Haijie Guo

Structure and luminescence properties of a novel yellow super long-lasting phosphate phosphor Ca6BaP4O17:Eu2+,Ho3+

A novel yellow-emitting long-lasting phosphate phosphor Ca6BaP4O17:Eu2+,Ho3+ is developed. The incorporation of Ho3+ ions, which act as trap centers, largely extends the thermoluminescence characteristics and evidently enhances the persistent luminescence behavior of the phosphor. Both the fluorescence and phosphorescence spectra of Ca6BaP4O17:0.02Eu2+,0.015Ho3+ reveal only one asymmetric broad emission band located at 553 nm, ascribed to the 5d–4f transitions of Eu2+ ions in two different Ca2+ sites. After 15 min of excitation, the initial long-lasting phosphorescence (LLP) intensity of Ca6BaP4O17:0.02Eu2+,0.015Ho3+ can reach about 0.13 cd m−2 and its LLP can last more than 47 h above the recognizable intensity level (0.32 mcd m−2), a phenomenon that is infrequent and excellent. Furthermore, the direct bandgap of about 4.081 eV for Ca6BaP4O17 provides a suitable bandgap for Eu2+ and Ho3+ ions. The results indicate that the phosphor has the potential to become a novel commercial LLP phosphor used in the field of emergency lighting and display. Detailed processes and a possible mechanism are studied and discussed.

Cyan emissive super-persistent luminescence and thermoluminescence in BaZrSi3O9:Eu2+,Pr3+ phosphors

We developed a novel long-lasting phosphorescence (LLP) material BaZrSi3O9:Eu2+,Pr3+. The crystal structure, electronic structure and photoluminescence of this phosphor have been investigated systematically. The highlight of this work is the admirable LLP performances of BaZrSi3O9:Eu2+,Pr3+, which could be observed by the naked eye for 15 hours in the dark after ceasing the irradiation source. Moreover, we have analyzed the reason why the BaZrSi3O9:Eu2+,Pr3+ phosphor has such excellent persistent luminescence by exploring the distribution of traps in BaZrSi3O9:Eu2+,Pr3+ with the help of thermoluminescence (TL) spectra. A series of the excitation duration, decay duration and temperature dependent TL experiments of BaZrSi3O9:Eu2+,Pr3+ were conducted, revealing the trapping and detrapping of electrons into and from the shallow traps through the conduction band and the deeper traps via the tunnelling process. According to the experimental results, the mechanism of the feasible persistent luminescence of BaZrSi3O9:Eu2+,Pr3+ was also proposed and illustrated in detail.

Photo-/cathodoluminescence and energy transfer properties of novel Ce3+ singly doped and Ce3+/Tb3+ codoped NaBaScSi2O7 phosphors

A series of novel Ce3+ singly doped and Ce3+/Tb3+ codoped color-tunable NaBaScSi2O7 phosphors were synthesized via the solid state method. The morphology, UV-vis reflectance, photoluminescence emission and excitation spectra, the lifetime and the thermal quenching properties were investigated in detail. NaBaScSi2O7:Ce3+ phosphors showed intense blue emission at about 425 nm, corresponding to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 spin orbit 4f ground states of Ce3+ ions, under optimal excitation of 349 nm. Under excitation of Ce3+ ions at 349 nm, the emission spectra of NaBaScSi2O7:Ce3+,Tb3+ phosphors consisted of a blue emission band of Ce3+ ions at 425 nm and a series of strong green emission lines at 488, 542, 582, and 625 nm due to the 5D4 → 7FJ (J = 6, 5, 4, and 3) characteristic transitions of Tb3+ ions. A possible energy transfer mechanism was proposed and ascribed to the dipole–dipole interaction on the basis of the experimental results and analysis. In addition, the cathodoluminescence properties of NaBaScSi2O7:Ce3+,Tb3+ were also studied in detail. The current results indicate that NaBaScSi2O7:Ce3+,Tb3+ can serve as a potential phosphor for application in UV-WLEDs and FEDs.

Controlling and revealing the trap distributions of Ca6BaP4O17:Eu2+,R3+ (R = Dy, Tb, Ce, Gd, Nd) by codoping different trivalent lanthanides

Thermoluminescence (TL) glow curves of Ca6BaP4O17:Eu2+,R3+ (R = Dy, Tb, Ce, Gd, Nd) samples were measured above room temperature in order to compare the trap distributions in the band gap. The observed phenomenon indicates that R3+ ions (R = Dy, Tb, Ce, Gd, Nd) have different effects on the trap properties of the Ca6BaP4O17:Eu2+ phosphor. The most shallow trap (0.620 eV) for the Tb3+ ion and the deepest trap (0.762 eV) for the Dy3+ ion eventually led to shorter duration (4.3 h and 1.2 h, respectively), while the appropriate trap depth (0.716 eV) for the Nd3+ ion makes the Ca6BaP4O17:Eu2+,Nd3+ sample show the longest afterglow duration (37.9 h). Codoping the Tb3+ ion slightly increases the instinct traps of the Ca6BaP4O17:Eu2+ sample and creates a new low-temperature TL peak corresponding to a relatively shallow trap leading to the strongest initial afterglow brightness (0.887 cd m−2), while codoping with other R3+ ions (R = Dy, Ce, Gd, Nd) creates new appropriate or inappropriate traps. By recording a series of long-lasting phosphorescence (LLP) spectra with various irradiated times and TL experiments with varying delay time after ceasing the UV irradiation, the trap distribution of the depth and shape was evaluated. The result provides a better understanding of the role of these trapping centers played in the persistent luminescence mechanism.

Ca6BaP4O17:Eu2+,Gd3+: a yellow emitting long-lasting phosphor with high brightness and long afterglow duration

A yellow emitting long-lasting phosphor based on a Ca6BaP4O17 matrix was successfully synthesized by means of solid state method in a reducing atmosphere. The persistent phosphor was characterized in detail by X-ray powder diffraction (XRD), photoluminescence (PL), persistent luminescence and decay curves as well as thermoluminescence (TL) spectra. When irradiated by ultraviolet light in advance, Ca6BaP4O17:0.02Eu2+,0.015Gd3+ phosphor shows a yellow persistent luminescence dominating at ∼553 nm at room temperature due to the 5d–4f transitions of Eu2+ ions and the chromaticity coordinate was calculated to be (0.431, 0.545), which is in the yellow region. Moreover, investigation of TL curves reveals that the incorporation of Gd3+ ion creates more appropriate energy traps and increases the density of intrinsic traps leading to the enhancement of afterglow. Consequently, the initial long lasting phosphorescence (LLP) intensity of Ca6BaP4O17:0.02Eu2+,0.015Gd3+ (0.21 cd m−2) is significantly enhanced by a factor of 2 compared with Ca6BaP4O17:0.02Eu2+,0.015Ho3+ and the sustained phosphorescence also can last approximately 19 h above the recognizable intensity level (0.32 mcd m−2). In order to study the trapping process and explain the mechanism of long afterglow, a schematic model is also proposed and discussed in detail.

Luminescent and magnetic properties of the afterglow phosphors GdSr2AlO5:RE3+ (RE3+ = Eu3+, Sm3+, Pr3+ and Dy3+)

Novel afterglow phosphors based on the GdSr2AlO5 host were prepared by solid-state reaction under reductive atmosphere. The photoluminescence, afterglow, thermoluminescence and magnetism properties of GdSr2AlO5:RE3+ (RE3+ = Eu3+, Sm3+, Pr3+ and Dy3+) were discussed in detail for the first time in this paper. By doping appropriate rare earth ions into the GdSr2AlO5 host, all phosphors showed a satisfactory long-wavelength afterglow phenomenon and excellent paramagnetism characteristics. The mass magnetic susceptibility value was determined to be approximately 4.4052 × 10−5 emu g−1 Oe−1.

Synthesis, structure and photoluminescence properties of Ce3+-doped SrSc2O4: a new scandate green-emitting phosphor with blue excitation

Novel Ce3+-doped SrSc2O4 phosphors were synthesized via a solid state method. The crystal structure and morphology were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The photoluminescence properties, quantum efficiency, CIE value and thermal stability of SrSc2O4:Ce3+ phosphors were investigated in detail to evaluate their application in LEDs. The photoluminescence results revealed that the phosphors have an excitation band in the blue region from 400 to 470 nm and a broad green emission band peaking at about 491 nm. The above results indicated that the phosphors can be effectively excited by blue light and could be promising candidates as green-emitting phosphors for application in LEDs.

Design, synthesis and characterization of near-infrared long persistent phosphors Ca4(PO4)2O:Eu2+,R3+ (R = Lu, La, Gd, Ce, Tm, Y)

Near-infrared (NIR) persistent phosphors have many remarkable optical advantages in bioimaging applications. However, only a very few activators and hosts have been found to exhibit NIR persistent luminescence. Here, we report Eu2+-activated phosphate NIR long persistent phosphors, Ca4(PO4)2O:Eu2+,R3+ (R = Lu, La, Gd, Ce, Tm, Y). All the codoped samples showed persistent luminescence peaking at a long wavelength of ca. 690 nm due to the 5d–4f transitions of Eu2+. The persistent luminescence of Ca3.97(PO4)2O:Eu0.012+,Y0.023+ can be detected for 2 h after the removal of excitation. As a key parameter of persistent phosphors, the trap depths can be tuned from 0.62 to 0.77 eV by codoping with trivalent rare earth ions. The I−1–t curves reveal that two effective traps contribute to long persistent luminescence of Y3+, Tm3+ and Ce3+ codoped samples, whereas there is only one effective trap for La3+, Gd3+ and Lu3+ codoped samples, which was confirmed by the thermoluminescence curves of the codoped samples. This investigation provided a new long persistent phosphor which enriches the activator and hosts of existing NIR long persistent phosphors for potential application in vivo bioimaging.

Enhancement of yellow persistent luminescence in Eu2+-doped β-Ba3P4O13 phosphor by Ga3+ codoping

Novel yellow emitting persistent phosphors β-Ba3P4O13:Eu2+ and β-Ba3P4O13:Eu2+,Ga3+ were successfully synthesized via solid-reaction method. Ga3+ was proved to be an effective codopant to enhance persistent luminescence in Eu2+-doped β-Ba3P4O13 phosphors. Eu2+–Ga3+ codoped β-Ba3P4O13 showed about 6 times longer persistence time than the Eu2+-singly-doped phosphors. The yellow long persistent luminescence (LPL) of β-Ba3P4O13:Eu2+,Ga3+ can persist about 10 h. The thermoluminescence (TL) analyses showed that co-doping Ga3+ can improve more appropriate traps leading to the enhancement of the afterglow. These results suggested that Ga3+ may play a critical role in creating traps in β-Ba3P4O13:Eu2+.

Cyan long lasting phosphorescence in green emitting phosphors Ba2Gd2Si4O13:Eu2+, RE3+ (RE3+ = Dy3+, Ho3+, Tm3+, Nd3+ and Tb3+)

Novel long lasting phosphorescence materials Ba2Gd2Si4O13:Eu2+, RE3+ (RE3+ = Dy3+, Ho3+, Tm3+, Nd3+ and Tb3+) are designed and prepared by a solid state reaction. Their photoluminescence emission spectra show a broad asymmetric green emitting band peaking at 517 nm, attributed to the 5d–4f transitions of Eu2+ ion occupying Ba2+ and Gd3+ site respectively. The Eu2+ ion occupying the Gd3+ site is not involved in the long lasting phosphorescence process, resulting in an interesting phenomenon that the green emitting phosphors show cyan phosphorescence. The experimental results show that co-doping the rare earth ions can improve the long lasting phosphorescence performance effectively, and with the moderate introduction of Nd3+ ion the most persistent cyan emission can last for 3 h approximately. This work shows up an interesting phosphorescence phenomenon, and provides a new and efficient candidate for long lasting phosphorescence materials.