Luminescence of Ce and Pr doped Sr2Mg(BO3)2 under VUV-UV and X-ray excitation

This report presents the luminescence properties of Ce³⁺ and Pr³⁺ activated Sr₂Mg(BO₃)₂ under VUV-UV and X-ray excitation. The five excitation bands of crystal field split 5d states are observed at about 46 729, 44 643, 41 667, 38 314 and 29 762 cm⁻¹ (i.e. 214, 224, 240, 261 and 336 nm) for Ce³⁺ in the host lattice. The doublet Ce³⁺ 5d→4f emission bands were found at about 25 840 and 24 096 cm⁻¹ (387 and 415 nm). The influence of doping concentration and temperature on the emission characteristics and the decay time of Ce³⁺ in Sr₂Mg(BO₃)₂ were investigated. For Pr³⁺ doped samples, the lowest 5d excitation band was observed at about 42017 cm⁻¹ (238 nm), a dominant band at around 35714 cm⁻¹ (280 nm) and two shoulder bands were seen in the emission spectra. The excitation and emission spectra of Ce³⁺ and Pr³⁺ were compared and discussed. The X-ray excited luminescence studies show that the light yields are ∼3200±230 and ∼1400±100 photons/MeV of absorbed X-ray energy for the samples Sr_1.86Ce_0.07Na_0.07Mg(BO₃)₂ and Sr_1.82Pr_0.09Na_0.09Mg(BO₃)₂ at RT, respectively.     

Characterization of the VUV excitation spectrum of BaZr(BO3)2:Eu

The excitation spectra of M (M=Si⁴⁺, Ti⁴⁺) and Eu³⁺ co-doped BaZr(BO₃)₂, BaZrO₃:Eu and La₂Zr₂O₇:Eu in the vacuum ultraviolet (VUV) regions of 110-300 nm are investigated and the host-lattice absorption are characterized. The result indicated that BaZr(BO₃)₂:Eu³⁺ phosphor has a strong absorption under the VUV excitation, and in the host-lattice excitation, the strong band at 130-160 nm could be due to the BO₃ atomic groups; the band at 160-180 nm is related to the excitation of Ba-O; 180-200 nm corresponds to the charge transfer (CT) transition of Zr-O. The band at 200-235 nm due to the CT band of Eu³⁺-O²⁻ and a bond valence study explained the observed weak CT band of Eu³⁺-O²⁻ in the excitation spectra of BaZr(BO₃)₂:Eu³⁺. The emission results show that Si⁴⁺ can sensitize luminescence in the host of BaZr(BO₃)₂:Eu but Ti⁴⁺ has no improvement effect on luminescence.     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Effect of concentration on the luminescence of Eu ions in nanocrystalline La2O3

Nanocrystalline powders with various Eu³⁺ concentration (from 1 to 10 mol %) doped La₂O₃ were prepared via a combustion route. Their structure and morphology were characterized using X-ray diffraction (XRD) and High-resolution transmission electron microscopy. The emission spectra of the as-synthesized samples show that the strongest emission position is centered at 626 nm corresponding to ⁵D₀→⁷F₂ transition of Eu³⁺ ions and the intensity change of 626 nm emission is considered as a function of ultraviolet (240 nm) irradiation time. The excitation spectra at 626 nm monitoring indicate that the charge transfer state band is varies with different Eu³⁺ ion concentration. These results are attributed to the surface defects of the nanocrystals.     

Novel green-emitting Na2CaPO4F:Eu phosphors for near-ultraviolet white light-emitting diodes

In this study, green-emitting Na₂CaPO₄F:Eu²⁺ phosphors were synthesized by solid-state reactions. The excitation spectra of the phosphors showed a broad hump between 250 and 450 nm; the spectra match well with the near-ultraviolet (NUV) emission spectra of light-emitting diodes (LEDs). The emission spectrum showed an intense broad emission band centered at 506 nm. White LEDs were fabricated by integrating a 390 nm NUV chip comprising blue-emitting BaMgAl₁₀O₁₇:Eu²⁺, green-emitting Na₂CaPO₄F:0.02 Eu²⁺, and red-emitting CaAlSiN₃:Eu²⁺ phosphors into a single package; the white LEDs exhibited white light with a correlated color temperature of 5540 K, a color-rendering index of 90.75, and color coordinates (0.332, 0.365) close to those of ideal white light.     

Emission analysis of Pr and Tm: Ca2Gd2W3O14 phosphors

5 mol% of Pr³⁺ and Tm³⁺ ions activated calcium gadolinium tungstate (Ca₂Gd₂W₃O₁₄₎ phosphors were synthesized by traditional solid state reaction method. Crystalline phase structure was identified from the X-ray diffraction (XRD) profiles. From the scanning electron microscopy (SEM) images, we have observed the agglomeration of the particles, and average grain size is around 40-300 nm. Using the energy dispersive X-ray analysis (EDAX) and Fourier transform infrared (FTIR) spectra, identified the elements and functional groups present in the prepared phosphors. The emission spectrum of Pr³⁺: Ca₂Gd₂W₃O₁₄ powder phosphors have shown an intense red emission at 615 nm with the excitation wavelength λ_exci=450 nm and thus these red color emitting powder phosphors are used as one of the components in the preparation of WLEDs. The excitation spectrum of Tm³⁺: Ca₂Gd₂W₃O₁₄ powder phosphor has shown a ligand to metal charge transfer (W-O) band (LMCT) within the WO₄²⁻ group. Emission spectrum of Tm³⁺: Ca₂Gd₂W₃O₁₄ phosphors have shown blue emissions at 453 nm (¹D₂→³F₄).     

Luminescence in trivalent rare earth activated Sr4Al2O7 phosphor

In this paper we report the combustion synthesis of trivalent rare-earth (RE³⁺ = Dy, Eu and Ce) activated Sr₄Al₂O₇ phosphor. The prepared phosphors were characterized by the X-ray powder diffraction (XRD) and photoluminescence (PL) techniques. Photoluminescence emission peaks of Sr₄Al₂O₇:Dy³⁺ phosphor at 474 nm and 578 nm in the blue and yellow region of the spectrum. The prepared Eu³⁺ doped phosphors were excited by 395 nm then we found that the characteristics emission of europium ions at 615 nm (⁵D₀?⁷F₂) and 592 nm (⁵D₀?⁷F₁). Photoluminescence (PL) peaks situated at wavelengths of 363 and 378 nm in the UV region under excitation at around 326 nm in the Sr₄Al₂O₇:Ce³⁺ phosphor.     

Color improvement of white-light through Mn-enhancing yellow-green emission of SrSi2O2N2:Eu phosphor for white light emitting diodes

Photoluminescence (PL) enhancement of SrSi₂O₂N₂:Eu and the resultant color improvement of white-light were investigated via co-doping Mn with Eu. We observed that a unique absorption of host lattice of SrSi₂O₂N₂ and its visible band emission peaked at around ∼550 nm for SrSi₂O₂N₂:Mn²⁺ in the wavelength range of 450-600 nm. This highly eye-sensitive ∼550 nm-peaked band emission of SrSi₂O₂N₂ doped with Mn²⁺ happens to overlap the 535 nm-peaked band emission of SrSi₂O₂N₂ doped with Eu²⁺, resulting in an intensified photoluminescence in a maximum by 355%. By combining this as-prepared Mn intensified SrSi₂O₂N₂:Eu phosphor with blue InGaN chip, the quality of white-light was improved to 93.3% for color rendering index and 3584 K for correlated color temperature.     

Characterization of host-lattice emission and energy transfer in BaMgAl10O17:Eu

Host-lattice emission, energy transfer and degradation processes are characterized in undoped and Eu-doped BaMgAl₁₀O₁₇. Undoped BaMgAl₁₀O₁₇ exhibits a broad emission centered at 265 nm when excited at wavelengths shorter than 190 nm. This emission is assigned to exciton recombination at Ba-O groups in the cation layer of the lattice. The emission exhibits excellent overlap with the excitation band of Eu²⁺ in this host, providing a means of host-to-activator energy transfer in the doped phosphor. The exciton emission is relatively stable to thermal damage, but undergoes a peak shift and significant decrease in intensity after exposure to VUV radiation. Heating of VUV-damaged materials in air leads to some repair of the spectral properties.     

Synthesis and luminescence properties of Eu, Ce and Tb-activated Sr3La2(BO3)4 under UV excitation

The spectroscopic properties in UV-excitable range for the phosphors of Sr₃La₂(BO₃)₄:RE³⁺ (RE³⁺=Eu³⁺, Ce³⁺, Tb³⁺) were investigated. The phosphors were synthesized by conventional solid-state reactions. The photoluminescence (PL) spectra and commission international de I'Eclairage (CIE) coordinates of Sr₃La₂(BO₃)₄:RE³⁺ were investigated. The f-d transitions of Eu³⁺, Ce³⁺ and Tb³⁺ in the host lattices are assumed and corroborated. The PL and PL excitation (PLE) spectra indicate that the main emission wavelength of Sr₃La₂(BO₃)₄:Eu³⁺ is 611 nm, and Sr₃La₂(BO₃)₄:Ce³⁺ shows dominating emission peak at 425 nm, while Sr₃La₂(BO₃)₄:Tb³⁺ displays green emission at 487, 542, 582 and 620 nm. These phosphors were prepared by simple solid-state reaction at 1000 °C. There are lower reactive temperature and more convenient than commercial phosphors. The Sr₃La₂(BO₃)₄:Tb³⁺ applied to cold cathode fluorescent lamp was found to emit green light and have a major peak wavelength at around 542 nm. These phosphors may provide a new kind of luminescent materials under ultraviolet excitation.     

KBaPO4:Ln (Ln=Eu, Tb, Sm) phosphors for UV excitable white light-emitting diodes

This letter reports the novel three emission bands based on phosphate host matrix, KBaPO₄ doped with Eu²⁺, Tb³⁺, and Sm³⁺ for white light-emitting diodes (LEDs). The phosphors were synthesized by solid-state reaction and thermal stability was elucidated by measuring photoluminescence at higher temperatures. Eu²⁺-doped KBaPO₄ phosphor emits blue luminescence with a peak wavelength at 420 nm under maximum near-ultraviolet excitation of 360 nm. Tb³⁺-doped KBaPO₄ phosphor emits green luminescence with a peak wavelength at 540 nm under maximum near-ultraviolet excitation of 370 nm. Sm³⁺-doped KBaPO₄ phosphor emits orange-red luminescence with a peak wavelength at 594 nm under maximum near-ultraviolet excitation of 400 nm. The thermal stabilities of KBaPO₄:Ln (Ln=Eu²⁺, Tb³⁺, Sm³⁺), in comparison to commercially available YAG:Ce³⁺ phosphor were found to be higher in a wide temperature range of 25-300 °C.     

Sol-gel growth of Gd2MoO6:Eu nanocrystalline layers on SiO2 spheres (SiO2@Gd2MoO6:Eu) and their luminescent properties

SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors. The XRD results demonstrate that the Gd₂MoO₆:Eu³⁺ layers on the SiO₂ spheres begin to crystallize after annealing at 600 °C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 600 nm), are not agglomerated, and have a smooth surface. The thickness of the Gd₂MoO₆:Eu³⁺ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀-⁷F₂ red emission at 613 nm) under the excitation of 307 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles.     

Spectroscopic properties of OH doped and Bi-rich Bi4Ge3O12 crystals by vertical Bridgman method

OH⁻ doped and Bi-rich Bi₄Ge₃O₁₂ (BGO) single crystals were grown by Vertical Bridgman (VB) method. The structure of these crystals was determined by XRD, and the emission spectra in visible and near infrared region (NIR) were measured at room temperature. The emission spectrum of Bi-rich BGO has extra peaks at 385, 367 and 357 nm, Bi-rich BGO after annealing in Ar at 500 °C for 5 h shows a significant emission band peaking around 1170 nm under 808 nm laser diodes (LDs) excitation, and OH⁻ doped BGO shows a noticeable emission band centered at about 1346 nm under 980 nm LDs excitation. A brief discussion is presented.     

Near-infrared luminescence of OH and Cl doped Bi4Ge3O12 crystals

OH⁻ and Cl⁻ doped Bi₄Ge₃O₁₂ (BGO) single crystals had been grown by Vertical Bridgman (VB) method. The structure of these crystals was determined by XRD, the transmittance and emission spectra in near infrared region (NIR) were measured at room temperature. 5% OH⁻ doped BGO shows a significant emission band peaking around 1181 nm under 808 nm laser diodes (LDs) excitation, and the 5% Cl⁻ doped BGO exhibits a relatively weak emission band as well. 100% and 5% OH⁻ doped BGO show noticeable emission band centered at about 1346 nm under 980 nm LDs excitation.     

Photoluminescence properties and effects of dopant concentration in Bi2ZnB2O7:Tb phosphor

A novel green phosphor, Tb³⁺ doped Bi₂ZnB₂O₇ was synthesized by conventional solid state reaction method. The phase of synthesized materials was determined using the XRD, DTA/TG and FTIR. The photoluminescence characteristics were investigated using spectrofluorometer at room temperature. Bi₂ZnB₂O₇:Tb³⁺ phosphors excited by 270 nm and 485 nm wavelengths. The emission spectra were composed of three bands, in which the dominated emission of green luminescence Bi₂ZnB₂O₇:Tb³⁺ attributed to the transition ⁵D₄ → ⁷F₅ is centered at 546 nm. The dependence of the emission intensity on the Tb³⁺ concentration for the Bi_2−xTb_xZnB₂O₇ (0.01 ≤ x ≤ 0.15) was studied and observed that the optimum concentration of Tb³⁺ in phosphor was 13 mol% for the highest emission intensity at 546 nm.     

Synthesis and luminescence properties of nanocrystalline Gd2O3:Eu by combustion process

The nanocrystalline Gd₂O₃:Eu³⁺ powders with cubic phase were prepared by a combustion method in the presence of urea and glycol. The effects of the annealing temperature on the crystallization and luminescence properties were studied. The results of XRD show pure phase can be obtained, the average crystallite size could be calculated as 7, 8, 15, and 23 nm for the precursor and samples annealed at 600, 700 and 800 °C, respectively, which coincided with the results from TEM images. The emission intensity, host absorption and charge transfer band intensity increased with increasing the temperature. The slightly broad emission peak at 610 nm for smaller particles can be observed. The ratio of host absorption to O²⁻-Eu³⁺ charge transfer band of smaller nanoparticles is much stronger compared with that for larger nanoparticles, furthermore, the luminescence lifetimes of nanoparticles increased with increasing particles size. The effects of doping concentration of Eu³⁺ on luminescence lifetimes and intensities were also discussed. The samples exhibited a higher quenching concentration of Eu³⁺, and luminescence lifetimes of nanoparticles are related to annealing temperature of samples and the doping concentration of Eu³⁺ ions.     

Effects of dopant concentrations on phosphorescence properties of Eu/Dy-doped Sr3MgSi2O8

Long afterglow Sr₃MgSi₂O₈: Eu, Dy phosphor with high brightness was prepared by sintering at high temperature and weak reductive atmosphere. The luminescent properties of this photoluminescent pigment were studied systematically by investigating concentration effects. The analytical results indicated that the main emission peaks appear at 482 nm. The excitation and emission spectra of this phosphor show that both of them are broadband. This is ascribed to the 4f⁷→4f⁶5d¹ transition of Eu²⁺ in the pigment matrix, which is in good agreement with the calculated value of 470 nm, and implies that luminescent centers Eu²⁺ occupy the deca-coordinated Sr²⁺ sites with the host of Sr₃MgSi₂O₈.     

Concentration effect of Tm on cathodoluminescence properties of SiO2:Tm and SiO2:Ho,Tm systems

Cathodoluminescence (CL) properties of SiO₂ powders activated with thulium (Tm³⁺) and holmium (Ho³⁺) ions prepared by a sol–gel process were investigated. Different molar concentrations of Tm³⁺ co-doped with Ho³⁺ were studied. The 460 nm peak was monitored and the influence of the beam energy and concentration of Tm³⁺ ions on the emission properties of this peak was also monitored. The peculiar behavior whereby the 460 nm emission peak decreases and the increase in the 705 and 865 nm peaks with the increase in the concentration of Tm³⁺ ions is reported. The relationship between the accelerating beam voltage and the CL intensity of the blue emission peak (460 nm peak) is established. Morphology, particle size and optical properties were characterized with Scanning electron microscopy (SEM), UV/VIS Lambda 750 S spectrometer and Auger electron spectroscopy (AES) equipped with Ocean Optics S2000, respectively.     

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⁺.     

Multiplication of electronic excitations in nanophosphors Lu2O3:Eu and Lu2O3:Tb

Luminescence properties of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanocrystalline powders with the particle size varying from 46 to 6 nm were studied under excitation by synchrotron radiation in the photon energy range (up to ∼22.5 eV) covering the region where the processes of multiplication of electronic excitation occur. It was found that the excitation spectra of Tb³⁺ emission from all Lu₂O₃:Tb³⁺ nanopowders have similar behavior, whereas the shape of the excitation spectra of Eu³⁺ emission from Lu₂O₃:Eu³⁺ nanopowders strongly depends on the particle size. The difference in the behavior of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanophosphor systems was explained by different mechanisms of the energy transfer from the host to Eu³⁺ or Tb³⁺ ions (either the hole or electron recombination mechanism, respectively), which are differently influenced by losses of electronic excitations near the particle surface.