Xinguo Zhang

Synthesis, photoluminescence and Judd–Ofelt analysis of red LiGd5P2O13 : Eu3+ phosphors for white LEDs

A series of LiGd5P2O13 : Eu3+ phosphors with a red-emitting band centered at 622 nm were synthesized by a high temperature solid-state reaction method. Structure-luminescence correlation, concentration quenching and thermal stability of the LiGd5P2O13 : Eu3+ phosphors were studied combined with XRD, PL/PLE spectra as well as temperature-dependent PL and decay curves. The optimum concentration of Eu3+ doped in the LiGd5P2O13 is 20 mol%, and the corresponding concentration quenching mechanism is found to be via dipole/quadrupole–quadrupole interactions. Intensity parameters (Ω2 and Ω4) and various radiative properties such as transition rates (A), branching ratios (β) and stimulated emission cross-section (δe) were calculated using the Judd–Ofelt theory. LiGd5P2O13 : Eu3+ exhibited good thermal stability and its emission intensity decreased slightly at temperature above 150 °C. The CIE coordinates of LiGd5P2O13 : 0.20Eu3+ (0.644, 0.339) are very close to s-RGB standard red (0.640, 0.330), and the corresponding quantum efficiency is 46.5%. The results show that LGPO : Eu3+ may be considered as a potential red emitting phosphor for NUV/blue pumped WLEDs.

Thermal quenching of luminescence of LiSr4(BO3)3:Eu2+ orange‐emitting phosphor

An orange-emitting phosphor, Eu(2+)-activated LiSr4(BO3)3, was synthesized using the conventional solid-state reaction. The photoluminescence excitation and emission spectra, and temperature dependence of the luminescence intensity of the phosphor were investigated. The results showed that LiSr4(BO3)3:Eu(2+) could be efficiently excited by incident light of 250-450 nm, and emits a strong orange light. With increasing temperature, the emission bands of LiSr4(BO3)3:Eu(2+) show an abnormal blue-shift with broadening bandwidth and decreasing emission intensity.

Near-UV-to-red light conversion through energy transfer in Ca2Sr(PO4)2:Ce3+,Mn2+ for plant growth

A series of Ca2Sr(PO4)2:Ce3+,Mn2+,Na+ phosphors were synthesized by a high-temperature solid-state reaction. Under ∼320 nm excitation, the Ce3+ and Mn2+ co-doped phosphors exhibit two emission bands peaking at 370 and 645 nm, which originate from 5d–4f and 3d–3d transitions of Ce3+ and Mn2+, respectively. Luminescence properties, diffuse reflectance spectra, and decay curves indicate that energy transfer (ET) occurs from Ce3+ to Mn2+, and the ET efficiency reaches its maximum (91%) at an Mn2+ content of 0.35. The diffuse reflectance spectrum shows that the co-doped phosphors have strong absorption around 320 nm, which is good for the anti-aging ability of an agricultural film. The absolute quantum efficiency of the Ca1.6Sr(PO4)2:0.15Ce3+,0.10Mn2+,0.15Na+ phosphor is ∼94%. And the co-doped phosphors exhibit good stability in water. Therefore, Ca2Sr(PO4)2:Ce3+,Mn2+,Na+ phosphors have potential application as a light conversion material in agricultural films.

Sol-gel synthesis and luminescent properties of red-emitting Y(P,V)O4:Eu3+ phosphors

Eu(3+)-activated Y(P,V)O4 phosphors were prepared by the EDTA sol-gel method, and the corresponding morphologies and luminescent properties were investigated. The sample particles were relatively spheroid with size of 2-3 µm and had a smooth surface. The excitation spectra for Y(P,V)O4:Eu(3+) consisted of three strong excitation bands in the 200-350 nm range, which were attributed to a Eu(3+)- O(2-) charge-transfer band and (1)A1-(1) T1/(1) T2 transitions in VO4(3-). The as-synthesized phosphors exhibited a highly efficient red luminescence at 613 nm due to the Eu(3+5) D0-(7) F2 electric dipole transition. With the increase in the V(5+)/P(5+) ratio, the luminescence intensity of the red phosphor under UV excitation was greatly improved due to enhanced VO4(3-) → Eu(3+) energy transfer.

Synthesis, luminescence and electron–vibrational interactions of UV-excitable green phosphor α-Na2Ca4(PO4)2SiO4:Eu2+

A series of UV-excitable α-Na2Ca4(PO4)2SiO4:Eu2+ green phosphors were synthesized by conventional solid-state reactions. The photoluminescence (PL) properties and concentration quenching of the as-prepared phosphors were investigated. All of the phosphors exhibited strong, broad absorption bands in the near-ultraviolet range and gave bright green emission upon excitation with 310 nm light. Eu2+ ions occupy 10-coordinated Ca(3) sites and 6-coordinated Ca(2) sites in α-Na2Ca4(PO4)2SiO4 and therefore generate two emission sub-bands at 497 and 557 nm. The electron–vibrational interaction (EVI) parameters, such as the Huang–Rhys factor, effective phonon energy and zero phonon line position, were calculated. With increasing Eu2+ concentration, the emission peak wavelength red-shifted from 510 to 530 nm, and the color hue can be tuned from green to yellowish green. The concentration quenching mechanism in α-Na2Ca4(PO4)2SiO4:Eu2+ consists of electric multipole–multipole interactions; these are most likely to be dipole–dipole and dipole–quadrupole interactions. These results indicate that α-NCPS:Eu2+ phosphors are promising candidates for application in NUV LEDs.

Strong green fluorescent hydrogels with Ba2MgSi2O7:Eu2+ phosphor embedded in cellulose

Non-cytotoxic and green-emitting fluorescent hydrogels were constructed from a cellulose solution containing Ba2 MgSi2 O7 :Eu2+ green phosphor in a NaOH/urea aqueous system. The structure, optical properties and cytotoxicity of these hydrogels were studied. The Ba2 MgSi2 O7 :Eu2+ phosphor particles were dispersed evenly in the cellulose hydrogel matrix. Good luminescent properties of Ba2 MgSi2 O7 :Eu2+ phosphor were maintained in the hydrogels, leading to strong green emission under ultraviolet excitation. Fluorescent hydrogels have no obvious cytotoxicity in a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) proliferation test, and have potential use in in vivo applications like optical imaging and drug delivery.

Luminescence, Energy Transfer and Tunable Color of Ce3+- and Tb3+-Activated Na3Gd(BO3)2 Phosphors

A series of blue Na3Gd(BO3)2:Ce3+ and blue-to-green color-tunable Na3Gd (BO3)2:Ce3+,Tb3+ phosphors were synthesized by the solid-state method. The luminescence, concentration quenching and energy transfer (ET) process of Na3Gd(BO3)2:Ce3+,Tb3+ were investigated. Both Ce3+ and Tb3+ occupy the Gd3+ site in the Na3Gd(BO3)2 host. Na3Gd(BO3)2:Ce3+ exhibits strong ultraviolet absorption and broadband blue emission. The Ce3+ sensitization effect on Tb3+ has been verified by the variation of PL/PLE spectra, the Ce3+ decay lifetimes and the energy transfer efficiency of Na3Gd(BO3)2:Ce3+,Tb3+ phosphors. The maximum Ce3+–Tb3+ ET efficiency has been calculated to be 95%. The emitting color of the obtained phosphors can be modulated from blue (0.179, 0.204) through bluish-green (0.271, 0.391) to green (0.349, 0.551) by properly changing the ratio of Ce3+/Tb3+.