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Domain switching for the APFA thin film. Topography images (a), PFM amplitude images (b), and PFM phase images (c) of an 8 × 8 μm² region. Vertical PFM amplitude (d) and (f) and phase (e) and (g) images superimposed on the topographic image, which were recorded after writing a square area with a voltage of −140 V (d) and (e) followed by a smaller central square with a voltage of +110 V (f) and (g) using a biased conductive tip. The blue and yellow regions in the phase images indicate the regions with out-of-plane polarization oriented upward and downward, respectively
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Organic single-component ferroelectrics, as an important class of metal-free ferroelectrics, are highly desirable because of their easy processing, mechanical flexibility, and biocompatibility. However, although nearly 50 years have passed since the discovery of photochromism in azobenzene-doped cholesteric liquid crystals, ferroelectricity has nev...
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... However, it is relatively difficult to realize typical ferroelasticity. There are two specific difficulties: first, this category of materials possess reversible solid phase transitions; second, the evolution of symmetric operations during the phase transition should strictly follow the Aizu rules (the 94 species of ferroelastic phase transition) [8][9][10][11][12]. In addition, most of the reported ferroelastic materials are hybridized with other physical properties, such as thermochromism, luminescence and ferroelectricity [13][14][15][16][17][18][19][20]. ...
Ferroelastics, the mechanism analogous to ferroelectrics and ferromagnets, have attracted comprehensive attention owing to the prominent spontaneous strain characteristics. However, the contingency of symmetry changes during the phase transition brings a great challenge to the construction of ferroelastic materials. Currently, most of the ferroelastics are accompanied by various physical properties, such as, photoluminescence and thermochromism. There are few reports of ferroelastics with a photochromic effect. Here, we have successfully designed two hybrid metal halide materials, (pyrrolidinium)PbBr3 (M-1) and (3-hydroxypyrrolidinium)4Pb3Br10 (M-2), both of which possess a two-step dielectric response. The organic-inorganic hybrid materials with structural adjustability provide a good design platform and great possibilities. Meanwhile, the strategy of introducing intermolecular H-bonds increases the phase transition temperature by about 90 K, and also achieves dimensional regulation (3D→2D). Exhilaratingly, the second phase transition of M-2 belongs to the \(2/mF\bar{1}\)-type ferroelastic phase transition according to the Aizu rules, and the ferroelastic domains are observed by a polarizing microscope, which strongly proves the ferroelasticity. Furthermore, we find that these compounds possess a photochromic effect. Especially, the photochromic phenomenon (from milky white to beige) of M-2 is more fascinating, making it a potential candidate for sensors and display devices. The key properties are well-verified through theoretical calculations with high consistency. In short, this photochromic ferroelastic might be a potential candidate in the fields of mechanical switches, shape memory and energy conversion.
Optically switchable field‐effect transistors (OSFETs) are non‐volatile photonic memory devices holding a great potential for applications in optical information storage and telecommunications. Solution processing of blends of photochromic molecules and π‐conjugated polymers is a low‐cost protocol to integrate simultaneously optical switching and charge transport functions in large‐area devices. However, the limited reversibility of the isomerization of photochromic molecules due to steric hindrance when embedded in ordered polymeric matrices represents a severe limitation and it obliges to incorporate as much as 20% in weight of the photochromic component, thereby drastically diluting the electronic function, limiting the device performance. Herein, a comparative study of the photoresponsivity of a suitably designed diarylethene molecule is reported when embedded in the matrix of six different polymer semiconductors displaying diverse charge transport properties. In particular, this study focuses on three semi‐crystalline polymers and three quasi‐1D polymers. It is found that 1% w/w of 1,2‐bis(5‐(3,5‐di‐tert‐butylphenyl)‐2‐methylthiophen‐3‐yl)cyclopent‐1‐ene in a blend with poly(indacenodithiophene‐co‐benzothiadiazole) is sufficient to fabricate OSFETs combining photo‐modulation efficiencies of 45.5%, mobilities >1 cm² V⁻¹s⁻¹, and photo‐recovered efficiencies of 98.1%. These findings demonstrate that quasi‐1D polymer semiconductors, because of their charge transport dominated by intra‐molecular processes, epitomize the molecular design principles required for the fabrication of high‐performance OSFETs.
The multifunctional tuning of solid-state dielectric switches constructed from organic-inorganic hybrid materials (OIHMs) has received great attention. In particular, molecular ferroelastics with dielectric phase transitions have considerable potential in the optical and electrical fields owing to their adjustable structures and physical features. However, it remains a challenge to effectively design ferroelastics with high phase transition temperature (Tc). We used [TTMA]2CdI4 (TTMA = tetramethylammonium, 1) as a template to continuously increase the molecular weight and change the structure of the hybrid material by modifying and extending the alkane chain in the cation. Therefore, a series of OIHMs were eventually developed: [TMEA]2CdI4 (TMEA = trimethylethylammonium, 2), [TMPA]2CdI4 (TMPA = trimethylpropylammonium, 3), and [TMIPA]2CdI4 (TMIPA = trimethyliso-propylammonium, 4). Among them, the Tc of ferroelastic 3 increased up to 387 K. DSC and temperature-related dielectric constant tests prove the occurrence of the phase transition for 1, 2, and 3. The structures further indicate that the phase transition is caused by the order-disorder cation motion. The extension of the alkyl chain greatly increases Tc and endows 3 with ferroelasticity at room temperature.
Since the switchable spontaneous polarization of ferroelectric materials endows it with many useful properties such as a large pyroelectric coefficient, switchable spontaneous polarization, and semiconductor, it has a wide range of application prospects, and the research of high-performance molecular ferroelectric materials has become a hot spot. We obtained a 0D organic-inorganic hybrid ferroelectric [(CH3)3NCH2CH2CH3]2FeCl4 (1) with well-defined ferroelectric domains and excellent domain inversion and exhibited a relatively large spontaneous polarization (Ps = 9 μC/m-2) and a Curie temperature (Tc) of 394 K. Furthermore, compound 1 belongs to the non-centrosymmetrical space group Cmc21 and has a strong second-harmonic generation signal. Interestingly, we also performed magnetic tests on 1, which confirmed that it is a magnetic material. This work provides clues for exploring the application of high-performance molecular ferroelectric materials in future multifunctional smart devices.
Considering nearly infinite design possibilities, organic second harmonic generation (SHG) molecules are believed to have long-term promise. However, because of the tendency to form dipole-antiparallel crystals that lead to zero macroscopic polarization, it is difficult to design a nonlinear optical (NLO) material based on organic molecules. In this manuscript, we report a new molecule motif that can form asymmetric organic solids by controlling the degree of hydrogen bonding through protonation. A conjugated polar organic molecule was prepared with a triple bond connecting an electron-withdrawing pyridine ring and an electron-donating thiophene ring. By controlling the degree of hydrogen bonding through protonation, two different crystal packing motifs are achieved. One crystallizes into the common dipole-antiparallel nonpolar P1̄ space group. The second crystallizes into the uncommon dipole-parallel polar P1 space group, in which the molecular dipoles are aligned along a single axis and thus exhibit a high macroscopic polarization in its solid-state form. Due to the P1 polar packing, the sample can generate second harmonic light efficiently, about three times the intensity of the benchmark potassium dihydrogen phosphate. Our findings show that crystal engineering by hydrogen bonding in a single molecular backbone can be used for controlling the macroscopic NLO properties.
The nonlinear optical response and photo-isomerization behavior of complexes, prepared by triple-ligand coupling Schiff reaction, were investigated by Z-scan and pump-probe with femtosecond pulse laser. Unlike the ligand, the complexes showed two-photon absorption at 800 nm with the effective two-photon absorption coefficient of 0.34 cm/GW, while a transition from saturable absorption to excited state absorption was observed at 400 nm with increasing excited light intensity. An optical-optical isomerization cycle model was adopted, and supported by the long lifetime as 1929 fs in complexes. The result indicated the multiple ligands coupling in AZO based materials can induced two-photon absorption, which is of significant for the field of photonics and nonlinear optics.
