Non-equilibrium defect chemistry in oxygen-rich zinc oxide nano-tetrapods synthesized using atmospheric pressure microplasma

We demonstrate the synthesis of zinc oxide (ZnO) nanoscale tetrapods (nTP) using an atmospheric pressure microplasma with a metal wire as a sacrificial electrode. ZnO nTPs were characterised using transmission electron microscopy (TEM), X-ray diffraction, ultraviolet-visible and photoluminescence spectroscopy. TEM clearly revealed that the nTP arm length was ∼65 nm and the diameter was around ∼11 nm, grown in the wurtzite phase along the {0001} direction containing polar surfaces. The lattice constants of the ZnO nanocrystals were evaluated using Rietveld refinement. A strong ultraviolet with weak visible-violet emission was observed though photoluminescence revealing that nTP crystals are free from intrinsic defects. Furthermore, the properties of the nTP crystals were compared with those of ZnO nanoparticles formed under different plasma conditions. A detailed formation mechanism based on optical emission spectroscopy and the crystal growth mechanism of the nTP were further discussed.