KEYWORDS: Raw materials, Electron microscopy, Molybdenum, Scanning electron microscopy, Spectroscopy, Transmission electron microscopy, Carbon nanotubes, Nanotechnology, Iodine, Nanowires
The processing of most one-dimensional nano-materials such as carbon-nanotubes is hampered by the fact that they are insoluble. Here we show how a significant portion (≈12wt%) of the as-produced Mo6S4.5I4.5 nanowires is stably dispersed in isopropanol as small diameter nanowire bundles. Sedimentation studies, performed combining experiments and theory, show the presence of three phases in the raw material: impurity material, insoluble and soluble nanowire bundles. A purification procedure is also discussed. The three phases has been characterized by UV-Vix-IR spectroscopy and XPS showing their intrinsic diversity.
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.
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.
In this research study carbon nanotubes were investigated as possible reinforcement agents to improve the mechanical and thermal properties of two different polymer matrix systems. The polymer matrices systems examined were polyvinyl alcohol and poly(9-vinyl cabazole). It was found by adding a range of mass fractions of carbon nanotubes that both Young's modulus and hardness as measured by nano-indentation increased dramatically for both matrices. Thermal properties were examined using differential scanning calorimetry and thermo gravimetric analysis. An increase of 82% in Young's modulus and 63% in hardness was observed for polyvinyl alcohol while adding approximately 1% by weight of multi walled carbon nanotubes. In the case of poly(9-vinyl cabazole) an increase of 200% in Young's modulus and 100% in hardness was achieved, by adding only 8% by weight of nanotubes. As far as the authors are aware this is the highest increase of mechanical properties observed when using carbon nanotube as a reinforcing agent. In addition the thermal properties varied significatly on introduction of the nanotubes. An increase of crystallinity was found for the semi-crystalline matrix, while a second phase appeared for the amorphous polymer when increasing the amount of multiwalled carbon nanotubes. Mechanical and thermal properties of the used polymer matrices could be significantly increased.
The nonlinear optical properties of multiwalled carbon nanotubes (MWNT) was investigated using femtosecond, picosecond and nanosecond laser pulses by the Z-scan and Degenerate four wave mixing techniques. Measurements show a significant third order nonlinear response in the both visible and near-infrared wavelengths regions, with Χ(3) values as high as 10-10 esu obtained on nanosecond and picosecond excitation and somewhat diminished Χ(3) values of the order of 10-12 esu obtained on femtosecond excitation. The temporal response at both picosecond and femtosecond excitation show a significant fast component indicating that electronic processes contribute to the third order nonlinear optical response of MWNT. This electronic role is highlighted by the observation of Van Hove singularities (VHS) in the density of states of MWNT. Unexpected visible luminescence from MWNT, observed on photo excitation at 1064 nm, is believed to arise from radiative transitions between energy states in the VHS. Our results shows that the presence of VHS enable efficient optical transitions in MWNT and furthermore the enhancement of the third order nonlinear optical response.
The low, medium and high concentration luminescence and luminescence-excitation spectra for alkyl substituted hexa-peri-hexabenzocoronene (HBC-C8,2) and hexa(4-n-dodecylphenyl) substituted hexa-peri-hexabenzocoronene (HBC-PhC12) are presented. A study of the concentration dependence of the optical properties of these self-assembling molecular nanowires, in the low to medium concentration regime, associates the spectrum at ~ 10-13 M with the single molecule, and indicates that previously published spectra of HBC's by others were the product of aggregation phenomena. The insertion of an exo-phenyl group between the HBC core and the alkyl side chains, as in HBC-PhC12, was found not to extend the conjugation, but did increase the inhomogeneous broadening of the single molecule luminescence. The continued presence of HBC-PhC12 single molecules, at high concentration, implies that HBC-C8,2 aggregates are thermodynamically more stable than HBC-PhC12 aggregates. In conclusion, the spectroscopic properties of both derivatives were found to be very sensitive to aggregation at low concentration and strongly correlated to the observed macroscopic physical properties.
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.
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.
The vibronic structure of the luminescence and luminescence-excitation spectra of alkyl substituted hexa-peri-hexabenzocoronene (HBC-C8,2) and hexa(4-n-dodecylphenyl) substituted hexa-peri-hexabenzocoronene (HBC-PhC12) are described and explained in terms of the collective behavior of molecules in a molecular nanowire structure. The low concentration species of HBC-C8,2(10-13 M) was found to have a homogeneouly broadeend emission expected for an isolated molecule. At medium (10-8 M) and high (10-6 M) concentration the HBC moelcules aggregate into nanowires and a change in the vibronic and electronic structure is observed. The addition of exo-phenyl groups, as in the case of HBC-PhC12, was found to increase the configurational coordinate displacement in the photo-exicted state by increasing the intermolecular vibronic coupling. These result coroelate well with the observed reduced photo-luminescence efficiency of the HBC-PhC12 nanowires.
In this work, mechanical properties of hybrid materials fabricated from nanotubes and commercially available polymers were investigated. It was found that, by adding various concentrations of arc discharge multiwall nanotubes, both Young’s modulus and hardness increased by factors of 1.8 and 1.6 at 1wt% in PVA and 2.8 and 2.0 at 8wt% in PVK, in reasonable agreement with the Halpin-Tsai theory. Furthermore, the presence of the nanotubes was found to nucleate crystallization of the PVA. This crystal growth is thought to enhance matrix-nanotube stress transfer. In addition, microscopy studies suggest extremely strong interfacial bonding in the PVA-based composite. This is manifested by the fracture of the polymer rather that the polymer-nanotube interface. The dependence of the polymer nanotube interfacial interaction on host polymer was studied by intercalating various polymers (PVA, PVP and PS) into single wall nanotube buckypaper. Even for short soak times, significant polymer intercalation into existing free volume was observed. Depending on the polymer and the level of intercalation tensile tests on intercalated sheets showed that the Young’s modulus, strength and toughness increased by factors of 3, 9 and 28, respectively. This indicates that the intercalated polymer enhances load transmission between nanotubes due the significant stress transfer. The level of stress transfer was observed to scale with polymer hydrophobicity as expected.
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 new route for nanotube-based applications in molecular electronics was developed. Individual polymer strands were assembled onto single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) by mechanical agitation. The SWNT hybrid systems have been characterized by electron microscopy (TEM, STM), optical absorption and Raman spectroscopy and a fully nondestructive technique, using electron paramagnetic resonance (EPR), has been developed to estimate the purity of MWNT soot and hybrids. It is demonstrated that solutions of the polymer are capable of suspending nanotubes indefinitely while the majority of the accompanying amorphous graphite precipitates out of solution. Electron microscopy and Raman scattering indicate that through an intercalation process, the ropes of SWNT are destroyed, resulting in individual nanotubes being well dispersed within the polymer matrix. Moreover, Raman and absorption studies suggest that the polymer interacts preferentially with nanotubes of specific diameters or a range of diameters. STM studies showed that the chiral angle of the underlying nanotube is reflected in the polymer coating, demonstrating that the lattice structure of the SWNT templates the ordering in the coating. This could lead to design of specific polymer architectures for selection of desired chiral angles, and hence specific electronic properties.
Visible photoluminescence from multiwalled carbon nanotubes (MWNT) was observed on excitation at 1064 nm. Strong nonlinear behavior of the photoluminescence was shown using power law dependence studies. The nonlinear response in MWNT was further investigated using degenerate four wave mixing. An ultrafast response was observed and the magnitude of the third order optical susceptibility, (chi) , was determined to be in the region of 1.2x10-10 esu. Van Hove singularities in the density of states were identified for the first time in MWNT using optical absorption spectroscopy. Optical transitions between the singularities coincide with the spectral region of the photoluminescence. We propose that a multiphoton absorption process, followed by up conversion luminescence, is responsible for nonlinear photoluminescence in MWNT. Photoluminescence from graphitic particles (GP) was also investigated. This is shown to result mainly from thermal behavior and well-known optical centers. Blackbody radiation was observed in the near infrared region in both materials with MWNT exhibiting lower blackbody temperatures than graphite under the same irradiation conditions.
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