Chemical solution synthesis and properties of nanoscale ferroelectrics, single phase and composite multiferroics

Abstract

Chemical solution syntheses are frequently used for the synthesis of many kinds of metal oxides. The cited advantages include the compositional flexibility, high phase purity at relatively low temperatures, low equipment and operation cost and for some cases, the compatibility with roll-to-roll processing. Here, we focus on solution based synthesis of nanoscale BaTiO3 besides multiferroics. First, the size limit for tetragonal BaTiO3 particles from solvothermal synthesis is investigated in view of biomedical applications in SHRIMPs (Second Harmonic resonance Imaging Probe). Down to 50 nm, the optimal nanocrystals produced strong SHG (second harmonic generation) signals, which is promising for their use in cell imaging. Second, solution-gel routes based on citrate complex precursors, are presented for the synthesis of magnetoelectric multiferroic materials. Single phase BiFeO3 is the most studied multiferroic, showing both ferroelectric and magnetic ordering at temperatures exceeding room temperature. The phase diagram displays many different phases, complicating the synthesis of BiFeO3. Especially for chemical solution deposited (CSD) thin films, limited knowledge is available. Therefore, we studied the phase formation and stability of thin film BFO from aqueous CSD and found that the substrate, buffer layers and substituents have a strong influence. Besides BFO, preliminary results on the hybrid improper ferroelectric, Ca3Mn2O7, are reported. Finally, composite thin films of magnetic and ferroelectric phases are reported to show enhanced magnetoelectric coupling through a high interface to volume ratio. Here, self-assembled composite films are obtained via CSD. As high resolution TEM shows, composites are formed after crystallizaton by phase separation between BaTiO3- or BiFeO3- CoFe2O4, resulting in highly textured films. In conclusion, chemical solution synthesis provides easily accessible routes to nanoscale ferroelectrics and multiferroic thin films, which hold great promise for diverse applications.