Temperature-responsive structural rearrangements of polyiodide species in carbon nanotube templates

The system of iodine-doped single-walled carbon nanotubes (I@SWCNTs) serves as an ideal model for investigating low-dimensional iodine species and charge transfer doping effects in SWCNTs. Herein, we present a temperature-dependent Raman spectroscopic study on this system, aiming to understand their thermal responses. Thermogravimetric analysis reveals a total encapsulated iodine content of approximately 25.6 wt% within SWCNTs of ∼1.4 nm. The origins of the Raman bands from confined iodine species are discussed in terms of different iodine lengths and configurations, including short I₃^-, linear and V-shaped I₅^-, long polyiodide chains (I_m^n-), as well as individual I₂ molecules. In addition, a transformation mechanism of confined iodine species under various temperatures is proposed and discussed in detail, accompanied by an analysis of the host-guest charge transfer during this process. Moreover, the dissociation energy of long polyiodide chains is also quantitatively estimated from the temperature-dependent full width at half maximum (FWHM) of the corresponding Raman band. At last, the reversibility of the thermal behavior of I@SWCNTs system after cooling-heating and heating-cooling cycle is compared. Our work provides deeper insights into the temperature-induced structural transformations of iodine in SWCNTs, its thermal stability and carrier concentration modulation on SWCNTs.