Synthesis And Nir Upconversion Luminescence Of β-NaYF₄:Yb³⁺,Tm³⁺/CdSe Nanoheterostructures

Upconversion luminescence nanomaterials can convert the near-infrared（NIR） into visible, even ultraviolet（UV）, which have become one of the most intensive research areas. Rare earth doped fluorides have draw intensive attentions because of their low phonon energy, chemical stability and good bio-compatibility. The spectral range in 600 to 1,100 nm（Red 600-700 nm, NIR 700-1,100 nm） is deemed to as “optical window” of biological tissues because of their lowest absorption. Compared with the conventional ultraviolet excitated organic fluorescent dyes and quantum dots, the NIR excitated upconversion luminescence nanomaterials can help reduce the optical damage to biological tissues, increase the depth of penetration and decrease autofluorescence background of biological tissues, making it more suitable for biological imaging. Yb/Tm co-doped Na YF4 nanocrystals can convert NIR into ultraviolet, blue, red and NIR. The intense ultraviolet and blue emissions can be absorbed by biological tissue causing optical damages. On the other hand, semiconductor quantum dots have a broad absortion spectrum and are capable to convert ultraviolet into visible. More importantly, quantum dots have size dependent quantum confinement effect. Here, we used Cd Se quantum dots as surface modifying agent of Na YF4:Yb3+,Tm3+ nanocrystals and synthesized nanoheterostructure Na YF4:Yb3+,Tm3+/Cd Se. Experimental results showed that the UV and blue emissions of Na YF4:Yb3+,Tm3+ nanocrystals were strongly suppressed, but ⁷97 nm NIR had been significantly enhanced. Na YF4:Yb3+,Tm3+/Cd Se nanoheterostructures have achieved NIR enhanced upconversion luminescence. In this thesis, the main results are outlined as followings:（1） High quality and monodisperse hexagonal Na YF4:Yb3+,Tm3+ nanocrystals were synthesized via high temperature thermal decomposition method. By manual or instrument precisely control the high temperature reaction time, we have got a range of different sizes, monodisperse and uniform β-Na YF4:Yb3+,Tm3+ nanocrystals, achieved β-Na YF4:Yb3+,Tm3+ nanocrystals controlled synthesis. The samples were composed of pure hexagonal phase Na YF4:Yb3+,Tm3+ nanocrystals measured by X-ray diffraction. Under 980 nm NIR excitation, the upconversion spectrum of Na YF4:Yb3+,Tm3+ nanocrystals indicated that there were intensive ultraviolet, blue, red, and NIR emissions, which revealed the upconvertion mechanism in Yb/Tm codoped system.（2） High quality and monodisperse wurtzite Cd Se and fluorescence enhanced core-shell Cd Se/Zn S quantum dots were synthesized via TOPO assisted oil phase method. By controlling the high temperature reaction time precisely, a range of different sizes and monodisperse Cd Se quantum dots were synthesized. The X-ray diffraction spectrum indicated that the samples were composed of pure wurtzite Cd Se nanocrystals. Under the excitation of 365 nm UV, the emissions of Cd Se quantum dots depended on the size of particles, exhibiting typical quantum confinement effect.（3） β-NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures were synthesized via a two-steps pyrochemical method. The X-ray diffraction spectrum showed that the samples were composed of both pure wurtzite Cd Se quantum dots and hexagonal Na YF4 nanocrystals. Under 980 nm NIR excitation, there were only ⁷97 nm NIR emissions emiting from β-NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures. Study results showed that energy transfer occured between Na YF4 nanocrystals and Cd Se quantum dots. The Yb3+ transfered the energy to Tm3+ after absorbed the NIR photons. Tm3+ could transfer the energy to the Cd Se partially, and then the excited Cd Se returned the energy back to Tm3+, resulting in quantities of electrons populated the 3H4 energy levels of Tm3+. That was the reason of the enhancement of ⁷97 nm NIR emission and the quenching of UV and blue emissions of Tm3+ in the β-NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures, without fluorescence of Cd Se quantum dots interestingly. The β-NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures achieved enhanced NIR-NIR upconversion luminescence. The excitation and emissions of β-NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures were both located at the “optial window” of biological tissues. Therefore, the β-NaYF₄:Yb³⁺,Tm³⁺/CdSe nanoheterostructures could be well applied to biology and biomedicine potentially.