An insight into defect relaxation in metastable ZnO reflected by a unique luminescence and Raman evolutions

Defects play a crucial role in semiconductors, but a facile method to observe defect variation inside semiconductors is still absent. Here, we provide an insight into defect relaxation in metastable ZnO nanoparticles, which are prepared via nonequilibrium laser ablation in liquid media, reflected by the Raman vibrations of surface optical (SO) and volume phonons, as well as by the evolution of luminescence. During the annealing process, the SO and volume phonon modes exhibit strong incompatibility and a unique "intermission" period in the temperature range from 300 °C to 400 °C, in which both the vibrations are completely suppressed in the Raman spectra. Combined with the corresponding defects-related photoluminescence spectra, it is demonstrated that there exists a delay between the reconstruction of the interfacial defects and annihilation of other intrinsic defects, including interstitial zinc and oxygen vacancy in the relaxation process, and that the sequence of different defects of ZnO in the order of instability is interfacial defect, interstitial zinc, and then oxygen vacancy. Such defect relaxation will deepen our understanding of some of the properties of semiconducting nanomaterials, including luminescence, photocatalysis, electronic transport, sensing and others.