Mechanical Properties of Functionalized Single‐Walled Carbon‐Nanotube/Poly(vinyl alcohol) Nanocomposites

Nanocomposites based on semi‐crystalline poly(vinyl alcohol) (PVA) and well‐dispersed chemically functionalized single‐walled carbon nanotubes are combined through simple mixing. The interaction between the nanotubes and the polymer matrix is studied using optical and thermal methods. Significant enhancement of the mechanical properties is obtained for the functionalized‐nanotube‐based composites. These results imply that promoting nanotube dispersion and strong interfacial bonding through adequate functionalization of nanotubes improves the load transfer from the matrix to the reinforcing phase.     

Vertically Aligned Nanocomposite Thin Films as a Cathode/Electrolyte Interface Layer for Thin‐Film Solid Oxide Fuel Cells

A thin layer of a vertically aligned nanocomposite (VAN) structure is deposited between the electrolyte, Ce_0.9Gd_0.1O_1.95 (CGO), and the thin‐film cathode layer, La_0.5Sr_0.5CoO₃ (LSCO), of a thin‐film solid‐oxide fuel cell (TFSOFC). The self‐assembled VAN nanostructure contains highly ordered alternating vertical columns of CGO and LSCO formed through a one‐step thin‐film deposition process that uses pulsed laser deposition. The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode. Low cathode polarization resistances of 9 × 10^?4 and 2.39 Ω were measured for the cells with the VAN interlayer at 600 and 400 °C, respectively. Furthermore, anode‐supported single cells with LSCO/CGO VAN interlayer demonstrate maximum power densities of 329, 546, 718, and 812 mW cm^?2 at 550, 600, 650, and 700 °C, respectively, with an open‐circuit voltage (OCV) of 1.13 V at 550 °C. The cells with the interlayer triple the overall power output at 650 °C compared to that achieved with the cells without an interlayer. The binary VAN interlayer could also act as a transition layer that improves adhesion and relieves both thermal stress and lattice strain between the cathode and the electrolyte.     

Structure–Property–Function Relationships in Nanoscale Oxide Sensors: A Case Study Based on Zinc Oxide

Chemical sensing on oxide sensors is a complex phenomenon involving catalytic activity as well as electronic properties. Thus, the properties of oxide sensors are highly sensitive towards structural changes. Effects like surface area, grain size, and, in addition, the occurrence of defects give separate contributions to the current. Structure–property–function relationships can be elucidated using a combination of state‐of‐the‐art analytical techniques. It is shown, that impurity atoms in the oxide lattice influence the performance of ZnO sensors more strongly than the other factors.     

Preparation and Gas Sensing Characteristics of Nanoparticulate p‐Type Semiconducting LnFeO3 and LnCrO3 Materials

Nanoparticulate perovskite‐type LnBO₃ (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; B = Cr, Fe) oxides are prepared by polyol‐mediated synthesis. X‐ray diffraction is used to confirm the structure of the materials, and scanning electron microscopy is applied to image the sample morphology. The powders are used for the preparation of suspensions that are deposited as thick films on comb‐type Pt electrodes on alumina substrates. Electrical characterization is performed by using high‐throughput impedance spectroscopy. The materials exhibit p‐type semiconductor behavior, and their sensitivity towards H₂, CO, NO, NO₂, and propylene in a temperature range from 200 to 500 °C is analyzed. In addition, the time‐resolved response of the LnFeO₃ materials is investigated. A correlation between the Ln–O binding energy and the sensing properties is observed.     

Deformation of Top‐Down and Bottom‐Up Silver Nanowires

Atomistic simulations are employed to probe the deformation behavior of experimentally observed top‐down and bottom‐up face‐centered cubic silver nanowires. Stable, <110> oriented nanowires with a rhombic and truncated‐rhombic cross section are considered, representative of top‐down geometries, as well as the multiply twinned pentagonal nanowire that is commonly fabricated in a bottom‐up approach. The tensile deformation of a stable, experimentally observed structure is simulated to failure for each nanowire structure. A detailed, mechanistic explanation of the initial defect nucleation is provided for each nanowire. The three geometries are shown to exhibit different levels of strength and to deform by a range of mechanisms depending on the nanowire structure. In particular, the deformation behavior of top‐down and bottom‐up nanowires is shown to be fundamentally different. The yield strength of nanowires ranging from 1 to 25 nm in diameter is provided and reveals that in addition to cross‐sectional diameter, the strength of the nanowires is strongly tied to the structure. This study demonstrates that nanowire structure and size may be tailored for specific mechanical requirements in nanometer‐scale devices.     

Transparent Nanocomposites of Radiopaque,Flame‐Made Ta2O5/SiO2 Particles in an Acrylic Matrix

Mixed Ta₂O₅‐containing SiO₂ particles, 6–14 nm in diameter, with closely controlled refractive index, transparency, and crystallinity are prepared via flame spray pyrolysis (FSP) at production rates of 6.7–100 g h^–1. The effect of precursor solution composition on product filler (particle) size, crystallinity, Ta dispersity, and transparency is studied using nitrogen adsorption, X‐ray diffraction, optical microscopy, high‐resolution transmission electron microscopy (HRTEM), and diffuse‐reflectance infrared Fourier‐transform spectroscopy (DRIFTS). Emphasis is placed on the transparency of the composite that is made with Ta₂O₅/SiO₂ filler and dimethylacrylate. Increasing Ta₂O₅ crystallinity and decreasing Ta dispersity on SiO₂ decreases both filler and composite transparencies. Powders with identical specific surface area (SSA), refractive index (RI), and Ta₂O₅ content (24 wt.‐%) show a wide range of composite transparencies, 33–78 %, depending on filler crystallinity and Ta dispersity. Amorphous fillers with a high Ta dispersity and an RI matching that of the polymer matrix lead to the highest composite transparency, 86 %. The composite containing 16.5 wt.‐% filler that itself contains 35 wt.‐% Ta₂O₅ has the optimal radiopacity for dental fillings.     

A Novel Polymeric Ionomer as a Potential Biomaterial: Crystallization Behavior,Degradation, and In‐Vitro Cellular Interactions

To evaluate the potential of polyester‐based ionomers as biomaterials, we have characterized them in terms of crystallization behavior, degradation, and in‐vitro cellular interactions. The polymers used are poly(butylene succinate)‐based ionomers (PBSis) with 1 to 5 mol‐% dimethyl 5‐sodium sulfoisophthalate. Even a few incorporated ionic groups significantly decreases the folding surface energy, indicating that folding into crystalline lamellae is more difficult for chains restricted by ionic aggregates. Transmission electron microscopy (TEM) does not reveal any distinct aggregation of ionic clusters following hydrolytic degradation, which suggests that the physical crosslinkage due to ionic interactions is vulnerable to hydrolysis. The in‐vitro cellular interactions of polyester‐based ionomers is assessed by the culture of human dermal fibroblasts with PBSi extracts or in direct contact with the PBSi films. Cells on PBSi films and in their extracts exhibit appropriate specific growth rates and normal metabolic function regardless of the incorporated ionic content compared with poly[(D ,L ‐lactic acid)‐co‐(glycolic acid)] (75:25, PLGA), which is well known to be biocompatible. The cells growing on PBSi films spread to a sufficient extent, displaying relatively active filopodial growth, as compared to that of parent PBS. These results suggest that the conspicuous topology and hydrophilic nature of the ionomer surface affect cellular interactions, and that this ionomer therefore has potential applications as a biomaterial.     

Air‐Stable n‐Type Organic Field‐Effect Transistors Based on Carbonyl‐Bridged Bithiazole Derivatives

An electronegative conjugated compound composed of a newly designed carbonyl‐bridged bithiazole unit and trifluoroacetyl terminal groups is synthesized as a candidate for air‐stable n‐type organic field‐effect transistor (OFET) materials. Cyclic voltammetry measurements reveal that carbonyl‐bridging contributes both to lowering the lowest unoccupied molecular orbital energy level and to stabilizing the anionic species. X‐ray crystallographic analysis of the compound shows a planar molecular geometry and a dense molecular packing, which is advantageous to electron transport. Through these appropriate electrochemical properties and structures for n‐type semiconductor materials, OFET devices based on this compound show electron mobilities as high as 0.06 cm² V^?1 s^?1 with on/off ratios of 10⁶ and threshold voltages of 20 V under vacuum conditions. Furthermore, these devices show the same order of electron mobility under ambient conditions.     

Thermal and Structural Characterizations of Individual Single‐, Double‐, and Multi‐Walled Carbon Nanotubes

Thermal conductance measurements of individual single‐ (S), double‐ (D), and multi‐ (M) walled (W) carbon nanotubes (CNTs) grown using thermal chemical vapor deposition between two suspended microthermometers are reported. The crystal structure of the measured CNT samples is characterized in detail using transmission electron microscopy (TEM). The thermal conductance, diameter, and chirality are all determined on the same individual SWCNT. The thermal contact resistance per unit length is obtained as 78–585 m K W^?1 for three as‐grown 10–14 nm diameter MWCNTs on rough Pt electrodes, and decreases by more than 2 times after the deposition of amorphous platinum–carbon composites at the contacts. The obtained intrinsic thermal conductivity of approximately 42–48, 178–336, and 269–343 W m^?1 K^?1 at room‐temperature for the three MWCNT samples correlates well with TEM‐observed defects spaced approximately 13, 20, and 29 nm apart, respectively; whereas the effective thermal conductivity is found to be limited by the thermal contact resistance to be about 600 W m^?1 K^?1 at room temperature for the as‐grown DWCNT and SWCNT samples without the contact deposition.     

Composition‐ and Shape‐Controlled Synthesis and Optical Properties of ZnxCd1–xS Alloyed Nanocrystals

Composition‐tunable Zn_xCd_1–xS alloyed nanocrystals have been synthesized by a new approach consisting of thermolyzing a mixture of cadmium ethylxanthate (Cd(exan)₂) and zinc ethylxanthate (Zn(exan)₂) precursors in hot, coordinating solvents at relatively low temperatures (180–210 °C). The composition of the alloyed nanocrystals was accurately adjusted by controlling the molar ratio of Cd(exan)₂ to Zn(exan)₂ in the mixed reactants. The alloyed Zn_xCd_1–xS nanocrystals prepared in HDA/TOP (HDA: hexadecylamine; TOP: trioctylphosphine) solution exhibit composition‐dependent shape and phase structures as well as composition‐dependent optical properties. The shape of the Zn_xCd_1–xS nanocrystals changed from dot to single‐armed rod then to multi‐armed rod with a decrease of Zn content in the ternary nanoparticles. The alloying nature of the Zn_xCd_1–xS nanocrystals was consistently confirmed by the results of high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), and UV‐vis absorption and photoluminescence (PL) spectroscopy. Further, the shape‐controlled synthesis of the ternary alloyed nanocrystals was realized by selecting appropriate solvents. Uniform nanodots in the whole composition range were obtained from TOPO/TOP solution, (TOPO: trioctylphosphine oxide) and uniform nanorods in the whole composition range were prepared from HDA/OA solution (OA: octylamine). The effect of the reaction conditions, such as solvent, reaction temperature, and reaction time, on the PL spectra of the alloyed Zn_xCd_1–xS nanocrystals was also systematically studied, and the reaction conditions were optimized for improving the PL properties of the nanocrystals.     

Clay Assisted Dispersion of Carbon Nanotubes in Conductive Epoxy Nanocomposites

Clay was introduced into single‐walled carbon nanotube (SWNT)/epoxy composites to improve nanotube dispersion without harming electrical conductivity or mechanical performance. Unlike surfactant or polymer dispersants, clay is mechanically rigid and known to enhance the properties (e.g., modulus, gas barrier, and flame retardation) of polymer composites. Combining nanotubes and clay allows both electrical and mechanical behavior to be simultaneously enhanced. With just 0.05 wt % SWNT, electrical conductivity is increased by more than four orders of magnitude (from 10^–9 to 10^–5 S cm^–1) with the addition of 0.2 wt % clay. Furthermore, the percolation threshold of these nanocomposites is reduced from 0.05 wt % SWNT to 0.01 wt % with the addition of clay. SWNTs appear to have an affinity for clay that causes them to become more exfoliated and better networked in these composites. This clay‐nanotube synergy may make these composites better suited for a variety of packaging, sensing, and shielding applications.     

Structural Transformations during Formation of Quasi‐Amorphous BaTiO3

A model of structural transformations of amorphous into quasi‐amorphous BaTiO₃ is suggested. The model is based on previously published data and on X‐ray photoelectron spectroscopy data presented in the current report. Both amorphous and quasi‐amorphous phases of BaTiO₃ are made up of a network of slightly distorted TiO₆ octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical‐charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO₆ network. Upon heating, the edge‐to‐edge and face‐to‐face connections between TiO₆ octahedra are severed and then reconnected via apices. Severing the connections between TiO₆ octahedra requires a volume increase, suppression of which keeps some of the edge‐to‐edge and face‐to‐face connections intact. Transformation of the amorphous thin films into the quasi‐amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO₆ octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non‐centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi‐amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.     

Atomistic Study of a CaTiO3‐Based Mixed Conductor: Defects,Nanoscale Clusters,and Oxide‐Ion Migration

Mixed oxide‐ion and electronic conductivity can be exploited in dense ceramic membranes for controlled oxygen separation as a means of producing pure oxygen or integrating with catalytic oxidation. Atomistic simulation has been used to probe the energetics of defects, dopant‐vacancy association, nanoscale cluster formation, and oxide‐ion transport in mixed‐conducting CaTiO₃. The most favorable energetics for trivalent dopant substitution on the Ti site are found for Mn³⁺ and Sc³⁺. Dopant‐vacancy association is predicted for pair clusters and neutral trimers. Low binding energies are found for Sc³⁺ in accordance with the high oxide‐ion conductivity of Sc‐doped CaTiO₃. The preferred location for Fe⁴⁺ is in a hexacoordinated site, which supports experimental evidence that Fe⁴⁺ promotes the termination of defect chains and increases disorder. A higher oxide‐ion migration energy for a vacancy mechanism is predicted along a pathway adjacent to an Fe³⁺ ion rather than Fe⁴⁺ and Ti⁴⁺, consistent with the higher observed activation energies for ionic transport in reduced CaTi(Fe)O_3–δ.     

Structural Modifications to Polystyrene via Self‐Assembling Molecules

We have previously reported that small quantities of self‐assembling molecules known as dendron rodcoils (DRCs) can be used as supramolecular additives to modify the properties of polystyrene (PS). These molecules spontaneously assemble into supramolecular nanoribbons that can be incorporated into bulk PS in such a way that the orientation of the polymer is significantly enhanced when mechanically drawn above the glass‐transition temperature. In the current study, we more closely evaluate the structural role of the DRC nanoribbons in PS by investigating the mechanical properties and deformation microstructures of polymers modified by self‐assembly. In comparision to PS homopolymer, PS containing small amounts (≤ 1.0 wt.‐%) of self‐assembling DRC molecules exhibit greater Charpy impact strengths in double‐notch four‐point bending and significantly greater elongations to failure in uniaxial tension at 250 % prestrain. Although the DRC‐modified polymer shows significantly smaller elongations to failure at 1000 % prestrain, both low‐ and high‐prestrain specimens maintain tensile strengths that are comparable to those of the homopolymer. The improved toughness and ductility of DRC‐modified PS appears to be related to the increased stress whitening and craze density that was observed near fracture surfaces. However, the mechanism by which the self‐assembling DRC molecules toughen PS is different from that of conventional additives. These molecules assemble into supramolecular nanoribbons that enhance polymer orientation, which in turn modifies crazing patterns and improves impact strength and ductility.     

Microphase‐Separated Donor–Acceptor Diblock Copolymers: Influence of HOMO Energy Levels and Morphology on Polymer Solar Cells

The synthesis of novel semiconducting donor–acceptor (D–A) diblock copolymers by means of nitroxide‐mediated polymerization (NMP) is reported. The copolymers contain functional moieties for hole transport, electron transport, and light absorption. The first block, representing the donor, is made up of either substituted triphenylamines (poly(bis(4‐methoxyphenyl)‐4′‐vinylphenylamine), PvDMTPA) or substituted tetraphenylbenzidines (poly(N,N′‐bis(4‐methoxyphenyl)‐N‐phenyl‐N′‐4‐vinylphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine), PvDMTPD). The second block consists of perylene diimide side groups attached to a polyacrylate backbone (PPerAcr) via a flexible spacer. This block is responsible for absorption in the visible range and for electron‐transport properties. The electrochemical properties of these fully functionalized diblock copolymers, PvDMTPA‐b‐PPerAcr and PvDMTPD‐b‐PPerAcr, are investigated by cyclic voltammetry (CV), and their morphology is investigated by transmission electron microscopy (TEM). All diblock copolymers exhibit microphase‐separated domains in the form of either wire‐ or wormlike structures made of perylene diimide embedded in a hole‐conductor matrix. In single‐active‐layer organic solar cells, PvDMTPD‐b‐PPerAcr reveals a fourfold improvement in power conversion efficiency (η = 0.26 %, short‐circuit current (I_SC) 1.21 mA cm^–2), and PvDMTPA‐b‐PPerAcr a fivefold increased efficiency (η = 0.32 %, I_SC = 1.14 mA cm^–2) compared with its unsubstituted analogue PvTPA‐b‐PPerAcr (η = 0.065 %, I_SC = 0.23 mA cm^–2).     

Piezoresistive Materials from Directed Shear‐Induced Assembly of Graphite Nanosheets in Polyethylene

A three‐dimensional conductive nanocomposite with an ordered conductive network and low percolation threshold has been successfully prepared by blending graphite nanosheets (GNs) with polyethylene on a two‐roll mill. The conductive nanosheets orient intensively in the composite, leading to highly anisotropic properties. The nanocomposite with the fraction of conductive nanosheets closest to the percolation threshold possesses a sharp positive pressure coefficient of resistivity, in which the abrupt transition can be attributed to compressive‐stress‐induced deformation of the conductive network. Such piezoresistive effects depend strongly on filler morphology, filler spatial arrangement, and filler concentration.     

Origin of the Reduced Fill Factor and Photocurrent in MDMO‐PPV:PCNEPV All‐Polymer Solar Cells

The photogeneration mechanism in blends of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) and poly[oxa‐1,4‐phenylene‐(1‐cyano‐1,2‐vinylene)‐(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene)‐1,2‐(2‐cyanovinylene)‐1,4‐phenylene] (PCNEPV) is investigated. The photocurrent in the MDMO‐PPV:PCNEPV blends is strongly dependent on the applied voltage as a result of a low dissociation efficiency of the bound electron–hole pairs. The dissociation efficiency is limited by low carrier mobilities, low dielectric constant, and the strong intermixing of the polymers, leading to a low fill factor and a reduced photocurrent at operating conditions. Additionally, electrons trapped in the PCNEPV phase recombine with the mobile holes in the MDMO‐PPV phase at the interface between the two polymers, thereby affecting the open‐circuit voltage and increasing the recombination losses. At an intensity of one sun, Langevin recombination of mobile carriers dominates over trap‐assisted recombination.     

Cover Picture: Structural Modifications to Polystyrene via Self‐Assembling Molecules (Adv. Funct. Mater. 3/2005)

The cover shows tensile failure of a sample of pure polystyrene (left), and a polystyrene sample with greater impact strength containing 1% by weight of dispersed nanoribbons (right), as reported in work by Stupp and co‐workers on p. 487. The nanoribbons are formed by self‐assembly of molecules known as dendron rodcoils (DRCs) in styrene monomer, resulting in the formation of a gel. This gel can then be polymerized thermally. We have previously reported that small quantities of self‐assembling molecules known as dendron rodcoils (DRCs) can be used as supramolecular additives to modify the properties of polystyrene (PS). These molecules spontaneously assemble into supramolecular nanoribbons that can be incorporated into bulk PS in such a way that the orientation of the polymer is significantly enhanced when mechanically drawn above the glass‐transition temperature. In the current study, we more closely evaluate the structural role of the DRC nanoribbons in PS by investigating the mechanical properties and deformation microstructures of polymers modified by self‐assembly. In comparision to PS homopolymer, PS containing small amounts (≤ 1.0 wt.‐%) of self‐assembling DRC molecules exhibit greater Charpy impact strengths in double‐notch four‐point bending and significantly greater elongations to failure in uniaxial tension at 250 % prestrain. Although the DRC‐modified polymer shows significantly smaller elongations to failure at 1000 % prestrain, both low‐ and high‐prestrain specimens maintain tensile strengths that are comparable to those of the homopolymer. The improved toughness and ductility of DRC‐modified PS appears to be related to the increased stress whitening and craze density that was observed near fracture surfaces. However, the mechanism by which the self‐assembling DRC molecules toughen PS is different from that of conventional additives. These molecules assemble into supramolecular nanoribbons that enhance polymer orientation, which in turn modifies crazing patterns and improves impact strength and ductility.     

Orientation Control of Linear‐Shaped Molecules in Vacuum‐Deposited Organic Amorphous Films and Its Effect on Carrier Mobilities

The molecular orientation of linear‐shaped molecules in organic amorphous films is demonstrated to be controllable by the substrate temperature. It is also shown that the molecular orientation affects the charge‐transport characteristics of the films. Although linear‐shaped 4,4′‐bis[(N‐carbazole)styryl]biphenyl molecules deposited on substrates at room temperature are horizontally oriented in amorphous films, their orientation when deposited on heated substrates with smooth surfaces becomes more random as the substrate temperature increases, even at temperatures under the glass transition temperature. Another factor dominating the orientation of the molecules deposited on heated substrates is the surface roughness of the substrate. Lower carrier mobilities are observed in films composed of randomly oriented molecules, demonstrating the significant effect of a horizontal molecular orientation on the charge‐transport characteristics of organic amorphous films.