Direct evidence of reversible energy transfer between Er3+ and Tm3+ ions in upconversion microcrystals

Lanthanide doped NaYF4 microcrystals were synthesized via a facile hydrothermal method. Multicolor upconversion luminescence was observed in NaYF4 microcrystals doped with Yb3+/Er3+, Yb3+/Tm3+, and Yb3+/Er3+/Tm3+ under the excitation of 980 nm infrared light. Importantly, the excitation power density dependence of upconversion emission intensity indicated clearly the energy transfer from Tm3+ to Er3+ ions under the excitation of low power density (5x10(2)-9x10(2) W/cm(2)). Meanwhile, the inverse energy transfer from Er3+ to Tm3+ ions under the excitation of relatively higher power density (4.1x10(4)-4.9x10(4) W/cm(2)) was also revealed. This was a direct evidence for reversible energy transfer between Er3+ and Tm3+ ions. Under the excitation of high power density (4.1x104-4.9x104 W/cm(2)), dark sensitizers were also motivated so that the bottleneck effect of high concentration Yb3+ ion doping was broken. This was the main reason for realizing high upconversion efficiency of the samples with heavy doping of Yb3+ ion.