Direct observation of melting behaviors at the nanoscale under electron beam and heat to form hollow nanostructures

We report the melting behaviours of ZnO nanowire by heating ZnO-Al ₂O₃ core-shell heterostructures to form Al ₂O₃ nanotubes in an in situ ultrahigh vacuum transmission electron microscope (UHV-TEM). When the ZnO-Al₂O₃ core-shell nanowire heterostructures were annealed at 600°C under electron irradiation, the amorphous Al₂O₃ shell became single crystalline and then the ZnO core melted. The average vanishing rate of the ZnO core was measured to be 4.2 nm s^-1. The thickness of the Al ₂O₃ nanotubes can be precisely controlled by the deposition process. Additionally, the inner geometry of nanotubes can be defined by the initial ZnO core. The result shows a promising method to obtain the biocompatible Al₂O₃ nanotubes, which may be applied in drug delivery, biochemistry and resistive switching random access memory (ReRAM).