Long-lived Upconversion Luminescence Of Mn²⁺-doped Fluoride ABF₃（A=K,Rb;B=Ca,Zn,Cd）

The transition metal Mn²⁺ ion belongs to the 3d⁵ electron configuration,exhibiting unique luminescent properties,such as broad single emission,tunable wavelength,long-lived lifetime and high chromaticity purity,etc.Therefore,Mn²⁺ ion shows important application prospects in fileds of lighting and display.However,Mn²⁺ ion has no intermediate metastable energy levels and has a large energy level mismatch with the commonly used sensitizer Yb³⁺ ion.Consequently,the upconversion（UC）of Mn²⁺ is usually weak or only exhibits luminescence at low temperatures.Theoretically,the realization of Mn²⁺ UC requires a close enough Yb³⁺-Mn²⁺ distance（⁵ ?）to form the unique luminescence center of exchange-coupled Yb³⁺-Mn²⁺ dimer.Nevertheless,the physical formation mechanism of the Yb³⁺-Mn²⁺ dimer is still unclear.Besides,how to design Yb³⁺/Mn²⁺ high-efficiency UC luminescence systems should be further explored.Therefore,the in-depth investigation on the formation mechanisam,luminescence characteristics and luminescence regulation with rare earth ions of the Yb³⁺/Mn²⁺ co-doped luminescence center(Yb³⁺-Mn²⁺ dimer)not only helps to deepen the understanding of the UC luminescence physics of Mn²⁺,but also helps to expand the application of UC luminescence of rare earth/transition metal ions,which has important scientific significance.For this issue,in this dissertation,the long-lived UC characteristics,UC mechanism,UC regulation with rare earth ions and UC application of Mn²⁺ in Yb³⁺/Mn²⁺ co-doped low phonon energy fluoride were investigated in detail.The dissertation is composed of six chapters.The first chapter introduces the research progress of Mn²⁺ UC luminescence.Particularly,the progress of Yb³⁺/Mn²⁺ co-doped systems and the unsolved scientific problems are summarized.The synthesis and characterizations of the samples are presented in chapter 2.For chapter 3-6,the UC characteristics,UC mechanism,UC regulation with rare earth ions and UC application of Mn²⁺ in fluoride ABF3（A=K,Rb;B=Ca,Zn,Cd）are systematically studied.The main achievements are shown as follows:（1）Long-lived UC luminescent material KCaF₃:Yb³⁺,Mn²⁺ has been synthesized by a high-temperature solid-state reaction method.Upon the excitation of 980 nm laser,KCaF₃:Yb³⁺,Mn²⁺ exhibits strong visible UC luminescence.Density functional theory（DFT）calculation and spectral analysis show that the random distribution nature of Mn²⁺ ions in the host lattice and the formation of unique {Yb-O-Mn} unit are the key factors for the efficient Mn²⁺ UC luminescence in this system.Given on the stable and efficient long-lived UC emission of KCaF₃:Yb³⁺,Mn²⁺,a data encoding/decoding prototype for information encryption and anti-counterfeiting with a fast authentication rate is proposed preliminarily.（2）UC luminescent material RbZnF₃:Yb³⁺,Mn²⁺ has been synthesized via a solvothermal method.Upon the excitation of 980 nm laser,RbZnF₃:Yb³⁺,Mn²⁺ not only exhibits visible UC luminescence of Mn²⁺,but also shows anomalous near-infrared（NIR）UC emission through Mn²⁺ heavy doping.By changing the doping concentration of Mn²⁺,the UC luminescence behavior can be well adjusted,and then a single-band NIR UC emission can be realized.The existence of Mn²⁺-Mn²⁺ dimer is confirmed by means of DFT calculations,crystallographic analysis,static and dynamic luminescence spectroscopy as well as the extend-X-ray absorption fine structure（EXAFS）characterization.Model of super-exchange coupled Yb³⁺-Mn²⁺-Mn²⁺ trimer is proposed to responsible for the anomalous NIR UC emission.（3）UC luminescent material KCaF₃:Yb³⁺,Mn²⁺,Ln³⁺（Ln=Er,Ho,Tm）with both long-lived and short-lived UC emissions has been synthesized by a solvothermal method.Under steady 980 nm laser excitation,a wide range of UC emission colors can be easily achieved by tuning the doping concentration of Mn²⁺ and Ln³⁺,whereas an un-adjustable long-lived yellow UC tailing is observed by the naked eye under dynamic excitation of 980 nm laser.Besides,study on the UC decay curves indicates the Ln³⁺ ions get a faster pump than Mn²⁺.The combination of visualized long-lived UC luminescence of Mn²⁺ ion,faster pumping of Ln³⁺ as well as tunable UC emission colors can significantly improve the anti-fake performance and the identify speed.This offers a new class of optical materials ideal for multilevel anti-counterfeiting with high-throughput rate of authentication.（4）UC luminescent material KCaF₃:Yb³⁺,Mn²⁺,Eu³⁺ has been synthesized via a solvothermal method.Intense Eu³⁺ UC emission is realized by controllable and efficient unidirectional energy transfer（ET）from Yb³⁺-Mn²⁺ dimer.By adjusting the concentration of Eu³⁺,the emission color can be welled tuned from yellow to orange and eventually obtain the pure UC of Eu³⁺.Notably,the UC lifetime of Eu³⁺ is quite long.Through ET rate analysis,the prolonged lifetime is ascribed to the genetic effect of the sustained resonance energy transfer from Yb³⁺-Mn²⁺ dimer(Mn²⁺).Moreover,pump power and temperature dependent UC luminescence further indicates the phosphor possesses excellent photochromic stability.