature solid-state synthesis of bulk templates, and ion-exchange and exfoliation reactions in solutions, requiring high cost and long processing time. Here we demonstrate the first single-step approach to directly access the MnO(2) monosheets, by the chemical oxidation of Mn(2+) ions in the presence of tetramethylammonium cations in an aqueous solution. Of importance is that this template-free reaction readily proceeds within a day at room temperature. The ability of the MnO(2) monosheets to self-assemble allows aggregation, to form layered structures with potassium cations and cationic tetrathiafulvalene analogues as intercalants. Furthermore, Langmuir-Blodgett (LB) films composed of the MnO(2) monosheets were successfully fabricated by the LB deposition method, in which about one layer of the monosheets was deposited for each process.