Advanced Materials

A grand challenge in microfabrication is the creation of simple and complex 3D microcomponents of varying composition from colloidal building blocks. Jennifer Lewis, Patrick Doyle, and co-workers demonstrate on p. 4734 that colloidal, silicon, and glass microcomponents can be rapidly patterned by stop flow lithography (SFL). This work will enable fundamental studies of granular packing as well as provide a low-cost route to MEMS devices. Cover artwork by R. Shepherd, S. Eisenmann, and J. A. Lewis.

Two-photon absorption (TPA) offers a rich playground for development of novel optical multifunctional technologies. Related experimental and theoretical methodologies are reviewed by Claudine Katan, Sergei Tretiak, and co-workers on p. 4641, with special attention to sources of errors that yield improper evaluation of TPA cross sections. Trends leading to large TPA responses are illustrated and thoroughly analyzed through a set of typical organic TPA-fluorophores including branched systems.

Materials science research and education in developing countries is the key to sustainable technological development. It is difficult to predict whether nanotechnology will make a difference in improving the quality of life both in developed and developing countries; nevertheless, materials science in general has undoubtedly fulfilled this role since prehistoric times and can continue to do so, largely because of its multidisciplinary approach and broad applications.

Two-photon absorption (TPA) offers a rich playground for development of novel optical multifunctional technologies. Related experimental and theoretical methodologies are reviewed with special attention on sources of errors that yield improper evaluation of TPA cross sections. Trends leading to large TPA responses are illustrated and thoroughly analyzed through a set of typical organic TPA-fluorophores including branched systems.

Stable, room-temperature ferromagnetism in carbon— more precisely, highly oriented pyrolytic graphite — with Curie temperature as high as 460 K is reported. The magnetization can be easily tuned by controlling the low-energy carbon ion implantation dose used to create the samples. Theory indicates that the ferromagnetic order likely results from itinerant π-electrons from defects (see figure).

Cu-Catalytic Growth of Ge Nanowires: Low temperature, deterministic growth characteristics are available by the Cu-catalytic growth. The low-temperature growth is accessible at as low as 200 °C on polymer substrates, and the epitaxial growth is also possible on single-crystalline substrates with the narrow diameter distribution of 7 nm, directly templated from those of Cu catalysts.

A patterned combinatorial library of peptide-grafted polymer affinity materials is screened for target protein binding. In situ light-directed synthesis is used to produce high-density libraries that allow for the screening in parallel within an area less than 1 cm². From these libraries, specific peptide/polymer combinations are identified with very high target protein affinity, enabling facile detection of the target at concentrations in the low picomolar range in the presence of excess competitor.

Nanoengineered microcapsules (shown in green) can be used to encapsulate vaccine antigens conferring protection of the cargo until the capsules are taken up by specialized antigen presenting cells in human blood such as dendritic cells (cell membrane in red, nucleus in blue). This technique offers potential applications for in vivo vaccine delivery.

We have demonstrated the ability to apply thin conformal films onto complex nanostructures using a polymer assisted deposition technique. Sequestering the metal by binding it to a polymer results in a bottom-up growth process that leads to conformal film deposition. We have deposited a thin film of the phosphor Eu:YVO₄ on 60 μm thick anodiscs^® with 200 nm pores resulting in highly luminescent nanostructures.

A polyelectrolyte is used as gate insulator material in organic field-effect transistors with self-aligned inkjet printed sub–micrometer channels. The small separation of the charges in the electric double layer at the electrolyte-semiconductor interface, which builds up in tens of microseconds, provides a very high transverse electric field in the channel that effectively suppresses short-channel effects at low applied gate voltages.

Macroporous ceramics are fabricated using emulsions stabilized with particles of various chemical compositions. Stabilization with particles hinders extensive droplet coalescence during solvent extraction, allowing for drying and sintering of the emulsions directly into macroporous materials in the absence of any chemical reaction.

3D superlattices of fluorescent gold nanoparticles are prepared at an air/water interface. The hexagonal assembly extends over several micrometers, and crystals of triangular morphology are imaged using the fluorescence emission of the monolayers protecting the nanoparticles (see figure). Surprisingly, the superlattices are fluorescent even though the fluorescein derivative is close to the metallic core and strong quenching is expected.

Subwavelength optical waveguides coated with porous silsesquioxane-palladium nanocomposites are used to detect hydrogen gas. This compact waveguide sensing platform shows extremely fast response times over short interaction lengths and can be reused by stripping/re-depositing the chemo-responsive nanoparticle coatings.

Positronium, a system consisting of an electron and its antimatter, a positron, offers a new technique to explore vertical accessibility and connectivity. Here, we show how this technique can be used to map out the vertical profile of mesoporous silica channel systems by comparing a standing (perpendicular to the substrate) 2D hexagonal with a lying (parallel to the substrate) 2D hexagonal mesoporous film.

Stop-flow lithography (SFL) is used for patterning colloidal building blocks into controlled structures (gears and other shapes) at rates that exceed 10³ min⁻¹ using an index-matched system composed of silica microspheres suspended in a photocurable acrylamide solution as shown in the figure. These structures are dried and then transformed, in batch, at elevated temperatures into microcomponents composed of porous or glassy silicon oxide or porous silicon via magnesiothermic reduction.

A nanoscale hybrid filler comprising single-walled carbon nanotubes and graphite nanoplatelets provides a synergistic effect in the enhancement of thermal conductivity of epoxy composites, which is ascribed to the formation of a more efficient percolating network with significantly reduced thermal interface resistance.

Close-packed gold-nanocrystal assemblies are selectively deposited into trenches (300 nm × 300 nm) fabricated by electron-beam lithography. The base of the trenches consists of highly doped silicon with 30 nm silica walls. The assemblies are formed using electric-field mediated deposition of8 nm gold spheres from organic solvents.

Carbon onions can be set in a state of heavy self-compression when irradiated with an electron beam. Annealing at high temperature relaxes the structure by an exchange of atoms between the shells. This gives us the possibility tostudy atom jumps quantitatively.

Photocurrent enhancement and improvement in solar cell performance are found when the interface between ZnO and polythiophene layers is modified with insulating alkanethiol self-assembled monolayers. This result is correlated with increased structural order in the polymer and interchain contributions in the photocurrent, suggesting that improved exciton diffusion or reduced electron-hole recombination can outweigh the impedance in electron transfer at the heterojunction interface.

Thin films of metal and polymer are patterned with varying geometry, moduli, and initial stresses to fold into complex 3D structures. In the schematic (top) the rigid segments, flexible hinges, and hollow areas are visible. In the optical and fluorescent micrograph below, cells were cultured on the self-assembled structures, and fluoresce green as they are alive.

Core/shell-structured silica/NaYF₄ nanospheres featuring uniform silica coatings are synthesized using a facile, user-friendly method. Multicolor nanospheres are produced by encapsulation of organic dyes or quantum dots in the silica shell, and upconversion fluorescence is generated based on fluorescence resonance energy transfer (FRET).

The combination of electrospun nanofibrous membranes and microfluidic channels dramatically improves the sensitivities of HIV immunoassays in microfluidic channels. Simplicity and effectiveness of the new system can potentially pave the way for inexpensive, portable and highly sensitive diagnostic devices.

Combining a piezoelectric/magnetostrictive laminated composite with a piezoelectric transformer (see figure) generates a giant magnetoelectric (ME) response. The output ME voltage is amplified by the additional piezoelectric transformer layer. The outstanding advantages of the proposed structure include giant ME voltage output, high efficiency, no power consumption, and short response time.

The average measured Hv for polycrystalline ReB₂ and Ti–B composites under different loads shows that the tendency of Hv to decrease becomes weak for large loads, and the measured hardness of ReB₂ is always lower than that of Ti–B composites. For comparison, the results from Chung et al. were inserted into the figure.

The effects of photonic properties on an organic photochemical reaction are traced out by monitoring the wavelength-dependent rate of photoisomerization of azobenzene tethered to silica opals of different sphere sizes. Optical enhancements arising from slow photons at both the blue- and red-edge of the photonic stop band are observed.

Solid-Pb-filled ZnO nanotubes are synthesized and tested for use as nanothermometers. The expansion of the filling with increasing temperature (see figure) — or the corresponding changes in capacitance — can be measured and related to temperature. The advantages of this nanothermometer are extremely low fabrication costs, superior reliability, and lower demands on structural integrity of the outer shell compared to nanothermometers based on liquid fillings.

Sub-micrometer layers of single-crystal diamond suitable for subsequent processing are fabricated. The method employed is a significant enabling technology for nanomechanical and photonic structures incorporating color centers. The process uses a novel double-implant process, annealing, and chemical etching to produce membranes. The thinnest layers achieved to date are 210 nm thick.

An effective multilayer interlayer technology for rapid transient-liquid-phase joining of advanced ceramics is demonstrated. Interlayers designed to exhibit rapid interdiffusion, chemical homogenization, and liquid film disappearance, enable rapid and reliable joining of ceramics at reduced temperatures while preserving high-temperature use capability. Processing times as short as 5 min at 1400 °C yield interlayers with a remelt temperature >2200 °C.

Whereas low molecular weight liquid crystals (LCs) are too soft to support the culture of fibroblast cells, gels formed by polystyrene microparticles dispersed in LCs can be mechanically tuned to support the cell culture. These self-supporting gels can be readily molded, thus facilitating their use for cell culture, and they contain micrometer-sized domains of LCs that can be oriented by interactions at the surfaces confining the gels.

Extended donor-acceptor oligothiophenes exhibit a narrow optical energy gap in combination with strong light absorption and good intermolecular π–π stacking in the solid state for efficient charge transport. Bilayer photovoltaic devices utilizing these oligomers as p-type semiconducting photosensitizers afford high power conversion efficiencies of up to 2.7%.

A novel nanoparticle hybrid is attained by the covalent attachment of iron oxide (γ-Fe₂O₃) nanoclusters onto the surface of a mutagenized cow pea mosaic virus (CPMV-T184C). Using a stepwise substrate-based integration, monodisperse CPMV-IO hybrids are anchored on a gold substrate. The physical and magnetic properties of individual CPMV-IO hybrids are qualitatively investigated by atomic/magnetic force microscopy (AFM/ MFM). During MFM characterization a ‘boundary-effect’ is observed at the CPMV/IO interface.

Helically twisted fibers can be produced by electrospinning liquid-crystalline cellulose solutions. Fiber topographies are studied by atomic force microscopy, scanning electron microscopy (see figure) and polarized optical microscopy. The fibers have a nearly universal pitch-to-diameter ratio and comprise both right- and left-handed helices.

Single-crystalline, cubic ZrO₂- and HfO₂ nanobelts have been fabricated via a direct thermal-decomposition process under atmospheric pressure. The formation of cubic nanobelts can be controlled by varying the dopant concentration of nanobelt precursors, heating temperatures, and heating rates. The lanthanide-doped nanobelts show a dual capability for both down- and up-conversion emissions (see picture) in the visible range.

CdSe quantum dots (QDs) with narrow size distribution are synthesized continuously in a pressurized high-temperature microreactor using supercritical hexane as the main solvent. The as-prepared QDs exhibit narrow photoluminescence emission (full-width at half-maximum as small as 25 nm, corresponding to σ = 4%), showing the advantages of supercritical-fluid synthesis compared to conventional high-boiling-point solvents.

2,8-Di(4-trifluoromethylphenyl)-5,11-di-n-octylindolo[3,2-b]carbazole (2TFMPOIND) has been synthesised by a Suzuki coupling reaction. Single crystal microribbons of 2TFMPOIND are then fabricated by a solution assembly process. The microribbons have novel mechanical, optical, and electrical properties.

The surfaces of alkaline earth oxides emit bright, colored light and have potential as thermally stable inorganic phosphors with adsorption-dependent optical properties. The doping of thermally stable MgO nanocubes with low-coordinated BaO surface elements (see figure) clearly demonstrates that chemical manipulation of the solid–gas interface provides an efficient means to adjust the optical properties of powders in controlled gas atmospheres.

Monodisperse ZnO colloidal nanocrystal clusters (CNCs) are synthesized through a rapid microwave-polyol process. The size of the clusters is tunable from 57 to 274 nm by varying the amount of Zn-complex precursors. The combination of unique secondary nanostructures, inter-nanocrystallites, and large internal surfaces make these ZnO CNCs ideal candidates for applications such as humidity sensing.

The orientation of microdomains in block copolymer thin films is controlled by two simple strategies that have been developed through controlling interfacial interactions by anchoring end-functionalized polymers to the substrate. Combined with solvent annealing, these methods provide a simple route for manipulating the orientation and lateral order of the copolymer microdomains.

Dimensionally and compositionally tuned bimetallic nanoparticles are synthesized in an atmospheric pressure microplasma reactor and found to catalyze carbon nanotube (CNT) growth at lower temperatures and higher growth rates than monometallic nanoparticles. Kinetic studies show that the activation energy for CNT growth is lowered substantially with nanoparticle alloys, demonstrating that intrinsic catalytic activity is intimately related to chemical composition.

Well-defined nanovoid and metal- nanocap arrays are fabricated on flexible polymer substrates with controlled sizes and morphologies, via trapping of inorganic particles at a polymer-film surface. The proposed method provides a platform to create an effective SERS substrate, for use in devices such as chemical and biomolecular sensors.

Small-fused acenes with an electron-donating alkoxyl naphthyl end-capper, which can tune the electronic properties and increase stacking by self-assembly, are synthesized by a simple process and fabricated into organic thin film transistors. In particular, hexyloxynaphthalene-end-capped anthracene, AN-ANE (see figure), shows field effect mobility three times that of pentacene under optimized conditions.

Single-walled carbon-nanotube (SWCNT) arrays are generated on catalyst patterns with controlled location, orientation, and spacing using a straightforward approach. Dip-pen nanolithography is used to fabricate catalyst patterns with sub-70 nm resolution, and SWCNTs are successfully grown on these patterns through chemical vapor deposition. The growth direction of the SWCNTs on quartz is controlled, and along the [100] crystallographic direction.

A new ion-transfer-based mechanism is proposed for the growth of CuTCNQ nanowires within the confines of an anodic aluminum oxide template. Remarkably, Cu ions are found to move across the growing solid single-crystalline nanowire at a very high rate with macroscopic distance, as schematically illustrated in the figure.

A small percentage of platinum added to silver/gold alloys that are then electrochemically dealloyed yields an extremely high surface area nanoporous metal, with pore size less than 4 nm, as shown in the figure (scale bar = 5 nm). The presence of surface platinum stabilizes the nanostructure to coarsening in air and electrolyte.

Electronics that offer fully elastic mechanical responses to large strain deformations are briefly reviewed here. We discuss progress in this emerging field of research, with an emphasis on approaches that use semiconductor nanomaterials in unusual structural layouts. Recent results demonstrate that high performance stretchable electronic/optoelectronic devices can be formed in this manner, with well-established classes of materials whose intrinsically fragile, brittle nature would otherwise preclude their use.