Fluorescence Enhancement Effect Of Rare Earth Doped NaYF₄ Nanocrystals

In the global tide of new technology revolution,Rare-earth(RE)doped luminescent nanomaterials have presented great potential applications in lighting,biology,solid-state laser,nano-optoelectronics,information science due to its unique spectral features.However,the low fluorescence efficiency restricted the developing of progress of rare earth doped luminescent nanomaterials.At the same time,the energy level structure of rare earth ions are very abundant and complex,this makes the research in the field full of challenges.It is significant to find out how to improve the efficiency of luminescence on the basic research and applications of the rare earth doped luminescent materials.In this dissertation,?-NaYF4:Yb3+/Er3+ nanoparticle is selected as the research object.By coating active shell and introducing metal surface plasmons,we can enhance the fluorescence efficiency.The main work is summarized as followd:(1)?-NaYF4:Yb3+/Er3+ nanostructures are prepared by high-temperature coprecipitation method.Then we coat NaYF4:Yb3+ shell onto the NaYF4:Yb3+/Er3+NPs.We research the influence of the upconversion fluorescence by inert layer and active layer.It is suggested that the upconversion fluorescence is greatly enhanced after the shell is added.The Yb3+ dopant concentration in the active shell can regulate the upconversion fluorescence and we found that the optimal doping concentration of Yb3+is 5%.(2)We build a suitable metal enhanced fluorescence system with NaYF4:Yb3+/Er3+ NPs.On the one hand,we use Au NPs,Au-Ag NPs and nanocones to investigate the influence of the LSPR wavelength overlapped with the emission wavelength.On the other hand,we used Au nanocones to investigate the influence of the overlapping of the LSPR wavelength with the excitation wavelength.Luminescence enhancement is obtained with enhancement Au,Au-Ag NPs and Au Nanocones.It is suggested that the excitation enhancement reached its maximum effect when the excitation wavelength is overlapped by the LSPR wavelength.For the coupling of the emission wavelength and LSPR wavelength,the enhancement effect is better when they overlapped more perfect.(3)P-NaYF4:Yb3+/Ln3+(Ln=Er3+,Tm3+)nanoplate are prepared by hydrothermal method.Single rare earth doped NaYF4 nanoplate with different size of Ag nanoparticles is designed for the investigation of metal enhanced upconversion luminescence effect.Luminescence enhancement is obtained with big Ag nanoparticles and luminescence quenching effect is also observed with small Ag nanoparticles.It is suggested that the excitation enhancement is the reason of the enhancement at both wavelength,and the emission enhancement is also involved.The absorption at the green range is larger than that at red range.As a result,the enhancement effect at 540 nm is greater than that at 650 nm.(4)A novel metal-enhanced fluorescence system is constructed.The hexagonal NaYF4:Yb3+/Er3+ nanodisk are prepared by hydrothermal method,then we coat SiO2 shell onto NaYF4:Yb3+/Er3+ NPs.Finally,the Ag is adhered to the core-shell structure.NaYF4:Yb3+/Er3+@SiO2@Ag core-shell hybrid particle is taken to investigate the influence of isolation distances on the metal enhanced upconversion luminescence and corresponding mechanism with a confocal microscopy setup.It is found that different isolation distances correspond to different enhancement results.They are less than 5 nm correspond to fluorescence quenching,and 12 nm is the best enhancement distance separately.The construction of the composite structure can effectively expand the understanding of metal enhanced fluorescence of Re doped nanomaterial.