Shiguo Han

Inorganic–organic hybrid switchable dielectric materials with the coexistence of magnetic anomalies induced by reversible high-temperature phase transition

An inorganic–organic hybrid compound that features a zero-dimensional inorganic structural framework, N-methylpiperidinium tetrabromoferrate(III) (compound 1, [C6H14N][FeBr4]), has been reported as a new switchable dielectric material with the coexistence of weak magnetic anomalies. It is noteworthy that 1 demonstrates remarkable dielectric responses, which can be tuned between two distinct dielectric states and reversibly switched by phase transition at ∼340 K (Tc). Its high-dielectric constants are at least 4 times as large as that of the low-dielectric state. Structural analysis reveals that 1 belongs to the monoclinic crystal system with the space group P21/c at room temperature, but transforms into an orthorhombic system with the space group Cmcm above Tc. The disordering of organic cations affords a driving force for high-temperature phase transition in 1 along with its switchable dielectric activities. Besides, weak magnetic anomalies were also found in 1 at the vicinity of its Tc, which should be reminiscent of another possible physical channel to the phase transition. This finding suggests that 1 might be a potential multifunctional material with the coexistence of switchable dielectric bistability and magnetic responses, which opens up new possibilities to develop molecular functional materials.

[(CH3)3NH]3Bi2I9: A Polar Lead-Free Hybrid Perovskite-Like Material as a Potential Semiconducting Absorber

Perovskite hybrids of lead organometal halides, most notably CH3 NH3 PbI3 , have shown extremely promising applications in the field of optoelectronics, because of their remarkable semiconducting and light-absorbing properties. However, two key issues-the toxicity of lead and the poor ambient instabilities-have restrained their further commercialization. Herein, we have designed a new stable polar lead-free hybrid material by utilizing the strategy of cation substitution, [(CH3 )3 N]3 Bi2 I9 (1), which adopts the 0D inorganic perovskite-like architecture by face-sharing BiI6 octahedra. It is interesting that 1 displays excellent absorbing properties with a narrow optical band gap of ≈2.0 eV, and positive temperature-dependent conductivity confirms its semiconducting behaviors. In addition, 1 has good phase stability against decomposition under ambient conditions, much superior to that of CH3 NH3 PbI3 . This work suggests the potential of 1 as a lead-free semiconducting absorber with high phase stability for photoelectric applications.

Lead-Free Hybrid Material with an Exceptional Dielectric Phase Transition Induced by a Chair-to-Boat Conformation Change of the Organic Cation

Hybrid organic-inorganic perovskite materials have demonstrated great potential in the field of photovoltaics and photoelectronics. On the basis of the high degree of structural flexibility and compatibility, diverse molecular functional materials have been assembled by modifying the length of the organic components and/or dimensionality of the inorganic frameworks. In this paper, we present a chiral lead-free organic-inorganic hybrid, (piperidinium)2SbCl5 (1), which follows the one-dimensional inorganic frameworks of the corner-sharing SbCl6 octahedra. Strikingly, 1 displays a dielectric phase transition at Tc = 338 K, changing from the chiral space group of P212121 to polar Pna21 upon heating. Crystal structure analyses reveal that an unusual thermally activated conformation change of the piperidinium cations affords the driving force to the phase transition of 1. That is, organic piperidinium moieties display a chairlike conformation below Tc, which transforms to a boatlike structure above Tc. Such an unprecedented change is strongly coupled to the dielectric transition along with notable steplike anomalies, which suggest that 1 could be used as a potential switchable dielectric material. Besides, the temperature-dependent conductivity and theoretical analysis of its electronic structure disclose the semiconducting behavior of 1. This study paves the pathway to the design of new lead-free semiconducting perovskites with targeted properties for optoelectronic application.

Switchable behaviors of quadratic nonlinear optical properties originating from bi-step phase transitions in a molecule-based crystal

Solid-state nonlinear optical (NLO) switches are recently emerging as a new class of promising stimuli-responsive materials for photoelectric application. Herein, we report an organic molecular crystal, N- methylcyclohexylamine picrate (1), which exhibits remarkable switching behaviors of quadratic NLO properties with a large contrast of ∼20 between its NLO-on and NLO-off states. This is almost comparable with the conventional photochromic counterparts, and suggests the great potential of 1 as a quadratic NLO-switching candidate. It is noteworthy that 1 undergoes bi-step structural phase transitions at T1 = 240 K and T2 = 285 K, which are closely associated with its NLO-switching activities. Above T1, disordering of anionic and cationic moieties leads to vanishing of the NLO effect, (i.e. NLO-off state). In contrast, below T1, stepwise frozen ordering of the structural moieties generates strong NLO activities, with the NLO response being ∼0.9 times that of KH2PO4. Such an order–disorder transformation accounts for the high-contrast NLO switching of 1. It is believed that this finding affords an effective strategy for designing new stimuli-responsive materials.

(C6H13N)2BiI5: A One-Dimensional Lead-Free Perovskite-Derivative Photoconductive Light Absorber

Lead-free organic-inorganic hybrid perovskites have recently attracted intense interest as environmentally friendly, low-cost, chemically stable light absorbers. Here, we reported a new one-dimensional (1D) zigzag chainlike light-absorbing hybrid material of (C6H13N)2BiI5, in which the corner-sharing octahedral bismuth halide chains are surrounded by organic cations of tetramethylpiperidinium. This unique zigzag 1D hybrid perovskite-derivative material shows a narrow direct band gap of 2.02 eV and long-lived photoluminescence, which is encouraging for optoelectronic applications. Importantly, it behaves as a typical semiconducting material and displays obvious photoresponse in the visible-light range. This work opens a potential pathway for the further application of 1D lead-free hybrids.

[C6H14N]PbBr3: An ABX3‐Type Semiconducting Perovskite Hybrid with Above‐Room‐Temperature Phase Transition

Organic-inorganic hybrid perovskites, with the formula ABX3 (A=organic cation, B=metal cation, and X=halide; for example, CH3 NH3 PbI3 ), have diverse and intriguing physical properties, such as semiconduction, phase transitions, and optical properties. Herein, a new ABX3 -type semiconducting perovskite-like hybrid, (hexamethyleneimine)PbBr3 (1), consisting of one-dimensional inorganic frameworks and cyclic organic cations, is reported. Notably, the inorganic moiety of 1 adopts a perovskite-like architecture and forms infinite columns composed of face-sharing PbBr6 octahedra. Strikingly, the organic cation exhibits a highly flexible molecular configuration, which triggers an above-room-temperature phase transition, at Tc =338.8 K; this is confirmed by differential scanning calorimetry (DSC), specific heat capacity (Cp ), and dielectric measurements. Further structural analysis reveals that the phase transition originates from the molecular configurational distortion of the organic cations coupled with small-angle reorientation of the PbBr6 octahedra inside the inorganic components. Moreover, temperature-dependent conductivity and UV/Vis absorption measurements reveal that 1 also displays semiconducting behavior below Tc . It is believed that this work will pave a potential way to design multifeatured perovskite hybrids by utilizing cyclic organic amines.

Thermochromism to tune the optical bandgap of a lead-free perovskite-type hybrid semiconductor for efficiently enhancing photocurrent generation

Thermochromic materials have recently attracted great attention due to their controllable and rich physicochemical properties. However, until now, no studies have been reported on thermochromic materials for photovoltaic and optoelectronic applications. Here we report a new lead-free hybrid semiconductor material, (C16H20N2)SbBr5 (1), which adopts the zero-dimensional (0-D) perovskite-type inorganic framework. Strikingly, the thermochromism in 1 leads to a wide tunable bandgap and superior photoelectric properties. Three distinct color-varying stages were first observed, i.e. colorless to yellow (I), yellow to red (II), and red to black brown (III). In particular, the figure-of-merits for thin-film photodetectors based on II-thermochromism were greatly improved, with the dark current lowered to one quarter and light photocurrent enhanced at least 12-fold. The photocurrent on/off switching ratio was thus improved by ∼50 times through thermochromism. As a new conceptual strategy to engineer the optical bandgap and meet specific photoelectric functions, our study paves the way for building high-performance optoelectronic devices based on thermochromic materials.

Dielectric phase transition triggered by the order–disorder transformation of cyclopropylamine in a layered organic–inorganic halide perovskite

Cyclic organic amines are emerging as excellent building blocks to assemble organic–inorganic hybrid phase transition materials due to their flexible cyclic structure. Herein, we used a three-membered ring organic amine, cyclopropylamine, assembling a layered organic–inorganic hybrid dielectric phase transition compound ([C3H5NH3]2[CdCl4], CPA) that displays a remarkable switchable dielectric response induced by an order–disorder transformation of the organic moiety. More specifically, the dielectric constant of CPA can be tuned between high- and low-dielectric states at ∼273 K, which demonstrates its potential application in a switchable dielectric field. In addition, theoretical analysis of electronic band structures suggests that CPA exhibits a direct-band-gap with the value of 5.20 eV. This solid-state structural phase transition triggered by ordering of a three-membered ring organic amine is reported for the first time, and it highlights a new potential strategy to design switchable dielectric materials.

(1,4-Butyldiammonium)CdBr4: a layered organic–inorganic hybrid perovskite with a visible-blind ultraviolet photoelectric response

Inspired by the breakthrough of three-dimensional hybrid perovskite CH3NH3PbI3, two-dimensional (2D) layered organic–inorganic halide perovskites are emerging as another promising class of hybrid materials for optoelectronic devices, such as photodetectors fabricated on lead halide perovskites. However, the majority of such 2D materials exhibit photosensitivity to visible light, while few candidates have been reported to exhibit visible-blind ultraviolet (UV) photoelectric response. Here, we present a new hybrid material, (1,4-butyldiammonium)CdBr4 (1), in which the corner-sharing CdBr6 octahedra construct the 2D perovskite-type inorganic frameworks. The optical bandgap (Eg) of 1 is estimated to be ∼3.45 eV. Particularly, 1 shows spectral-selective photoconductivity, that is, it is sensitive to UV-light illumination below 360 nm but almost blind to the standard visible light (above 400 nm), disclosing the potential of 1 for visible-blind ultraviolet photodetection. Further theoretical analyses of its electronic structure and energy gap disclose that the inorganic perovskite architecture dominates the optical bandgap. It is believed that this work provides a potential route for the design and fabrication of new 2D hybrid perovskite materials with a UV photoelectric response.

A Molecular Ferroelectric Showing Room-Temperature Record-Fast Switching of Spontaneous Polarization

Fast switching of spontaneous polarization (Ps ) is one of the most essential requirements for ferroelectrics used in the field of data storage. However, in contrast to inorganic counterparts, the low operating frequency (<500 Hz) for molecular ferroelectrics severely hinders their large-scale applications. Herein, for the first time, we achieved the room-temperature fastest switching of the Ps in a new molecular ferroelectric, N-methylmorpholinium trinitrophenolate (1), which displays notable ferroelectricity (Ps =3.2 μc cm-2 ). Strikingly, electric polarizations of 1 have been switched under a record-high frequency of 263 kHz, and this performance remains stable without any obvious fatigue after ca. 2×105 switching cycles. To our knowledge, 1 is the first organic ferroelectric to switch polarization at such a high operating frequency, exceeding the majority of organic ferroelectrics, which opens up new possibilities for its potential in the field of non-volatile memory.