In a recent report [Sun et al., Science 312, 1119 (2006)], the partially filled material inside multiwalled carbon nanotubes (MWNTs) was shown to have shrunk and deformed in the axial direction under $300\mathrm{kV}$ electron irradiation. In this experiment, $100\mathrm{MeV}$ ${\mathrm{Au}}^{7+}$ ion irradiation was performed to study the deformation and defects in uniformly nickel filled MWNTs with high-resolution transmission electron microscopy and Raman spectroscopy. We propose that high-pressure induced torsion in confined nickel could possibly result in successive compressions and expansions of the tubes, leading to axial buckling of MWNTs. The tangential Raman $G$ band systematically upshifts as the ion fluence increases, attributed to the torsional strain. In contrast to a square root dependence of the buckling wavelength $\left(\lambda \right)$ on the radius $\left(r\right)$ and thickness $\left(t\right)$ of the tubes $[\lambda =3.5{\left(rt\right)}^{1∕2}]$, as predicted by theoretical models, the exponential fit of the data that assumes $\lambda \propto e^{\sqrt{(r∕t)}}$ also produces an excellent fit.

• Received 25 January 2007

DOI:https://doi.org/10.1103/PhysRevB.76.014108