Effect of excitation wavelength and europium doping on the optical properties of nanoscale zinc oxide

Abstract

The present study demonstrates a red light-emitting nano-phosphor material tailored by doping europium (Eu⁺³) ions in zinc oxide (ZnO), prepared using a solution based co-precipitation method. Instead of using acetates or nitrates based precursor for Eu⁺³ doping, here we directly used europium oxide (Eu₂O₃) as a precursor. The precursor showed a limited amount of solubility only up to 3% in an alcoholic solution. No phase change of any kind in x-ray diffraction (XRD) patterns indicates effective Eu⁺³ doping in ZnO. Also, the broadening of the XRD peaks confirms the reduction of size to the nanoscale. Further, the optical properties of pure ZnO and Eu⁺³-doped ZnO are elucidated using UV–Visible and photoluminescence (PL) spectroscopy. A redshift of 10 nm in the absorption edge from pure ZnO to 3% Eu⁺³-doped ZnO concentration is detected, indicating that Eu⁺³ ions occupy impurity trap levels below the conduction band. A regular increase in the excitation wavelength from 190 nm to 270 nm confirms that 226 nm excitation wavelength is the onset point for the emission from Eu⁺³ ions. The Eu⁺³-doped ZnO nanoparticles exhibit two emission peaks at 584 nm and 613 nm corresponding to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu⁺³ ions at low excitation wavelengths of 254 nm and 270 nm. Out of these two peaks, 613 nm peak was the most intense suggesting that mostly ⁵D₀→⁷F₂ transitions are taking place. The intensity ratio of ⁵D₀→⁷F₂ / ⁵D₀→⁷F₁ for both excitation wavelengths of 254 nm and 270 nm is always greater than one confirms the efficient emission of red color from Eu⁺³ ions. In addition to that, the samples exhibit a high color purity value of 83.78 %, with CIE coordinates (0.60, 0.40) lying closer to the ideal red color CIE coordinates at an excitation wavelength of 254 nm.