In real electronic devices the elevated operating temperature of the active medium with respect to the "standard" room temperature (21-23°C) is a direct result of Joule heating and acts as a limiter to device performance and lifetime. It has been shown for discrete devices that as the active area is reduced the device is less susceptible to Joule heating. Therefore smaller devices may be driven at higher current densities for a longer period of time than similar devices with a larger active area. This is important for electronic display applications where the display brightness, which is proportional to current density and the display lifetime, is critical. We report on how a porous alumina membrane, filled with nickel using a pulsed electro-deposition technique, was used as a nano-structured anode in polymer light-emitting diodes. Devices made using mechanically polished nickel-filled membranes were tested. Electrical data are presented and the uniformly filled porous alumina based devices sustained higher current densities, than equivalent conventional evaporated metallic sheet-electrode devices. It was found that the reproducibility and rectification ratios of the uniformly filled nickel devices represent a significant improvement on similar copper-filled devices.
Single-wall carbon nanotubes (SWCNT) are severely restricted in their applications, as they exist in rope-like bundles. Recently, J. Coleman et al. demonstrated a spectroscopic method to monitor bundle dissociation in low concentration NT-polymer composites. The method relies on the measurement of the ratio of free-polymer to the nanotube-bound polymer in the SWCNT-polymer solutions via luminescent spectroscopy. A theory has been developed to transform this data into the bundle surface area, which is of course related to the bundle size. This method clearly shows that individual, isolated SWCNT are stable in low concentration dispersions. The main aim of this work is to better understanding of the physics behind polymer-SWCNT interactions, the binding scheme, and the magnitude of the polymer-SWCNT binding energy. In an effort to broaden the understanding of the physical processes governing the NT de-bundling a wide range of suitable polymers and short-chain molecules have been examined. We found a strong dependence of the concentration at which individual NTs become stable with the nature of the dispersant molecule.
A derivative of poly (m-phenylene vinylene-co-2, 5-dioctyloxy-p-phenylene vinylene) is synthesized using two types of condensation polymerisation reactions, namely Horner-Eammons and Wittig, in three different solvents, toluene, dimethhylformamide (DMF) and chlorobenzene. Although the derivatives formed by each reaction have the same chemical composition it is found that their chemical and optical properties vary. NMR studies show that the morphologies of the polymers are different, due to the differences in cis-trans bond ratio of the vinylene bonds. It is found that the derivatives produced by Horner condensation contain a majority of trans bonds and these derivatives show different spectral characteristics to the Wittig derivatives. The ability of the polymers to disperse carbon nanotubes is also studied. Here, not only is the synthetic route important, but the solvent used also plays a role. The derivatives that are produced by the Horner condensation route in DMF or chlorobenzene are found to have the best binding capabilities with carbon nanotubes.
Organic light emitting diodes were fabricated using a novel electrode structure. The anode structure comprised of a metallic nanowire array and was fabricated by electroplating porous aluminium oxide with copper. These devices were compared with devices with a conventional planar anode structure. The light emitting polymer Poly[(4-methylphenyl) imino-4,4'-diphenylene-(4-methylphenyl)imino-1,4-phenylene-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-ethenylene-1,4-phenylene)] was used as the emissive material in single layer devices of structure Cu: TPD(4M)-MEH-PPV: Al, where the aluminium was negatively biased with respect to the copper. The DC current-voltage characteristics of both device types are presented. The electroluminescence spectra are also presented. We found that due to the reduction in active area in the nanowire device from that of the planar device the current density reached in the nanowire array anode device exceeded that in the planar anode device by a factor of eight. Similarly a relative increase in the electroluminescence intensity was also observed.
The change in morphology of a polymer matrix upon the introduction of carbon nanotubes is characterized in this study. Multi-walled carbon nanotubes were dispersed in the conjugated copolymer poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV) to produce a composite material. Photoluminescence (PL) measurements show a reduction in PL efficiency as the nanotube content is increased. Electron microscopy studies have shown an ordering of the polymer around the nanotubes allowing a layer thickness of 25nm to be estimated. This observed thickness agrees well with the expected value of 55nm calculated using a model relating the PL decrease to the changes in conformation that result from polymer - nanotube interactions. Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR) techniques have been employed to investigate how the polycrystallinity of the polymer is affected due to the presence of nanotubes. The results indicate an increase in polymer crystallinity occurs due to an interfacial interaction between the polymer and the nanotube.
Thanks to their cheap processability, organic optoelectronic devices are believed to gradually gain a non-negligible place on the market. However, their performances remain low, mainly because of the poor electron transport in conventional polymers used in such devices. Carbon nanotubes, with a bulk conductivity as high as 10E5 S/m, could therefore be seen as potential candidates to address this important issue. In this work, we have studied the use of a carbon nanotube and polymer composite as an active layer in organic light-emitting diodes and organic photovoltaic devices. Enhanced brightness was achieved using the composite as an electron-transport layer in organic light-emitting diodes, the best efficiency being obtained for those devices with a nanotube content of 1.2 %. Secondly, we have studied the use of the polymer and carbon nanotube composite as the active layer in organic photovoltaic cells. Photocurrents in such devices were greater than that of the cells without carbon nanotubes. It is believed that carbon nanotube composites could act as efficient transport media for charges, which were originally dissociated. This study has demonstrated that carbon nanotubes can be used as functional materials in organic optoelectronic devices and enhance the charge transport, hence the efficiency in such devices.
A thin film preparation technique leading to reduced polaron formation in thin films of the polymer poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV) was used to prepare thin films with significantly improved photoluminescence efficiency. This was achieved through increased interchain separation in films prepared using this technique. Photoinduced absorption measurements were performed to study the nature of the increased photoluminescence efficiency. The electrical transport properties of PmPV films prepared using this preparation technique were measured using both direct and alternating current measurement techniques and found to be improved for positive charge carriers and unchanged for negative charge carriers relative to conventional preparation techniques. The relative permittivity was shown to be greater in this film type, due to the longer delocalisation lengths resulting from increased interchain separation in these films. Fabrication of single layer light emitting devices utilising PmPV prepared using this technique were found to be significantly brighter and to have longer device operating lifetimes.
Experimental measurements of nonlinear optical extinction of nanosecond laser pulses by a set of conjugated co-polymer/multi walled carbon nanotube composites dispersed in solution is reported here. The polymer poly(meta-phenylenevinylene-co-2,5-dioctyloxy-para-phenylenevinylene) and multi walled carbon nanotube composites were varied according to nanotube mass content. The fabrication technique employed to produce the composite material is discussed. The experiments were performed using an open aperture Z-scan with 6 ns gaussian pulses at 532 nm from a frequency doubled, Q-switched Nd:Yag laser. The nonlinear optical extinction of the incident pulses displays enhanced dissipation of the incident light for lower incident intensities relative to increasing multi walled carbon nanotube content. Either the multi walled carbon nanotubes or the polymer dominates the nonlinear response of the composite depending on the relative mass of polymer to nanotube. Mechanistic implications of the optical dissipation are also discussed and investigated via angular dependent scattering measurements.
We have fabricated two conjugated organic polymer-multiwalled carbon nanotube (MWNT) composites and measured the MWNT content of these two hosts using electron paramagnetic resonance (EPR). These polymers were poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV) and poly(9,9-di-n-octylfluorenyl-2,7'-diyl) (PFO). These polymers both disperse MWNTs efficiently but differ in that PFO also suspends graphitic nanoparticles. The fraction of available MWNTs suspended in PmPV rises with increasing polymer mass before saturating at approximately 50% by mass for an optimum soot to polymer mass ratio of 1:4. The optimum settling time for PFO composites was 96 hrs after which 35% of available MWNTs remained suspended. Finally the host polymers were removed by Buchner filtration and the remaining residues were investigated with transmission electron microscopy (TEM). PFO also suspends graphitic nanoparticles with a maximum diameter of approximately 100 nm, which can be attributed to the structure of the polymer itself.
KEYWORDS: Composites, Polymers, Polyurethane, Transmission electron microscopy, Scanning electron microscopy, Artificial muscles, Microscopy, Single walled carbon nanotubes, Crystals, Analytical research
Today, many materials are being investigated as possible artificial muscle devices. Nanotubes and conducting polymers are two of the most attractive materials for this application, because of their low operating voltage. In this research, a number of materials are investigated, including nanotube based polymer composites. Methods of characterisation include thermal analysis using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), hot stage microscopy and polarized light microscopy were used to evaluate the morphology of the composites. Fourier transform infrared spectroscopy was used as a compliment to the DSC and hot stage microscopy to examine the crystallinity. Gel permeation chromatography (GPC) was employed to determine the effect of the nanotubes on the molecular weight of the polymer. Since the application of this research is a biomedical device, the biocompatibility of the composites was examined using contact angle analysis and cytotoxicity tests. In summary, results to date indicate that these materials have promise as possible artificial muscle devices.
Experimental measurements of optical limiting of nanosecond laser pulses by two distinctly different polymer and carbon nanostructure composite materials dispersed in solution is reported here. The polymer poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene) was used to form exclusive multi walled carbon nanotube and polymer composites. The polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) was used to form composites consisting of multi walled carbon nanotubes, other clearly defined carbon nanoparticles and polymer. The fabrication technique and material characterization steps are described, where it was found that the carbon nanostructures were stably dispersed in the polymer matrix in both cases. A range of each of these composites was prepared and varied according to carbon nanostructure mass content. The optical limiting experiments were performed using an open aperture Z-scan apparatus with 6 ns gaussian pulses at 532 nm from a frequency doubled Q-switched Nd:Yag laser. In the poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene) and exclusive multi walled carbon nanotube composite either the multi walled carbon nanotubes or the polymer dominates the nonlinear response depending on the relative mass of polymer to nanotube. In the other material saturation of the optical limiting was reached at carbon nanostructure mass percentages in excess of 3.8%, relative to the polymer mass, while the polymer exhibited no response of its own. Furthermore, the scattering of high intensity light from the materials was qualitatively probed and its angular dependence investigated. The nature of the carbon nanostructure inclusions in each material was found to significantly influence the scattering response of the composites.
We have studied the effects of using a composite fabricated from carbon nanotubes and a host polymer, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-viny lene) (PmPV), as an electron-transport layer in organic light-emitting diodes. Double layer devices using this composite as an electron-transport layer, triple layer devices with a composite electron-transport layer and poly(9-vinylcarbazole) (PVK) as a hole-transport layer, as well as poly(2,5-dimethoxy-1,4-phenylene-vinylene-2-methoxy-5(2'-eth ylhexyloxy)-1,4-phenylene-vinylene (M3EH-PPV) single layer devices were prepared. Current-voltage-luminance and electroluminescent spectral measurements were performed using six different nanotube powder to polymer mass ratios (0, 2, 4, 8, 16, and 32%) for all device structures studied. DC transport and photoluminescence behavior of the polymer-nanotube composite were also investigated. Although a potential barrier is introduced at the M3EH-PPV/composite interface, a significant increase in efficiency was observed using the composite. The best efficiency was obtained for those devices with an electron-transport layer of mass ratio 8%. In addition, on doping with nanotubes, electron conductivity in the composite increased by over four orders of magnitude with little quenching of photoluminescence.
A nonlinear optical study in poly (arylene ethynylene)(PAE) and poly (arylene vinylene) (PAV) copolymers for optical switching is presented. The principle aim of this work is to establish the polymer properties, which can be exploited in the design of materials useful for optical switching. A systematic study of the polymer parameters in solution and solid state are analysed in view of their performance as nonlinear materials. Insight is gained into the material performance, which is then related back to the structure. Wavelength dependent values of the third order nonlinear absorption coefficient (beta) and the third order nonlinear refraction coefficient (eta) 2, for all four polymers are measured using the Z-scan technique. Suitability for optical switching applications is ascertained from the linear and nonlinear absorption. Two figures of merit are presented, enabling the suitability of the materials for optical switching to be ascertained, in the wavelength region spanning 465nm through 685nm. Molecular mechanical modeling is employed to provide a preliminary view of the polymer structure to aid the understanding of the nonlinear optical and film forming properties of the polymers. Correlations are drawn between the PAE and PAV polymers, enabling the possible synthesis of the ideal material for optical switching devices at visible and near infra red wavelengths, utilizing solid state organic materials.
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