Understanding the microscopic processes that govern the charge-induced deformation of carbon nanotubes

While carbon nanotubes have technological potential as actuators, the underlying actuation mechanisms remain poorly understood. We calculate charge-induced stresses and strains for electrochemical actuation of carbon nanotubes with different chiralities and defects, using densityfunctional theory and various tight-binding models. For a given deformation mode the concept of bonding and anti-bonding orbitals can be redefined depending on the sign of a differential band structure stress. We use this novel theoretical framework to analyse orbital contributions to the actuation. These show charge asymmetric behavior which is due to next-nearest-neighbor hopping, while Coulombic contributions account for approximately charge-symmetric isotropic deformations. In the typical case of a (10, 10) tube strains around 0.1% with 1 nN force along the tube axis are obtained. Defects and functional groups have negligible influence on the actuation. In multi-wall tubes we find charge inversion on the inner tubes due to Friedel-type oscillations which could lead to a slight magnification of charge-induced strains. Finally, we consider photo actuation of nanotubes and predict that transitions between van-Hove singularities can be expected to expand the tubes.