The use of Raman Spectroscopy to Study Carbon Nanotubes

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Abstract

This project has studied different types of carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) using Raman spectroscopy. Bundled CNTs, such as single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) have been isolated in poly(vinyl alcohol) (PVA) fibres using blow spinning so that the CNTs with different lengths are well aligned in the PVA fibres. Radial breathing modes (RBMs) of the isolated nanotubes has been characterised in terms of their frequency, intensity, and linewidth. The intensity of the RBM is comparable to its G band and most of the metallic tubes show a high RBM intensity with abroad G- band, while most of the semiconducting tubes show a different behaviour. The interlayer spacing between inner and outer layers of isolated DWNT was determined when both tubes were in resonance with the same laser energy. The inner tube can be accommodated in different outer tubes and a small interlayer spacing affects the interband transition energy. Small inner nanotubes were found in DWNTs using both transmission electron microscopy (TEM) and Raman spectroscopy. A high RBM frequency with splitting of the D band is a characteristic feature to identify small nanotubes. However, some artefacts have similar features to the RBM peaks. These artefacts can be distinguished from the nanotube spectra. For example, the spectra of remaining catalyst do not have D, G, and 2D bands, and the intermediate frequency modes appear only with specific RBM frequencies. The D and G bands were shown to be sensitive to the surrounding environment. Tight-binding calculations have shown that some chiralities of small tubes have a band gap close to the 633 nm laser energy, while another has a band gap outside the laser energy range. The mechanical interface of CNTs/PVA and CNTs/epoxy composite films were also monitored using the Raman 2D band shift of the CNTs per unit tensile strain. The stress transfer between the inner and outer shells of DWNTs/PVA was found to be variable. The average internal stress transfer (𝑘𝑖) factor is 0.73, similar to that of MWNTs and few-layer graphene. Multi-walled carbon nanotubes (MWNTs) and unfunctionalised and functionalised graphene nanoribbons (GNRs) have also been isolated using organic solvents and the structure of the isolated nanocarbons was determined. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have shown the ribbons are unzipped longitudinally but many of them are only partially open. The PVA and epoxy matrices were also used with both the GNRs and MWNTs. The stress-induced Raman band shift in the GNR and MWNT composites has shown to be variable depending on the structure of the nanomaterials. The unfunctionalised GNR composites have a better interface than the MWNT composites, while the alkyl-functionalised GNRs were found to have a poor mechanical interface in the composites.

Keyword(s)

Carbon nanotubes; Graphene nanoribbons ; Raman spectroscopy

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