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

Abstract

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 2 . 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-athom thick layer. At small diameters, by comparing the computed Eshelby twist executed by the one-atom thick layers with the one predicted by pure rolling of the mono-layer, 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.

Ewald summation on a helix: A route to self-consistent charge density-functional based tight-binding objective molecular dynamics

“…3(c), the untwisted (rolled-up) morphology does not correspond to a metastable state, in agreement with previous predictions. 43 The analogous non-SCC simulations predicts a twist angle of 13.51 • /cell and a length of 2.520 Å/cell. Our present simulation predicts a very similar twist angle of 13.52 • /cell, and a significantly differing (0.3% strain) length of 2.513 Å/cell.…”

“…Ideal values for θ , and T corresponding to the rolled-up construction can be obtained for any nanotube by following the approach detailed in Ref. 43. It is beyond the scope of this paper to review the twocenter symmetry-adapted non-orthogonal TB and the well established SCC-DFTB.…”

“…It has recently been obtained with non-SCC-DFTB objective MD that a general (n, m) nanotube can lose the translational periodicity predicted by this rolledup construction due to a shear strain manifested as an intrinsic structural twist; for such cases, simulations relying on translational symmetry would become even more demanding. 43 The rich objective symmetry which characterizes this class of materials, however, can drastically reduce their computational costs, if adequately exploited. Following the screwdislocation procedure described before 29,43 we calculate the optimal morphology of a (4, 2) BN nanotube, Fig.…”

“…43 The rich objective symmetry which characterizes this class of materials, however, can drastically reduce their computational costs, if adequately exploited. Following the screwdislocation procedure described before 29,43 we calculate the optimal morphology of a (4, 2) BN nanotube, Fig. 3(a), using a computational cell consisting of six B-N dimers (12 atoms) positioned along the roll-up vector, Fig.…”

Symmetry-adapted phonon analysis of nanotubes

“…First, our approach based on OS provides a tight link to deformation of the nanotubes. This is critical to go beyond exclusively axial-load-free and twisting-moment-free nanotubes; in fact, as recent work shows, the load-and moment-free structure of chiral nanotubes likely does not correspond to the assumed highly-symmetric configuration [VBSF + 10, ZAD11,AD12]. Relaxing or applying such loads is readily accomplished using the OS framework [Jam06,DJ07].…”