Synthesis And Luminescent Properties Of Na₂SnO₃ Self-activated Long-lasting Phosphors

The long-lasting luminescent material is a kind of photoluminescent material,which can absorb and store energies under excitation of the external light source,and continue emission after the removal of excitation source.In recent years,due to its excellent long-lasting luminescent performance,it has received extensive attention in the fields of lighting,display,bioimaging,high-energy ray detection,multi-dimensional optical memory and fluorescent anti-counterfeiting,and so on.In this thesis,new types of long-lasting phosphors with alkali metal stannate Na₂SnO₃ as matrix were synthesized.The effects of synthetic conditions and the rare earth ions doping on the luminescent properties of phosphors were investigated.The main content shows as follows:1.Na₂SnO₃ self-activated long-lasting phosphor and its synthetic conditions optimization.A series of sodium stannate compounds were synthesized via high temperature solid state method by controlling the reaction temperatures?750,800,850,900,950 and 1000??and atmospheric conditions?air or N₂ flow?.With increasing temperature,the as-obtained Na₂SnO₃ bulk crystals show a tendency of orientation growth and gradually ordered occupancy of sites resulting in structural evolution from triclinic to monoclinic system.All the samples have broadband self-activated luminescence properties in the visible light region.The emission color dramatically depends on the synthetic temperature and atmospheric condition.A gradual red shift from blue to yellowish-green is observed in air while from blue to cyan under nitrogen flow with increasing sintering temperature from 750 to 1000?.No matter what luminescence is,all the samples possesses blue afterglow after the removal of UV excitation.Electron paramagnetic resonance?EPR?,X-ray photoelectron spectroscopy?XPS?and thermoluminescence?TL?spectroscopy,etc.are carried out to figure out the possible defects in the lattices and the distribution of the corresponding levels within the band gap.Possible mechanism about the intrinsic defects-induced self-activated luminescence and long lasting afterglow were proposed.2.Ti-doping sensitized luminescence and Na₂Sn_1-xTi_x O₃ self-activated phosphor.A series of Na₂Sn_1-xTi_xO₃?0.01?x?0.09?solid solutions self-activated phosphors were synthesized by high temperature solid state method.The results imply that the introduction of Ti⁴⁺weakens the dependence of the self-activated luminescence on the synthetic conditions.The emission wavelength does not migrate with the change of the sintering temperatures and atmospheric conditions.Moreover,the luminescence intensity,thermal stability and quantum efficiency?QE?are significantly improved,along with remarkably enhanced moisture resistance ability.Prepared at 1000? in air atmosphere,the luminescence intensity of the optimal Na₂Sn_0.96Ti_0.04O₃ is about 10 times higher than that of Na₂SnO₃,and QE of the former is enhanced to 90%.In addition,as the Ti⁴⁺content increases gradually,the excitation and emission spectra are slightly red-shifted and blue-shifted,respectively,indicating gradual reduced Stokes shift.Through experimental characterizations by XRD,EPR,XPS,etc.,we propose that the incorporation of Ti⁴⁺is beneficial to stabilize the matrix lattice,reduce structural vibration and reduce the number of surface non-radiative transition centers,leading to the improvement of the luminescence stability and QEs.3.Rare earth ions-doped color-adjustable long-lasting phosphor Na₂SnO₃:RE³⁺ .A series of rare earth doped long lasting phosphors Na₂SnO₃:RE³⁺ ?RE=Tm,Sm,Eu,Dy?were synthesized.Tm³⁺ ,Sm³⁺ ,Eu³⁺ and Dy³⁺ show their respective characteristic f-f transitions in Na₂SnO₃with blue,orange,orange-red and white emissions under 254 nm excitation.Long-lasting luminescence with the same color of emissions was also observed in all the RE³⁺ -doped phosphors.In addition,single-doped single-phase white light-emitting phosphors can be realized under 254 nm excitation by controlling the Sm³⁺ or Dy³⁺ doping concentration and the relative intensity ratios of the self-activated luminescence and the rare earths' emissions.