Advanced Functional Materials

Nanoscale strain inhomogeneity is shown to play a fundamental role on the spectral properties of InGaN–GaN epilayers. For layers grown above a certain critical thickness, a strong and spatially varying strain profile accompanies a nonplanar surface morphology (see figure and cover), which is associated with a transition from planar 2D to a Stranski-Krastanow-like 2D–3D growth mode.

Well-defined dots and arrays composed of brightly emitting CdTe nanocrystals in a poly(vinylalcohol) (PVA) matrix are obtained using inkjet printing (see figure and inside cover). Dot uniformity is achieved using ethylene glycol–water as a solvent. The use of PVA as a matrix improves both the morphology and the emission properties of the printed nanocrystal films.

Nothing is so constant: Change is an ever-present fact of life, and the engine that drives scientific research. In this Editorial, the Deputy Editor looks at the role of Advanced Functional Materials in communicating the ever-evolving breakthroughs of materials science, and overviews some of the changes in the journal over the past year.

Well-defined dots and arrays composed of brightly emitting CdTe nanocrystals in a poly(vinylalcohol) (PVA) matrix are obtained using inkjet printing (see figure and inside cover). Dot uniformity is achieved using ethylene glycol–water as a solvent. The use of PVA as a matrix improves both the morphology and the emission properties of the printed nanocrystal films.

Two new ruthenium complexes are synthesized, characterized, and used as photosensitizers in dye-sensitized solar cells (CYC-P1 and CYC-P2 in figure). The overall efficiencies of the sensitized cells are 6.01 and 3.42 %: a cis-di(thiocyanato)-bis(2,2′-bipyridyl)-4,4′-dicarboxylate ruthenium(II) (N3)-sensitized device has an efficiency of 7.70 %.

Nanoscale strain inhomogeneity is shown to play a fundamental role on the spectral properties of InGaN–GaN epilayers. For layers grown above a certain critical thickness, a strong and spatially varying strain profile accompanies a nonplanar surface morphology (see figure and cover), which is associated with a transition from planar 2D to a Stranski-Krastanow-like 2D–3D growth mode.

Top-down and bottom-up silver nanowires are mechanically characterized using molecular-dynamic simulations. The mechanical response and operant deformation mechanisms are shown to be strongly geometry dependent (see figure). A size effect on the tensile strength is presented for nanowires ranging from 2 to 25 nm in diameter.

The metallocyclodextrin Ru^II complex [Ru(dcb)₂(α-CD-5-bpy)]Cl₂ with surface-anchoring groups is designed, synthesized, and tested in nanocrystalline titania solar cells (see figure). The supramolecular dye shows enhanced short-circuit photocurrent, photovoltage, and power-conversion efficiencies attributed to host–guest interactions between the supramolecular moiety and the iodide/triiodide redox couple.

ZnO–SnO₂ hollow spheres and hierarchical nanosheets (see figure) are successfully synthesized using an aqueous solution containing ZnO rods, SnCl₄, and NaOH using a simple hydrothermal method. Both hollow spheres and hierarchical nanosheets show higher photocatalytic activities in the degradation of methyl orange than that of ZnO rods or SnO₂.

Thermoplastic siloxane copolymers are investigated for use as nanoimprint lithography resists. Poly(dimethylsiloxane) (PDMS) imparts the materials with many properties favorable for imprinting (see figure), including low surface energy for easy mold release without delamination and high silicon content for chemical-etch resistance; thus, they can be used for further pattern transfer to underlying layers. Feature sizes of 50 nm, and possibly below, can be imprinted.

Blue-light-emitting polyfluorene diodes have low stability: the polyfluorene rapidly degrades to green-light-emitting products (see figure). Upon electrochemical degradation, a green band at 485 nm develops. This band is different from the 535 nm band usually assigned to fluorenone defects. The product of degradation has a crosslinked character and its emission may be related to intramolecular excimers.

Biomolecule entrapment within electrochemically polymerized conducting polymers can provide electrically active biomaterials with enhanced cellular attachment. Polypyrrole(PPy)/nerve growth factor (NGF) coatings decrease the impedance amplitude of microelectrodes by almost two orders of magnitude, and PPy/NGF films yield a significant population of differentiated PC-12 cells (see figure), indicating successful delivery of NGF.

The effect of nanotube functionalization on the coefficients of thermal expansion (CTE) of SWNT–epoxy nanocomposites is revealed (see figure). The CTE values of the functionalized SWNT–epoxy composites below the glass-transition temperature can be reduced by 52 and 42 % through simple mechanical chopping and oxidization methods, respectively.

A new red-light-emitting dopant, DCQTB (see figure), is synthesized for organic light-emitting diode applications. Compared with the known dopant DCJTB, DCQTB exhibits red-shifted fluorescence but blue-shifted absorption, and a comparable fluorescence quantum yield. Such unique characteristics lead to an improved red color purity and luminous efficiency in DCQTB-based organic light-emitting diodes.

Oriented thin films of poly(3-hexylthiophene-2,5-diyl) (P3HT) with varying molecular weights are grown by directional epitaxial crystallization. P3HT with a weight-average molecular weight M_w = 7.3 kDa behaves like an oligomeric system, whereas, for M_w ≥ 18.8 kDa, it shows the periodic lamellar structure of a semicrystalline polymer as a result of chain folding (see figure).

Photoinduced changes of the ferroelectric properties of an azobenzene-functionalized liquid-crystalline polymer are investigated in detail using both photostationary and time-resolved measurements (see figure). The results give an insight into the effect of trans–cis isomerization on the Curie temperature and ferroelectric properties and the molecular mechanisms leading to this “photoferroelectric effect”, respectively.

Electroluminescence efficacy of 2,7- bis(phenylethenyl)fluorenes is strongly affected by the choice of substituents at the 9-position of the fluorene unit, and upon the terminal phenyl groups. Shortening the conjugation length by removal of the ethenyl units to give 2,7-diphenyl-9,9-dialkylfluorene (OFPh) derivatives decreases both emission intensity and color purity (see figure), although color purity in the OFPhs could be improved by dilution in poly(methylmethacrylate) matrices.

Li insertion has been used to study the crystallization behavior of mesoporous TiO₂ films (see figure) prepared by a novel poly(ethylene-co-butylene)-b-poly(ethylene oxide) block-copolymer template. Such polymers allow the attainment of high crystallinity in the films by annealing at temperatures of up to 650 °C, whilst maintaining the 3D mesoporous structure.

Morphological control of mesoporous silica thin film pore structure through guided placement and directionality using lithographically patterned (100)-orientated substrates and a dip-coating method is demonstrated (see figure). The channels within the aligned films are filled with cobalt, allowing high- resolution imaging of the pore using electron microscopy. Channel dimensions control pore packing leading to some variations in pore direction and d-spacing.

A corona-charging process is used to stably store charge in the bulk of an organosilsesquioxane-based gate dielectric (see figure). The stored charge is used to controllably shift the threshold voltage and modulate the current for an organic field-effect transistor fabricated on top of the dielectric. A unipolar logic gate is demonstrated on the charge- patterned resin using FETs with identical dimensions, as schematically depicted in the figure.

High-efficiency (8.4 %) dye-sensitized solar cells are prepared, based on a new high molar extinction coefficient ion- coordinating ruthenium sensitizer (see figure) and nonvolatile organic-solvent electrolyte, which show excellent stability during long-term accelerated testing under 80 °C in the dark or visible-light soaking at 60 °C (IPCE: incident photo-to-current conversion efficiency).

Photoelectron emission microscopy is used to probe morphological and surface work-function changes during the thermally induced martensitic phase transformation in a polycrystalline NiTiCu thin film. Low-temperature martensite phase has a micrometer-scale wrinklelike structure on the surface (see figure). When the sample is heated above the transition temperature, the image becomes dark and the wrinklelike structure disappears.