Temperature dependent phonon renormalization in metallic nanotubes

Raman spectroscopy is a powerful non destructive technique for the characterization of carbon materials, and is a fundamental tool in the recent advances in the understanding of single wall carbon nanotubes (SWNTs). Raman experiments probe the optical phonons, allowing to assess the vibrational properties of the analyzed materials. A strong interplay exists between temperature (T ) and phonon frequencies. Indeed, because of anharmonic effects in the atomic oscillations, an increase in T usually results in a downshift of the phonon energy and a lifetime reduction [1]. For Raman active modes, this corresponds to a downshift and a broadening of the Raman peaks [1]. The temperature dependence of the Raman spectra is extremely effective for the evaluation of the local heating in a variety of electronic devices [2, 3], and provides valuable information for the characterization of nanomaterials [4]. Since SWNTs are at the center of nanotechnology research, a thorough investigation and understanding of the temperature effects on their Raman spectra is needed, especially in view of their foreseen application in high current nanodevices. Several groups reported changes of the Raman spectra of single-, doubleand multi-wall tubes as a function of T. Some focussed on the G band [5, 6, 7, 8, 9, 10, 11, 12, 13]. Others considered the position [5, 6, 7, 8, 11, 12, 14] and the intensity [14] of the Radial Breathing Modes (RBM). A few reported the temperature evolution of the 2D [5, 13] and 2D’ modes [5]. However, the different components of the G band, which crucially distinguish metallic from semiconducting nanotubes, were not independently studied, in order to ascertain if those would have a different temperature evolution in semiconducting and metallic SWNTs, thus fingerprinting each material. Here we present an extensive set of temperaturedependent measurements of the G and G peaks in metallic and semiconducting SWNTs. We show that there is a significant difference in the measured trends. We detect a re-normalization of the G peak, ruled by the variation in the electronic temperature, in metallic SWNTs. Furthermore, we prove that this can only be explained by considering the dynamic nature of the phonons and the resulting breakdown of the Born150