NIR upconversion luminescence of NaYF₄:Yb³⁺, Tm³⁺/CdSe nanoheterostructures

NaYF₄:Yb³⁺, Tm³⁺/CdSe nanoheterostructures with NIR-to-NIR upconversion luminescence were synthesized by a two-step improved wet-chemical method. X-ray diffraction (XRD) characterization shows that the nanoheterostructures are composed of both pure hexagonal phase NaYF₄:Yb³⁺, Tm³⁺ and wurtzite phase CdSe. Upconversion emission spectra under the excitation of 980 nm near-infrared laser show that 797 nm emission from Tm³⁺ ions has been enhanced greatly, while neither ultraviolet nor blue emissions of Tm³⁺ appear compared with that of pure NaYF₄:Yb³⁺, Tm³⁺. It indicates that the energy transfer occurs between NaYF₄:Yb³⁺, Tm³⁺ and CdSe. Under 980 nm excitation, the energy absorbed by Yb³⁺ ions transfers to Tm³⁺ ions, then transfers to CdSe partially, and finally the excited CdSe returns the energy back to the corresponding energy levels of Tm³⁺. These processes lead to the quenching of UV and blue emissions and the enhancement of 797 nm emission of Tm³⁺ ions in the NaYF₄:Yb³⁺, Tm³⁺/CdSe nanoheterostructures. The excitation (980 nm) and emission (797 nm) are both located in the NIR spectral range (700-1100 nm) that is referred as the “optical window” of biological tissues because of the minimum of light scattering and autofluorescence of tissue and a relatively large penetration depth. Therefore, the present NaYF₄:Yb³⁺, Tm³⁺/CdSe nanoheterostructures could have potential applications in biology and biomedicine fields.