Eu3+ - Doped ZnO Quantum Dots: Structure, Vibration Characteristics, Optical Properties, JO Theory, and Energy Transfer Process

Abstract

This article studies the synthesis, structural, vibrational, and optical properties of Eu³⁺-doped ZnO quantum dots (QDs) and investigates the energy transfer mechanism from ZnO host to Eu3+ ions using Reisfeld’s approximation. Eu³⁺-doped ZnO QDs at varying concentrations (0–7%) were successfully prepared using a wet chemical method. The successful doping of Eu³⁺ 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 Eu³⁺ 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 (Eu³⁺), confirming the successful doping of Eu³⁺ 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 Eu³⁺ incorporation, indicating strong coupling between Eu³⁺ 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 5D₀→7F₂ transition of Eu³⁺ ions under UV excitation. Using Judd-Ofelt (JO) analysis, the intensity parameters (Ω₂, Ω₄, Ω₆) were derived, providing insights into the asymmetry of the Eu³⁺ ion's local environment and the radiative transition probabilities. The JO theory was further applied to calculate important optical parameters such as branching ratios and radiative lifetimes, which confirmed the high luminescence efficiency of Eu³⁺-doped ZnO QDs. Energy transfer processes between the ZnO host and Eu³⁺ dopants were examined, showing efficient sensitization of Eu³⁺ through excitation of the ZnO host, with an optimal Eu³⁺ doping level maximizing luminescence. These results highlight the potential of Eu³⁺-doped ZnO QDs for applications in photonic devices, light-emitting diodes (LEDs), and bio-imaging due to their tunable optical properties and efficient energy transfer mechanisms.