Advanced Materials

AAO films with a novel six-membered ring structure were prepared by constant current anodization. The six-membered ring contains smaller pores, formed at the triple junction sites of the larger pores. The interpore distance was reduced from the usual 85 nm to 50 nm, doubling the usual pore density. These novel AAO film structures with high pore density may have applications in the preparation of mosaic nanowire arrays and photonic devices.

End-modified poly(β-amino ester)s, easy-to-synthesize degradable polymers, are able to deliver DNA to primary human cells at levels comparable to adenovirus and two orders of magnitude better than the commonly used non-viral vector, polyethylenimine. Small structural changes are found to affect multiple steps of gene delivery including the DNA binding affinity, nanoparticle size, intracellular DNA uptake, and final protein expression. In vivo, these polymer modifications enhance DNA delivery to ovarian tumors.

The use of porphyrins for studies in nonlinear optics has advanced significantly in recent years. The review summarizes the progress in the synthesis of suitable material, their nonlinear optical characteristics and utility for applications.

A thermally reversible culture substrate with a topographically active surface of aligned nanofibers is able to induce cytoskeletal alignment and nucleus elongation. These morphological changes induce myogenic differentiation in stem cells. The differentiated cells can be recovered in sheet form by thermally induced dissolution of the substrate. A surface able to provide topographical cues to create an aligned stem cell sheet, activated to differentiate to a specific lineage, can have a significant impact on engineering of tissue constructs for regenerative medicine applications.

A hetero-bifunctional gelator composed of two different polymerizable groups, acryl and diacetylene, and used to gelate an acrylic monomer has been prepared. The photopolymerization of the organogel formed in hexyl methacrylate produces a free-standing film, in which fluorescent polydiacetylene nanofibers are embedded in a poly(hexyl methacrylate) matrix (see figure).

The electrical instability of organic field-effect transistors is investigated. We observe that the threshold-voltage shift (see figure) shows a stretched- exponential time dependence under an applied gate bias. The activation energy of 0.6 eV is common for our and all other organic transistors reported so far. The constant activation energy supports charge trapping by residual water as the common origin.

The biomolecular mechanism of nickel carcinogenicity is driven by intracellular Ni cation. The role of nickel catalyst in single-wall carbon nanotube toxicity will therefore depend on its bioavailability, which is highly uncertain due to encapsulation by carbon shells. This article measures the material-specific Ni release into extra- and intracellular physiological fluid phases and suggests practical techniques for managing the metals contribution to SWNT toxicity.

Well-aligned nanocylinder formation (see figure) is carried out by using conventional sputtering deposition. Various metal-silicide–silicon and metal-germanide–germanium systems exhibit uniform phase separation with ordered cylinder arrays in a matrix with periodic spacing from 1.9 to 20.2 nm. There is a correlation between the periodic spacing of the cylinders and the eutectic temperatures in these systems.

Silver gets fat and thin: Ag nanoparticles photocatalytically deposited on rutile TiO₂ single crystals exhibit multicolor photochromism. Under monochromatic visible light, particles resonant with the light are photoelectrochemically dissolved, and re-deposited on the TiO₂ substrate or non-resonant particles. These size-selective dissolution and growth are responsible for the wavelength-dependent color changes.

Electric-field-triggered “two-state switching” between two arbitrary structural colors (see figure) in the entire visible region at certain temperatures is shown by a stimuli-responsive chromic hydrogel. The external rapid tuning in the structural color of this hydrogel is successfully achieved by introducing a periodically ordered interconnecting porous structure.

Hydrogenography optically monitors hydrogen ab- and desorption simultaneously on thousands of alloy compositions under exactly the same experimental conditions. In this way the hydride-formation enthalpy map of the Mg–Ni–Ti–H system is determined from a compositional gradient thin film. The figure shows that the region of interesting hydrogen-storage materials is in dark blue, with enthalpies of around –40 kJ (mol H₂)^–1.

Single-walled carbon nanotubes decorated with Pd nanoparticles on thin poly(ethylene terephthalate) sheets serve as building blocks for the fabrication of mechanically flexible hydrogen sensors (left frame of the figure) with excellent sensing performance. The sensitivities (i.e., the change of resistance) of the as-fabricated sensors are comparable to (or even higher than) the sensors built on rigid substrates and the responses are reversible (right frame of the figure).

Bi-layered hybrid nanostructures of polythiophene (PTh) and metal nanotubes are fabricated by a sequential electrochemical synthetic method. Huge enhancement of photoluminescence of PTh single nanotubes coated with nanometer-scale Cu, Ni, or Co metal is observed by using a laser confocal microscope, and can be explained by the effect of surface plasmon resonance. Bright light emission from a single strand of a PTh/Cu hybrid nanotube is observed (see figure).

Core-crosslinked nanocylinders with well-defined shape and narrowly-dispersed size are prepared by using S₂Cl₂ to react with the alkenyl functionalities of the PNb-g-PIp-based core domain of cylindrical core-shell brush block copolymer, PNb-g-PIp-b-PtBA (see figure). This strategy potentially may be further extended by using brush copolymer templates having different compositional, structural and topological features for the synthesis of a broad variety of crosslinked nanostructures with diverse shapes and sizes.

End-modified poly(β-amino ester)s, easy-to-synthesize degradable polymers, are able to deliver DNA to primary human cells at levels comparable to adenovirus and two orders of magnitude better than the commonly used non-viral vector, polyethylenimine. Small structural changes are found to affect multiple steps of gene delivery including the DNA binding affinity, nanoparticle size, intracellular DNA uptake, and final protein expression. In vivo, these polymer modifications enhance DNA delivery to ovarian tumors.

A solvothermal reaction of a cupric salt and tetrazole in acetonitrile generates a tetrazolate (tta)- and cyano-bridged homometallic mixed- valence copper(I,II) molecular ferrimagnet, [Cu₅(tta)₆(CN)₂] (see figure), that exhibits a field-induced transition from a 3D antiferromagnetic state to a ferrimagnetic state at 7.9 K.

Microfluidic devices were fabricated from silk fibroin using an aqueous molding process adapted from soft lithography. The process results in mechanically robust, patent devices with macroscopic fluidic connections. Hepatocytes were seeded in the device and perfused for up to five days during which they exhibited liver-specific functions equivalent to that observed for standard tissue culture substrates.

A new type of lightly crosslinked shape-memory polymer that contains reversibly associating side-groups is reported. H-bonding interactions stabilize mechanically strained states at low temperatures. The materials' shape recovery rate exhibits Arrhenius-like temperature dependence due to the dynamics of H-bond dissociation.

Fabrication of calcium phosphate microtubes is performed utilizing a simple dissolution-reprecipitation mechanism, to form amorphous microtubes with sizes of the order of 10 to 20μm. Raman microspectroscopy gives more understanding of the degradation and formation mechanism.

Conductive polymer chains were directly grafted on a variety of organic substrates containing aromatic functionalities. In this one-step mechanism, the covalent linkages between the substrates and the conducting polymers are created. The enhanced adhesion by grafting enables high-resolution patterning (down to 60 nm) of grafted PEDOT films using standard lithographic techniques without any modification of the patterning processes.

New evidence of a pentacene “bulk phase”, which shows a different layer orientation from the “thin-film phase”, was discovered. Thin films prepared on surface-hydrophobicity-controlled polymer/SiO₂ dielectrics, were studied by using 2D grazing-incidence X-ray diffraction.

Titanium oxide and silicon oxide nanoparticles (TiO₂ and SiO₂ NPs) are selectively self-assembled on the sidewalls of single-walled carbon nanotubes (SWNTs) grown on a p-type Si substrate by using a simple UV irradiation in an aqueous media.

Copolymers of poly(styrene-co-4-bromostyrene) (PBr_xS) with adjustable monomer sequence distribution of styrene (S) and 4-bromostyrene (4-BrS) and composition (x = content of 4-BrS) are prepared by brominating parent polystyrene (PS). While bromination reaction carried out above the theta temperature (Θ) leads to PBr_xS with random monomer distribution, bromination of PS performed at temperatures below Θ produces PBr_xS having a random-blocky character.

Reactions of N,N-bis-[1(2)H-tetrazol-5-yl]amine (H₂Bta) with several high nitrogen molecules result in new nitrogen rich energetic salts (see figure). Densities, melting points, decomposition temperatures and elemental analyses are reported. Based on their calculated heats of formation, detonation properties, and thermal and hydrolytic stabilities, these stable compounds hold promise for energetic applications.

Polycrystalline BiFeO₃nanoparticles (size 80–120 nm) are prepared by a simple sol–gel technique. Such nanoparticles are very efficient for photocatalytic decomposition of organic contaminants under irradiation from ultraviolet to visible frequencies. The BiFeO₃ nanoparticles also demonstrate weak ferromagnetism of about 0.06 μ_B/Fe at room temperature, in good agreement with theoretical calculations.

Silver grids are utilized to exclude the expensive use of indium tin oxide (ITO) in conjugated polymer photovoltaic devices. The grids are generated by electroless deposition from elastomeric microfluidic channels onto transparent substrates. The organic photovoltaic devices demonstrated here, with minimized series resistance, are confirmed to have characteristics comparable to devices exploiting ITO.

The ring-opening process in diarylethene molecular switches is suppressed with decreasing temperature, leading to the complete absence of the photoreaction below a cutoff temperature. By contrast, the reverse ring-closure process shows no significant temperature dependence above 115 K. Reversibility of the photoprocesses can be thus controlled by temperature (see figure).

A hydrogel-capped hair-cell flow microsensor, which closely mimics a superficial neuromast of a fish, is introduced. By encapsulating the hair sensor into the artificial hydrogel cupula a dramatic increase in hair-sensor sensitivity to the oscillating and the steady flow is achieved. It opens the way toward the remote monitoring of the underwater environment by autonomous, unmanned microvehicles with self-navigating capability.

The temperature and pressure dependencies of the magnetization, M(T,P), of cubic [Ru₂(O₂CMe)₄]₃[Cr(CN)₆] reveals that T_c reversibly increases with increasing pressure up to 83 % to 59 K at 12.8 kbar (2.1 K kbar^–1), and the pressure dependence of the hysteresis, M(H,P), shows that the anomalous constricted hysteresis observed at ambient pressure disappears at higher pressure (see figure).

The charge carrier transport in solution-processible copper tetrabenzoporphyrin field-effect transistors (FETs) (see figure) is studied and characterized between room temperature and 4.2 K. The insulator-to-metal transition is reached at a carrier density of 3 × 10¹² cm^–2, approximately independent of the source–drain voltage.

Nanoscale parallel line-space patterns are imprinted into a spin-on organosilicate thin film that contains a second phase pore generating material (porogen). Imprints are converted into nanoporous low-dielectric patterns by vitrification at 430 °C where the porogen is volatilized and creates nanoscale pores (2.2 nm) in the vitrified organosilicate network (see figure). The results are well-defined, high-modulus, nanoporous patterns with an estimated dielectric constant of k∼2.4.

Post-annealing studies in different atmospheres combined with high resolution x-ray studies on epitaxial ZnO and Zn_1–xCo_xO films revealed that the c-axis lattice parameter decreases when interstitial zinc is removed from ZnO. The ‘c' parameter change reverses sign when samples are re-annealed in a Zn-rich atmosphere. Stronger ferromagnetism occurs in Zn_1–xCo_xO film for higher ‘c' parameters and hence higher Zn interstitial concentrations.

Pressure-responsive macromolecules that change their conformation in response to variations in the film pressure can be used as model systems for studying the mechanism of the fluid transport on nanometer length scales. The ability to visualize pressure-sensitive macromolecules in motion enables quantitative measurements of the local film pressure and the friction coefficient with the substrate.

A conducting scaffold of carbon nanotube architecture has been used to anchor ZnO nanoparticles. The interaction between functionalized carbon nanotubes (f-CNT) and photoexcited ZnO nanoparticles results in direct electron transfer between the two systems. The role of f-CNT in facilitating the charge collection and charge transport in the f-CNT-ZnO composite film is elucidated by photoelectrochemical measurements.

A new nanostructured material with tunable viscoelastic properties is fabricated using a 2D on 1D approach. The material differs from traditional carbon nanotube (CNT) re-enforced materials in that it has an integrated, hard, amorphous diamond (a-D) surface that can spread point stresses evenly to the underlying CNT supports (see figure). The film is unique in that it is both hard and wear-resistant but still maintains viscoelastic properties.

Pt nanoparticle catalysts are used to synthesize Si nanowires. The standard deviations of the starting colloid and the resulting wire diameters are essentially the same, whereas the wires were 22 % larger than the particles. The figure shows current–voltage plots of a silicon nanowire different gate voltages.

Biomimetic syntheses of silicas using block copolypeptides containing a hydrophilic lysine block are reported. Porous silica platelets or spheres can be made using block copolypeptides as templates. The results show that biomimetic synthesis under benign conditions may be a potential route for assembling nanostructured materials.

The catalytic synthesis of three-dimensional hierarchical carbon nanotube composites was based on the electrodeposition of iron and the subsequent iron-catalyzed growth of carbon nanotubes from cyclohexane. Electrochemical studies provided evidence for the strong interaction between the graphite foil, the long primary CNTs, and the short secondary CNTs, which are essential for the application of the hierarchical nanocomposite in fuel cell catalysis and in sensor systems.

We present theory and experiment demonstrating a scheme to harvest singlet excitons over 25 nm away from a donor-acceptor interface using resonant energy transfer. Improvement in materials choice could yield effective diffusion lengths as large as 40 nm using long-range transfer, while minimizing the energy loss to less than 0.1 eV making this a promising approach for developing highly efficient organic photovoltaics.

The initially hydrophobic surface of as-prepared luminescent stain-etched porous Si powder is made hydrophilic by means of the physical adsorption of different nonionic surfactants. Unsaturated linear hydrocarbons (undecylenic acid and oleyl alcohol) are found to provide the best durability of aqueous suspensions of porous Si powder, maintaining the ability of excited Si nanocrystals to interact with molecular oxygen by means of electronic energy transfer.

Stimuli responsive magnetic emulsions are produced by using “smart” microgels as stabilizers. The oil droplets in the emulsions are magnetic, can be separated with a static magnetic field and are destabilized by an ac magnetic field. This allows for a remote control on separation and stability of the emulsion.

Two dimensional metal nanoparticle arrays with a size gradient (see figure) are generated by shadow deposition of a finely-collimated low energy cluster beam under an oblique incident configuration. The length of the gradient region is tunable from hundreds of nanometers to several micrometers, by adjusting the incident angle of the cluster beam. The size of the nanoparticles changes by about one order of magnitude across the gradient region.

Carbon nanotubes are assembled into dense, vertically aligned arrays on substrates by using capillary forces. Modulating the interactions between the nanotubes and the underlying substrate provides control over the assembly of the nanotube array; capillary forces can either cause bending (left) or translation (right) of the nanotube array. The use of this densification strategy enables a four-fold increase in current density through nanotube arrays.

Thermo and pH dual-responsive nanoparticles encapsulating an anti-cancer drug (paclitaxel) were assembled from a diblock copolymer comprised of a hydrophilic poly(N-isopropylacrylamide-co-acrylic acid) block and a hydrophobic polycaprolactone block. These nanoparticles aggregated at body temperature under a slightly acidic pH of 6.9 (see figure), and a faster drug release was found to be associated with higher temperature and lower pH, both of which are advantageous for tumor-targeted anti-caner drug delivery.

Nanostructure-based leaf-like polyaniline (PANI) (see figure) has been fabricated successfully in the presence of surfactant F127. It is found that the building blocks of leaf-like PANI can be tuned from nanofibers (ca. 25 nm in diameter) to nanotubes (ca. 100 nm in diameter) with increasing monomer concentration. The proposed synthetic method will provide insight into the synthesis of oriented nanostructures of polymer materials by the soft-template strategy.

Conjugated dendrimers are promising optoelectronic materials as their modular architecture enables independent tuning of the optical, electronic, and processing properties. Dendrimer distributed feedback lasers are demonstrated, and an improvement in slope efficiency from 0.3 % to 1.6 % is seen as the dendrimer generation is increased. Simple fabrication of the lasers by soft lithography is also shown, giving a high slope efficiency of 14 %. The combination of high efficiency dendrimer gain media and soft lithographic fabrication techniques makes conjugated dendrimers attractive materials for organic semiconductor lasers.

AAO films with a novel six-membered ring structure were prepared by constant current anodization. The six-membered ring contains smaller pores, formed at the triple junction sites of the larger pores. The interpore distance was reduced from the usual 85 nm to 50 nm, doubling the usual pore density. These novel AAO film structures with high pore density may have applications in the preparation of mosaic nanowire arrays and photonic devices.

Three simple, controlled steps are all it takes to synthesize the title pentacene analogue DBTDT (see figure). The material's high ionization potential, high thermal and photostability, high mobilities, and an on/off ratio larger than 10⁶ at a substrate temperature of ca. 36 °C, as reported here, suggest that DBTDT will be extremely valuable for applications in plastic organic electronics.

Epitaxial growth of vertical In₂O₃ (111) nanowires with hexagonal cross-section on a-plane sapphire is reported. The figure showing the vertical wires is viewed at 45° from the substrate normal. These wires taper gradually towards the tops and they start to bend near the tips. Because of the low-temperature growth, the wires possess a new hexagonal symmetry and a shortest photoluminescence wavelength among those that have been reported for In₂O₃ nanowires.

Mesoporous titania nanotubes with tunable dimension have been fabricated within the pores of alumina membranes by a simple sol-gel templating method. The 3D network structures of these mesoporous nanotubes (see figure) can provide both electron pathway and lithium ion pathway which benefit their applications in a high rate rechargeable lithium battery.

FePt nanoparticles inside ordered mesoporous silica SBA-15 were synthesized via incipient wetness impregnation and subsequent thermal treatments in reductive atmosphere. The porous matrix is used to prevent sintering and agglomeration during the fcc–FePt to fct–FePt phase transition. According to magnetic measurements and Mössbauer spectroscopic studies, the resulting FePt@SBA-15 composites are ferromagnetic.

A highly efficient approach for metal filling of TiO₂nanotube layers is demonstrated. Electrochemical self-doping is used to switch selectively the bottom of TiO₂ nanotubes into a conductive state. Subsequent metal electrodeposition therefore starts at the tube bottom and continues in a homogenous and controllable manner up the tubes. The approach essentially allows to deposit all material classes into TiO₂ nanotubes and therefore provides the basis for a large variety of functional TiO₂ nanotubes – composite materials.

The effect of a MFI membrane nanostructure change on n-hexane/ 2,2-dimethylbutane (DMB) permeation is described. The figure shows a) DMB transporting through the defects in an empty membrane, b) DMB permeation is blocked due to the initial defects closing by crystal expansion by n-hexane, c) both n-hexane and DMB permeation are blocked when the membrane is exposed to DMB first. The pores shrink and trap DMB so that almost all transport is blocked.

A new class of organic species, dibenzotetrathiafulvalene bisimides (see figure), were designed and synthesized by using a facile and general synthetic strategy. The new bisimides possess a diverse library of derivatives with various functionalities at the imide rings, and they show promise for use in a range of organic electronic applications.

Large-scale generation of multiwalled carbon nanotubes (MCNTs) is efficiently achieved through a supercritical fluid technique employing carbon dioxide as the carbon source. Nanotubes with diameters ranging from 10 to 20 nm and lengths of several tens of micrometers are synthesized (see figure). The supercritical-fluid-grown nanotubes also exhibit field-emission characteristics similar to MCNTs grown by chemical-vapor deposition.

Single-crystalline arrays of PbS, PbSe, and PbTe nanowires (Figure: PbS) with diameters ranging from 40-200 nm and lengths up to 100 μm have been synthesized by a chemical vapor transport approach. Electrical and thermal characterization was performed to investigate their potential as thermoelectric materials. Compared to bulk, the nanowires exhibit reduced thermal conductivity below 100 K by up to 3 orders of magnitude, suggesting that they may be promising thermoelectric materials.

A suspended PMMA quasi-3D PC slab with a hexagonal array of air holes fabricated by using a single-step electron-beam lithography process is shown in the figure (PMMA = polymethylmethacrylate). The extension of the single-step lithography technique to the fabrication of 3D multilayer nanostructures with photonic band gaps in the visible-light region is demonstrated.