Showing papers in "Advanced Functional Materials in 2002"

Controlled growth of ZnO nanowires and their optical properties

Abstract: This article surveys recent developments in the rational synthesis of single-crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power-dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short-wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 1010 cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single-crystalline, well-facetted nanowires can function as self-contained optical resonance cavities. This argument is further supported by our recent near-field scanning optical microscopy (NSOM) studies on single nanowires.

A low-bandgap semiconducting polymer for photovoltaic devices and infrared emitting diodes

Abstract: A novel low-bandgap conjugated polymer (PTPTB, E-g = similar to1.6 eV), consisting of alternating electron-rich N-dodecyl-2,5-bis(2'-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units, is introduced for thin-film optoelectronic devices working in the near infrared (NIR). Bulk heterojunction photovoltaic cells from solid-state composite films of PTPTB with the soluble fullerene derivative [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) as an active layer shows promising power conversion efficiencies up to 1% under AM1.5 illumination. Furthermore, electroluminescent devices (light-emitting diodes) from thin films of pristine PTPTB show near infrared emission peaking at 800 nm with a turn on voltage below 4 V. The electroluminescence can be significantly enhanced by sensitization of this material with a wide bandgap material such as the poly(p-phenylene vinylene) derivative MDMO-PPV.

Organization of Matter on Different Size Scales: Monodisperse Nanocrystals and Their Superstructures

Abstract: Advanced colloidal syntheses enable the preparation of monodisperse semiconductors and magnetic alloy nanocrystals. They can be further used as building blocks for the fabrication of ordered assemblies: two-dimensional and three-dimensional arrays and colloidal supercrystals. This article reviews our recent activities in these fields. A theoretical description of the evolution of an ensemble of nanoparticles in a colloidal solution is applied to the problem of control over the nanocrystal monodispersity.

Electrochromic Window Based on Conducting Poly(3,4‐ethylenedioxythiophene)–Poly(styrene sulfonate)

Abstract: A short survey of technological aspects of electrochromism with various electroactive species is given. Different approaches with inorganic and organic materials have been pursued in the past. So far widespread usage of this technology for large area applications has not been achieved. Nevertheless one major technical product, self-darkening rear-view mirrors for cars, is already well established. This article reviews some research results on electroactive polythiophenes, especially poly(3,4-alkylenedioxythiophenes). Some promising results with the commercially available electrically conducting polymer Baytron P (PEDT/PSS) are presented. It is demonstrated that an all solid-state electrochromic multilayer assembly based on a polymeric electrochromic material might be close to technical realization. The coating of large area substrates with aqueous poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) dispersion can be a way to an economically viable product.

Synthesis of a Novel, Biodegradable Electrically Conducting Polymer for Biomedical Applications

Abstract: We report the synthesis and characterization of a novel biomaterial that possesses the unique properties of being both electrically conducting and biodegradable; and thus capable of electronic interfacing with tissue. This polymer was synthesized from conducting oligomers of pyrrole and thiophene that are connected together via degradable ester linkages. We demonstrate that this polymer is conductive, degradable, and biocompatible.

Synthesis and Characterization of Uniform Nanowires of Trigonal Selenium

Abstract: This article describes a soft, solution-phase approach to the large-scale synthesis of uniform nanowires of trigonal selenium (t-Se) with lateral dimensions controllable in the range of ~10 to ~800 nm, and lengths up to hundreds of micrometers. These highly anisotropic, one-dimensional (1D) nanostructures were directly nucleated and grown from aqueous solutions without the help of any physical templates, such as channel-like structures etched in porous materials, or scaffolds assembled from surfactants or block-copolymers. The 1D morphology of the product was solely determined by the linear characteristics of the building blocks—i.e., the extended, helical chains of atoms contained in the crystalline lattice of t-Se. A blue shift was observed for the bandgap and interchain transition of these nanowires when their diameters were reduced from ~32 to ~10 nm. The photoconductivity of individual nanowires has also been measured using the four-probe method, and an increase by ~150 times was found when the sample was taken from the dark and exposed with ~3 lW lm ‐2 tungsten light. Since no exotic seeds were involved in this synthetic process, every nanowire (including both ends) should be made entirely of pure selenium, crystallized in the trigonal phase. We believe the protocol described here can be scaled up for the high-volume production of t-Se nanowires that can subsequently serve as the physical or chemical templates to generate 1D nanostructures of various kinds of functional materials. The synthetic strategy itself, may also be extendable to other systems containing chain-like building blocks. The single crystallinity and absence of kinks and other related defects in these nanowires should make them particularly useful in fabricating nanoscale electronic, optical, or mechanical nanodevices.

Role of dimensionality on the two-photon absorption response of conjugated molecules: The case of octupolar compounds

Abstract: A comparative study of the two-photon absorption (TPA) properties of octupolar compounds and their dipolar one-dimensional counterparts is presented on the basis of correlated quantum-chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground-state and excited-state wavefunctions of three-arm octupolar systems are built from a linear combination of the corresponding single-arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge-transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.

Spectroscopic Studies of Photoexcitations in Regioregular and Regiorandom Polythiophene Films

Abstract: Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.

Opal Circuits of Light—Planarized Microphotonic Crystal Chips

Abstract: We present a novel technique coined directed evaporation-induced self-assembly (DEISA) that enables the formation of planarized opal-based microphotonic crystal chips in which opal crystal shape, size, and orientation are under synthetic control. We provide detailed synthetic protocols that underpin the DEISA process and formulate directed self-assembly strategies that are suited for the fabrication of opal architectures with complex form and designed optical functionality. These developments bode well for the utilization of opal-based photonic crystals in microphotonic crystal devices and chips.

Template-Based Growth of Various Oxide Nanorods by Sol–Gel Electrophoresis

Abstract: The ability to form oxide nanorods is of great interest in a number of areas. In this paper, we report the template-based growth of nanorods of several oxide ceramics, formed by means of a combination of sol–gel processing and electrophoretic deposition. Both single metal oxides (TiO2, SiO2) and complex oxides (BaTiO3, Sr2Nb2O7, and Pb(Zr0.52Ti0.48)O3) have been grown by this method. Uniformly sized nanorods of about 125–200 nm in diameter and 10 μm in length were grown over large areas with near unidirectional alignment. Desired stoichiometric chemical composition and crystal structure of the oxide nanorods was readily achieved by an appropriate procedure of sol preparation, with a heat treatment (700 °C for 15 min) for crystallization and densification.

Quadratic Nonlinear Optical Properties of N-Aryl Stilbazolium Dyes**

Abstract: Trans-4'-(dimethylamino)-N-R-4-stilbazolium hexafluorophosphate (R = Me, Me 1, Ph, Ph 2, 2,4-dinitrophenyl, DNPh 3, 2-pyrimidyl, Pym 4, Scheme 1) were prepd. Their electronic absorption spectra show intense, visible intramol. charge-transfer bands, the energy (Emax) of which decreases in the order R = Me > Ph > DNPh > Pym. This trend arises from the steadily increasing electron deficiency of the pyridinium ring, a phenomenon also obsd. in cyclic voltammetric and 1H NMR data. Fluorescence-free 1st hyperpolarizability .beta. values of [1-4]PF6 were measured by using femtosecond hyper-Rayleigh scattering (HRS) with MeCN solns. and a 1300. nm laser, and static 1st hyperpolarizabilities .beta.0 were obtained by application of the two-state model. The HRS results indicate that the N-aryl chromophores in [2-4]PF6 have considerably larger .beta.0 values than their N-Me counterpart in [1]PF6, with a .apprx. 10-fold increase in .beta.0 obsd. in moving from [1]PF6 to [4]PF6 (25 .fwdarw. 230 .times. 10-30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes .DELTA..mu.12 (10.9-14.8 D), which were used to calc. .beta.0 according to the two-state equation .beta.0 = 3.DELTA..mu.12(.mu.12)2/2(Emax)2 (.mu.12 = transition dipole moment). With the exception of [1]PF6, the Stark-derived .beta.0 values are in reasonable agreement with those from HRS. However, the increase in .beta.0 in moving from [1]PF6 to [4]PF6 is only 2-fold for the Stark data (90 .fwdarw. 185 .times. 10-30 esu). The obsd. trend of increasing .beta.0 in the order [1]PF6 < [3]PF6 < [2]PF6 < [4]PF6 arises from a combination of decreasing Emax and increasing .DELTA..mu.12, with only a slight increase in .mu.12 between [1]PF6 and [4]PF6. Probably the .beta.0 values for [3]PF6 are lower than expected due to the steric effect of the ortho-NO2 group, which causes twisting of the DNPh ring out of the plane of the stilbazolium unit. A single crystal x-ray structure shows that [2]PF6 crystallizes in the space group Cc, with head-to-tail alignment and almost parallel stacking of the pseudo-planar stilbazolium portions of the cations to form polar sheets within a polar bulk structure. [2]PF6 is essentially isostructural with the related Schiff base salt trans-4-[(4-dimethylaminophenyl)iminomethyl]-N-phenylpyridinium hexafluorophosphate ([8]PF6). Second harmonic generation (SHG) studies on [2]PF6 and [8]PF6 using a 1907. nm laser and sieved powd. samples (53-63 .mu.m) afforded efficiencies of 470 and 240 times that of urea, resp. Under the same conditions, the well-studied compd. [1]p-MeC6H4SO3 gave an SHG efficiency of 550 times that of urea. [on SciFinder (R)]

Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

Abstract: After a brief review on electro-optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single- and multiple-dendron-modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three-fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site-isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r33 = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double-end functionalization of a three-dimensional phenyl-tetracyanobutadienyl (Ph-TCBD)-containing NLO chromophore with thermally crosslinkable trifluorovinylether-containing dendrons.

Large, Switchable Electrochromism in the Visible Through Far‐Infrared in Conducting Polymer Devices

Abstract: Advanced materials with large and dynamic variation in thermal properties, sought for urgent defense and space applications, have heretofore been elusive. Conducting polymers (CPs) have shown some intrinsic variation of mid- to far-infrared (IR) signature in this respect, but the practical utilization of this has remained elusive. We report herein the first significant IR electrochromism in any material, to our knowledge, in the 0.4 through 45 μm region. This is seen in practical CP devices in the form of thin (<0.5 mm), flexible, entirely solid-state, variable area (1 cm2 to 1 m2) flat panels. Typical properties include: very high reflectance variation; switching times <2 s; cyclabilities of 105 cycles; emittance variation from 0.32 to 0.79; solar absorptance variation from 0.39 to 0.79; operating temperatures of –35 to +85 °C; durability against γ-radiation to 7.6 Mrad, vacuum to 10–6 torr, and simulated solar wind (e.g., 6.5 × 1016 e/cm2 @ 10 keV).

Capillary Force Lithography: Large‐Area Patterning, Self‐Organization, and Anisotropic Dewetting

Abstract: This article gives an overview on a new lithographic technique called capillary force lithography for large-area patterning. The technique simply involves placing a polydimethylsiloxane mold on a polymer film, which is then heated above the glass-transition temperature of the polymer. Various useful microstructures can be obtained by sequential applications of the technique through self-organization. Dewetting, which can be observed in capillary force lithography for relatively thin films, is also described as a new pathway for realizing anisotropic dewetting.

Strain Response from Polypyrrole Actuators under Load

Abstract: The strain generated by an actuator material when subjected to an external force is a key performance parameter. However, very little is known about the performance of polymer actuator materials when subjected to external loads. Increasing external loads were applied to polypyrrole films and the actuator response was found to decrease linearly. Analysis of the effects of the applied load on actuator strain showed that the rate of strain decrease with increasing load depended upon the difference in the Young’s modulus of the polymer in the doped and undoped states. A simple analytical model shows good agreement with the experimental data.

1,2-Dithienylethene Photochromes and Non-destructive Erasable Memory

Abstract: Monitoring changes in ultraviolet-visible (UV-vis) absorption is not a viable method to process information for photochromic memory media due to the readout signal interfering with the photochromism. Only by monitoring the changes in other photophysical properties accompanying the photoisomerization reaction (refractive index, optical rotation, or luminescence, for example) can non-destructive, all photon-mode photochromic memory be realized. We have investigated several such systems based on 1,2-dithienylcyclopentene derivatives, which have a backbone that we consider to be currently the most promising of the photochromes. The two readout signals highlighted in this article are luminescence and optical rotation. The luminescent systems rely on porphyrinic chromophores tethered to the photochrome directly or through dative bonds. When the macrocycles are irradiated with light at wavelengths outside the absorption range of the photochrome, luminescence is only observed when the 1,2-dithienylcyclopentene backbone exists in its open-state. The self-assembly of a chiral photochromic metallo-helicate allows for stereoselective ring-closing of the 1,2-dithienylcyclopentene backbone providing a change in optical rotation that can be used as a readout signal. In the article, we also describe the use of ring-opening metathesis polymerization (ROMP) to fabricate well-ordered photochromic homopolymers possessing identical photochromic properties as their monomers.

Structural Differences Between Biogenic Amorphous Calcium Carbonate Phases Using X-ray Absorption Spectroscopy**

Abstract: We compare the organization of the first coordination shells around the calcium ion in biogenic ACC phases from three different sources. The results show that although the three biogenic samples have the same chemical composition, which is referred to collectively under the name “amorphous calcium carbonate”, they are structurally different from one another. These differences may be attributed to the diverse modes of formation of such biogenic materials and may account for their known variations in stability.

Tunable, Gap‐State Lasing in Switchable Directions for Opal Photonic Crystals

Abstract: A photonic crystal laser that is tunable throughout the visible in three-dimensionally switchable directions is demonstrated. This photo-pumped laser utilizes a dye-infiltrated, single-crystal SiO2 opal having incomplete bandgaps. Our results support a gap-state-enhanced distributed feedback mechanism for lasing. Three different types of wavelength tunability are demonstrated, each applicable over a different frequency range and involving either single or multiple bandgaps. The many independent laser cavities that exist in one photonic crystal are demonstrated by simultaneously obtaining lasing in various colors and directions from an opal crystal. The observation of characteristic laser emission lines provides a new spectroscopy for characterizing intra-gap photonic states, which may be useful for developing the photonic crystal analogues of electronic circuitry.

Synthesis and Characterization of MgO Nanowires Through a Vapor‐Phase Precursor Method

Abstract: This paper describes a vapor-phase, precursor method that generates MgO nanowires at relatively low temperatures (800–900 °C) as compared to previous approaches (≥ 1200 °C) that directly use MgO as the source material. Magnesium diboride (MgB2) powders were used as the precursor, which slowly decomposed into Mg and MgB4 under a constant flow of argon gas at temperatures as low as 700 °C. The Mg vapor subsequently reacted with trace O2 contained in the reaction system and formed MgO vapor. Under appropriate conditions, the MgO vapor could reach supersaturation, condense onto the surface of a solid substrate that was placed on top of the precursor powders, and grow into highly anisotropic nanostructures. Depending on the distance between the MgB2 source and the location on the solid substrate, a gradient in the MgO concentration existed. As a result, MgO nanostructures with different morphologies were formed on the solid support, including whiskers, tapered nanowires whose diameters were continuously reduced from ∼200 to 20–30 nm over a distance of ∼50 μm, and nanowires having uniform diameters (15–20 nm) and lengths up to ∼30 μm. When a mixture of dihydrogen and argon gases was used as the reaction atmosphere, MgO nanowires with a uniform cross-section of ∼150 nm in diameter could grow several millimeters long without branching.

Biomimetic crystallization of calcium carbonate spherules with controlled surface structures and sizes by double-hydrophilic block copolymers

Abstract: Big CaCO3 spherules with controlled surface structures and sizes ranging from several hundreds of nanometers and micrometers can be easily fabricated through a slow gas–liquid diffusion reaction at room temperature by using double-hydrophilic block copolymers (DHBCs) as crystal modifiers. The influence of the DHBCs with different functionalities such as carboxyl, partially phosphated, and phosphorylated groups on the crystallization and structure of calcite formation was investigated. The morphology evolution process and the early stage of the big spherical superstructures were followed. Such big spherules with complex surface structure made of calcite rhombohedra are not easily produced by conventional solution growth methods, and furthermore show high potential for chromatographic purposes due to their exposition of multiple calcite faces and the huge particle sizes suitable for chromatographic column packings.

Emergent Nanostructures: Water-Induced Mesoscale Transformation of Surfactant-Stabilized Amorphous Calcium Carbonate Nanoparticles in Reverse Microemulsions

Abstract: The water-induced crystallization of alkylbenzenesulfonate-coated amorphous calcium carbonate (ACC) nanoparticles in water-in-isooctane sodium bis(2-ethylhexyl)sulfosuccinate (NaAOT) microemulsions at a [H2O]/[NaAOT] molar ratio (w) of 10:1 produces a range of organized hybrid surfactant–vaterite nanostructures depending on the water droplet/ACC nanoparticle number ratio, n = [H2O]/[CaCO3]. The crystalline nanostructures develop within primary aggregates of the surfactant-stabilized ACC nanoparticles by in situ mesoscale transformation, which is mediated by the extent of coupling at the surfactant–inorganic interface. Strong coupling in the presence of low amounts of water (n = 34) gives monodisperse spheroidal aggregates of densely packed 5 nm diameter surfactant-coated vaterite nanoparticles, whereas weak interactions at n = 3400 produce discrete vaterite nanoparticles, 130 nm in size. Significantly, intermediate levels of coupling produce anisotropic nanostructures such as spindle-shaped aggregates of 18 nm sized surfactant-coated vaterite nanoparticles (n = 170) and high-aspect-ratio bundles of co-aligned 10 nm wide twisted vaterite nanofilaments (n = 340). Adding excess aqueous CO32– to the microemulsion droplets inhibits the growth of the nanofilaments, whereas excess Ca2+ has no effect. The results show that the transformation pathways are determined by the extent of water penetration into the ACC cores and electrostatic interactions at the mineral–surfactant interface, and indicate that complex hybrid nanostructures can be assembled in situ when these processes are coupled synergistically at the mesoscopic level. Such observations could be of generic importance in nanochemistry and biomineralization.

Templated Synthesis and Structural Study of Densely Packed Metal Nanostructures in MCM‐41 and MCM‐48

Abstract: A template synthetic method to prepare densely packed metal nanostructures in functionalized (MCM)-41 and MCM-48 is described. The intrachannel surface of host silica has been functionalized to carry positive charges for the accommodation of highly concentrated and negatively charged metal complexes. After reduction, Au and Pt nanowire bundles in MCM-41 as well as Pd nanowire networks in MCM-48 are formed. The Pt nanowire bundles in MCM-41 are observed to grow along a preferred direction and stack along Pt {111} planes relative to the pore wall of the host. Furthermore, bimetallic AuPt alloy nanowire bundles in MCM-41 have also been prepared and characterized.

Thermoplastic elastomer hydrogels via self-assembly of an elastin-mimetic triblock polypeptide

Abstract: Synthetic polymers consisting of well-defined blocks of compositionally dissimilar monomers undergo microscopic phase separation in the solid state and in selective solvents to afford ordered microstructures that display unique, technologically significant properties in comparison to blends of the respective homopolymers. [1] However, the synthetic repertoire of these materials has been limited to tapered blocks of uniform sequence, which potentially restricts the functional complexity of the resulting microstructures. Genetic engineering of synthetic polypeptides enables preparation of block copolymers composed of complex sequences in which the individual blocks may have different mechanical, chemical, or biological properties. [2‐6] The segregation of the protein blocks into compositionally, structurally, and spatially distinct domains should occur in analogy with synthetic block copolymers, affording ordered structures on the nanometer to micrometer size range. The utility of these protein materials depends on the ability to functionally emulate or enhance the materials properties of conventional polymer systems, while retaining the benefits of greater control over the sequence and microstructure that protein engineering affords for the construction of materials. We report herein the genetically directed synthesis and characterization of a triblock copolymer 1 that is derived from an elastin-mimetic polypeptide sequence in which the respective blocks exhibit different mechanical properties (Scheme 1). Moreover, the sequences of the respective blocks were chosen such that the polypeptide undergoes reversible microscopic phase separation from aqueous solution to form a thermoplastic elastomer hydrogel above a lower critical solution temperature Tt.

Biomaterials by Design: Layer‐By‐Layer Assembled Ion‐Selective and Biocompatible Films of TiO2 Nanoshells for Neurochemical Monitoring

Abstract: Titania nanoshells with an external diameter of 10–30 nm and a wall thickness of 3–5 nm were prepared by dissolving the silver cores of Ag@TiO2 nanoparticles in a concentrated solution of ammonium hydroxide. The nanoshells were assembled layer-by-layer (LBL), with negatively charged poly(acrylic acid) (PAA) to produce coatings with a network of voids and channels in the interior of the film. The diameter of the channels in the titania shells was comparable to the thickness of the electrical double layer in porous matter (0.3–30 nm). The prepared nanoparticulate films demonstrated strong ion-sieving properties due to the exclusion of some ions from the diffuse region of the electrical double layer. The permeation of ions could be tuned effectively by the pH and ionic strength of a solution between “open” and “closed” states. The ion-separation effect was utilized for the selective determination of one of the most important neurotransmitters, dopamine, on a background of ascorbic acid. Under physiological conditions, the negative charge on the surface of TiO2 facilitated the permeation of positively charged dopamine through the LBL film to the electrode, preventing the access of the negatively charged ascorbic acid. The deposition of the nanoshell/polyelectrolyte film resulted in a significant improvement to the selectivity of dopamine determination. The prepared nanoshell films were also found to be compatible with nervous tissue secreting dopamine. Although the obtained data demonstrated the potential of TiO2 LBL films for implantable biomedical devices for nerve tissue monitoring, the problem of electrode poisoning by the by-products of dopamine reduction has yet to be resolved.

Photoresponsive Hydrogel Microstructure Fabricated by Two‐Photon Initiated Polymerization

Abstract: A photoresponsive polymeric hydrogel cantilever that deflects under illumination has been fabricated by using two-photon three-dimensional lithography. The hydrogel was prepared from a comonomer solution containing acryloylacetone, acrylamide, and N,N′-methylene bisacrylamide. The photoresponse of the cantilever was activated by photoexcitation of acetylacetone groups at 244 nm. Deflection of the cantilever by ∼ 45° was effected upon UV irradiation for 20 min.

Synthesis and Characterization of Crystalline Ag2Se Nanowires Through a Template‐Engaged Reaction at Room Temperature

Abstract: Single-crystalline Ag2Se nanowires have been successfully synthesized through a template-engaged topotactic reaction in which nanowires of trigonal selenium were transformed into Ag2Se by reacting with aqueous AgNO3 solutions at room temperature (RT). An interesting size-dependent transition between two crystal structures has also been observed for this newly synthesized one-dimensional system: The Ag2Se nanowires adopted a tetragonal structure when their diameters were less than ∼40 nm; an orthorhombic structure was found to be more favorable as the diameter of these nanowires was increased beyond 40 nm. Since this reaction can be carried out at ambient pressure and temperature, it should be straightforward to scale up the entire process for the high-volume production of Ag2Se nanowires with well-controlled sizes and crystal structures. These highly uniform nanowires of single-crystalline Ag2Se are potentially useful as photosensitizers, superionic conductors, magnetoresistive compounds, or thermoelectric materials. This work also represents the first demonstration of a template-engaged process capable of generating single-crystalline nanowires from the solution-phase and at RT.

Fabrication of DAST Channel Optical Waveguides

Abstract: In order to use the large, electro-optic coefficient of a nonlinear optical ionic crystal, 4-(p-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST), a channel optical waveguide structure is needed. We successfully fabricated a waveguide using two methods: by a dry-etching technique and by photo-bleaching. Because DAST has a large optical loss, parts of the waveguide should be composed of a transparent polymer. We used photolithography and a reactive ion etching method to fabricate a serially grafted (conjunct) waveguide of DAST with a transparent polymer waveguide. The waveguide was also fabricated by photobleaching, whereby the refractive indices of the crystal’s a- and b-axes were decreased by degrading the crystal. The cladding part of the DAST waveguide was photobleached by irradiating with UV light. The under- and over-cladding layers of these channel waveguides were composed of a UV-cured resin that did not dissolve the DAST crystal. The loss of the crystal waveguide for the crystal b-direction was around 10 dB/cm, due to the scattering loss of the DAST single crystal.

Silica-carbon nanocomposites: A new concept for the design of solar absorbers

Abstract: To create materials that are composites or hybrids structured on the nanometer scale or the meso-domain, respectively, is one of the major tasks in modern materials science. In this paper, we demonstrate general strategies on how to obtain these nanocomposites founded on the knowledge about ordered mesoporous materials. One strategy involves the formation of the composite by performing a chemical reaction in the pores of a pre-formed ordered mesoporous silica while the other strategy uses compounds that first mold their porous environment in the silica and in a succeeding step react to the final composite. As a model system, here, we present the formation of porous silica–carbon hybrid materials. Besides this more general question, we also tackle the task of finding a suitable application for the obtained nanocomposites. We chose an application as selective solar-absorber materials.