Ahmad J. Ghandour

High-pressure studies of carbon nanotubes

Considering carbon nanotubes as a rolled-up graphene sheet, the basic properties of the Raman scattering of nanotubes can be predicted from the Raman of graphite. We expect increased pressure coefficients if the nanotubes are impermeable to the pressure medium and the same as graphite if they are filled; also if they collapse at high pressures. The literature is reviewed to see how well these predictions are borne out in practice. New experiments are reported, which confirm the crucial effects of the nature of the pressure-transmitting medium (solvent), which is unexpected, and the resonances with the Raman excitation energy (expected). Key open questions include the mechanism of the solvent effect and the influence of bundling.

Raman excitation spectroscopy of carbon nanotubes: effects of pressure medium and pressure

Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added – water, hexane, sulphuric acid – and when the nanotubes are unbundled in water with surfactant and sonication. The shift is about 2–3 cm−1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100 meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously.

Effect of chemical environment on high-pressure Raman response of single-walled carbon nanotubes

The pressure-induced Raman peak shifts for single-walled carbon nanotubes (SWNTs) have been studied using different solvents as hydrostatic pressure-transmitting fluids (PTF). We suggest that the variation in the nanotube response to hydrostatic pressure with different PTF is evidence that the common solvents used are able to penetrate the interstitial spaces in the nanotube bundle. With hexane, we find the surprising result that the individual nanotubes appear unaffected by hydrostatic pressures (i.e. a flat Raman response) up to 0.7 GPa. Qualitatively similar results have been obtained with butanol. Following the approach of Amer et al. [M. S. Amer, M. S. El-Ashry and J. F. Maguire, J. Chem. Phys. 121 2752 (2004)], we speculate that this is due to the inability of SWNTs to adsorb some solvents onto their surface at low pressure.