Crystallinity and morphology dependent luminescence of Li-doped Y2−xGdxO3:Eu thin film phosphors

The influence of lithium doping on the crystallization, the surface morphology, and the luminescent properties of pulsed laser deposited Y_2−xGd_xO₃:Eu³⁺ thin film phosphors was investigated. The crystallinity, the surface morphology, and the photoluminescence (PL) of films depended highly on the Li-doping and the Gd content. The relationship between the crystalline and morphological structures and the luminescent properties was studied, and Li⁺ doping was found to effectively enhance not only the crystallinity but also the luminescent brightness of Y_2−xGd_xO₃:Eu³⁺ thin films. In particular, the incorporation of Li and Gd into the Y₂O₃ lattice could induce remarkable increase in the PL. The highest emission intensity was observed Li-doped Y_1.35Gd_0.6O₃:Eu³⁺ thin films whose brightness was increased by a factor of 4.6 in comparison with that of Li-doped Y₂O₃:Eu³⁺ thin films.     

Improved luminescent properties of red-emitting Ca0.54Sr0.16Eu0.08Gd0.12(MoO4)0.2(WO4)0.8 phosphor for LED application by charge compensation

The red-emitting Ca_0.54Sr_0.16Eu_0.08Gd_0.12(MoO₄)_0.2(WO₄)_0.8 phosphor is improved in the emission charateristics by charge compensation, of which chromaticity coordinates (CIE) are x=0.66 and y=0.33. Three approaches to charge compensation are investigated, namely (a) 3Ca²⁺/Sr²⁺→2Eu³⁺/Gd³⁺+vacancy, (b) 2Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+M⁺(M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ employed as a charge compensator) and (c) Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+N⁻ (N⁻ is a monovalent anion like F⁻, Cl⁻, Br⁻ and I⁻ employed as charge compensation ions). Through photoluminescent spectra analyzing the radiative and non-radiative relaxation mechanisms of luminescent system are obtained. Under 20 mA forward-bias current, one red-emitting LED is made by combining 390-405 nm-emitting LED chip and the phosphor. The red-emitting phosphor has broad prospects in LED application field.     

Highly enhanced photoluminescence and X-ray excited luminescence of Li doped Gd2O3:Eu thin films

Transparent Li-doped Gd₂O₃:Eu³⁺ thin-film phosphors were prepared by a modified sol-gel method. The effect of the Li⁺ ions on luminescent properties of the thin film was investigated. The results indicated that incorporation of Li⁺ ions into Gd₂O₃ lattice could result in a remarkable increase on photoluminescence or X-ray excited luminescence, and the strongest emission was observed from Gd_1.84Li_0.08Eu_0.08O_3−δ film, in which the intensity was increased by a factor of 1.9 or 2.3 in comparison with that of Gd_1.92Eu_0.08O₃ film. And it could be achieved the highest intensity for sintering the Gd_1.84Li_0.08Eu_0.08O_3−δ film at 700 °C. Such a temperature is much lower than the typical solid-state reaction temperature for its powder phosphors. This kind of transparent thin-film phosphors may promise for application to micro X-ray imaging system.     

Synthesis of Gd2Ti2O7:Eu, V phosphors by sol-gel process and its luminescent properties

A red-emitting phosphor material, Gd₂Ti₂O₇:Eu³⁺, V⁴⁺, by added vanadium ions is synthesized using the sol-gel method. Phosphor characterization by high-resolution transmission electron microscopy shows that the phosphor possesses a good crystalline structure, while scanning electron microscopy reveals a uniform phosphor particle size in the range of 230-270 nm. X-ray photon electron spectrum analysis demonstrates that the V⁴⁺ ion promotes an electron dipole transition of Gd₂Ti₂O₇:Eu³⁺ phosphors, causing a new red-emitting phenomenon, and CIE value shifts to x=0.63, y=0.34 (a purer red region) from x=0.57, y=0.33 (CIE of Gd₂Ti₂O₇:Eu³⁺). The optimal composition of the novel red-emitting phosphor is about 26% of V⁴⁺ ions while the material is calcinated at 800  °C. The results of electroluminescent property of the material by field emission experiment by CNT-contained cathode agreed well with that of photoluminescent analysis.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.     

Luminescence studies on ZnO-P2O5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Zinc phosphate glasses doped with Gd₂O₃:Eu nanoparticles and Eu₂O₃ were prepared by conventional melt-quench method and characterized for their luminescence properties. Binary ZnO-P₂O₅ glass is characterized by an intrinsic defect centre emission around 324 nm. Strong energy transfer from these defect centres to Eu³⁺ ions has been observed when Eu₂O₃ is incorporated in ZnO-P₂O₅ glasses. Lack of energy transfer from these defect centres to Eu³⁺ in Gd₂O₃:Eu nanoparticles doped ZnO-P₂O₅ glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between the luminescent centre and Eu³⁺ ions. Both doped and undoped glasses have the same glass transition temperature, suggesting that the phosphate network is not significantly affected by the Gd₂O₃:Eu nanoparticles or Eu₂O₃ incorporation.     

Morphology and crystalline-phase-dependent luminescence of Li-doped Gd2O3:Eu3+ thin films grown by pulsed laser deposition

Gd₂O₃:Eu³⁺ and Li-doped Gd₂O₃:Eu³⁺ luminescent thin films have been grown on Si(100) substrates using pulsed laser deposition. The films grown at different deposition conditions show different crystalline and morphology structures and luminescent characteristics. Although both cubic and monoclinic crystalline structures were observed in both Gd₂O₃:Eu³⁺ and Li-doped Gd₂O₃:Eu³⁺ films, the cubic structure becomes more dominant for Li-doped Gd₂O₃:Eu³⁺ films. The photoluminescence brightness data obtained from Li-doped Gd₂O₃:Eu³⁺ films indicate that Si(100) is a promising substrate for growth of high-quality Li-doped Gd₂O₃:Eu³⁺ thin-film red phosphor. In particular, the incorporation of Li⁺ ions into the Gd₂O₃ lattice induced a change of crystallinity and enhanced surface roughness. Two major factors to determine photoluminescence brightness for Li-doped Gd₂O₃:Eu³⁺ films were crystalline phase and surface roughness. The highest emission intensity was observed with Gd_1.84Li_0.08Eu_0.08O₃, whose brightness was a factor of 2.1 larger than that of Gd₂O₃:Eu³⁺ films. This phosphor is promising for applications in flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Red emission in B- and Li-doped SrAl2O4:Eu phosphor under UV excitation

Samples of SrAl₂O₄:Eu³⁺ doped with B³⁺ and SrAl₂O₄:Eu³⁺ co-doped with B³⁺ and Li⁺ have been prepared by the solid-reaction method. The influence of B³⁺ and Li⁺ contents on luminescence property has been investigated. It is found that the substitution of B³⁺ for Al³⁺ greatly improves red emission intensity at 591, 615 and 701 nm. The dopant Li⁺ as charge compensator in SrAl₂O₄:Eu³⁺, B³⁺ can further enhance luminescence intensity. The strongest red emission is obtained in the Sr(Al_1.9, B_0.1)O₄:Eu_0.02³⁺, Li⁺_0.02 sample. The developed phosphors can be efficiently excited by ultraviolet (UV) light from 350 to 480 nm, which indicates that B³⁺ and Li⁺ co-doped SrAl₂O₄:Eu³⁺ is a good candidate phosphor applied in solid-state lighting in conjunction with white UV light-emitting diodes (LEDs).     

Photoluminescence properties of Eu3+-doped Gd2MoB2O9 phosphors for LED-based solid-state-lighting

Investigation was performed on luminescent properties of novel Gd_2−x Eu _x MoB₂O₉ (0.02≤x≤2.0) phosphors. The excitation spectra consist of broad band and intense narrow lines. The 4f-4f transitions are situated in a favorable position for excitation by GaN chip emission. The emission spectra consist of transitions from the ⁵D₀ level to the lower ⁷F manifold, and the emission shows no concentration quenching at higher doping level. The decay time spectra of the ⁵D₀ → ⁷F₂ emission are recorded. Under 395 nm excitation, the intensity of ⁵D₀ → ⁷F₂ transition of GdEuMoB₂O₉ is 1.2 times stronger than that of commercial Eu³⁺:Y₂O₂S phosphor. Gd₂MoB₂O₉:Eu³⁺ phosphors are promising candidates for near-UV-based solid-state-lighting (SSL).     

White light emitting from single phased K2Ca1−x−yP2O7: xEu, yMn phosphors under UV excitation

A single phased white light emitting phosphors K₂Ca_1−x−yP₂O₇: xEu²⁺, yMn²⁺ were synthesized by solid state reaction method. The Effective energy transfer occurs in this phosphor due to the large spectral overlap between the emission of Eu²⁺ and the excitation of Mn²⁺. The emission hue of K₂Ca_1−x−yP₂O₇: xEu²⁺, yMn²⁺ from blue to white light can be obtained by tuning the Eu²⁺/Mn²⁺ content ratio. The energy transfer mechanism from Eu²⁺ to Mn²⁺ in this phosphor was carefully investigated and demonstrated to be via the dipole–quadrupole interaction.     

Synthesis and characterization of novel red emitting nanocrystal Gd6WO12:Eu phosphors

Novel nanosized Gd₆WO₁₂:Eu³⁺ phosphors were synthesized via a co-precipitation reaction. The crystal structure and morphology of the phosphors were characterized by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). It was found that the resultant powders show a regular and sphere-like shape with average particle size of 60 nm. Intrinsic red emission originating from Eu³⁺ was observed while excited at the W⁶⁺→O²⁻ and Eu³⁺→O²⁻ charge transfer bands or f-f absorption bands. The color coordinates of the phosphors were calculated to be x=0.625, y=0.375. The concentration dependence of the luminescence was studied, and optimum doping concentration for obtaining maximum emitting intensity was confirmed to be around 12 mol%. It was also found that the electric dipole-dipole interaction plays an important role for quenching luminescence of Eu³⁺.     

Synthesis and Eu concentration-dependent photoluminescence of Gd2−xEuxO3 nanowires

The nanowire growth behavior and photoluminescence characteristics of red-emitting oxide phosphor Gd_2−xEu_xO₃ have been investigated in the function of activator (Eu³⁺) concentrations (x=0.08, 0.12, 0.16, 0.20, and 0.24). Nanowires of Gd_2−xEu_xO₃ phosphor were prepared by the dehydration of corresponding hydroxides Gd_1−x/2Eu_x/2(OH)₃ obtained by the hydrothermal reaction. Highly uniform nanowires of 20-30 nm in diameter can grow up to several tens of micrometers in length. A number of defects on the surface of Gd_1.92Eu_0.08O₃ nanowires, which are induced during structural transformation from hexagonal hydroxide to cubic oxide, strongly decrease the luminescence efficiency in comparison with that of the bulk phosphor. In contrast, the photoemission intensity of nanowires is significantly improved with increasing Eu³⁺ content (x) of Gd_2−xEu_xO₃ solid solution. The highest relative emission intensity of nanowires is observed when the x value is close to x=0.20. This content is much higher than the optimal concentration of Eu³⁺ (x=0.08-0.10) for the bulk Gd₂O₃:Eu powder.     

Luminescence and energy transfer in Eu, Mn co-doped Li4SrCa(SiO4)2 for white light-emitting-diodes

Li₄(Sr_0.96Eu_0.04)(Ca_1 − xMn_x)(SiO₄)₂ phosphors were synthesized by solid-state reactions and photoluminescence (PL) properties were investigated. These phosphors have intense absorption in n-UV region, which is suitable for excitation of UV LEDs. The orange-reddish emission of Mn²⁺ can be adjusted by changing the Mn²⁺/Eu²⁺ ratio. Energy transfer from Eu²⁺ to Mn²⁺ is observed. Li₄(Sr_0.96Eu_0.04)(Ca_1 − xMn_x)(SiO₄)₂ phosphors could be used in white LEDs.     

Synthesis and electrochemical performances of Li4Ti4.95Al0.05O12/C as anode material for lithium-ion batteries

Spinel compounds Li₄Ti_5−xAl_xO₁₂/C (x=0, 0.05) were synthesized via solid state reaction in an Ar atmosphere, and the electrochemical properties were investigated by means of electronic conductivity, cyclic voltammetry, and charge-discharge tests at different discharge voltage ranges (0-2.5 V and 1-2.5 V). The results indicated that Al³⁺ doping of the compound did not affect the spinel structure but considerably improved the initial capacity and cycling performance, implying the spinel structure of Li₄Ti₅O₁₂ was more stable when Ti⁴⁺ was substituted by Al³⁺, and Al³⁺ doping was beneficial to the reversible intercalation and deintercalation of Li⁺. Al³⁺ doping improved the reversible capacity and cycling performance effectively especially when it was discharged to 0 V.     

Influence of Eu doping content on photoluminescence of Gd2O3:Eu phosphors prepared by liquid-phase reaction method

Europium-doped cubic Gd₂O₃:Eu³⁺ nanoparticles containing various activator content in the range of 5-15 wt% were synthesized by a liquid-phase reaction method to investigate the influence of Eu³⁺ loading on the optical properties of phosphors by using XRD, TEM, BET, spectrometer and fluorometer. The size of Gd₂O₃:Eu³⁺ powders was in the range 21-41 nm. The phosphors showed an initial increase in luminescence and then a subsequent decrease with further doping (above 10 wt%). The decay time was reduced with increasing Eu loading; however, it decreased significantly above the 10% Eu doping. From spectroscopic studies, the Eu³⁺ doping ion distribution was uniform and homogeneous up to the 10 wt% loading because no concentration quenching effect was observed. However, further Eu³⁺ doping above 10 wt% reduced the luminescence due to the concentration quenching effect, as deduced from the shortening of the decay time.     

Photoluminescence properties of Sr1-xSi2O2N2: Eux as green to yellow-emitting phosphor for blue pumped white LEDs

This study evaluated potential applications of green to yellow-emitting phosphors (Sr_1−xSi₂O₂N₂: Eu²⁺_x) in blue pumped white light emitting diodes. Sr_1-xSi₂O₂N₂: Eu²⁺_x was synthesized at different Eu²⁺ doping concentrations at 1450 °C for 5 h under a reducing nitrogen atmosphere containing 5% H₂ using a conventional solid reaction method. The X-ray diffraction patterns of the prepared phosphor (Sr_1-xSi₂O₂N₂: Eu²⁺_x) were indexed to the SrSi₂O₂N₂ phase and an unknown intermediate phase. The photoluminescence properties of these phosphors (Sr_1−xSi₂O₂N₂: Eu²⁺_x) showed that the samples were excited from the UV to visible region due to the strong crystal field splitting of the Eu²⁺ ion. The emission spectra under excitation of 450 nm showed a bright color at 545-561 nm. The emission intensity increased gradually with increasing Eu²⁺ doping concentration ratio from 0.05 to 0.15. However, the emission intensity decreased suddenly when the Eu²⁺ concentration ratio was >0.2. As the doping concentration of Eu²⁺ was increased, there was a red shift in the continuous emission peak. These results suggest that Sr_1-xSi₂O₂N₂: Eu₂⁺_x phosphor can be used in blue-pumped white light emitting diodes.     

Luminescent properties of R doped Sr2SiO4:Eu(R=Li, Na and K) red-emitting phosphors for white LEDs

Sr₂SiO₄:Eu³⁺ and Sr₂SiO₄:Eu³⁺ doped with R⁺(R⁺=Li⁺, Na⁺ and K⁺) phosphors were prepared by conventional solid-state reaction and investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. XRD patterns and SEM reveal that the optimal firing condition for Sr₂SiO₄:Eu³⁺ was 1300 °C for 4 h. The excitation and emission spectra indicate that the phosphor can be effectively excited by ultraviolet (395 nm) and blue (466 nm) light and emits intense red light peaked at around 614 nm corresponding to the ⁵D₀→⁷F₂ transitions of Eu³⁺. In the research work, the effect of R⁺ contents on luminescence property and the Eu³⁺ concentration quenching process have also been investigated. The Eu³⁺ concentration quenching mechanism was verified to be a multipole-multipole interaction and the critical energy-transfer distance was calculated to be around 14.6 Å. The dopant R⁺(R⁺=Li⁺, Na⁺ and K⁺) as charge compensator in Sr₂SiO₄:Eu³⁺ can further enhance luminescence intensity, and the emission intensity of Sr₂SiO₄:Eu³⁺ doping Li⁺ is higher than that of Na⁺ or K⁺.     

KGd(MoO4)2:Eu as a promising red phosphor for light-emitting diode application

A red phosphor KGd(MoO₄)₂:Eu³⁺ was prepared by solid-state reaction technique at high temperature. Its photoluminescent property was investigated and the optimum concentration of Eu³⁺ doped in the KGd(MoO₄)₂ is 25 mol%. Compared with Y₂O₂S:0.05Eu³⁺, the obtained KGd(MoO₄)₂:Eu³⁺ shows wider excitation band around 400 nm, higher intensity of Eu^{3+ 5}D₀–⁷F₂ emission upon excitation 393 nm, and the CIE chromaticity coordinates (x = 0.655, y = 0.345) are closer to the standard of National Television Standard Committee (NTSC). The optical properties of KGd(MoO₄)₂:Eu³⁺ suggest that it is an efficient red-emitting phosphor for light-emitting diode applications.     

Preparation and luminescent properties of BaCa2Si3O9:Eu

A blue emitting phosphor of the triclinic BaCa₂Si₃O₉:Eu²⁺ was prepared by the combustion-assisted synthesis method and an efficient blue emission ranging from the ultraviolet to visible was observed. The luminescence and crystallinity were investigated using luminescence spectrometry and X-ray diffractometry (XRD), respectively. The emission spectrum shows a single intensive band centered at 445 nm, which corresponds to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The excitation spectrum is a broad extending from 260 to 450 nm, which matches the emission of ultraviolet light-emitting diodes (UV-LEDs). The critical quenching concentration of Eu²⁺ in BaCa₂Si₃O₉:Eu²⁺ phosphor is about 0.05 mol. The corresponding concentration quenching mechanism is verified to be a dipole-dipole interaction. The CIE of the optimized sample Ba_0.95Ca₂Si₃O₉:Eu_0.05²⁺ was (x, y)=(0.164, 0.111). The result indicates that BaCa₂Si₃O₉:Eu²⁺ can be potentially useful as a UV radiation-converting phosphor for white light-emitting diodes (LEDs).     

Preparation and optical properties of Ce activated (Ca1 − x,Srx)Al2Si2O8 phosphors

(Ca_1 − x, Sr_x)Al₂Si₂O₈:0.06Ce³⁺, M⁺ (M⁺ = Li⁺, Na⁺, K⁺) phosphors have been prepared by conventional solid-state reaction method. The structural and optical properties of the phosphors were characterized by X-ray diffraction (XRD) technique and spectrophotometer, respectively. A regular variation was found among the XRD patterns of (Ca_1 − x, Sr_x)Al₂Si₂O₈:0.06Ce³⁺ phosphors based on the changing of Sr content. With the increase of Sr content, the maximum of emission band presented slight blue shifts (~ 15 nm). The luminescence intensity of CaAl₂Si₂O₈:0.06Ce³⁺ and SrAl₂Si₂O₈:0.06Ce³⁺ were significantly enhanced when K⁺ and Li⁺ were incorporated, respectively.