Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results

TL;DR

This paper uses van der Waals density functional calculations and analytical methods to study the bonding in semiconducting nanotube crystals, revealing vdW interactions consistent with experiments and proposing a framework for modeling nanotube bundling.

Contribution

It introduces a combined ab initio and analytical approach to quantify nanotube bonding, demonstrating the effectiveness of vdW-DF in predicting intertube interactions and separations.

Findings

vdW-DF predicts nanotube intertube bonding strength consistent with experiments.

The wall-to-wall separation matches experimental data.

Nanotube-crystal binding energy can be approximated by pairwise vdW interactions.

Abstract

The dispersive interaction between nanotubes is investigated through ab initio theory calculations and in an analytical approximation. A van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to determine and compare the binding of a pair of nanotubes as well as in a nanotube crystal. To analyze the interaction and determine the importance of morphology, we furthermore compare results of our ab initio calculations with a simple analytical result that we obtain for a pair of well-separated nanotubes. In contrast to traditional density functional theory calculations, the vdW-DF study predicts an intertube vdW bonding with a strength that is consistent with recent observations for the interlayer binding in graphitics. It also produce a nanotube wall-to-wall separation which is in very good agreement with experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal binding energy can be approximated by a sum of nanotube-pair interactions when these are calculated in vdW-DF. This observation suggests a framework for an efficient implementation of quantum-physical modeling of the CNT bundling in more general nanotube bundles, including nanotube yarn and rope structures.