Guifang Ju

Persistent luminescence and its mechanism of Ba5(PO4)3Cl:Ce3+,Eu2+

We investigated the persistent luminescence in europium-doped barium chloroapatite upon codoping with cerium. Luminescence properties of the phosphors, including photoluminescence, luminescence decay, and thermoluminescence are systematically studied. This material holds the key to the unsettled questions in the field of persistent luminescence. We formulated a model of persistent luminescence on the basis of the experimental results. In our model, some Eu2+ ions are oxidized to Eu3+ under short UV excitation, and the released electrons are trapped at positive substitutional defect introduced by Ce3+. The persistent luminescence arises from the recombination of these trapped electrons with the photo-ionized Eu3+ ions. The effects of Coulomb attraction between the substitutional defect and trapped electron, along with electron–electron repulsion, construct a metastable exciton-like trap. To a first approximation, the third ionization energy and ionic radius of lanthanide codopant can be used to rationalize...

Multifunctional near-infrared emitting Cr3+-doped Mg4Ga8Ge2O20 particles with long persistent and photostimulated persistent luminescence, and photochromic properties

Here we report a series of multifunctional Cr3+-doped magnesium gallogermanate particles. Several minutes of ultraviolet (UV) light excitation can lead to strong near-infrared (NIR) long persistent luminescence (LPL) at 600–850 nm with a long lasting time of at least 25 h. After the disappearance of LPL, NIR photostimulated persistent luminescence (PSPL) can be rejuvenated repeatedly several times by an external photostimulus (visible or NIR light). Meanwhile, the material shows photochromic (PC) properties, i.e., the surface color can change reversibly between white/pale green and rosybrown with high fatigue resistance by alternating UV and visible light irradiation. When the temperature is increased to 300 °C, the induced rosybrown surface color can also be bleached. This new multifunctional material has potential applications in a wide range of fields, such as in vivo bio-imaging, optical information write-in and read-out media, erasable optical memory media and optical sensors. The continuous trap distribution and modulation were characterized. The optimum trap depth was determined to be 0.51–0.86 eV. In addition, we gained insight into the mechanism of multifunctional properties and the interconnection between them.

Novel La3GaGe5O16 : Mn4+ based deep red phosphor: a potential color converter for warm white light

New non-rare-earth red phosphor La3GaGe5O16 : Mn4+ was prepared successfully via the traditional solid state reaction method. In this paper, we reported this red phosphor La3GaGe5O16 : Mn4+ as a promising red converter for warm white light under excitation at UV or blue light. The phase and microstructure of this red phosphor were characterized with aids of the XRD and SEM; the luminescent performance was investigated by the diffuse reflection spectra, photoluminescence (PL) spectra, and temperature-dependent PL/decay measurements. The excitation band of La3GaGe5O16 : Mn4+ phosphor covers a broad spectral region from 250 nm to 500 nm, which match well with the commercial near-UV and blue LEDs and can give intense bright red emission. The crystal field strength (Dq) and the Racah parameters (B and C) are calculated to evaluate the nephelauxetic effect of Mn4+ suffering from the La3GaGe5O16 host. The concentration quenching of Mn4+ in La3GaGe5O16 : Mn4+ occurs at a low content of 0.25% due to the dipole–dipole interaction in this work. We gained insight into the temperature-dependent relative emission intensity of La3GaGe5O16 : 0.25% Mn4+ phosphor, and we also reported the luminescence quenching temperature and the activation energy for thermal quenching (ΔE). In future, WLEDs will be made when this red phosphor was applied to the package of a UV/blue LED chip and YAG : Ce.

Reversible colorless-cyan photochromism in Eu2+-doped Sr3YNa(PO4)3F powders

Photochromic materials have attracted increasing interest as optical switches and erasable optical memory media. Here, we report on a novel reversible colorless-cyan photochromic powder material Sr3YNa(PO4)3F:Eu2+. The photochromic properties including coloring and bleaching are characterized by reflectance spectra after the powder is irradiated by different wavelengths of light for different irradiation times or processed by thermal treatment. The results show that it can be colored by short wavelength light (200–320 nm) and bleached by longer wavelength light (320–800 nm) or thermal treatment at 240 °C. The optimal doping concentration of Eu2+ is determined to be about 0.5 mol%. It shows high fatigue resistance in photochromism performance. Electrons in 4f ground levels excited by short wavelength irradiation to higher 5d states of Eu2+ and then captured by traps are responsible for the coloring process. Correspondingly, the release of trapped electrons from two kinds of traps with different depths causes the initially fast and later slow bleaching processes. The critical role of charge carrier motions between two different traps is discussed. The colorless-cyan photochromism can be explained semi-quantitatively on the basis of obtained results. A tentative model was proposed to illustrate the PC mechanism.

Luminescence properties of the pink emitting persistent phosphor Pr3+-doped La3GaGe5O16

The pink persistent phosphor La3GaGe5O16:Pr3+ was prepared successfully via a traditional solid state reaction method. In this paper, the La3GaGe5O16 host is proved to be a new self-activated luminescent host lattice on the basis of excitation and emission spectra. The La3GaGe5O16 host gives a blue emission under excitation at 260 nm. In the emission spectrum for La2.99GaGe5O16:0.01Pr3+, two dominant peaks arise from a 3P0 → 3H4 transition in the blue region and a 1D2 → 3H4 transition in the red region, respectively. However, concentration quenching of Pr3+ in La3GaGe5O16:Pr3+ occurs at a low Pr3+ content of 0.02 in this work, which is mainly due to a dipole–dipole interaction. La3GaGe5O16:Pr3+ phosphors exhibit pink persistent luminescence after short UV-irradiation. The optimal concentration of Pr3+ for the long persistent luminescence in the La3GaGe5O16 host is experimentally about 0.01. The long persistent luminescence of La3GaGe5O16:Pr3+ is discussed in accordance with the afterglow decay and thermoluminescence analysis. A model is proposed based on the experimental results to explain the mechanisms of the photoluminescence and afterglow phenomenon.

Preparation and characterization of a long persistent phosphor Na 2 Ca 3 Si 2 O 8 :Ce 3+

A novel host lattice Na2Ca3Si2O8 was used for synthesizing long persistent phosphors for the first time. A blue-emitting long persistent phosphor was prepared successfully via a traditional high temperature solid-state reaction method. The phase structure was checked by XRD. The photoluminescence and persistent luminescence decay properties of Ce3+-doped samples were studied systematically. The defects acting as traps were investigated by thermoluminescence. It demonstrated that the doping Ce3+ ions into the Na2Ca3Si2O8 host not only largely enriched the intrinsic traps but also introduced abundant new extrinsic traps which play a determining role in the generation of persistent luminescence. The origin of the persistent luminescence was analyzed and a related mechanism was systematically discussed based on a schematic diagram as well.

La 3 GaGe 5 O 16 :Cr 3+ phosphor: the near-infrared persistent luminescence

Long persistent phosphors in the near-infrared (NIR) region have attracted much attention due to the potential application in in vivo imaging. La3GaGe5O16:Cr3+ phosphor presents a NIR long persistent luminescence after the short UV-irradiation. La3GaGe5O16 host also exhibits a cyan persistent luminescence. The optimal concentration of Cr3+ in La3GaGe5O16 is experimentally about 0.01 and the afterglow time can last more than 30 min. The estimated trap depth which varies continuously as a function of delay time is evidence for the presence of a continuous trap distribution. In order to improve the performance of afterglow luminescence of the La3GaGe5O16:Cr3+, we modified composition around Cr3+ by adjusting the Ge/O content. La3GaGe5O16:Cr3+ is shown to be a new near-infrared persistent phosphor potentially suitable for in vivo imaging due to its 650nm-750nm emission range.

Luminescent properties of a green long persistent phosphor Li 2 MgGeO 4 :Mn 2+

A novel rare-earth ion free phosphor, Mn2+-activated Li2MgGeO4, was prepared by a high temperature solid-state reaction method. The phase structure was identified by XRD. We reported on its photoluminescence and long persistent luminescence properties. The green emission with a broad band centered at 525 nm corresponds to the 4T1(4G)–6A1(6S) transition of the Mn2+ ions. After irradiation by 254 nm UV light, green long persistent luminescence can be observed and last at least 5 h. Thermoluminescence was studied and the long persistent luminescence mechanism was also discussed in detail.

Trap distribution tailoring guided design of super-long-persistent phosphor Ba2SiO4:Eu2+,Ho3+ and photostimulable luminescence for optical information storage

Here, we report a novel long-persistent luminescence (LPL) phosphor, Ba2SiO4:Eu2+,Ho3+. The crystal structure, photoluminescence, photoluminescence excitation and LPL decay behaviors were studied systematically. The developed LPL phosphor Ba2SiO4:Eu2+,Ho3+ shows super-LPL, which remains above 2.97 mcd m−2 even 24 h after the removal of UV irradiation source. The outstanding LPL performance is mainly derived from the tailoring of trap distribution via Ho3+ co-doping. 15 days after UV irradiation, the sample still shows observable green luminescence by 980 nm stimulation. By means of photostimulated luminescence (PSL), excitation duration, decay duration and excitation temperature dependent thermoluminescence spectra, the continuous trap distribution, trap distribution modulation and dynamics of electron motion are characterized. The role of co-dopant Ho3+ serving as electron traps and the tailoring of trap distribution are revealed. The fabrication of Ba2SiO4:Eu2+,Ho3+ phosphor film allows optical information write-in after passing UV light through a photomask (projection printing method) and then the stored information is read out by thermal stimulation or 980 nm photostimulation, which holds great promise for optical information storage. Moreover, combined with the experimental and DFT calculation results, a tentative schematic diagram is proposed for the illustration of LPL and PSL mechanism in Ba2SiO4:Eu2+,Ho3+.

Photoluminescence properties of Ce 3+ and Tb 3+ -activated Ba 2 Mg(PO 4 ) 2

The luminescence and energy transfer of cerium and terbium codoped Ba2Mg(PO4)2 were investigated. The phosphor was prepared by a solid-state reaction at high temperature. The photoluminescence properties were investigated under UV excitation. From a powder X-ray diffraction analysis, the formation of single-phased Ba2Mg(PO4)2 with a monoclinic structure was confirmed. The photoluminescence excitation spectrum of Ba2Mg(PO4)2:Ce3+ shows that the excitation band from 220 to 350 nm and the broad-band UV-blue emission are attributed to the allowed 4f ↔ 5d transitions of Ce3+. Upon excitation of Ce3+ band, Ba2Mg(PO4)2:Ce3+,Tb3+ phosphors exhibit both Ce3+ emission band the characteristic Tb3+ emission lines. Also, efficient energy transfer from Ce3+ to Tb3+ in Ba2Mg(PO4)2 was verified by observing the excitation spectra of Ba2Mg(PO4)2:Ce3+,Tb3+. An energy level scheme is constructed to depict the energy transfer process.