Pressure-Induced Selectivity for Probing Inner Tubes in Double- and Triple-Walled Carbon Nanotubes: A Resonance Raman Study

The dependence of the radial breathing modes (RBMs) and the tangential mode (G-band) of triple-wall carbon nanotubes (TWCNTs) under hydrostatic pressure is reported. Pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high-pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters because their pressure response is different. Furthermore, it was possible to observe similar RBM profiles for the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressure-induced deformations. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure.