Stabilization of carbon nanotubes by filling with inner tubes: An   optical spectroscopy study on double-walled carbon nanotubes under   hydrostatic pressure
  The stabilization of carbon nanotubes via the filling with inner tubes is demonstrated by probing the optical transitions in double-walled carbon nanotube bundles under hydrostatic pressure with optical spectroscopy. Double-walled carbon nanotube films were prepared from fullerene peapods and characterized by HRTEM and optical spectroscopy. In comparison to single-walled carbon nanotubes, the pressure-induced redshifts of the optical transitions in the outer tubes are significantly smaller below $\sim$10 GPa, demonstrating the enhanced mechanical stability due to the inner tube already at low pressures. Anomalies at the critical pressure P$_d$$\approx$12 GPa signal the onset of the pressure-induced deformation of the tubular cross-sections. The value of P$_d$ is in very good agreement with theoretical predictions of the pressure-induced structural transitions in double-walled carbon nanotube bundles with similar average diameters. 
B. Anis, K. Haubner, F. B\"orrnert, L. Dunsch, M. H. R\"ummeli, and C.   A. Kuntscher
cond-mat.mes-hall cond-mat.mtrl-sci