Structural investigation and anti-Stokes emission of scandium oxide nanocrystals activated with trivalent erbium

The structural and emission (Stokes and anti-Stokes) properties of Sc2O3: Er3+ nanocrystals doped with 0.1, 1, and 10 mol % Er3+ were investigated. The nanocrystalline powders were characterized using X-ray scattering as well as transmission and scanning electron microscopy. The samples showed a very porous, open microstructure with the particles having a narrow distribution of sizes (10-60 nm). Furthermore, the mechanisms responsible for the anti-Stokes emission (lambda(exc) = 980 nm) were elucidated. We observed that the processes responsible for populating the green (H-2(11/2), S-4(3/2)) and red (F-4(9/2)) emitting states were dependent upon the concentration of the dopant ion. In 0.1 mol % nanocrystalline Sc2O3: Er3+, upconversion was determined to occur via excited state absorption while in the 10 mol % sample, energy transfer upconversion was the dominant mechanism. An enhancement of the red anti-Stokes emission from the F-4(9/2) --> I-4(15/2) transition was observed in Sc2O3: Er3+ nanocrystals as a function of Er3+ concentration. This was the result of two independent processes responsible for directly populating the F-4(9/2) state and bypassing the green emitting levels (H-2(11/2) and S-4(3/2)). Furthermore, the red enhancement was found to be more pronounced compared to identically doped Y2O3: Er3+ nanocrystals. An explanation for this phenomenon is presented and discussed.