Direct observation of boron nitride nanocage growth by molecular beam nitridation and liquid-like motion of Fe–B nanoparticles

Abstract

The growth of connected chains of BN nanocages has been achieved by the molecular beam nitridation of nano-sized Fe–B particles. The particles were exposed to ammonia under high vacuum conditions at a temperature of 1000 °C. The experiment was performed in an ultrahigh vacuum chamber built into the column of a high resolution transmission electron microscope enabling direct observation of the process. BN nanocages were observed to form around the Fe–B particles, followed by expulsion of the particles from the cages. Expulsion occurred at a critical shell thickness in a liquid-like manner, despite the experiment being conducted well below the Fe–B liquidus and the particles remained crystalline throughout the experiment. Following expulsion, fresh BN shells formed around the expelled particles, which remained in contact with the previous shell under the influence of surface tension. Chains of connected nanocages were thus formed. The observations provide direct confirmation of the possibility of nanostructure formation by a liquid-like flow of nanocrystalline particles.