Ultra-fast oscillation of cobalt clusters encapsulated inside carbon nanotubes

摘要

Using molecular dynamics (MD) simulations, the authors have studied the oscillatory characteristics of the 2Co@CNT oscillator systems. Each of these oscillator systems consists of a hosting carbon nanotube (CNT) and two encapsulated cobalt (Co) clusters, and oscillations are initiated by prescribing an initial kinetic energy to each of the two cobalt clusters. The non-symmetric oscillation mode, in which the two cobalt clusters always move towards the same direction, was found to be stable over a wide range of initial energy. However, the symmetric oscillation mode, in which the two cobalt clusters move towards or away from each other, bouncing off each other in each oscillation, is stable only when the initial kinetic energies are lower than a threshold value. Above this threshold, the oscillation becomes increasingly unstable with the increasing initial kinetic energy. The instability is found to take place through transferring energy from the translational motion to the rotational motion of the cobalt clusters, due to the fact that the impact of the cluster-cluster collisions can be slightly off-center, causing the clusters to roll and rock. The rocking motion of the cobalt clusters serves as the channel for the energy transfer. The rocking motion can be retarded and may even be eliminated by reducing the hosting CNT diameter. But a smaller hosting CNT does not always lead to more stable translational oscillation. There apparently exists an optimal CNT for a given size of clusters for stabilizing the translational oscillation. A hosting CNT that is too much smaller than optimum causes severe cobalt-carbon atomic interactions, which lead to losses of energy from the ordered translational motion of clusters to disordered thermal motions of the atoms.