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

• Published in: Annalen der Physik Vol. 534; no. 5
• Main Authors: Kim, Hong Hee, Lee, Heebin, Kang, Jeung Ku, Choi, Won Kook
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.05.2022
• Subjects: Conduction bands; Donors (electronic); Electron paramagnetic resonance; oxygen vacancies; Photoluminescence; Quantum dots; Relaxation time; Thin films; Valence band; Zinc oxide; zinc vacancy‐hydrogen complexes; ZnO quantum dots
• ISSN: 0003-3804; 1521-3889
• DOI: 10.1002/andp.202100382

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 VZn‐H complexes at ≈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 (T2) estimated from the peak‐to‐peak line width ΔHpp in the EPR signal is ≈17.5–52.5 ns, which is two orders of magnitude longer than known values for bulk or thin‐film ZnO. EPR peaks g = 1.964, 1.960, and 1.956 of ZnO quantum dots with sizes of 3.9, 4.9, and 13.2 nm emerge only under irradiation with light shorter than 400 nm, which is explicitly correlated with the photo‐excited electrons trapped in the conduction band or the shallow donors participating in green luminescence.