Understanding the microscopic processes that govern the charge-induced deformation of carbon nanotubes
L. Pastewka, P. Koskinen,
C. Elsässer, M. Moseler
Physical Review B, Band 80, Seiten 155428-1-155428-16 (2009)
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 density-functional theory and various tight-binding models. For a given deformation mode the concept of bonding and antibonding orbitals can be redefined depending on the sign of a differential band-structure stress. We use this theoretical framework to analyze 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 multiwall 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 photoactuation of nanotubes and predict that transitions between van-Hove singularities can be expected to expand the tubes.