Photoluminescence and Electron Paramagnetic Resonance Spectroscopy for Revealing Visible Emission of ZnO Quantum Dots

Both PL and EPR are simultaneously adopted to systematically elucidate the defect centers of green luminescence (GL) as well as the EPR peak g = 1.96 of ZnO and the relationship between them. The PL of ZnO QDs reveals that GL of 2.21-2.31 eV disappears at excitation wavelengths > 400 nm. This is related to the electronic transition from conduction band (CB), or shallow donor defect centers, to deep defects of V-Zn-H complexes at approximate to 0.9 eV above the valence band (VB). The EPR peak g = 1.96 emerged only when irradiated with light shorter than 400 nm, which is explicitly correlated with the electrons trapped in the CB or the shallow donors participating in the GL. A spin-spin relaxation time (T-2) estimated from the peak-to-peak line width Delta H-pp in the EPR signal is approximate to 17.5-52.5 ns, which is two orders of magnitude longer than known values for bulk or thin-film ZnO.

Automated summary

In this study, both PL and EPR techniques were used to investigate the defect centers and their relationship with the EPR peak g = 1.96 in ZnO. It was found that the disappearance of green luminescence was related to electron transitions from the conduction band or shallow donor defect centers to deep defects. The presence of the EPR peak g = 1.96 was correlated with the light irradiation and trapped electrons in the conduction band. Additionally, the spin-spin relaxation time estimated from the EPR signal was two orders of magnitude longer than that of bulk or thin-film ZnO.