Intrinsic Temperature Dependence of Raman-Active Modes in Individual Isolated Single- and Double-Walled Carbon Nanotubes.

The intrinsic temperature dependence of Raman-active modes in carbon nanotubes (CNTs), particularly the radial breathing mode (RBM), has been a topic of a long-standing controversy. In this study, we prepared suspended individual CNTs to investigate how their Raman spectra depend on temperature and to understand the effects of environmental conditions on this dependency. We analyzed the intrinsic temperature dependence of the main Raman-active modes, including the RBM, the moiré-activated R feature, and the G-band in double-walled carbon nanotubes (DWCNT) and single-walled carbon nanotubes (SWCNTs) after complete desorption of air. The inner tube of the DWCNT, like the desorbed SWCNTs, was free from environmental influences, resulting in minimal temperature-induced RBM frequency shifts. We show that the larger RBM shift of SWCNTs upon initial heating is not intrinsic but is due to air desorption. The R feature, attributed to moiré-activated phonon scattering and nondispersive in nature, demonstrated a quasi-linear temperature dependence, akin to the G-band but with a lower temperature coefficient. The G-band, which was largely unaffected by environmental conditions, exhibited a consistent temperature coefficient across SWCNTs, DWCNTs, and small SWCNT bundles.