PEG Assisted Hydrothermal Synthesis of β-Na YF_4:Yb~(3+),Er~(3+) Microrods for Upconversion Photoluminescence Display

β-Na YF4:Yb3+,Er3+ microrods with average length of 10 μm and diameter of 2.5 μm were synthesized through hydrothermal method. Based on the obtained microrods, transparent and flexible film with upconversion luminescence property was prepared through inorganic-organic composition method by using co-polymerization product of isobornylmethacrylate and urethane acrylate as the polymer substrate. The prepared film shows excellent optical transmission in the visible region with average transmission of 81.3% at high doping concentration(2 wt%) of β-Na YF4:Yb3+,Er3+ microrods. By directing and moving NIR laser beam toward the prepared film with high scanning speed, visible light is effectively emitted and various images are able to display.     

Flexible and Stretchable Lithium‐Ion Batteries and Supercapacitors Based on Electrically Conducting Carbon Nanotube Fiber Springs

The construction of lightweight, flexible and stretchable power systems for modern electronic devices without using elastic polymer substrates is critical but remains challenging. We have developed a new and general strategy to produce both freestanding, stretchable, and flexible supercapacitors and lithium‐ion batteries with remarkable electrochemical properties by designing novel carbon nanotube fiber springs as electrodes. These springlike electrodes can be stretched by over 300 %. In addition, the supercapacitors and lithium‐ion batteries have a flexible fiber shape that enables promising applications in electronic textiles.     

Experimental and Theoretical Studies of Serpentine Microstructures Bonded To Prestrained Elastomers for Stretchable Electronics

Stretchable electronic devices that exploit inorganic materials are attractive due to their combination of high performance with mechanical deformability, particularly for applications in biomedical devices that require intimate integration with human body. Several mechanics and materials schemes have been devised for this type of technology, many of which exploit deformable interconnects. When such interconnects are fully bonded to the substrate and/or encapsulated in a solid material, useful but modest levels of deformation (<30–40%) are possible, with reversible and repeatable mechanics. Here, the use of prestrain in the substrate is introduced, together with interconnects in narrow, serpentine shapes, to yield significantly enhanced (more than two times) stretchability, to more than 100%. Fracture and cyclic fatigue testing on structures formed with and without prestrain quantitatively demonstrate the possible enhancements. Finite element analyses (FEA) illustrates the effects of various material and geometric parameters. A drastic decrease in the elastic stretchability is observed with increasing metal thickness, due to changes in the buckling mode, that is, from local wrinkling at small thicknesses to absence of such wrinkling at large thicknesses, as revealed by experiment. An analytic model quantitatively predicts the wavelength of this wrinkling, and explains the thickness dependence of the buckling behaviors.     

Flexible Solid‐State Supercapacitor Based on Graphene‐based Hybrid Films

A flexible solid‐state asymmetric supercapacitor based on bendable film electrodes with 3D expressway‐like architecture of graphenes and “hard nano‐spacer” is fabricated via an extended filtration assisted method. In the designed structure of the positive electrode, graphene sheets are densely packed, and Ni(OH)₂ nanoplates are intercalated in between the densely stacked graphenes. The 3D expressway‐like electrodes exhibit superior supercapacitive performance including high gravimetric capacitance (≈573 F g^‐1), high volumetric capacitance (≈655 F cm^‐3), excellent rate capability, and superior cycling stability. In addition, another hybrid film of graphene and carbon nanotubes (CNT) is fabricated as the negative electrodes for the designed asymmetric device. In the obtained graphene@CNT films, CNTs served as the hard spacer to prevent restacking of graphene sheets but also as a conductive and robust network to facilitate the electrons collection/transport in order to fulfill the demand of high‐rate performance of the asymmetric supercapacitor. Based on these two hybrid electrode films, a solid‐state flexible asymmetric supercapacitor device is assembled, which is able to deliver competitive volumetric capacitance of 58.5 F cm^‐3 and good rate capacity. There is no obvious degradation of the supercapacitor performance when the device is in bending configuration, suggesting the excellent flexibility of the device.     

Mesoporous NiCo2O4 Nanowire Arrays Grown on Carbon Textiles as Binder‐Free Flexible Electrodes for Energy Storage

Binary metal oxides has been regarded as a promising class of electrode materials for high‐performance energy storage devices since it offers higher electrochemical activity and higher capacity than mono‐metal oxide. Besides, rational design of electrode architectures is an effective solution to further enhance electrochemical performance of energy storage devices. Here, the advanced electrode architectures consisting of carbon textiles uniformally covered by mesoporous NiCo₂O₄ nanowire arrays (NWAs) are successfully fabricated by a simple surfactant‐assisted hydrothermal method combined with a short post annealing treatment, which can be directly applied as self‐supported electrodes for energy storage devices, such as Li‐ion batteries, supercapacitors. The as‐prepared mesoporous NiCo₂O₄ nanowires consist of numerous highly crystalline nanoparticles, leaving a large number of mesopores to alleviate the volume change during the charge/discharge process. Electrode architectures presented here promise fast electron transport by direct connection to the growth substrate and facile ion diffusion path provided by both the abundant mesoporous structure in nanowires and large open spaces between neighboring nanowires, which ensures every nanowire participates in the ultrafast electrochemical reaction. Benefiting from the intrinsic materials and architectures features, the unique binder‐free NiCo₂O₄/carbon textiles exhibit high specific capacity/capacitance, excellent rate capability, and cycling stability.     

Highly Sensitive Non‐Classical Strain Gauge Using Organic Heptazole Thin‐Film Transistor Circuit on a Flexible Substrate

A non‐classical organic strain gauge as a voltage signal sensor is reported, using an inverter‐type thin‐film transistor (TFT) circuit, which is able to sensitively measure a large quantity of elastic strain (up to ≈2.48%), which approaches an almost folding state. Novel heptazole‐based organic TFTs are chosen to be incorporated in this gauge circuit; organic solid heptazole has small domain size in general. While large crystal domain‐pentacene TFTs seldom show sufficient current variation upon mechanical bending for tensile strain, these heptazole TFTs demonstrate a significant variation for the same strain condition as applied to pentacene devices. In addition, the pentacene channel does not recover to its original electric state after bending but heptazole channels are very elastic and reversible, even after going through serious bending. More interesting is that the heptazole TFTs show only a little variation of signal current under horizontal direction strain, while they make a significant amount of current decrease under vertical direction strain. Utilizing the anisotropic response to the tensile bending strain, an ultrasensitive voltage output strain gauge composed of a horizontally and vertically oriented TFT couple is demonstrated.     

Unusually High Optical Transmission in Ca:Ag Blend Films: High‐Performance Top Electrodes for Efficient Organic Solar Cells

Highly transparent electrodes are demonstrated based on thermally evaporated calcium:silver blend thin–films, which show unusually high transmission well above the expectations from bulk material properties and thin film optics. These electrodes exhibit a low sheet resistance of 27.3 Ω/, combined with an extraordinarily high mean transmittance of 93.0% in the visible spectral range (σ_dc/σ_opt = 186.7), superior to the commonly used inorganic electrodes made from indium tin oxide (ITO). Additionally, the metal blend electrode is flexible, showing a constant sheet resistance down to a bending radius of 10 mm and can be employed on top of organic devices without causing damage to the organic material. The spontaneously formed unique microstructure of a polycrystalline Ag network with randomly distributed nanoapertures, surrounded by a calcium shell, enables broadband transmittance enhancement due to amplified plasmonic coupling. Consequently, top‐illuminated organic solar cells using such metal blend electrodes achieve a power conversion efficiency of 7.2% (which defines a new record for top illuminated organic solar cells) and even exceed the efficiency of similar bottom‐illuminated reference solar cells (6.9%) employing common ITO electrodes.     

Flexible and Transparent Nanocomposite of Reduced Graphene Oxide and P(VDF‐TrFE) Copolymer for High Thermal Responsivity in a Field‐Effect Transistor

A new class of temperature‐sensing materials is demonstrated along with their integration into transparent and flexible field‐effect transistor (FET) temperature sensors with high thermal responsivity, stability, and reproducibility. The novelty of this particular type of temperature sensor is the incorporation of an R‐GO/P(VDF‐TrFE) nanocomposite channel as a sensing layer that is highly responsive to temperature, and is optically transparent and mechanically flexible. Furthermore, the nanocomposite sensing layer is easily coated onto flexible substrates for the fabrication of transparent and flexible FETs using a simple spin‐coating method. The transparent and flexible nanocomposite FETs are capable of detecting an extremely small temperature change as small as 0.1 °C and are highly responsive to human body temperature. Temperature responsivity and optical transmittance of transparent nanocomposite FETs were adjustable and tuneable by changing the thickness and R‐GO concentration of the nanocomposite.