Synthesis And The Properties Investigations Of Rare Earth Upconversion Materials

Up-conversion(UC)nano-or micro-materials,which can convert low-energy photons into high-energy photons(anti-Stokes process),have aroused increasing research interests over the past decade due to their attractive prospect for applications in the fields of bioanalystics,compact solid-state lasers,optical telecommunication,and display technologies.In particular,the energy transfer up-conversion encompasses the benefits of high conversion efficiency,large anti-stokes shifts,sharp emission bandwidths,and long excited-state lifetimes by merely lamp excitation or continuous wave lasers with moderate excitation densities.In the past decade,great progress has been achieved in rare earth(RE)ions doped UC inorganic micro-/nanoparticles.However,there are still some basic research needed to be done on this area,such as,desining of synthsis process,tuning of UC luminescences,improving the UC properties,and so on.Based on the above statement,this thesis mainly focused on the design and synthesis of multiple RE ions doped UC materials,the analysis of their structures through XRD,TEM,HRTEM,XPS,and FT-IR,and the manipulation of their luminescent properties UC spectra.The hydrothermal method,molten salt process,and micro-wave assisted process were used for the synthesis of crystals.Through analysis the size,phase,structure,and morphology of the crystals obtained in different conditions,the growth mechanisms were also investigated.Most importantly,the luminescent properties were tuned and analyzed through changing the doping ions,doping concentrations,and the host materials properties.A nano-composite based on rare earth oxide UC nanocrystals was also prepared,the UC properties and the drug release properties were also investigated.Specifically,this thesis is mainly focus on the following five aspects.Firstly,the BaYFs and ?-NaLuF4 nanocrystals were synthesized through a facile hydrothermal method and hydrothermal conversion process,respectively.The influences of different parameter on the final structure and morphology of the two products have been well investigated.And the growth mechanisms were also proposed.Additionally,the UC properties of Yb3+/Er3+,Yb3+/Tm3+,and Yb3+/Ho3+ doped BaYFs and ?-NaLuF4 nanoparticles were also discussed.Under 980 nm laser excitation,the nanocrystals exhibit respective strong UC luminescences.The luminescence mechanisms for the doped lanthanide ions were also analyzed in detail.Secondly,well-defined LiYF4:Yb3+,Er3+/Tm3+/Ho3+ and NaLuF4:Yb3+,Er3+/Tm3+/Ho3+micro-crystals were successfully prepared by a surfactant-free molten salt method for the first time.The phases and morphologies were tuned trough manipulating the reaction parameters,from which the growth mechanisms were also proposed.Upon 980 nm laser diode excitation,multi-color UC light was successfully achieved by properly tuning the sensitizer(Yb3?)concentration in the host matrix.UC mechanisms in the co-doping and tri-doping LiYF4 and?-NaLuF4 samples were analyzed in detail based on the emission spectra and the plot of luminescence intensity to pump power.The as-obtained abundant luminescence colors in a much wide region contribute themselves great potential applications in various fields,and provides an effective and facile approach to gain a desired color by manipulating the sensitizer concentration.Thirdly,pure hexagonal NaYF4:Yb3+,Ln3+(Ln = Er,Tm,Ho)crystals were obtained for the first time through a facile microwave assisted method at relatively low temperature(160 ?)and atmospheric pressure within only 50 min.A possible growth mechanism for the phase transformation under this MW condition has been proposed.The UC emission intensities of hexagonal NaYF4:Yb3+,Ln3+(Ln = Er,Tm,Ho)are increased by 10-12 times compared to those of cubic phase.Moreover,for the first time,we introduced Bi3+ ion into?-NaYF4:20%Yb3+,2%Ln3+ crystals.As expected,the UC emission of?-NaYF4:Yb3+,Ln3+,Bi3+ are about 10-40 times higher than those of Bi3+ free samples.XRD results gives evidence that tri-doping of Bi3+ ions can tailor the local crystal field and dissociate the Yb3+ and Ln3+ ion clusters,which is the main reason for the UC enhancement.This designed MW reflux method for the synthesis of ?-NaYF4:20%Yb3+,2%Ln3+ can be applied to prepare other rare earth fluorides.The markedly enhanced UC luminescence through Bi3+ doping also provides an effective way to gain highly bright UC emissions.Finally,the drug loading and release properties were analyzed in two systems.The second system is mesoporous silica composites.Gd2O3:Yb3+,Er3+ assembled mesoporous silica nanocomposites with 2D hexagonal(MCM-41)and 3D cubic(MCM-48)network have been successfully prepared via a one-step sol-gel procedure.Then,gold nanocrystals were integrated with the amino groups functioned nanocomposites through the electrostatic interaction.Upon 980 nm NIR laser irradiation,a wavelength-dependent enhancement of the UC intensities is observed due to the surface plasmon resonance effect of attached gold nanoparticles.DOX loading and release properties of the two systems were tested.The photothermal effect of gold nanocrystals also induces a rapid DOX release under laser irradiation.Such novel multifunctional anti-cancer drug delivery systems,which combine hyperthermia with the chemotherapeutic drugs by synergistic effect should be of high potential in cancer therapy.