Xiao-jun Wang

White light emitting diode by using α-Ca2P2O7:Eu2+, Mn2+ phosphor

The α-Ca2P2O7:Eu2+, Mn2+ phosphors show two emission bands peaking at around 416 (blue) and 600nm (orange), originating from the allowed f-d transition of Eu2+ and the forbidden T14-A16 transition of Mn2+, respectively, under near ultraviolet (UV) excitation at 400nm. Spectroscopy and fluorescence lifetime measurements demonstrate that energy transfer from Eu2+ to Mn2+ performs with transfer efficiency as high as 65% for Mn2+ concentration of 12mol%. The authors have fabricated a white light emitting diode (LED) through the integration of GaN near-UV chip and two phosphor blends (α-Ca2P2O7:Eu2+, Mn2+ blue-orange phosphor and Ba2SiO4:Eu2+ green phosphor) into a single package. The white LED shows color rendering index of 78, luminescent efficiency of 9lm∕W, and low color point variation against forward-bias currents.

Enhanced red phosphorescence in nanosized CaTiO3:Pr3+ phosphors

The red photoluminescence and phosphorescence originating from D21-H43 transition of Pr3+ in CaTiO3 nanoparticles are studied as a function of Pr3+ concentrations. The nanophosphors exhibit both longer persistence time of 30min and higher quenching concentration of 0.4mol% than the bulk (10min and 0.1mol%). The initial phosphorescence in the nanophosphor is an order of magnitude stronger than that in the bulk at the corresponding quenching concentrations. Phosphorescence decay patterns and diffused reflectance spectra before and after ultraviolet exposure indicate the existence of more traps contributing to phosphorescence in the nanoparticles.

Long lasting yellow phosphorescence and photostimulated luminescence in Sr3SiO5 : Eu2+ and Sr3SiO5 : Eu2+, Dy3+ phosphors

We report the observation of long lasting yellow phosphorescence and photostimulated luminescence (PSL) in Sr3SiO5 : Eu2+ and Sr3SiO5 : Eu2+, Dy3+ phosphors. The decay patterns of phosphorescence and thermoluminescence curves demonstrate that introduction of Dy3+ into Sr3SiO5 : Eu2+ can generate a large number of shallow traps and deep traps. The generated deep traps prolong the phosphorescence up to 6 h after UV irradiation. The PSL is studied under 808 nm excitation. Slow rising and falling edges of the emission in Sr3SiO5 : Eu2+, Dy3+ are observed, showing a retrapping process by the generated shallow traps due to co-doping Dy3+.

Red phosphorescence in Sr4Al14O25: Cr3+,Eu2+,Dy3+ through persistent energy transfer

Cr3+, Eu2+, and Dy3+ codoped Sr4Al14O25 has been synthesized by solid-state reaction. Long persistent phosphorescence in red of Cr3+ and in blue of Eu2+ has been observed in this system with persistence times of over 2h for the red and 10h for the blue. Red phosphorescence is performed through persistent energy transfer from Eu2+ to Cr3+, converting the blue to the red. Concentration effect is analyzed based on energy transfer. The calculated results are in good agreement with the experimental data. The different decay patterns of the red and blue phosphorescence are measured and discussed.

New function of the Yb3|[plus]| ion as an efficient emitter of persistent luminescence in the short-wave infrared

The trivalent ytterbium (Yb3+) ion has been extensively used as an emitter in short-wave infrared (SWIR) lasers, a sensitizer to activate other lanthanide ions for up-conversion luminescence, and a spectral converter in Ln3+-Yb3+ doubly doped quantum cutting phosphors. Here we report a new function of the Yb3+ ion—as an efficient emitting center for SWIR persistent luminescence. We have developed the first real SWIR persistent phosphor, MgGeO3:Yb3+, which exhibits very-long persistent luminescence at around 1000 nm for longer than 100 h. The MgGeO3:Yb3+ phosphor is spectrally transparent to visible/near-infrared light (~400–900 nm) and is a promising ultraviolet-to-SWIR spectral convertor. The MgGeO3:Yb3+ phosphor also exhibits a photostimulated persistent luminescence capability, where the SWIR persistent emission in an ultraviolet-light pre-irradiated sample can be rejuvenated by low-energy light (white or red light) stimulation. The MgGeO3:Yb3+ phosphor is expected to have promising applications in biomedical imaging, night-vision surveillance and photovoltaics.

Optical properties of trivalent europium doped ZnO:Zn phosphor under indirect excitation of near-UV light

Eu3+, Li+-codoped ZnO:Zn phosphor with intense Eu3+ emissions upon indirect excitation of near-UV light has been synthesized under reducing condition. Steady-state and time-resolved photoluminescence and diffuse reflectance spectra are measured to investigate properties of the luminescence. The results suggest that there exists prominent energy transfer from ZnO host to Eu3+ ions. A series of energy levels as temporary storage of excitation energy play a crucial role on this energy transfer process. Two kinds of Eu3+ sites in Eu3+, Li+-codoped ZnO:Zn are distinguished based on the emission and excitation spectra in comparison with pure Eu2O3.

Photon cascade emission and quantum efficiency of the 3P0 level in Pr3+-doped SrAl12O19 system

Abstract Photon cascade emission has been reported in Pr3+-doped SrAl12O19 system, providing a unique method for direct measuring the quantum efficiency of 3P0 in this material. The 1S0→1I6 fluorescence intensity can be taken as a measure of the initial 3P0 population. Thus the quantum efficiency of 3P0 can be calculated using the intensity ratio of the total emission from 3P0 and the emission of 1S0→1I6. The efficiency decreases as the concentration of Pr increases, indicating the effect of concentration quenching. The quantum efficiency is still substantially less than 100% even in dilute sample due to the thermal excitation of 1I6 and 3P1 states.

Photoluminescent properties of yellow emitting Ca1−xEuxSi2O2−δN2+2δ/3 phosphors for white light-emitting diodes

Ca1?xEuxSi2O2??N2+2?/3 phosphors with Eu2+ concentration (x) in the range of 0?0.25 are synthesized by solid-state reaction. With increasing x, the energy dispersive spectra show that the O/N ratio increases from 0.54 to 1.43 and the emission and excitation spectra show a redshift due to increased splitting of the low-lying 4f65d1 state of Eu2+. An additional excitation band appears at around 460?nm for high doping concentrations of Eu2+, perfectly matching the emission wavelength of blue light-emitting diodes (LEDs). The phosphors can be excited in the near-UV to blue region to emit a broad yellow band in the range 543?562?nm, which depends on x. A white LED is fabricated by combining the Ca0.85Eu0.15Si2O2??N2+2?/3 phosphor with a blue GaN chip. Under 20?mA forward-bias current, a CIE chromaticity coordinate of (0.3396, 0.3474), the corresponding colour temperature (Tc) of 5223?K, the colour rendering index of 74 and luminous efficiency of 20?lm?W?1 are obtained.

Terpolymer-based SIPN coating with excellent antifogging and frost-resisting properties

We prepared an effective antifogging/frost-resisting coating by forming a semi-interpenetrating polymer network (SIPN) on the basis of a linear, random terpolymer poly(2-(dimethylamino)ethyl methacrylate-co-N-vinylpyrrolidone-co-methyl methacrylate), poly(DMAEMA-co-NVP-co-MMA), and a network of poly(ethylene glycol dimethacrylate). The excellent antifogging/frost-resisting property was mainly attributed to a balanced hydrophilicity/hydrophobicity of the terpolymer with the optimal DMAEMA/NVP/MMA molar ratio at 40/30/30. Compared to our previous work using poly(DMAEMA-co-MMA), the terpolymer-based coating demonstrated excellent antifogging property against both low- and high-temperature moist air, by eliminating the lower critical solution temperature related to the DMAEMA segments in the binary copolymer. By monitoring the coating thickness change during the fogging/frosting test, it appeared that water molecules could rapidly be absorbed into and desorbed from the terpolymer-based coating, implying long-term effectiveness of the antifogging/frost-resisting coating.

Spectroscopic investigation of CaAl12O19:M3+ upon UV/vacuum–UV excitation: a comparison with SrAl12O19:M3+ (M = Pr,Cr)

In order to evaluate the possibility of using Cr³⁺ co-doping to modify the emission properties of Pr³⁺-based quantum cutting phosphors, which emit one ultraviolet (UV) and one visible photon from each vacuum-UV photon absorbed, into phosphors that emit two visible photons, the optical spectroscopies between 150 and 750 nm of CaAl₁₂O₁₉:M³⁺ (M = Pr,Cr) were investigated and compared with those of SrAl₁₂O₁₉:M³⁺ using synchrotron radiation as one of the excitation sources. When Pr³⁺ is excited to its 4f5d states, cascade emission of UV photons from the ¹S₀ state followed by visible photons from the ³P₀ state are both observed in (Ca,Sr)Al₁₂O₁₉:Pr³⁺. Similar to SrAl₁₂O₁₉:M³⁺, there exist desirable spectral overlaps between Pr³⁺ ¹S₀ emissions and the Cr³⁺ higher multiplet absorptions in CaAl₁₂O₁₉, which should be favourable to the energy transfer from Pr³⁺ ¹S₀ to Cr³⁺, converting the UV photon from Pr³⁺ ¹S₀ into the red Cr³⁺ emission. However, the host absorption band and the Cr³⁺-related charge transfer band have an unfavourable overlap with the Pr³⁺ 4f5d states in the vacuum-UV region, preventing efficient selective Pr³⁺ excitation under radiation of Xe discharge, a prerequisite for the quantum cutting process.