Self-Assembled Liquid-Crystalline Membranes Form Supramolecular Hydrogels via Hydrogen Bonding.

Developing simple methods to organize nanoscale building blocks into ordered superstructures is a crucial step toward the practical development of nanotechnology. Bottom-up nanotechnology using self-assembly bridges the molecular and macroscopic, and can provide unique material properties, different from the isotropic characteristics of common substances. In this study, a new class of supramolecular hydrogels comprising 40 nm thick linear polymer layers sandwiched between nanolayers of self-assembled amphiphilic molecules are prepared and studied by nuclear magnetic resonance spectroscopy, scanning electronic microscopy, small angle X-ray diffraction, and rheometry. The amphiphilic molecules spontaneously self-assemble into bilayer membranes when they are in liquid-crystal state. The hydrogen bonds at the interface of the nanolayers and linear polymers serve as junctions to stabilize the network. These hydrogels with layered structure are facile to prepare, mechanically stable, and with unique temperature-dependent optical transparency, which makes it interesting in applications, such as soft biological membranes, drug release, and optical filters.

Nanocomposite hydrogels for strain sensing based on optical and electrical signals: a review.

Unlike metals, ceramics, or plastics, hydrogels are semi-solid, hydrophilic polymer networks with high water content. Embedding nanostructures or nanomaterials into hydrogels can impart special properties to the composites, such as anisotropy, optical or electrical properties. With the development of nanomaterials and advanced synthetic methods, nanocomposite hydrogels have attracted growing research interest in recent years due to their desirable mechanical properties, optical/electrical functions, reversibility, stimulus sensitivity, and biocompatibility. They have opened up a broad range of applications as stretchable strain sensors in mapping of strain distributions, motion detection, health-monitoring, and flexible skin-like devices. This minireview summarizes the recent developments in nanocomposite hydrogels as strain sensors based on optical and electrical signals. The strain sensing performances and their dynamic properties are discussed. Appropriate embedding of nanostructures or nanomaterials in hydrogels and engineering of interactions between nanomaterials and polymer networks can yield significant improvements in the performance of strain sensors.

Biosynthesis of highly branched gold nanoparticles through structural engineering of fatty acids.

Biomimetic approaches have been used to develop inorganic nanomaterials with complex morphologies and functions. Fatty acids are among the most important and decomposable biomolecules in nature. However, the controlled synthesis of branched gold nanoparticles using these biomolecules has not been reported. Herein, we demonstrate a strategy to produce highly branched gold nanoparticles through structural engineering of fatty acids. Furthermore, we developed a method for tailoring fatty acid molecules by altering their aliphatic chains to facilitate the morphological evolution of gold nanoparticles from spherical to branched shape. It is found that the growth of the nanoparticles is sensitive to characteristics of fatty acids, such as saturation degrees. The growth of the nanoparticle is visualized by high-speed atomic force microscopy. The reaction mechanisms and growth processes of branched gold nanoparticles are proposed. This work may serve as a cornerstone to the design in a biomimetic fashion for the controllable synthesis of metallic nanomaterials.

Co-Self-Assembly of Amphiphiles into Nanocomposite Hydrogels with Tailored Morphological and Mechanical Properties.

As one of the most amazing aspects of life, all living organisms are formed by self-assembly, a fundamental biological design process in which ordered nanostructures are assembled from small parts. For example, most of the biological tissues contain structurally soft and hard parts that are usually hierarchically organized at nano or micro levels to achieve specific functions. Hydrogels are one of the most promising soft materials owing to their potential applications in building of biological tissues and stretchable sensors. In this work, a series of hydrogels are synthesized through the co-self-assembly of two types of amphiphiles in their aqueous solution prior to polymerization. Soft and hard parts with nanostructures of different order parameters are incorporated into the hydrogels. The hydrophilic segment (as soft phases) of the polymer network provides water absorption, fluid flow, and softness, whereas the hydrophobic segment (as hard phases) provides strength and tearing and fracture resistance. Appropriate soft/hard nanostructures and their interfaces allow for the tailoring of the desired morphological and mechanical properties, including a different wetting ability, toughness, energy dissipation, self-recovery, and fracture resistance arising from their nanostructures. This work provides insights into the design of nanostructured anisotropic hydrogels with controlled morphological and mechanical properties.

Gold clay from self-assembly of 2D microscale nanosheets.

Nature has always demonstrated incredible ability to create amazing materials such as soft clay which are built from nanoplatelet packing structures. It is challenging to produce artificial clays owing to the difficulty in obtaining large volume fractions of nanoplatelets and the lack of structural control in layer-by-layer packing. Here, single-crystalline Au nanosheets are synthesized by controlled growth in the bilayer membranes of succinic acid surfactants. Then, a self-assembly strategy is used to make {111}-oriented gold nanostructures at the liquid-liquid interface. The stiffness of the nanosheet assemblies are six orders of magnitude softer than bulk gold. The Au nanosheet aggregates show high plasticity and deformable into macroscale free-standing metallic architectures. They show a stress/strain-dependent conductivity owing to morphological changes. Our study provides valuable insights on the chemical synthesis of 2D nanostructures as well as for the self-assembly strategy on fabrication of mouldable metals for producing free-standing metallic architectures with microscale resolutions.

Designing Responsive Photonic Crystal Patterns by Using Laser Engraving.

Soft photonic crystals are periodic nanostructures that have attracted much attention for their applications in sensors, owing to their tunable structural colors in response to external stimuli. Patterned photonic crystals provide a novel strategy for constructing high-performance photonic materials with unique structures and functions. In this work, laser engraving is used for the first time to design patterns on a layered photonic hydrogel. This approach is based on the integration of laser power and chemical modifications to embed different polymer composites (polyelectrolyte and neutral polymers) along a prescribed laser-printed path. The polyelectrolyte and neutral composites show differential swelling or shrinking, causing a mechanical instability in the layered hydrogel. The resultant soft polymeric materials appear as synchronous tuning in the photonic band gaps in response to external stimuli. This approach is favorable for designing responsive photonic crystals with controllable optical properties and 3D shape transformation. Moreover, it is of great use in developing advanced photonic crystals for applications in sensors, soft actuators, and drug release.

A chemically inert bismuth interlayer enhances long-term stability of inverted perovskite solar cells.

Long-term stability remains a key issue impeding the commercialization of halide perovskite solar cells (HPVKSCs). The diffusion of molecules and ions causes irreversible degradation to photovoltaic device performance. Here, we demonstrate a facile strategy for producing highly stable HPVKSCs by using a thin but compact semimetal Bismuth interlayer. The Bismuth film acts as a robust permeation barrier that both insulates the perovskite from intrusion by undesirable external moisture and protects the metal electrode from iodine corrosion. The Bismuth-interlayer-based devices exhibit greatly improved stability when subjected to humidity, thermal and light stresses. The unencapsulated device retains 88% of its initial efficiency in ambient air in the dark for over 6000 h; the devices maintain 95% and 97% of their initial efficiencies after 85 °C thermal aging and light soaking in nitrogen atmosphere for 500 h, respectively. These sound stability parameters are among the best for planar structured HPVKSCs reported to date.

Light-induced mechanical response in crosslinked liquid-crystalline polymers with photoswitchable glass transition temperatures.

Energy conversion of light into mechanical work is of fundamental interest in applications. In particular, diligent molecular design on nanoscale, in order to achieve efficient photomechanical effects on macroscopic scale, has become one of the most interesting study topics. Here, by incorporating a "photomelting" azobenzene monomer crosslinked into liquid crystalline (LC) networks, we generate photoresponsive polymer films that exhibit reversible photoswitchable glass transition temperatures (Tg) at room temperature (~20 °C) and photomechanical actuations under the stimulus of UV/visible light. The trans-to-cis isomerization of azo chromophores results in a change in Tg of the crosslinked LC polymers. The Tg of the polymer network is higher than room temperature in the trans-form and lower than room temperature in the cis-form. We demonstrate the photoswitchable Tg contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in Tg of the film is proposed.

[6,6]-Phenyl-C61-Butyric Acid Methyl Ester/Cerium Oxide Bilayer Structure as Efficient and Stable Electron Transport Layer for Inverted Perovskite Solar Cells.

Stability issues and high material cost constitute the biggest obstacles of a perovskite solar cell (PVSC), hampering its sustainable development. Herein, we demonstrate that, after suitable surface modification, the low-cost cerium oxide (CeO x) nanocrystals can be well dispersed in both polar and nonpolar solvents and easily processed into high-quality electron transport layers (ETLs). The inverted PVSC with the configuration of "NiMgLiO/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/CeO x" has achieved a high efficiency up to 18.7%. Especially, the corresponding devices without encapsulation can almost keep their initial PCEs in 30% humidity-controlled air in the dark for 30 days and also show no sign of degradation after continuous light soaking and maximum power point tracking for 200 h in a N2 atmosphere. These results have been proved to be associated with the dual functions achieved by the PCBM/CeO x bilayer ETLs in both efficient electron extraction and good chemical shielding. Furthermore, an all inorganic interfacial layer based PVSC with the configuration of "NiMgLiO/MAPbI3/CeO x" has also achieved a promising efficiency of 16.7%, reflecting the potential to fabricate efficient PVSCs with extremely low cost.

Decoupling dual-stimuli responses in patterned lamellar hydrogels as photonic sensors.

This work developed a photonic hydrogel that is responsive to, and can distinguish between two stimuli of stress and pH. Patterning is used to locally change the chemistry of a one-dimensional (1D) photonic gel, such that the native region is responsive to mechanical stress while the chemically modified region is responsive to both mechanical stress and pH. By combining the optical signals in the native region and the modified region, one can distinguish the stimuli between pH and stress. Specifically, the native 1D photonic gel is composed of periodically aligned polymeric bilayers in a soft polyacrylamide (PAAm) network. The chemical modification is done by partially hydrolyzing PAAm into sodium polyacrylic acid in some patterned regions, which imparts pH sensitivity, in addition to the stress sensitivity, to these regions.

Enhanced Stability of Perovskite Solar Cells through Corrosion-Free Pyridine Derivatives in Hole-Transporting Materials.

The molecular structure of pyridine derivatives is critical to perovskite solar cell performance, especially stability. Most of the pyridine additives easily form complexes with perovskite. A new pyridine additive with a long alkyl chain substituted at its o-position does not corrode perovskite. The stability of devices containing this additive is the highest among the investigated cells.

Consecutive Morphology Controlling Operations for Highly Reproducible Mesostructured Perovskite Solar Cells.

Perovskite solar cells have shown high photovoltaic performance but suffer from low reproducibility, which is mainly caused by low uniformity of the active perovskite layer in the devices. The nonuniform perovskites further limit the fabrication of large size solar cells. In this work, we control the morphology of CH3NH3PbI3 on a mesoporous TiO2 substrate by employing consecutive antisolvent dripping and solvent-vapor fumigation during spin coating of the precursor solution. The solvent-vapor treatment is found to enhance the perovskite pore filling and increase the uniformity of CH3NH3PbI3 in the porous scaffold layer but slightly decrease the uniformity of the perovskite capping layer. An additional antisolvent dripping is employed to recover the uniform perovskite capping layer. Such consecutive morphology controlling operations lead to highly uniform perovskite in both porous and capping layers. By using the optimized perovskite deposition procedure, the reproducibility of mesostructured solar cells was greatly improved such that a total of 40 devices showed an average efficiency of 15.3% with a very small standard deviation of 0.32. Moreover, a high efficiency of 14.9% was achieved on a large-size cell with a working area of 1.02 cm(2).

Selective Deposition of Insulating Metal Oxide in Perovskite Solar Cells with Enhanced Device Performance.

We report a simple methodology for the selective deposition of an insulating layer on the nanoparticulate TiO2 (np-TiO2) mesoporous layer of perovskite solar cells. The deposited MgO insulating layer mainly covered the bottom part of the np-TiO2 layer with less coverage at the top. The so-called quasi-top-open structure is introduced to act as an efficient hole-blocking layer to prevent charge recombination at the physical contact of the transparent conducting oxide with the perovskite. This leads to an open-circuit voltage higher than that of the reference cell with a compact TiO2 hole-blocking layer. Moreover, such a quasi-top-open structure can facilitate the electron injection from perovskite into the np-TiO2 mesoporous layer and improve the spectral response at longer wavelength because of the less covered insulating layer at the top. This work provides an alternative way to fabricate perovskite solar cells without the need to use a conventional compact TiO2 layer.

High-Quality Mixed-Organic-Cation Perovskites from a Phase-Pure Non-stoichiometric Intermediate (FAI)1- x -PbI2 for Solar Cells.

A high-quality mixed-organic-cation perovskite (MA)x (FA)1- x PbI3 is prepared from a phase-pure non-stoichiometric intermediate complex (FAI)1- x -PbI2 . The phase-pure (FAI)1- x -PbI2 probably facilitates homogenous nucleation and modulates the growth kinetics during the crystallization of (MA)x (FA)1- x PbI3 . This strategy can be expected to pave the way for the development of mixed-organic-cation perovskite solar cells.

Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers.

The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking.

Mechano-actuated ultrafast full-colour switching in layered photonic hydrogels.

Photonic crystals with tunability in the visible region are of great interest for controlling light diffraction. Mechanochromic photonic materials are periodically structured soft materials designed with a photonic stop-band that can be tuned by mechanical forces to reflect specific colours. Soft photonic materials with broad colour tunability and fast colour switching are invaluable for application. Here we report a novel mechano-actuated, soft photonic hydrogel that has an ultrafast-response time, full-colour tunable range, high spatial resolution and can be actuated by a very small compressive stress. In addition, the material has excellent mechanical stability and the colour can be reversibly switched at high frequency more than 10,000 times without degradation. This material can be used in optical devices, such as full-colour display and sensors to visualize the time evolution of complicated stress/strain fields, for example, generated during the motion of biological cells.