Quasi‐Amorphous Inorganic Thin Films: Non‐Crystalline Polar Phases

Quasi‐amorphous thin films of BaTiO₃, SrTiO₃, and BaZrO₃ are the only known examples of inorganic, non‐crystalline, polar materials. The conditions under which they are formed and the origin of their polarity set these materials apart from other classes of inorganic materials. The most important feature of the quasi‐amorphous phase is that the polarity is the result of the orientational ordering of local bonding units but without any detectable spatial periodicity. This mechanism is reminiscent of that observed in ferroelectric polymers and permits compounds that do not have polar crystalline polymorphs, such as SrTiO₃ and BaZrO₃, to form polar non‐crystalline solids. In the present report, we provide an overview of the essential features of these materials including preparation, structure, and chemical composition. The report also reviews our current level of understanding and offers some guidelines for further development and application of non‐crystalline inorganic polar materials.     

Conjugated‐Polyelectrolyte‐Functionalized Reduced Graphene Oxide with Excellent Solubility and Stability in Polar Solvents

Conjugated‐polyelectrolyte (CPE)‐functionalized reduced graphene oxide (rGO) sheets are synthesized for the first time by taking advantage of a specially designed CPE, PFVSO₃, with a planar backbone and charged sulfonate and oligo(ethylene glycol) side chains to assist the hydrazine‐mediated reduction of graphene oxide (GO) in aqueous solution. The resulting CPE‐functionalized rGO (PFVSO₃‐rGO) shows excellent solubility and stability in a variety of polar solvents, including water, ethanol, methanol, dimethyl sulfoxide, and dimethyl formamide. The morphology of PFVSO₃‐rGO is studied by atomic force microscopy, X‐ray diffraction, and transmission electron microscopy, which reveal a sandwich‐like nanostructure. Within this nanostructure, the backbones of PFVSO₃ stack onto the basal plane of rGO sheets via strong π–π interactions, while the charged hydrophilic side chains of PFVSO₃ prevent the rGO sheets from aggregating via electrostatic and steric repulsions, thus leading to the solubility and stability of PFVSO₃‐rGO in polar solvents. Optoelectronic studies show that the presence of PFVSO₃ within rGO induces photoinduced charge transfer and p‐doping of rGO. As a result, the electrical conductivity of PFVSO₃‐rGO is not only much better than that of GO, but also than that of the unmodified rGO.     

A Fluid Liquid‐Crystal Material with Highly Polar Order

An anomalously large dielectric permittivity of ≈10⁴ is found in the mesophase temperature range (MP phase) wherein high fluidity is observed for a liquid‐crystal compound having a 1,3‐dioxane unit in the mesogenic core (DIO). In this temperature range, no sharp X‐ray diffraction peak is observed at both small and wide Bragg angles, similar to that for a nematic phase; however, an inhomogeneous sandy texture or broken Schlieren one is observed via polarizing optical microscopy, unlike that for a conventional nematic phase. DIO exhibits polarization switching with a large polarization value, i.e., P = 4.4 µC cm^?2, and a parallelogram‐shaped polarization–electric field hysteresis loop in the MP phase. The inhomogeneously aligned DIO in the absence of an electric field adopts a uniform orientation along an applied electric field when field‐induced polarization switching occurs. Furthermore, sufficiently larger second‐harmonic generation is observed for DIO in the MP phase. Second‐harmonic‐generation interferometry clearly shows that the sense of polarization is inverted when the +/? sign of the applied electric field in MP is reversed. These results suggest that a unidirectional, ferroelectric‐like parallel polar arrangement of the molecules is generated along the director in the MP phase.     

Polar Side Chains Enhance Processability,Electrical Conductivity,and Thermal Stability of a Molecularly p‐Doped Polythiophene

Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field‐effect transistors, solar cells, and thermoelectric generators. However, many dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid‐state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g₄2T‐T)—a polythiophene—in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) is explored. Doping of p(g₄2T‐T) with F4TCNQ results in an electrical conductivity of up to 100 S cm^?1. Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.     

Quantitative High‐Dynamic‐Range Electron Diffraction of Polar Nanodomains in Pb2ScTaO6

Highly B‐site ordered Pb₂ScTaO₆ crystals are studied as a function of temperature via dielectric spectroscopy and in situ high‐dynamic‐range electron diffraction. The degree of ordering is examined on the local and macroscopic scale and is determined to be 76%. Novel analysis of the electron diffraction patterns provides structural information with two types of antiferroelectric displacements determined to be present in the polar structure. It is then found that a low‐temperature transition occurs on cooling at ≈210 K that is not present on heating. This phenomenon is discussed in terms of the freezing of dynamic polar nanodomains where a high density of domain walls creates a metastable state.     

Electrostatic Properties of Ideal and Non‐ideal Polar Organic Monolayers: Implications for Electronic Devices

Molecules in (or as) electronic devices are attractive because the variety and flexibility inherent in organic chemistry can be harnessed towards a systematic design of electrical properties. Specifically, monolayers of polar molecules introduce a net dipole, which controls surface and interface barriers and enables chemical sensing via dipole modification. Due to the long range of electrostatic phenomena, polar monolayer properties are determined not only by the type of molecules and/or bonding configuration to the substrate, but also by size, (dis‐)order, and adsorption patterns within the monolayer. Thus, a comprehensive understanding of polar monolayer characteristics and their influence on electronic devices requires an approach that transcends typical chemical designs, i.e., one that incorporates long‐range effects, in addition to short‐range effects due to local chemistry. We review and explain the main uses of polar organic monolayers in shaping electronic device properties, with an emphasis on long‐range cooperative effects and on the differences between electrical properties of uniform and non‐uniform monolayers.     

Asymmetrical Ladder‐Type Donor‐Induced Polar Small Molecule Acceptor to Promote Fill Factors Approaching 77% for High‐Performance Nonfullerene Polymer Solar Cells

In this work, an effectual strategy of constructing polar small molecule acceptors (SMAs) to promote fill factor (FF) of nonfullerene polymer solar cells (PSCs) is first reported. Three asymmetrical SMAs of IDT6CN , IDT6CN‐Th , and IDT6CN‐M , which own large dipole moments, are designed and synthesized. The PSCs based on three polar SMAs exhibit apparently higher FFs compared with their symmetrical analogues. The asymmetrical design strategy accompanied with side chain and end group engineering makes IDT6CN‐Th ‐ and IDT6CN‐M ‐based nonfullerene PSCs achieve high power conversion efficiency with FFs approaching 77%.     

Ultrahigh‐Water‐Content,Superelastic, and Shape‐Memory Nanofiber‐Assembled Hydrogels Exhibiting Pressure‐Responsive Conductivity

High‐water‐content hydrogels that are both mechanically robust and conductive could have wide applications in fields ranging from bioengineering and electronic devices to medicine; however, creating such materials has proven to be extremely challenging. This study presents a scalable methodology to prepare superelastic, cellular‐structured nanofibrous hydrogels (NFHs) by combining alginate and flexible SiO₂ nanofibers. This approach causes naturally abundant and sustainable alginate to assemble into 3D elastic bulk NFHs with tunable water content and desirable shapes on a large scale. The resultant NFHs exhibit the integrated properties of ultrahigh water content (99.8 wt%), complete recovery from 80% strain, zero Poisson's ratio, shape‐memory behavior, injectability, and elastic‐responsive conductivity, which can detect dynamic pressure in a wide range (>50 Pa) with robust sensitivity (0.24 kPa^?1) and durability (100 cycles). The fabrication of such fascinating materials may provide new insights into the design and development of multifunctional hydrogels for various applications.