Modulation Of Upconversion Luminescence In Rare-Earth Doped NaYF₄Nanocrystals

Rare-earth doped upconversion luminescent materials have a great deal of potential applications in many fields, especially in biology, biomedicine, catalyst, and solar energy. Although hexagonal NaYF4:Yb/Er(Tm) nanomaterial is one of the most efficient upconversion(UC) luminescent materials, the application of NaYF4:Yb/Er(Tm) is still greatly constrained because of lower UC efficiency. In this paper, several methods have been developed to improve the emission efficiency of hexagonal NaYF4:Yb/Er(Tm) nanomaterial.As the sensitizer of UC luminescence, Yb3+ions could absorb near-infrared light and transfer energy to activator. Firstly, The UC intensity of hexagonal NaYF4:Yb/Er is investigated by varying the doping concentrations of YV3+ions in this paper. Upconversion emission spectra suggested that under the same power density, emission intensity of green and red emission decreased with the improvement of doping concentration of Yb3+. The presence of more Yb3+ions in the lattice makes the distance between Yb3+and Er3+ions shorter and the cross-relaxation between Er3+and Yb3+will be increasedSecondly, the NaYF4:Yb/Er nanoparticles are coated with SiO2shell and Yb3+ion are doped into the SiO2shell. The UC intensity is investigated by changing coating conditions. The results suggested that the UC intensity of NaYF4:Yb/Er didn’t increase after coated with SiO2shell by varying TEOS quantity, but increased by varying reaction time. The attempt to dope Yb3+ions into the SiO2shell is unsuccessful because NaYF4:Yb,Er nanomaterial was destroyed and became oxyfluorides after reacting and the UC intensity also decreased.Thirdly, the NaYF4:Yb/Er nanoparticles are coated with NaYF4and NaYF4:Yb shell. The experiment results suggested that the UC intensity of NaYF4:Yb/Er increased after coated both NaYF4and NaYF4:Yb shell, especially the NaYF4:Yb shell. The NaYF4:Yb shell serves two purposes; i) to protect the luminescing Er3+ions from the non-radiative decay and ii) to transfer NIR absorbed radiation to the luminescing core. But the NaYF4shell could only protect the luminescing Er3+ions from the non-radiative decay.Moreover, we reported a chemical synthesis method for generating NaYF4:Yb,Tm(Er)/CdS and NaYF4:Yb,Tm(Er)/CdSe nano-heterodimers for the first time. The UC intensity of NaYF4:Yb,Tm nanoparticles was also investigated after introducing CdS(CdSe) quantum dots. The results suggested that the emission intensity of475nm decreased dramatically, but the emission intensity of693nm and802nm increased greatly when comparing nanocrystals of NaYF4:Yb,Tm/CdS(CdSe) with NaYF4;Yb,Tm. But the emission intensity of NaYF4:Yb.Er/CdS(CdSe) decreased compared to NaYF4:Yb,Er. The difference between NaYF4:Yb,Er and NaYF4:Yb,Tm results from the different energy level distribution between Er3+and Tm3r after introducing quantum dots. The theory was also founded that fluorescence resonance energy transfer between NaYF4:Yb,Tm and CdS quantum dots in NaYF4:Yb.Tm/CdS heterodimers ocurred and caused the enhancement of red and near-infared(NIR) emission intensity of NaYF4:Yb,Tm...