Microscopic investigation of laser-induced structural changes in single-wall carbon nanotubes

Extensive excited-state molecular dynamics simulations of femtosecond laser-induced structural transformation in single-walled carbon nanotubes (SWNTs) are presented. We considered in the simulation two limiting cases; one where only a short portion of the tube is irradiated and the other where the whole tube is affected by the laser pulse. We have analyzed the role of chirality (zigzag versus armchair and some chiral tube cases) in the damage threshold as a function of tube diameter. Nontrivial dependence of the damage threshold as a function of diameter has been found. We find that for equal laser parameters, zigzag SWNTs are on average equally stable with respect to laser excitation as armchair SWNTs, but their stabilities show different dependencies on diameter. Due to the higher stiffness of the (n,n) tubes in the direction perpendicular to its axis as compared to the (n,0) tubes, we find the formation of standing waves in the nanotube wall for zigzag and not for armchair tubes. We also studied the role of the laser pulse duration and show that in general longer laser pulses increase the damage threshold. This result is rationalized in terms of electron-ion relaxation times. Implications of laser-induced structural transformations are analyzed.