Two color up-conversion emissions of Er3+-doped Al2O3 nanopowders prepared by non-aqueous sol-gel method

Er3+-doped Al2O3 nanopowders have been prepared by the non-aqueous sol-gel method using the aluminum isopropoxide as precursor, acetylacetone as a chelating agent, nitric acid as a catalyzer, and hydrated erbium nitrate as a dopant under isopropanol environment. The different phase structure, including three crystalline types of (Al, Er)(2)O-3 phases, alpha, gamma, theta, and an Er-Al-O stoichiometric compound phase, Al10Er6O24, was observed for the 0.01-0.5 mol% Er3+-doped Al2O3 nanopowders at the sintering temperature of 1,000 degrees C. The green and red up-conversion emissions centered at about 523, 545 and 660 nm, corresponding respectively bto the H-2(11/2), S-4(3/2)-> I-4(15/2) and F-4(9/2)-> 4 I-15/2 transitions of Er3+, were detected by a 978 nm semiconductor laser diodes excitation. With increasing Er3+ doping concentration from 0.01 to 0.1 mol%, the intensity of the green and red emissions increased with a decrease of the intensity ratio of the green to red emission. When the Er3+ doping concentration rose to 5 mol%, the intensity of the green and red emissions decreased with an increase of their intensity ratio. The maximum intensity of both the green and red emissions with the minimum of intensity ratio was obtained, respectively, for the 0.1 mol% Er3+-doped Al2O3 nanopowders composed of a single alpha-(Al,Er)(2)O-3 phase. The intensity ratio of the green emission at 523 and 545 nm increased monotonously for all Er3+ doping concentrations. The two-photon absorption up-conversion process was involved in the green and red up-conversion emissions of the Er3+-doped Al2O3 nanopowders.