Upconversion nanocrystals

Appl. Phys. Express 5, 092601 (2012)

Lanthanide-doped nanocrystals that can efficiently upconvert infrared radiation into visible light are being studied with a view to applying them in the display, DNA-detection and bio-imaging areas. Now Xiaojie Xue and co-workers from the Toyota Technological Institute, Japan have observed ultraviolet emission from Tb³⁺/Yb³⁺-codoped KY₃F₁₀ nanocrystals that were excited with light from a 976-nm laser diode. KY₃F₁₀ nanocrystals doped with x% Tb³⁺ and 5% Yb³⁺ were synthesized by a hydrothermal method and had an average size of about 45 nm. The emission spectrum of the nanocrystals featured both strong ultraviolet emission at 381 nm and weaker green emission at 544 nm. The emission decay time for the ultraviolet emission was 1.9 ms. The intensity ratio between the ultraviolet–blue emission from the ⁵D₃ level and the green emission from the ⁵D₄ level decreased with increasing Tb³⁺ concentration. The researchers say that this concentration-dependent phenomenon is evidence of cross-relaxation, and indicates possible upconversion mechanisms based on the energy transfer from Yb³⁺ to Tb³⁺ and cross-relaxation processes between Tb³⁺ pairs.