Ping-Ping Shi

Precise Molecular Design of High-Tc 3D Organic–Inorganic Perovskite Ferroelectric: [MeHdabco]RbI3 (MeHdabco = N-Methyl-1,4-diazoniabicyclo[2.2.2]octane)

With the flourishing development of (CH3NH3)PbI3, three-dimensional (3D) organic-inorganic perovskites with unique structure-property flexibility have become a worldwide focus. However, they still face great challenges in effectively inducing ferroelectricity. Despite the typical 3D perovskite structure and the ability of dabco (1,4-diazabicyclo[2.2.2]octane) to trigger phase transition, unfortunately [H2dabco]RbCl3 adopts a nonpolar crystal structure without ferroelectricity. Within the larger RbI3 framework, we assemble N-methyl-1,4-diazoniabicyclo[2.2.2]octane (MeHdabco) obtained by reducing the molecular symmetry of dabco into a new 3D organic-inorganic perovskite. As expected, MeHdabco bearing a molecular dipole moment turns out to be vital in the generation of polar crystal structure and ferroelectric phase transition occurring at 430 K. It is the first time that the dabco component has been successfully wrapped into a 3D cage to achieve ferroelectricity even through there is intensive research on dabco. This precise molecular design strategy based on the modification of molecular symmetry provides an efficient route to enrich the family of 3D organic-inorganic perovskite ferroelectrics. Intriguingly, the iodine-doped crystal can exhibit intense saffron yellow luminescence with a high quantum yield of 17.17% under UV excitation, extending its application in the field of ferroelectric luminescence and/or multifunctional devices.

De Novo Discovery of [Hdabco]BF4 Molecular Ferroelectric Thin Film for Nonvolatile Low-Voltage Memories

To date, the field of ferroelectric random access memories (FeRAMs) is mainly dominated by inorganic ferroelectric thin films like Pb(Zr,Ti)O3, which suffer from the issues of environmental harmfulness, high processing temperatures, and high fabrication costs. In these respects, molecular ferroelectric thin films are particularly advantageous and thus become promising alternatives to the conventional inorganic ones. For the prospect of FeRAMs applications, they should fulfill the requirements of effective polarization switching and low-voltage, high-speed operation. Despite recent advancements, molecular ferroelectric thin films with such high performance still remain a huge blank. Herein we present the first example of a large-area continuous biaxial molecular ferroelectric thin film that gets very close to the goal of application in FeRAMs: [Hdabco]BF4 (dabco = diazabicyclo[2.2.2]octane). In addition to excellent film performance, it is the coexistence of a low coercive voltage of ∼12 V and ultrafast polarization switching at a significantly high frequency of 20 kHz that affords [Hdabco]BF4 considerable potential for memory devices. Particularly, piezoresponse force microscopy (PFM) clearly demonstrates the four polarization directions and polarization switching at a low voltage down to ∼4.2 V (with an ∼150 nm thick film). This innovative work on high-performance molecular ferroelectric thin films, which can be compatible with wearable devices, will inject new vitality to the low-power information field.

A Switchable Molecular Dielectric with Two Sequential Reversible Phase Transitions: [(CH3)4P]4[Mn(SCN)6]

A new organic-inorganic hybrid switchable and tunable dielectric compound, [(CH3)4P]4[Mn(SCN)6] (1), exhibits three distinct dielectric states above room temperature and undergoes two reversible solid-state phase transitions, including a structural phase transition at 330 K and a ferroelastic phase transition with the Aizu notation of mmmF2/m at 352 K. The variable-temperature structural analyses disclose that the origin of the phase transitions and dielectric anomalies can be ascribed to the reorientation or motion of both the [(CH3)4P](+) cations and [Mn(SCN)6](4-) anions in solid-state crystals.

Switchable Nonlinear Optical and Tunable Luminescent Properties Triggered by Multiple Phase Transitions in a Perovskite-Like Compound

A new perovskite-like inorganic-organic hybrid compound [Et3(n-Pr)P][Cd(dca)3] (1) (where [Et3(n-Pr)P]+ is the propyltriethylphosphonium cation and dca is a dicyanamide ligand) was discovered to undergo three reversible phase transitions at 270 K (T1), 386 K (T2), and 415 K (T3), respectively. The variable-temperature single-crystal X-ray structural analyses reveal that these sequential phase transitions originate from the deformations of the [Cd(dca)3]- frameworks and the concomitant reorientations of the [Et3(n-Pr)P]+ guest cations. It is found that 1 possesses a sensitive nonlinear optical (NLO) switching at T2 with a large contrast of ∼40 within a narrow temperature range of ∼7 K. Furthermore, 1 shows intriguing photoluminescence (PL) property, and the PL intensity suffers a plunge near T3. The multiple phase transitions, switchable NLO and tunable luminescent properties simultaneously exist in this inorganic-organic perovskite-like hybrid compound, suggesting its great potential application in molecular switches and photoelectric field.

Dielectric and structural phase transition of [Ni(dmit)2]− salt with (4-ethoxyanilinium)([18]crown-6) supramolecular cation

The hydrogen-bonding supramolecule with 4-ethoxyanilinium and [18]crown-6 is introduced to [Ni(dmit)2]− salt. The arrangement of supramolecular cations formed a three-dimensional structure with the one-dimensional channel filled with [Ni(dmit)2]− anions. The temperature-dependent structural analyses and DSC measurement disclose the first-order phase transition occurred around 285 K, where the lattice parameters show an abrupt change without a space group change. The title compound crystallizes in No. 1 space group P-1 and is piezoelectrically active with d33 value of 4.8 pC N−1. The frequency- and temperature-dependent dielectric constants and potential energy calculation are consistent with the forward–backward motion of the ethoxyl group in the cation.

Perovskite-type organic–inorganic hybrid NLO switches tuned by guest cations

Three organic–inorganic hybrid analogues, ([Et3(n-Pr)P][Mn(dca)3]) 1, ([Et3(CH2CHCH2)P][Mn(dca)3]) 2, ([Et3(CH2OCH3)P][Mn(dca)3]) 3, [dca = dicyanamide, N(CN)2−], show similar three-dimensional perovskite frameworks, in which the guest phosphonium cations occupy the cavities. Compounds 1 and 3 belong to the orthorhombic noncentrosymmetric space group P212121 at room temperature, while compound 2 crystallizes in the monoclinic centrosymmetric space group P21/c. Differential scanning calorimetry (DSC) and dielectric measurements confirmed the phase transitions in compounds 1, 2 and 3, where subtle structural distinctions of guest cations affect the crystal lattices, phase transition temperatures and physical properties. Interestingly, compounds 1 and 3 are SHG active at room temperature and can be used as NLO switches tuned by guest cations and triggered by temperature.

Dielectric and nonlinear optical dual switching in an organic–inorganic hybrid relaxor [(CH3)3PCH2OH][Cd(SCN)3]

A new organic–inorganic hybrid compound [(CH3)3PCH2OH][Cd(SCN)3] (1) has been synthesized, which exhibits a reversible phase transition at 248.5 K confirmed by differential scanning calorimetry. The phase transition in 1 is from a centrosymmetric space group Pmcn to a non-centrosymmetric space group P21, so that 1 exhibits a switchable second harmonic generation (SHG) effect between SHG-on and SHG-off states. This phase transition also displays switchable dielectric behaviors between high and low dielectric states accompanied by the remarkable dielectric relaxation described by the Cole–Cole equation. Variable-temperature single-crystal X-ray diffraction analyses reveal that the origin of the phase transition can be attributed to the motion or reorientation of the [(CH3)3PCH2OH]+ cations and the movement of (SCN)− ions in solid-state crystals. These superior physical properties suggest that 1 could be a potential switchable dielectric and NLO relaxor-type material, which provides a new approach to design novel multiple switch materials.

A Molecular Polycrystalline Ferroelectric with Record-High Phase Transition Temperature

An outstanding advantage of inorganic ceramic ferroelectrics is their usability in the polycrystalline ceramic or thin film forms, which has dominated applications in the ferroelectric, dielectric, and piezoelectric fields. Although the history of ferroelectrics began with the molecular ferroelectric Rochelle salt in 1921, so far there have been very few molecular ferroelectrics, with lightweight, flexible, low-cost, and biocompatible superior properties compared to inorganic ceramic ferroelectrics, that can be applied in the polycrystalline form. Here, a multiaxial molecular ferroelectric, guanidinium perchlorate ([C(NH2 )3 ]ClO4 ), with a record-high phase transition temperature of 454 K is presented. It is the rectangular polarization-electric field (P-E) hysteresis loops recorded on the powder and thin film samples (with respective large Pr of 5.1 and 8.1 µC cm-2 ) that confirm the ferroelectricity of [C(NH2 )3 ]ClO4 in the polycrystalline states. Intriguingly, after poling, the piezoelectric coefficient (d33 ) of the powder sample shows a significant increase from 0 to 10 pC N-1 , comparable to that of LiNbO3 single crystal (8 pC N-1 ). This is the first time that such a phenomenon has been observed in molecular ferroelectrics, indicating the great potential of molecular ferroelectrics being used in the polycrystalline form like inorganic ferroelectrics, as well as being viable alternatives or supplements to conventional ceramic ferroelectrics.

Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical Applications

Though dominating most of the practical applications, inorganic ferroelectric thin films usually suffer from the high processing temperatures, the substrate limitation, and the complicated fabrication techniques that are high-cost, energy-intensive, and time-consuming. By contrast, molecular ferroelectrics offer more opportunities for the next-generation flexible and wearable devices due to their inherent flexibility, tunability, environmental-friendliness, and easy processability. However, most of the discovered molecular ferroelectrics are uniaxial, one major obstacle for improving the thin-film performance and expanding the application potential. In this Perspective, we overview the recent advances on multiaxial molecular ferroelectric thin films, which is a solution to this issue. We describe the strategies for screening multiaxial molecular ferroelectrics and characterizations of the thin films, and highlight their advantages and future applications. Upon rational and precise design as well as optimizing ferroelectric performance, the family of multiaxial molecular ferroelectric thin films surely will get booming in the near future and inject vigor into the century-old ferroelectric field.

A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric

Organic-inorganic hybrid perovskite, [CH3NH3]PbI3, holds a great potential for next-generation solar devices. However, whether the ferroelectricity exists in [CH3NH3]PbI3 and results in the ultrahigh performance is not at present clear. Beyond that, no hybrid lead iodide perovskite ferroelectric has yet been found. Here, using precise molecular modifications, we successfully designed a room-temperature hybrid perovskite ferroelectric, [(CH3)3NCH2I]PbI3. Because of the high-symmetry and nearly spherical shape of [(CH3)4N]+ cation, [(CH3)4N]PbI3 crystallizes in a centrosymmetric space group P63/ m at room temperature and undergoes a structural phase transition at 184 K. Accompanied by the introduction of halogen atoms on the cation from F to I, the phase transition temperature gradually increases to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH3)3NCH2I]-I and the largest volume of [(CH3)3NCH2I]+ among these compounds might be possible reasons for the stabilization of ordered [(CH3)3NCH2I]+ cation array and further reservation of its ferroelectricity at relatively high temperature. This work provides an efficient molecular design strategy toward the targeted harvest of room-temperature organic-inorganic perovskite ferroelectrics, and should inspire further exploration of the interplay between structure and ferroelectricity. The discovery of lead iodide perovskite ferroelectric also offers a foothold to the possibility for the existence of ferroelectricity in [CH3NH3]PbI3.