In this paper, the upconversion luminescent properties of Y2O3:Er3+(1%)/Yb3+(4%) nanoparticles with different sizes (13−55 nm) and its corresponding bulk material as a function of excitation power were studied under 978-nm excitation. Red (4F9/2 → 4I15/2), green (2H11/2, 4S3/2 → 4I15/2), and blue (2H9/2 → 4I15/2) transitions were observed. The results indicated that the relative intensity of the blue as well as the red to the green increased gradually with decreasing particle size. As a function of excitation power, the slope in the ln−ln plot for the red emission changed between 2.0 and 1.0 and gradually decreased with increasing particle size, which was attributed to competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the particle size decreased to 13 nm, a three-photon populating process occurred for the green emission. As the excitation power varied in different paths, gradually increasing or gradually decreasing, a hysteresis loop appeared in the power dependence of emission intensity, which was mainly caused by a local thermal effect induced by laser irradiation. The intensity ratio of 2H11/2 → 4I15/2 to 4S3/2 → 4I15/2 (R HS ) varied complicatedly with excitation power, which was theoretically explained considering the thermal distribution and relaxation processes. Two novel cross-relaxation paths were proposed on the basis of the variation of R HS under excitation at different wavelengths, 488 and 978 nm.

In this paper, the upconversion luminescent properties of Gd2O3:Er3+,Yb3+ nanowires as a function of Yb concentration and excitation power were studied under 978-nm excitation. The results indicated that the relative intensity of the red emission (4F(9/2)-4I(15/2)) increased with increasing the Yb3+ concentration, while that of the green emission (4S(3/2)/2H(11/2)-4I(15/2)) decreased. As a function of excitation power in ln-ln plot, the green emission of 4S(3/2)-4I(15/2) yielded a slope of approximately 2, while the red emission of 4F(9/2)-4I(15/2) yielded a slope of approximately 1. Moreover, the slope decreased with increasing the Yb3+ concentration. This was well explained by the expanded theory of competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the excitation power density was high enough, the emission intensity of upconversion decreased due to thermal quenching. The thermal effect caused by the exposure of the 978-nm laser was studied according to the intensity ratio of 2H(11/2)-4I(15/2) to 4S(3/2)-4I(15/2). The practical sample temperature at the exposed spot as a function of excitation power and Yb3+ concentration was deduced. The result indicated that at the irradiated spot (0.5 x 0.5 mm2) the practical temperature considerably increased.

One-dimensional pure cubic Y(2)O(3)/Eu(3+) nanocrystals (NCs) were synthesized by a hydrothermal method at various temperatures. The NCs prepared at 130 degrees C yielded nanotubes (NTs) with wall thickness of 5-10 nm and outer diameter of 20-40 nm. The NCs prepared at 170 and 180 degrees C yielded nanowires (NWs) with diameters of approximately 100 and approximately 300 nm, respectively. Their luminescent properties, including electronic transition processes, local environments surrounding Eu(3+) ions, electron-phonon coupling, and UV light irradiation induced spectral changes have been systematically studied and compared. The results indicate that the Y(2)O(3)/Eu(3+) NTs and NWs have strong red (5)D(0)-(7)F(2) transitions. The fluorescence lifetime of (5)D(1)-(7)F(1) hardly changes in different samples, while that of (5)D(0)-(7)F(2) decreases a small amount in Y(2)O(3)/Eu(3+) NTs. The (5)D(0)-(7)F(2) lines originate from the emissions of Eu(3+) ions occupying one C(2) site, like that in the bulk powders. The phonon sideline with a frequency shift of 40-50 cm(-1) appears at the low-energy side of the (7)F(0)-(5)D(0) zero phonon line. The relative intensity of the sideline to zero phonon line increases by varying from NTs to NWs, and the spectral position of the phonon sideline shifts red. The UV light irradiation induced spectral change in the charge-transfer band was studied. The results indicate that the spectral change is dependent on sample size and is wavelength selective. A detailed model was proposed to explain the light-induced spectral change.