Ultrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t)-α, with α in the range of 0.7 to 1. The PL emission of S-NTs in D2O solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.
In this study we examine the interaction of both cis and trans poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene (PmPV-co-DOctOPV) with C60 in solution From the presented data it is clear that there is an interaction between the HE PmPV and C60. Just what this interaction is however is not as clear. A possible explanation that fits the available data involves the HE PmPV wrapping around the C60 molecules, similar to the effect observed by Dalton et al with Carbon Nanotubes. In theory the close proximately of the coils to the C60 molecules may allow for charge transfer or energy transfer between to the two molecules. If this theory is correct it would explain the why the absorption spectra of HE-PmPV at the different loaded fraction displays a negative deviation for the expected values. It may be speculated that due to the coiling the C60 molecules are prevented from absorbing photons of light, consequently resulting in a reduction in it's contribution to the overall intensity. This theory would also explain the increased quenching effect observed in the luminescence spectra at the same percentage weights, since the close proximity of the coils to the C60 molecules allows for charge or energy transfer between the two.
The production of small diameter (0.7-1.2nm) and high purity single walled carbon nanotubes using a gas-phase catalytic approach has aroused considerable interest in the chemistry of this unique material. Most recently it has been proposed that tubes produced in this manner can be cut by simply grinding them in a soft organic material such as g-cyclodextrin. The results reported on such cutting techniques however concentrated upon microscopy thereby limiting the degree of information, which could be deduced about the type of interaction between the two materials. In this study electronic and vibrational spectroscopy as well as Differential Scanning Calorimetry has been performed upon a ground mixture of the aforementioned single walled carbon nanotubes and γ-Cyclodextrin. The mixture was prepared by grinding in a 30:1 ratio γ-cyclodextrin and single walled carbon nanotubes for approximately two hours with the drop-wise addition of ethanol (1ml) in the first 10 minutes. A similar ground mixture of g-Cyclodextrin and multi walled carbon nanotubes was also prepared to help asses the type and degree of interaction between the single walled carbon nanotubes and the γ-Cyclodextrin. Absorption spectroscopy showed changes to the electronic structure of both the single walled carbon nanotubes and the γ-Cyclodextrin, while evidence from Raman spectroscopy indicates that the cyclodextrins are absorbed via van der Waals forces along the length of the tube inducing a compressive strain. No such evidence for an interaction with multi walled carbon nanotubes was observed suggesting the possibility of a diameter selective interaction. Finally as a comparison a sample containing 5mg of tubes was refluxed in an aqueous solution of γ cyclodextrin (0.3M) for ~72 hour similar to early studies preformed on C60 and γ cyclodextrin
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.
Single wall carbon nanotubes are insoluble in most organic solvents such as toluene. Improvements in the solubility of the single wall carbon nanotubes are however seen as a result of specific interactions with molecules such as terphenyl and anthracene. Suspensions formed in toluene with these molecules and the single wall carbon nanotubes are seen to be stable over prolonged periods. Spectroscopic analysis clearly shows an interaction between the carbon nanotubes and the molecules. It is proposed in this study that the use of these more simple molecular systems may help elucidate the nature and extent of the interaction in more complex composite based systems.
Interactions between Arc Discharge single walled carbon nanotubes within polymer composites have been well documented. Here hybrid systems of the conjugated organic polymer poly (p-phenylene vinylene-co-2,5-dioctyloxy -m-phenylene vinylene) (PmPV) and carbon nanotubes produced by Gas-Phase Decomposition of CO (HiPco) process are explored using Raman spectroscopy. Laser excitation wavelengths of 514.5nm and 633nm are employed to determine the specific nature of interaction. Also presented at laser energies 1.96eV and 2.4eV, are hybrid systems of Arc Discharge SWNTs at similar mass fractions to enable a direct comparison of solubility of each nanotube type within the polymer composite to be made. Comparisons are also made between the two hybrid systems in relation to range of nanotube diameters selected at 514.5 and 633nm.
The excited state properties of C60 thin films have been probed in the temperature range 77-273K using Raman spectroscopy. The change in the Raman pentagonal pinch mode of C60 (whose position is largely independent of temperature) was monitored as a function of the excitation intensity at 514.5nm. This mode, normally positioned at 1469cm-1, was seen to shift reversibly to a lower Raman frequency with increasing laser intensity. Two excited state species have been identified. The first, at 1466cm-1 has been associated with the molecular triplet of C60 as determined from measurements preformed in solution. The second species at 1463cm-1, has been speculated to be an excited state co-operative involving two or more excited states in the solid and is seen to be intrinsic to solid state C60 below the phase transition, as similar measurements in solution show no Raman evolution beyond 1466cm-1. This species observed at 1463cm-1 has previously been reported in the depolymerisation of solid state C60, as well as in reversible processes in C60 crystals and has been characterised by a nonlinear photo-luminescence and photoconductivity. It is further proposed in this study that this excited state is analogous to a number of highly conducting states recently reported for C60. The data presented highlights the existence of a highly non-linear delocalised excited state species at low temperatures which is intrinsic to solid state C60.
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.
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.