Carbon nanotube-containing photorefractive polymer composites operating at telecommunication wavelengths

The field dependences of photocurrent, the two-beam coupling gain coefficient, and the grating formation time constant in polymer composites made from polyvinylcarbazole (PVK) and single-wall carbon nanotubes (SWNTs) were measured under the conditions of one-photon SWNT excitation with continuous laser radiation at a wavelength of 1550 nm. Carbon nanotubes are responsible for optical electronic absorption up to ～2000 nm in this composite. The dependence of the quantum efficiency of generation of mobile charge carriers on the electric field E ₀ as determined from the photocurrent coincides with the curves calculated via the Onsager equation expanded to the (E ₀)⁴ term, at a quantum yield of thermalized electron-hole pairs of η₀ = 0.07 and a charge separation distance in the pair of r ₀ = 9.8 Å. An analysis of the photorefractive characteristics showed that the admixture of fullerene C₆₀ in an amount of 3 wt % to the PVK composite with 0.26 wt % SWNT leads to a twofold increase in the beam-coupling gain coefficient. In the PVK-matrix composite containing 0.26 wt % SWNT and 3 wt % C₆₀, the beam-coupling gain coefficient Γ of a 1550-nm laser beam and the net gain Γ-α are 32 and ～27 cm^?1, respectively, at a constant field of E ₀ = 140 V/μm.     

Thermal and dielectric characterization of multi-walled carbon nanotubes−thermoplastic polyurethanes composites

Multi-walled carbon nanotubes–thermoplastic polyurethanes composites were characterized by means of differential scanning calorimetry and dielectric relaxation spectroscopy. The composite is characterized by two glass transition temperatures T _g . The T _g associated with the soft segment decreases by increasing of carbon nanotubes content, while carbon nanotubes content has practically no effect on the value of the T _g associated with the hard segments. It was observed that rising the temperature and carbon nanotubes content resulted in the increased of both the dielectric permittivity and the loss factor. The presence of carbon nanotubes produces an enhancement of charge carriers trapping, increasing the electrical conductivity. The electrical conductivity of the composite was found to exhibit an insulator to conductor transition at a carbon nanotubes critical content, i.e., the percolation threshold, near 6 wt %.     

The nature of the field dependence of drift mobility in molecularly doped polymers

A direct experimental comparison is performed for the field dependences of drift and effective mobilities of holes in a polar molecularly doped polymer (polycarbonate containing 30 wt % p-diethylaminobenzaldehyde diphenylhydrazone), which are measured by the time-of-flight method during the bulk irradiation of polymer samples with a pulse of fast electrons. A numerical simulation of the time-of-flight experiments is conducted with the use of the multiple-trapping model with the Gaussian energy trap distribution. The parameters of the model are determined during independent measurements. It is shown that, because of the nonequilibrium state of the transport of holes under the conditions of the time-of-flight experiment, the true Poole–Frenkel constant (0.27 (μm/V)^1/2) makes up only a part of the experimentally measured value (0.39 (μm/V)^1/2).     

Infrared photorefractive composites based on polyvinylcarbazole and carbon nanotubes

Photorefractive materials based on unplasticized polymers that have a high glass-transition temperature and the frozen random orientation of chromophores were prepared by layer casting. Under these conditions, only the third-order susceptibility has a nonzero value, increasing with an increase in the conjugation-chain length and reaching considerable values in the case of nanosized molecules, such as single-wall carbon nanotubes (SWNTs). In SWNT-containing polyvinylcarbazole, the photoelectric sensitivity and photorefractive characteristics were measured at 1064 nm. Using electric-field induced second-harmonic generation, the third-order susceptibility was estimated. In a composite containing 0.26 wt % SWNT, the diffraction grating is displaced by 5° relative to the interference pattern, the result that is presumably due to the close mobility of unlike charge carriers. Therefore, the beam-coupling gain coefficient Γ and the net gain Γ-α have low values, which are 53 and 42 cm^?1, respectively, at 115 V/μm.     

Intrachain versus Interchain Electron Transport in Poly(fluorene‐alt‐benzothiadiazole): A Quantum‐Chemical Insight

Poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) [F8BT], displays very different charge‐transport properties for holes versus electrons when comparing annealed and pristine thin films and transport parallel (intrachain) and perpendicular (interchain) to the polymer axes. The present theoretical contribution focuses on the electron‐transport properties of F8BT chains and compares the efficiency of intrachain versus interchain transport in the hopping regime. The theoretical results rationalize significantly lowered electron mobility in annealed F8BT thin films and the smaller mobility anisotropy (μ_∥/μ_⊥) measured for electrons in aligned films (i.e. 5–7 compared to 10–15 for holes).     

Properties of Film Materials Based on Composite Nanofibers from Aliphatic Copolyamide and Carbon Nanotubes for Tissue Engineering

The investigation of the dependence of effective viscosity on shear rate for a water-alcohol solution of an aliphatic copolyamide and its mixtures with single-wall carbon nanotubes reveals that additives of the nanoparticles in the amount of 0.5 wt % lead to a substantial reduction in the effective viscosity as the shear rate rises. The measurement of the surface tension and electrical conductivity of the solutions bearing 0.1–2.0 wt % of the nanotubes allows one to choose an optimal mode for electrospinning of the composite nanofibers based on the aliphatic copolyamide. The introduction of carbon nanofibers reduces the specific resistance of the material to 8.9 × 10⁹ Ω m, but increases the elastic modulus. The lack of cytotoxicity of the resulting materials and the high proliferative activity of human dermal fibroblasts on their surface allow one to use the film materials based on the composite nanofibers in cell technologies and as matrices for tissue engineering.     

Functional composites based on polybenzimidazole and graphite nanoplates

A procedure was developed for preparing stable dispersions of graphite nanoplates with the concentration of up to 25 mg mL^–1 by two-step ultrasonic treatment of graphite in N-methyl-2-pyrrolidone. A series of elastic films based on poly-2,2′-p-oxydiphenylene-5,5′-bisdibenzimidazole oxide with the filler content of up to 45 wt % were prepared from such dispersions. Introduction of the nanoadditive into the matrix of the heterocyclic polymer results in 47% enhancement of the tensile strength of the materials and in an increase in the temperature of the 10% weight loss by 52–81°C. In addition, the films are characterized by high electrical conductivity reaching 480 S cm^–1 for the composite with 45 wt % graphite nanoplates and exhibit tensoresistive properties, which allows using them in various electrotechnical devices and fabric engineering structures.     

Effect of single-wall carbon nanotubes on the properties of polymeric gel electrolyte dye-sensitized solar cells

A hybrid of polymer/dispersed single-wall carbon nanotubes was utilized in networking a novel composition of gel electrolyte in dye-sensitized solar cells. The gel is composed of polyethylene glycol, polyvinyl pyrrolidone, single-wall carbon nanotubes, and I^?/I₃ ^? as electrolyte. Formation of the less conductive polyiodide species in electrolyte was prohibited by the addition of single-wall carbon nanotubes leading to the excellent photovoltaic behavior of the cell under simulated standard illumination of the fabricated device owing to the increased open circuit voltage (0.47 V). Electrochemical impedance spectroscopy was employed to quantify the charge transport resistance and the electron lifetime at the TiO₂ conduction band. Charge transport resistances at the TiO₂/dye/electrolyte interface were determined for the cells consisting of the non-gel reference and our new gel electrolytes, and it was indicated that the charge recombination between injected electrons and electron acceptors (I₃ ^?) in the redox electrolyte was remarkably retarded. Electrochemical parameters obtained by the fitting showed all of the resistances increased as compared to liquid electrolyte dye-sensitized solar cells that can be related to the increase in viscosity of the gel, which hinders the ionic transportation through the electrolyte. These results were also confirmed by the electron lifetime analyses. The characteristic peak shifted to a lower frequency in the Bode phase plot for the cell containing gel electrolyte which is an indication of a longer electron lifetime in comparison with that of the cell containing very conventional liquid electrolyte.     

Engineered Molecular Chain Ordering in Single‐Walled Carbon Nanotubes/Polyaniline Composite Films for High‐Performance Organic Thermoelectric Materials

Single‐walled carbon nanotubes (SWNTs)/polyaniline (PANI) composite films with enhanced thermoelectric properties were prepared by combining in situ polymerization and solution processing. Conductive atomic force microscopy and X‐ray diffraction measurements confirmed that solution processing and strong π–π interactions between the PANI and SWNTs induced the PANI molecules to form a highly ordered structure. The improved degree of order of the PANI molecular arrangement increased the carrier mobility and thereby enhanced the electrical transport properties of PANI. The maximum in‐plane electrical conductivity and power factor of the SWNTs/PANI composite films reached 1.44×10³ S cm^?1 and 217 μW m^?1 K^?2, respectively, at room temperature. Furthermore, a thermoelectric generator fabricated with the SWNTs/PANI composite films showed good electric generation ability and stability. A high power density of 10.4 μW cm^?2 K^?1 was obtained, which is superior to most reported results obtained in organic thermoelectric modules.     

Enhanced performance of field-effect transistors based on C<Subscript>60</Subscript> single crystals with conjugated polyelectrolyte

Contact resistance at the interface between metal electrodes and semiconductors can significantly limit the performance of organic field-effect transistors, leading to a distinct voltage drop at the interface. Here, we demonstrate enhanced performance of n-channel field-effect transistors based on solution-grown C₆₀ single-crystalline ribbons by introducing an interlayer of a conjugated polyelectrolyte (CPE) composed of poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] dibromide (PFN⁺Br^?). The PFN⁺Br^? interlayer greatly improves the charge injection. Consequently, the electron mobility is promoted up to 5.60 cm² V^?1 s^?1 and the threshold voltage decreased dramatically with the minimum of 4.90 V.     

Synthesis and electrochemical properties of nanocomposites containing poly(3,6-bis(phenylamino)-2,5-dichlorobenzoquinone and multiwalled carbon nanotubes

The method of synthesizing nanocomposites based on multiwalled carbon nanotubes and a new polymer poly(3,6-bis(phenylamino)-2,5-dichlorobenzoquinone) containing polyaniline chains with electroactive substituents in the N-position is developed, and the electrochemical properties of the composites are studied. The in situ oxidative polymerization of N-substituted aniline performed in the presence of multiwalled carbon nanotubes makes it possible to design an organized, effective structure of the conducting composite material with enhanced electrochemical capacity and stable capacity parameters during long cycling in protic (1 mol/L H₂SO₄) and aprotic (1 mol/L LiClO₄ in propylene carbonate) electrolytes.     

Preparation of a polyacetylene composite with single-walled carbon nanotubes and study of its electrochemical properties

A composite consisting of single-walled carbon nanotubes (2.4 wt %) covered by polyacetylene has been prepared. Polyacetylene synthesized directly on single-walled carbon nanotubes is a defectless long-chain polymer composed of trans and cis units with a sufficient length (60% trans and 40% cis isomers). An appreciable acceleration of the lithium ion intercalation-deintercalation corresponds to an increase in the conductivity of the composite material near the percolation threshold. The polyacetylene layer on single-walled carbon nanotubes plays the role of an electrochemically active ion-conductive membrane suitable for the transport of Li⁺ cations to the surface of nanotubes.     

Signal‐Enhanced Amperometric Immunosensor Based on Ferrocene‐Branched Poly(allylamine)/Multiwalled Carbon Nanotubes Redox‐Active Composite

A signal‐enhanced immunosensor has been developed by self‐assembling Au NPs onto a ferrocene‐branched poly(allylamine)/multiwalled carbon nanotubes (PAA‐Fc/MWNTs) modified electrode for the sensitive determination of hepatitis B surface antigen (HBsAg) as a model protein. The formation of PAA‐Fc/MWNTs composite not only effectively avoided the leakage of Fc and retained its electrochemical activity, but also enhanced the conductivity and charge‐transport properties of the composite. Further adsorption of Au NPs into the PAA matrix provided both the interactive sites for the immobilization of hepatitis B surface antibody (HBsAb) and a favorable microenvironment to maintain its activity. Tests performed with this immunosensor showed a specific response to HBsAg in the range of 0.1–350.0 ng mL^?1 with a detection limit of 0.03 ng mL^?1.     

Coaxial conducting polymer nanotubes: polypyrrole nanotubes coated with polyaniline or poly(<Emphasis Type="Italic">p</Emphasis>-phenylenediamine) and products of their carbonisation

Polypyrrole nanotubes were prepared by the oxidation of pyrrole with iron(III) chloride in a reaction mixture containing methyl orange. They were subsequently coated with polyaniline or poly(p-phenylenediamine) in situ during the oxidation of respective monomers in their presence. A part of the coaxial nanotubes was deprotonated using ammonia solution. The conductivity of polypyrrole nanotubes of 60 S cm^?1, was reduced after the coating, and again after the deprotonation, but maintained at a level above 10^?4 S cm^?1. Infrared and Raman spectra reflect the presence of the polymer overlayer deposited on the polypyrrole template. Thermogravimetric analysis was used as a tool for the analytical carbonisation of samples in an inert nitrogen atmosphere. The conversion of conducting polymers to nitrogen-containing carbon nanotubes was confirmed using Raman spectra.     

Photoelectric,nonlinear optical,and photorefractive properties of composites based on poly(<Emphasis Type="Italic">N</Emphasis>-vinylcarbazole) and gallium phthalocyaninate

Poly(N-vinylcarbazole) layers containing tetra-5-crown-5-gallium phthalocyaninate (R₄Pc)Ga(OH) are shown to possess photoelectric and photorefractive sensitivity at a wavelength of 1064 nm. This effect is associated with the formation of supramolecular ensembles of (R₄Pc)Ga(OH) molecules with electronic optical absorption in the near-IR range and nonlinear optical properties. For the composite containing 5 wt % (R₄Pc)Ga(OH) supramolecular ensembles, the dependence of the quantum efficiency of mobile-charge photogeneration on electric field E ₀ is well fit by the Onsager equation expanded to E ₀ ³ at a quantum yield of electron-hole pairs of φ₀ = 0.9 s with an initial separation radius of r ₀ = 9.8 Å susceptibility χ⁽³⁾ equal to 1.85 × 10^?10 esu is measured via the well-known method of electric-field-induced second-harmonic generation. Two-beam-coupling gain coefficient Γ is found to be 80 cm^?1 at E ₀ = 120 V/μm.     

Conductive polymer nanocomposites containing <Emphasis Type="Italic">in situ</Emphasis> ultra-fine metal particles

A conductive polymeric composite containing in situ ultra-fine metal particles is prepared by melt blending. Incorporation of elastomeric nano-particles and carbon nanotubes hinders the coalescing of metal particles and causes a shift to the breakup direction in the breakup/coalescence equilibrium of metal particles. The prime metal particles (about 26 μm) are in situ converted into the ultra-fine metal particles (UFMP, about 932 nm). The network of carbon nanotubes has been improved due to in situ ultra-fine metal particles and the percolation threshold of the composite with 1.96 vol% UFMP is only 0.25 vol% carbon nanotubes.     

The effect of length of single‐walled carbon nanotubes (SWNTs) on electrical properties of conducting polymer–SWNT composites

DC conductivity of conjugated polymer‐single‐walled carbon nanotube (SWNT) composite films has been measured for different SWNT concentrations. The composite was prepared by dispersing SWNTs in the poly (3‐octylthiophene), P3OT matrix already dissolved in xylene. The conductivity of the composite films showed a rapid increase as the SWNT concentration increases beyond a certain value. This behavior is explained in terms of percolating paths provided by the SWNTs in the volume of polymer matrix. To investigate the effect of length of nanotubes on the percolation conductivity, different SWNT samples were employed with similar diameter but varying tube lengths. It was found that the conductivity of the composite films is strongly dominated by the length of the nanotubes. Lower percolation limit and high conductivity value of composite films is observed for longer nanotubes. Furthermore, the conductivity is observed to be dependent on the size of the host polymer molecule also. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 89–95, 2010     

Near-IR photorefractive composites based on oxidized single-wall carbon nanotubes

The photoelectric, nonlinear optical, and photorefractive characteristics of polymer composites made from unplasticized polyvinylcarbazole (PVK) and single-wall carbon nanotubes containing surface oxygen groups (at C:O ratio of 77.8:22.2%) were analyzed. The dependences of the quantum efficiency of charge-carrier generation on the applied electric field E measured at 1064 and 1550 nm coincide and are approximated by the Onsager equation expanded to the E ⁴ term, at π₀ = 0.012 and r ₀ = 9.8 Å. The third-order optical nonlinearity determined by the EFISH technique at a nanotube content of 0.26 wt % is 2 × 10⁵ pm²/V² or 2.3 × 10^?29 C⁴ m/J³ in SI units. The pattern of photorefractive kinetic curves indicates that the mean free path of holes is longer than the mean range of electrons. The photorefractive net gain coefficient of the signal beam measured at 1064 nm in a field of E = 170 V/μm is Γ ? α = 55 cm^?1 (α = 10 cm^?1 is the optical absorption coefficient at 1064 nm). At 1550 nm, the net gain coefficient measured in a field of E = 265 V/μm is Γ ? α = 55.7 cm^?1 (α = 3.3 cm^?1 at 1550 nm). In the presence of oxygen groups, there is no transfer of electrons photogenerated in carbon nanotubes to the external acceptor C⁶⁰. This effect can be associated with an increase in the ionization potential of nanotubes by almost 0.8 eV as a result of oxidation.     

Secondary doping in poly(3,4‐ethylenedioxythiophene):Poly(4‐styrenesulfonate) thin films

The electrical and structural properties of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of selected secondary dopants are studied in detail. An improvement of the electrical conductivity by three orders of magnitude is achieved for dimethyl sulfoxide, sorbitol, ethylene glycol, and N,N‐dimethylformamide, and the secondary dopant concentration dependence of the conductivity exhibits almost identical behavior for all investigated secondary dopants. Detailed analysis of the surface morphology and Raman spectra reveals no presence of the secondary dopant in fabricated films, and thus the dopants are truly causing the secondary doping effect. Although the ratio of benzenoid and quinoid vibrations in Raman spectra is unaffected by doping, the phase transition in PEDOT:PSS films owing to doping is confirmed. Further analysis of temperature‐dependent conductivity reveals 1D variable range hopping (VRH) charge transport for undoped PEDOT:PSS, whereas highly conductive doped PEDOT:PSS films exhibit 3D VRH charge transport. We demonstrate that the charge ‐ hopping dimensionality change should be a fundamental reason for the conductivity enhancement. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1139–1146     

Electron transport of holes in molecularly doped polycarbonate and its radiation-induced conductivity

The salient features of charge transport in a typical molecularly doped polymer (polycarbonate + 30 wt % DEH hydrazone) were studied by time-of-flight and nonsteady-state radiation-induced conductivity measurements. It was shows that the mobility of holes (major carriers) is due to dispersive transport in the temperature range 296–353 K covering the glass transition temperature at an observation time of up to a few seconds. The appearance of a plateau on the current transient, presumably manifesting the establishing of quasiequilibrium (Gaussian) transport, is the artifact of the time-of-flight technique when the charge carrier generation takes place at the sample surface. All of the obtained results can be satisfactorily rationalized in terms of the Rose-Fowler-Weisberg model with a uniform set of parameters of the model. Such an approach is compatible with the basic concepts of the radiation chemistry of condensed phase (the Onsager theory and the Langevin recombination mechanism), structural features of a disordered medium (transport zone, structural traps), and rotational diffusion of small molecules or their molecular groups in vitrified polymers.