Double- and triple-walled carbon nanotubes (DWNTs and TWNTs) consist of coaxially-nested two and three single-walled carbon nanotubes (SWNTs). They act as the geometrical bridge between SWNTs and multi-walled carbon nanotubes (MWNTs), providing an ideal model for studying the coupling interactions between different shells in MWNTs. Within this context, this article comprehensively reviews various synthetic routes of DWNTs’ and TWNTs’ production, such as arc discharge, catalytic chemical vapor deposition and thermal annealing of pea pods (i.e., SWNTs encapsulating fullerenes). Their structural features, as well as promising applications and future perspectives are also discussed.

Isolated linear carbon chains (LCCs) encapsulated by multiwalled carbon nanotubes are studied under hydrostatic pressure ($P$) via resonance Raman scattering. The LCCs’ spectroscopic signature C band around $1850{\mathrm{cm}}^{-1}$ softens linearly with increasing $P$. A simple anharmonic force-constant model not only describes such softening but also shows that the LCCs’ Young’s modulus ($E$), Grüneisen parameter ($\gamma$), and strain ($ϵ$) follow universal $P^{-1}$ and $P^2$ laws, respectively. In particular, $\gamma$ also presents a unified behavior for all LCCs. To the best of our knowledge, these are the first results reported on such isolated systems and the first work to explore universal $P$-dependent responses for LCCs’ $E$, $ϵ$, and $\gamma$.