Showing papers in "Advanced Materials in 2004"

Electrospinning of Nanofibers: Reinventing the Wheel?†

Abstract: Electrospinning provides a simple and versatile method for generating ultrathin fibers from a rich variety of materials that include polymers, composites, and ceramics. This article presents an overview of this technique, with focus on progress achieved in the last three years. After a brief description of the setups for electrospinning, we choose to concentrate on the mechanisms and theoretical models that have been developed for electrospinning, as well as the ability to control the diameter, morphology, composition, secondary structure, and spatial alignment of electrospun nanofibers. In addition, we highlight some potential applications associated with the remarkable features of electrospun nanofibers. Our discussion is concluded with some personal perspectives on the future directions in which this wonderful technique could be pursued.

Exploitation of Localized Surface Plasmon Resonance

Abstract: Recent advances in the exploitation of localized surface plasmons (charge density oscillations confined to metallic nanoparticles and nanostructures) in nanoscale optics and photonics, as well as in the construction of sensors and biosensors, are reviewed here. In particular, subsequent to brief surveys of the most-commonly used methods of preparation and arraying of materials with localized surface plasmon resonance (LSPR), and of the optical manifestations of LSPR, attention will be focused on the exploitation of metallic nanostructures as waveguides; as optical transmission, information storage, and nanophotonic devices; as switches; as resonant light scatterers (employed in the different near-field scanning optical microscopies); and finally as sensors and biosensors.

White Organic Light‐Emitting Devices for Solid‐State Lighting

Abstract: White organic light-emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid-state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid-crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid-state lighting sources are also noted.

Inkjet Printing of Polymers: State of the Art and Future Developments

Abstract: Inkjet printing is considered to be a key technology in the field of defined polymer deposition. This article provides an introduction to inkjet printing technology and a short overview of the available instrumentation. Examples of polymer inkjet printing are given, including the manufacturing of multicolor polymer light-emitting diode displays, polymer electronics, three-dimensional printing, and oral dosage forms for controlled drug release. Special emphasis is placed upon the utilized polymers and conditions, such as polymer structure, molar mass, solvents, and concentration. Studies on viscoelastic fluid jets and the formation of viscoelastic droplets under gravity indicate that strain hardening is the key parameter that determines the inkjet printability of polymer solutions.

Microwave Absorption Enhancement and Complex Permittivity and Permeability of Fe Encapsulated within Carbon Nanotubes

Abstract: CNT/crystalline Fe nanocomposites (see Figure) have excellent microwave-absorption characteristics. This absorption property is shown to result from the confinement of crystalline Fe in carbon nanoshells, deriving mainly from magnetic rather than electric effects-the complex permittivity and permeability depend both on the shape and phase of the CNT/Fe nanocapsulates.

Form and Function in Multilayer Assembly: New Applications at the Nanoscale

Abstract: New frontiers in materials and polymer science include the development of assembly processes that are flexible, allow the access and implementation of nanoscale structure and order, can provide access to a broad range of materials systems, and yet can be implemented at relatively low cost. The ability to fine tune the composition of nanostructured thin films on the nanometer length scale, when combined with inexpensive patterning and templating routes, provides a powerful tool for nano- and microscale assembly of devices and novel new material systems. The layer-by-layer electrostatic assembly technique is a rich, versatile, and significantly inexpensive approach to the formation of thin films via alternating adsorption of positively and negatively charged species from aqueous solutions; this method has also been extended to include the alternation of polymers with hydrogen-bond donor and acceptor groups. Polymer organic and organic/inorganic thin films formed using this technique may contain a number of different functional groups, including electro-optic, electroluminescent, conducting, and dielectric layers, and functional organic and inorganic nanoparticles. Newly developed materials systems based on alternating layer methods will be addressed, as well as a number of frontier areas and future areas for the use of these systems, ranging from nanomechanical composites to electrochemical devices and templated deposition of functional microspheres. Both new functionalities incorporated within these materials and new means of patterning and templating these structures in two and three dimensions will be addressed in this review.

Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer

Abstract: The authors show that efficient bulk-heterojunction solar cells can be made using ZnO nanoparticles and a conjugated polymer. These cells can be processed from soln. and exhibit an incident photon to current conversion efficiency up to 40 %. Nanocryst. ZnO (nc-ZnO) of approx. 5 nm diam. was synthesized and used. As the n-type semiconductor. Complexes with MDMO-PPV were used, with Aluminum and a PEDOT-PSS/ITO-coated glass hole-conducting electrode. The incident photon to current conversion efficiency (IPCE)closely resembles the absorption spectrum of the MDMO-PPV:nc-ZnO layer on glass, and reaches a value of 40 % at the absorption max. MDMO-PPV. Integration of this spectral response with the solar spectrum (AM1.5G, normalized 100 mW cm-2) affords an est. of the short-circuit c.d. of Jsc = 3.3 mA/cm2 under AM1.5 (1 sun) conditions. C.d.-voltage (J-V) measurements, carried out in the dark reveal excellent diode behavior, with electron current dominating the c.d. in forward bias. TEM micrographs showed intimate mixing of the ZnO and the MDMO-PPV, with the majority of the polymer domains smaller than a few tens of nanometers, which has been shown to be the exciton diffusion length in a similar PPV polymer.

Silica Particles: A Novel Drug-Delivery System †

Abstract: In recent decades, significant advances in drug-delivery systems have enabled more effective drug administration. To deliver drugs to specific organs, a range of organic systems (e.g., micelles, liposomes, and polymeric nanoparticles) have been designed. They suffer from limitations, including poor thermal and chemical stability, and rapid elimination by the immune system. In contrast, silica particles offer a biocompatible, stable, and “stealthy” alternative. Bioactive molecules can be easily encapsulated within silica particles by combining sol-gel polymerization with either spray-drying or emulsion chemistry. Spray-drying faces challenges, including low yield, surface segregation, and size limitations. In contrast, sol-gel emulsions enable the production of nanoparticles with homogeneous drug distribution, and permit ambient temperature processing, necessary for handling biologicals. Independent control of the size and release rate can be readily achieved. Preliminary in-vivo experiments reveal enhanced blood stability of the nanoparticles, which, coupled with sustained release of anti-tumor agents, show good potential for cancer treatment.

Electrospinning Nanofibers as Uniaxially Aligned Arrays and Layer‐by‐Layer Stacked Films

Abstract: The conventional procedure for electrospinning has been modified to generate nanofibers as uniaxially aligned arrays over large areas. The key to the success of this method was the use of a collector composed of two conductive strips separated by an insulating gap of variable width. Directed by electrostatic interactions, the charged nanofibers were stretched to span across the gap and became uniaxially aligned arrays. Two types of gaps have been demonstrated: void gaps and gaps made of a highly insulating material. When a void gap was used, the nanofibers could readily be transferred onto the surfaces of other substrates for various applications. When an insulating substrate was involved, the electrodes could be patterned in various designs on the solid insulator. In both cases, the nanofibers could be conveniently stacked into multi-layered architectures with controllable hierarchical structures. This new version of electrospinning has already been successfully applied to a range of different materials that include organic polymers, carbon, ceramics, and composites.

Solution-Processed, Organophilic Membrane Derived from a Polymer of Intrinsic Microporosity

Abstract: A polymer with a rigid, randomly contorted molecular structure (see Figure), incorporating fused rings connected by spiro-centres, may be precipitated or cast from solution to give microporous powders and membranes stable up to temperatures of 350 °C, with apparent surface areas > 600 m2 g–1. Organophilic membranes may be formed, as demonstrated by the separation of phenol from water by pervaporation.

Patterning: Principles and Some New Developments

Abstract: This article provides an overview of various patterning methodologies, and it is organized into three major sections: generation of patterns, replication of patterns, and three-dimensional patterning. Generation of patterns from scratch is usually accomplished by serial techniques that are able to provide arbitrary features. The writing process can be carried out in many different ways. It can be achieved using a rigid stylus; or a focused beam of photons, electrons, and other energetic particles. It can also be accomplished using an electrical or magnetic field; or through localized add-on of materials such as a liquid-like ink from an external source. In addition, some ordered but relatively simple patterns can be formed by means of self-assembly. In replication of patterns, structural information from a mask, master, or stamp is transferred to multiple copies with the use of an appropriate material. The patterned features on a mask are mainly used to direct a flux of radiation or physical matter from a source onto a substrate, whereas a master/stamp serves as the original for replication based on embossing, molding, or printing. The last section of this article deals with three-dimensional patterning, where both vertical and lateral dimensions of a structure need to be precisely controlled to generate well-defined shapes and profiles. The article is illustrated with various examples derived from recent developments in this field.

Intracellular Delivery of Quantum Dots for Live Cell Labeling and Organelle Tracking

Abstract: Experimental PEI (MW 5000 g/mol) was purchased from Hyperpolymers (Frei-burg, Germany). All other chemicals, if not stated otherwise, were from Fluka and used without further purification. One gram of PEI was suspended in 50 mL acetone and the mixture was cooled to 0 C. Methacryloyl chloride (0.83 mL dissolved in 35 mL acetone) was added dropwise to the stirred suspension within 20 min. After 30 min at 0 C, 150 mL of methanol and 20.4 mL of HEA were added to give a clear solution. The solvents were removed under reduced pressure, the PEI-MA/HEA stock solution was further diluted with HEA to control the PEI content, and 1 mg of the photoinitiator Irgacure 651 (Ciba) was dissolved in 1 mL of the solution. 20 lL of the this mixture was spread on a commercial glass slide previously modified with methacryloyloxy-propyltrimethoxysilane using a standard procedure [20]. The slide was then covered with another glass slide, previously coated with a polypropylene film. The liquid layer between the two slides was UV cured in the UV reactor Heraflash from Heraeus-Kul-zer (Hanau, Germany) for 180 s to give a transparent film. The films were then washed with water/methanol/triethylamine (TEA) (1:1:1 v/v/v), water/methanol (1:1 v/v), and water, than immersed in a solution of 250 mg AgNO 3 in 8 mL water for 30 s, washed with water, and finally added to 50 mL of an aqueous solution of ascorbic acid (10 mg mL ±1). The modification of the films with PEG was performed as follows: The samples were washed with the water/methanol/TEA mixture, methanol, and acetone, then immersed into a solution of 2 g cyanuric chloride in 10 mL acetone at room temperature overnight, rinsed with acetone and chloroform, and finally left in a solution of 2 g O-2-aminoethyl-O¢-methoxy polyethylene glycol (MW 5000 g/ mol) in 10 mL chloroform for 24 h at room temperature. Prior to use the films were thoroughly rinsed with chloroform and immersed in a large amount of water for at least 24 h. UV-vis measurements were carried out with the photospectrometer Lambda 11 from Perkin Elmer. AFM images were recorded with a Nanoscope III scanning probe microscope using Si cantilevers with a fundamental resonance frequency of around 200 kHz. TEM measurements were carried out using a LEO 912 transmission electron microscope applying an acceleration voltage of 120 kV. The silver content was determined using the flame-atom absorption spectrometer Var-io 6 …

Novel Citric Acid‐Based Biodegradable Elastomers for Tissue Engineering

Abstract: Tissue engineering often requires the use of a three dimensional scaffold for cells to grow on and differentiate properly. Generally, the ideal scaffold should be biocompatible, biodegradable, allow for proper cell loading, be cell-responsive and regulate the cell multiplication and differentiation, and possess mechanical and physical properties that are suitable for the target application. As many tissues in the body have elastomeric properties, successful tissue engineering of these requires the development of compliant biodegradable scaffolds. Despite the recognized importance of mechanical stimuli on tissue development, there has been a dearth of research into the design and evaluation of elastomeric biodegradable scaffolds. The few materials that have been reported in the literature require complex and costly synthesis procedures, which translate into higher manufacturing costs and hinder the commercial and clinical implementation of tissue engineering. Herein we describe the synthesis and characterization of a novel biodegradable elastomer, poly(1,8-octanediol-co-citric acid) (POC), that has potential for use in tissue engineering, in particular the engineering of small-diameter blood vessels. Development of materials for this application is important as cardiovascular disease affecting blood vessels is the principal cause of death in the U.S.A. POC has the following advantages: non-toxic monomers, relatively simple synthesis that can be carried out under mild conditions without addition of toxic catalysts or crosslinking reagents (making it a good candidate for drug delivery and cost-effective scale-up), controllable mechanical and biodegradation properties, easy processing, and inherent surface affinity for various cell types.

Preparation of the Novel Nanocomposite Co(OH)2/ Ultra‐Stable Y Zeolite and Its Application as a Supercapacitor with High Energy Density

Abstract: A Co(OH)(2)/zeolite nanocomposite with an extraordinarily high specific capacitance is described. Nanometer-sized Co(OH)(2) whiskers on the zeolite surface permit high electrochemical accessibility and a fast diffusion rate of electrolyte through the material, resulting in a specific capacitance approaching the theoretical limit. increasing the Co(OH)(2) loading increases whisker growth (see Figure), until bulk Co(OH)(2) begins to nucleate and grow in solution (arrows).

Dual‐Scale Roughness Produces Unusually Water‐Repellent Surfaces

Abstract: Super-hydrophobicity can be achieved on relatively smooth surfaces. Short, wide pillars on slightly rough surfaces are shown to produce super-hydrophobic surfaces (see Figure) where neither the pillars nor the slight roughness suffice alone. This use of two length scales to create super-hydrophobic surfaces directly mimics the mechanism used by some plants including the lotus.

High‐Conductivity Elastomeric Electronics

Abstract: Flexible electronic circuits have recently gained widespread interest for numerous applications including flexible displays (electronic paper) [1,2] and wearable electronics. [3,4] Despite substantial improvements over rigid devices, current flexible electronics cannot fully conform to their surroundings, due to the inability of metals and conductive polymers to stretch significantly. [5,6] Electronics that could undergo stretching as well as bending would provide additional degrees of freedom in range of motion and prevent damage to both devices and surroundings by matching their mechanical impedances. Currently , to produce a material that is both elastomeric and conductive , metal particles are embedded in an elastomer, such as silicone. [7] This approach is widely used to form intercon-nections between rigid materials, but the conductivity of such connectors is lower than that of metals and tends to change significantly with strain. [8] Additionally, miniaturization is limited because microfabricated features must be larger than the particle size to ensure conductivity. Here we demonstrate a general strategy to construct elastomeric electronics using mi-crofabricated tortuous wires encased in a silicone elastomer. After a single iteration of geometric optimization, these wires were able to accommodate linear strains of up to 54 % while maintaining stable conductivity. This approach to elastomeric electronics dramatically improves current performance, enabling a range of applications such as forming a direct interface with delicate living tissues or withstanding extreme stress and vibration. Our strategy for constructing elastomeric electronics is based on the fact that many metals, despite being unable to stretch significantly, are able to bend if their cross sections are sufficiently small. While the common helical spring has been used for centuries to produce a net elongation based on bending , the potential of this simple device to produce stretchable integrated circuits has been largely unexplored. To directly test the hypothesis that integrated circuits made of springs could form the basis of stretchable elastomeric electronics, we used standard lithography techniques [9±11] to embed straight or spring-shaped metal wires in an elastomer (Fig. 1). Circuits, formed by connecting a conductivity meter to exposed contact pads at the ends of the wires, were subjected to linear strain until the point of electrical failure (strain at failure). For ease of microscale manufacture, springs were fabricated in the form of 2D oscillations instead of 3D coils. Gold was the chosen metal, based on its high conductivity, malleability, and chemical inertness. [12] Poly(dimethylsiloxane) (PDMS) was chosen to form the elastomeric …

High‐Performance n‐ and p‐Type Single‐Crystal Organic Transistors with Free‐Space Gate Dielectrics

Abstract: [17] The diameters of a large number of spheres (typically ~ 200 spheres) were obtained from scanned TEM images processed using ImageJ and tabulated. The size distribution, defined as the standard deviation divided by the mean sphere diameter, was subsequently evaluated as such for all samples reported. The shell thickness cannot be obtained by simply taking the difference in mean diameters between the coated and bare spheres because shrinking of the cores can occur as a result of condensation of unreacted Si-OH groups when the spheres are re-dispersed in a basic solution for coating. [21] The quantum yield was determined as follows: the optical densities of the index matched coated spheres in a toluene/ethanol mixture and an appropriate reference dye in methanol were closely matched at the wavelength of excitation. To ensure that no reabsorption of the dye emission occurs, the optical densities were always maintained at a value below 0.1 at the excitation wavelength. The photo-luminescence spectra of both the sample of coated spheres and the reference dye were acquired using a SPEX Fluorolog 1680 spectrometer. Comparison of their corresponding integrated emission allowed the quantum yield of the sample to be determined. [22] Although the quantum yields of as-synthesized core±shell CdSe/ZnS NCs used were as high as 38 %, subsequent loss of the original surface ligands due to cap-exchange with AP and APS can lead to diminished quantum yields. Furthermore, the decline in the quantum yield due to processing is very dependent on the quality and thickness of the ZnS shell on the NCs, which can vary from sample to sample. [23] The standard deviation is more significant than the absolute value of the ratio due to the curvature of the microsphere, which may introduce inherent systematic error into the WDS measurement. [24] R. Study of intrinsic transport properties in single-crystal organic semiconductors has the potential to yield fundamental insights into the behavior of plastic transistors for flexible electronics. [1±4] The organic field-effect transistors (OFETs) that facilitate these studies are, however, complex structures whose properties depend on interactions between the semiconductor , gate dielectric, and electrodes. [5±7] Carrier trapping , charge doping, molecular reorientation, dipole formation , and a range of possible chemical interactions are among the many phenomena that can occur at the semiconductor/di-electric interface and degrade device performance. [8±11] We introduce an unusual device design that entirely avoids these effects by replacing the standard solid dielectric layer …

Recent advances in hydrogen storage in metal-containing inorganic nanostructures and related materials

Abstract: An overview of recent advances in the application of non-carbonaceous nanostructured and composite materials in hydrogen storage is presented in this review. The main focus is on complex hydrides, non-graphitic nanotubes, and other porous composite and framework materials, since carbon nanotubes have been the subject of numerous other reviews. Recent advances in the area of alanates show a promising reversible absorption capability of up to 5 %, closing in on the projected Department of Energy (DOE) target of 6 %. Non-carbon nanotubes mainly showed a sorption capacity of 1–3 wt.-%, although a promising level of 4.2 wt.-% is shown by boron nitride nanotubes after collapse of their walls. Other interesting materials included here are lithium nitride and porous metallo-organic frameworks.