Garrett F. Farrell

Spectroscopic analysis of single-walled carbon nanotubes and semiconjugated polymer composites.

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 HiPco SWNTs are explored using UV/vis/NIR and Raman spectroscopy at 514.5 and 632.8 nm to determine specific interactions. An examination of the radial breathing modes at 514.5 nm shows similar tube diameters of 1.28 and 1.35 nm selected for both the arc discharge and HiPco composites. The corresponding G lines of both composites show no specific type of tubes being selected. At 514.5 nm, the G line of the HiPco composite (1% mass fraction) shows contributions from semiconducing and metallic tubes, and the arc discharge composite (1% mass fraction) is dominated by semiconducting nanotubes. At 632.8 nm, the G line of the HiPco composite (1% mass fraction) is dominated by semiconducting tubes, and the arc discharge composite (1% mass fraction) shows strong contributions from metallic tubes. This finding is a strong indication that the selection process is dependent on tube diameter rather than backbone structure. The solubility limits of both composites are determined by investigating the G lines of both composites and have been found to be greater than 1% mass fraction by weight for the arc discharge composite and greater than 0.1% mass fraction by weight for the HiPco composite.

Stokes/anti‐Stokes Raman Spectroscopy of HiPco Single‐Wall Carbon Nanotubes

An analysis of the Raman spectra of single‐walled HiPco carbon nanotube powder using laser energies of 1.92 eV, 2.4 eV, 2.5 eV and 2.7 eV, including a comparison of the Stokes and anti‐Stokes contributions is presented. The diameter distribution was determined to be 0.8 – 1.2 nm from the spectral positioning of the Radial Breathing Modes. The diameter distribution is consistent with that determined by NIR absorption spectroscopy. At all excitation energies the profile of the G‐line (≈ 1580cm−1) indicates a predominance of semiconducting tubes although at 2.4 eV there is some indication of some contribution from metallic tubes. In all cases the anti Stokes line was weak and the Stokes/anti Stokes ratio was at least an order of magnitude indicating at most weak resonance enhancement in either.

Fluorescence concentration studies of HiPco SWNTs and semi-conjugated polymers

Photoluminescence intensity PL measurements were taken for a range of PmPV concentrations, in which HiPco single walled carbon nanotubes (SWNTs) at 100%, 10%, 1%, 0.1%, 0.01% and 0% mass fractions were added. The PL intensity of the composite was shown to decrease for all mass fractions, relative to the polymer up to 1.56x10-3g/l of PmPV, above which there is an initial increase in the composite emission yield with respect to the polymer. This increase is associated with an interaction within the composite, which results in a decrease in polymer aggregate formation, which has been shown to quench intensity yields. Within the concentration range studied 5.9x10-8g/l to 2g/l the photoluminescence intensity yield for each system is highly non linear. Previously the ratio of the maximum PL intensity of the composite, which includes both, bound and unbound polymer chains, and the maximum PL intensity of the polymer, which includes only unbound polymer chains was plotted as a function of polymer concentration. From this the authors calculated the amount of free polymer within each composite and derived a model, which showed that as the polymer concentration is lowered the bundles break up until isolated SWNTs are stable at low concentrations. In particular for their 100% mass fraction polymer/HiPco SWNT it was shown that individual nanotubes are stable in solutions ~3x10-5kg/m3. Here we utilize this approach and results indicate that as the mass fraction of nanotubes in reduced, individual nanotubes are stable at higher polymer concentrations. In particular for our 100% mass fraction results indicate that below ~1.5x10-4g/l individual nanotubes are stable. This result indicates that the choice of polymer and or solvent has a significant effect on the debundling and aggregation within these systems.

Spectroscopic analysis of the intermolecular interactions of gamma cyclodextrin and carbon nanotubes

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

Excited state properties of C 60 revisited: a Raman study

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-situ Raman spectroscopy of electrically generated species in fullerene thin films

Organic materials have, in recent decades, been shown to be insulators, semiconductors, or even metallic when doped and the prospect of cheap, easily fabricated devices has attracted much interest. Primitive devices have been demonstrated and yet potentially competitive performance has been limited to polymer light emitting diodes. The recent report that lattice expanded C60 single crystals can be made superconducting, with a transition temperature of 117K, by the injection of charge via a FET type geometry has once again highlighted the potential of C60 in the development of molecular electronic devices. In light of the aforementioned report it is essential that a true understanding of the inter- and intramolecular processes in terms of their contribution to the electronic transport be obtained. In this study the current voltage characteristics of C60 thin film sandwich structures fabricated by vacuum deposition on indium tin oxide (ITO) with an aluminium top electrode are presented and discussed. A strongly non-linear behavior and a sharp increase in the device conductivity was observed at relatively low voltages (~2V), at both room and low temperatures (20K). At room temperature the system is seen to collapse, and in situ Raman measurements indicate a solid state reduction of the fullerene thin film to form a polymeric state. The high conductivity state was seen to be stable at elevated voltages and low temperatures. This state is seen to be reversible with the application of high voltages. At these high voltages the C60 film was seen to sporadically emit white light at randomly localized points analogous to the much documented electroluminescence in single crystals. Moreover the evidence suggests that this highly conducting species maybe similar in nature to a high intensity optically excited species. It is further speculated that the species recently reported in the superconducting lattice expanded C60 single crystals may also be analogous to the highly conducting species observed here.

Physical interactions between HiPco SWNTs and semiconjugated polymers

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.

Electronic transfer studies of fullerene/polymer hybrids

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 its 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 Physical Interactions between HiPCo SWNTs and Semi‐Conjugated Polymers

Hybrid systems of the conjugated organic polymer poly (p‐phenylene vinylene‐co‐2,5‐dioctyloxy ‐m‐phenylene vinylene) (PmPV) and HiPco SWNT are explored using spectroscopic and thermal techniques to determine specific interactions. Vibrational spectroscopy indicates a weak interaction and this is further elucidated using Differential‐Scanning Calorimetry and Temperature Dependent Raman Spectroscopy. Two distinct transitions in region of −60°C and + 60°C are investigated.

The generation of a carbon nanotube-cyclodextrin complex

Abstract : In this study the intermolecular interactions of small diameter (^ 0.7 nm) carbon nanotubes and gamma-cyclodextrin were examined. Four samples of gamma cyclodextrin and HiPco carbon nanotubes were prepared. The fast, by grinding the tubes and the cyclodextrin (1:30 ratio) together in a dry mixture, the second was prepared in a similar fashion but was ground in the presence of water (1 ml). Finally an aqueous solution of gamma-cyclodextrin (0.3 M) and HiPco carbon nanotubes (5 mg) was prepared by refluxing for 100 hours, forming a pale yellow solution from which a number of crystals were produced, both the solution and the recrystallised material were analysed. The samples were analysed using UV-Vis-NIR and Raman spectroscopy. The results presented are the first spectroscopic evidence of an intermolecular interaction between gamma-cyclodextrin and single wall nanotubes.