Ab initio simulations of excited carrier dynamics in carbon nanotubes

Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hv=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient electron-electron scattering. At room temperature, the excitation gap is reduced to nearly half its initial value after ~230 fs, where coupling to phonons starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling.Comment: 8 pages, 3 figures, 1 EPAPS supplementary fil

Microscopic Mechanism of the Helix-to-Layer Transformation in Elemental Group VI Solids

We study the conversion of bulk Se and Te, consisting of intertwined a helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our ab initio calculations reveal that previously unknown and unusually stable \delta - and \eta-2D allotropes may form in an intriguing multi-step process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers <0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting.Comment: accepted by Nano Letter