Charge transfer and negative curvature energy in magnesium boride nanotubes

Abstract

Using first-principles calculations based on density functional theory, we study the energetics and charge transfer effects in ${\mathrm{MgB}}_{x}$ nanotubes and two-dimensional (2D) sheets. The behavior of adsorbed Mg on 2D boron sheets is found to depend on the amount of electron transfer between the two subsystems. The amount is determined by both the density of adsorbed Mg as well as the atomic-scale structure of the boron subsystem. The degree of transfer can lead to repulsive or attractive Mg-Mg interactions. In both cases, model ${\mathrm{MgB}}_{x}$ nanotubes built from 2D ${\mathrm{MgB}}_{x}$ sheets can display negative curvature energy: a relatively unusual situation in nanosystems where the energy cost to curve the parent 2D sheet into a small-diameter nanotube is negative. Namely, the small-diameter nanotube is energetically preferred over the corresponding flat sheet. We also discuss how these findings may manifest themselves in experimentally synthesized ${\mathrm{MgB}}_{x}$ nanotubes.