Tunable Broadband Upconversion Luminescence From Yb³⁺/Mn²⁺ Codoped Perovskite Fluoride ABF₃

In recent years, rare earth(RE) ions doped upcoversion(UC) luminescence materials have gained increasing attention and gradually become as the research hotspot in solids luminescence fields due to their great potential applications in solar cells, bio-label, bio-imaging, three dimensional displays and anti-counterfeiting. However, RE-doped UC materials always show fixed multiple sharp emission bands, the applications are limited. The approach of codoping with Yb3+/Mn2+ pairs into a single host lattice provides a novel strategy to realize Mn2+-tunable broadband UC emission upon excitation of a 976 nm laser diode(LD), showing promising to make up for the deficiencies of UC materials based on RE ions. However, up to now, Mn2+-UC emission has been achieved only in some limited bulk Yb3+/Mn2+ codoped systems at quite low temperatures(< 100 K), the practical applications are limited. In this dissertation, Yb3+/Mn2+ codoped perovskite fluoride ABF3(A = Na+, K+, Rb+ and Cs+; B = Mg2+, Zn2+, Mn2+ and Cd2+) are designed based on the luminescent theory and the characteristics emission of Mn2+. The room-temperature UC luminescence properties of ABF3:Yb3+,Mn2+ are investigated in detail.The dissertation is composed of six chapters. Chapter 1 introduces the history of UC luminescence, advances in RE ions and transition metal ions doped UC materials, and then the research topic of this dissertation is proposed. Chapter 2 introduces the sample preparation and measurents. Chapters 3-6 provide a detail investigation on Stokes and UC luminescence properties of Yb3+/Mn2+ codoped perovskite fluoride ABF3(A = Na+, K+, Rb+ and Cs+; B = Mg2+, Zn2+, Mn2+ and Cd2+). The main achievements are shown as follows:(1) The Yb3+/Mn2+ codoped perovskite fluoride KZn F3:Yb3+,Mn2+ samples are synthesized by a facile solvothermal method, which show quite intense room-temperature(RT) UC luminescence upon excitation of a 976 nm LD. The UC emission band is centered at 585 nm and identified as the 4T1(4G)â†'6A1(6S) transitions of Mn2+. Based on the crystal structure analysis, excitation spectrum and fluorescence lifetimes measurements, a ground state absorption(GSA)/excited-state absorption(ESA) UC mechanism based on the exchange coupled Yb3+–Mn2+ dimer is proposed for Mn2+-UC luminescence.(2) A series of perovskite fluorides ABF3:Yb3+,Mn2+(A = K, Rb and Cs; B = Mg, Zn and Cd) are synthesized by a modified solvothermal method. Upon excitation of a 976 nm LD, the ABF3:Yb3+, Mn2+(A = K, Rb and Cs; B = Mg, Zn and Cd) samples show quite intense RT UC luminescence. Since the different crystal field strength of Mn2+ in these systems, the emission wavelength of Mn2+ can be tuned from 550 to 610 nm. The influences of crystal structure, pump power and environmental temperature on UC luminescence properties of ABF3:Yb3+, Mn2+(A = K, Rb and Cs; B = Mg, Zn and Cd) samples are investigated in detail.(3) A near-infrared(NIR) emission band at approximately 770 nm is realized in KZn F3:Mn2+ and KZn F3:Yb3+,Mn2+ via the strategy of heavily doping with Mn2+ ions. Based on the structure analysis, excitation and emission spectra, and luminescence decay curves measurements, a exchange coupled Mn2+-Mn2+ dimer model and Yb3+-Mn2+-Mn2+ trimer model are proposed to interpret the Stokes and UC emission NIR emissons, respectively. Since the NIR emission band is located at the bio-optical-window, the KZn F3:Yb3+,Mn2+ materials show great potential applications in high-resolution and high-penetration bioimaging.(4) A series of Mn2+ solely doped or Yb3+/Mn2+ codoped perovskite fluoride ABF3(A = Na+, K+, Rb+ and Cs+; B = Mg2+, Zn2+, Mn2+ and Cd2+) samples are synthesized via a hydrothermal or solvothermal method. Resutls show that a NIR emission band(centred at 760~860 depending on composition) can be realized in these samples via heavily doping with Mn2+ ions. A detailed investigation of the crystal structure, density functional theory(DFT) calculations, X-ray absorption fine-structure(XAFS) analysis, fluorescence lifetime analysis and excitation and emission spectroscopy measures of KMg1-xF3:x Mn2+ samples show that the tunable NIR emission can be ascribed to the 6 6 4 4 6 6 6 61 1 1 1A( S) T( G) ®A( S) A( S) transitions of Mn2+-Mn2+ dimers that are formed by the aggregation of antiferromagnetic-coupled Mn2+ ions via superexchange interactions. A comprehensive understanding of the nature of the Mn2+ NIR emission is key for using these materials in bioapplications or advanced photonic devices with enhanced properties and manufacturability.