Advanced Materials

The cover illustrates a polymer semiconductor highway for efficient transport of both electrons and holes. On p. 478, Samson A. Jenekhe, Mark D. Watson, and co-workers have demonstrated high-mobility single-component ambipolar field-effect transistors, by utilizing a new polymer semiconductor, and integrated them into complementary inverters. Polymer semiconductors with good ambipolar charge transport provide a simpler way to realize complementary circuits and other devices and functions in organic electronics.

Ultrahigh-aspect-ratio microneedles can be fabricated via “drawing lithography”, a novel technique in which a thermosetting polymer is directly drawn from a two-dimensional solid surface without the need for a mask and light irradiation. Kwang Lee and co-workers demonstrate this technique on p. 483. The inside cover shows a scanning electron microscopy (SEM) image of three-dimensional structures with ultrahigh aspect ratios, potentially suitable as drug-delivery devices that could replace hypodermic syringes.

Differentiation of myoblast cells onto polyelectrolyte multilayer films (PEMs). Physico-chemical, mechanical, and biological properties of PEMs for biomaterials coatings, tissue engineering and regenerative medicine.

A chiral nematic liquid crystal/chiral ionic liquid composite with unique electro-optical characteristics is reported. The composite can be switched electrically between three different light states: transparent, scattering, and mirror reflecting (see images). Moreover, the reflection bandwidth can be controlled accurately and reversibly by adjusting the intensity of the electric field applied.

Magnetic carbon nanotube (CNT) arrays are grown by in situ encapsulation of ferromagnetic iron carbide nanoparticles within the tube walls (see figure). The resulting structures, combining the inherently unique characteristics of aligned and open-ended CNTs with the fact that magnetic functionality is provided from “inside the walls”, are promising functional units in highly efficient flow systems, for example, as parallel magnetic nanoreactors and in biological separations.

High-performance ambipolar transistors and inverters are demonstrated using a new donor–acceptor copolymer semiconductor. The ambipolar transistors show electron and hole mobilities of up to 0.04 and 0.003 cm² V⁻¹ s⁻¹, respectively. Voltage transfer curves of the inverters made on common gold electrodes showed sharp switching with gain of 30.

Drawing lithography is a novel fabrication technique in which a thermosetting polymer is directly drawn from a two-dimensional solid surface without the need for a mask and light irradiation (see figure). Drawing lithography differs from traditional lithography techniques, such as photolithography, in that it is based on the inherently planar geometries of a two-dimensional substrate in a three-dimensional microstructure.

Three-dimensional photonic crystals with an omnidirectional bandgap at visible frequencies can have significant impact on solid-state lighting and solar-energy conversion. Using a procedure based on multistep electron-beam lithographic processing, a 9-layer log-pile photonic crystal is fabricated composed of 70-nm-wide titanium dioxide rods with 250-nm lattice spacing that exhibit a stacking direction bandgap between 400 nm and 500 nm (see image).

A shape-engineered trilayer ferromagnetic/non-magnetic/ferromagnetic nanostructure is lithographically fabricated to support four stable flux-closure domain configurations at remanence. Quantitative phase measurements using electron holography reveal the detailed magnetization reversal process and demonstrate how four domain states with distinct vortex chirality combinations can be controllably achieved by systematic manipulation of the applied magnetic field (see image).

Self-healing polymer coatings are formed by electrospinning bead-on-string healing agent filled capsules onto a substrate followed by infilling of a polymer matrix. Upon damage to the self-healing coating, liquid healing agents are released from the ruptured beads passivating the damaged region, preventing it from corroding as shown through microscopy, electrochemistry, and corrosion testing.

Design and control of nanometer-scale electronic phase separation are demonstrated in high-quality manganite superlattices composed of a ferromagnetic metal and an antiferromagnetic insulator (see figure). Bicritical competition of these phases sensitively controls the magnetic and electronic properties of the superlattices as a whole. A magnetic field can effectively tune the volume fraction of each phase and consequently the position of the phase domain boundaries.

Blue photoluminescene (PL) from chemically derived graphene oxide has been observed. The PL is attributed to radiative recombination of electron–hole pairs in isolated “molecular” sp² domains, which are present within the carbon–oxygen sp³ matrix in chemically derived graphene oxide. The PL intensity is correlated to the evolution of sp² domains during reduction.

A multilevel biomemory device displays two pairs of nonvolatile write, read, and erase functions (see figure). The device consists of recombinant azurin/cytochrome c. Azurin is recombined with a cysteine residue to enhance the stability of the self-assembled protein on a gold surface. Then, cytochrome c is adsorbed onto the immobilized azurin layer. The verified memory characteristic of the proposed device implies that the metalloprotein is a very promising molecule for the development of a new type of memory device.

A mesoporous silica film acts as a template and a potential equalizer between the edge/defect sites and the basal plane of a graphene sheet. Using an electrochemical deposition method of CdSe on these graphene sheets covered with a silica film results in CdSe quantum dots that are evenly distributed in regular hexagonal arrays (see figure).

Natural materials are hierarchically structured nanocomposites. A bottom-up multiscale approach to model the mechanical response of the chitin-based mineralized cuticle material of Homarus americanus is presented, by combining quantum-mechanical ab initio calculations with hierarchical homogenization. The simulations show how the mechanical properties are transferred from the atomic scale through a sequence of specifically designed microstructures to realize optimal stiffness.

Two multiaryl-substituted pyridine derivatives, applied as electron transport materials (ETMs; see figure) with good hole- blocking ability in high-performance deep-blue OLEDs, are characterized. The maximum current efficiencies of the devices based on the new compounds are over 70% higher than previously reported devices using traditional ETMs instead. The CIE coordinates of the devices based on the new compounds are close to the NTSC standards of (0.14, 0.08) for blue emission.

Direct structuring of photocrosslinkable polyspirobifluorene copolymers by holographic lithography to yield insoluble, organic, distributed feedback (DFB) lasers with thresholds as low as 2 µJ cm⁻² is reported (see figure). These DFB lasers (X-LEP layer in figure) are embedded in the first optimized, multilayered slab-waveguide organic light-emitting diode with good luminance efficiency of ∼6 cd A⁻¹ and concomitant low laser thresholds. The effect of electrical excitation on the optically pumped laser is investigated for the first time.

Enhanced Seebeck coefficients and power factors – important for the conversion of heat to electrical energy – are obtained in polyaniline/carbon nanotube (PANI/CNT) composites in which PANI coats CNT networks (see figure). The values are several times larger than those of either of the individual components. This new approach has potential for synthesizing high-performance thermoelectric materials.

Polymer-controlled crystallization can access a variety of unique mineral superstructures (see figure). Emerging new strategies for morphogenesis and controlled crystal growth of minerals — e.g., selective adsorption and mesoscale transformation to highly ordered superstructures, the combination of a synthetic hydrophilic polymer with an insoluble matrix or a substrate, and controlled crystallization in a mixed solvent — are highlighted.