High-Pressure Optical Microspectroscopy Study on Single-Walled Carbon Nanotubes Encapsulating C60

The high-pressure behavior of single-walled carbon nanotubes (SWCNTs) filled with C-60 molecules, so-called C-60@SWCNT peapods, has been investigated by optical spectroscopy using nitrogen and argon as pressure transmitting media. Peapods with a high filling ratio were prepared by the sublimation method and characterized by high-resolution transmission electron microscopy. Optical transmission measurements under high pressure were conducted on films of SWCNTs and C-60@SWCNT peapods. As with SWCNTs, the absorption bands observed for peapods exhibit a red-shift under pressure. The relative energy shifts of the optical transitions under pressure are higher than those for empty SWCNTs, demonstrating enhanced hybridization and/or symmetry breaking effects. We find an anomaly in the pressure-induced shifts of the optical transitions at P-c1 approximate to 6.5 GPa (P-c1 approximate to 5.5 GPa) when using nitrogen (argon) as pressure transmitting medium. The anomaly signals the deformation of the nanotube from circular to an oval shape. The value of Pc, is in good agreement with theoretical predictions of the pressure-induced deformation for highly filled peapods with similar average diameter. A plateau in the pressure-induced shifts with an onset at P-c2 = 12-13 GPa indicates a saturation of the pressure-induced effects above this critical pressure.