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.     

Long lasting phosphorescent properties of Ti doped ZrO2

A novel long-lasting afterglow phosphor ZrO₂:Ti is prepared by the conventional solid-stated method and their luminescent properties are investigated. A bluish white strong and broad emissive band, which is attributed to originate from the recombination of electrons trapped by F⁺ centers and the holes created in Valence band, is observed under 254 nm UV irradiation. The identical color long afterglow, which lasts about 1 h, is found in the ZrO₂:Ti phosphor after removing the 254 nm UV light. The mechanism of the long lasting phosphorescence (LLP) has been discussed based on the thermoluminescence (TL) results. The replacement of Zr by Ti produces more anion vacancies, resulting in the enhanced photoluminescence (PL) and LLP of ZrO₂:Ti sample. These results indicate that ZrO₂:Ti phosphor has potential promising applications.     

Enhanced photoluminescence properties of Zn2SiO4:Mn co-activated with Y/Li under VUV excitation

A series of green phosphors Zn_1.92−2xY_xLi_xSiO₄:0.08Mn²⁺ (0≤x≤0.03) were prepared by solid-state synthesis method. Phase and lattice parameters of the synthesized phosphors were characterized by powder X-ray diffractometer (XRD) and the co-doped effects of Y³⁺/Li⁺ upon emission intensity and decay time were investigated under 147 nm excitation. The results indicate that the co-doping of Y³⁺/Li⁺ has favorable influence on the photoluminescence properties of Zn₂SiO₄:Mn²⁺, and the optimal photoluminescence intensity of Zn_1.90Y_0.01Li_0.01SiO₄:0.08Mn²⁺ is 103% of that of commercial phosphor when the doping concentration of Y³⁺/Li⁺ is 0.01 mol. Additionally, the decay time of phosphor is much shortened and the decay time of Zn_1.90Y_0.01Li_0.01SiO₄:0.08Mn²⁺ is 3.39 ms, shorter by 1.83 ms than that of commercial product after Y³⁺/Li⁺ co-doping.     

EPR and photoluminescence properties of green light emitting LaAl11O18:Mn phosphors

The LaAl₁₁O₁₈:Mn²⁺ powder phosphor has been prepared using a self-propagating synthesis. Formation and homogeneity of the LaAl₁₁O₁₈:Mn²⁺ phosphor has been verified by X-ray diffraction and energy dispersive X-ray analysis respectively. The EPR spectra of Mn²⁺ ions exhibit resonance signals with effective g values at g≈4.8 and g≈1.978. The signal at g≈1.978 exhibits six-line hyperfine structure and is due to Mn²⁺ ions in an environment close to tetrahedral symmetry, whereas the resonance at g≈4.8 is attributed to the rhombic surroundings of the Mn²⁺ ions. It is observed that the number of spins participating in resonance for g≈1.978 increases with decreasing temperature obeying the Boltzmann law. Upon 451 nm excitation, the photoluminescence spectrum exhibits a green emission peak at 514 nm due to ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. The crystal field parameter Dq and Racah inter-electronic repulsion parameters B and C have been evaluated from the excitation spectrum.     

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.     

Combustion synthesis and luminescence properties of NaY1−xEux(WO4)2 phosphors

The red phosphors NaY_1−xEu_x(WO₄)₂ with different concentrations of Eu³⁺ were synthesized via the combustion synthesis method. As a comparison, NaEu(WO₄)₂ was prepared by the solid-state reaction method. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. The results show that the red light emission intensity of the combustion synthesized samples under 394 nm excitation increases with increase in Eu³⁺ concentrations and calcination temperatures. Without Y ions doping, the emission spectra intensity of the NaEu(WO₄)₂ phosphor prepared by the combustion method fired at 900 °C is higher than that prepared by the solid-state reaction at 1100 °C. NaEu(WO₄)₂ phosphor synthesized by the combustion method at 1100 °C exhibits the strongest red emission under 394 nm excitation and appropriate CIE chromaticity coordinates (x=0.64, y=0.33) close to the NTSC standard value. Thus, its excellent luminescence properties make it a promising phosphor for near UV InGaN chip-based red-emitting LED application.     

Luminescence and EPR studies of Mn-activated SrAl12O19 phosphor prepared by facile combustion approach

Manganese-activated strontium hexa-aluminate (SrAl₁₂O₁₉) phosphor has been prepared at low temperature (500 °C) and in a very short time (<5 min) by urea combustion route. Powder X-ray diffraction pattern showed the presence of hexagonal SrAl₁₂O₁₉ phase. Scanning electron microscopy (SEM) indicated the presence of several particles with sizes of 200 nm. The luminescence of Mn²⁺ activated SrAl₁₂O₁₉ exhibits a strong green emission peak around 515 nm from the synthesized phosphor particles under excitation (451 nm). The luminescence is assigned to a transition from the upper ⁴T₁→⁶A₁ ground state of Mn²⁺ ions. EPR investigations also indicated the presence of Mn²⁺ ions in the prepared material. From the observed EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) constant indicates that there exists a moderately covalent bonding between Mn²⁺ ions and the surrounding ligands. The variation of zero-field splitting parameter (D) with temperature is measured and discussed. The mechanism involved in the generation of a green emission has been explained in detail.     

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.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

EPR and luminescence properties of LiGa5O8:Mn green emitting phosphor

Green emitting LiGa₅O₈:Mn powder phosphor has been prepared in a short time by solution combustion method. Powder X-ray diffraction pattern indicated a dominant phase of LiGa₅O₈ with another secondary LiGaO₂ phase. Morphology aspects were studied by using field emission scanning electron microscopy. Upon UV light excitation (296 nm), the phosphor exhibits a strong green luminescence (510 nm), which corresponds to the ⁴T₁→⁶A₁ transition of Mn²⁺ ions in an environment close to tetrahedral symmetry. EPR spectrum exhibits resonance signals characteristic of Mn²⁺ ions. It is observed that the spin-Hamiltonian parameters g and A do not vary with temperature. The magnitude of the hyperfine splitting constant (A) in the present study indicates that there exists a moderately covalent bonding between Mn²⁺ ions and the surrounding ligands. The zero-field splitting parameter (D), spin concentration (N) and paramagnetic susceptibility (χ) have also been evaluated.     

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.     

Photoluminescence and EPR studies of BaMgAl10O17:Eu phosphor with blue-emission synthesized by the solution combustion method

A blue phosphor, BaMgAl₁₀O₁₇:Eu²⁺, has been synthesized in the furnace at a temperature of 500 °C by solution combustion method. The formation of the as-prepared BaMgAl₁₀O₁₇:Eu²⁺ phosphor was confirmed by the powder X-ray diffraction technique. The EPR spectrum exhibited an intense resonance signal centered at g=4.63 at 150 mT along with a number of resonances in the vicinity of g>2.0 and g<2.0. The number of spins participating in resonance (N) and the susceptibility (c) for the resonance signal at g=4.63 have been calculated as a function of temperature. The excitation spectrum of BaMgAl₁₀O₁₇:Eu²⁺ phosphor showed a strong peak near 336 nm (4f⁷ (⁸S)→5d¹(t_2g) transition) with a staircase like structure in the region 376-400 nm owing to crystal field splitting of the Eu²⁺ d-orbital. The 336 nm excitation produced a broad blue emission at 450 nm corresponding to 4f⁶5d→4f⁷ transition. PL studies reveal the two emission centers one at 450 nm and the other at 490 nm in this phosphor.     

High luminance of new green emitting phosphor, Mg2SnO4:Mn

Mg₂SnO₄, which has an inverse spinel structure, was adopted as the host material of a new green emitting phosphor. Luminescence properties of the manganese-doped magnesium tin oxide prepared by the solid state reaction were investigated under vacuum ultraviolet (VUV) ray and low-voltage electron excitation. The Mg₂SnO₄:Mn phosphor exhibited green luminescence with the emission spectrum centered at 500 nm due to spin flip transition of the d-orbital electron associated with the Mn²⁺ ion. Optimum Mn concentration of Mg₂SnO₄:Mn under VUV excitation with 147 nm wavelength and electron beam excitation with 800 V excitation voltage are 0.25 and 0.6 mol%, respectively. The emission intensities of Mg₂SnO₄:Mn phosphors under the two excitation sources are higher than those of Zn₂SiO₄:Mn and ZnGa₂O₄:Mn phosphors. At 0.25 mol% of Mn concentration, on the other hand, the decay time is shorter than 10 ms.     

Luminescence and EPR studies of Eu doped BaAl12O19 blue light emitting phosphors

Europium doped BaAl₁₂O₁₉ powder phosphors have been synthesized by combustion process within few minutes. The phosphors have been characterized by XRD, SEM, FT-IR, EPR and PL techniques. The EPR spectrum exhibits an intense resonance signal at g=1.96 characteristic of Eu²⁺ ions. In addition to this two weak resonance signals have been observed at g=2.28 and g=4.86. The population of the spin levels (N) for the resonance signal at g=1.96 is calculated as a function of temperature. By post-treating the phosphor at 1350 °C under a reducing atmosphere, it is observed that the population of spin levels has been increased five times. The excitation spectrum shows a peak at 326 nm with a shoulder at 290 nm. Upon excitation at 326 nm, the emission spectrum exhibits a well defined broad band with maximum at 444 nm emitting a blue light corresponding to 4f⁶5d→4f⁷ transition. The luminescence intensity also has been enhanced to 60% by post-treating the phosphor at 1350 °C under a reducing atmosphere.     

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).     

Room-temperature ferromagnetism in Sn1−xMnxO2 nanocrystalline thin films prepared by ultrasonic spray pyrolysis

Sn_1−xMn_xO₂ (x=0.01-0.05) thin films were synthesized on quartz substrate using an inexpensive ultrasonic spray pyrolysis technique. The influence of doping concentration and substrate temperature on structural and magnetic properties of Sn_1−xMn_xO₂ thin films was systematically investigated. X-ray diffraction (XRD) studies of these films reflect that the Mn³⁺ ions have substituted Sn⁴⁺ ions without changing the tetragonal rutile structure of pure SnO₂. A linear increase in c-axis lattice constant has been observed with corresponding increase in Mn concentration. No impurity phase was detected in XRD patterns even after doping 5 at% of Mn. A systematic change in magnetic behavior from ferromagnetic to paramagnetic was observed with increase in substrate temperature from 500 to 700 °C for Sn_1−xMn_xO₂ (x=0.01) films. Magnetic studies reveal room-temperature ferromagnetism (RTFM) with 3.61×10⁻⁴ emu saturation magnetization and 92 Oe coercivity in case of Sn_1−xMn_xO₂ (x=0.01) films deposited at 500 °C. However, paramagnetic behavior was observed for the films deposited at a higher substrate temperature of 700 °C. The presence of room-temperature ferromagnetism in these films was observed to have an intrinsic origin and could be obtained by controlling the substrate temperature and Mn doping concentration.     

Optical properties of Mn-activated Na2SnF6 and Cs2SnF6 red phosphors

Alkaline hexafluorostantanate red phosphors Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺ are synthesized by chemical reaction in HF/NaMnO₄ (CsMnO₄)/H₂O₂/H₂O mixed solutions immersed with tin metal. X-ray diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space group D_4h¹⁴ (Na₂SnF₆:Mn⁴⁺) and a trigonal symmetry with the space group D_3d³ (Cs₂SnF₆:Mn⁴⁺). Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck-Condon analysis of the PLE data yields the Mn⁴⁺-related optical transitions to occur at ∼2.39 and ∼2.38 eV (⁴A_2g→⁴T_2g) and at ∼2.83 and ∼2.76 eV (⁴A_2g→⁴T_1g) for Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺, respectively. The crystal field parameters (Dq) of the Mn⁴⁺ ions in the Na₂SnF₆ and Cs₂SnF₆ hosts are determined to be ∼1930 and ∼1920 cm⁻¹, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose-Einstein occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of Mn⁴⁺-activated alkaline hexafluoride salts.     

Simultaneous three-photon absorption induced ultraviolet upconversion in Pr:Y2SiO5 crystal by femtosecond laser irradiation

Near-infrared to ultraviolet upconversion luminescence was observed in the Pr³⁺:Y₂SiO₅ crystal with 120 fs, 800 nm infrared laser irradiation. The observed emissions at around 270 nm and 305 nm could be assigned to 5d → 4f transitions of Pr³⁺ ions. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to simultaneous three-photon absorption induced upconversion luminescence.     

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.     

Luminescent properties of Eu-activated Sr3B2SiO8: A red-emitting phosphor for white light-emitting diodes

Single-phased Sr₃B₂SiO₈:Eu³⁺ phosphor was prepared by a solid-state method at 1020 °C. The luminescence spectra showed that Sr₃B₂SiO₈:Eu³⁺ phosphor can be effectively excited by near ultraviolet light (393 nm) and blue light (464 nm). When excited at 393 or 464 nm Sr₃B₂SiO₈:Eu³⁺ exhibited the main emission peaks at 611 and 620 nm, which resulted from the supersensitive ⁵D₀→⁷F₂ transition of Eu³⁺. The luminescence intensity of Sr₃B₂SiO₈:Eu³⁺ at 611 and 620 nm reached the maximum when the doping content of Eu³⁺ was 4.5 mol%. Its chromaticity coordinates (0.646, 0.354) were very close to the NTSC standard values (0.67, 0.33). Thus, Sr₃B₂SiO₈:Eu³⁺ is considered to be an efficient red-emitting phosphor for long-UV InGaN-based light-emitting diodes.