Mn-Activated Transparent Glass-Ceramics For Photonic Applications

Over the past decades,red-emitting phosphors have found widespread applications in solid-state lighting,display,and spectral converters for solar cells.It is commonly used to modulate the color temperature and color rendering index of phosphor-converted white light-emitting diodes(WLEDs).In general,rare-earth(RE)ions,such as Eu3+ and Sm3+,are widely used as activators for red-emitting phosphors.Nevertheless,the phosphors activated by these trivalent rare-earth ions commonly exhibit a 4f-4f emission that could be only effectively excited by near-UV and blue light corresponding to the narrow f-f transition lines,which strongly limit their practical applications.In comparison,transition metal ions(such as Mn2+and Mn4+)doped transparent glass-ceramics(GC),as well as phosphors,have shown to be alternative red-emission phosphors,benefited by their tunable optical spectra with high efficiency.However,the use of Mn4+as an activator in the production of red phosphors for solid-state reaction-based WLEDs represents a significant challenge because of the instability of Mn4+at high temperatures.In this thesis,we designed a novel method to synthesis Mn2+ and Mn4+ doped luminescent materials based on the crystallization of the glass and investigated systematically their optical properties.In chapter three,we studied the synthesis and optical properties of Mn2+activated monolithic luminescent glass-ceramics.For solid-state lighting,the monolithic phosphor plate is often used as it is convenient for manufacturing,while in practical light-emitting devices only a thin luminescent layer is used for more efficient excitation and light extraction.In this chapter,Mn2+-doped 60SiO2-8Na2O-20ZnO-12Ga2O3 glass and glass-ceramic were developed by the traditional melt-quenching technique.We observe that the crystallization of ?-Zn2SiO4 nanocrystals takes place on the glass surface with controllable thickness after heat treatment.The glass samples show typical red emission peaking at ?=620 nm that can be ascribed to the spin-forbidden 4Tlg(G)?6Alg(S)transition of Mn2+(d5)located in the octahedral coordination site of the glass host.After surface crystallization,this red emission is retained and a new green emission at 528 nm is observed through the control of the crystallization temperature and duration,thus offering tunable emission characteristics promising for the lighting application.This change in the visible emission is interpreted in terms of the change of coordination state of Mn2+from octahedral in a glass matrix to tetrahedral in the surface precipitated ?-Zn2SiO4 crystals.In chapter four,we investigated the fabrication of Mn4+doped GCs and their optical properties.Recently,Mn4+ has stimulated the continued search of robust hosts and efficient synthetic methods to stabilize Mn4Gcenters with strong photoluminescence.In this chapter,we demonstrate a facile synthetic method for Mn4+ doped glass-ceramic(GC)based on crystallization-induced oxidation state change in an oxide glass.The parent glass with a formula of LiNaGe4O9 is fabricated by melt-quenching and crystallization is induced by thermal treatment in air.Oxidation of Mn2+ in glass to Mn4+ in the GC is confirmed by both optical spectroscopy and electron paramagnetic resonance(EPR)measurements.After thermal treatment,the characteristic reddish photoluminescence(PL)of Mn2+ in the glass centered at 611 nm disappears and a strong photoluminescence peak at 660 nm attributed to Mn4+ is observed.The conversion to Mn4+after crystallization in the examined system may have strong implications for the synthesis of Mn4+ doped phosphors which always require rigorous control of the redox equilibrium during synthesis.In chapter five,we synthesized an Mn4+ doped Li2Ge4O9 glass-ceramic by controlling the devitrification of the glass sample by thermal treatment.From electron paramagnetic spectroscopy and optical spectroscopy,we confirmed the presence of Mn4+ ion in the dilithium tetragermanate Li2Ge4O9 glass-ceramic after the thermal treatment at a temperature less than Tc.A sharp red emission peaked at 668 nm due to Mn4+is observed.UV-Vis-IR absorption spectroscopy was employed to study the optical properties.The red-shift of the direct optical bandgap Egopt from 3.81 to 2.55 eV is observed by increasing the Mn4+ concentration.In addition,the dispersion parameters,as well as the transition strength(Ed),refraction indices(n,n?),and oscillator wavelength(?0),were determined by using Wemple-Didomenico single oscillator model.In our theoretical investigation,we employed different models to extrapolate the correlation between both the energy gap and the refractive index.Our results of this chapter suggest that,besides the favorable thermal stability,the synthetic method demonstrated here for Li2Ge4O9:Mn4+might be extended for the synthesis of other phosphors containing activators with a high oxidation state.