The Controlled Synthesis and Spectral Modulation of Rare-earth Doped Oxide Up-conversion Micro/nanocrystals

Author:Zhao Xiao Qi

Supervisor:guo chong feng


Degree Year:2018





Benefiting from their large anti-Stokes shifts,long luminescence lifetimes,and superior physical and chemical stabilities,rare earth-doped up-conversion(UC)micro/nanocrystals show immense application prospects in numerous fields,including multi-color displays,security printing,and biomedicine.Rare earth ions(RE3+)usually possess plentiful ladder-like arranged metastable f electronic energy levels,which endows RE3+ with various energy transfer paths and abundant f-f transitions.As a consequence,RE3+-containing UC materials are capable of presenting our desired emission ranging from ultraviolet(UV)to near infrared(NIR)regions.The UC emission color and intensity can be efficiently tuned via selectively populating or quenching specific excited states based on varied energy transfer pathways.In comparison to fluoride hosts,RE3+-doped oxide UC materials possess the superior chemical and physical stabilities and lower toxicity.Along with the deepening development of nanotechnology,the research priority for RE3+-doped oxide UC materials is gradually transforming from bulk phosphor to micro/nanoparticles in order to satisfy the requirement in biomedicine and other high-tech areas.In this thesis,Gd2O3,Y2O3,CeO2 and Bi2O3 are employed as host materials,accompanying with Er3+,Ho3+ and Tm3+ as activators.A series of UC micro/nanocrystals with specific morphologies and outstanding luminescent properties are synthesized by a facile co-precipitation process.The main content of this thesis includes four chapters as follows:In the first section of the third chapter,RE2O3:Yb3+/Er3+(RE = Gd,Y)3D nanoflower are synthesized.The results show that the flower-like morphology is formed by the self-assembly of nanosphere precursors.With the increase of Yb3+ ions concentrations,the R/G ratios monotonically increase,resulting in a color-tunable UC emission from yellow-green to red.The red-to-green UC emission intensity(R/G)ratio of flower-like sample with the larger surface area is far greater than that of sphere-like sample,which is attributed to the surface quenching effect.In the second section,red,green and blue(RGB)UC emissions are obtained in Yb3+/Ln3+(Ln = Er,Ho,Tm)co-doped Gd2O3 UC phosphors with flower-like morphology.The spectral color purities of corresponding RGB UC emissions are further modulated by means of regulating the doping concentration,annealing temperature,excitation power density and pulse-width of 980 nm laser.Additionally,our experimental Iesuits have aemonstiated the apphcations as-prepared spectiahy pure RGB up-converted materials in fingerprint recognition and multi-color printing.In the fourth chapter,our work is focused on the influence of cross-relaxation(CR)process and excitation pulse-width on UC emission.In the first section,it can be found that the conversion of UC emission color from green to red is effectively realized through adjusting the probability of Tm3+-Er3+,Ho3+-Er3+ and Er3+-Er3+ CR processes and modulating the excitation pulse-width of 980 nm laser.In the second section,our work is focused on the influence of Ho3+-Ce3+ CR processes and excitation pulse-width on UC emission of Ho3+.The experimental results show that the UC R/G ratio of Ho3+ gradually increases with increasing the doping concentration of Ce3+ and enlarging the excitation pulse-Iidth of 980 nm laser.In the first section of the fifth chapter,CeO2:Yb3+/Er3+ samples with nanosphere and nanoflower morphologies are synthesized.The results show that the flower-like morphology is formed by the self-assembly process of nanosphere precursors,and elevating synthesis temperature and increasing the dosage of urea can be both conducive to the morphology evolution from nanosphere to nanoflower.Increasing the doping concentration of Yb3+,decreasing the particle size of nanospheres,and the morphology transition from nanosphere to nanoflower can all result in the rise of the UC R/G ratios of Er3+.In the second section,CeO2:Yb3+/Tm3+ with partial hollow core-shell structure is synthesized.The results show that the core-shell structure is formed by the agglomeration of a great many small single crystal particles.With the increase of Yb3+ doping concentration,the energy back transfer(EBT)processes from Er3+ to Yb3+ more effectively take place,leading to the decreased proportion of the UC emissions originating from 1G4 and 3H4 in whole spectra.Additionally,our experimental results reveal that CeO2:18%Yb3+/1%-Tm3+ up-converted material can effectively kill Escherichia coli upon 980 nm excitation,and its potential application in bio-tissue imaging is also preliminarily assessed.In the first section of the sixth chapter,BI2o3:Yb3+/Er3+ sample with nanoflower morphology is synthesized.The experimental results show that Bi2O3 and Bi2O3:Yb3+/Er3+ possess three crystalline phases:monoclinic,cubic and tetragonal phases.For Bi2O3 blank sample,elevating annealing temperature can be conducive to the phase evolution from tetragonal tomonoclinic.For Yb3+/Er3+ co-doped Bi2O3,increasing the annealing temperature and Yb3+doping concentration both facilitate the phase evolution from tetragonal to cubic.The essential reasons for the phase evolution processes in Bi2O3 and Bi2O3:Yb3+/Er3+ are the different phase-transition temperatures and phase stabilities under the varied doping conditions.Upon 980 nm excitation,Bi2O3:Yb3+/Er3+ all show red UC emissions,which can atributed to the radiation reabsorption of green emission of Er3+ from Bi2O3 host.Furthermore,the UC emission of cubic samples is found to be stronger than that of tetragonal samples.In the second section,Bi2O3:Yb3+/Tm3+ sample with nanoflower morphology is synthesized.The experimental results show that Bi2O3 blank sample possesses monoclinic phase,while Bi2O3:Yb3+/Tm3+ have two crystalline phases:cubic and tetragonal phases.Under the excitation of 980 nm laser,Bi2O3:χ%Yb3+/1%Tm3+ all show deep-red to near-infrared UC emission.With the increase of Yb3+ doping concentration,the energy back transfer process from Tm3+ to Yb3+ more effectively take place,leading to the decrease of I795/I693.