Yahong Jin

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.

The long persistent luminescence properties of phosphors: Li2ZnGeO4 and Li2ZnGeO4:Mn2+

The blue emitting long persistent phosphor (LPP) Li2ZnGeO4 and green emitting LPP Li2ZnGeO4:Mn2+ were newly prepared by a high temperature solid-state reaction method. The blue and green afterglow with duration of 5 and 8 h were confirmed to originate from host emission and d–d transitions of Mn2+ ions, respectively. The density of traps with suitable depth was significantly increased with the incorporation of Mn2+ ions. The energy transfer from host emission to Mn2+ ions was observed. The motion of charge carriers and a possible LAG mechanism based on a tentative model were illuminated and discussed in detail.

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.

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.

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.

Persistent luminescence in the self-activated K2Zr(BO3)2

This study reports a cyan emitting self-activated persistent phosphor K2Zr(BO3)2. The material is synthesized by solid state reaction method. The persistent phosphor was characterized in detail by X-ray powder diffraction, diffuse reflectance, photoluminescence, persistent luminescence and thermoluminescence spectra. After UV irradiation, the K2Zr(BO3)2 phosphor shows a cyan persistent luminescence dominating at ∼485 nm. Both the fluorescence and persistent luminescence are due to charge transfer emission from the central zirconium ion to oxygen in the ligand. The deconvolution of the thermoluminescence curve reveals that there are four traps responsible for the persistent luminescence. The depth of the dominant trap is 0.66 eV. Based on experimental results, the trapping and detrapping processes of the charge carriers are discussed. A rudimentary energy level scheme was proposed to explain the mechanisms of persistent luminescence as well as photoluminescence.

Sr3YLi(PO4)3F:Eu2+,Ln3+: colorless-magenta photochromism and coloration degree regulation through Ln3+ co-doping

Herein, novel photochromic materials, Sr3YLi(PO4)3F:Eu2+,Ln3+, have been prepared by a conventional high-temperature solid-state reaction. Eu2+ singly doped Sr3YLi(PO4)3F showed reversible colorless-magenta photochromism when alternately irradiated by UV and visible light (or thermal treatment). Thus, Sr3YLi(PO4)3F:Eu2+ might be a potential candidate in various fields such as in imaging, sensors, photo-switches, and erasable optical storage media. The optimal Eu2+ doping concentration is experimentally determined to be about 0.5 mol%. The coloring and bleaching processes are attributed to electron trapping accompanied by the generation of F-centers and detrapping by different traps, respectively. Thus, these processes open a window for us to design a simple way to regulate and control the photochromic performance just by adjusting the doping concentration of Eu2+ ions or co-doping Ln3+ ions. On the basis of thermoluminescence, a schematic of a photochromic mechanism has also been proposed.