Influence of co-doping different rare earth ions on CaGa2S4: Eu, RE(RE=Ln) phosphors

Photoluminescent phosphors CaGa₂S₄: Eu²⁺, RE³⁺ (RE³⁺ including all rare earth ions except for Sc³⁺, Pm³⁺, Eu³⁺ and Lu³⁺) were prepared by sintering at high temperature in a reductive atmosphere, and their luminescent properties were studied intensively. The influences of co-doping rare earth ions on their luminescent properties were also investigated. No remarkable differences were found from excitation spectra of co-doped phosphors CaGa₂S₄: Eu²⁺, RE³⁺ in contrast with that of phosphor CaGa₂S₄: Eu²⁺, but there were a few differences in emission spectra of Ce³⁺, Pr³⁺ or Ho³⁺ co-doped phosphors. Phosphors CaGa₂S₄: Eu²⁺, RE³⁺ (RE=Ce, Pr, Gd, Tb, Ho and Y) had persistent afterglow, and very short afterglow was shown for Nd³⁺ or Er³⁺ co-doped phosphors, but no long afterglow appeared when auxiliary activator was La³⁺, Sm³⁺, Dy³⁺, Tm³⁺ or Yb³⁺. Among the phosphors with long-lasting phosphorescence, in our experiments, CaGa₂S₄: Eu²⁺, Ho³⁺ had the longest and the highest brightness long yellow afterglow. Thermo-luminescence of all co-doped phosphors was measured to find the answer of different influences from different rare earth auxiliary activators.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

Thermoluminescent properties of Eu and RE co-doped phosphors CaGa2S4:Eu, RE (RE=Ln, excluding Pm, Eu and Lu)

The thermo-luminescence (TL) of rare earth ions RE³⁺ (RE=Ln, excluding Pm, Eu and Lu) co-doped phosphors CaGa₂S₄:Eu²⁺, RE³⁺ was studied between room temperature and 300 °C, and 3D thermo-luminescence of the phosphors were measured from room temperature to 400 °C. The basic material CaGa₂S₄:Eu²⁺, showed at least two bands in the TL glow curve. Changing the auxiliary activator RE³⁺ (rare earth ion), intensities and the positions of the TL glow curve peaks were affected significantly. For the phosphors with long afterglow, auxiliary activator such as Ce³⁺, Pr³⁺, Gd³⁺, Tb³⁺, Ho³⁺, or Y³⁺ created some new defects in these compounds at lower trap levels and enhanced their TL intensities. The Nd³⁺ or Er³⁺ auxiliary activator only enhanced TL intensities to a low extent, so these two phosphors have short persistent luminescence at room temperature. TL intensities of La³⁺, Sm³⁺, Tm³⁺ or Yb³⁺ co-doped phosphors were suppressed greatly and no afterglow was shown. The relationship between auxiliary activators and corresponding thermo-luminescence curves of phosphors CaGa₂S₄:Eu²⁺, RE³⁺ are discussed in detail. According to our results, suitable activation energy and enough high corresponding trap density are necessary for the phosphor with long afterglow.     

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⁺.     

Photoluminescence of Rare Earth Phosphors Na0.5Gd0.5WO4: RE3+ and Na0.5Gd0.5(Mo0.75W0.25)O4: RE3+ (RE=Eu, Sm, Dy)

Two series of phosphors, Na_0.5Gd_0.5WO₄: RE³⁺ and Na_0.5Gd_0.5(Mo_0.75W_0.25)O₄: RE³⁺ (RE?=?Eu, Sm, Dy) have been synthesized by hydrothermal process to obtain the high purity, which have been characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM). The results suggest that Na_0.5Gd_0.5(Mo_0.75W_0.25)O₄: RE³⁺ phosphors are more easily to crystallize than Na_0.5Gd_0.5WO₄: RE³⁺ ones. Both of them present the characteristic luminescence of Eu³⁺, Sm³⁺ and Dy³⁺. Especially the photoluminescent properties of Na_0.5Gd_0.5WO₄: x%Eu³⁺ (Sm³⁺) can be obtained to show white luminescence as the suitable doping concentration of Eu³⁺ or Sm³⁺.     

Blue and red long lasting phosphorescence (LLP) in β-Zn3(PO4)2:Mn,Zr

Multi-color long lasting phosphorescent (LLP) phenomenon in β-Zn₃(PO₄)₂:Mn²⁺,Zr⁴⁺ was systematically investigated. It is found that the red (λ_Em=616 nm) LLP performance of Mn²⁺ such as brightness and duration is largely improved, and that the blue (λ_Em=475 nm) LLP of Zr⁴⁺ with lower intensity appears when Zr⁴⁺ ions are co-doped into the matrix. The fluorescence, phosphorescence and thermoluminescence (TL) spectra show that Mn²⁺ ion is solely expected as a luminescent center, while Zr⁴⁺ ion not only acts as a luminescent center, but also induces an electron trap (Trap_Zr) associated with a TL peak at 344 K. The trap depth for Trap_Zr is 0.25 eV, while that for the intrinsic trap is 0.38 eV, associated with a dominant peak at 385 K for Zn₃(PO₄)₂:Mn²⁺. The Zr⁴⁺-induced trap with suitable depth is responsible for the improvement of the red LLP of Mn²⁺ ion and the appearance of the blue LLP of Zr⁴⁺ ion. The LLP mechanism is also investigated.     

Photoluminescence studies of NaCaPO4:RE (RE=Dy, Mn or Gd)

The purpose of the present study is to develop an understanding of photoluminescence properties of Dy³⁺, Mn²⁺ or Gd³⁺doped NaCaPO₄ phosphors, which have served as efficient phosphors in many industrial applications. The phase formation was confirmed by the X-ray powder diffraction (XRD) measurement. Photoluminescence (PL) excitation spectrum measurement of NaCaPO₄:Dy³⁺ shows this phosphor can be efficiently excited by near-ultraviolet (UV) light from 300 to 400 nm and presents dominant luminescence band centered at 480 nm (blue) and 573 nm (yellow). The PL excitation of NaCaPO₄:Mn²⁺ and Gd³⁺ under UV wavelength shows the emissions at 520 and 313 nm, respectively. A scanning electron microscope (SEM) shows an average crystallite size in sub-micrometer range. The obtained results show that the phosphors have the potential for application in the lamp industry and medical applications.     

Structural, luminescent and thermal properties of blue SrAl2O4:Eu, Dy phosphor filled low-density polyethylene composites

The performance of nanophase luminophors is usually compromised by environmentally induced degradation. In this study, composites of low density polyethylene (LDPE) with various concentrations of the blue-emitting europium and dysprosium co-doped strontium aluminate (SrAl₂O₄:Eu²⁺,Dy³⁺) phosphor were investigated. The blue long-lasting phosphorescence of the composites was observed in the dark after removal of the excitation light. X-ray diffraction analysis revealed the presence of the SrAl₂O₄ phase in the composites. PL spectra of the composites have two sets of peaks, major broad bands peaking at about 4855 Å and minor ones at wavelengths between 4115 and 4175 Å, attributed to the 4f-5d transition of Eu²⁺. DSC and TGA results show that the introduction of the phosphor in LDPE matrix caused a slight reduction in the crystallinity of LDPE but a significant increase in the stability of the composites.     

Blue-emitting long-lasting phosphor, Sr3Al10SiO20:Eu,Ho

A novel blue-emitting long-lasting phosphor Sr₃Al₁₀SiO₂₀:Eu²⁺,Ho³⁺ is prepared by the conventional high-temperature solid-state technique and their luminescent properties are investigated. XRD, photoluminescence (PL) and thermoluminescence (TL) are used to characterize the synthesized phosphors. These phosphors are well crystallized by calcinations at 1500-1600 °C for 3 h. The phosphor emits blue light and shows long-lasting phosphorescence after it is excited with 254/365 nm ultraviolet light. TL curves reveal the introduction of Ho³⁺ ions into the Sr₃Al₁₀SiO₂₀:Eu²⁺ host produces a highly dense trap level at appropriate depth, which is the origin of the long-lasting phosphorescence in this kind of material. The long-lasting phosphorescence lasts for nearly 6 h in the light perception of the dark-adapted human eye (0.32 mcd/m²). All the results indicate that this phosphor has promising potential practical applications.     

A novel blue-emitting long-lasting proyphosphate phosphor Sr2P2O7:Eu,Y

By introducing the Y³⁺ into Sr₂P₂O₇:Eu²⁺, we successfully prepared a kind of new phosphor with blue long-lasting phosphorescence by the high-temperature solid-state reaction method. In this paper, the properties of Sr₂P₂O₇:Eu²⁺,Y³⁺ were investigated utilizing XRD, photoluminescence, luminescence decay, long-lasting phosphorescence and thermoluminescence (TL) spectra. The phosphor emitted blue light that was related to the 4f⁶5d¹-⁸S_7/2 transition of Eu²⁺. The bright blue phosphorescence could be observed by naked eyes even 8 h after the excitation source was removed. Two TL peaks at 317 and 378 K related to two types of defects appeared in the TL spectrum. By analyzing the TL curve the depths of traps were calculated to be 0.61 and 0.66 eV. Also, the mechanism of LLP was discussed in this report.     

Green emitting long lasting phosphorescence (LLP) properties of Mg2SnO4:Mn phosphor

A novel long-lasting phosphorescence phosphor, Mn²⁺-activated Mg₂SnO₄, has been synthesized and its optical properties have been investigated. The Mg₂SnO₄:Mn²⁺ emits green light with high luminance, upon UV irradiation, centered at 499 nm from the spin forbidden transitions of the d-electrons in Mn²⁺ ions. The CIE chromaticity coordinates of the Mg₂SnO₄:Mn²⁺ phosphor are x=0.0875 and y=0.6083 under 254 nm UV excitation. The phosphorescence can be observed by the naked eyes (0.32 mcd/m²) in the dark clearly for over 5 h after the 5 min UV irradiation. Thermoluminescence has been studied and the mechanism of the long-lasting phosphorescence has been discussed.     

Gd, N-codoped trititanate nanotubes: Preparation, characterization and photocatalytic activity

Trititanate nanotubes were prepared using hydrothermal method and then co-doped with Gd³⁺ and N through ion-exchanging with H⁺. They were characterized by X-ray photoelectron spectra (XPS), high-resolution transmission electronmicroscopy (HRTEM), X-ray diffraction (XRD), UV-vis diffusion reflection spectra (UV-vis DRS) and photoluminescence (PL) spectra. The photocatalytic activities were investigated with Rhodamine B as the model pollutant. The results indicated that synergistic reaction occurred when codoping with Gd³⁺ and N and the photocatalytic activities of TiO₂ were enhanced significantly under visible light irradiation.     

Effects of Gd and Lu co-doping on the long afterglow properties of yellowish-orange phosphor Y2O2S:Ti, Mg

A new long afterglow phosphor Y₂O₂S:Ti⁴⁺, Mg²⁺ co-doped with Gd³⁺ and Lu³⁺ was synthesized by solid-state reaction in inert gas ambient. Its properties were systematically analyzed by X-ray diffraction (XRD), luminescence spectra, afterglow decay curves and thermoluminescence (TL) spectra. It was found that the long afterglow performance of Y₂O₂S:Ti⁴⁺, Mg²⁺ such as brightness and persistent time was largely improved when co-doped with Gd³⁺ and Lu³⁺. By analyzing the TL curve the activation energy E were calculated to be 0.64 eV for 388 K peak and 0.98 eV for 508 K peak, and the trap intensity related to 388 K peak is much stronger than that related to 508 K peak. The mechanism of the long afterglow was also discussed in this paper.     

The luminescence of CaYBO4:RE (REEu, Gd, Tb, Ce) in VUV-visible region

Phosphors CaYBO₄:RE³⁺ (RE=Eu, Gd, Tb, Ce) were synthesized with the method of solid-state reaction at high temperature, and their vacuum ultraviolet (VUV)-visible luminescent properties in VUV-visible region were studied at 20 K. In CaYBO₄, it is confirmed that there are two types of lattice sites that can be substituted by rare-earth ions. The host excitation and emission peaks of undoped CaYBO₄ are very weak, which locate at about 175 and 350-360 nm, respectively. The existence of Gd³⁺ can efficiently enhance the utilization of host absorption energy and result in a strong emission line at 314 nm. In CaYBO₄, Eu³⁺ has typical red emission with the strongest peak at 610 nm; Tb³⁺ shows characteristic green emission, of which the maximum emission peak is located at 542 nm. The charge transfer band of CaYBO₄:Eu³⁺ was observed at 228 nm; the co-doping of Gd³⁺ and Eu³⁺ can obviously sensitize the red emission of Eu³⁺. The fluorescent spectra of CaYBO₄:Ce³⁺ is very weak due to photoionization; the co-addition of Ce³⁺-Tb³⁺ can obviously quench the luminescence of Tb³⁺.     

Luminescence properties of a new Mn-activated red long-afterglow phosphor

A new phosphor, CaZnGe₂O₆:Mn²⁺, which emits red long-lasting phosphorescence centered at 648 nm upon UV light excitation, is prepared by the conventional high-temperature solid-state method and its luminescent properties are systematically investigated in this paper. XRD, photoluminescence, thermoluminescence spectra and afterglow decay curve are used to characterize the synthesized phosphor. This phosphor is well crystallized by calcination at 1150 °C for 3 h and possesses excellent performance. The color coordinate values of this phosphor are x=0.64, y=0.26 under 250 nm UV light excitation. Under 250-nm UV light irradiation, this phosphor shows obvious long-lasting phosphorescence that can be seen with the naked eye in the dark clearly after the irradiation source has been removed for more than 3 h. The possible mechanism of this red-light-emitting long-afterglow phosphor is also investigated based on the experiment results.     

Infrared-to-visible up-conversion luminescence and energy transfer of RE3+/Yb3+(RE = Ho,Tm) co-doped SrIn2O4

Near-infrared excited up-conversion phosphors of RE³⁺/Yb³⁺(RE = Ho, Tm) co-doped SrIn₂O₄ were synthesized by a solid-state reaction method. X-ray diffraction analysis revealed the phase composition of those samples, and the up-conversion spectroscopic properties were studied in terms of up-conversion emission spectra. Under 980 nm near-infrared laser excitation, strong green emission with the peak at 546 nm was observed in SrIn₂O₄: Ho³⁺/Yb³⁺, which can be assigned to the characteristic ⁵S₂(⁵F₄) → ⁵I₈ transition of Ho³⁺. Furthermore, SrIn₂O₄: Tm³⁺/Yb³⁺ showed bright blue emission with the peak at 486 nm, which is associated with the ¹G₄ → ³H₆ transition of Tm³⁺. The UC power studies indicated that the luminescence of SrIn₂O₄: Ho³⁺/Yb³⁺ and SrIn₂O₄: Tm³⁺/Yb³⁺ are attributed to two-photon and three-photon process, respectively. The possible UC luminescence mechanism and energy transfer in SrIn₂O₄: RE³⁺/Yb³⁺ were discussed.     

Magnetic properties of CoFe1.9RE0.1O4 nanoparticles (RE=La,Ce, Nd,Sm, Eu,Gd, Tb,Ho) prepared in polyol

Highly crystalline CoFe_1.9RE_0.1O₄ ferrite nanoparticles, where RE=La, Ce, Nd, Sm, Eu, Gd, Tb, and Ho, have been synthesized by forced hydrolysis in polyol. X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), ⁵⁷Fe Mössbauer spectrometry, Co K-edge X-ray absorption spectroscopy and magnetic measurements using a SQUID magnetometer were employed to investigate the effect of the substitution RE³⁺ ions for Fe³⁺ ones on the structure, the microstructure, the chemical homogeneity, and the magnetic properties of the cobalt ferrite system. All the produced particles are superparamagnetic at room temperature. Nevertheless, the substitution causes reduction of the blocking temperature which is mainly ascribed to partial cation exchange among the spinel-like sublattices of CoFe₂O₄ induced by the insertion of the relatively large RE³⁺ ions. The low-temperature saturation magnetization and coercivity appear to be greatly affected by the nature of RE³⁺ ions—maxima values were found for Gd³⁺ and Eu³⁺, respectively.     

Red long-lasting luminescence in clinoenstatite

A long-lasting phosphor Ca_0.2Zn_0.9Mg_0.9Si₂O₆:Eu²⁺, Dy³⁺, Mn²⁺ was prepared by a sol-gel method. Nanoparticles crystallizing in a clinoenstatite structure were obtained. Long persistent phosphorescence in the red has been observed with persistence time over one hour at 680 nm and was attributed to Mn²⁺ emission. The persistent luminescence is suggested to involve Eu²⁺ as a sensitizer, Dy³⁺ or Dy³⁺-related defect as a trap center and Mn²⁺ as the luminescent center. However, the details of the mechanism are still under further investigation.     

Energy transfer in LaMgB5O10:Pr,Mn under VUV excitation

Luminescent properties of Pr³⁺ or Mn²⁺ singly doped and Pr³⁺, Mn²⁺ co-doped LaMgB₅O₁₀ are investigated by synchrotron radiation VUV light. When LaMgB₅O₁₀:Pr³⁺ is excited at185 nm, the photon cascade emission between 4f levels of Pr³⁺ is observed. In the excitation spectra of LaMgB₅O₁₀:Mn²⁺ monitoring the 615 nm emission of Mn²⁺, several excitation bands in a spectral range from 330 to 580 nm are recorded, among which the most intense band is centered at 412 nm (⁶A_1g→⁴E_g-⁴A_1g). This band has considerable spectra overlap with the 410 nm emission (¹S₀→¹I₆) of Pr³⁺, which is favorable for energy transfer from Pr³⁺ to Mn²⁺. Such energy transfer is observed in the co-doped sample, converting the violet emission (410 nm) of Pr³⁺ into the red emission (615 nm) of Mn²⁺. The concentration dependence of transfer efficiency is also investigated.     

Magnetic properties of RE0.7Ca0.3Mn0.95Fe0.05O3 (RE=Sm and Gd) manganites at low temperature

The magnetic properties of RE_0.7Ca_0.3Mn_0.95Fe_0.05O₃ perovskite with rare-earth cations (RE=Sm and Gd) were investigated by means of X-ray diffraction, Mössbauer spectroscopy, and low temperature (4.2-266 K) magnetization measurements. Structural characterization of these compounds shows that they both have orthorhombic (Pbnm) structure. The Mössbauer spectra show clear evidence of local structural distortion of the Mn(Fe)O₆ octahedron, which is based on the non-zero nuclear quadrupole interactions for high-spin Fe³⁺ ions. It was found that the local structural distortion increases significantly when Sm³⁺ is replaced by Gd³⁺. This distortion is attributed to the Jahn-Teller coupling strength as estimated from the Mössbauer effect results. The magnetic results indicate that the Curie temperature decreases as a result of replacing Sm by Gd. This is due to the decrease of the average A-site cationic radius 〈r_A〉. The rapid increase of magnetization at low temperature indicates the magnetic ordering of rare earth ions at the A-site.