Effect of Eu²⁺ doping on photoluminescence of Al<inf>2</inf>O<inf>3</inf>:Tb³⁺ functional coatings formed by plasma electrolytic oxidation

Abstract

Multifunctional coatings of Al2O3 co-doped Tb3+ or Tb3+/Eu2+ were synthesized by the rapid and facile plasma electrolytic oxidation method on pure aluminium substrate. SEM & EDS analysis revealed porous structure independent on the doping concentration, with uniform distribution of co-doped rare-earth elements and incorporated concentrations proportional to the rare-earth particles concentrations added to the basic electrolyte. The gamma phase of Al2O3 increases to the expense of the alpha phase of Al2O3 with rising content of Tb3+. The addition of Eu2+ caused no further phase transformations. The photoluminescence (PL) excitation spectrum of Al2O3:Tb3+ was featured by sharp peaks (due to inter-configurational 4f transitions of Tb3+) and orders of magnitude larger broad-band in the middle ultraviolet region ascribed to the 4f→4f5d transitions. The PL emission spectrum was featured by the typical 4f→4f transitions of Tb3+, with maximum green emission at 541 nm and intensity proportional to the doping concentration. As the Eu2+ emission and Tb3+ excitation overlap, the concentration dependent energy transfer Eu2+→Tb3+ (of dipole-dipole character) occurred, resulting in the 20x increase in total PL and 3.2x increased Tb3+ PL. The PL emission color of Al2O3:Tb3+ was green and concentration independent, while of Al2O3:Eu2+/Tb3+ the color was in the domain of cold white with high correlated color temperature, which slightly decreased with increasing Eu2+ concentration.