The Upconversion And Quantum Cutting Of Rare Earth Doped Fluoride Core Multishell Nanocrystals

Rare earth doped nano materials were widely utilized in bio-imaging,bio-sensor and solar cell due to these special optical properties.Especially in solar cell field,rare earth doped materials can increase the efficiency of solar cell by solving two types of loss effect: through upcovnerion,the photons below silicon bandgap can be transferred to the absorption range of silicon;quantum cutting can cut one high energy photon into two lower energy phtotons with quatum efficiency over 100%,and in this process,the photons exceeding the absorption range can be cut into two lower energy photons,which can be absorbed by solar cell completely.However,currently most of the studies were focusing on bulk materials.Based on this,in this thesis,we used rare earth doped core shell nanocrystals to study the upcovnerion of the photons below the silicon bandgap;quantum cutting of the high energy in the ultra-violent range;and combined upconverion and quantum cutting into one core shell materials.The study of transferring the photons below the silicon bandgap（>1100nm）to the range upon its bandgap through Er³⁺,Ho³⁺ and Tm³⁺ ions upconverion process.We first studied the upconversion of Er³⁺ in three different matrix NaYF₄,Li YF₄ and NaGd F₄,and the result showed NaYF₄ was the best.We then optimized the Er³⁺ dopant concentration,and 10% was the best.We coated three matrix with homogeneous epitaxial growth,respectively.All three kinds of core shell nanoparticles showed enhancement after coating the shell.Among them,the emission intensity of NaYF₄:10%Er³⁺@NaYF₄ was the strongest.The quantum yield was as high as 3.9% by an integration sphere,which was the highest among all the reports.Then,we did the same studies on Ho³⁺ at 1157 nm and Tm³⁺ at 1213 nm.NaYF₄ was chosen as the matrix to combine Er³⁺,Ho³⁺,Tm³⁺ upconversion together through a core multishell structure,which employed an inert layer of NaYF₄ in the middle of each active layer to eliminate the cross relaxation between them.The structure of core multishell nanoparticles was NaYF₄:10%Er³⁺@NaYF₄@ NaYF₄:10%Ho³⁺@NaYF₄@NaYF₄: 1%Tm³⁺@NaYF₄.We compared it with the core multishell without inert layer between active layers,NaYF₄:10%Er³⁺ @NaYF₄:10%Ho³⁺@NaYF₄:1%Tm³⁺@NaYF₄ and the co-doped core shell nanoparticles NaYF₄:10%Er³⁺,10%Ho³⁺,1%Tm³⁺@NaYF₄.With excitation at 1523 nm,1157nm and 1213 nm,the enhancement were 1.9 folds,16.7 folds;2.1 folds,14.5 folds;1.4 folds,6.7 folds,respectively.Furthermore,we studied the upconverion mechanism of Er³⁺,Ho³⁺,Tm³⁺ single ion center.The excitation spectrum showed 270 nm response of the photons below silicon bandgap.We developed the quantum cutting processes of Tb³⁺-Yb³⁺ couple,Pr³⁺-Yb³⁺ couple and Er³⁺ single doped system in NaYF₄.We controlled the Tb³⁺ doping concentration of 2%,then varied Yb³⁺ concentration.We synthesized NaYF₄:2%Tb³⁺,x%Yb³⁺（x=0、20、40、60、80）with the same size by changing the oleic acid amount.The quantum cutting emission increased with Yb³⁺ concentration from 0 to 20%,after that,the intensities decreased with Yb³⁺ concentration from 20% to 80%.The dependencies of intensities with power densities were measured,leading to the quantum cutting mechanism of Tb³⁺-Yb³⁺.We calculated the theoretical quantum cutting efficiency of 183.7%.At last,we coated inert shells on NaYF₄:2%Tb³⁺,x%Yb³⁺（x=0、20、40、60、80）series nanoparticles.The luminescence of all the samples enhanced comparing with the core,and the emission tendency changed from the core,and the emission increased with improving Yb³⁺ doping concentration,and the sample with 80% Yb³⁺ showed the strongest intensity.We combined upconversion and quantum cutting processes together in one single particle with core multishell structure.NaYF₄:10%Er³⁺@Na Lu F₄@ NaYF₄:2%Tb³⁺,20%Yb³⁺@NaYF₄ was synthesized,which employed Na Lu F₄ as the middle isolation layer to stop the cross relaxation between each active layer.Notably,we can observe the core multishell structure with TEM directly,due to the mass difference between Lu and Y.NaYF₄:10%Er³⁺@NaYF₄ was ultilized as a reference standard to quantify the quantum yield of upconversion process,which appeared as around 3.6%.For the quantum cutting,rhodamine 6G was chosen as the standard with the quantum yield of 130%.