Eu-doped ZnO quantum dots: structure, vibration characteristics, optical properties, and energy transfer process.

This article studies the synthesis, as well as the structural, vibrational, and optical properties of Eu-doped ZnO quantum dots (QDs) and investigates the energy transfer mechanism from the ZnO host to Eu ions using Reisfeld's approximation. Eu-doped ZnO QDs at varying concentrations (0-7%) were successfully prepared using a wet chemical method. The successful doping of Eu ions into the ZnO host lattice, as well as the composition and valence states of the elements present in the sample, were confirmed through X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. XRD results demonstrated the crystalline nature of the ZnO QDs, revealing their wurtzite (WZ) structure with no secondary phases. XPS analysis provided further confirmation of the presence of Eu ions within the ZnO host, with clear signals corresponding to the Zn, O, and Eu elements. The valence states of Eu were verified as trivalent (Eu), confirming the successful doping of Eu ions, as evidenced by the characteristic Eu 3d peaks in the XPS spectra. Raman spectroscopy (RS) was employed to analyze the vibrational modes, revealing shifts in ZnO lattice vibrations due to Eu incorporation, indicating strong coupling between Eu ions and the ZnO host. Optical properties were studied using UV-Vis absorption, photoluminescence (PL) spectroscopy, and PL decay spectroscopy, showing a significant enhancement of red emission, attributed to the D → F transition of Eu ions under UV excitation. Using Judd-Ofelt (JO) analysis, the intensity parameters ( , , ) were derived, providing insights into the asymmetry of the Eu ion's local environment and the radiative transition probabilities. Energy transfer processes between the ZnO host and Eu dopants were examined, showing efficient sensitization of Eu through excitation of the ZnO host, with an optimal Eu doping level maximizing luminescence. Eu-doped ZnO QDs, which emit in the visible light region and are non-toxic, have great potential for applications in photonic devices, light-emitting diodes, and bioimaging.