Origin of axial and radial expansions in carbon nanotubes revealed by ultrafast diffraction and spectroscopy.

The coupling between electronic and nuclear degrees of freedom in low-dimensional, nanoscale systems plays a fundamental role in shaping many of their properties. Here, we report the disentanglement of axial and radial expansions of carbon nanotubes, and the direct role of electronic and vibrational excitations in determining such expansions. With subpicosecond and subpicometer resolutions, structural dynamics were explored by monitoring changes of the electron diffraction following an ultrafast optical excitation, whereas the transient behavior of the charge distribution was probed by time-resolved, electron-energy-loss spectroscopy. Our experimental results, and supporting density functional theory calculations, indicate that a population of the excited carriers in the antibonding orbitals of the nanotube walls drives a transient axial deformation in ∼1 ps; this deformation relaxes on a much longer time scale, 17 ps, by nonradiative decay. The electron-driven expansion is distinct from the phonon-driven dynamics observed along the radial direction, using the characteristic Bragg reflections; it occurs in 5 ps. These findings reveal the nonequilibrium distortion of the unit cell at early times and the role of the electron(phonon)-induced stress in the lattice dynamics of one-dimensional nanostructures.