Ultraviolet Persistent Luminescence And Up-conversion Charging Of Persistent Phosphors

Persistent luminescence(Pers L)refers to the phenomenon that the material continues to emit light after the removal of external excitation light source.The wavelength of current Pers L materials is mainly located in the visible and near-infrared spectral region,with very few in the ultraviolet(UV)band.UV light has important application in many fields such as photochemistry,anti-counterfeiting,and disinfection.The realization of UV Pers L will promote the development of photocatalysis,photoenergy therapy and other related fields.Additionally,the UV Pers L materials usually require high-energy excitation light like X-ray,which brings great difficulties to the practical application of Pers L technology.In the recent years,searching for UV Pers L that can be achieved by using low-energy excitation light has been a hotspot.Herein,a series of Pers L phosphors based on Pr³⁺and Gd³⁺ions are developed.An up-conversion excitation process is used to fulfill the energy traps in the Pers L phosphors.We have obtained a narrowband-UVB(NB-UVB,309?313 nm)Pers L upon illumination by a blue light-emitting diode(LED).The main results are as follows:(1)The Lu₃Al Mg₂Si₂O₁₂:Pr³⁺Pers L phosphor is prepared by the high-temperature solid-phase method.After being excited by UV lamp,three Pers L emissions of Pr³⁺ion locating in the UV,visible and near-infrared regions are achieved.The effect of thermal quenching on different emission energy levels is explored through temperature-dependent steady-state emission spectroscopy and thermoluminescence measurement.The reason for that the UV Pers L is more likely to be thermally quenched than the visible Pers L is as follows:The energy level of 4f5d energy level is deeper than ¹D₂ when compared with the bottom of conduction band.(2)By introducing Pr³⁺ion as a sensitizer into Gd³⁺ion-activated phosphor,we prepare Lu₂Si O₅:Pr³⁺,Gd³⁺phosphor and realize a NB-UVB Pers L with a sharp 313 nm emission,which corresponds to the transition of ⁶P_7/2?⁸S_7/2in Gd³⁺ion.The Pers L can be lasted for more than1 h.Our spectroscopic investigations verify that Pr³⁺ion can act as an efficient sensitizer for activating the NB-UVB emission.The energy transfer process between Pr³⁺and Gd³⁺ion is illustrated by an energy level diagram.Through thermoluminescence measurements,we reveal the distribution of the shallow and deep traps.(3)We combine the two processes of up-converted luminescence and Pers L,and propose a novel up-conversion charging(UCC)design to fill the traps in phosphors.The visible Pers L emission from Pr³⁺ion is achieved in the Gd₃Ga₅O₁₂:Pr³⁺phosphor after the high-lying traps in the UV region are filled by subsequently absorbing two excitation photons from a blue LED.Spectroscopic investigations reveal that the UCC intermediate state is ³P₀ state of Pr³⁺ion in the phosphor.This work may provide a major step toward developing the UCC design,which enables new properties of Pers L to be discovered.(4)We have achieved a NB-UVB Pers L in Lu₃Al₂Ga₃O₁₂:Pr³⁺,Gd³⁺phosphor upon blue LED illumination.Our investigations indicate that the net population in the traps in the phosphor is dominated by a competition between a non-linear excitation and a photostimulated releasing of electrons from traps.The competitive process can be manipulated by using the same blue LED with different power densities.The phosphor can be charged by a high-power illumination but discharged by a low-power illumination.A rate equation is also proposed to describe the effect of photo-stimulated detrapping on the UCC process.Through an imaging by the Ofil Scalar camera,we further demonstrate an advantage of the NB-UVB in the optical tagging field and provide a major step toward further development of optical tagging applications.