Chengmin Ji

Solid‐State Reversible Quadratic Nonlinear Optical Molecular Switch with an Exceptionally Large Contrast

Exceptional nonlinear optical (NLO) switching behavior, including an extremely large contrast (on/off) of ∼35 and high NLO coefficients, is displayed by a solid-state reversible quadratic NLO switch. The favorable results, induced by very fast molecular motion and anionic ordering, provides impetus for the design of a novel second-harmonic-generation switch involving molecular motion.

Deep-ultraviolet transparent phosphates RbBa2(PO3)5 and Rb2Ba3(P2O7)2 show nonlinear optical activity from condensation of [PO4](3-) units.

It is challenging to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance between deep-UV transparency and high NLO activity. Known deep-UV NLO materials are almost exclusively limited to borates, except few newly discovered phosphates despite their small NLO activities. Here we report two asymmetric phosphates, RbBa2(PO3)5 (I) and Rb2Ba3(P2O7)2 (II), which feature [PO3]∞ chains and [P2O7](4-) dimers formed by condensation of [PO4](3-) units, respectively. Remarkably, I achieves the desired balance, with the shortest deep-UV absorption edge at 163 nm and the largest NLO activity of 1.4 × KDP (KH2PO4) in deep-UV NLO phosphates. According to first-principles calculations, the enhanced macroscopic SHG response of I can be attributed to the [PO3]∞ chains which exhibit significantly larger microscopic SHG coefficients as compared with the [P2O7](4-) dimers.

N-Isopropylbenzylammonium tetrafluoroborate: an organic dielectric relaxor with a tunable transition between high and low dielectric states

An organic dielectric relaxor, N-isopropylbenzylammonium tetrafluoroborate (1), with tunable dielectric properties between high and low dielectric states, has been successfully constructed through the prominent strategies of reversible structure transitions associated with ordering and reorientation of molecular motions. 1 undergoes an exceptional order–disorder solid state phase transition at 201 K, which was confirmed by the variable temperature single crystal X-ray diffraction analysis, thermal analysis and dielectric measurements. Owing to the ordering of the pendulum-like motion of methyl groups in the N-isopropylbenzylammonium (N-IPBA) cations and the reorientation of the molecular dipoles, 1 demonstrates a phase transition from a space group of C2/m at room temperature to C2/c at low temperature. Emphatically, the striking anisotropy of the dielectric response was investigated, revealing the crucial role of the ordering and orientation motions of N-IPBA cations. Moreover, the significant dielectric relaxation behaviour, attributed to the orientational polarization of the molecules’ dipoles, was observed and discussed. Such distinctive dielectric performances suggest that 1 might be a potential switchable relaxor-type dielectric material.

A Photoferroelectric Perovskite-Type Organometallic Halide with Exceptional Anisotropy of Bulk Photovoltaic Effects.

Perovskite-type ferroelectrics composed of organometallic halides are emerging as a promising alternative to conventional photovoltaic devices because of their unique photovoltaic effects (PVEs). A new layered perovskite-type photoferroelectric, bis(cyclohexylaminium) tetrabromo lead (1), is presented. The material exhibits an exceptional anisotropy of bulk PVEs. Upon photoexcitation, superior photovoltaic behaviors are created along its inorganic layers, which are composed of corner-sharing PbBr6 octahedra. Semiconducting activity with remarkable photoconductivity is achieved in the vertical direction, showing sizeable on/off current ratios (>10(4) ), which compete with the most active photovoltaic material CH3 NH3 PbI3 . In 1 the temperature-dependence of photovoltage coincides fairly well with that of polarization, confirming the dominant role of ferroelectricity in such highly anisotropic PVEs. This finding sheds light on bulk PVEs in ferroelectric materials, and promotes their application in optoelectronic devices.

Exploring a Lead-free Semiconducting Hybrid Ferroelectric with a Zero-Dimensional Perovskite-like Structure

Perovskite lead halides (CH3 NH3 PbI3 ) have recently taken a promising position in photovoltaics and optoelectronics because of remarkable semiconducting properties and possible ferroelectricity. However, the potential toxicity of lead arouses great environmental concern for widespread application. A new chemically tailored lead-free semiconducting hybrid ferroelectric is reported, N-methylpyrrolidinium)3 Sb2 Br9 (1), which consists of a zero-dimensional (0-D) perovskite-like anionic framework connected by corner- sharing SbBr6 coordinated octahedra. It presents a large ferroelectric spontaneous polarization of approximately 7.6 μC cm(-2) , as well as notable semiconducting properties, including positive temperature-dependent conductivity and ultraviolet-sensitive photoconductivity. Theoretical analysis of electronic structure and energy gap discloses a dominant contribution of the 0-D perovskite-like structure to the semiconducting properties of the material. This finding throws light on the rational design of new perovskite-like hybrids, especially lead-free semiconducting ferroelectrics.

A sequentially switchable molecular dielectric material tuned by the stepwise ordering in diisopropylammonium trifluoromethanesulfonate

A novel switchable and tunable molecular dielectric material, diisopropylammonium trifluoromethanesulfonate (1), which undergoes two reversible second-order solid state phase transitions at 200 K (TcL) and 340 K (TcH) respectively, has been successfully synthesized and grown as bulk crystals. The differential scanning calorimetry (DSC) measurements, and dielectric and variable-temperature single-crystal X-ray diffraction studies confirmed the stepwise phase transitions. 1 exhibits a remarkable temperature-dependent dielectric behavior, which could be tuned through three distinctive states triggered by the temperature change. Temperature-dependent single crystal structural analyses of 1 reveal a collection of exceptionally distinct and synchronous molecular motions of both the cationic and anionic moieties in 1, and that the interesting stepwise ordering of cations and anions are mainly responsible for its switchable and tunable dielectric properties. All of this demonstrates its potential application as a switchable and tunable molecular dielectric material.

Ultrahigh Pyroelectric Figures of Merit Associated with Distinct Bistable Dielectric Phase Transition in a New Molecular Compound: Di‐n‐Butylaminium Trifluoroacetate

Ultrahigh pyroelectric figures of merit are achieved in a new phase-transition material, di-n-butylaminium trifluoroacetate, of which the peak values are an order of magnitude larger than those of their inorganic counterparts. Such an attractive behavior of pyroelectric detectivity is strongly related to its distinct bistable dielectric behavior, which recalls excellent thermoelectric response in organic molecular phase-transition systems.

Switchable dielectric behaviour associated with above room-temperature phase transition in N-isopropylbenzylammonium dichloroacetate (N-IPBADC)

A bulk transparent single-crystal of N-isopropylbenzylammonium dichloroacetate (N-IPBADC) with sizes of 15 × 15 × 10 mm3, which possesses switchable dielectric permittivities above room temperature, has been successfully grown by the slow solution cooling method. A reversible second-order solid-state phase transition at 366 K was confirmed by thermal analyses, including differential scanning calorimetry (DSC) and specific heat (Cp), dielectric measurements, variable-temperature single-crystal X-ray diffraction and powder X-ray diffraction (PXRD). In particular, order–disorder transformations of the dichloroacetate moieties in N-IPBADC from room temperature to high temperature phases have been revealed to induce the distinct dielectric anomaly along with dielectric anisotropic properties above room temperature, up to 366 K. The successful discovery of N-IPBADC would potentially pave a new way to explore new above room-temperature phase transition materials.

An organic–inorganic hybrid co-crystal complex as a high-performance solid-state nonlinear optical switch

Solid-state nonlinear optical (NLO) switches have attracted great interest, while high performance solid-state nonlinear optical (NLO) switches still remain scarce. Herein, we firstly present a high-performance solid-state NLO switch based on an organic–inorganic hybrid co-crystal complex, [H2dabcoCl2][FeCl3(H2O)3] (1, dabco = 1,4-diazabicyclo[2.2.2]octane). It is found that 1 exhibits a moderately large NLO response of ∼0.31 pm V−1, a superior switching contrast (∼25) and a highly tunable repeatability, which may guarantee its potential device applications. In addition, microscopic crystal structural analyses reveal that its NLO switching is attributed to the order–disorder transformation of the dabco cation, cooperating with the reorientational displacement of the inorganic [FeCl3(H2O)3] component. Owing to the broader designability of organic–inorganic hybrids, this work opens up an attractive approach for exploring new high-performance NLO switches.

Bulk crystal growth and characterization of imidazolium L-tartrate (IMLT): a novel organic nonlinear optical material with a high laser-induced damage threshold

Bulk transparent organic nonlinear optical (NLO) single-crystals of imidazolium L-tartrate (IMLT), with a low near-UV cutoff wavelength at 235 nm and a large powder second harmonic generation (SHG) efficiency, being 4.5 times larger than that of KH2PO4 (KDP), have been successfully grown by the slow cooling method. Kurtz and Perry powder test reveals that IMLT is a phase-matchable NLO material with good optical transmittance in the entire visible region. The laser-induced damage threshold experiments show that the grown IMLT bulk crystals possess an excellent resistance to laser radiation with a high threshold up to 7.45 GW cm−2, much larger than those of several known inorganic and organic NLO materials. Furthermore, the thermal properties associated with its high laser-induced damage threshold, including the specific heat and thermal expansion coefficients, have been investigated thoroughly as a function of temperature. The intrinsic origin of the laser-induced damage was also analyzed based on studying the surface morphologies triggered with the laser-induced damage using an optical microscope. All the findings in the present work indicate that IMLT has a potential application as a useful NLO candidate.