Polarization Characteristics And Domain Engineering Of Molecular Ferroelectric Materials

Ferroelectrics exhibit spontaneous electric polarization,the direction of which can be reversed by inverting the external electric field.The ferroelectricity and the abundant related phenomena like dielectric,piezoelectric,pyroelectric,and electro-optic properties of ferroelectrics have been explored in numerous applications,including ferroelectric random access memory(FRAM),surface acoustic wave,capacitors,sensors,photovoltaics,and so on.Despite years of efforts to develop new ferroelectric materials,currently inorganic ceramic ferroelectrics such as barium titanate(BTO)and lead zirconate titanate(PZT)are still the most studied and widely used ones with excellent performance characteristics.Not insignificant,however,is the fact that these materials contain environmentally harmful heavy metals and usually require highcost,high-temperature processing and high energy consumption.As the alternative or the supplement,molecular ferroelectrics have been intensively studied to overcome the drawbacks because of their many advantages such as low cost,light weight,easy processing,mechanical flexibility,and environmental friendliness.Hence,molecular ferroelectric materials have always been one of the frontier and hot areas of high-tech research.Polarization ordering and switching dynamics are among the most important issues in the physics of ferroelectrics,which should be addressed before considering any practical applications for molecular ferroelectrics.Only in this way can we design better molecular ferroelectrics with good performance to replace inorganic ferroelectric materials.Through molecular design and crystal engineering,we successfully synthesized several molecular ferroelectrics in this thesis.Using piezoresponse force microscopy(PFM),the structure and dynamics of ferroelectric domains were analyzed and discussed in detail.Moreover,the polarization switching performance of two ferroelectrics was systematically studied.(I)In Chapter Two,we demonstrate a potential hybrid molecular ferroelectric,(Ph3PO)2Mn Br2,which exhibits intensely green triboluminescence,being promising candidate for next generation of optical waveguides and light displays.Using PFM,the presence of ferroelectric domains in(Ph3PO)2Mn Br2 thin films is confirmed.We also provide evidence for the reversible switching of the ferroelectric domains by poling with DC biases.(II)In Chapter Three,we present a biaxial molecular ferroelectric thin film of [Hdabco]Cl O4(dabco = 1,4-diazabicyclo[2.2.2]octane),which was fabricated by a simple and effective aqueous solution process.The growth begins with organized dendritic crystals,which finally merge together to form a continuous film.With this high-quality thin film,the well-defined rectangular P-E hysteresis loops are successfully achieved even at 10 k Hz.Moreover,using PFM,we clearly observed the coexistence of 180° and non-180° ferroelectric domains,and provided a direct experimental proof that 180° ferroelectric switching and non-180° ferroelastic switching are both realized;that is,a flexible alteration of the polarization axis direction can occur in the thin film by applying an electric field.The excellent film-forming ability,the multiaxial characteristic,and the ultrafast polarization switching make it an ideal candidate for applications in next-generation flexible electronics.(III)Chapter Four shows a molecular ferroelectric thin film of [Hdabco]Re O4.Remarkably,it displays not only the highest Curie temperature of 499.6 K but also the fastest polarization switching of 100 k Hz among all reported molecular ferroelectrics.Combined with the large remanent polarization values(~9 ?C/cm2),the low coercive voltages(~10 V)and the unique multiaxial ferroelectric nature,[Hdabco]Re O4 becomes a promising and viable alternative for data storage applications on next generation flexible devices,wearable devices and bionics.(IV)In Chapter Five,recently,a plastic crystal of quinuclidinium perrhenate([HQ]Re O4)was reported to have feasibility of controlling the crystallographic orientation in the grown crystal,but the corresponding temperature window is only about 22 K(345-367 K).Such a narrow window and uncertain ferroelectricity at room temperature would extremely limit its potential application.In this Chapter,we prepared large area high-quality polycrystalline thin film of [HQ]Re O4 and observed the ferroelectricity in the temperature range from 298 K to 367 K for the first time.A local PFM measurement was also employed to study the mechanisms of multi-axial polarization rotation and domain dynamics.By extending the ferroelectric temperature window to room temperature and the extraordinary thin-film processability,[HQ]Re O4 would certainly become a suitable candidate for next generation ferroelectric materials.