Synthesis and luminescence properties of highly uniform SiO2@LaPO4:Eu3+ core–shell phosphors

SiO₂@LaPO₄:Eu³⁺ core–shell phosphors have been successfully synthesized by a one-step and economical wet-chemical route at low temperature. The as-obtained products were characterized by means of photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). The SEM, EDS and XPS analysis indicate that SiO₂@LaPO₄:Eu³⁺ core–shell phosphors can only be synthesized in a pH range of 8–11 and the possible mechanism has been proposed. The XRD results demonstrate that the structure of LaPO₄:Eu³⁺ layers is transferred into monoclinic phase from hexagonal phase after annealing at 800 °C for 2 h. The SiO₂@LaPO₄:Eu³⁺ phosphors show strong orange–red luminescence under ultraviolet excitation. The relative emission intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles, and the optimum concentration for Eu³⁺ was determined to be 5 mol% of La³⁺ in SiO₂@LaPO₄ phosphors.     

Luminescent properties of Sr2Al2SiO7:Ce3+,Eu2+ phosphors for near UV-excited white light-emitting diodes

The Sr₂Al₂SiO₇:Eu²⁺, Ce³⁺ phosphors were synthesized by a high temperature solid-state reaction. Effective energy transfer occurs in Ce³⁺ and Eu²⁺ co-doped Sr₂Al₂SiO₇ due to large spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺ ions. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺ ions. The critical distance has been estimated to be about 1.83 nm by spectral overlap method. Furthermore, the developed phosphors can generate lights from blue to green region under the excitation of UV radiation by appropriately tuning the activator content. The Sr₂Al₂SiO₇:Eu²⁺,Ce³⁺ phosphors are promising phosphors for warm-white-light-emitting diode because of its effective excitation in the near ultraviolet range.     

Synthesis and luminescence properties of monodisperse SiO2@SiO2:Eu3+ microspheres

Monodisperse core–shell structured SiO₂@SiO₂:Eu³⁺ microspheres were synthesized in a seeded growth way. In that way, a thin shell of Eu³⁺-doped silica was grown on the prepared monodisperse silica colloids. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR), thermal analysis (TGA-DSC) and photoluminescence (PL) spectroscopy. The results reveal that the SiO₂ spheres have been successfully coated by SiO₂:Eu³⁺ phosphors and the obtained SiO₂@SiO₂:Eu³⁺ particles have perfect spherical shape with narrow size distribution. Additionally, the monodisperse SiO₂@SiO₂:Eu³⁺ microspheres exhibit considerably strong photoluminescence (PL) of Eu³⁺ under the excitation of 393 nm compared with the SiO₂:Eu³⁺ samples with polydispersed or irregular shapes and sizes obtained by base-catalyzed Stöber method. Furthermore, the PL intensity increases with the increasing of Eu³⁺ concentration in SiO₂ microspheres shell, and concentration quenching occurs when Eu³⁺ concentration exceeds 5.0 mol%.     

Color tunable Sr2SiO4:Eu2+ phosphors through the modification of crystal structure

Eu²⁺ doped Sr₂SiO₄ phosphors were prepared through a solid-state reaction method. The phase-composition and photoluminescence of the obtained phosphors were systematically studied in terms of calcination temperature, Eu and Ba doping. High calcination temperature promoted the phase transformation from α′–Sr₂SiO₄ (orthorhombic) to β–Sr₂SiO₄ (monoclinic), while the doping of Eu or Ba ions could stabilize α′–Sr₂SiO₄ phase due to their long bond length with oxygen. Small amount of Eu/Ba doping prefers to occupy Sr(I) sites in the crystal lattice of Sr₂SiO₄, acting as nucleation sites for both α′– and β–Sr₂SiO₄ phases. After nucleation, Eu²⁺ ions distribute equally in the two sites. Through structural modification, the Sr₂SiO₄:Eu²⁺ phosphors could be controlled to emit different colors in a wide range, from blue to yellow, making them good candidates for tuning the chromaticity in application.     

Photoluminescence spectra tuning of Eu2+ activated orthosilicate phosphors used for white light emitting diodes

Divalent europium activated alkaline earth orthosilicate M₂SiO₄ (M = Ba, Sr, Ca) phosphors were synthesized through solid-state reaction technique and their luminescent properties were investigated. Photoluminescence emission spectra of Sr₂SiO₄:Eu²⁺ phosphor was tuned by substitution of Sr²⁺ with 10 mol% Ca²⁺ or Mg²⁺. Two emission bands originated from the 4f–5d transition of Eu²⁺ ion doped into different cation sites in the M₂SiO₄ host lattice were observed under ultraviolet excitation. The Sr₂SiO₄:Eu²⁺ phosphor showed a blue and a green broad emission bands peaked around 475 and 555 nm with some variation for different Eu²⁺ doping concentration. When 10 mol% of Sr²⁺ was substituted by Ca²⁺ or Mg²⁺, the blue emission band blue-shifted to 460 nm and the green emission band shifted to even longer wavelength. An energy loss due to energy transfer from one Eu²⁺ to another Eu²⁺ ion, changing of the crystal field strength and covalence in the host lattice together were assigned for the tuning effect. With an overview of the excitation spectra and the emission spectra in blue and green-yellow color, these co-doped phosphors can become a promising phosphor candidate for white light-emitting-diodes (LEDs) pumped by ultraviolet chip.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Abstract

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Dependence of optical properties on the composition of (Ba1−x−ySrxEuy)Si2O2N2 phosphors for white light emitting diodes

BaSi₂O₂N₂: Eu²⁺ is an efficient phosphor because of its high quantum yield and quenching temperature. Partial substitution of Ba²⁺ by Sr²⁺ is the most promising approach to tune the color of phosphors. In this study, a series of (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ (x = 0.0–0.97, y = 0.00–0.10) phosphors are synthesized via high-temperature solid-state reactions. Intense green to yellow phosphors can be obtained by the partial substitution of the host lattice cation Ba²⁺ by either Sr²⁺ or Eu²⁺. The luminescent properties and the relationships among the lowest 5d absorption bands, Stokes shifts, centroid shifts, and the splitting of Eu²⁺ are studied systematically. Then, based on (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors and near-ultraviolet (∼395 nm)/blue (460 nm) InGaN chips, intense green–yellow light emitting diodes (LEDs) and white LEDs are fabricated. (Ba_0.37Sr_0.60)Si₂O₂N₂: 0.03Eu²⁺ phosphors present the highest efficiency, and the luminous efficiency of white LEDs can reach 17 lm/w. These results indicate that (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors are promising candidates for solid-state lighting.     

Luminescence enhancement of Tb3+ in Sr3SiO5:Eu2+ phosphor

Sr₃SiO₅ phosphors co-doped with Eu²⁺ and Tb³⁺ were prepared by a conventional solid-state reaction method. The prepared Sr₃SiO₅:Eu²⁺,Tb³⁺,Li⁺ phosphors had characteristic luminescent spectra excited under near-UV excitation in which both the broadband spectrum assigned to Eu²⁺ and the line spectrum assigned to Tb³⁺ are observed, although Tb³⁺ is inactive with this photon energy in general. For Eu²⁺–Tb³⁺ codoped Sr₃SiO₅, energy transfer process takes place and the mechanism is ascribed to the overlap between the shorter Eu²⁺ luminescence band from the Sr₃SiO₅ crystal structure with two Sr sites and ₅D⁴ energy level of Tb³⁺ ion. Due to the energy transfer, PL intensity of Eu²⁺ emission increased about 26 %. We suggest that this enhancement mechanism could shed light on the potential applications in white light-emitting diodes excited by near-UV light. In addition, the emission peak position near the orange region indicates that our system is a step towards a new class of wavelength sources for artificial lighting with improved PL intensity and lower energy consumption.     

Effects of RE3+ as a co-dopant in blue-emitting long-lasting phosphors, Sr3Al10SiO20:Eu2+

A series of novel long-lasting phosphors, Sr₃Al₁₀SiO₂₀:Eu²⁺,RE³⁺, were prepared and studied. Under UV irradiation, broad-band emission long-lasting phosphorescence located at 466 nm was observed in all of these phosphors at room temperature. The effects of RE³⁺ as a co-dopant in Sr₃Al₁₀SiO₂₀:Eu²⁺ were discussed in conjunction with the afterglow decay curves and thermoluminescence (TL) spectra. Quantitative TL spectra revealed that the introduction of RE³⁺ ions into Sr₃Al₁₀SiO₂₀:Eu²⁺ host produces a highly dense trapping level at appropriate depth (335 K), which is considered to be responsible for the long-lasting phosphorescence at room temperature.     

Luminescent properties of Sr2P2O7:Eu,Mn phosphor under near UV excitation

The phosphors in the system Sr_2−x−yP₂O₇:xEu²⁺,yMn²⁺ were synthesized by solid-state reactions and their photoluminescence properties were investigated. These phosphors have strong absorption in the near UV region, which is suitable for excitation of ultraviolet light emitting diodes (UVLEDs). The orange-reddish emission of Mn²⁺ in these phosphors can be used as a red component in the tri-color system and may be enhanced by adjusting the Mn²⁺/Eu²⁺ ratio. The energy transfer from Eu²⁺ to Mn²⁺ is observed with a transfer efficiency of ∼0.45 and a critical distance of ∼10 Å. The results reveal that Sr_2−x−yP₂O₇:xEu²⁺,yMn²⁺ phosphors could be used in white light UVLEDs.     

Nanorods of white light emitting Sr2SiO4:Eu2+: microemulsion-based synthesis,EPR, photoluminescence,and thermoluminescence studies

White light emitting Sr₂SiO₄:Eu²⁺ nanoparticles were prepared using reverse micellar route using Tergitol as a surfactant. The systems were characterised by X-ray diffraction, scanning electron microscopy (SEM), photoluminescence, thermoluminescence (TL), and electron paramagnetic resonance (EPR) spectroscopy. SEM shows the formation of silicate nanorods. Two emission bands of bluish-green at 490 nm (S(I)) and of orange-red at 605 nm (S(II)) were observed. The two emission bands are assigned to the 4f–5d transition of Eu²⁺ ions in two different cation sites in α′-Sr₂SiO₄ orthorhombic lattices. Gamma-irradiated Sr₂SiO₄:Eu showed the presence of three TL glow peaks at 437, 487 K and weak peak at 540 K; however, no glow was observed in the undoped sample. Reduction of Eu³⁺ to Eu²⁺ is confirmed by EPR spectroscopy.     

Sol-gel composition of multicomponent hybrid precursors to long afterglow of CaxSr1 − xAl2O4: Eu phosphors

Ca_xSr_1 − xAl₂O₄: Eu²⁺ photoluminescent materials with high brightness and long afterglow were in situ synthesized by hybrid precursor assembly sol-gel technology in a reductive atmosphere. The particle size of luminescent materials is in the range of 30-60 nm by the estimation of XRD. And SEM shows that there exists uniform morphology and microstructure owing to the hybrid precursors. The influence of co-doping Ca²⁺ and Sr²⁺ on the luminescence of the phosphor was studied. Their excitation and emission spectra were very similar to that of SrAl₂O₄: Eu²⁺ phosphors and all of them have long afterglow phenomenon. Changing the co-doping concentrations of Ca²⁺ and Sr²⁺ in Ca_xSr_1 − xAl₂O₄: Eu²⁺ phosphors, the luminescent intensities are different. When the proportion of Ca and Sr is 6 to 4, the phosphor reaches the strongest emitting intensity.     

Green emission enhancement of Ba2SiO4:Eu2+ phosphor via Gd co-doping and charge compensation

A series of new green-emitting Ba_2?x?2ySiO₄:xEu²⁺, yGd³⁺, yR⁺ (R = Li, Na or K) phosphors were synthesized by the solid-reaction method. X-ray diffraction (XRD) and fluorescence spectrophotometer are utilized to characterize the crystal structure and luminescence properties of the as-synthesized phosphors, respectively. The XRD patterns reveal that the doping of Gd³⁺, Eu²⁺ and R⁺ ions have no significant influence on the Ba₂SiO₄ phase. The green emission of Eu²⁺ ion associated with 4f⁶5d¹ → 4f⁷ can be obtained by 396 nm UV excitation source, which match well with the emission wavelength of UV-LEDs chip (380–420 nm). Moreover, the effect of charge compensator ions (Li⁺, Na⁺ or K⁺) on the luminescence intensity of (Ba, Gd)₂SiO₄:Eu²⁺ phosphors were also investigated. When introducing the Li⁺ ions into the (Ba, Gd)₂SiO₄ host lattices, the as-prepared phosphors show the strongest emission. The emission intensity of Ba_1.95SiO₄:0.04Eu²⁺, 0.005Gd³⁺, 0.005Li⁺ is about 1.39 times than that of Ba_1.96SiO₄:0.04Eu²⁺. Furthermore, the mechanism of energy transfer and concentration quenching of Ba_1.982?xSiO₄:xEu²⁺, 0.009Gd³⁺, 0.009Li⁺ phosphors are also discussed.     

Single phased Sr<Subscript>3</Subscript>La(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>:Eu<Superscript>2+</Superscript>/Mn<Superscript>2+</Superscript> phosphors: solid state synthesis,tunable luminescence and potential applications in white light LEDs

A series of Sr₃La(PO₄)₃:Eu²⁺/Mn²⁺ phosphors were synthesized by a solid state reaction. The phase and the optical properties of the synthesized phosphors were investigated. The XRD results indicate that the doped Eu²⁺ and Mn²⁺ ions do not change the phase of Sr₃La(PO₄)₃. The peak wavelengths of Eu²⁺ single doped and Eu²⁺/Mn²⁺ codoped Sr₃La(PO₄)₃ phosphors shift to longer wavelength due to the larger crystal field splitting for Eu²⁺ and Mn²⁺. The increases of crystal field splitting for Eu²⁺ and Mn²⁺ are induced by the substitution of Sr²⁺ by Eu²⁺ and Mn²⁺ in Sr₃La(PO₄)₃ host. Due to energy transfer from Eu²⁺ to Mn²⁺ in Sr₃La(PO₄)₃:Eu²⁺/Mn²⁺ phosphors, tunable luminescence was obtained by changing the concentration of Mn²⁺. And the white light was emitted by Sr₃La(PO₄)₃:3.0 mol%Eu²⁺/4.0 mol%Mn²⁺ and Sr₃La(PO₄)₃:3.0 mol%Eu²⁺/5.0 mol%Mn²⁺ phosphors.     

Luminescence and afterglow in Sr2SiO4:Eu, RE [RE = Ce, Nd, Sm and Dy] phosphors—Role of co-dopants in search for afterglow

Luminescence of Eu²⁺ in Sr₂SiO₄:Eu²⁺, RE³⁺ [RE = Ce, Nd, Sm and Dy] phosphors was studied with a view to obtain an afterglow phosphor. The synthesized phosphors were characterized by powder X-ray diffraction (XRD), diffuse reflectance, photo- and thermoluminescence spectroscopic techniques. Afterglow was observed only with Dy³⁺ co-doped phosphor. The observed afterglow with Dy³⁺ co-doping originated from the formation of suitable traps which was supported by thermoluminescence results.     

Preparation and photoluminescence properties of Sr4Al14O25:Eu2+ phosphors synthesized via the microemulsion route

Strontium aluminate phosphors doped with europium ions (Sr₄Al₁₄O₂₅:Eu²⁺) were successfully synthesized via the microemulsion route. In comparison with the traditional solid-state reaction process, the calcination temperature of Sr₄Al₁₄O₂₅:Eu²⁺ phase in this study was lowered to 1,100 °C when the flux was added. In addition, the particle size of Sr₄Al₁₄O₂₅:Eu²⁺ phosphors prepared via the microemulsion route was greatly reduced to 50 nm. The lowered synthesis temperature and reduced particle size are attributed to nano-scaled micelles formed in the microemulsion system. The emission and excitation intensity of Sr₄Al₁₄O₂₅:Eu²⁺ phosphors were increased with an increase in the synthesis temperature. In addition, the rise in the calcination temperature lowered the afterglow characteristics of Sr₄Al₁₄O₂₅:Eu²⁺ phosphors. The microemulsion route was demonstrated to be an more effective process than the solid-state reaction process for preparing Sr₄Al₁₄O₂₅:Eu²⁺ phosphors.     

Synthesis of high intensity green emitting (Ba,Sr)SiO4:Eu2+ phosphors through cellulose assisted liquid phase precursor process

Green emitting phosphor (Ba_1−x,Sr_1−x)SiO₄:2xEu²⁺, x = 0.03, 0.05, 0.1, and 0.15 were synthesized through a Liquid Phase Precursor Process (LPP). Liquid phase precursor method is reported to result in phosphors with markedly increased emission intensities compared to other synthesis methods. Here microcrystalline cellulose (MCC) and hydroxypropyl cellulose (HPC) templates were studied to achieve high efficiency green phosphors. The phase formation was investigated by XRD analysis which showed the conformation of the Ba₂SiO₄ (JCPDS card number 761631) phase. The obtained samples exhibited broad excitation spectra with maximum at 430 nm and a green emission centered around 520 nm. An optimized dopant concentration was selected and the effect of two different types of cellulose, i.e. MCC and HPC templates on the emission properties was considered. It was found that the samples synthesized using HPC and fired at 1050 °C under a reducing atmosphere, showed a high intensity of almost 2 times that of the MCC sample. Further experiments were conducted by varying viscosity, particle weight, and molecular weight of the HPC template. It was found that the green emission from the (Ba,Sr)SiO₄:Eu²⁺ increased with the increase in viscosity and molecular weight of the template.     

Influence of calcination temperature on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol–gel-combustion processing

The detailed preparation process of Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ phosphor powders with red long afterglow by sol–gel-combustion method in the reducing atmosphere is reported. X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the effects of synthesis temperature on the crystal characteristics, morphology and luminescent properties of the as-synthesized Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors. The results reveal that Sr₃Al₂O₆ crystallizes completely when the combustion ash is sintered at 1200 °C. The excitation and the emission spectra indicate that the excitation broad-band lies chiefly in visible range and the phosphor powders emit strong light at 618 nm under the excitation of 472 nm. The light intensity and the light-lasting time of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors are increased when increasing the calcination temperatures from 1050 to 1200 °C. The afterglow of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors sintered at 1200 °C lasts for over 600 s when the excited source is cut off. The red emission mechanism is discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions.     

Characterization of various Eu2+ sites in Ca2SiO4:Eu2+ and Ba2SiO4:Eu2+ by high-pressure spectroscopy

Photoluminescence of Ba₂SiO₄ and Ca₂SiO₄ activated with Eu²⁺ was investigated at various temperatures (from 10 K to 300 K) and pressures (from ambient to 200 kbar). At ambient pressure and room temperature, under UV excitation both phosphors yielded a green emission band with maxima at 505 nm and 510 nm for Ba₂SiO₄ and Ca₂SiO₄, respectively. The energies of these bands depended on pressure; the pressure shifts were ?12:55 cm^?1/kbar for Ba₂SiO₄:Eu²⁺; and ?5:59 cm^?1/kbar for Ca₂SiO₄:Eu²⁺. In the case of Ca₂SiO₄:Eu²⁺, we observed additional broadband emission at lower energies with a maximum at 610 nm (orange band). The orange and green emission in Ca₂SiO₄:Eu²⁺ had different excitation spectra: the green band could be excited at wavelengths shorter than 470 nm, whereas the orange band — at wavelengths shorter than 520 nm. The pressure caused a red shift of orange emission of 7.83 cm^?1/kbar. The emission peaked at 510 nm was attributed to the 4f⁶5d→4f⁷(⁸S₇₌₂) transition of Eu²⁺ in the β — Ca₂SiO₄:Eu²⁺ phase, whereas the emission peaked at 610 nm — to the γ — Ca₂SiO₄:Eu²⁺ phase. The emission of Ba₂SiO₄:Eu²⁺ peaked at 505 nm was attributed to the 4f⁶5d→ 4f⁷(⁸S_7/2) transition of Eu²⁺ in the β — Ba₂SiO₄ phase.