Nanotechnology makes the smallest Archimedean screw (Nanowerk News) Nanotechnology researchers have reported a new material phenomenon, occurring in faceted double-walled nanotubes (DWNTs), where minute mechanical manipulations induce pronounced global superstructure reconfiguration (Nano Letters, "Smallest Archimedean Screw: Facet Dynamics and Friction in Multiwalled Nanotubes").

For monochiral DWNTs that exhibit axially aligned facets even the slightest interwall rotation induces significant circumferential facet revolution, and minor interwall telescoping can lead to complete unfaceting. Similar manipulations applied to bichiral DWNTs result in global screw-like motion of their elongated helical facets reminiscent of an Archimedean screw.

Bichiral DWNT facet rotation. Perspective view of the bichiral (70,70)@(77,74) DWBNNT at two interwall configurations θ/z of 0.2°/2.4 Å (left) and 0.2°/3.2 Å (right). These correspond to configurations close to maximum and minimum potential energy, respectively. Blue and red atom false coloring represents high and low interlayer energy, respectively. Facet dynamics during a pull-out simulation at an interwall velocity of 0.01 Å/ps is reported. (© ACS)

Importantly, these superstructure evolutions under coaxial sliding open new collective energy dissipation channels that enhance interwall dynamic friction.

This, in turn, suggests that the relative abundance of faceting in multiwalled boron-nitride nanotubes plays a central role not only in their enhanced torsional stiffness but also in the significantly higher interwall friction that they exhibit with respect to MWCNTs.

Despite their remarkable structural similarity, faceting is more commonly observed in multiwalled boron-nitride nanotubes (MWBNNTs) than in their carbon counterparts (MWCNTs). According to the authors, this can be attributed to three important factors:

1) stronger long-range dispersive attractive interactions exhibited by the former that provide higher interwall adhesion thus favoring facet formation; 2) softer ZA modes of h-BN30 that allow for sharper vertices thus promoting the formation of wider planar facet regions; and 3) higher interwall chiral angle correlation exhibited by MWBNNTs over MWCNTs that induces extended lattice registry patterns between adjacent tube shells and dictates the nature of the facets.

The screw-like motion of the faceted helical pattern, which establishes the smallest realization of an Archimedean screw, has the potential to achieve directional transport of weakly adsorbed molecules along the surface of the tube.

"Several other, more speculative but highly intriguing, consequences of the striking facet evolutions discussed in our paper can be envisioned," write the scientists. "First, we have shown that facet dynamics strongly depend on the relative chirality of adjacent nanotube walls. Therefore, the interwall pulling force trace should encode information about the identity of the various tube shells."

This, in turn, opens new opportunities for novel material characterization techniques that may provide access to the specific sequence of chiralities of successive nanotube walls. Furthermore, electronic effects, not discussed in this work, may also exhibit unexpected behavior.