Advanced Materials

Novel donor-acceptor nanoassemblies (see cover) are prepared using pristine single-walled carbon nanotubes (SWNTs) as electron-acceptor components in supramolecular “polymer wraps”, with poly(methyl methacrylate) carrying porphyrin units (H₂P) as excited-state electron donors (see Figure). In these novel donor–acceptor ensembles, SWNTs quench the photoexcited H₂P chromophores, resulting in the creation of long-lived radical ion pairs.

Silicon nanocrystals are candidates for silicon-based light-emitting devices and non-volatile memories—provided that size control can be realized. In recent years, various approaches of narrowing the size distribution of silicon nanocrystals have been developed by introducing new synthetic techniques, such as incorporation into SiO₂ matrices to form superlattices (see Figure). The most promising recently developed approaches are reviewed, and their potential for silicon-nanocrystal applications is discussed.

Manganese oxide octahedral molecular sieves with increasing nanoscale tunnel sizes are systematically synthesized using hydrothermal treatment of Na-birnessite under increasing pH conditions. OMS-5 (2×4), OMS-6 (2×3), and OMS-7 (1×1; see Figure) tunnel structures have been synthesized at pH values of 13, 7.0, and 1.0, respectively.

Protein gradients on substrates are prepared using a microstamping technique. An Au pattern is transferred from a patterned silicon stamp to a polymeric substrate, and exposed polymer regions are then selectively activated for protein immobilization. The protein gradient is achieved by varying the spacing between the exposed polymer regions (see Figure).

The feasibility of solid-state dye-sensitized solar cells as a low-cost alternative to amorphous silicon cells is demonstrated. Such a cell (see Figure) with a record efficiency of over 4 % under simulated sunlight is reported, made possible by using a new organic metal-free indoline dye (see Figure) as the sensitizer with high absorption coefficient.

A protein field-effect transistor (Pro-FET) based on the blue-copper protein azurins (see Figure) and operating at room temperature and ambient pressure is demonstrated. The transfer characteristics of the Pro-FET exhibit a pronounced resonance due to the switch from behaving as a n-metal oxide semiconductor FET (n-MOSFET) to a p-MOSFET. Carrier transport through the device is explained in terms of an equilibrium between the two possible oxidation states of the redox site (Cu¹⁺ and Cu²⁺).

Novel light-emitting diodes have been constructed using electroluminescent nanocapsules (ELCs) as the emitting layer (see Figure). The ELCs comprise electroluminescent perylene as a core with electron- and hole-transfer moieties in a polymeric shell, and were prepared by emulsion copolymerization, a technique that allows the properties of devices to be tuned via the appropriate selection of matrix polymer and other components.

Magnetic resonance imaging (MRI) signal enhancement is reported using a coating grown by layer-by-layer assembly. A macromolecular conjugate of hyaluronan with Gd-coordinated diethylenetriaminepentaacetic acid is used with chitosan to coat polymeric interventional tools. The MRI visibility of such Gd-loaded coatings can be easily tailored by varying the number of layers (see Figure).

Liquid-feed flame spray pyrolysis allows the synthesis of 20 nm diameter hexagonal Y₃Al₅O₁₂ nanopowders (see Figure) that show no evidence of a yttrium aluminum garnet (YAG) phase. The nanopowders can be easily formed into green bodies that sinter to the YAG phase at nearly full density. This novel material offers potential for making high-quality YAG materials including ceramic lasers with exceptional control of processing parameters.

A new method for fluorescence imaging based on a polymeric photobase generator containing oxime-urethane groups was developed via the use of fluorescamine, a prefluorescent amino-group dye. The fluorescent image obtained is stable and has a high contrast and good resolution (see Figure). In addition, it is possible that the fluorescent image can be erased or restored by the alternative treatment of base and acid.

Strong ultrasonic agitation leads to more than a 10³-fold increase in the rate of attachment of zeolite microcrystal monolayers to glass with molecular linkages compared to the rate by reflux and stirring. The degree of close packing also increases tremendously (see Figure).

Spherical-particle assemblies (see Figure) are fabricated by micropatterning of methanol droplets containing SiO₂. Hydrophilic regions of a patterned self-assembled monolayer (SAM) are covered with methanol solution containing SiO₂ particles and immersed in decahydronaphthalene. The particles assemble to form micropatterns of spherical-particle assemblies on the hydrophilic regions of the SAM.

Wide-wavelength tunability, from the blue to the near-infrared spectral regions, and spectrally narrow emission are achieved by embedding a silicon-rich oxide thin film in an optical microcavity structure. The low processing temperatures for these devices suggest compatibility with complementary metal oxide semiconductor fabrication techniques, indicating the potential of this material for integrated optoelectronic applications.

Strategically functionalized 5,11-dialkylindolo[3,2-b]carbazoles (see Figure) represent a novel class of high-mobility p-channel semiconductors for organic thin-film transistors. Specifically, 2,8-dichloro-substituted 5,11-didodecylindolo[3,2-b]carbazole, which forms highly crystalline films with large interconnecting terrace-like layered domain structures, has exhibited mobilities reaching 0.14 cm² V^–1 s^–1 and current on/off ratios of 10⁷ under ambient conditions.

A simple process was developed to prepare fluorescent nanocrystal–polymer complexes with flexible processability. From these complexes one may facilely fabricate macroscopic fluorescent bulk in different shapes and fluorescent microspheres. The Figure is a photoluminescene image of the resultant composites.

Traditional vanadate glasses are known to be totally black (see inset Figure), while their crystal counterparts are promising laser materials due to their large stimulated-emission cross-section. In this work, a novel transparent vanadate glass (see main Figure) is realized for the first time. Furthermore, when doped with neodymium, its large stimulated-emission cross-section and low phonon energy demonstrate potential to be utilized in optical applications, especially in laser-related fiber optics.

Pyrolysis of poly(norbornenyldecaborane) that has been electrostatically spun provides a route to non-woven mats of boron-carbide/carbon ceramic nanofibers with narrow distributions and controllable dimensions (see Figure). This approach allows the fabrication of composite ceramic fibers with varying composition, which could be tailored to suit specific applications.

Li⁺ Intercalation into TiO₂-B nanowires is reported up to a composition of Li_0.91TiO₂-B (specific charge capacity: 305 mA h g^–1) and with a superior rate capability to nanoparticulate anatase and bulk TiO₂-B (see Figure). The combination of high capacity for Li⁺ intercalation, high rate capability, and a potential of ∼1.6 V, renders the intercalated nanowires interesting as anodes for rechargeable lithium batteries and negative electrodes for supercapacitors.

Highly ultramicroporous single-walled carbon nanohorns (see Figure), porosity-designed by HNO₃ treatment, show dramatically enhanced storage capacity of supercritical methane, which can be adsorbed into internal and interstitial spaces of the single-wall carbon nanohorns. This enhanced storage capacity of methane may result from the developed ultramicroporosity, which can promote the adsorptivity of methane.

Novel donor-acceptor nanoassemblies (see cover) are prepared using pristine single-walled carbon nanotubes (SWNTs) as electron-acceptor components in supramolecular “polymer wraps”, with poly(methyl methacrylate) carrying porphyrin units (H₂P) as excited-state electron donors (see Figure). In these novel donor–acceptor ensembles, SWNTs quench the photoexcited H₂P chromophores, resulting in the creation of long-lived radical ion pairs.

Photostable naphthalocyanines (see Figure) can act as optical limiters, simultaneously absorbing nanosecond laser pulses at 532 nm and 1064 nm. In these naphthalocyanines, where M can correspond to Zn, InCl, or In(p-trifluoromethylphenyl), the pattern of substitution and the nature of the substituents were found to be critical factors in achieving such a combination of linear and nonlinear optical properties.

A photonic crystal structure with a refractive index that gradually changes in the specific direction of the crystal is demonstrated (see Figure). The colloidal photonic crystal was obtained by infiltration of a monomer and organic dopant with a high refractive index into the colloidal opal, followed by interfacial gel polymerization. The resulting colloidal crystal has a gradually varying stop-band at different positions in the crystal when the incident light is normal to the {111} crystallographic axis.

A new method for biomimetic silica fabrication is reported. Organized poly(ethyleneimine) rapidly catalyzes and directs the shape of silica in the hydrolysis of tetramethoxysilane under ambient conditions. The resultant silica displays a diversity of controllable morphologies, such as flower, plate, bundle, leaf, and sphere (see Figure, scale bar represents 1 μm).

A plant pathogenic fungus, Fusarium oxysporum, can be used as a biological model system for the extracellular bioleaching of hollow spherical silica nanoparticles (see Figure) from sand. The room-temperature synthesis of oxide nanomaterials using microorganisms starting from potential waste materials could lead to eco-friendly and economically viable methods for the large-scale synthesis of nanomaterials.

The adsorption of β-sheet-rich fibrils produced in vitro via the self-assembly of polypeptides onto surfaces is found to be governed by electrostatic interactions. Chemically patterned substrates are used to create lateral micropatterns of fibrils and hybrid microstructures of fibrils with inorganic nanoparticles (see Figure). β-sheet-rich fibrils are promising as functional biomaterials.

Polymers with lowered bandgaps (see Figure) are produced from monomers containing a bisfulleroid as an electron-acceptor and thiophene as an electron-donor, fused by a thieno[3,4-b]pyrazine bridge. The bandgaps could reach as low as 0.25 eV. Electron flow from the poly(thiophene) backbone to the fullerene through the covalently bound thieno[3,4-b]pyrazine unit in the ground state makes this polymer a good candidate for the fabrication of photovoltaics and photoconductors.

A new metastable intermolecular composite (MIC) which employs KMnO₄ nanoparticles as the oxidizer has been developed for energetic-material applications. The nanoparticles are synthesized by a spray-drying process, and a strong correlation between pressurization rates (measurement schematic shown in Figure) for several MIC combinations and the fraction of reactive oxygen present in the combustion product is observed.

Pentacene films are thermally evaporated onto amorphous carbon-coated mica substrates held at room temperature. The crystal structure and morphology of the films are analyzed using electron microscopy and diffraction, and a new orthorhombic structure is characterized for films below a critical thickness (see Figure). Evidence that the orthorhombic structure is thermodynamically stable at low film thickness due to its low (001) surface energy is obtained.

Single-crystalline nanorods and nanowires are grown using a hydrothermal system with the assistance of polymer additives. Tetragonal perovskite PbZr_0.52Ti_0.48O₃ nanorods (see Figure) are grown in the presence of poly(vinyl alcohol), while nanowires are formed in the presence of poly(vinyl alcohol) and poly(acrylic acid). The polymers are absorbed on the axial side of the particles, reducing the surface energy and inducing oriented growth.

New MoS₂–C₆₀ composite layered crystals built of alternating MoS₂ and C₆₀ molecular layers and with unusual diffraction contrast (see Figure) have been grown via a catalyzed transport reaction using fullerene C₆₀. A model structure of these layered composites is proposed based on transmission electron microscopy, electron diffraction, X-ray diffraction, and photoelectron spectroscopy. A growth mechanism is proposed based on electrostatic interactions, considering optical absorption by MoS₂ and possible charge transfer to C₆₀.

Submicrometer laminated Fe/SiO₂ soft magnetic composites (see Figure) have been fabricated by controlled deformation and subsequent sol–gel coating processes. The bulk materials made of these laminates show flat permeability spectra up to 50 MHz, about two orders of magnitude higher than those made of the parent powders. In addition, the composites have large saturation fields, and thus can be used in high-power devices.

Novel sulfide lithium-ion conducting materials are synthesized by crystallization of mechanically milled Li₂S–P₂S₅ glasses. High ambient-temperature conductivities (σ) compared with other electrolytes (see Figure) and a low conduction activation energy are achieved by the formation of a highly conductive new crystalline phase.

Silicon nanowires develop a uniform synthetic bone coating under an electric-field bias in simulated body fluid. The hydroxyapatite-coated nanowires (see Figure) support fibroblast attachment and proliferation, and remain responsive to electrical bias even after coating, providing a system for electric-field modulated tissue regeneration.

New types of monodisperse, micrometer-sized, hollow particles (see Figure) are obtained by encapsulation of emulsion droplets in solid shells and dissolution of the cores. A facile fabrication method is used in which, by simply tuning the thickness of the shells, different types of particles with tunable properties are obtained.

Metal nanoparticles on carbon nanotubes (CNTs) are expected to be efficient catalysts for several industrially important processes. Here, the successful preparation of well-dispersed Ru–CNT nanocomposites—which are subsequently demonstrated to be very active for the hydrogenation of benzene to cyclohexane—using RuCl₃·H₂O as the precursor is reported. The Figure shows a transmission electron microscopy image of nano- composites used four times as a catalyst.