Advanced Materials

On page 3511, Sunkook Kim, InSeo Kee, Sang Yoon Lee, and co-workers report the integration of low-temperature color filters, thin-film encapsulation, and red, green, blue organic light-emitting diode (OLED) microcavities into a display device. The device is demonstrated to be mechanically flexible, and to have low power consumption and high outdoor readability. The study offers a promising possibility for the implementation of flexible displays, with potentially important technical implications.

Leaves, such as those of Corokia cotoneaster, manage their ultra-hydrophibic interactions with water drops by microstructuring their surfaces with long hairs. On page 3565, Joerg C. Tiller and co-workers report achievement of the same effect by preparing an artificial leaf that fully mimics the texture of the hairy leaf surface. The structure was prepared performing a multistep moulding technique that uses a natural tooth as template. Images courtesy of Christoph P. Fik, Christian Krumm, Ulrich Hellinger, and Joerg C. Tiller.

Terpyridinesare interesting photo- and redox-active building blocks that can be used to functionalize surfaces. This molecular engineering approach promises unique possibilities to self-assemble functional devices, molecular electronic structures, switches, and responsive surfaces. Trends and recent progress in this area of research are highlighted and provide a state-of-the-art overview of the developments in the past decade.

Vladimir Bulović, Karen K. Gleason, and co-workers demonstrate monolithic fabrication of organic photovoltaic circuits directly on a variety of common paper substrates using oxidative chemical vapor deposition to vapor print conformal conductive polymer electrodes. The paper photovoltaic arrays produce >50 V, power common electronic displays in ambient indoor lighting, and can be tortuously flexed and folded without loss of function.

Organic photovoltaic circuits are monolithically fabricated directly on a variety of common paper substrates using oxidative chemical vapor deposition to vapor print conformal conductive polymer electrodes. The paper photovoltaic arrays produce >50 V, power common electronic displays in ambient indoor lighting, and can be tortuously flexed and folded without loss of function.

A novel cell-accumulation technique has been developed using highly biocompatible nanofilms by layer-by-layer assembly for the rapid construction of thick layered tissues with a well-controlled layer number and thickness. Furthermore, 3D tissues with highly developed blood capillary networks (over 1 cm² of layered tissues) were also constructed by sandwiching endothelial cells between the layered tissues. Such a simple and rapid methodology would be useful for tissue engineering and drug assessment.

A low-power flexible organic light-emitting diode (OLED) display device based on low temperature color filters with a thin film encapsulated RGB OLED microcavity is achieved with high contrast ratios (up to 150 000:1 in dark ambient) and low power consumption (34% power saving compared to polarization film). Furthermore, a mechanical 10 000 times folding experiment with a folding radius of 1 mm demonstrates the mechanical reliability of the flexible OLED device.

Liposomes of hydrogenated soy phosphatidylcholine (PC) lipids spontaneously self-assemble in close-packed layers on solid surfaces to form boundary lubricants that reduce the coefficient μ of sliding friction between them down to μ ≈ 10⁻⁴ – 2 × 10⁻⁵, at pressures up to ca. 12 MPa. This strikingly low value is attributed to hydration-lubrication by the PC headgroups exposed at the surfaces of the gel-phase vesicles.

Hexagonal graphene flakes: Equiangular hexagon-shaped graphene domains are synthesized by methane chemical vapor deposition on copper surface in a controlled manner. These graphene domains possess either armchair or zigzag edge, and are formed via nucleation and growth mechanism. Left image is a typical AFM image of one graphene flake, and right image indicates the armchair or zigzag edge of hexagonal graphene.

A novel template was designed for photocontrollable drug release. Multiple functional groups can be loaded and unloaded onto the template via UV irradiation. Importantly, the template can inhibit loaded drug being uptaken by normal cells. However, upon the irradiation of UV light to the target site, such as tumor tissues, the loaded drug can be released from the template quickly and thereafter endocytosed by target cells to achieve the desirable cure effect.

Pattern array of CVD-grown single layer graphene (SLG) is demonstrated by using micro-patterned PDMS stamp. These ultrathin SLG electrodes can use as the source and drain for organic field effect transistors. The devices have high hole mobilities exceeding 10 cm²/Vs, high on-off current ratios larger than 10⁷ and a low threshold voltage for switching.

The single domain insulator-to-metal transition in VO₂ nanobeam triggered by coupling Joule heating and external strain is utilized to fabricate the high performance electromechanical switch, which can be controlled by stretching or compressing the substrate, and the device has the potential application in high sensitivity strain monitor and micro/nano-systems action control such as dynamic logic gate.

Hydrophilic flower-like CuS superstructures synthesized by a hydrothermal route can serve as an efficient 980 nm laser-driven photothermal agent for ablation of cancer cells, because the CuS superstructures exhibit greatly enhanced photothermal effects upon irradiation by a 980 nm laser. This finding demonstrates the great potential application of the CuS superstructures in the photothermal ablation of in vivo tumor tissues.

A novel reprecipitation route is combined with an evaporation process for the synthesis of self-assembled mesocrystalline thin films with at least five hierarchical levels including pattern assembly, fan-shaped patterns, dendrites, fibers with branches, lamellae, and nanoparticles. The hierarchical thin films presented here show several similarities to biological structures such as fern structures.

The first observation of near-band-edge electroluminescent emission from the colloidal quantum dots (QDs) consisting of CuInS₂-ZnS alloyed (ZCIS) cores and ZnSe/ZnS double shells is reported. Bright and organic-free electroluminescence (EL) emission peaked at red (λ = 623 nm), yellow (λ = 594 nm), and green (λ = 560 nm) wavelengths is observed from the heavy-metal-free QD light-emitting diodes (QD-LEDs). The onset voltages, luminance output uniformity, and efficiencies of the heavy-metal-free QD-LEDs are characterized.

A new class of memristors based entirely on soft and liquid-based matter is presented. The devices are formed by interfacing liquid metal electrodes with water-based gel doped with polyelectrolytes. They memorize the last state of bias polarization on the basis of the asymmetric conductance of the hydrogel-oxide-liquid metal stack. Such devices may find application in circuit-live tissue interfaces, soft robotics, and neuromorphic systems.

Ultrahydrophobic surfaces that mimic the surface of a Corokia cotoneaster leaf were created by using cow teeth as natural template. After UV curing of an acrylate resin and removing the template, a highly dense hairy fur-like structure with high aspect ratio could be obtained. Subsequent modification with silica nanoparticles and fluorosilane afforded a water contact angle of some 170°.

A novel, bipolar polymeric host based on a poly(aryl ether) containing phosphine oxide units in the electron-transporting main chain and carbazole units in the hole-transporting side chains is designed and synthesized for blue electrophosphorescence. This polymeric host possesses a bipolar character and a high E_T of 2.96 eV. The efficiency of blue-emitting PhPLEDs based on this polymeric host doped with FIrpic reaches 23.3 cd A⁻¹ (EQE = 10.8%).

Sandwich-like, graphene-based mesoporous titania (G-TiO₂) nanosheets possess thin thickness, large aspect ratio, and mesoporous structure and show enhanced electrical conductivity. Such unique features provide numerous open channels for the access of electrolyte and facilitate the fast diffusion of lithium ions during the cycling processes. The graphene layer within each nanosheet can act as a mini-current collector, which is favorable for the fast electron transport in the electrode. As a consequence, G-TiO₂ nanosheets exhibit a ultrahigh rate capability and excellent cycle performance, holding great potential as a high-rate anode material for lithium storage.

Highly stretchable 2D buckled polypyrrole film is prepared by a simple electropolymerization method on pre-strained Au-coated elastic substrate. This electrode retains its electrochemical properties in Mg batteries after 2000 stretching cycles with 30% strain applied. A combination of exceptional stretchability and electrochemical properties makes these novel conducting polymer films promising materials for many technological applications.

Mo₂C-coated vanadium composites are presented as an example of a new class of metallic membranes for hydrogen separations. Unlike conventional platinum- group catalysts, Mo₂C does not alloy with the bulk metal at temperatures >350 °C, allowing for operation without risk of embrittlement or loss of catalyst activity. Optimal permeabilities approach that of palladium, at a far lower cost.

Spin-coated small-molecule organic light-emitting diodes (OLEDs) based on green-emitting Ir(mppy)₃-doped 4,4'-bis(9-carbazolyl)-biphenyl (CBP) blended with electron- and hole-transporting molecules are demonstrated to have very high efficiencies (up to 60 lm/W and 69 Cd/A without any outcoupling enhancement, which can double with a high-quality microlens array). An additional electron-transporting/hole-blocking BPhen layer is thermally evaporated on the active layer, followed by the LiF/Al cathode.

This Research News article is a survey of the tremendous amount of literature published from 2002 to 2010 that exhibits polymeric solar cells based on the well-known bulk heterojunction based on P3HT and PCBM. The power-conversion-efficiency discrepancy is correlated with the solar-cell-fabrication parameters.