High Pressure Induced Binding Between Linear Carbon Chains and Nanotubes

TL;DR

This study uses atomistic simulations to show that high pressure causes covalent bonding between linear carbon chains and nanotubes, leading to irreversible vibrational frequency shifts.

Contribution

It reveals the mechanism of covalent bond formation under pressure between LCCs and CNTs, which was not previously understood.

Findings

High pressure induces inhomogeneous deformation of CNTs with LCCs.

Reactive sites form covalent bonds at the LCC-CNT interface.

Bond formation causes irreversible vibrational frequency red shifts.

Abstract

Recent studies of single-walled carbon nanotubes (CNTs) in aqueous media have showed that water can significantly affect the tube mechanical properties. CNTs under hydrostatic compression can preserve their elastic properties up to large pressure values, while exhibiting exceptional resistance to mechanical loadings. It was experimentally observed that CNTs with encapsulated linear carbon chains (LCCs), when subjected to high hydrostatic pressure values, present irreversible red shifts in some of their vibrational frequencies. In order to address the cause of this phenomenon, we have carried out fully atomistic reactive (ReaxFF) molecular dynamics (MD) simulations for model structures mimicking the experimental conditions. We have considered the cases of finite and infinite (cyclic boundary conditions) CNTs filled with LCCs (LCC inside CNTs) of different lengths (from 9 up to 40 atoms). Our results show that increasing the hydrostatic pressure causes the CNT to be deformed in an inhomogeneous way due to the LCC presence. The LCC-CNT interface regions exhibit convex curvatures, which results in more reactive sites, thus favoring the formation of covalent chemical bonds between the chain and the nanotube. This process is irreversible with the newly formed bonds continuing to exist even after releasing the external pressure and causing an irreversibly red shift in the chain vibrational modes from 1850 to 1500 cm$^{-1}$.