Direct evidence of reversible energy transfer between Er^(3+) and Tm^(3+) 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 yb^3＋/Er^3＋, yb^3＋/Tm^3＋, and yb^3＋/Er^3＋/Zm^3＋ under the excitation of 980 nm infrared light. Importantly, the excitation power density dependence of upconversion emission intensity indicated clearly the energy transfer from Tm^3＋ to Er^3＋ ions under the excitation of low power density （5× 10^2× 10^2 W/cm^2）. Meanwhile, the inverse energy transfer from Er^3＋ to Tm^3＋ ions under the excitation of relatively higher power density （4.1 × 10^4.9× 10^4 W/cm^2） was also revealed. This was a direct evidence for reversible energy transfer between Er^3＋ and Tm^3＋ ions. Under the excitation of high power density （4.1 ×10^4-4.9× 10^4 W/cm^2）, dark sensitizers were also motivated so that the bottleneck effect of high concentration Yb^3＋ ion doping was broken. This was the main reason for realizing high upconversion efficiency of the samples with heavy doping of Yb^3＋ ion.