Controlling Multicolor Upconversion In Rare Earth Ions Doped Nanocrystals

Lanthanide ions doped upconversion materials,with advantages such as sharp emission bands,large Stokes-shift,minimal photobleaching and multi-color tunability,have a great application prospect in the fields of multi-color display,anticounterfeiting,coding and decoding,and bio-imaging.In recent years,tuning the upconversion luminescence?UCL?from lanthanide ions to obtain special emission features?such as single band red UCL,single band near-infrared UCL,dual-color orthogonal UCL and so on?,has become the research hotspot of upconversion.Among the various upconversion materials,fluoride with Er³⁺as the activators(such as NaYF₄:Yb³⁺/Er³⁺)have gained the most attention due to their relatively high efficiencies,tunable UCL properties and controllable synthesis feature.The emission spectra of Er³⁺in different hosts are very similar,with a strong green emission and a weaker red emission,showing green color as a whole.In practical applications,to meet the demands of different target purposes,the UCL feature from Er³⁺doped materials usually need to be tailored.Although many methods have been developed to tuning the UCL spectra and colors from Er³⁺ions,there are still some problems,including luminescence quenching under high doping concentration,impure emission color,unclear tuning mechanism,uncontrollable sample shape and size,insufficient emission intensity,difficulty to combine red upconversion into core-shell upconversion nanocrystals?UCNPs?.As an important constitution of primary colors,red UCL from Er³⁺,with its wavelength located in the“transparent window”of bio-imaging?600-1700 nm?,plays an indispensible role in bio-imaging and multi-color display.Thus,it is of great significance to investigate the red UCL of Er³⁺and to develop some new methods to obtain red emission from Er³⁺doped materials.Based on the above consideration,in this thesis,a new way was proposed to get red UCL from KMgF3:Yb³⁺/Er³⁺via a nonequivalent substitution strategy.The energy transfer processes between Mn and Er³⁺as well as the different position of ⁴T1 energy level on the tuning of UCL from Er³⁺in cubic perovskite ABF3:Yb³⁺/Er³⁺/Mn²⁺?A=K,Cs;B=Zn,Cd?have also been investigated.In addition,by using soft chemical synthesis method,the red emission from Er³⁺ions has been incorporated into core-shell UCNPs.With ternary 808/980/1550 nm or binary 808/980 nm excitations,multicolor-tunable emission profiles,including red-blue,red-green and red-green-blue,were successfully realized in the multilayer core–shell UCNPs.This thesis consists of six chapters.Chapter 1 briefly introduces the fundamental theory of upconversion,the constitution of upconversion materials,the advanced applications,and various emission tuning methods.Then the research topic of this thesis was established.Chapter 2 described the experimental conditions in the thesis in detail.Chapter 3-5exhaustively investigated the UCL from Er³⁺ions in ABF3 hosts and the construction of red color–based multicolor upconversion in core-shell UCNPs.Chapter 6 shows the conclusion of this thesis.The main research contents are listed as following:?1?Yb³⁺/Er³⁺co-doped KMgF₃ were synthesized by a facile hydrothermal method,and a single band red UCL was obtained under the excitation of 980 nm laser.The analyzing of the ions radius,the charge matching and the crystal structure in KMgF₃ stated that Yb³⁺/Er³⁺ions will preferentially substitute the Mg²⁺ions site.Further study about the upconversion emission and energy level of Er³⁺showed that the single-band red upconversion(⁴F_9/2?⁴I_15/2)profile is very stable under different excitation power.The relationship between upconversion intensity and the pump power indicated that the red emission is a two-photon upconversion process.Similar upconversion phenomena can be found in other ABF3 host?KZnF3 and KCdF3?,and their red and green emission also belong to a two-photon process.Eight substitution geometry models of KMgF3:Er³⁺were built,and then the formation energy under different models were calculated using DFT calculation,showing that models with the shortest distance between the Er³⁺ions have the lowest formation energy.This suggests that Er³⁺will stay together to achieve the lowest systematic energy,indicating the aggregation of Er³⁺.The lifetime of Er³⁺in the ABF3?KMgF3,KZnF3 and KCdF3?with different ions doping content were further measured,verifying the aggregation of Yb³⁺/Er³⁺.Finally,the red upconversion mechanism was proposed as following:the aggregation of Yb³⁺/Er³⁺will form after the nonequivalent substitution of Mg²⁺and K⁺by Er³⁺and Yb³⁺ions in KMgF3,which will cause strong cross-relaxation between Yb³⁺/Er³⁺,weakening the green emission and enhancing the red upconversion,leading to the final single band red profile in KMgF₃:Yb³⁺/Er³⁺.?2?Mn²⁺doped ABF₃:Yb³⁺/Er³⁺/xMn²⁺?A=K,Cs;B=Zn,Cd?were obtained using a facile hydrothermal method.By studying the green and red UCL spectra from KZnF₃:0.5%Yb³⁺,0.5%Er³⁺,xMn²⁺and the change rule of decay lifetime under different Mn²⁺doping content,the tuning mechanism of Er³⁺using Mn²⁺doping in KZnF₃ host were established.The energy transfer process between Er³⁺and Mn²⁺was also proved from the perspective of spectra and decay lifetime.Furthermore,the influence of the energy level position ⁴T1 in tuning Er³⁺upconversion emission was inspected from UCL spectra and lifetime.The results shows that the energy transfer process between Mn²⁺and Er³⁺will change if the ⁴T₁ energy level of Mn²⁺was higher than the green emitting level of Er³⁺ions,and the higher Mn²⁺energy level position will facilitate the green emission with a low Mn²⁺doping content.?3?The photon blocking effect was demonstrated to be an efficient strategy to realize multi-color orthogonal upconversion in core-shell UCNPs.Based on this strategy,a series of red color-based“smart”UCNPs were synthesized.First,the thickness of inert interlayer and outer blocking layers as well as the concentration of Yb³⁺ions were optimized.Then,by employing red-emitting NaErF4:0.5%Tm³⁺as the core,blue-emitting NaYbF4:0.5%Tm³⁺and NaYbF₄:0.5%Tm³⁺@NaYF₄:50%Nd³⁺/10%Yb³⁺and green-emitting NaYF₄:80%Yb³⁺/2%-Ho³⁺and NaYF₄:80%Yb³⁺/2%Ho³⁺@NaYF₄:50%Nd³⁺/10%Yb³⁺as the blocking shell,UCNPs with red-blue and red-green orthogonal UCL profiles under ternary 808/980/1550 nm excitation were successfully synthesized.The emission colors can be well maintained under different excitation power densities.The relationship between UCL intensity and pump power density indicated that the blue UCL is a three-photon process and the green and red emissions are both two-photon processes.Furthermore,by adopting blue-emitting core-shell NaYF₄:20%Nd³⁺/20%Yb³⁺@NaYF₄:30%Yb³⁺/0.5%Tm³⁺as the core and 980 nm excitation power dependent NaYbF4:1%Ho³⁺layer as the blocking shell,UCNPs capable of emitting tunable red/green/blue colors can be readily constructed and synthesized.These excitation sensitive multi-colors emitting UCNPs show a great promise in various applications including display,anti-counterfeiting,coding and decoding,and bio-imaging.