Xitao Liu

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.

Characterization and strong piezoelectric response of an organometallic nonlinear optical crystal: CdHg(SCN)4(C2H6SO)2

Bulk single crystals of CdHg(SCN)4(C2H6SO)2 (CMTD) have been grown by the temperature-lowering method. Refractive index measurements reveal that the refractive index nx along the c axis is much smaller than the ny and nz indices in the ab-plane. The detailed powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicate that the crystal is a phase matchable nonlinear optical material which exhibits a strong SHG effect, 1.2 times that of KTiOPO4. The complete set of dielectric, elastic and piezoelectric constants of CMTD have been measured and the results show that the crystal possesses excellent piezoelectric properties of d36 = 20.40 and d14 = 17.80 pC N−1 (much larger than those of α-SiO2). The crystal exhibits a neutral layered structure consisting of the asymmetric HgS4 tetrahedron and CdN4O2 octahedron which are connected by π-conjugated –SCN– ligands. The physical properties have been interpreted on the basis of the crystal structure combined with theoretical calculations. The strong piezoelectric response and high optical SHG efficiency make CMTD a promising candidate for both piezoelectric and nonlinear optical applications.

Strong enhancement of second harmonic generation in nonlinear optical crystals: 2-amino-3-nitropyridinium halides (Cl, Br, I)

Three nonlinear optical (NLO) crystals of 2-amino-3-nitropyridinium halides (Cl, Br, I), which are assembled by anchoring the organic NLO chromophores into the halide anion frameworks through multicenter hydrogen bonds, have been successfully grown. Their optical transmittance window and cut-off wavelength have been investigated by UV-visible-NIR studies. The Kurtz–Perry powder measurements disclose a large enhancement of their NLO activities, i.e., second harmonic generation (SHG) efficiencies of 2-amino-3-nitropyridinium iodide are 1.5 times that of KDP, while the corresponding values for the compounds of 2-amino-3-nitropyridinium chloride and 2-amino-3-nitropyridinium bromide are about 15 and 10 times, respectively. Further structure analyses and theoretical calculations reveal that such a distinct difference in the NLO behaviours results from their diverse coplanar alignments and dipole-dipole superposition, which is expected to originate from the discrepant size of halide anions. It is believed that the enhancement of the NLO response would indicate a potential pathway to design new functional materials in the family of organic chromophores.

[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.

Above-room-temperature switching of quadratic nonlinear optical properties in a Bi–halide organic–inorganic hybrid

Organic–inorganic hybrid nonlinear optical (NLO) switches have recently attracted intensive attention as potential candidates for optical applications. Herein, by introducing an organic triamine cation, we obtain a lead-free Bi-based organic–inorganic hybrid, (C4H16N3)BiBr6 (1), which behaves as a promising above-room-temperature modulator of quadratic NLO responses. More importantly, 1 possesses a remarkable NLO switching contrast of up to ∼35 and a notable switching repeatability. Additionally, microscopic single-crystal structural analyses reveal that the coordinated distortion of inorganic [BiBr6]3− octahedra and the order–disorder transformation of the flexible organic triamine cations synergistically contribute to its NLO switching behavior. We believe that this work will promote the development of high-performance NLO switching materials based on devisable organic–inorganic metal–halide hybrids.

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.