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

TL;DR

This study investigates how different pressure media and pressure influence the Raman spectra of carbon nanotubes, revealing shifts in resonance energy and frequency that help distinguish effects of pressure from those of the medium.

Contribution

It provides a detailed analysis of how various pressure media and applied pressure affect the Raman spectra of carbon nanotubes, enabling clearer separation of these effects.

Findings

Peaks shift by 2-3 cm-1 in RBM frequency with different media.

Resonance energy shifts up to 100 meV depending on medium and pressure.

Orthogonal shifts allow separation of pressure and medium effects.

Abstract

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 100meV 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.