A new lower limit for the ultimate breaking strain of carbon nanotubes

We apply immense strain to ultralong, suspended, single-walled carbon nanotubes while monitoring their Raman spectra. We can achieve strains up to 13.7 ± 0.3% without slippage, breakage, or defect formation based on the observation of reversible change in Raman spectra. This is more than twice that of previous observations. The rate of G band downshift with strain is found to span a wide range from -6.2 to -23.6 cm^-1/% strain. Under these immense strains, the G band is observed to downshift by up to 157 cm ^-1 (from 1592 to 1435 cm^-1). Interestingly, under these significant lattice distortions, we observe no detectable D band Raman intensity. Also, we do not observe any broadening of the G band line width until a threshold downshift of δω_G > 75 cm^-1 is achieved at high strains, beyond which the fwhm of the G band increases sharply and reversibly. On the basis of a theoretical nonlinear stress-strain response, we estimate the maximum applied stress of the nanotubes in this study to be 99 GPa with a strength-to-weight ratio of almost 74 000 kN·m/kg, which is 30 times that of Kevlar and 117 times that of steel.