Electromechanical strain response of phosphorene nanotubes

Nanomaterials that undergo structural or other property changes upon application of external stimuli are called stimuli responsive materials and are particularly suited for drug delivery, biosensing or artificial muscle applications. Two-dimensional (2D) black phosphorus is an ideal material for such applications due to its remarkable electromechanical response. Given that one-dimensional (1D) black phosphorus nanotubes (PNTs) are calculated to be energetically stable, it is possible that they can undergo similar electromechanical responses to their 2D counterparts, allowing their potential application as nanochannel devices for drug delivery. Using first-principles density functional theory, we investigated the electromechanical response of different-sized PNTs upon charge injection. Upon hole injection, the diameter of the PNTs expands up to a maximum of 30.2% for a (0,15) PNT that is 0.24 nm in diameter. The PNTs become highly p-doped as the valence band maximum crosses the Fermi level and undergoes switching from a direct to indirect band gap. The mechanism behind the electromechanical response was determined through analysis of the structural deformations, charge density distribution and Bader partial charges. It was shown that injection of charge alters the Young’s Modulus of the PNTs, as hole injection weakens the structural integrity of the nanotube, allowing a greater electromechanical response, with PNT-15 showing the largest decrease in the Young’s Modulus of 15.34%. These findings show that 1D PNTs are promising materials for the development of nanoelectromechanical actuators which could be used for drug delivery, energy harvesting or similar applications.