Fabrication of luminescent SrWO4 thin films by a novel electrochemical method

Highly crystallized SrWO₄ thin films with single scheelite structure were prepared within 60 min by a cell electrochemical method. X-ray diffraction analysis shows that SrWO₄ thin films have a tetragonal structure. Scanning electron microscopy examinations reveal that SrWO₄ grains grow well in tetragonal tapers and grains like flowers or bunches, which can usually form by using the electrolysis electrochemical method, have disappeared under cell electrochemical conditions. X-ray photoelectron spectra and energy dispersive X-ray microanalysis examinations demonstrate that the composition of the film is consistent with its stoichiometry. These SrWO₄ films show a single blue emission peak (located at 460 nm) using an excitation wave of 230 nm. The speed of cell electrochemical method can be controlled by changing temperature. The optimum treatment temperature is about 50-60 °C.     

Hydrothermal synthesis and photoluminescence of SrWO4:Tb novel green phosphor

Tb³⁺-doped SrWO₄ phosphors with a scheelite structure have been prepared by hydrothermal reaction. X-ray powder diffraction, field-emission scanning electron microscopy, photoluminescence excitation and emission spectra and decay curve were used to characterize the resulting samples. Scanning electron microscopy image showed that the obtained SrWO₄:Tb³⁺ phosphors appeared to be nearly spherical and their sizes ranged from 1 to 3 μm. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under ultraviolet light excitation. Because 12 at.% SWO₄:Tb³⁺ phosphor exhibits intensive green emission under 254 nm excitation in comparison with the commercial green fluorescent lamp phosphor (LaPO₄:Ce,Tb), the excellent luminescence properties make it a new promising green phosphor for fluorescent lamps application.     

Photoluminescence properties of NaGd(MoO4)2:Eu nanophosphors prepared by sol-gel method

NaGd(MoO₄)₂:Eu³⁺ (hereafter NGM:Eu) phosphors have been prepared by sol-gel method. The properties of the resulting phosphors are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curve. The excitation spectra of NGM:Eu phosphors are mainly attributed to O → Mo charge-transfer (CT) band at about 282 nm and some sharp lines of Eu³⁺ f-f transitions in near-UV and visible regions with two strong peaks at 395 and 465 nm, respectively. Under the 395 and 465 nm excitation, intense red emission peaked at 616 nm corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺ are observed for 35 at.% NGM:Eu phosphors as the optimal doping concentration. The luminescence properties suggest that NGM:Eu phosphor may be regarded as a potential red phosphor candidate for near-UV and blue light-emitting diodes (LEDs).     

Synthesis and optical properties of KCaPO4:Eu phosphor

A series of Eu²⁺ doped KCaPO₄ phosphors were prepared by high temperature solid state reaction and an efficient blue-green emission was observed. The photoluminescence (PL) spectrum of the phosphor appeared one asymmetric peak under near-ultraviolet (n-UV) excitation and two emission bands at 480 nm and 540 nm were obtained using Gaussian fit, which was because Eu²⁺ ions inhabited two different Ca²⁺ sites: Eu(I) and Eu(II) in the host lattice, respectively. The excitation spectrum was a broadband extending from 250 to 450 nm, which matched well with the emission of ultraviolet light-emitting diodes (UV LEDs). The effect of Eu²⁺ concentration on the emission intensity of KCaPO₄:Eu²⁺ phosphor was investigated in detail.     

Hydrothermal synthesis and luminescence properties of octahedral LiYbF4: Er microcrystals

LiYbF₄: Er³⁺ octahedral microcrystals have been successfully prepared through a facile hydrothermal method assisted with EDTA (ethylenediaminetetraacetic acid). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric and differential scanning calorimeters (TG-DSC), photoluminescence (PL) spectra are used to characterize the samples. Under 976 nm excitation, the upconversion (UC) luminescence emission spectra of LiYbF₄: Er³⁺ microcrystals show the characteristic Er³⁺ emissions. The results show that the infrared light emissions at 792 nm of ⁴I_9/2 → ⁴I_15/2 are dominantly strong unusually, while the green emissions at 526 and 545 nm assigned to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2, respectively, and the red emission at 667 nm of ⁴F_9/2→⁴I_15/2 are relatively weaker. Most importantly, the samples show more efficient luminescence with further heat treatment.     

Near-ultraviolet excitable Ca4Y6(SiO4)6O: Ce,Tb white phosphors for light-emitting diodes

The present investigation aims to demonstrate the potentiality of Tb³⁺ and Ce³⁺ co-doped Ca₄Y₆(SiO₄)₆O phosphors. By incorporation of Ce³⁺ into Ca₄Y₆(SiO₄)₆O: Tb³⁺, the excitation band was extended from short-ultraviolet to near-ultraviolet region. The energy transfer from Ce³⁺ to Tb³⁺ in Ca₄Y₆(SiO₄)₆O host was investigated and demonstrated to be a resonant type via a dipole–dipole mechanism with the critical distance of 10.2 Å. When excited by 352 nm, Ca₄Y₆(SiO₄)₆O: Ce³⁺, Tb³⁺ exhibited a brighter and broader violet-blue emission (421 nm) from the Ce³⁺ and an intense green emission (542 nm) from the Tb³⁺. Combining the two emissions whose intensities were adjusted by changing the doping levels of the co-activator, an optimized white light with chromaticity coordinates of (0.278, 0.353) is generated in Ca₄Y₆(SiO₄)₆O: 2% Ce³⁺, 8% Tb³⁺, and this phosphor could be potentially used in near-ultraviolet light-emitting diodes.     

Synthesis and blue to near-infrared quantum cutting of Pr/Yb co-doped Li2TeO4 phosphors

Near-infrared (NIR) quantum cutting luminescent materials Li₂TeO₄ doped with Pr³⁺ and Yb³⁺ were synthesized by solid-state reaction method. The dependence of Yb³⁺ doping concentration on the visible- and NIR-emissions, decay lifetime, and quantum efficiencies of the phosphors are investigated. Quantum cutting down-conversion involving 647 nm red emission and 960-1050 nm broadband near-infrared emission for each 487 nm blue photon absorbed is realized successfully in the resulting phosphors, of which the process of near-infrared quantum cutting could be expressed as ³P₀(Pr³⁺) → ²F_5/2(Yb³⁺) + ²F_5/2(Yb³⁺). The maximum quantum cutting efficiency approaches up to 166.4% in Li₂TeO₄: 0.3 mol%Pr³⁺, 1.8 mol%Yb³⁺ sample corresponding to the 66.4% value of energy transfer efficiency.     

A novel blue-emitting NaSrPO4:Eu phosphor for near UV based white light-emitting-diodes

A novel blue-emitting phosphor based on a phosphate host matrix, NaSrPO₄:Eu²⁺, was prepared by a conventional solid-state reaction method. The NaSrPO₄:Eu²⁺ phosphor was efficiently excited at wavelengths of 250-450 nm, which is suitable for the emission band of near ultraviolet (n-UV) light-emitting-diode (LED) chips (350-430 nm). The NaSrPO₄:Eu²⁺ phosphor exhibits a strong blue emission peaking at 453 nm and broadly weak green and red emission bands up to 700 nm. The effect of the activated Eu²⁺ concentration on the emission intensity of the NaSrPO₄:Eu²⁺ was also investigated. Here, a phosphor-converted LED (pc-LED) was fabricated and exhibits bright blue emission under a forward bias of 20 mA. All of these characteristics suggest that the NaSrPO₄:Eu²⁺ phosphors could be applicable to n-UV based white LEDs.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Synthesis and upconversion luminescence properties of Yb/Er codoped BaGd2(MoO4)4 powder

Synthesis and upconversion luminescence properties of the new BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor were reported in this paper. The phosphor powder was obtained by the traditional high temperature solid-state method, and its phase structure was characterized by the XRD pattern. Based on the upconversion luminescence properties studies, it is found that, under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits intense green upconversion luminescence, which is ascribed to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺. While the observed much weaker red emission is due to the non-radiative relaxation process of ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_15/2 transition originating from the same Er³⁺. The concentration quenching effects for both Yb³⁺ and Er³⁺ were found, and the optimum doping concentrations of 0.5 mol% Yb³⁺ and 0.08 mol% Er³⁺ in the new BaGd₂(MoO₄)₄ Gd³⁺ host were established.     

Hydrothermal synthesis and vacuum ultraviolet-excited luminescence properties of novel Dy-doped LaPO4 white light phosphors

Novel LaPO₄:Dy³⁺ white light phosphors with monoclinic system were successfully synthesized by hydrothermal method at 240 °C. The strong absorption at around 147 nm in excitation spectrum was assigned to the host absorption which suggested that the vacuum ultraviolet-excited energy was efficiently transferred from the host to the Dy³⁺ ion. The f-d transition of Dy³⁺ ion was observed locating at 182 nm. Under 147 nm excitation, La_1−xPO₄:xDy³⁺ phosphor exhibited two emission bands locating at 571 nm (yellow) and 478 nm (blue) which corresponded to the hypersensitive transitions ⁴F_9/2-⁶H_13/2 and ⁴F_9/2-⁶H_15/2. It was the two emission bands that lead to the white light.     

Preparation and luminescence properties of Mn-doped ZnGa2O4 nanofibers via electrospinning process

One-dimensional Mn²⁺-doped ZnGa₂O₄ nanofibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. SEM results indicated that the as-formed precursor fibers and those annealed at 700 °C are uniform with length of several tens to hundred micrometers, and the diameters of the fibers decrease greatly after being heated at 700 °C. Under ultraviolet excitation (246 nm) and low-voltage electron beams (1–3 kV) excitation, the ZnGa₂O₄:Mn²⁺ nanofibers presents the blue emission band of the ZnGa₂O₄ host lattice and the strong green emission with a peak at 505 nm corresponding to the ⁴T₁–⁶A₁ transition of Mn²⁺ ion.     

A promising red phosphor MgMoO4:Eu for white light emitting diodes

Motivated by the need for new red phosphors for solid-state lighting applications Eu³⁺-doped MgMoO₄ was prepared by solid-state reaction and its excitation and emission spectra were measured at room temperature. In addition, the effects of firing temperature and Eu³⁺ doping concentration on the PL intensities were also investigated. Compared with Y₂O₂S:0.05Eu³⁺, the obtained Mg_0.80MoO₄:Eu³⁺_0.20 phosphor shows a stronger excitation band near 400 nm and intensely red-emission lines at 616 nm correspond to the forced electric dipole ⁵D₀ → ⁷F₂ transitions on Eu³⁺ under 394 nm light excitation. The CIE chromaticity coordinates (x = 0.651, y = 0.348) of Mg_0.80MoO₄:Eu³⁺_0.20 close to the NTSC (National Television Standard Committee) standard values, and therefore may find application on near UV InGaN chip-based white light emitting diodes.     

Novel blue-emitting phosphor NaMg4(PO4)3:Eu, Ce with energy transfer

A blue-emitting phosphor of NaMg₄(PO₄)₃:Eu²⁺, Ce³⁺ was prepared by a combustion-assisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Photoluminescence excitation spectrum measurements show that the phosphor can be excited by near UV light from 230 to 400 nm and presents a dominant luminescence band centered at 424 nm due to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions at room temperature. Effective energy transfer occurs in Ce³⁺/Eu²⁺ co-doped NaMg₄(PO₄)₃ due to large spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺-Eu²⁺ co-doped NaMg₄(PO₄)₃ powders can possibly be applied as blue phosphors in the fields of lighting and display.     

White upconverted luminescence of Ho/Yb/Tm tri-doped Gd2Mo3O9 phosphors

Yb³⁺/Tm³⁺/Ho³⁺ tri-doped Gd₂Mo₃O₉ phosphors were synthesized by the high-temperature solid-state method. Under 980 nm near-infrared excitation, the white-light emission can be observed, which is consists of the blue, green, and red UC emissions. The green and red emission at 547 nm and 660 nm originated from the transition of Ho³⁺ (⁵S₂, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈) and the blue emission at 475 nm attributed to the transition of Tm³⁺ (⁵G₄ → ⁵H₆). In this experiment, we selected the optimum concentration ratio of the three rare earths for the bright white emission. The Commission internationale de L’Eclairage (CIE) coordinates for the samples were calculated, and chromaticity coordinates were very close to white light regions. We find that the calculated CIE color coordinates of the Yb³⁺/Tm³⁺/Ho³⁺ tri-doped Gd₂Mo₃O₉ phosphors changed with the incident pump power from 400 mW/cm² to 1000 mW/cm². The upconversion luminescence mechanism of the samples was discussed on its spectral. The white light may be proved to be a candidate material for applications in various fields.     

Synthesis and photoluminescence properties of MgAl2O4:Mn hexagonal nanoplates

MgAl₂O₄:Mn²⁺ hexagonal nanoplates have been synthesized via a simple two-step method. The nanoplates have uniform hexagonal morphology with an average edge length of 1 μm and thickness of 30 nm. X-ray diffraction and various microscopic techniques indicate that MgAl₂O₄:Mn²⁺ nanoplates are single-crystal with multilayered morphology. The formation mechanism has also been discussed. Photoluminescence (PL) spectrum of the MgAl₂O₄:Mn²⁺ nanoplate shows a broad green emission band centered at 568 nm, which is assigned to the ⁴T₁ → ⁶A₁ transition of Mn²⁺ ion. The MgAl₂O₄:Mn²⁺ nanoplate is a promising candidate for efficient nanoscale optical material.     

Near UV and blue-based LED fabricated with Ca8Zn(SiO4)4Cl2:Eu as green-emitting phosphor

Green-emitting phosphor Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ has been prepared by the solid state reaction method and there luminescence properties are investigated. The excitation spectrum of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ shows an intense excitation band in the blue centered at 450 nm and emits with a maximum at 505 nm. The concentration quenching mechanism is studied and verified to be the energy transfer among the nearest-neighbor ions. Upon 450 nm excitation, the emission intensity of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ is much stronger than the green emitting Ca₃SO₄Cl₂:Eu²⁺ phosphor and even higher than YAG:Ce³⁺. This excitation spectrum range matches UV and blue light-emitting diodes (LEDS) chips very well, suggesting Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ could be a promising green emitting phosphor candidate for LED devices.     

Synthesis and photoluminescence properties of SrLu2O4:Eu superfine phosphor

Superfine powder SrLu₂O₄:Eu³⁺ was synthesized with a precursor prepared by an EDTA - sol-gel method at relatively low temperature using metal nitrate and EDTA as starting materials. The heat decomposition mechanism of the precursor, formation process of SrLu₂O₄:Eu³⁺and the properties of the particles were investigated by thermo-gravimetric (TG) - differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) analyses. The results show that pure SrLu₂O₄:Eu³⁺ superfine powder has been produced after the precursor was calcinated at 900 °C for 2 h and has an elliptical shape and an average diameter of 80-100 nm. Upon excitation with 250 nm light, all the SrLu₂O₄:Eu³⁺ powders show red and orange emissions due to the 4f-4f transitions of Eu³⁺ ions. The highest photoluminescence intensity at 610 nm was found at a content of about 6 mol% Eu³⁺. Splitting of the ⁵D₀-⁷F₁ emission transition revealed that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite by substituting Lu³⁺ ions.     

Photoluminescence and low-voltage cathodoluminescence characteristics of blue phosphor, ZnGa2O4:M (M = Na, Ag)

Photoluminescence and low-voltage cathodoluminescence characteristics of ZnGa₂O₄ phosphor doped with monovalent ions has been studied. Monovalent ions such as Na⁺ and Ag⁺ are incorporated into ZnGa₂O₄ lattices in order to increase the concentration of oxygen vacancies in the spinel lattice. By doping low concentrations of monovalent ions (Na⁺, Ag⁺) into ZnGa₂O₄, the self-activated blue luminescence originated from oxygen vacancies is enhanced. Also, the blue luminescence intensity is enhanced more along with a good color purity by annealing ZnGa₂O₄:Na⁺ in a reducing atmosphere, which is due to increasing the concentration of oxygen vacancies even more. The luminescence band at the UV region (λ_max=360 nm) does not become the major luminescence band by introducing Na⁺ ion into the ZnGa₂O₄ lattice, while the UV luminescence band becomes the major one by annealing the undoped ZnGa₂O₄ in a reducing atmosphere.     

Infrared to visible upconversion luminescence in Er/Yb co-doped CeO2 inverse opal

Infrared to visible upconversion luminescence has been investigated in Er³⁺/Yb³⁺ co-doped CeO₂ inverse opal. Under the excitation of 980 nm diode lasers, visible emissions centered at 525, 547, 561, 660 and 680 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (547, 561 nm), ⁴F_9/2 → ⁴I_15/2 (660 and 680 nm), respectively. The effect of photonic band gap on the upconversion luminescence intensity was also obtained. Additionally, the upconversion luminescence mechanism was studied. The dependence of Er³⁺ upconversion emission intensity on pump power reveals that it is a two-photon excitation process.