Probing high-pressure properties of single-wall carbon nanotubes through fullerene encapsulation

The high pressure behavior of bundled $1.35\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ diameter single wall carbon nanotubes (SWNT) filled with ${\mathrm{C}}_{70}$ fullerenes (usually called peapods) has been investigated by Raman spectroscopy and compared with the corresponding behavior of the nonfilled SWNT. We show experimentally that two reversible pressure-induced transitions take place in the compressed bundle SWNT. The first transition, in the $2\char21{}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ range, is in good correspondence with predictions of the thermodynamic instability of the nanotube circular cross section for the studied tube diameter. An interaction between the fullerenes and the tube walls is then observed at about $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which evidences a progressive deformation of the tube cross section. The second transition takes place at pressures between 10 and $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and is evidenced by two effects by a strong frequency downshift of the Raman transverse modes and the concomitant disappearance of the fullerenes Raman modes in peapods. The pressure at which the second transition takes place is strongly dependent on the nature of the pressure transmitting medium. We also report irreversible effects at high pressure as the shortening of the tubes, the formation of nanostructures and the disappearance of the ${\mathrm{C}}_{70}$ Raman signal in some cases. Transmission electron microscopy studies are also reported supporting these transformations.