Elizabeth Gregan

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.

Use of Raman Spectroscopy in the Investigation of Debundling of Single Walled Carbon Nanotubes

Samples of raw nanotubes are compared to those deposited from solutions to examine separation of nanotube bundles. Single wall nanotubes bundles produced by the arc-discharge and HiPco methods were solubilised in toluene, DMF and 1,2 dichloroethane. Resonant Raman spectroscopy was used to determine if debundling of the tubes sample occurred. The results showed some degree of debundling, best for the 1,2 dichloroethane solvent, which also shows long term solubility.

Stokes/anti-Stokes Raman spectroscopy of high-pressure carbon oxide (HiPco) single-walled 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 0.8-1.2nm 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 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.

The Use of Raman Spectroscopy in the Characterization of Single Walled Carbon Nanotubes

Carbon nanotubes, long, thin cylinders of carbon are intriguing structures and have sparked much excitement in recent years. It is clear that understanding the fundamental properties of nanotubes is necessary to get a realisation of the full potential of these materials. Processing and purification on a mass scale will need to be carried out if these materials are to have large scale industrial applications and move beyond the laboratory bench. Highly sensitive, capital intensive equipment such as TEM and AFM, have been used to identify the state and morphology of tube samples as well as their levels of purity, however for large scale industrial applications there is a need for routine, high throughput, cost effective methods to give the same kind of detail. In this thesis organic solvents are used to debundle single walled nanotubes. The debundling is confirmed and the degree measured using absorption spectroscopy and also the capitally intensive and low throughput techniques, TEM and AFM. Although Raman spectroscopy is a well established technique for characterising tube bundles and single tubes little is know about what subtle changes occur in the Raman spectrum as one goes from a highly bundled state to a lower degree of bundling. In this thesis these changes are examined and the use of previously unidentified spectral markers for the degree of debundling are identified. Finally, using Raman spectroscopy a temperature dependent study was carried out. Mean free paths due to plasmon-phonon scattering in nanotube bundles and electron-phonon scattering in debundled tubes are calculated and found to be different.

Single wall carbon nanotubes as templates for organic molecules.

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.

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.

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.

Interaction of SWNT with Simple Dye Molecules

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 dye molecules such as terphenyl and anthracene. Suspensions formed in toluene with these dye 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 dye molecules. It is proposed in this study that the use of these more simple molecular systems may provide a route towards selectively processing different types of tubes.

The use of single-wall carbon nanotubes as templates for organic molecules

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 oligomers such as terphenyl and anthracene. Suspensions formed in toluene with these oligomers 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 oligomers. 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.