Research On Upconversion Luminescence Properties Of Rare-Earth Doped Nanocrystals And PMMA Composites

The lanthanide-doped nanocrystals have drawn considerable attention not only for their unique optical properties but also for the potential applications in the fields of vivo imaging,solar cells and volumetric displays.Especially,lanthanide-doped oxides and fluorides nanocrystals have been regarded as one of the most excellent host lattices for performing upconversion luminescence,due to their good optical transparency,high chemical stability and low phonon energy.At the same time,the composite materials of nanocrystals and polymers will further promote the widespread application.The YF₃: Yb/Er,the mixture of ?-NaYF₄ and ?-NaYF₄ and ?-NaYF₄:Yb/Er samples were synthesized by a simple hydrothermal method.The phase conversions from YF₃ to ?-NaYF₄,as well as ?-NaYF₄ to ?-NaYF₄ crystal,accompanied by morphology manipulation and upconversion luminescence enhancement have been demonstrated by simply tuning the Zn²⁺ ion doping concentration.Zn²⁺ ion occupies the Y³⁺ ion crystal lattice site leading to positive vacancies and lower energy barrier in the reaction of forming NaYF₄.Besides,we also investigated the upconversion luminescence intensity enhancement of the samples.Y₂O₃: Yb/Tm/Gd@Y₂O₃ nanocrystals with core-shell structure were prepared by homogeneous precipitation method and then annealed at a high temperature.The nanocrystals were characterized by transmission eletctron microscopy?TEM?,X-ray diffraction?XRD?,and luminescent spectra.Upconversion spectra and excitation power dependence indicate that the energy transfer from Tm³⁺ to Gd³⁺ plays a crucial role in populating the excited states of Gd³⁺ ion.The shortest wavelength of upconversion emission at 310 nm in Y₂O₃ matrix is converted from the infrared region?980 nm?.Meanwhile,the core-shell structure greatly improved the upconversion fluorescence intensity of Gd³⁺ ion in Yb³⁺-Tm³⁺-Gd³⁺ codoped system.Yb³⁺ and Er³⁺/Tm³⁺ codoped Gd₂O₃ core-shell nanocrystals were synthesized by a homogeneous precipitation method in our work.Under 980 nm laser excitation,a great enhancement of the upconversion emissions in Gd₂O₃: Yb/Er?Tm?has been observed by coating the homogeneous shell.Meanwhile,energy-transfer processes and excited absorption are discussed on the basis of our experimental results.The NaYF₄/PMMA composites were synthesized by bulk polymerization,and NaYF₄ nanoparticles were uniformly dispersed in PMMA polymer.The doping of nanoparticles improves the thermal stability of composites,and the light transmittance is not significantly reduced.At the same time,with the NaYF₄ nanoparticles content increasing,the upconversion emission intensity of the composites achieves a enhancement,and the green light is emitted under 980 nm excitation.