Helical BN and ZnO nanotubes with intrinsic twisting: An objective molecular dynamics study

We investigate helical single-walled nanotubes of BN and ZnO described with density-functional based tight-binding models. The employed objective molecular dynamics computational framework accounts for the helical instead of the translational symmetry and allows for simulating chiral nanotubes as the result of the nanomechanical process of a nearly axial glide. At large diameters, by comparing the microscopic strain stored in the tube wall with the continuum predictions, we observe the invalidity of the continuum shell idealization of the one-atom thick layer. At small diameters, comparing the computed Eshelby twist executed by the one-atom thick layers with the one predicted by pure rolling, we find that a large catalog of nanotubes store intrinsic twists. This unusual intrinsic twist effect is shown to be dependent on chirality and diameter, as part of the general trend to depart from the standard rolled-up construction. While changes in the electronic structures and Young's modulus are dominated by curvature, the shear elastic constants vary both with curvature and chirality.