Advanced Materials

The structural transition from a crystalline solid to its melt is central to understanding crystal nucleation and growth, but it is difficult to image directly on the atomic scale. On p. 2716, Peter Schall and co-workers use novel thermosensitive colloidal systems as models to obtain new insight into this structural transition. The pictures show computer reconstructions obtained from the 3D imaging of a real colloidal crystal-fluid interface. Cover design by Rob Bekker, www.robbekker.com.

On p. 2727, Heiko Jacobs and co-workers report on the liquid-liquid-solid self-tiling process where inorganic semiconductor dies follow a stepwise reduction in energy. The material is first preoriented at a liquid/liquid interface, then transferred in an epitaxy-like self-assembly process to a solderpatterned substrate to form close-packed domains. The inside cover shows a variety of differently shaped receptor domains and components with a large-area, 3D, tiled region in the background.

A novel nanofabrication method based on nitrogen passivation by hydrogen in GaAsN is presented. This approach combines a masked hydrogenation process with a very sharp H forefront in GaAsN. This allows embedding a GaAsN nanometer-sized region in a GaAs barrier, resulting in the formation of ordered arrays of nanoemitters with marked zero-dimensional spectroscopic characteristics.

The degradation in magnetic properties in very thin film complex oxides is studied using SrTiO₃(001)/La_1-xSr_xCoO₃, providing unequivocal evidence for nanoscopic interfacial magnetic phase separation. Electron microscopy and spectroscopy reveal that this occurs due to inhomogeneity in local hole doping, driven by subtle, depthwise variations in the Sr and O stoichiometry. Simple thermodynamic and structural arguments for the origin of these variations are provided.

The crystal/melt interface is central to the understanding of crystal nucleation and the morphological stability of crystal growth, but it is difficult to study experimentally. Micrometer-sized stimuli-dependent colloidal particles are assembled into large crystals using temperature fields. The structure of the colloidal crystal/melt interface is imaged directly in three dimensions. Interface reconstructions allow direct connection of the structure and interface free energy.

Highly efficient red phosphorescentdopants in organic light-emitting devices have been explored by using a cyclometalated iridium complex with a fully methylated phenyl ring and a quinoline ring as well as a sterically crowded ancillary ligand. The red phosphorescent devices with these dopants give extremely high external quantum efficiencies.

A method for self-tiling and electrically connecting microscopic semiconducting tiles with high speed and yield is presented. Assembly is driven by a stepwise reduction of interfacial free energy at a triple interface. We discuss design rules to produce highly periodic domains on rigid, flexible, and curved substrates. A fault-tolerant microconcentrator solar cell is demonstrated.

A simple route for the fabrication of a highly ordered surface pattern consisting of reduced graphene oxide (RG-O) platelets is developed, in which RG-O platelets modified with poly(ionic liquid) (PIL) undergo a spontaneous self-assembly process as a result of the controlled solvent evaporation.

Hybrid metal sulfide/polymer solar cell active layers are fabricated employing an approach based upon the in-situ thermal decomposition of a single source metal xanthate precursor in a semiconducting polymer film. The nanomorphology of the film, the charge photogeneration yield at the donor-acceptor heterojunction and device performance are shown to be dependent upon the annealing temperature. Photovoltaic devices based upon such layers are shown to exhibit power conversion efficiencies of ∼2.2% under AM1.5 solar illumination thus demonstrating the potential of such nanocomposite films for photovoltaic device applications.

Activated carbons with promising performance in capacitors are produced from fungi via a hydrothermal assistant pyrolysis approach. This study introduces a facile strategy to discover carbonaceous materials and triggers interest in exploring fungi for material science applicatons.

Copper(I) oxide (Cu₂O) inverse woodpile structures with a complete photonic bandgap (PBG) at 765 nm are created by using single prism holographic litho­graphy (HL) to form a polymeric template, followed by Cu₂O electrodeposition and template removal. This is the first realization of an inverse woodpile structure exhibiting a complete PBG at or near the visible wavelengths.

A new method for investigating light-emitting property in organic devices is demonstrated. We apply the ambipolar light-emitting transistors (LETs) to directly observe the recombination zone, and find a strong link between the transistor performance and the zone size. This finding unambiguously indicates that the light emission comes from the electric-field-induced p-i-n homojunction in ambipolar LETs.

Interface engineering by combination of TiO_x and a conjugated polyelectrolyte as an electron transport layer modifies the electrical contact between a metal electrode and an organic active layer with well-aligned frontier orbital energy levels for efficient charge transport. This results in remarkable improvements in the device performance of inverted polymer solar cells and polymer light-emitting diodes.

A new type of dye-sensitized solar cell with a multilayered photoanode is prepared by a simple film-transfer technique. The absorption spectrum is broadened and the photocurrent density is increased. An efficiency of 11.05% is achieved, which is much higher than what has been achieved using other methods under similar conditions.

Daniel G. Anderson and coworkers, on p. H189, synthesized a library of cationic polymers that drastically affect the morphology and behavior of monocyte/macrophage cells in vitro when immobilized on a surface, and modulate early inflammatory reactions in vivo. Certain polymers are found capable of mitigating the foreign body responses.

Selective encapsulation of hydrophobic molecules in the nanoshell of multilayer polysaccharide capsules is presented by Catherine Picart, Rachel Auzély-Velty, and co-workers on page H200 . The capsules open new avenues for applications in nanomedicine: as carrier systems for hydrophobic molecules entrapped in the shell as well as hydrophilic ones loaded in the aqueous cavity.

This article reports on recent progress in the development of advanced nanoscale photoreactive, magnetic and multifunctional materials applicable to brain cancer diagnostics, imaging, and therapy, with an emphasis on the latest contributions and the novelty of the approach, along with the most promising emergent trends.

Microfluidic devices for point-of-care diagnostics are being realized in response to a pressing need for diagnostic tests for diseases that are not covered by current technology. This Progress Report details the requirements of point- of-care diagnostics and the technological components that can be used to develop such microfluidic devices. Specifically, materials, surface treatments, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the perspective of various biosensing concepts are addressed. Finally, the integration of components into accurate and reliable devices is discussed.

Jennifer A. Lewis and co-workers present 3D biomimetic microvascular networks of nearly arbitrary design on page H178. The networks are fabricated using omnidirectional printing of a fugitive ink within a photopolymerizable hydrogel matrix. After printing and curing, the ink is removed from the matrix leaving behind hierarchical, bifurcating channels that mimic natural microvasculature. (Image credit: W. Wu and J. A. Lewis, University of Illinois.)

3D biomimetic microvascular networks of nearly arbitrary design are patterned by omnidirectional printing of a fugitive organic ink into a photopolymerizable hydrogel matrix. This novel approach hinges critically on tailoring the chemical and rheological properties of the fugitive ink as well as the photopolymerizable hydrogel reservoir and fluid filler. These hydrogel-based, microvascular constructs may find potential application in 3D cell culture, tissue engineering, organ modeling, and autonomic healing.

Microelectrodes based on poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) are able to record the oxidation of catecholamines released from chromaffin cells during exocytosis with a high signal-to-noise ratio. This result represents a new capability for organic electronics that could lead to devices that interface with the nervous system in novel ways.

A library of cationic polymers, poly(beta-amino alcohols) with a great chemical diversity are synthesized using combinatorial polymerization. These polymers, when immobilized on a surface, drastically affect the behavior of monocyte/macrophage cells in vitro and early inflammatory reactions in vivo. Certain polymers are found capable of mitigating the foreign-body responses.

Lanthanide-doped nanophosphors stimu­lated by high-energy photons or β⁺-particles display characteristic 590, 615, and 692 nm emissions of the Eu³⁺ activator. These radioluminescent nanophosphors, produced by a thermo degradation process and coated with poly(ethylene glycol), serve as imaging probes as demonstrated in vivo with matrigel inclusions detected by both a custom X-ray luminescence and a conventional small animal optical imaging system.

A versatile method to selectively encapsulate water-insoluble molecules in the nanoshell of layer-by-layer capsules made exclusively of polysaccharides is described. This relies on the very high affinity of hydrophobic molecules for an alkylated hyaluronic acid used as a polyanionic partner of a chitosan derivative for the capsule synthesis. The hydrophobic molecules entrapped in the nanoshell can be delivered intracellularly in dendritic cells.

More than ever, now is the opportunity to develop portable microfluidic diagnostic devices with high performance characteristics. In their review, on page H151, Emmanuel Delamarche and co-workers discuss why and present recent work on materials, reagents, and functional elements that can be integrated into microfl uidic devices for diagnostics. (Image credit: Matthias Zepper)