Luminescent properties of (Y,Gd)BO3:Bi,RE (RE=Eu, Tb) phosphor under VUV/UV excitation

Bi³⁺- and RE³⁺-co-doped (Y,Gd)BO₃ phosphors were prepared and their luminescent properties under vacuum ultraviolet (VUV)/UV excitation were investigated. Strong red emission for (Y,Gd)BO₃:Bi³⁺,Eu³⁺ and strong green emission for (Y,Gd)BO₃:Bi³⁺,Tb³⁺ are observed under VUV excitation from 147 to 200 nm with a much broader excitation region than that of single Eu³⁺-doped or Tb³⁺-doped (Y,Gd)BO₃ phosphor. Strong emissions are also observed under UV excitation around 265 nm where as nearly no luminescence is observed for single Eu³⁺-doped or Tb³⁺-doped (Y,Gd)BO₃. The luminescence enhancement of Bi³⁺- and RE³⁺-co-doped (Y,Gd)BO₃ phosphors is due to energy transfer from Bi³⁺ ion to Eu³⁺ or Tb³⁺ ion not only in the VUV region but also in the UV region. Besides, host sensitization competition between Bi³⁺ and Eu³⁺ or Tb³⁺ is also observed. The investigated phosphors may be preferable for devices with a VUV light 147-200 nm as an excitation source such as PDP or mercury-free fluorescent lamp.     

The luminescent properties of CaYBO4:Ln(Ln=Eu, Tb) under UV-VUV range

The luminescent properties of CaYBO₄:Ln(Ln=Eu³⁺, Tb³⁺) were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) region. The CT band of Eu³⁺ at about 245 nm blue-shifted to 230 nm in VUV excitation spectrum; the band with the maximum at 183 nm was considered as the host lattice absorption. For the sample of CaYBO₄:0.08Tb³⁺, the bands at about 235 and 263 nm were assigned to the f-d transitions of Tb³⁺ and the CT band of Tb³⁺ was calculated according to Jφrgensen's theory. Under UV and VUV excitation, the main emission of Eu³⁺ corresponding to the ⁵D₀-⁷F₂ transition located at about 610 nm and two intense emission of Tb³⁺ from the ⁵D₄-⁷F₅ transition had been observed at about 542 and 552 nm, respectively. With the incorporation of Gd³⁺ into the host lattice of CaYBO₄, the luminescence of Tb³⁺ was enhanced while that of Eu³⁺ was decreased because of their different excitation mechanism.     

Luminescence of LiTb(PO3)4:Sm3+ and energy transfer from Tb3+ to Sm3+ under vacuum ultraviolet–ultraviolet excitation

The photoluminescence (PL) of LiTb(PO₃)₄, LiGd_0.97Sm_0.03(PO₃)₄, and LiTb_0.97Sm_0.03(PO₃)₄ under vacuum ultraviolet (VUV)/ultraviolet (UV) excitation were studied. We observed the VUV–UV sensitization of Sm³⁺ emission (561 nm, 601 nm, 649 nm, and 710 nm) by Tb³⁺ in LiTb(PO₃)₄:Sm³⁺, which leads to the yellow light emission (486 nm, 546 nm, 561 nm, 587 nm, 601 nm, 621 nm, 649 nm, and 710 nm) of LiTb(PO₃)₄:Sm³⁺ phosphor under UV and VUV excitation. The emission is a result of partial energy transfer from Tb³⁺ to Sm³⁺, which is discussed in detail in terms of the excitation and emission spectra and decay curves.     

VUV-UV excited luminescent properties of LnCa4O(BO3)3:RE (Ln=Y, La, Gd; Re=Eu, Tb, Dy, Ce)

Calcium lanthanide oxyborate doped with rare-earth ions LnCa₄O(BO₃)₃:RE³⁺ (LnCOB:RE, Ln=Y, La, Gd, RE=Eu, Tb, Dy, Ce) was synthesized by the method of solid-state reaction at high temperature. Their fluorescent spectra were measured from vacuum ultraviolet (VUV) to visible region at room temperature. Their excitation spectra all have a broadband center at about 188 nm, which is ascribed to host absorption. Using Dorenbos’ and Jφrgensen's work [P. Dorenbos, J. Lumin. 91 (2000) 91, R. Resfeld, C.K. Jφrgensen, Lasers and Excite States of Rare Earth [M], Springer, Berlin, 1977, p. 45], the position of the lowest 5d levels E(Ln,A) and charge transfer band E_ct were calculated and compared with their excitation spectra.Eu³⁺ and Tb³⁺ ions doped into LnCOB show efficient luminescence under VUV and UV irradiation. In this system, Ce³⁺ ions do not show efficient luminescence and quench the luminescence of Tb³⁺ ions when Tb³⁺ and Ce³⁺ ions are co-doped into LnCOB. GdCOB doped with Dy³⁺ shows yellowish white light under irradiation of 254 nm light for the reason that Gd³⁺ ions transfer the energy from itself to Dy³⁺. Because of the existence of Gd³⁺, the samples of GdCOB:RE³⁺ show higher excitation efficiency than LaCOB:RE³⁺ and YCOB:RE³⁺, around 188 nm, which indicates that the Gd³⁺ ions have an effect on the host absorption and can transfer the excitation energy to the luminescent center such as Tb³⁺, Dy³⁺ and Eu³⁺.     

Vacuum Ultraviolet Excited Photoluminescence Properties of Novel Na3Y9O3（BO3）8：Tb^3＋ Phosphor

The novel vacuum ultraviolet （VUV） excited Na3 Y9O3 （BO3）8：Tb^3＋ （NYOB：Tb^3＋） green phosphor is prepared. Strong VUV photoluminescence and high quenching concentration of Tb^3＋ （20 wt%） are observed in NYOB： Tb^3＋ and the strong emission are correlated with the unique layer-type structure of NYOB. All the characteristic 4 f - 5d transitions of Tb^3＋ and the host absorption band in VUV region are identified in the excitation spectrum. Based on the results, the energy levels scheme of Tb^3＋ in NYOB：Tb^3＋ is first established. This newly developed NYOB：Tb^3＋ phosphor shows excellent optical properties when compared with the commercial Zn2SiO4：Mn^2＋ and would be a potential VUV-excited green phosphor.     

A novel green phosphor GdCaAlO4:Tb for PDP application

A novel green-emitting nano-sized phosphor, Tb³⁺-doped GdCaAlO₄ was synthesized with a precursor prepared by citrate sol-gel method at relatively low temperature. Powder X-ray diffraction (XRD) analysis confirmed the formation of GdCaAlO₄. Field-emission scanning electron microscopy (FE-SEM) observation indicated a narrow size-distribution of about ∼100 nm for the particles with a spherical shape. Upon excitation with near UV and vacuum ultraviolet (VUV) light, the phosphor showed strong green-emission peaked at around 546 nm, corresponding to the ⁵D₄→⁷F₅ transition of Tb³⁺, and the highest photoluminescence (PL) intensity at 546 nm was found at a content of about 12 mol% Tb³⁺. As the Tb³⁺ concentration increases, the fast diffusion of energy among terbium ions toward traps or impurities resulting in a decrease of the lifetime. The optical properties study suggests that it is a potential candidate for plasma display panels (PDPs) application.     

New promising phosphors Ba3InB9O18 activated by Eu/Tb

Borate Ba₃InB₉O₁₈ (BIBO) has been adopted as a host material for phosphors for the first time. Lanthanide ions (Eu³⁺/Tb³⁺)-doped BIBO phosphors have been synthesized by solid-state reaction and luminescent properties investigated under ultravoilet (UV) excitation. For red phosphor BIBO:Eu, dominant emission peaking at 590 nm was attributed to ⁵D₀→⁷F₁ transition of Eu³⁺, which confirmed that the local site of Eu³⁺ occupied by In³⁺ ion in BIBO crystal lattice is at inversion symmetry center. Optimum Eu³⁺ concentration of BIBO:Eu under UV excitation with 227 nm wavelength is around 40%. The green phosphor BIBO:Tb showed bright green emission at 550 with 232 nm light excited and optimal of Tb³⁺ concentration measured in BIBO is about 8%. The corresponding luminescence mechanisms of Ln-doped BIBO (Ln=Eu³⁺/Tb³⁺) were analyzed. The luminescent intensity of Tb³⁺ can be significantly improved by co-doping of Bi³⁺ in the BIBO:Tb lattice. The likely reason was proposed in terms of the different interactions of the host lattice with these ions, and of these ions with each other.     

Charge compensation on the luminescence properties of ZnWO4:Tb phosphors via hydrothermal synthesis

A phosphor Tb³⁺-doped ZnWO₄ (ZWO:Tb) phosphors were prepared by a hydrothermal method. X-ray powder diffraction (XRD) analysis revealed that the as-obtained sample is pure ZnWO₄ phase. The excitation and emission spectra indicated that the phosphor could be well excited by ultraviolet light (272 nm) and emit blue light at about 491 nm and green light at about 545 nm. Significant energy transfer from WO₄²⁻ groups to Tb³⁺ ions has been observed. Two approaches to charge compensation are investigated: (a) 2Zn²⁺ = Tb³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Zn²⁺ = 2Tb³⁺ + vacancy. Compared with two charge compensation patterns in the ZnWO₄:Tb³⁺, it has been found that ZnWO₄:Tb³⁺ phosphors used Li⁺ as charge compensation show greatly enhanced bluish-green emission under 272 nm excitation.     

Synthesis and optimum luminescence of monodispersed spheres for BaWO4-based green phosphors with doping of Tb

Monodispersed spheres for Tb³⁺-doped BaWO₄ (BWO:Tb) phosphors were prepared by a hydrothermal method. X-ray diffraction (XRD) and field-emission scanning electron microscopy were used to characterize the resulting samples. Emission and excitation spectra were studied using xenon excited spectroscopic experiments at room temperature. Because 12 at% BWO:Tb phosphor exhibits intensive green emission under 254 nm excitation in comparison with the commercial green fluorescent lamp phosphor (LaPO₄:Ce,Tb), it is considered to be a new promising green phosphor for fluorescent lamps application.     

A novel Dy3+-doped GdPO4 white-light phosphors under vacuum ultraviolet excitation for Hg-free lamps application

Novel Dy3+-doped GdPO4 white light phosphors with monoclinic system were successfully synthesised by hydrothermal method at 240 ℃. This paper investigates the luminescence properties of white-light Gd1-xPO4 : xDy3+ under vacuum ultraviolet (VUV) excitation. The strong absorption at around 147 nm in excitation spectrum energy can be transferred to the energy levels of Dy3+ ion from the host absorption. Additionally, this white light phosphors are activated by a single Dy3+ ion and with a lower preparation temperature, which tend to decrease the consumption of rare earth resource and energy. Therefore, the luminescence of Gd1-xPO4 : xDy3+ under VUV excitation is effective, and proves to be promising in application to mercury-free lamp.     

A novel Dy3+-doped Gd(PO3)3 white-light phosphors under VUV excitation for Hg-free lamps application

Novel Dy³⁺-doped Gd(PO₃)₃ white light phosphors each with an orthorhombic system are successfully synthesized by solid-state reaction. The luminescence properties of white-light Gd_1-x(PO₃)₃:xDy³⁺ (0<x≤ 0.25) under vacuum ultraviolet (VUV) excitation are investigated. The strong absorption at around 147 nm in excitation spectrum energy can be transferred to the energy levels of Dy³⁺ ion from the host absorption. Additionally, the white light phosphor is activated by a single Dy³⁺ ion. Therefore, the luminescence of Gd_1-x(PO₃)₃:xDy (0<x≤ 0.25) under VUV excitation is effective, and it has the promise of being applied to mercury-free lamp.     

Correlation of photoluminescence of (Y, Ln)VO4:Eu (Ln=Gd and La) phosphors with their crystal structures

We have studied the photoluminescence (PL) of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors and the correlation of the PL of those phosphor with their crystal structure. It is found that (Y, Gd)VO₄:Eu³⁺ phosphors have the same crystal structure as YVO₄:Eu³⁺, which is tetragonal with a little different lattice parameters. In the case of (Y, La)VO₄:Eu³⁺ phosphors, however, the gradual change from tetragonal to monoclinic structure of host lattice was observed as the amount of La ion increased. To investigate the PL property of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors, vacuum ultraviolet (VUV) and ultraviolet (UV) excitation were used. The favorable crystal structure for the PL intensity of orthovanadate phosphor under 147 and 254 nm excitation was tetragonal containing Gd ion and under 365 nm excitation was monoclinic containing La ion which might have the lowest site symmetry for Eu³⁺ ion.     

Efficient VUV sensitization of Eu3+ emission by Tb3+ in potassium rare‐earth double phosphate

The luminescence properties of K₃Tb(PO₄)₂ activated by Eu³⁺ were studied at excitation over the 120–300 nm wavelength range. It is demonstrated that Tb³⁺ ions, exhibiting a strong absorption band in the vacuum‐ultraviolet (VUV), can provide efficient sensitisation of Eu³⁺ emission in this wave length range, giving rise to intense red luminescence at 150 nm excitation. A proof is given for the concept of VUV sensitisation enabling the engineering of luminescence materials with enhanced conversion efficiency of VUV radiation into visible light. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and luminescence properties of Tb:NaGd(WO4)2 novel green phosphors

Tb³⁺:NaGd(WO₄)₂ (Tb:NGW) phosphors with different Tb³⁺ concentrations have been synthesized by a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the Tb:NGW phosphors. XRD analysis confirmed the formation of NGW with scheelite structure. SEM study showed that the obtained Tb:NGW phosphors appeared to be nearly spherical and their sizes ranged from 1 to 1.5 μm. The excitation spectra of these systems showed an intense broad band with maximum at 270 nm related to the O→W ligand-to-metal charge-transfer state. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under UV light excitation. Analysis of the photoluminescence spectra with different Tb³⁺ concentrations revealed that the optimum dopant concentration for Tb³⁺ is about 15 at% of Tb³⁺ ions in Tb:NGW phosphors.     

Double emitting phosphor NaSr4(BO3)3:Ce, Tb for near-UV light-emitting diodes

Blue and green double emitting phosphor, Ce³⁺ and Tb³⁺ co-doped NaSr₄(BO₃)₃, was synthesized in a weak reducing atmosphere by a conventional high temperature solid-state reaction technique. For comparison, Ce³⁺ or Tb³⁺ singly doped NaSr₄(BO₃)₃ was also prepared. The emission and excitation spectra of all samples have been investigated. NaSr₄(BO₃)₃:Tb³⁺ excitation includes a strong absorption at about 240 nm and some weak sharp lines in near-ultraviolet (n-UV) spectral region. The excitation of Ce³⁺ and Tb³⁺ co-doped NaSr₄(BO₃)₃ shows a strong broad band absorption in the n-UV region from the contribution of Ce³⁺, which makes it suitable for excitation by a n-UV LED chip. The emission of NaSr₄(BO₃)₃:Ce³⁺,Tb³⁺ consists of a blue emission band from Ce³⁺ and a green emission from Tb³⁺ under the excitation of n-UV light. Energy transfer between Ce³⁺ and Tb³⁺ is also discussed, and the relative intensity of blue emission and green emission could be tuned by adjusting the concentration of Ce³⁺ and Tb³⁺. The phosphor NaSr₄(BO₃)₃:Ce³⁺,Tb³⁺ could be considered as a double emission phosphor for n-UV excited white light-emitting diodes.     

Effective energy transfer between Ceand Eu in CaAl2Si2O8 host under vacuum ultraviolet illumination

We presented the energy transfer from Ce³⁺ to Eu²⁺ in CaAl₂Si₂O₈ host. The Ce³⁺-doped CaAl₂Si₂O₈ phosphor had a strong emission band at 378 nm under the vacuum ultraviolet (VUV) light. This emission spectrum of Ce³⁺ well overlapped with the excitation spectrum of Eu²⁺ under the UV illumination. As a result, the energy transfer from Ce³⁺ to Eu²⁺ in CaAl₂Si₂O₈ matrix was observed under VUV excitation, which resulted in a significant enhancement of the emission peak intensity at 446 nm. More details about the luminescent properties were presented.     

Luminescence efficiencies of self- and Tm activated CaWO4 under vacuum ultraviolet radiation excitation

Luminescence efficiency of self-activated CaWO₄ under 147 nm vacuum ultraviolet (VUV) radiation excitation is about 90% of that of BaMgAl₁₀O₁₇:Eu²⁺ (BAM), the commercial blue plasma display panel (PDP) phosphor. However, the color purity and the particle size of the former needs substantial modification before it can be considered for application in PDP. CaWO₄:Tm exhibits Tm³⁺ emission peaks in the blue region due to energy transfer from WO₄ to Tm³⁺ ions but the overall emission intensity under 147 nm excitation is reduced when compared to that of CaWO₄.     

Enhanced luminescence and degradation resistance in Tb modified Yttrium Borate core-nano silica shell phosphor under UV and VUV excitation

Composition variation in optimized solid state reaction conditions has been done to achieve intense green emission in YTb_xBO₃ phosphor under UV and VUV (147 nm resonant Xe*, 172 nm Xe₂* excimer band) excitation. Inert interface layer created by fabricating a shell of silica nanoparticles over individual phosphor grain protected the phosphor surface from deterioration and oxidation of luminescent ion (Tb³⁺) thus completely arresting phosphor degradation. At optimum Tb content of 20 mol%, integrated photoluminescence intensity of developed YTb_xBO₃ phosphor is four times higher than commercial green YBT. With short decay time of 4 ms, YTb_xBO₃ core-nano silica shell green emitting phosphor has great application potential in PDP panel and phosphor coated Xe lamps.     

Feasibility Study of VUV Sensitization Effect of Tb3+

The possibility to use Tb³⁺ as luminescence sensitizer for enhancement of the conversion efficiency of vacuum-ultraviolet (VUV) radiation into visible light was examined. We studied the luminescence properties of K₃Tb(PO₄)₂ and Ba₃Tb(PO₄)₃ activated by Eu³⁺, and of SrAl₁₂O₁₉ co-doped with Mn²⁺ and Tb³⁺ at excitation over the 120 to 300 nm wavelength range. It is shown that Tb³⁺ ions, exhibiting a strong absorption band in the VUV, can provide efficient sensitization of Eu³⁺ and Mn²⁺ emissions for excitation in this spectral range, giving rise to intense red and green luminescence, respectively. This study provides a proof for the concept of VUV sensitization, which enables the engineering of luminescence materials with improved efficiency for excitation from a noble gas discharge.     

Cooperative energy transfer and frequency upconversion in Yb3+-Tb 3+ and Nd 3+-Yb 3+-Tb 3+ codoped GdAl3(BO3)4 phosphors

Polycrystalline GdAl₃(BO₃)₄ phosphors codoped with Yb³⁺/Tb³⁺ and/or Nd³⁺/Yb³⁺/Tb³⁺ have been synthesized by combustion method. Upon excitation with a 980 nm laser diode, an intense green upconversion luminescence has been observed in GdAl₃(BO₃)₄:Yb,Tb phosphor. The quadratic dependence of the luminescence on the pump-laser power indicating a cooperative energy transfer process. Meanwhile, it is noticed that upon excitation with 808 nm laser diode, intense luminescence has clearly been detected in GdAl₃(BO₃)₄:Nd,Yb,Tb phosphor. The luminescence intensity exhibits also a quadratic dependence on incident pump-laser power. However, no green-emission has been observed in GdAl₃(BO₃)₄ phosphors codoped with Yb³⁺/Tb³⁺ or Nd³⁺/Tb³⁺ respectively upon excited at 808 nm laser diode. A proposed upconversion mechanism involving energy transfer from Nd³⁺ to Yb³⁺, and then a cooperative energy transfer process from two excited Yb³⁺ to Tb³⁺ has been presented.