Synthetic Metals (v.154, #1-3)
In-situ polymerized polyaniline films 6. FTIR spectroscopic study of aniline polymerisation by M. Trchová; I. Šeděnková; J. Stejskal (1-4).
Polyaniline was prepared by the oxidative polymerization of aniline hydrochloride with ammonium peroxydisulfate in aqueous medium. The progress of polymerization was monitored in situ by FTIR spectroscopy after depositing a droplet of the reaction mixture on a ZnSe crystal. The growth of polyaniline film at the crystal surface, as well as changes proceeding in the surrounding aqueous phase, is reflected in the spectra. The processes occurring during polymerization are discussed with the help of differential spectra.
Keywords: Polyaniline; Thin films; Infrared spectroscopy;
Preparation of conducting core-shell nanoparticles based on dendritic hexadecaaniline-C60-poly(dimethylsiloxane) conjugates by Taizoon Canteenwala; Prashant A. Padmawar; Sambhaji Patil; Manas Haldar; Long Y. Chiang (5-8).
A new class of macromolecular core-shell-like elastomeric conducting nanoparticles based on triblock (A16)5-C60-PDMS-C60-(A16)5 analogous dendritic structure were fabricated using flexible poly(dimethylsiloxane) (PDMS) as the elastomeric segment in linking with conducting C60-(hexadecaaniline)5 [C60(A16)5] subunits. In polar solvents, the PDMS segments coagulated into nanosized particles encapsulated with self-assembled C60(A16)5 shell. Spherical particles in a diameter of 50–80 nm were achieved and observed in scanning electron micrographs.
Keywords: Cyclosulphatedfullerene; Hexadecaaniline; nitrofullerene; hexadecaanilinofullerene; Poly(dimethylsiloxane); Conducting core-shell nanoparticles;
Measuring the Tilt Angle of ODTMS Self-Assembled Monolayers on Al Oxide Surfaces by L. Thomsen; B. Watts; D.V. Cotton; P.C. Dastoor (9-12).
Adsorbed octadecyltrimethoxysilane (ODTMS) on native aluminium oxide substrates have been studied with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to determine the average tilt angle of the surface film. Previous experiments have shown that the time-dependant adsorption isotherm of ODTMS on Al exhibits oscillatory adsorption. In this work we present the findings of our tilt angle studies for Al samples dipped in a 0.75% ODTMS solution at various times during oscillatory adsorption. It clearly can be seen that the alignment of the surface film is related to the coverage, hence for a minimum in the film coverage (at 65 s dip time) the tilt angle of the ODTMS film is found to be ∼50° relative to the surface normal. Below the minimum in the film coverage (at 30 s dip time) the tilt angle has been found to be ∼41° whereas for samples dipped above the minimum film coverage (at 80 and 95 s) the angles are ∼46° and ∼45° respectively. These measurements indicate a strong correlation between film alignment and film coverage during oscillatory adsorption.
Keywords: X-ray absorption spectroscopy; Polycrystalline surfaces; Organic/inorganic interfaces; Coatings; Self-assembly using surface chemistry; Order-disorder phase transitions;
Doping effect for ionic conductivity in DNA film by Yasumitsu Matsuo; Kazutaka Sugita; Seiichiro Ikehata (13-16).
We have investigated the role of ions such as H+ and Rb+ in the electrical conductivity of DNA films. On the basis of the relative humidity dependence of electrical conductivity, the electrical conductivity of DNA films is found to strongly depend on relative humidity h and its dependence comprises three regions: (I: h < 17%), (II: 17% < h < 59%), and (III: h > 59%). Moreover, the 1H-NMR absorption line becomes narrow in regions (II) and (III) due to the appearance of mobile protons. These results indicate that mobile protons play an important role in the electrical conductivity in regions (II) and (III). Furthermore, based on these results, it is deduced that the electrical conductivity in region (II) is caused by proton hopping from one PO4 tetrahedron to another accompanied by the breaking and rearrangement of the hydrogen bond. In addition, we have also observed that the electrical conductivity increases by doping rubidium ions into DNA film.
Keywords: Nuclear magnetic resonance spectroscopy; Other conjugated and/or conducting polymers; Semiconducting films;
DNA-modified Carbon Nanotubes for Self-assembling and Biosensing Applications by Pingang He; Sinan Li; Liming Dai (17-20).
We have demonstrated that specific DNA sequences could be covalently immobilized onto acid-oxidized and plasma-activated carbon nanotubes. While various functional supramolecular structures could be prepared by self-assembling of the acid-oxidized carbon nanotubes attached with DNA chains of complementary sequences, the DNA-immobilized aligned carbon nanobues have been demonstrated to be significant for sensing complementary DNA and/or target DNA chains of specific sequences with a high sensitivity and selectivity.
Keywords: Carbon nanotube; DNA; Self-assembling; Biosensing;
Synthesis of Well-defined Block Copolymer Electrolytes Using Atom Transfer Radical Polymerization by S.M. Chang; H.W. Liao; C.L. Lin; J.T. Lee (21-24).
A series of well-defined hydrophobic-hydrophilic di-blocks has been synthesized under environmentally friendly conditions through atom transfer radical polymerization (ATRP). The hydrophilic block is based on methoxy-capped oligo(ethylene glycol) methacrylate (nEGMA, n = 4∼5, 8∼9) and the hydrophobic component is PMMA block. ATRP prepared PMMA-Br (Mn 12000 Mw/Mn 1.37) was applied as a macroinitiator. Two solvent systems, acetone and mixed solvents of toluene/H2O (1/10), were employed. The acetone mediated polymerization gave the copolymer good control of molecular weight distribution (1.2) and rapid conversion. The toulene/H2O mediated polymerization provided the copolymer with a higher molecular weight but a less narrow polydispersity. The maximum conductivity was measured as 4.8 × 10−5 S/cm at 90 °C with di-block coplymer, (PMMA)x-b-(PnEGMA)y (n = 8∼9, x/y ratio 3/1, contained ∼67 wt.% PEGMA), doped with Li salt ([EO:Li+] = 20:1). The conductivity at room temperature was ca 2 × 10−6 S/cm.
Keywords: Atom transfer radical polymerization (ATRP); Co-polymer; Electrolyte;
The Development and Characterisation of Conducting Polymeric-based Sensing Devices by S. Brady; K.T. Lau; W. Megill; G.G. Wallace; D. Diamond (25-28).
This project is focused on developing novel smart textiles based on conducting polymer coatings deposited on a foam substrate. This process renders the foam conductive, while retaining the tactile properties of the original material. The foam is a soft, porous and conductive solid, made by chemically oxidising pyrrole in the presence of a polyurethane foam. The 3-D structure of this material means it is sensitive to pressures exerted from all three dimensions, making it attractive for use in wearable sensors for sport and medical applications. A potential application under investigation is the development of a smart insole for patients with Diabetes Mellitus, who require constant monitoring of the pressure exerted underfoot during walking or standing in order to reduce the risk of damaging their feet due to excess pressure being applied. Future applications for this material may also lie in the area of wearable electronic components, whereby the material can be fabricated to produce resistors, capacitors, etc. The gas sensing capabilities of this material were also investigated. Results have shown that this smart fabric based chemical sensor offers higher selectivity towards ammonia over other volatile organic vapours.
Keywords: Polypyrrole; Wearable sensors; Smart textiles; Pressure sensors; Chemical sensors;
Conductive Fibre Prepared From Ultra-High Molecular Weight Polyaniline for Smart Fabric and Interactive Textile Applications by D. Bowman; B.R. Mattes (29-32).
Panion™ fibre is prepared from a high molecular weight version of chlorine and defect free polyaniline (PANI). It is processed into textile fibre by continuous wet spinning techniques. The room temperature DC conductivity for unstretched and stretched (5x at 373 K) high molecular weight samples was 72 and 725 Ω−1 cm−1, respectively. Resistivity measurements of Panion™, from 4 to 350 K, indicate that the fibre manufacturing process and the polymer synthetic route both serve to minimize structural disorder in the solid-state. The transport resides on the metallic side of the metal-insulator (M-I) phase boundary for unstretched and stretched fibre samples prepared from high Mw PANI emeraldine base. The monofilament fibre can be formed into yarns for weaving fabric, braided into 2- and 3- dimensional structures, or used to form knitted fabrics, all with tunable electronic functionality.
Keywords: Tensile drawing; Transport measurements; Conductivity; Metal-insulator phase transitions; Polyaniline; Fibres;
Towards fabrication of synthetic metal nanowires by D.V. Cotton; L. Thomsen; B. Watts; P.C. Dastoor (33-36).
We characterise the adsorption of a simple organofunctional silane (PTMS) on a böhmite (γ-Al2O3·H2O) substrate and compare it to adsorption on the native oxide of aluminium. The adsorption isotherm of PTMS on böhmite is found to resemble the isotherm for the adsorption on the native oxide in structure but with a significantly reduced coverage. The contrast in adsorption properties on the two substrates provides a method of patterning potentially suitable for the fabrication of a synthetic metal nanowire.
Keywords: Manipulation of surface structure and morphology; Self-assembly using surface chemistry; Organic/inorganic interfaces; Photoelectron spectroscopy; Scanning electron microscopy;
Electrospun polymer nanofiber sensors by D. Aussawasathien; J.-H. Dong; L. Dai (37-40).
Using the electrospinning technique, we have prepared lithium perchlorate (LiClO4) doped-polyethylene oxide (PEO) electrospun nanofibers for humidity sensing and camphosulfonic acid (HCSA) doped-polyaniline (PANI)/polystyrene (PS) electrospun nanofibers for sensing hydrogen peroxide (H2O2) and glucose. The diameters of these as-prepared polymeric nanofibers are in the range of 400–1000 nm. Owing to the large surface area and good electrical properties intrinsically associated with these nanoscale functional polymer fibers, we achieved significantly enhanced sensitivity for the nanofiber sensors in respect to their corresponding film-type counterparts. While scanning electron micrographs showed some distortions for the LiClO4 doped-PEO nanofibers after the humidity measurements, no observable morphological change was seen for the HCSA doped-PANI/PS nanofiber sensors after the measurements of H2O2 and/or glucose.
Keywords: Electrospun nanofibers; Lithium perchlorate doped-polyethylene oxide; Camphosulfonic acid doped-polyaniline/polystyrene; Sensors; Humidity sensing; Glucose sensing;
Temperature and Time Dependence of Heat Treatment of RR-P3HT/PCBM Solar Cell by Kanzan Inoue; Ross Ulbricht; Pallavi C. Madakasira; William M. Sampson; Sergey Lee; Jose Gutierrez; John Ferraris; Anvar A. Zakhidov (41-44).
A detailed study of the postproduction heat treatment of organic solar cells based on regio-regular poly(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) is presented. The efficiency of the devices was significantly improved by postproduction heat treatment and both the optimal annealing temperature and optimal annealing time were determined. The phase separation of PCBM and RR-P3HT into bi-continuous network structure occurs within a very short period of time and is very stable.
Keywords: Phase-segregated composite interfaces; PCBM; Poly(3-hexylthiophene); Organic photovoltaic; Cells;
ESR observations of electric-field-induced polarons in regioregular poly(3-alkylthiophene)s in MIS diode structures by K. Marumoto; Y. Muramatsu; S. Ukai; H. Ito; S. Kuroda (45-48).
Electron spin resonance (ESR) measurements have been performed on metal-insulator-semiconductor (MIS) diodes formed with a regioregular poly(3-alkylthiophene) (RR-PAT) film as a semiconductor in order to study the spin states of the field-induced charge carriers in the RR-PAT film. The structure of the MIS diodes fabricated is Al/Al2Al2O3(insulator)/RR-PAT(semiconductor)/Au. We have successfully observed an electric-field-induced ESR signal by applying voltage across the Al and Au electrodes of the MIS diode. The ESR intensity increases under negative bias applied to the Al electrode owing to the accumulation of holes at the interface between Al2O3 and RR-PAT. The observed ESR signal is consistent with that of positive RR-PAT polarons detected by light-induced ESR measurements on RR-PAT/C60 composites. Therefore, the present results directly show that the field-induced carriers are polarons.
Keywords: Electron spin resonance; Semiconducting films; Metal-oxide-semiconductor (MOS) structures; Polythiophene and derivatives; Fullerenes and derivatives;
Organic photovoltaic effects using CuPc and C60 depending on layer thickness by Sung Woo Hur; Hyun Seok Oh; Yong Cheul Oh; Dong Hoe Chung; Joon Ung Lee; Jong Wook Park; Tae Wan Kim (49-52).
We have studied photovoltaic effects in ITO/CuPc/Al and ITO/CuPc/C60/BCP/Al devices to investigate the effect of layer thickness of CuPc, C60, and BCP exciton blocking layer. The thickness of CuPc single layer was varied from 10 nm to 50 nm using thermal vapor deposition, and we obtained optimum current density-voltage characteristics of ITO/CuPc/Al for 40 nm thick CuPc layer. From the thickness-dependent photovoltaic effects in CuPc/C60 heterojunction devices, higher power conversion efficiency was obtained for ITO/20 nm CuPc/40 nm C60/Al, which has a thickness ratio (CuPc:C60) of 1:2. The BCP layer was introduced as an exciton blocking layer, and there was a pronounced improvement of conversion efficiency with the use of BCP layer.
Keywords: Solar cell; Photovoltaic effect; Exciton blocking layer;
Design, Synthesis and Properties of Low Band Gap Polyfluorenes for Photovoltaic Devices by Erik Perzon; Xiangjun Wang; Fengling Zhang; Wendimagegn Mammo; Juan Luis Delgado; Pilar de la Cruz; Olle Inganäs; Fernando Langa; Mats R. Andersson (53-56).
Two new alternating polyfluorene copolymers containing different low band gap segments have been designed and synthesized for use in optoelectronic devices. The low band gap segments consist of electron donating and accepting moieties which induce a partial charge transfer in the polymer backbone and thereby, a lower band gap. Results obtained from characterization of the new materials, called APFO-Green1 and APFO-Green2, include absorption and emission at extended wavelengths as well as high stability. Furthermore, blends of the new copolymers with different fullerene derivatives exhibit unprecedented photovoltaic behavior at long wavelengths.
Keywords: Low-bandgap conjugated polymers; Solar cells; UV-Vis-NIR absorption; Fullerenes; Coupling reactions;
A New Approach to the Synthesis of Nanocrystal Conjugated Polymer Composites by A.A.R. Watt; H. Rubinsztein-Dunlop; P. Meredith (57-60).
A novel one pot process has been developed for the preparation of PbS nanocrystals in the conjugated polymer poly 2-methoxy, 5-(2 - ethyl-hexyloxy-p-phenylenevinylene) (MEH-PPV). Current techniques for making such composite materials rely upon synthesizing the nanocrystals and conducting polymer separately, and subsequently mixing them. This multi-step technique has two serious drawbacks: templating surfactant must be removed before mixing, and co-solvent incompatibility causes aggregation. In our method, we eliminate the need for an initial surfactant by using the conducting polymer to terminate and template nanocrystal growth. Additionally, the final product is soluble in a single solvent. We present materials analysis which shows PbS nanocrystals can be grown directly in a conducting polymer, the resulting composite is highly ordered and nanocrystal size can be controlled.
Keywords: Conducting polymer; Nanocrystal; Poly(phenylene vinylenes);
The role of polymers in the luminescence conversion of sunlight for enhanced solar cell performance by B.S. Richard; A. Shalav (61-64).
The application of luminescent devices to existing silicon solar cells has the potential to bridge the gap between first and third generation photovoltaics (PV) and enhance the energy conversion efficiency. Two mechanisms that limit the efficiencies of conventional solar cells are, firstly, the transmission of sub-bandgap light and, secondly, the thermalisation of charge carriers generated by the absorption of photons with an energy greater than the silicon bandgap (E g ) of the semiconductor. In this paper we discuss ways of reducing these losses via the application of passive optical devices called up- and down-converters, respectively. Down-conversion (DC) results in the generation of more than one lower energy photon (>E g ) being generated per incident high-energy photon (>2E g ), while up-conversion (UC) generates one photon with energy >E g for every two or more sub-bandgap (<E g ) photons absorbed. A final process, down-shifting (DS), is similar to DC except that the external quantum efficiency of the DS process is less than unity. For a low cost and large area PV product, the luminescent centres are incorporated into large area polymer sheets. The desirable properties of such polymer hosts are compared for the three scenarios of UC, DC and DS.
Keywords: Solar cells; Photoluminescence; Non-linear optical methods; Waveguides; Components; polarisers; windows;
Optimizing Organic Solar Cells in Both Space and Energy Domains by Sam-Shajing Sun; Carl E. Bonner (65-68).
Optimization of organic solar cells in both space and energy domains has been preliminarily investigated, both experimentally and theoretically, in order to minimize the ‘photon loss’, the ‘exciton loss’ and the ‘carrier loss’. In spatial domain optimization, for instance, thin films of -donor-bridge-acceptor-bridge- (-DBAB-) type block copolymers exhibited much better photoluminescence quenching and photoconductivity in comparison to corresponding donor/acceptor simple blend films. This enhancement of photo-electronic conversion was attributed mainly to the improved spatial bicontinuous morphologies in the -DBAB- block copolymer. In energy domain optimization, theoretical simulation reveals that, an optimum exciton-charge conversion can be achieved when the LUMO level offset of donor/acceptor is close to the sum of exciton binding energy and the charge separation reorganization energy.
Keywords: Polymer solar cells; Spatial optimizations; Energy optimizations; Conjugated block copolymers;
Hydrogen generation using PPy-FMS modified PVDF membrane and other substrates by Chunming Yang; Jun Chen; Gerhard F. Swiegers; Chee O. Too; Gordon G. Wallace (69-72).
Conducting polypyrrole (PPy) containing ferrocene monosulfonate (FMS) as counter-ion, was successfully electrodeposited onto Pt-coated PVDF membrane, Pt-coated PVDF hollow fiber, stainless steel mesh, and carbon fiber paper. Post-polymerisation cyclic voltammograms of these PPy-FMS-modified substrates showed that ferrocene had been incorporated into the polymer. Scanning electron microscopy (SEM) was employed to examine the morphology of PPy-FMS synthesized at different conditions. Hydrogen gas generation in 1.0 M H2SO4 using these different PPy-FMS-modified substrates demonstrated that the PPy-FMS-modified Pt/PVDF membrane exhibited the greatest catalytic effect.
Keywords: Hydrogen generation; Pyrrole; Ferrocene; PVDF membrane;
Syntheses of polythiophene and titania nanotube composites by Ming-De Lu; Sze-Ming Yang (73-76).
A composite of polythiophene (PT) and titania nanotubes was synthesized. X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), fourier transformed infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) were used to characterize these nanocomposites. The interaction between polythiophene and titania was analyzed by XPS and TGA. The XPS spectra of the composites show that the Ti2p peak shifts to a lower binding energy and S2p peak shifts to a higher binding energy. The TGA results also show that phase segregation occured when the nanocomposites contained 35% polythiophene.
Keywords: Polythiophene and derivatives; Scanning electron microscopy; X-ray diffraction;
Photoconductivity excitation spectrum for stretch-oriented PPV by Paulo B. Miranda; Lucas F. Santos; Francoise T. Reis; Luis V.A. Scalvi (77-80).
We have obtained the photoconductivity (PC) excitation spectrum for a stretch-oriented poly(paraphenylene vinylene) film over a wide spectral range (up to 5 eV). The measurements were performed in the surface cell configuration with the electric field parallel or perpendicular to the stretch direction. Although the sample had a stretch ratio of ∼4, the dark conductivity and the steady-state photoconductivity were both about 40 and 20 times higher with the electric field parallel to the average chain direction, respectively. However, the shape of the PC excitation spectrum was independent of field direction and did not show a significant rise in the ultraviolet, as is usually observed for measurements in the photodiode configuration. The implications of these results to the charge photogeneration mechanism in conjugated polymers are discussed.
Keywords: Photoconductivity; Poly(phenylene vinylene) and derivatives; Semiconducting films;
Photophysical study of two alternating polyfluorene copolymers exhibiting dual fluorescence by J. Morgado; A. Charas; J.M.G. Martinho; L. Alcácer (81-84).
We present a photophysical study of two fluorene alternating coporymeis in dilute solutions. The UV-Vis absorption spectra of the copolymers show two absorption bands in the 300–600 nm wavelength range. In spite of the alternating nature of the copolymers the fluorescence spectrum, recorded by excitation within the higher energy absorption band, is also composed of two bands. The excitation spectra show that the lower energy emission band has contributions from both absorption bands, while the picosecond time-resolved fluorescence studies do not provide evidence for any energy transfer process, meaning that, if it occurs, is faster than the time resolution of our set up (≈5 ps).
Keywords: Optical absorption and emission spectroscopy; Time-resolved fast spectroscopy; Other conjugated and/or conducting polymers;
Quadrimolecular recombination kinetics of photogenerated positive polarons and C60 − radical anions in regioregular poly(3-alkylthiophene)/C60 composites measured by light-induced ESR by K. Marumoto; T. Sakamoto; Y. Muramatsu; S. Kuroda (85-88).
Light-induced electron spin resonance (LESR) measurements have been performed to study recombination kinetics of photogenerated charge carriers in the composites of regioregular poly(3-alkylthiophene) (RR-PAT) and C60. Two transient LESR signals due to photogenerated positive RR-PAT polarons and C60 − radical anions are remarkably enhanced for the excitation energy of ∼1.8 eV. We observe an ∼ I ex 0.25 power dependence of the prompt decay LESR component on excitation-light intensity I ex. This behavior is well explained by a model of quadrimolecular recombination (QR) of the photogenerated charge carriers. The QR is dominant at higher I ex while bimolecular recombination (BR) is relatively important at lower I ex. The time decay of the LESR intensity is also well explained by the QR model. The temperature dependence of the QR rate constant shows a crossover from tunneling to hopping behavior with increasing temperature; the activation energy for the hopping of 0.10 eV is obtained. In the regiorandom PAT/C60 composites, only BR is observed.
Keywords: Electron spin resonance; Semiconducting films; Polythiophene and derivatives; Fullerenes and derivatives; Solar cells;
Preparation and proton transport property of N,N′- diethyldithiooxamidatocopper coordination polymer by Y. Nagao; T. Kubo; K. Nakasuji; R. Ikeda; T. Kojima; H. Kitagawa (89-92).
A novel proton-conductive copper coordination polymer, (H5C2)2 dtoaCu (R2 dtoaH2 = dithiooxamide derivatives), was synthesized. From X-ray powder diffraction measurements, the crystal structure of (H5C2)2 dtoaCu was found to be similar to that of a two-dimensional (2-D) coordination polymer (HOH4C2)2 dtoaCu. This title coordination polymer was revealed to be a proton conductor by the relative humidity (RH) dependence of AC conductivity measurements. The proton conductivity (σ p) was 4.2 × 10−6 S cm−1 under the RH of 100% and the ionic transport number was more than 0.99. The σ p of (HOH4 dtoaCu was two orders of magnitude higher than that of (H5C2)2 dtoaCu, which would be derived from the existence of-OH groups in the alkyl substituent R.
Keywords: Transport measurements; Ion conductivity; Conjugated conducting polymers; Fuel cells;
Towards processable polyether-functionalized poly(3’-styrylterthiophenes) by D.K. Grant; D.L. Officer (93-96).
We have investigated the chemical and electrochemical polymerizations of polyether-functionalized 3’-styrylterthiophene monomers. A comparison of the UV/VIS/NIR spectra of the polymerization products formed from four of these styrylterthiophenes that were either unsubstituted or incorporated decyloxy groups at the 4 and 4”-positions, provides evidence for the formation of significantly longer-chain materials from the decyloxy-substituted compounds.
Keywords: Polythiophene and derivatives; Special-purpose functionalized conjugated polymers; Electrochemical polymerization; UV-Vis-NIR absorption; Coupling reactions;
Synthesis and properties of polyfluorenes with phenyl substituents by L.J. Lindgren; X. Wang; O. Inganäs; M.R. Andersson (97-100).
Two new, alkoxyphenyl-substituted polyfluorenes, poly(9,9-bis(4′-hexyloxyphenyl)fluorene) (PPF-1) and poly(9,9-bis(4′-hexyloxyphenyl)fluorene-alt-9,9-bis(4′-(2″-(2″′-methoxyethoxy)ethoxy)phenyl)fluorene) (PPF-2), were synthesised using the Suzuki coupling reaction. The polymers were designed to obtain stable and efficient materials for light-emitting applications. Both polymers exhibit high PL-efficiencies and show PL- and EL-emission in the blue spectral region, with emission maxima in a narrow range around 427 nm. Annealing (200 °C, 2 h) of polymer films exposed to normal indoor light in ambient atmosphere, results in an additional emission band at higher wavelengths, indicating degradation by photo oxidation.
Keywords: Polyfluorenes; Light sources; Photoluminescence; Electroluminescence; Coupling reactions;
Origin of Free Charge Carrier Generation of the Second-order Process Photocurrent in Conjugated Polymer Studied by Two-Correlated-Pulse Technique by Y Nishihara; A. Matsuda; A. Fujii; M. Ozaki; E.L. Frankevich; K. Yoshino (101-104).
Kinetics of decay of polaron pairs as precursors of free charge carriers at photoexcitation of poly(3-octylthiophene) under applied electrical field have been observed directly within a picosecond time domain by using the two-correlated-pulse laser technique. Experiments have shown that polaron pairs were generated without any noticeable delay after the light absorption. At high enough excitation intensity, free charge carriers have been shown to take part in the second-order processes photocurrent produced by the recombination of two neighboring polaron pairs. It has been clarified that polaron pairs with two kinds of lifetime τ fast , τ delay exists, and τ delay decreases by high electrical field strength, and that polaron pair is the main origin of the second-order photocurrent. The direct observation of photoluminescence within a picosecond time domain has been also discussed.
Keywords: Photoconductivity; Photoluminescence; Polythiophene and derivatives;
Fabrication of patterned polypyrrole on fluoropolymers for pH sensing applications by W. Prissanaroon; N. Brack; P.J. Pigram; P. Hale; P. Kappen; J. Liesegang (105-108).
Patterns of polypyrrole (PPy) have been fabricated on flexible poly(tetrafluoroethylene) (PTFE) films using a combination of micro-contact printing, electroless deposition of copper and electropolymerisation of pyrrole. A patterned elastomeric stamp was used to deliver a nitrogen-containing silane coupling agent to an argon plasma-pretreated PTFE surface. The surface was subsequently activated by PdCl2 and immersed in an electroless copper plating bath allowing selective metallisation. Electropolymerisation of pyrrole was performed on copper-patterned PTFE resulting in the formation of micrometer-scale PPy structures. Each modification and deposition step was characterised by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Potentiometric measurements have demonstrated that PPy-patterned PTFE has potential application as a pH sensor. The response times of the sensors have also been investigated. This study offers an interesting alternative to conventional polymer-based pH sensors in terms of production and mechanical support.
Keywords: Polypyrrole; Electrochemical polymerization; XPS; TOF-SIMS; Microcontact printing; pH sensing;
Self-assembled multilayer films based on functionalized poly(thiophene)s by Y. Takeoka; Y. Iguchi; M. Rikukawa; K. Sanui (109-112).
We study the layer-by-layer assembly and properties of polyelectrolyte multilayers containing cationic and anionic polythiophene derivatives, poly-2-(3-thienyloxy)propyltriethylammonium chloride (PTPA), poly(thiophene-3-acetic acid) (PTAA), and poly(sodium-2-(4-methyl-3-thiehyloxy)ethane sulfonate) (P-SMTE). These polymers were synthesized by chemical oxidation with FeCl3 and soluble in aqueous media. Two types of multilayers have been prepared by sequential adsorption of polyanions and polycations on charged surfaces: PTPA/PTAA and PTPA/P-SMTE. The linear relationship of the UV-vis adsorption spectra and QCM observed in these multilayer systems indicated that each layer was successfully deposited to form layer-by-layer films. FT-IR results also revealed that the multilayer structure was formed by electrostatic interaction.
Keywords: Polythiophene and derivatives; Self-organization in macromolecules;
Studies on electrical transport properties of a novel n-type polymer containing tripylborane and fluorene moieties by N. Sato; H. Ogawa; F. Matsumoto; Y. Chujo; T. Matsuyama (113-116).
An organoboron polymer, poly[(2,4,6-triisopropylphenyl)borylenevinylene-2,7-fluorenylenevinylene], has been synthesized by hydro-boration polymerization with 2,4,6-triisopropylphenylborane and 2,7-diethynylfluorene. The polymer is π-electron deficient due to boron atom contained in its main-chain and the π-conjugation length in it is extended via the vacant p-orbital of the boron atom. It has been found that the polymer shows an electrical conductivity of about 10−6 Scm−1 by n-doping with triethylamine at RT. By continuous photo-excitation of ITO-Polymer-Au cells, it has been proved that the polymer is an n-type conductor. In addition, by pulsed photo-excitation of the cell, it has been verified that the drift mobility of electron is about five times higher than that of hole. The n-type electrical transport properties of the polymer have been also associated with an anodic wave that appears at about −1.17 V in cyclic voltammograms and is ascribable to n-doping.
Keywords: Conjugated polymers; Conducting polymers; Transport measurements; Photoconductivity;
Towards functionalised terthiophene-based polymers by Sanjeev Gambhir; Klaudia Wagner; David L. Officer (117-120).
Terthiophenes functionalised with nitro, amino, cyano, carboxyl, decylcarboxylate, acetic and acrylic acid groups were synthesized using Suzuki coupling. The electrochemical polymerization and spectroscopic data of the films deposited from substituted terthiophene modified at the 3′-position with electron-donating (NH2) and electron-withdrawing (NO2, COOC10H21) groups have been investigated. In addition, effect of alkoxy groups on the electrochemical production of substituted poly(terthiophenes) and characterization of the resulting polymer film was studied.
Keywords: Coupling reactions; Electrochemical polymerization; UV-Vis-NIR absorption;
Synthesis and Optical Properties of Soluble Conjugated Poly(p-Diethynylbenzene)s by Chenjun Shi; Kaixia Yang; Yong Cao (121-124).
A series of novel poly(p-diethynylbenzene)s (PDB) has been prepared by the oxygen-coupling polycondensaton of aryl acetylenes, for the first time. Different side chains including n-decyloxy, n-oclyl, methoxy, and methyl groups were attached to the rigid-rod polymer main chain in order to change the band gap of the polymer. The polymers have good solubility in organic solvents and narrow MWD (∼1.5). Like poly(p-phenyleneethynylene)s (PPEs), poly(p-diethynylbenzene)s (PDBs) have high photoluminescence in solution and the solid state. The photoluminescence quantum yields of PDBs are can reach 12.5%. The studies of UV and photoluminescence indicate that aggregation and intermolecular interactions influence the photophysical properties. The electrochemical study showed the polymers have a good electron-accepting property (Eox = l.l∼1.4 V vs SEC) induced by the electron-withdrawing effect of the triple bonds.
Keywords: Photoluminescence; Electroluminescence; Conducting polymers; Coupling reactions; Electrochemical; Light-emitting diode;
Preparation of hyperbranched copolymers containing triphenylamine and divinylbenzene units by Susumu Tanaka; Tomoaki Iso; Jun-ichi Sugiyama; Kazuhiko Takeuchi; Mitsuru Ueda (125-128).
In the presence of palladium(II) acetate, the coupling reaction of tris(4-bromophenyl)amine 1 and p-divinylbenzene 2 gave a hyperbranched conjugated copolymer 4 in a one step process and in a high yield. Polymer 4 was soluble in organic solvents such as THF and chloroform. GPC indicated that it had an average molecular weight of 5.1 × 103. A cast film had an anodic peak at 1.05 V vs Ag wire. It was blue above the oxidation potential and light-yellow in the neutral state.
Keywords: Transition-metal-catalyzed reactions; Coupling reactions; Hyperbranched polymers; Electrochemical doping; UV-VIS-NIR absorption; Photoluminescence;
Spectral shape of photoluminescence in luminescent π conjugated polymers by T. Kobayashi; T. Endo; M. Rikukawa; K. Sanui; H. Kunugita; K. Ema (129-132).
In polyacetylene and polydiacetylene oligomers, photoluminescence (PL) is induced from the dipole-forbidden state by vibronic coupling. The assumption that the PL in polythiophene (PT) and poly(p-phenylene vinylene) (PPV) also includes this induced component, consistently explains many experimental results. In this work, we focus on the temperature dependence of the time-integrated and time-resolved PL spectra of a PT derivative and unsubstituted PPV, and demonstrate the validity of the above model for their temperature dependence. Applying this model to the hitherto unresolved drastic PL spectral change of unsubstituted PPV, we propose that the spectral change and the spectral shape itself can be understood as resulting from a contribution to the PL from the 2A g state coupled with a change in the ordering of the 2A g and the 1B U states at low temperatures.
Keywords: (Photoluminescence; Polythiophene and derivatives; Poly(phenylene vinylene) and derivatives; Time-resolved fast spectroscopy);
Strong luminescence intensity modulation near a metal-organic interface by C.A.M. Borges; C.A. Rodrigues; R.M. Faria; F.E.G. Guimarães (133-136).
The dependence of the luminescence intensity on the distance between an ultra thin PPV probe layer and a metal surface is investigated. We have employed a new Layer-by-Layer method, which allows control of layer deposition at the monolayer level. A precise variation of the metal/PPV separation over the entire substrate surface was achieved by the deposition of an inert spacer layer. A strong dependence of the PPV emission on the spacer layer and metal thickness is observed. The effect of the morphology of the metal surface is also discussed. These results are explained in terms of a competing short range radiationless energy transfer and intensity modulation due to interference effects produced by the optical cavity formed by metal and polymer films.
Keywords: Optical Properties; Photoluminescence; PPV; Metal electrode effect; Interface; Optical interference;
Electroluminescence and photoluminescence in poly(di-n-octylfluorene) light-emitting diodes operated at elevated temperatures by H. Cheun; M.J. Winokur (137-140).
Both electroluminescence (EL) and photoluminescence from conventional poly(di-n-octylfluorene) (PF8) light emitting diodes (LEDs) has been monitored in situ during thermal cycling at temperatures up to 410 K. Even after the PF8 polymer undergoes a thermotropic phase transition to a liquid crystal phase these LEDs continue to operate although at reduced currents. This behavior is likely caused by changes in the hole transport layer and/or polymer interfaces and not by the phase transition itself. Overall EL output is diminished but PL measurements indicate no evidence of PF8 degradation. The step-wise thermal cycling of a working device produces a decrease in the relative EL efficiency at higher applied bias voltages but results in a pronounced EL efficiency increase at voltages near the device turn-on threshold. EL emission and measured device currents are both time- and temperature-dependent.
Keywords: Photoluminescence; Electroluminescence; Other conjugated and/or conducting polymers; Liquid crystalline phase;
Electrical Effect in Organic Thin-Film Transistors Using Polymerized Gate Insulators by Vapor Deposition Polymerization (VDP) by S.W. Pyo; D.H. Lee; J.R. Koo; J.H. Kim; J.H. Shim; J.S. Kim; Y.K. Kim (141-144).
We report that the organic thin film transistors were fabricated by the organic gate insulators transistor with vapor deposition polymerization (VDP) processing. In order to form polyimide as a gate insulator, vapor deposition polymerization process was also introduced instead of spin-coating process, where polyimide film was co-deposited by high-vacuum thermal evaporation from 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 4,4′-oxydianiline (ODA), and cured at 150 °C for 1 hour followed by 200 °C for 1 hour. Details on the explanation of organic thin-film transistors (OTFTs) electrical characteristics of 6FDA-ODA as gate insulators fabricated thermal co-deposition method.
Keywords: Organic thin-film transistor; Vapor deposition polymerization; Gate insulator;
Enhanced color purity and stability from polymer/nanoporous silica nanocomposite blue light-emitting diodes by Jong Hyeok Park; Tae-Ho Kim; Jae-Woong Yu; Jai Kyeong Kim; Young Chul Kim; O. Ok Park (145-148).
Photo-oxidation plays an important role in the lifetime of PLEDs fabricated from PFs. In addition, most of these PLEDs suffer from a degradation of the device under operation; the degradation is documented in the formation of a low-energy emission band at 2.2–2.3 eV, which turns the desired blue emission color into an undesired blue-green emission. With the aim of simultaneously overcoming all these drawbacks, a blue light-emitting polymer/mesoporous silica nanocomposite was prepared by blending poly(9,9-dioctylfluorene) with surface modified nanoporous silica. This nanocomposite film shows dramatically increased photo-stability and color purity.
Keywords: Light sources; Organic/inorganic interfaces; Semiconducting films;
Vertical type organic light emitting device using thin-film ZnO electrode by Hiroyuki Iechi; Masatoshi Sakai; Kenji Nakamura; Masaaki Iizuka; Masakazu Nakamura; Kazuhiro Kudo (149-152).
We propose a new type organic light emitting transistor (OLET) combining static induction transistor (SIT) with double hetero junction type organic light emitting diodes (OLED) using n-type zinc oxide (ZnO) films which works as a transparent and electron injection layer. The device characteristics of newly developed OLED and ZnO-SIT showed relatively high luminance of about 500 cd/m2 at 7.6 mA/cm2 and is able to control by gate voltage as low as 1 V, respectively. The crystal structure of the ZnO films as a function of Ar/O2 flow ratio and the basic characteristics of the OLET depending on the ZnO sputtering conditions are investigated. The results obtained here show that the OLET with using ZnO film is a suitable element for flexible sheet displays.
Keywords: Organic semiconductors based on conjugated molecules; Light sources; Switches; Sputtering; Evaporation and sublimation; X-ray emission; Diffraction and scattering;
Indium-tin oxide anodes modified by self-assembly for light-emitting diodes based on blue-emitting polyfluorenes by J. Morgado; N. Barbagallo; V. Bodrozic; A. Teixeira; A.C. Femandes; R. Giintner; U. Scherf; F. Cacialli; L. Alcacer (153-156).
We present the electro-optical properties of light-emitting diodes (LEDs) based on two blue-emitting polyfluorenes, with aluminum cathodes and having indium-tin oxide (ITO) anodes modified by self-assembled monolayers (SAM) of benzenesulphenyl chloride derivatives. The modification of ITO surface was confirmed by contact angle measurements. Electroabsorption measurements were used to assess the modification of ITO work function upon SAM formation. We find that the use of a benzenesulphenyl chloride derivative containing a nitro group is as effective at improving LEDs characteristics as the use of a hole-injection layer of well known PEDOT:PSS.
Keywords: Self-assembly using surface chemistry; Electroabsorption; Other conjugated and/or conducting polymers;
New Phosphorescent ppy-based Iridium Complexes Containing Electron-withdrawing Groups by Hyosook Jang; Chang Hwan Shin; Nam Gwang Kim; Kyu Young Hwang; Youngkyu Do (157-160).
For the development of blue-emitting Ir(III) complexes, a set of new ligands, 2-(3-Y-phenyl)-4-methoxypyridine where Y denotes an electron-withdrawing cyanyl (cpmpH) or acetyl group (apmpH), were designed and synthesized. The phosphorescent Ir complexes, IrL2(pic) (L = cpmp; 1, apmp; 2, pic = 2-picolinate) were synthesized via chloro-bridged dimers, [IrL2Cl]2 and characterized by NMR spectra and elemental anaylses. 1 and 2 show bluish-green light with the PL maxima of 476 and 481 nm, respectively and blue-shifted emission compared to green phosphorescent Ir(ppy)2(pic). The solution PL efficiency of 1 and 2 is 0.12 and 0.09, respectively.
Keywords: Phosphorescence; Photoluminescence; Iridium;
Study of the interaction of tris-(8-hydroxyquinoline) aluminum (Alq3) with potassium using vibrational spectroscopy: Examination of the possible isomerization upon K-doping by Y. Sakurai; T. Yokoyama; Y. Hosoi; H. Ishii; Y. Ouchi; G. Salvan; A. Kobitski; T.U. Kampen; D.R.T. Zahn; K. Seki (161-164).
The geometrical structure of potassium doped Alq3 (tris-(8-hydroxyquinoline) aluminum) and its interaction with potassium were studied using infrared reflection-absorption spectroscopy (IRRAS), surface enhanced Raman scattering and DFT calculations. The major aim of this study was to examine the isomerization of Alq3 molecules from the meridional to the facial form upon alkali-metal doping, which has been theoretically predicted by other researchers. The observed spectra showed significant changes after potassium deposition. The calculated IR spectra of the K-Alq3 complex for the meridional and facial forms were very different, and that the meridional form in reasonable agreement with the observed spectrum. This demonstrates that the Alq3 molecule is not converted to a facial isomer by potassium deposition, but retains the meridional form, in contrast to reported theoretical predictions. We also found that the calculated IR spectrum of the K-Alq3 complex with Alq3 in its meridional form is much different from that of the isolated anion in the same isomeric form. We consider that the observed significant changes of spectra are due to the modification of electron distribution by the complexation with the K atom. Even though vibrational spectra of alkali-metal doped organic materials are usually interpreted on the basis of an isolated anion, the results presented here show that care should be taken in interpreting the spectra of doped organic materials, and that the presence of the counter ion needs to be taken into account. The observed Raman spectra and theoretical calculations of the Raman spectra show similar trends when compared to the IRRAS results. The present study demonstrates that vibrational spectroscopy can be used as a sensitive tool for discriminating subtle differences between isomers as well as between complexes and isolated anions.
Keywords: Infrared and Raman spectroscopy; Organic/inorganic interfaces;
Structural and Morphological Properties of 3,4,9,10-PeryleneTetraCarboxylic DiAnhydride Films on Passivated GaAs(100) Substrates by G. Salvan; S. Silaghi; B. Paez; T.U. Kampen; D.R. T. Zahn (165-168).
Raman spectroscopy is employed to investigate in situ the structural properties and the morphology of 3,4,9,10-Perylene-TetraCarboxylic DiAnhydride (PTCDA) films deposited onto S-GaAs(100) substrates at various temperatures. Additional two-dimensional Raman mapping measurements are carried out ex situ to extract information at the microscopic level. Complementary atomic force microscopy studies reveal that all the films consist of islands. The crystalline nature of these islands is proven by the observation of libronic phonon-like modes in the Raman spectra characteristic to the molecular crystal. The decrease in the phonon band widths observed at elevated substrate temperatures is related to an increase in the size of the crystalline domains and improvement of crystallinity. The PTCDA films consist of a mixture of two polymorphs α and β having the same symmetry and very similar lattice parameters. An estimation of the α- to β- content at microscopic level is obtained from the deconvolution of an internal Raman mode with C-H deformation character following a previously introduced model.
Keywords: Raman Spectroscopy; Atomic force microscopy; Organic semiconductors based on conjugated molecules (not polymers) (anthracenes; perylenes,etc); Evaporation and sublimation; Other heterostructures (organic/inorganic heterostructure); Polycrystalline thin films;
Synthesis and properties of chiral π-conjugated polymer by Y.S. Gal; K.T. Lim; S.Y. Shim; S.H. Kim; K. Koh; S.H. Jang; S.H. Jin (169-172).
The polymerization of 2-propynyl-tetra-O-β-glucopyranoside, an acetylene derivative having optically active moieties, was performed by such transition metal catalysts as PdCl2, (NBD)PdCl2, [Rh(NBD)Cl]2, PtCl2, WCl6, and MoCl5. The polymerization proceeded well to give a moderate yield of polymer, regardless of the bulky substituents. The spectroscopic data for the poly(2-propynyl-tetra-O-ß-glucopyranoside) indicated that the polymer has a conjugated polymer backbone system bearing the corresponding chiral substituents. The electrical conductivity of the iodine-doped polymer were in the range of 7.4 × 10−4–2.0 × 10−3 S/cm. The poly(2-propynyl-tetra-O-ß-glucopyranoside) prepared by [Rh(NBD)Cl]2 showed more intense CD signal at around 350 nm than those of the same polymers prepared by other transition metal catalysts. The polymer thin films exhibited reversible electrochemical behaviors between the doping and undoping peaks.
Keywords: Polyacetylene and derivatives; Transition-metal-catalysed reactions; Electrochemical methods; UV-Vis-NIR absorption; Photoluminescence;
Synthesis and Characterization of Novel PPV-Derivatives Containing Bulky Bicylcoalkylsilylphenyl Substituent by S.H. Jin; H.U. Seo; M.K. Joo; D.S. Koo; W.S. Shin; J.Y. Do; Y.I. Kim; Y.S. Gal (173-176).
We synthesized and characterized a new series of soluble conjugated electroluminescent (EL) polymers with high molecular weight using the Gilch polymerization method. The resulting EL polymers showed excellent solubility and good film forming property. The number average molecular weight (Mn) and polydispersities of the polymers were in the range of 79,000–429,000 and 1.05–7.80, respectively. The UV-visible absorption spectra showed at around 428–490 nm and their maximum PL emission spectra appeared at around 527–598 nm. As the increasing MEH-PPV content in copolymers, the UV-visible adsorption peaks of the copolymers were red-shifted from 440 nm to 490 nm.
Keywords: Polymer LED; PPV; Electroluminescence; Gilch polymerisation;
Metallic electrical conduction in alkaline metal-doped pentacene by Yutaka Kaneko; Tomoaki Suzuki; Yasumitsu Matsuo; Seiichiro Ikehata (177-180).
We have prepared a cesium-doped pentacene film and a rubidium-doped pentacene film and have investigated their electrical properties in order to obtain a new molecular conductive film based on pentacene. It is found that the electrical conductivity depends strongly on the doping temperature. A high electrical conductivity is obtained by annealing of pentacene film doped with the alkaline metal. As a result, the electrical conductivities equal 74 S/cm and 47 S/cm in the cesium-doped pentacene film and in the rubidium-doped pentacene film, respectively. Moreover, we have observed that these conductivities increase with decreasing temperature and show metallic behavior below room temperature. Furthermore, it is found that the electrical resistivity shows anomalous behavior at around 80 K in the cesium-doped pentacene film and at around 90 K in the rubidium-doped pentacene film.
Keywords: Electrochemical doping; Single crystalline thin films; Metallic films;
Theoretical study on two-photon absorption for symmetric molecular systems composed of charged groups linked with a π-conjugated bridge by R. Kishi; M. Nakano; S. Yamada; K. Kamada; K. Ohta; T. Nitta; K. Yamaguchi (181-184).
Two-photon absorption (TPA) properties of symmetric molecular systems composed of charged groups linked with a π-conjugated bridge are investigated using the time-dependent perturbation theory with the single and double excitation CI method in the INDO/S approximation in order to clarify the effect of introducing charges on the TPA property. It turns out that the introduction of charges causes the red shift and significant enhancement of TPA cross section in the first peak position as compared to that in the neutral molecule with a similar framework. This is primarily ascribed to be the increase of transition moment between the one-photon-allowed, first excited state and the two-photon allowed, higher excited state for the charged molecules because of the localization of contributing MOs in the both-end regions.
Keywords: Nonlinear optics; Two-photon absorption; Orbital interaction; π-Conjugation bridge; Hyperpolarizability;
Synthesis of C60-diphenylaminofluorene dyads with two-photon absorbing characteristics by Prashant A. Padmawar; Taizoon Canteenwala; Sarika Verma; Loon-Seng Tan; Guang S. He; Paras N. Prasad; Long Y. Chiang (185-188).
Linear A-sp 3 -D molecular conjugates as 9,9-dialkyl-2-diphenylaminofluorene (DPAF-C,)-C60 monoadducts were demonstrated using methanoketo unit as a linker to bridge the DPAF donor moiety and the fullerene acceptor chromophore together within a short distance of roughly 2.0 Å. Target products of C60-diphenylaminofluorene dyads C60(>DPAF-C2) and C60(>DPAF-C18) were synthesized using Bingel cycloproanation reaction from the corresponding 7-α-bromoacetyl-9,9-dialkyl-2-diphenylaminofluorene precursors. Both dyads C60(>DPAF-C2) and C60(>DPAF-C18) were characterized by spectroscopic methods and simultaneous two-photon excitation measurements, showing large two-photon absorption cross-sections in the nanosecond regime.
Keywords: Fullerene derivative; Diphenylaminofluorene; Two-photon absorption; Donor-acceptor conjugate; Photonic organics;
Preparation and nanoscopic internal structure of single-walled carbon nanotube-ionic liquid gel by H.B. Kim; J.S. Choi; S.T. Lim; H.J. Choi; H.S. Kim (189-192).
As an effort to expand their applicability of both room temperature ionic liquid (RTIL) and single-walled carbon nanotube (SWNT), we prepared a gel-like paste using imidazolium-based ionic liquid (1-n-butyl-3-methylimidazolium tetrafluoroborate; [Bmim] BF4) and SWNT, and investigated their internal structure and viscoelastic characteristics. The suspension containing 1 wt% of SWNT was prepared via grinding it in an agate mortar. A black paste (gel) was then obtained from its centrifugation. Rheological properties of a purified SWNT- ionic liquid ([Bmim] BF4) gel were measured using a rotational rheometer equipped with a parallel plate geometry, and then compared with those of unpurified SWNT-ionic liquid gel from both steady shear and oscillatory shear mode tests. We also examined their effects of a well-dispersed SWNT in a suspending medium on rheological properties, resulting in gel-state even at a very low filler concentration due to the nano-dimension of SWNT and its interaction with a novel ionic liquid. From this study, it is found that the characteristic rheological behaviors, such as high shear thinning, dynamic yield stress, and frequency independent dynamic behavior, not only support the gel formation due to the uniform dispersion of SWNT in the ionic liquid medium, but also explain the formation of network structure induced by a strong interaction between ionic liquid and SWNT. In addition, we also observed that such a gel structure was followed by a shear induced secondary structure in a high shear rate region.
Keywords: SWNT; Ionic liquid; Gel; Rheology Network structure;
Electrical response of Poly(styrene)/carbon black conductive polymer composites (CPC) to methanol, toluene, chloroform and styrene vapors as a function of filler nature and matrix tacticity by J.F. Feller; Y. Grohens (193-196).
We have studied the sensing properties of several conductive polymer composites (CPC) to saturated solvent vapors (methanol, toluene, chloroform and styrene) as a function of carbon black (CB) structure and poly(styrene) PS crystal Unity. It appears that the combination of these two previous parameters allows obtaining very different electrical signature for the four vapors studied, confirming the importance of the carbon black conductive pathways morphologies structuring during the process. This study shows that selectivity of the CPC toward the vapors can be obtained not only by changing the matrix chemical nature but also by tailoring the morphology of the CB network.
Keywords: Semi conductive films; Graphite and related compounds; Detectors; Solution processing; Glass surfaces;
Temperature Dependent Spectroscopic studies of HiPco SWNT composites. by S.M. Keogh; T.G. Heddermann; G.F. Farrell; M. Ruether; E. Gregan; M. McNamara; G. Chambers; H.J. Byrne (197-200).
Hybrid systems of the conjugated organic polymer poly(p-phenylene vinylene-co-2,5-dioctyIoxy-m-phenylene vinylene)(PmPV) and HiPco SWNTs are explored using spectroscopic and thermal techniques to determine specific interactions. Vibrational spectroscopy indicates a weak interaction and this is further elucidated using Differential Scanning Calorimetry, Temperature Dependent Raman Spectroscopy and Temperature Dependent Infrared Spectroscopy of the raw materials and the composite. An endothermic transition is observed in the DSC of both the polymer and the 0.1% HiPco composite in the region of 60 °C. Also observed in the DSC of the composite is a doubly peaked endotherm at -39 °C and -49 °C, which does not appear in the polymer film. The Raman of the polymer backbone upon increasing the temperature to 60 °C shows diminished phenyl Ag modes at 1557 cm−1 and 1575 cm−1, with an increase in the relative intensity of the vinyl mode at 1630 cm−1. This change in the Raman of the polymer is translated to the Raman of the composite upon increase to 60 °C, where the spectrum becomes abruptly dominated by nanotubes. The Raman of the composite shows no change in the lower temperature regions, however infrared suggests that the transition at -39 °C derives from an interaction with the polymer side chains. The composite at -39 °C shows a change in the absorption of the polymer side chain aryl-oxide linkage at 1250 cm−1. while Raman suggests a change to the polymer backbone configuration at 60 °C. Implications are discussed.
Keywords: Semi-comjugated polymers; SWNT Composites; Differential Scanning Calotimetry; Infrared and Raman Spectroscopy;
Spectroscopically selective nanovolume Raman imaging of one single walled carbon nanotube by Han Athalin; Serge Lefrant (201-204).
In this paper, we report on a new procedure to analyze individual single walled carbon nanotube (SWNT), based on an improved use of a confocally selective nanovolume Raman set-up coupled to a refined signal treatment. This technique permits to further improve the resolution in both vertical and lateral dimensions. To acquire spectrally selected SWNT, we registered the spatial distribution of the emitted photons in x,y and z vectors with unprecedented accuracy and were able to resolve details in the specimen with near-field resolution in 3D. The SWNTs are localized with an accuracy better than 17 run in the lateral x,y and better than 2 nm in the z direction. In this letter we report what we believe to be the first evidence of selective nanovolume Raman spectroscopy of carbon nanotubes.
Keywords: Infrared and Raman spectroscopy; Carbon nanotubes; Metal/semiconductor interfaces; Laser spectroscopy;
Silver Nanoparticle Self-organization into Dendritic Fractals by M.E. Brennan; A.M. Whelan; J.M. Kelly; W.J. Blau (205-208).
Organic reagent assisted self assembly of shaped silver nanoparticles into structured linear arrays and dendritic patterns, is reported. Controlled synthesis of nanoparticle shape is used to prepare color tunable silver nanoparticles. Treatment of the silver nanoparticles with selected organic reagents at certain concentrations enabled the determination of the type of structure or pattern generated as well as control over the length scale over which the assembly is generated. Polyacrylamide treatment of the silver nanoparticles was used to produce linear and branched arrays on the nanometer to micron scale, whereas treatment with thiol compounds enabled the production of dendritic patterns with dimensions ranging from microns to millimeters.
Keywords: Self-organization; Macromolecules;
The influence of added ionic salt on nanofiber uniformity for electrospinning of electrolyte polymer by Chang Kee Lee; Sun I. Kim; Seon Jeong Kim (209-212).
We have investigated the morphology of electrospinning nanofiber dependence on applied voltage and the influence of added ionic salts on nanofiber uniformity. For the ionic salt concentration of 0.01 mol%, the applied electric field of 10 ∼ 25 kV, and a 15-cm spinning distance, the experimental results showed that uniform nanofibers which did not contain any beads were synthesized, while the current of electrospinning process had two orders of amount higher than the non-added ionic salt droplet current. Also the fiber diameter distribution exhibited a very narrow. We obtained uniform fibers with diameters of around 70 nm and, exhibited electric current value for obtaining uniform high-quality nanofibers. The morphology of the electrospun nanofibers was investigated using field emission scanning electron microscopy and the electric current was measured using a digital multimeter.
Keywords: Droplet current; Ionic salt; Electrospinning; PAMPS;
Synthesis of conducting polyaniline in semi-IPN based on chitosan by Su Ryon Shin; Sang Jun Park; Seoung Gil Yoon; Geoffrey M. Spinks; Sun I. Kim; Seon Jeong Kim (213-216).
The chitosan/polyaniline (PANi) semi-interpenetrating network polymers (semi-IPN)s were prepared under different pH conditions. Changes of structural properties were investigated using Fourier transform infrared (FT-IR) spectroscopy and Solid-state 13C NMR. The spectroscopic studies revealed that the PANi had converted to its pH-switched, emeraldine salt (ES) form when prepared from solutions of pH<l. The surface conductivity of the semi-IPN was dependent on the chitosan/PANi weight ratio and on the pH.
Keywords: Chitosan; Polyaniline; Semi-IPN; pH-condition;
Nano-sized tetralanthanoid-copper complex ions containing oximato bridges by Sohei Ueki; Muhamad Sahlan; Takayuki Ishida; Takashi Nogami (217-220).
Complexation of LnIII(hfac)3, LnIII(OAc)3, and [CuII(dmg)2]2− in the presence of Ph4P+ gave [Ln4Cu1]-type pentanuclear complexes (dmg = dimethylglyoximate dianion; Ln = La, Nd, Eu, Gd, Tb). Single-crystal X-ray diffraction analyses revealed that four Ln(IIl) ions formed a rectangular array around the copper ion at its inversion center. The [Ln4Cu1] core structures are essentially the same regardless of the different Ln ions. From the magnetic susceptibility measurements, the Cu(dmg)2 dianion moiety has been shown to play a role of the ferrimagnetic coupler among the Gd spins. The out-of-phase ac susceptibility of the [Tb4Cu1] complex exhibited anomalous frequency- dependence below 4 K, indicating blocking behavior potentially related to a single-molecule magnet.
Keywords: Magnetic measurements; Molecule-based magnet; Rare earth metal; Lanthanide; Single-molecule magnet;
Optical properties of carbon nanotube-PPVcomposites: influence of the PPV conversion temperature and nanotube concentration by E. Mulazzi; R. Perego; H. Aarab; L. Mihut; E. Faulques; S. Lefrant; J. Wéry (221-224).
Optical absorption, photoluminescence, Raman scattering spectra of carbon nanotube-PPV composite films are studied as function of precursor conversion temperature in the range 120 °C–300 °C, of precursor aging and of nanotube concentrations. Dramatic changes are observed in the optical spectra at 120 °C PPV conversion temperature with respect to those obtained at 300 °C. It is found that the effect of the low conversion temperature on all the optical spectra is similar to that of increasing the nanotube concentration in standard PPV. In addition, from the changes in the PL spectra of the different composite films, taken at 300 K and 77 K, we can prove that the charge migration process between segments is the more important mechanism for the energy transfer from short to long segments. A theoretical model is also given, which is able to reproduce all the optical spectra and their changes as function of the conversion temperature and nanotube concentration in good agreement with the experimental data.
Keywords: Poly(para-phenylene vinylene) and derivatives; single-walled nanotubes; Optical absorption and emission spectroscopy;
Multicomponent Micropatterns or Carbon Nanotubes by Lingchun Li; Junbing Yang; Richard Vaia; Liming Dai (225-228).
The excellent optoelectronic, mechanical, and thermal properties of carbon nanotubes have made them very attractive for a wide range of potential applications. However, many applications require the growth of aligned/micropatterned carbon nanotubes. Based on our previous work on the aligned and micropatterned growth of carbon nanotubes, we have recently developed a novel approach towards the micropattern construction of perpendicularly-aligned carbon nanotubes by simply pressing a Scotch tape pre-patterned with a non-adhesive layer onto a non-patterned aligned carbon nanotube film, followed by peeling off the Scotch tape from the quartz substrate in a dry state. In conjunction with the region-specific surface modification, this dry contact transfer method has enabled us to produce various multicomponent carbon nanotube micropatterns in which different components are interposed in an intimate fashion. Examples include micropatterns with self-assembled non-aligned carbon nanotubes interdispersed into the discrete areas in the patterned structure of aligned carbon nanotubes and flexible polymer films with embedded aligned carbon nanotube networks. More recently, we have also developed a simple but effective template-free electroplating method for region-selective deposition of cobalt and nickel nanoparticles for patterned growth of carbon nanotubes. These carbon nanotube micropatterns in which multicomponents are interposed in a controllable fashion should be of significance to many nanotube based multifunctional systems.
Keywords: Carbon nanotube; Alignment; Micropatterning; Electrodeposition; Multicomponent micropatterns; Polymer; Nanocomposite film;
Surface Modification of Aligned Carbon Nanotube Arrays for Electron Emitting Applications by A. Patil; R. Vaia; L. Dai (229-232).
We have previously developed a simple pyrolytic method for large-scale production of aligned carbon nanotube arrays perpendicular to the substrate. These aligned carbon nanotube arrays can be transferred onto various substrates of particular interest (e.g. polymer films for organic optoelectronic devices) in either a patterned or non-patterned fashion. The well-aligned structure is important, which not only allows us to prepare aligned coaxial nanowire of carbon nanotubes sheathed with polymers by electrochemical polymerization of conducting polymers on the individual nanotubes but also enables us to develop a facile approach for modification of carbon nanotube surfaces via plasma activation, followed by chemical reactions characteristic of the plasma-induced functionalities. These surface modification methods are particularly attractive, as they allow surface characteristics of the aligned carbon nanotubes to be tuned to meet specific requirements for particular applications while their alignment structure can be largely retained. The aligned carbon nanotubes with tunable surface characteristics thus prepared are of great significance to various practical applications, especially for the use of carbon nanotubes as electron emitters in flat panel displays. Particularly, we found that hexane-plasma coating reduced the turn-on electric field E to, coupled with a concomitant increase in the emission current at a constant V; the turn-on electric field decreased from E to = 2.5 V/μm, characteristic of the pristine aligned carbon nanotubes, to E to = 1.5 V/μm with a significantly increased emission current after the treatment of n-hexane plasma at 30 W, 250 KHz and under a monomer pressure of 0.65 Torr for 2 min. In this paper, we will discuss the effects of the surface modification on the electron-emitting properties of the aligned carbon nanotubes by presenting some examples from our recent work.
Keywords: Aligned carbon nanotube; Nanotube electron emitter; Plasma deposition; Flat panel display; Surface modification;
Synthesis and properties of multi-nuclear ruthenium (II) complexes of bis(2,2′ -bibenzimidazole) by Jun Yin; Ronald L. Elsenbaumer (233-236).
Conjugated metallopolymers in which metal sites are in electronic communication through a conjugated organic backbone have been attracting growing interest. Bis(2,2′-bibenzimidazole) has been prepared as the conjugated backbone. The multinuclear Ru complexes were synthesized by the complexation of the conjugated bis(2,2′-bibenzimidazole) with Ru(bpy)2Cl2. The deprotonation form of the dinuclear Ru complexes may show enhanced electron transport ability between metal centers through the conjugated bibenzimidazole backbones. Deprotonation also allows the synthesis of tetranuclear Ru complexes. The preparations, spectroscopic and electrochemical properties of these multinuclear Ru complexes were investigated.
Keywords: Heterocycle synthesis; Organic semiconductors based on conjugated molecules; UV-Vis-NIR absorption; Electrochemical methods;
Recombination kinetics of photoexcitations in films of MEH-PPV and MEH-PPV/C60 mixtures by E. Gershman; T. Drori; C. Herzog; Y. Eichen; E. Ehrenfreund (237-240).
We report on the recombination kinetics of photoexcited polarons and triplet excitons in films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)- 1,4-phenylene vinylene) (MEH-PPV) and MEH-PPV/C60 mixtures. Experimentally, the dependence of the polaron and exciton photoin- duced absorption bands on the excitation intensity and modulation frequency was measured, covering a dynamic range of 4–5 orders of magnitude. We have found that in all cases the recombination kinetics can be successfully described as a “bimolecular dispersive” recom- bination, with the following characteristics of the photoinduced absorption, (a) Sublinear modulation frequency dependence (ω−α , α < l) at high frequencies of both in-phase and quadrature components, (b) Sublinear excitation intensity dependence (I γ ,γ < l), with decreasing ex- ponent as the modulation frequency decreases. The exponent α and the average recombination time vary with the polymer molecular weight.
Keywords: Photoexcited polarons; Triplet excitons; Dispersive bimolecular recombination;
A new BEDT-TTF salt and polypyrrole films containing the chiral polyoxometalate [H4Co2Mo10O38]6− by E. Coronado; S. Curreli; C. Giménez-Saiz; C.J. Gómez-García; J. Roth (241-244).
The chiral polyoxometalate [H4Co2Mo10O38]6− has been used in the preparation of hybrid materials in the form of crystals or polymeric films. A new radical salt of the donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) containing the two enantiomers of this polyoxometalate has been obtained by electrocrystallization. This radical salt shows semiconducting behavior with room temperature conductivity of 9 S.cm−1 and activation energy of 40 meV. Polypyrrole films doped with a racemic mixture of polyoxometalates and also the enantiomerically pure (+)589-[H4Co2Mo10O38]6− have been prepared electrochemically. Circular dichroism experiments suggest that the chiral polyoxometalates do not induce macroasymmetry into the polypyrrole chains during polymerization. Conductivities at room temperature of these polymer films range between 36 and 49 S.cm−1.
Keywords: Bis(ethylenedithio)tetrathiafulvalene; Polypyrrole; Chiral polyoxometalates; Hybrid materials; Radical salts; Electrical properties;
New magnetic conductors and superconductors based on BEDT-TTF and BEDS-TTF by E. Coronado; S. Curreli; C. Giménez-Saiz; C.J. Gómez-García (245-248).
Here we present our last results in the search for multifunctionality in conducting molecular materials obtained by combining paramagnetic molecular complexes with organic donors to produce paramagnetic molecular metals and superconductors. The structural and physical characterization of two new molecular paramagnetic superconductors ET4[H3OM(C2O4)3].PhBr (MIII = Cr and Fe) are presented (ET = BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene). These salts present a superconductor transition at Tc = 1.5 and 4.0 K, respectively and very anisotropic critical fields Hc2a ≈ Hc2b ≈0.5 T and HC2C ≈ 8 T for the Fe derivative. We will also present the structural and physical characterization of two new metallic paramagnetic systems: ET4[H3OM(C2O4)3].PhCl (M = CrIII and FeIII) and five different new semiconducting paramagnets obtained with the donor bis(ethylenediselena)tetrathiafulvalene (BEDS-TTF = BEST) and [M(C2O4)3]3− (M = CrIII and FeIII) complexes.
Keywords: Magnetic conductors; Magnetic superconductors; BEDT-TTF; BEDS-TTF; Tris(oxalate)metallate complexes;
Field induced phase observed above 25T in (DMET-TSeF)2I3 by K. Oshima; T. Kambe; T. Sasaki; R. Kato (249-252).
We found that the Field Induced Spin Density Wave (FISDW) phase in (DMET-TSeF)2I3 has an apparent high threshold field of about 25 T in spite of the very similar room temperature crystal structure and the Fermi surface compared to the salts studied so far with anions AuCl2, AuBr2 and Aul2 which have threshold fields well below 10 T. The X-ray crystal structural studies down to 90 K do not show any special change in the structure. Possible reasons for the origin of the differences are discussed, and the sliding motion of the FISDW condensates at lower magnetic fields is proposed to be the most probable reason.
Keywords: Transport measurements; Magneto resistance; Metal-insulator phase transitions; Organic superconductors;
Nuclear spin-lattice relaxation in κ-(BETS)2FeBr4 by S. Fujiyama; M. Takigawa; J. Kikuchi; K. Kodama; T. Nakamura; E. Fujiwara; H. Fujiwara; H.-B. Cui; H. Kobayashi (253-256).
We report results of 77Se NMR study for a newly found field-induced superconductor, κ-(BETS)2FeBr4. The nuclear spin lattice relaxation rate (1/T1) above 25 K has a linear relation with temperature, 1/T 1 = aT+b, which can be analyzed in a framework of normal metal with dipolar fields from uncorrelated 3d local moments to the Se nuclei. However, 1/T1 has an evident kink at 25 K and shows a remarkable decrease within a narrow temperature range of 2.5 K. Below 22.5 K, we found that 1/T 1 follows a power law 1/T 1∼T 1.43, although the material is still conductive. The temperature where 1/T 1 shows a kink agrees with the temperature where the temperature devivative of the resistivity dρ/dT diverses. We discuss a possible reconstruction of the electronic state, which may attain magnetic correlation below this temperature.
Keywords: Nuclear magnetic resonance spectroscopy; Superconducting phase transition; Organic superconductors;
Inter-chain Coulomb-lattice relaxation and multicriticality in charge transfer organic complexes by J. Kishine; T. Ohara; T. Luty; K. Yonemitsu (257-260).
We discuss the neutral-to-ionic phase transition and emergence of the multi-criticality in the quasi-one-dimensional charge-transfer salt TTF-CA under pressure. We stress that subtle interplay of Coulomb and lattice processes may be quite sensitive to pressure. Emergence or disappearance of the multi-critical points in a series of charge-transfer salts are understood through this interplay. What's behind is coexistence and coupling of the non-symmetry-breaking and symmetry-breaking order parameters.
Keywords: Equilibrium thermodynamics and statistical mechanics; Order-disorder phase transitions;
Crystal structures and conductivities of new hydrogen-bonded system: CnDT-EDO-TTF and BzDT-EDO-TTF by H. Suzuki; S. Ichikawa; K. Yamashita; S. Kimura; H. Mori; Y. Nishio; K. Kajita (261-264).
A new series of RDT-EDO-TTF [R = C5, C6, C7, C8; (cis-l,2-cycloalkylene-l,2-dithio)ethylenedioxytetrathiafulvalene and R = Bz; (benzodithio)ethylenedioxytetrathiafulvalene] donors containing cycloalkylene or benzene, and ethylenedioxy groups as a source of the steric hindrance and the intermolecular hydrogen bonds has been synthesized. The charge transfer (CT) salts, β″-(RDT-EDO-TTF)2(PF6)x (R = C5, C6, C7, C8, and Bz), have the similar β″-type donor arrangement due to the intermolecular hydrogen-bonded network (CH…O) in the two-dimensional (2D) plane. The C6DT-EDO-TTF molecule containing the flexible cyclohexylene group affords the 2:1 salts, which shows the metallic behaviors down to 1.3 K with a variety of resistivities at room temperature, 3.0 × 10−3∼5.0 × 10−2 ohm cm. On the other hand, the BzDT-EDO-TTF donor with the fixed functional benzo group gives the non-stoichiometry complexes, which exhibit the similar metallic behavior down to 1.3 K with the largest RRR = 40∼115 among the RDT-EDO-TTF salts.
Keywords: Organic conductors based on radical cation and/or anion salts; Transport measurements; Conductivity; Hall effect; Magnetotransport; X-ray emission; Diffraction and scattering;
Magnetic oscillations, disorder and the Hofstadter butterfly in finite systems by James G. Analytis; Stephen J. Blundell; Arzhang Ardavan (265-268).
We present numerical calculations of a tight-binding model applied to a finite square lattice in the presence of a perpendicular magnetic field. The persistent current associated with each eigenstate is calculated, the chirality of which is determined by whether the eigenstate exists within the bulk or localised to the edges of the lattice. This treatment allows us to extract oscillations in the magnetization, which are analogous to de Haas-van Alphen oscillations. We consider the influence of short range disorder and long range potential modulations on these systems.
Keywords: Computer simulations; Molecular dynamics; Lattice dynamics; Electron density; Excitation spectra calculations;
ESR Study on λ-(BEDT-TSF)2Fe1-xGaxCl4 by T. Kizawa; S. Kawamata; T. Suzuki; E. Negishi; H. Matsui; N. Toyota; T. Ishida (269-272).
Electron spin resonance experiments with 15 GHz microwave have been performed on λ-(BEDT-TSF)2Fe1-xGaxCl4 (x = 0 and 0.4) single crystals at the various different temperatures down to 1.5 K. For λ-(BEDT-TSF)2FeCl4, the g-value do not show the symmetric angular dependencies of the magnetic field direction with respect to the conducting ac-plane at several temperatures above 20 K. This means that the observed signals are not mainly due to π-electrons in BEDT-TSF molecules but to (d-electrons of Fe ions. The line width for H//a* and H//c increases with decreasing temperature from 200 to 70 K. This might be correlated to the anomaly in the dielectric response at 70 K reported earlier. For λ-(BEDT-TSF)2Fe0.6Ga0.4Cl4, the large enhancement of the g-value below 120 K and the peak of the line width at around 95 K are observed. These anomalies suggest the possible anomaly in the electronic states as in the case of λ-(BEDT-TSF)2FeCl4. An enhancement of the line width associated with the antiferromagnetic transition is observed both for λ-(BEDT-TSF)2FeCl4 and λ-(BEDT-TSF)2Fe0.6Ga0.4Cl4.
Keywords: Electron spin resonance; Metal-insulator phase transitions; Magnetic phase transitions; Organic conductors based on radical cation and/or anion salts;
Melting of charge order in (DI-DCNQI)2Ag by pressure by T. Itou; K. Miyagawa; K. Kanoda; K. Hiraki; T. Takahashi; K. Murata; T. Matsumoto (273-276).
We showed how the charge order in (DI-DCNQI)2Ag is melted by pressure, by investigations of the NMR spectra and the resistivity parallel and perpendicular to the conducting c-axis. When the system is pressurized, the second-order-transition temperature of the charge ordering is lowered and the disproportionation ratio is depressed. Under higher pressures, the transition changes into first-order one and eventually it vanishes at 21.6 kbar. At a low-pressure and high-temperature region, temperature dependence of ρ // and ρ ⊥ are contrasting. The melting mechanism of the charge ordering is very likely due to the “dimensional crossover”. Furthermore, in the high-pressure metallic region, curious T 3 dependence of the resistivity is observed, implying unprecedented electron-electron scattering mechanism.
Keywords: Transport measurements; Nuclear magnetic resonance spectroscopy; Metal-insulator phase transition; Organic conductors based on radical cation and/or anion salts;
Physical properties and crystal structures of MeDA-TTP salts by Hiroyuki Nishikawa; Hiromichi Sekiya; Daisuke Watanabe; Takeshi Kodama; Koichi Kikuchi; Isao Ikemoto; Jun-ichi Yamada (277-280).
A dithiane analogue of MeDH-TTP [2-isopropylidene-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene], MeDA-TTP [2-isopropylidene-5-(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene], has been prepared, and crystal structures and electrical properties of MeDA-TTP radical salts have been investigated. Although all the salts are isostructural, conducting behavior differs between the salt with an octahedral anion and the one with a tetrahedral anion; the salts with octahedral anions are metallic, but the ClO4 salt is semiconductive. A dithiane analogue of DODHT [(1,4-dioxane-2,3-diyldithio)dihydrotetrathiafulvalene], DADOT [2-(1,3-dithiane-2-ylidene)-4,5-(1,4-dioxane-2,3-diyldithio)-1,3-dithiole] has also been synthesized. Crystal structure of the DADOT salt is completely different from that of DODHT superconductors.
Keywords: Organic conductor; Crystal structure; Reduced π-electron system; Dithiane analogues;
DMRG study of the impurity–induced transition from the spin-Peierls state to the antiferromagnetic state by Tomoyuki Hara; Takeo Kato; Akira Terai (281-284).
We investigate impurity effects in the spin-Peierls systems coupled with an interchain exchange interaction. The density matrix renormalization group method is used. The interchain interaction is treated within the mean field approximation. In the impurity-free case, the ground state makes a first-order transition from the spin-Peierls (SP) state to the antiferromagnetic (AF) state at a critical strength of the interchain interaction. Impurities are doped to the spin-Peierls system which is located close to the SP-AF phase boundary in the phase diagram. A single impurity suppresses the bond alternation and induces a staggered magnetic moment locally around the doped site. If the impurity density is larger than a critical value, the spin-Peierls system makes a transition to the antiferromagnetic state. It is shown that a two-impurity problem is essential to understand the physical mechanism of the phase transition.
Keywords: Semi-empirical models and model calculations; Computer simulations; Structural phase transition; Magnetic phase transition;
Synthesis and Characterization of a Six-Coordinate Silicon (IV) Complex with a Tripodal Phenalenyl-Based Hexadentate Ligand by S. Samanta; M.E. Itkis; R.W. Reed; R.T. Oakley; F.S. Tham; R.C. Haddon (285-288).
We report the preparation and solid state characterization of a silicon (IV) salt based on a new redox active tripodal hexadentate ligand [H3L = l,l,l-Tris(l′-oxophenalenyl-9′-N-methyl) ethane]. X-ray analysis of the complex reveals a short Si-N bond distance (av 1.878(16)). The compound is shown to undergo three reversible one-electron reduction steps (without decomposition) due to the formation of radical, anion and dianion of the phenalenyl moiety.
Keywords: Radical; Tripodal ligand; Disproportionation potential; Unpaired electron;
Investigation of interlayer coherency and angular-dependent magnetoresistance oscillations in magnetic graphite intercalation compounds by K. Enomoto; T. Yamaguchi; T. Terashima; T. Konoike; M. Nishimura; S. Uji; T. Enoki; M. Suzuki; I.S. Suzuki (289-292).
We investigated the interlayer coherency and angular-dependent magnetoresistance oscillations (AMROs) in magnetic graphite intercalation compounds (GICs), stage-1 FeCl3 GIC (S = 5/2) and stage-5 MoCl5 GIC (S = 1/2), where sufficiently strong π-d coupling is expected. Shubnikov-de Haas (SdH) oscillations were observable in a limited field-angle region. Standard AMRO was observed for both GICs, but the background of the angular-dependent magnetoresistance cannot be interpreted by the semi-classical model, based on the Boltzmann coherent transport theory. The resistance peak in magnetic fields parallel to the conduction layers is observed only for stage-5 MoCl5 GIC, demonstrating coherent transport. The results are discussed in terms of magnetic scattering in the characteristic stacking structure of GICs.
Keywords: Transport measurements; Magnetotransport; Graphite and related compounds;
Magnetism of novel organic heterospin composite systems in supramolecular complexes of nitronyl nitroxide by Y. Kanzaki; T. Ise; D. Shiomi; K. Sato; T. Takui (293-296).
The authors have shown from theoretical calculations that a one-dimensional molecular assemblage composed of ground-state singlet biradicals and S = 1/2 monoradicals has three kinds of ground states; two with seemingly ferrimagnetic spin alignments and one with a spinless (diamagnetic) state. The energy preference of the three states depends on the relative ratios of the intra- and intermolecular magnetic interactions. This gives a quantum switching device, in which a conversion of the spin state is controlled by external perturbation such as pressure and magnetic field. As a model system for the magnetic switching device, we have designed and synthesized organic coordination compound of an anionic biradical (1 −) with a singlet (S = 0) ground state and a Na+-15-crown-5-ether derivative of nitronylnitroxide monoradical (2), 2,5-Bis(1 -oxyl-3-oxido-dihydroxy-4,4,5,5-tetramethyl imidazolin-2-yl)phenol-Sodium(4-(1 -oxyl-3-oxido-dihydroxy-4,4,5,5-tetramethyl imidazolin-2-yl)-15-crown-5) Salt, [Na2] + 1 − . Possible occurrence of the exotic ferrimagnetic spin alignment in the complex salt [Na2] + 1 − is discussed from magnetic susceptibility measurements. A novel 18-crown-6-ether substituted nitronyl nitroxide monoradical 3 has also been synthesized as a building block for organic ferrimagnet. The intermolecular interaction of 3 was found to be antiferromagnetic from magnetic susceptibility measurements.
Keywords: Organic ferrimagnets; Supramolecular crystal engineering; Magnetic measurements; Magnetic phase transitions;
Nitronyl nitroxide triradical as a model compound for generalized ferrimagnetism by T. Ise; D. Shiomi; K. Sato; T. Takui (297-300).
A novel nitronyl nitroxide triradical, p-triNN (1), has been synthesized, in which a π-conjugated biradical with a singlet ground state and a doublet monoradical are united by σ-bonds. Two kinds of intramolecular exchange interactions, one between the biradical and monoradical entities J(σ) through the σ-bonds and the other within the biradical moiety J(π) through the π-conjugation, were examined by paramagnetic susceptibility χ m for the sample dispersed in an organic polymer film. The magnitude of J(σ) (2J(σ)/k B ≤ −0.01 K) was found to be much weaker than that of J(π) (2J(π)/k B = −70.0 K). This result indicates that the triradical p-triNN (1) can be used as a building block for generalized ferrimagnets, which from theoretical calculations have been predicted to exhibit an exotic spin alignment.
Keywords: Magnetic measurements; Electron spin resonance;
Low-temperature structure and ESR study of quasi-one-dimensional ferromagnet γ-p-NPNN by T. Kambe; K. Kajiyoshia; K. Oshimaa; M. Tamurab; M. Kinoshitac (301-304).
The low-temperature structure analysis and low-temperature ESR were performed in the γ-phase of p-NPNN, which is considered to be as a quasi-one-dimensional ferromagnet above the antiferromagnetic ordering temperature (0.65 K without a static magnetic field). No structural phase transition was observed below 30 K, but we found the large thermal shrinkage between the intra-chain p-NPNN molecules along c-axis. This result suggests that the one-dimensionality of the ferromagnetic chain is effectively enhanced at lower temperature, since the geometry through NO-NO2 contact gives rise to the strong ferromagnetic interaction. Below 0.6 K, we succeeded in observing the antiferromagnetic resonance (AFMR) using the low frequency (∼300 MHz) and low-temperature (0.5 K) ESR. The frequency-field relation is well reproduced by the two-sublattice model with orthorhombic anisotropy.
Keywords: X-ray emission; Diffraction and scattering; Magnetic measurement; Magnetic phase transition;
Millimetre-wave studies on the high-spin molecules Cr10(OMe)20(O2CCMe3)10 and Cr12O9(OH)3(O2CCMe3)15 by S. Sharmin; A. Ardavan; S.J. Blundell; A.I. Coldea (305-308).
We report millimetre-wave electron spin resonance (ESR) measurements on single crystals of the high-spin molecules Cr10(OMe)20(O2CCMe3)10 and Cr12O9(OH)3(O2CCMe3)15 within a temperature range of 1.4 K to 50 K and in magnetic fields of up to 5 Tesla. In our experiments it is possible to vary the orientation of the magnetic field with respect to the crystal axes, and thus to study the ESR lineshapes as a function of both temperature and angle. Our results confirm that Cr10(OMe)20(O2CCMe3)10 behaves as a single-molecule magnet with S = 15 and D = -0.03 K, while Cr12O9(OH)3(O2CCMe3)15 has S = 6 and D ∼ 0.1 K. A comparison of the experimental spectra with numerical simulations gives good agreement at low temperatures. At higher temperatures, we observe a narrowing of the ESR spectrum that is not explained by simple models.
Keywords: Electron spin resonance;
A proposal of spin-and charge-modulated open-shell nonlinear optical systems by M. Nakano; B. Champagne; E. Botek; R. Kishi; T. Nitta; K. Yamaguchi (309-312).
The second hyperpolarizabilities (γ) for three spin states (doublet, quartet and sextet) of π-conjugated molecules with a charged defect are investigated as a model of novel spin- and charge- modulated nonlinear optical systems. It turns out from the UCCSD(T) results that a charged defect makes the γ ˙ negative in sign for low and intermediate spin states (doublet and quartet) while the highest spin, sextet, state exhibits remarkably enhanced positive γ value.
Keywords: Nonlinear optics; Hyperpolarizability; Open-shell system; Spin multiplicity; Radical;
Theoretical studies of magnetic interaction in π-radical thiol and Gold hybrid systems by Mitsutaka Okumura; Yasutaka Kitagawa; Takashi Kawakami; Takeshi Taniguchi; Kizashi Yamaguchi (313-316).
Nanoscale magnetic material is becoming an important topic in nanoscience and magnetism. Especially, gold nanoparticles chemisorbed by alkanethiols have drawn much attention due to the size effect on their electronic structure. However, magnetic interaction of gold nanoparticles derivatized by magnetically active ligands has not been investigated in detail. In order to elucidate the magnetic interaction in gold nanoparticles chemisorbed by alkanethiol systems, small gold cluster and π-radical hybrid model systems were examined using the hybrid DFT method. In this research, the effective exchange integrals (Jab) of model systems were calculated for qualitative understanding of the intramolecular magnetic interaction. From these calculations, it was found that there was a possibility of controlling the magnetic interaction of these hybrid model systems using electron/hole doping.
Keywords: Ab initio quantum chemical methods and calculations; Density functional calculation; Models of surface and interface chemistry and physics; Heterojunction;
Theoretical studies on magnetic interaction in one-dimensional CunBrm (S = 1/2) chains by T. Kawakami; T. Taniguchi; Y. Kitagawa; T. Matsumoto; Y. Kamada; T. Sugimoto; M. Okumura; K. Yamaguchi (317-320).
Theoretical calculations for molecular-based magnetic materials have been carried out and successfully revealed magnetic interaction. Recently, Sugimoto and his co-workers reported synthesis and experimental studies for salts of TTF-derivative donors and CunBrm (S = 1/2) chains. In these crystals, their magnetic properties depend on structure changing. In this paper, we focus on magnetic interaction in one- dimensional CunBrm chains. Here, we introduced the universal model structure for their real crystals and four parameters (r, a, b and c) in it can fix the structure. The J ab values under each parameter value were studied by using of UHF, UB3LYP and UCCSD(T) methods. Magnetic interactions in all shapes of CunBrm chains were investigated.
Keywords: Ab initio quantum chemical methods and calculations; Density functional calculation; Organic conductors based on radical cation and/or anion salts;
A quantum chemical study of magnetic interactions in phenazine derivatives for chemical or photo-induced organic magnets by Takeshi Taniguchi; Yuji Watazu; Mitsuo Shoji; Takashi Kawakami; Mitsutaka Okumura; Kizashi Yamaguchi (321-324).
Donor-radical model systems for chemical or photo-induced magnets are proposed, and their electronic states and intramolecular magnetic interactions are investigated by Kohn-Sham hybrid density functional theory (KS-HDFT). 5,10-dihydrophenazine is employed as a donor and nitronyl- or imino-nitroxide radicals as stable radical substituents. The models mostly provide ferromagnetic intramolecular spin alignment in hole doping. In addition, the inductive effect on intramolecular magnetic interactions is examined by substituting the 5- and 10-positions of dihydrophenazine by several electron-withdrawing and -donating groups. The model compounds exhibit appropriate magnetic interactions and ionization potentials to allow them to serve as components of chemical or photo-induced magnets.
Keywords: Donor-radical system; Stable organic radicals; Ab initio MO calculation; Effective exchange integral;
Structural and electronic properties of substituted terthiophenes by Pawel Wagner; David L. Officer (325-328).
Several 3′-(2-arene-l-yl-vinyl)-[2,2′;5′,2′′]terthiophenes and their thiophene analogues were synthesised by Wittig, Wadsworth-Emmons or Knovenagel condensations in good to moderate yields and their electronic absorption and emission spectra were investigated. The spectra are consistent with the presence of two independent chromophores in the ground state together with a single emission from the excited state. There would appear to be exceptions in the analogous cross-linked terthiophene derivatives. A charge transfer effect was also observed for the terthiophene with an indandione substituent.
Keywords: UV-Vis absorption; Polythiophenes;
Single molecule electrical transport using self-assembled-monolayers by V. Burtman; A.S. Ndobe; X. Jiang; Z.V. Vardeny (329-332).
The understanding of current flow through molecular conductors requires an interdisciplinary approach to molecular electronics. New reproducible experiments with better assembling, including more sophisticated fabrication processes and structural control as a feedback for assembling of molecular devices, and designing molecular electronic devices with pre-defined properties are essential for this goal. Any interpretation of molecular transport experimental data requires information on the number of molecular wires per device, device structure and geometry, and the nature of the molecular-electrode bonding. The lack of this knowledge is the pivotal problem in molecular electronics. We have thus embarked in developing approaches to fabricate molecular electronic devices through self-assembled-monolayer (SAM) of molecular wires and insulating molecules in solid-state solutions that enable fine structural control, along with development of spectroscopic and electrical transport tools to study these structures. We used ellipsometry and reflectance spectroscopy to verify that the SAM films can be tuned by changing the ratio, r of molecular conductors (wires, such as Me-BDT) to insulating molecules (spacers, such as PT). We used surface titration for estimating the number of molecular wires per device. The single molecule electrical transport process in the vertical sandwich configuration SAM diodes with small r-value (r < 10−3) that is based on isolated molecular wires, reveals a Fowler-Nordheim type tunneling injection mechanism. However this behavior is not observed in devices fabricated with large r-values.
Keywords: Self-assembled monolayers; Single molecule transport; Single molecule diodes; Organic/inorganic interfaces; Reflection spectroscopy; Ellipsometry;
Author Index of Volume 154 (333-334).
Subject Index of Volume 154 (335-342).