Upconversion dynamics in Er3+ doped nanocrystalline YAlO3

We report the luminescence and upconversion spectra of nanocrystalline YAIO(3) doped with trivalent erbium at concentrations of 5.0, 1.0 and 0.1 mol.%. The powder samples were prepared using a solution combustion reaction method, and the resulting YAlO3 nanocrystals show, under wide-angle X-ray diffraction, a size in the range 20-10 nm. Efficient green and red emissions are observed at room temperature under continuous-wave pumping at 980 nm. A weak emission can also be detected in the blue at 410 nm. The upconversion dynamics were studied measuring the decay times and the pump-power dependence of the transitions to the I-4(15/2) ground state starting from the H-2(11/2), S-4(3/2) and F-4(9/2) excited states. Excited-state absorption (ESA) is found to be responsible for the higher energy (H-2(11/2), S-4(3/2) --> I-4(15/2)) green transitions. On the other hand, for the F-4(9/2) --> I-4(15/2) red transition a competing energy-transfer upconversion (ETU) mechanism is found, which accounts for the more than 100-fold increase in intensity of the red emission on passing from the lowest (0.1 mol.%) to the highest (5 mol.%) erbium concentration