Enhanced near-infrared upconversion luminescence of NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures

NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures were synthesized by an improved two-step wet-chemical route. Upon the excitation from a 980-nm near-infrared (NIR) laser, an intense 797 nm NIR emission from Tm³⁺ ions was observed in the nanoheterostructures while neither ultraviolet (UV) nor blue emissions of Tm³⁺ ions appeared in the upconversion (UC) spectrum. It indicated that the energy transfer (ET) occurred between NaYF₄:Yb³⁺,Tm³⁺ and CdSe. Under 980 nm irradiation, the excitation energy absorbed by Yb³⁺ ions transferred to Tm³⁺ ions successively, then transferred to CdSe partially from the ¹G₄ state of Tm³⁺ ions, and finally the excited CdSe returns the energy back to the ³F_2,3 levels of Tm³⁺ ions. Both the excitation (980 nm) and the emission (797 nm) are located in the NIR spectral range (700-1100 nm) that is referred as the "optical window" of biological tissues. Therefore, the nanoheterostructures could have great potential in applications of biology and biomedicine.