Contruction And Upconversion Property Regulation Of Er³⁺ Doped Fluoride Core-shell Nanocrystals

Lanthanide doped fluoride upconversion nanocrystals have been widely used in the fields of spectral conversion,solar cells,fluorescent display,biological or cell imaging,sensing and detection,super-resolution microscopy and micro-nano laser due to their unique optical properties.Although upconversion nanocrystals have excellent properties,such as large anti-Stokes shift,high stability,non-bleaching,low detection background,high signal-to-noise ratio and strong tissue penetration,there are still some key problems remained to be solved.Especially,low upconversion efficiency,induced due to the inherent narrowband and low absorption coefficients of lanthanide ions,indeed restricts the further application of upconversion nanoparticles.To construct highly efficient upconversion nanosystem with enhanced absorption capacity,this project takes Er³⁺ doped fluoride upconversion system as basic research object,and mainly focus on new structure design,concentration quenching suppression,novel energy transfer pathway design and optimization,etc.After that,we also performed systematic investigations on the applications of solar cells,biological cell marker and imaging,temperature sensor,based on efficient upconversion nanoparticles with strong emission and functional designs.The organic dye sensitized energy cascaded upconversion nanosystem was constructed to expand the spectral response range and the photon absorption capacity of rare earth ions.Based on the key points of energy transfer efficiency inside this organic-inorganic hybrid system,energy transfer distance and energy matching?spectral overlap?are taking into consideration.Then,IR 783 that can chelate with the surface of nanoparticles and transfer excited energy to lanthanide ions effectively was screened as antenna molecule.Meanwhile,Nd³⁺ ions which match well with IR 783 dye,was selected as the energy transfer ions,which was placed in the outmost shell layer after doping concentration optimization.This kind of design could not only avoid the cross-relaxation between Nd³⁺ and Er³⁺ ion inside the core,but also shorten the energy transfer distance between IR 783 dye molecules and Nd³⁺ ion.With the help of organic dye antenna,the emission intensity of energy cascaded upconversion nanosystem(IR783 sensitized Na YF4:20%Yb³⁺,2%Er³⁺@Na YF4:30%Nd³⁺)saw an enhancement of 11 times and the quantum yield was calculated to 3.43%.Finally,the photoelectric conversion efficiency of dye-sensitized solar cells was improved by 13.1% when equipped with this broadband strong absorption upconversion layer.An efficient multispectral band absorbing Er³⁺ ion sensitization upconversion system with enhanced absorption capacity and luminescence intensity was constructed through the suppression of fluorescence concentration quenching.It was proved that excitied state energy transfer to the surface quenching sites induced great energy losses inside Na YF4:x%Er³⁺ nanoparticles.This pathway was restricted by the precisely control of inert shell layer,wihch can isolate active lanthanide ions with surface quenching centers spatially.Then,the optimal doping concentration of Er³⁺ ions inside core could be increased from 10% to 100% with enhanced emission intensity as the inert shell thickness reaches 6 nm.The emitter number and the photon absorption capacity of upconversion nanocrystal would increase continuously with the increasing of Er³⁺ ion doping concentration.And the upconversion process would transform from excited state absorption to energy transfer upconversion,leading to the enhancement of upconversion efficiency.In addition,the emission spectrum of a single core-shell structured nanoparticle is proved to be a kind of partially linearly polarized light,which show a dependency relationship with length/diameter ratio.Excited state energy confinement of Er³⁺ ions sensitized upconversion system was achieved through introducing various energy trapping centers.Due to the restraining of excited state energy transfer to the surface quenching center,31.1 times enhancement of emission intensity was obtained while the size of this core-shell nanoparticle maintains below 20 nm.Moreover,by regulating the trapping center energy level,the modulation of energy transfer and back-transfer pathway as well as the transfer efficiency is realized in Er³⁺ sensitized upconversion nanosystem.Then,the population in 4F9/2/ 2H11/2/ 4S3/2 energy levels,which is corresponding to the emission peak of red/green light,can be tuned easily.Therefore,the luminescence output color can be adjusted gradually from red to green?R/G: 0.05 ? 30?.Moreover,after targeted surface modification through ligand exchange and multiple EDC reactions,mitochondria labeling imaging and tracing of Hela cell were realized with a Pearson correlation coefficient of 0.71.A broad range thermometry from cryogenic to room temperature was constructed based on a dual-mode upconversion nanoprobe.The temperature sensing performance of Yb³⁺/Er³⁺ co-doped nanoparticles was studied on the consideration of the Boltzmann distribution of related thermally coupled energy levels.It was found that the energy difference??E?will affect the population distribution of the higher energy level greatly,especially at low temperature?<150 K?.In order to make up the performance shortage of Er³⁺ ions,Tm³⁺ ions with a much smaller ?E??288 cm-1?was introduced.A ultrasmall multi-layer core-shell structured nanoprobe(? 15 nm,?-Na Yb F4:Tm³⁺@CaF₂ @Na YF4:Yb³⁺/Er³⁺@CaF₂)was constructed to integrate these two temperature sensing ions.And finally,a high sensitivity of 3.06% K-1 at a wide temperature range of 10-295 K was achieved in this dual mode temperature sensor with a tiny temperature uncertainty of 0.16 K.