Tunable luminescence of Dy3+ single-doped and Dy3+/Tm3+ co-doped tungsten borate glasses

RE³⁺ (RE³⁺ = Tm³⁺, Dy³⁺) ion single and co-doped tungsten borate glasses for white light emitting diodes (LEDs) were prepared by melt quenching method. Emission and excitation spectra of the glasses were measured. The color of luminescence can be tuned by changing the composition of glass matrix or the concentrations of Tm³⁺ and Dy³⁺ ions. White light emission can be achieved from 0.5Dy³⁺ single-doped 15WO₃–25La₂O₃–60B₂O₃ and 0.4Tm³⁺/1.5Dy³⁺ co-doped 50WO₃–25La₂O₃–25B₂O₃ glasses. In addition, energy transfers between Tm³⁺ and Dy³⁺ were also analyzed. The Dy³⁺/Tm³⁺ co-doped tungsten borate glasses may be potential candidates for white LED application.     

Luminescence properties and energy transfer studies of a color tunable BaY2Si3O10:Tm3+,Dy3+ phosphor

A series of color-tunable and white light emitting phosphors BaY₂Si₃O₁₀:Tm³⁺,Dy³⁺ were synthesized by a high temperature solid-state reaction, and their phase structure, photoluminescence properties, and energy transfer processes between rare-earth ions were investigated in detail. Upon UV excitation, white light emission depending on dopant concentrations could be achieved by integrating a blue emission band located at 458 nm and an orange one located at 576 nm attributed to Tm³⁺ and Dy³⁺ ions, respectively. In addition, the energy transfer process between Tm³⁺ and Dy³⁺ ions was demonstrated to be a resonant type via a dipole–quadrupole mechanism. Preliminary studies showed that the phosphor might be promising as a single-phased white-light-emitting phosphor for UV chip pumped white-light LEDs.     

2 μm emission properties and hydroxy groups quenching of Tm3+ in germanate-tellurite glass

Tm³⁺ activated germanate-tellurite glasses with good thermal stability and anti-crystallization ability were prepared. Efficient 2 μm fluorescence was observed in the optimal concentration Tm³⁺ doped glass and the corresponding radiative properties were investigated. For Tm³⁺: ³F₄ → ³H₆ transition, high spontaneous radiative transition probability (260.75 s⁻¹) and large emission cross section (7.66 × 10⁻²¹ cm²) were obtained from the prepared glass. According to Dexter's and Forster's theory, energy transfer microscopic parameters were computed to elucidate the observed 2 μm emissions in detail. Besides, the effect of hydroxy groups quenching was also quantificationally investigated based on simplified rate equations. Results demonstrate that the optimal concentration Tm³⁺ doped germanate-tellurite glass possessing excellent spectroscopic properties might be an attractive candidate for 2 μm laser or amplifier.     

Photoluminescence properties of emission-tunable Ca9Y(PO4)7: Tm,Dy phosphor for white light emitting diodes

A white-emitting Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor has been successfully prepared by conventional high-temperature solid-state reaction. X-ray diffraction (XRD) and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The excitation and emission spectra show that all the Tm³⁺ and Dy³⁺ co-doped Ca₉Y(PO₄)₇ samples can be effectively excited by UV light and then emit blue and yellow light simultaneously. Furthermore, the emission and color coordinate of as-obtained samples pumped by 365 nm are able to be adjusted around white light by varying the doping concentrations of Tm³⁺ and Dy³⁺. So, the as-fabricated single-composition Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor will have a promising application in the area of white light emitting diodes.     

Energy transfer based photoluminescence spectra of co-doped (Dy3+ + Sm3+): Li2O-LiF-B2O3-ZnO glasses for orange emission

The present paper brings out the results concerning the preparation and optical properties of Sm³⁺ and Dy³⁺ each ion separately in different concentrations (0.3, 0.5, 1.0 and 1.5 mol.%) and also together doped (x mol.% Dy³⁺ + 1.5 mol.% Sm³⁺): Li₂O-LiF-B₂O₃-ZnO (where x = 0.5, 1.0 and 1.5 mol.%) glasses by a melt quenching method. Structural and thermal properties have been extensively studied for those glasses by XRD and TG/DTA. The compositional analysis has been carried out from FTIR spectral profile. Optical absorption spectral studies were also carried out. Sm³⁺: LBZ glasses have displayed an intense orange emission at 603 nm (⁴G_5/2 → ⁶H_7/2) with an excitation wavelength at 403 nm and Dy³⁺: LBZ glasses have shown two emissions located at 485 nm (⁴F_9/2 → ⁶H_15/2; blue) and 574 nm (⁴F_9/2 → ⁶H_13/2; yellow) with an excitation wavelength at 385 nm. Remarkably, it has been identified that the significant increase in the reddish orange emission of Sm³⁺ ions and diminished yellow emission pertaining to Dy³⁺ ions in the co-doped LBZ glass system under the excitation of 385 nm which relates to Dy³⁺ ions. This could be due energy transfer from Dy³⁺ to Sm³⁺. The non-radiative energy transfer from Dy³⁺ to Sm³⁺ is explained in terms of their emission spectra, donor lifetime, energy level diagram and energy transfer characteristic factors. These significantly enhanced orange emission exhibited glasses could be suggested as potential optical glasses for orange luminescence photonic devices.     

Optical characterization of YAl3(BO3)4:Dy3+–Tm3+ phosphors under near UV excitation

Dy³⁺ and Tm³⁺ co-doped YAl₃(BO₃)₄ (YAB) phosphors were prepared by solid-state reaction method at 1200 °C/3 h. The average crystallite size was determined as 52.09 nm from the X-ray diffraction measurements. Upon 352 and 359 nm near ultra violet excitation, the YAB:Dy³⁺–Tm³⁺ phosphors exhibit Dy³⁺:⁴F_9/2 → ⁶H_J (J = 15/2, 13/2, 11/2) and Tm³⁺:¹D₂ → ³F₄ transitions with different luminescence intensity. The photoluminescence emission and decay measurements revealed the energy transfer from Dy³⁺ to Tm³⁺ ions under 359 nm excitation only. The energy transfer between Dy³⁺ and Tm³⁺ takes place in Dy³⁺–Tm³⁺ clusters through exchange interaction mechanism. The Commission International de I’Eclairage chromaticity coordinates of YAB:Tm³⁺ phosphor (λ_ex = 359 nm) were found very close to the European Broadcasting Union and National Television Standard Committee illuminants. The emission color of the studied phosphors could be tuned from blue-to-white as a function of excitation wavelength. The YAB:Dy³⁺–Tm³⁺ phosphors can be used as potential candidates in display technology.     

Preparation and luminescence characteristics of LiYF4: Tm3+/Dy3+ single crystals for white-light LEDs

Tm³⁺ /Dy³⁺ co-doped LiYF₄ single crystals were synthesized by using vertical Bridgman method in sealed Pt crucibles. When excited by a proper UV-light, the crystals show blue emission band centered at 485 nm, which overlaps between the transition of Tm³⁺ (¹G₄ → ³H₆) and Dy³⁺ (⁴F_9/2 → ⁶H_15/2) ions, and yellow band of 573 nm ascribed to Dy³⁺ (⁴F_9/2 → ⁶H_13/2) ions. Both chromaticity coordinates and photoluminescence intensity vary with the excitation wavelengths and the concentration of rare earth dopants. A white light can be achieved from Tm³⁺ (0.6 mol%), Dy³⁺ (2.25 mol%) co-doped LiYF₄ crystal with chromaticity coordinates of x ≈ 0.2836, y ≈ 0.3229, and color temperature T _c = 8419 K by the excitation of a 350 nm light. It indicates that this crystal can be a potential candidate for the UV-light excited white-light emitting diodes.     

Tm3+ doped Ga–As–S chalcogenide glasses and fibers

Tm³⁺ doped Ga–As–S chalcogenide glass samples were produced using As₂S₃ pure glass as starting materials. Their photoluminescence properties were characterized and strong emission bands were observed at 1.2 μm (¹H₅ → ³H₆), 1.4 μm (³H₄ → ³F₄) and 1.8 μm (³F₄ → ³H₆) under excitation wavelengths of 698 nm and 800 nm. The thulium and gallium concentrations were optimized to achieve the highest photoluminescence efficiency. From the optimal composition, a Tm³⁺ doped Ga–As–S fiber was drawn and its optical properties were studied.     

Optical properties of the Tm and energy transfer between Tm→Pr ions in P2O5-CaO-SrO-BaO phosphate glass

A P₂O₅-CaO-SrO-BaO phosphate glass doped with Tm³⁺ and glasses doped with (Tm³⁺, Pr³⁺) were used for this study. The photo-luminescence behaviors of Tm³⁺ and Pr³⁺ in phosphate glass were investigated by absorption, excitation and emission spectroscopy. The energy transfer between Tm³⁺ and Pr³⁺ in phosphate glasses (which exhibit a variety of transfer efficiencies) was studied. The experimental quantum efficiencies of the luminescence of Tm³⁺ η₀ and (Tm³⁺, Pr³⁺) doped phosphate glasses were measured to give η/η₀ = 0.447, 0.305, and 0.179 for (0.4 mol% Pr³⁺, 1.0 mol% Tm³⁺), (0.8%Pr³⁺, 1.0%Tm³⁺) and (1.6 mol% Pr³⁺, 1.0 mol% Tm³⁺), respectively. In order to verify the nature of the ion coupling in our phosphate glass system, we applied the Inokuti-Hirayama model. The non-radiative energy transfer rate from Tm³⁺ to Pr³⁺, transfer efficiencies, and the donor-acceptor distance have been calculated and compared with obtained experimental data. As usual, the efficiency and the probability of energy transfer increase with the concentration of the acceptor.     

White-emitting phosphors Ca6La2Na2(PO4)6F2:Dy and luminescence enhancement through Ce → Dy energy transfer

Ce³⁺ and Dy³⁺ activated fluoro-apatite Ca₆La₂Na₂(PO₄)₆F₂ with chemical formulas Ca₆La_2−xLn_xNa₂(PO₄)₆F₂ (Ln = Ce³⁺, Dy³⁺) were prepared by a solid state reaction technique at high temperature. The vacuum-ultraviolet (VUV) and ultraviolet (UV) spectroscopic properties are investigated. The results indicate that Ce³⁺ ions show the lowest 5d excitation band at ∼305 nm and a broad emission band centered at ∼345 nm. Dy³⁺ ions exhibit intense absorption at VUV and UV range. White-emitting under 172 nm excitation is obtained based on two dominant emissions from Dy³⁺ ions centered at 480 and 577 nm. In addition, the energy transfer from Ce³⁺ to Dy³⁺ in the co-doped samples are observed and discussed.     

Comparative study of Er3+ and Tm3+ co-doped YOF and Y2O3 powders as red spectrally pure upconverters

We prepared Er³⁺ and Tm³⁺ co-doped yttrium oxyfluoride (YOF) powder by combustion synthesis and we observed that under near-infrared (λ = 980 nm) laser excitation the characteristic green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) emission of Er³⁺ was suppressed by energy transfer (ET) mechanisms between Tm³⁺ and Er³⁺. The ET process observed in YOF was much more efficient than that observed in standard Y₂O₃ powder prepared under similar conditions. YOF combines the superior mechanical and thermal properties of oxides with low phonon energy of fluorides. Our results show that this material is a serious candidate for use as a red upconversion phosphor.     

Multicolor up conversion emission and color tunability in Yb/Tm/Ho triply doped heavy metal oxide glasses

Multicolor and white light emissions have been achieved in Yb³⁺, Tm³⁺ and Ho³⁺ triply doped heavy metal oxide glasses upon laser excitation at 980 nm. The red (660 nm), green (547 nm) and blue (478 nm) up conversion emissions of the rare earth (RE) ions triply doped TeO₂–GeO₂–Bi₂O₃–K₂O glass (TGBK) have been investigated as a function of the RE concentration and excitation power of the 980 nm laser diode. The most appropriate combination of RE in the TGBK glass host (1.6 wt% Yb₂O₃, 0.6 wt% Tm₂O₃ and 0.1 wt% Ho₂O₃) has been determined with the purpose to tune the primary colors (RGB) respective emissions and generate white light emission by varying the pump power. The involved infrared to visible up conversion mechanisms mainly consist in a three-photon blue up conversion of Tm³⁺ ions and a two-photon green and red up conversions of Ho³⁺ ions. The resulting multicolor emissions have been described according to the CIE-1931 standards.     

Multicolor upconversion of Er3+/Tm3+/Yb3+ doped oxyfluoride glass ceramics

Er³⁺/Tm³⁺/Yb³⁺ tridoped oxyfluoride glass ceramics was synthesized in a general way. Under 980 nm LD pumping, intense red, green and blue upconversion was obtained. And with those primary colors, multicolor luminescence was observed in oxyfluoride glass ceramics with various dopant concentrations. The red and green upconversion is consistent with ⁴F_9/2 → ⁴I_15/2 and ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ respectively. While the blue upconversion originates from ¹G₄ → ³H₆ transition of Tm³⁺. This is similar to that in Er³⁺/Yb³⁺ and/or Tm³⁺/Yb³⁺ codoped glass ceramics. However the upconversion of Tm³⁺ is enhanced by the energy transfer between Er³⁺ and Tm³⁺.     

White light emission in Dy-doped lead fluorophosphate glasses

Lead fluorophosphate (PbFPDy: P₂O₅ + K₂O + Al₂O₃ + PbF₂ + Na₂O + Dy₂O₃) glasses doped with different Dy³⁺ ion concentrations have been prepared and characterized through Raman, absorption, emission and decay rate measurements. Free-ion Hamiltonian model for energy level analysis and Judd–Ofelt theory for spectral intensities have been used to analyze the spectroscopic properties of Dy³⁺ ions in lead fluorophosphate glasses. The chromaticity coordinates were calculated from emission spectra and analyzed with Commission International de I’Eclairage color diagram and appear in the white light region under ultraviolet excitation. The decay rates for ⁴F_9/2 level have been measured and are found to be deviated from exponential to non-exponential nature with increase in Dy³⁺ ion concentration. The non-exponential decay rates have been fitted with the Inokuti–Hirayama model for S = 6, which revealed that dipole–dipole mechanism is responsible for the energy transfer processes through Dy³⁺–Dy³⁺ interaction.     

Tunable luminescence properties and energy transfer of single-phase Ca<Subscript>4</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>O: Dy<Superscript>3+</Superscript>, Eu<Superscript>2+</Superscript> multi-color phosphors for warm white light

The novel Ca_4?x(PO₄)₂O: xDy³⁺ and Ca_4?x?y(PO₄)₂O: xDy³⁺, yEu²⁺ multi-color phosphors were synthesized by traditional solid-state reaction. The crystal structure, particle morphology, photoluminescence properties and energy transfer process were investigated in detail. The X-ray diffraction (XRD) results demonstrate that the products showed pure monoclinic phase of Ca₄(PO₄)₂O when x < 0.1. The scanning electron microscopy (SEM) indicated that the phosphors were grain-like morphologies with diameters of ~ 3.7–7.0 μm. Under excitation of 345 nm, Dy³⁺-doped Ca₄(PO₄)₂O phosphors showed multi-color emission bands at 410, 481 and 580 nm originated from oxygen vacancies and Dy³⁺. Interestingly, Ca₄(PO₄)₂O: Dy³⁺, Eu²⁺ phosphors exhibited blue emission band at 481 nm and broad emission band from 530 to 670 nm covering green to red regions. The energy transfer process from Dy³⁺ to Eu²⁺ was observed for the co-doped samples, and the energy transfer efficiency reached to 60% when Eu²⁺ molar concentration was 8%. In particular, warm/cool/day white light with adjustable CCT (2800–6700 K) and high CRI (R_a > 85) can be obtained by changing the Eu²⁺ co-doping contents in Ca₄(PO₄)₂O: Dy³⁺, Eu²⁺ phosphors. The optimized Ca_3.952(PO₄)₂O: 0.04Dy³⁺, 0.008Eu²⁺ phosphor can achieve the typical white light with CCT of 4735 K and CRI of 87.     

Single phase cool white light emitting novel Ca9Al(PO4)7:Dy3+ nanophosphor under NUV excitation

A series of novel single-phase white light-emitting Dy³⁺-doped Ca₉Al(PO₄)₇ nanophosphors was successfully synthesized at 1100 °C via solution combustion route. X-ray diffraction (XRD) and Rietveld refinement analysis of Ca₉Dy_0.03Al_0.97(PO₄)₇ sample confirmed that this phosphor had a trigonal crystal structure with space group R3c(161). Meanwhile, as-observed from the transmission electron microscopy (TEM) study; particles of Ca₉Dy_0.03Al_0.97(PO₄)₇ sample were found to have a quadrilateral shape with crystallite sizes around 40–60 nm which were also confirmed by the Debye Scherrer equation. Under near-ultraviolet (NUV) excitation at 350 nm, photoluminescence (PL) emission spectra of nanocrystalline Ca₉Al(PO₄)₇:Dy³⁺ phosphors showed two peaks at 481 nm and 572 nm corresponding to ⁴F_9/2?→?⁶H_15/2 and ⁴F_9/2?→?⁶H_13/2 transitions, respectively. The optimum concentration was found to be x?=?0.03 mol. The critical energy transfer distance was calculated to be 20 Å and further Huang analysis concluded the exact mechanism, i.e. dipole–dipole interactions responsible for concentration quenching in Ca₉Dy_xAl_(1?x)(PO₄)₇ samples. Furthermore, the Commission Internationale de I’Eclairage (CIE) chromaticity coordinates of Ca₉Dy_0.03Al_0.97(PO₄)₇ nanophosphor was calculated to be (0.260, 0.297) and this nanophosphor had correlated color temperature (CCT) of 11,332 K which is located in a cool white area. Existing results indicate that Ca₉Dy_0.03Al_0.97(PO₄)₇ nanophosphor may be considered as a favorable candidate in NUV-based single-phase cool white light-emitting diodes (WLEDs).

     

Effects of excitation wavelength and Y3+ content on luminescent properties of YMO4:Dy3+ (M = V,P) phosphors induced by ultraviolet excitation

Y_0.99VO₄:0.01Dy³⁺, Y_0.99PO₄:0.01Dy³⁺ and Y_xVO₄:0.01Dy³⁺ phosphors were synthesized by chemical co-precipitation method. All the samples were characterized by X-ray powder diffraction (XRD) and photoluminescence spectroscopy. XRD results show that the samples only have single tetragonal structure and the crystallinity of Y_0.99VO₄:0.01Dy³⁺ phosphor is higher than that of Y_0.99PO₄:0.01Dy³⁺ phosphor when the heat treatment process is same. Photoluminescence excitation spectra results show that the Y_0.99VO₄:0.01Dy³⁺ and Y_0.99PO₄:0.01Dy³⁺ phosphors can be efficiently excited by ultraviolet light from 250 nm to 380 nm, the former have a wide Dy³⁺–O^2? charge transfer band ranging from 260 nm to 350 nm including a peak at 310 nm, the latter have four peaks at 294 nm, 326 nm, 352 nm and 365 nm. Emission spectra of all the samples exhibit a strong blue emission (483 nm) and another strong yellow emission (574 nm). Moreover, the yellow-to-blue emission intensity ratio and color temperature of emission of Dy³⁺ are strongly related to excitation wavelength in Y_0.99PO₄:0.01Dy³⁺ phosphor, but it is almost not in Y_0.99VO₄:0.01Dy³⁺ phosphor. For Y_xVO₄:0.01Dy³⁺ (x = 0.94, 0.97, 0.99, 1.01, 1.03) phosphors, with increasing value of x, the body color of phosphor changes from yellow to white and the strongest peak in excitation spectra shifts a little to shorter wavelength. It is detrimental to luminous intensity when Y³⁺ content deviate stoichiometric ratio, but the influence of Y³⁺ on the color temperature of emission of YVO₄:Dy³⁺ phosphor is slight.     

Photon-avalanche upconversion of Ho in NaYF4 and enhancement of violet, blue and green emissions induced by sensitization of Tm

Ho³⁺ singly doped and Ho³⁺/Tm³⁺ co-doped hexagonal NaYF₄ powders have been synthesized by a solid-state reaction method. Under excitation of 671 nm diode laser, upconverted blue, green and red emission bands are observed in Ho³⁺ singly doped sample. Temporal evolution and excitation power dependent behavior for the green emission are explored, indicating that a photon-avalanche mechanism is responsible for the upconversion processes in Ho³⁺ singly doped hexagonal NaYF₄ sample. With the introduction of Tm³⁺, the intensities of both blue and green emissions of Ho³⁺ are efficiently enhanced, which are attributed to two energy transfer processes from Tm³⁺ to Ho³⁺, i.e., ³F₄ (Tm³⁺) + ⁵I₈(Ho³⁺) → ³H₆ (Tm³⁺) + ⁵I₇(Ho³⁺) and ¹G₄ (Tm³⁺) + ⁵I₈(Ho³⁺) → ³H₆ (Tm³⁺) + ⁵F₃(Ho³⁺). The result offers a new sensitization approach to enhance the upconversion efficiency of Ho³⁺ under 671 nm excitation.     

Ultraviolet upconversion luminescence of Gd3+ and Eu3+ in nano-structured glass ceramics

Ultraviolet multiphoton upconversion emissions of Eu³⁺ (⁵H_3–7, ⁵G_2–6, ⁵L₆ → ⁷F₀) and Gd³⁺ (⁶I_J, ⁶P_J → ⁸S_7/2) are studied in the Eu³⁺ (or Gd³⁺) doped SiO₂–Al₂O₃–NaF–YF₃ precursor glasses and glass ceramics containing β-YF₃ nanocrystals, under continuous-wavelength 976 nm laser pumping. It is experimentally demonstrated that energy transfer from Yb³⁺ to Tm³⁺, then further to Eu³⁺ or Gd³⁺ is responsible for the upconversion process. Compared to those in the precursor glasses, the upconversion emission intensities in the glass ceramics are greatly enhanced, owing to the participation of rare earth ions into the low-phonon-energy environment of β-YF₃ nanocrystals. Hopefully, the studied glass ceramics may find potential applications in the field of ultraviolet solid-state lasers.     

Powder synthesis and white light-emitting properties of Eu3+ co-doped K2CaP2O7:Dy3+ phosphors with energy transfer between Dy3+ and Eu3+

A series of white-emitting K₂CaP₂O₇:Dy³⁺ and K₂CaP₂O₇:Dy³⁺, Eu³⁺ phosphors were synthesized via a solid-state method, and Eu³⁺ was co-doped in K₂CaP₂O₇:Dy³⁺ to improve its white light performance. The influences of preparation temperature and Dy³⁺/Eu³⁺ concentration on the crystal structure and photoluminescence characteristics were investigated. XRD results indicate that K₂CaP₂O₇:Dy³⁺ samples prepared above 700 °C matches the standard K₂CaP₂O₇ phase. Under excitation of 349 nm, K₂CaP₂O₇:Dy³⁺ phosphor exhibited characteristic emission peaks at 487 nm (blue) and 579 nm (yellow), and white emission was realized through combining these blue and yellow emissions. After co-doping Eu³⁺ ions, the co-luminescence of Dy³⁺/Eu³⁺ with energy transfer between Dy³⁺and Eu³⁺ were demonstrated. The chromaticity of white light was controlled by changing the ratio of Dy³⁺/Eu³⁺ concentrations, which lead to a warm white light. Therefore, the results indicate that K₂CaP₂O₇:Dy³⁺, Eu³⁺ powders have a potential application in w-LEDs as single-component white-emitting phosphor.