Tailored pore size mesoporous silica, incorporating different concentrations of transition metal-based catalysts, has been used as platforms for the growth of carbon nanotubes by the catalytic chemical vapor deposition method. Both compositional surface analysis by EDX/SEM combinatory techniques and thermo gravimetric analysis were employed to characterize the samples prior to CNT growth. The CNTs produced were characterized using Raman Spectroscopy, high resolution SEM and TEM. Raman spectroscopy showed good quality highly graphitic CNTs and indicated the presence of crystalline graphitic carbon, microcrystalline graphite as well as amorphous carbon in the carbon nanotube layer. TEM and HI RES SEM images matched diameters of the carbon nanotubes to the corresponding pores of the matrices. Comparison of the carbon nanotube diameters to porous properties of the mesoporous silica confirmed probable growth from within the pores. The density of the carbon nanotubes was found to be high for higher metal concentrations for the same pore diameters. Fe and Co were confirmed to be better catalysts, compared to Ni, for growth of carbon nanotubes by the catalytic chemical vapour method.
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.
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.
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.
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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