具有钙钛矿结构的分子铁电体研究进展

Recent progress in molecular ferroelectrics with perovskite structure

铁电材料是一类重要的功能性材料,由于其特殊的可翻转的自发极化,铁电材料在能源、信息、传感、医学等领域都有着广泛的应用.自20世纪以来,具有钙钛矿结构的铁电材料由于其优异的性质一直活跃在应用的舞台上,而其中应用较为广泛的几乎都是无机钙钛矿材料.近年来,随着科学技术的发展,人们发现了众多具有钙钛矿结构的分子铁电体.这些新型的分子铁电体具有较低的声阻抗、容易进行结构设计和调控、容易实现多种功能特性等优点,有望在柔性电子学、薄膜器件等应用中成为无机钙钛矿铁电材料的有益补充.本文结合分子钙钛矿铁电体的设计思路,对近年来其在压电、能隙调控、光电等功能特性方面取得的进展进行了介绍.

Discovered in 1920 by Valasek, a molecular crystal of Rochelle salt(potassium sodium tartrate tetrahydrate) displayed abnormal electrical hysteresis, named as ferroelectric effect. After that, ferroelectrics became an important class of functional materials, due to their unique reversible spontaneous polarization characteristics and other fascinating dielectric,thermoelectric and piezoelectric properties. Although the first reported ferroelectric material is an organic material, the later discovered inorganic materials almost dominated the fields of ferroelectrics for both fundamental studies and practical applications. Among them, inorganic perovskite ceramics received rapid development and showed great application potential in a wide-range of applications such as non-volatile memories, capacitors, sensors, energy convertors, etc., due to their high phase transition temperature and excellent ferroelectric/piezoelectric properties.Comparing to inorganics, molecular materials possess advantages as low acoustic impedance, good flexibility of structure design and composition modification, simple solution synthesis and easy realization of functional properties.Thus, molecular ferroelectrics are expected to be an irreplaceable complement to conventional inorganics, especially in applications like flexible electronics and thin film devices. In the large family of molecular materials, some adapt a similar structure of perovskite with partial substitution of metal ions by organic molecules. These hybrid materials have attracted widespread attention because of their extraordinary optoelectronic and photovoltaic performance. Since last decade, the ferroelectric related properties of molecular perovskite materials were also largely improved. The combination of highly adaptive perovskite structures and variable composition attributes hybrid perovskites them unparalleled potential in rational design of ferroelectricity and piezoelectricity.In this review, we firstly introduced the structural characteristics and dimensionalities of different molecular perovskite ferroelectrics. The next part, we focused on the current design strategy toward rational synthesis of molecular perovskite ferroelectrics, especially on how to tune polarization performance and introducing multiple polar axial properties. In the third part, we briefly introduced the latest progress of molecular piezoelectrics and the role of perovskite structure,molecular design, halogen-bonding and morphotropic phase boundary(MPB) in dramatically improving the piezoelectric coefficient d33. The fourth part of this review is an application-driven example of tuning band gap in molecular perovskite ferroelectrics, for better optoelectronic and photovoltaic performance. In the last part, a new class of perovskite materials,the metal-free perovskite, is introduced. Without the limitation of metal ions, the metal-free system is believed to have more potential with even better related properties.In summary, the ferroelectric, piezoelectric, and optoelectronic properties of perovskite molecular ferroelectric materials have been greatly developed. This article introduces several design strategies of perovskite molecular ferroelectrics,including "quasi-spherical" theory, H/F substitution and hydrogen bonding introduction method. Meanwhile, we combine comprehensive examples to introduce its recent application and the progress made in other functional characteristics such as band gap regulation and photo electricity. This review provides macroscopic view and general introduction of the evolution and recent progress in molecular perovskite ferroelectrics materials and is suitable for readers in a broad area of research.