Energy transfer in LaMgB5O10:Pr,Mn under VUV excitation

Luminescent properties of Pr³⁺ or Mn²⁺ singly doped and Pr³⁺, Mn²⁺ co-doped LaMgB₅O₁₀ are investigated by synchrotron radiation VUV light. When LaMgB₅O₁₀:Pr³⁺ is excited at185 nm, the photon cascade emission between 4f levels of Pr³⁺ is observed. In the excitation spectra of LaMgB₅O₁₀:Mn²⁺ monitoring the 615 nm emission of Mn²⁺, several excitation bands in a spectral range from 330 to 580 nm are recorded, among which the most intense band is centered at 412 nm (⁶A_1g→⁴E_g-⁴A_1g). This band has considerable spectra overlap with the 410 nm emission (¹S₀→¹I₆) of Pr³⁺, which is favorable for energy transfer from Pr³⁺ to Mn²⁺. Such energy transfer is observed in the co-doped sample, converting the violet emission (410 nm) of Pr³⁺ into the red emission (615 nm) of Mn²⁺. The concentration dependence of transfer efficiency is also investigated.     

Characterization and luminescent properties of SiO2:ZnS:Mn and ZnS:Mn nanophosphors synthesized by a sol-gel method

ZnS and SiO₂-ZnS nanophosphors, with or without different concentration of Mn²⁺ activator ions, were synthesized by using a sol-gel method. Dried gels were annealed at 600 °C for 2 h. Structure, morphology and particle sizes of the samples were determined by using X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The diffraction peaks associated with the zincblende and the wurtzite structures of ZnS were detected from as prepared ZnS powders and additional diffraction peaks associated with ZnO were detected from the annealed powders. The particle sizes of the ZnS powders were shown to increase from 3 to 50 nm when the powders were annealed at 600 °C. An UV-Vis spectrophotometer and a 325 nm He-Cd laser were used to investigate luminescent properties of the samples in air at room temperature. The bandgap of ZnS nanoparticles estimated from the UV-Vis data was 4.1 eV. Enhanced orange photoluminescence (PL) associated with ⁴T₁→⁶A₁ transitions of Mn²⁺ was observed from as prepared ZnS:Mn²⁺and SiO₂-ZnS:Mn²⁺ powders at 600 nm when the concentration of Mn²⁺ was varied from 2-20 mol%. This emission was suppressed when the powders were annealed at 600 °C resulting in two emission peaks at 450 and 560 nm, which can be ascribed to defects emission in SiO₂ and ZnO respectively. The mechanism of light emission from Mn²⁺, the effect of varying the concentration on the PL intensity, and the effect of annealing are discussed.     

Photophysical and photoluminescence properties of co-activated ZnS:Cu,Mn phosphors

ZnS:Cu,Mn phosphors were prepared by conventional solid state reaction with the aid of NaCl-MgCl₂ flux at 900 °C. The samples were characterized by X-ray powder diffraction, UV-vis absorbance spectra and photoluminescence spectra. All samples possess cubic structure. Cu has a much stronger effect on the absorption property of ZnS than Mn. Incorporation of Mn into ZnS host only slightly enhances the light absorption, while addition of Cu remarkably increases the ability of absorption due to ground state Cu⁺ absorption. The emission spectra of the ZnS:Cu,Mn phosphors consist of three bands centered at about 452, 520 and 580 nm, respectively. Introduction of Mn significantly quenches the green luminescence of ZnS:Cu. The excitation energy absorbed by Cu is efficiently transferred to Mn activators non-radiatively and the Mn luminescence can be sensitized by Cu behaving as a sensitizer (energy donor).     

Synthesis, characterization and optical properties of water-soluble ZnS:Mn nanoparticles

ZnS nanoparticles with Mn²⁺ doping (0.5-20%) have been prepared through a simple chemical method, namely the chemical precipitation method. The structure of the nanoparticles has been analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and UV-vis spectrometer. The size of the particles is found to be 3-5 nm range. Photoluminescence spectra were recorded for undoped ZnS nanoparticles using an excitation wavelength of 320 nm, exhibiting an emission peak centered at around 445 nm. However, from the Mn²⁺-doped samples, a yellow-orange emission from the Mn²⁺⁴T₁-⁶A₁ transition is observed along with the blue emission. The prepared Mn²⁺-doped sample shows efficient emission of yellow-orange light with the peak emission 580 nm with the blue emission suppressed. The maximum PL intensity is observed only at the excitation energy of 3.88 eV (320 nm). Increase in stabilizing time up to 48 h in de-ionized water yields the enhancement of emission intensity of doped (4% Mn²⁺) ZnS. The correlation made through the concentration of Mn²⁺ versus PL intensity resulted in opposite trend (mirror image) of blue and yellow emissions.     

Synthesis and photoluminescence of water-soluble Mn-doped ZnS quantum dots

The water-soluble Mn²⁺-doped ZnS quantum dots (Mn:ZnS d-dots) were synthesized by using thioglycolic acid (TGA) as stabilizer in aqueous solutions in air, and characterized by X-ray powder diffraction (XRD), UV-vis absorption spectra and photoluminescence (PL) emission spectroscopy. The sizes of Mn:ZnS d-dots were determined to be about 2 nm using XRD measurements and the UV-vis absorption spectra. It was found that the Mn²⁺⁴T₁ → ⁶A₁ emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn²⁺ concentration, and showed a maximum when Mn²⁺ doping content was 1.5%. If Mn²⁺ concentration continued to increase, namely more than 1.5%, the Mn²⁺⁴T₁ → ⁶A₁ emission intensity would decrease. In addition, the effects of TGA/(Zn + Mn) molar ratio on PL were investigated. It was found that the peak intensity ratio of Mn²⁺⁴T₁ → ⁶A₁ emission to defect-states emission showed a maximum when the TGA/(Zn + Mn) molar ratio was equal to 1.8.     

Optical properties of annealed Mn-doped ZnS nanoparticles

Cysteine stabilized ZnS and Mn²⁺-doped ZnS nanoparticles were synthesized by a wet chemical route. Using the ZnS:Mn²⁺ nanoparticles as seeds, silica-coated ZnS (ZnS@Si) and ZnS:Mn²⁺ (ZnS:Mn²⁺@Si) nanocomposites were formed in water by hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The influence of annealing in air, formier gas, and argon at 200-1000 °C on the chemical stability of ZnS@Si and ZnS:Mn²⁺@Si nanoparticles with and without silica shell was examined. Silica-coated nanoparticles showed an improved thermal stability over uncoated particles, which underwent a thermal combustion at 400 °C. The emission of the ZnS@Si and ZnS:Mn²⁺@Si passed through a minimum in photoluminescence intensity when annealed at 600 °C. Upon annealing at higher temperatures, ZnS@Si conserved the typical emission centered at 450 nm (blue). ZnS:Mn²⁺@Si yielded different high intensity emissions when heated to 800 °C depending on the gas employed. Emissions due to the Mn²⁺ at 530 nm (green; Zn₂SiO₄:Mn²⁺), 580 nm (orange; ZnS:Mn²⁺@Si), and 630 nm (red; ZnS:Mn²⁺@Si) were obtained. Therefore, with a single starting product a set of different colors was produced by adjusting the atmosphere wherein the powder is heated.     

SrS:Ce/ZnS:Mn-A di-band phosphor for near-UV and blue LED-converted white-light emitting diodes

The SrS:Ce/ZnS:Mn phosphor blends with various combination viz 75:25, 50:50 and 25:75 were assign to generate the white-light emission using near-UV and blue-light emitting diodes (LED) as an excitation source. The SrS:Ce exhibits strong absorption at 427 nm and the corresponding intense emission occurs at 480 and 540 nm due to electron transition from 5d(²D)−4f(²F_5/2, _7/2) of Ce³⁺ ion as a result of spin-orbit coupling. The ZnS:Mn excited under same wavelength shows broad emission band with λ_max=582 nm originates due to 3d (⁴G−⁶S) level of Mn²⁺. Photoluminescence studies of phosphor blend excited using near-UV to blue light confirms the emitted radiation varies from cool to warm white light in the range 430-600 nm, applicable to LED lightings. The CIE chromaticity coordinate values measured using SrS:Ce/ZnS:Mn phosphor blend-coated 430 nm LED pumped phosphors in the ratio 75:25, 50:50 and 25:75 are found to be (0.235, 0.125), (0.280, 0.190) and (0.285, 0.250), respectively.     

Synthesis and luminescence properties of Tb:NaGd(WO4)2 novel green phosphors

Tb³⁺:NaGd(WO₄)₂ (Tb:NGW) phosphors with different Tb³⁺ concentrations have been synthesized by a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the Tb:NGW phosphors. XRD analysis confirmed the formation of NGW with scheelite structure. SEM study showed that the obtained Tb:NGW phosphors appeared to be nearly spherical and their sizes ranged from 1 to 1.5 μm. The excitation spectra of these systems showed an intense broad band with maximum at 270 nm related to the O→W ligand-to-metal charge-transfer state. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under UV light excitation. Analysis of the photoluminescence spectra with different Tb³⁺ concentrations revealed that the optimum dopant concentration for Tb³⁺ is about 15 at% of Tb³⁺ ions in Tb:NGW phosphors.     

The effect of Y co-doping on the persistent up-conversion luminescence of the ZrO2:Yb,Er nanomaterials

The effect of the defects due to the charge compensation obtained with the yttrium co-doping to the ZrO₂:Yb³⁺,Er³⁺ up-converting phosphors was studied. The materials were prepared with the combustion method. The materials purity was analyzed with the FT-IR spectroscopy. The crystal structure was studied with the X-ray powder diffraction and the crystallite sizes were estimated with the Scherrer formula. Up-conversion luminescence was excited at room temperature with an IR-laser at 970 nm. The up-conversion luminescence spectra showed red (650-685 nm) and green emission (520-560 nm) due to the ⁴F_9/2→⁴I_15/2 and (²H_11/2,⁴S_3/2)→⁴I_15/2 transitions of Er³⁺, respectively. Persistent up-conversion luminescence was observed both in the Yb³⁺,Er³⁺ and Y³⁺,Yb³⁺,Er³⁺ doped materials.     

Spectral analysis of Pr-, Sm- and Dy-doped transparent GeO2-BaO-TiO2 glass ceramics

In this paper, we present the photoluminescence properties of Pr³⁺-, Sm³⁺- and Dy³⁺-doped germanate glasses and glass ceramics. From the X-ray diffraction measurement, the host glass structure was determined. These glasses have shown strong absorption bands in the near-infrared (NIR) region. Compared to Pr³⁺-, Sm³⁺- and Dy³⁺-doped glasses, their respective glass ceramics have shown stronger emissions due to the Ba₂TiGe₂O₈ crystalline phase. For Pr³⁺-doped glass and glass ceramic, emission bands centered at 530 nm (³P₀→³H₅), 614 nm (³P₀→³H₆), 647 nm (³P₀→³F₂) and 686 nm (³P₀→³F₃) have been observed with 485 nm (³H₄→³P₀) excitation wavelength. Of them, 647 nm (³P₀→³F₂) has shown bright red emission. Emission bands of ⁴G_5/2→⁶H_5/2 (565 nm), ⁴G_5/2→⁶H_7/2 (602 nm) and ⁴G_5/2→⁶H_9/2 (648 nm) for the Sm³⁺:glass and glass ceramic, with excitation at ⁶H_5/2→⁴F_7/2 (405 nm) have been recorded. Of them, ⁴G_5/2→⁶H_7/2 (602 nm) has shown a bright orange emission. With regard to the Dy³⁺:glass and glass ceramic, a bright fluorescent yellow emission at 577 nm (⁴F_9/2→⁶H_13/2) has been observed, apart from ⁴F_9/2→⁶H_11/2 (667 nm) emission transition with an excitation at 454 nm (⁶H_15/2→⁴I_15/2) wavelength. The stimulated emission cross-sections of all the emission bands of Pr³⁺, Sm³⁺ and Dy³⁺:glasses and glass ceramics have been computed based on their measured full-width at half-maxima (FWHM, Δλ) and lifetimes (τ_m).     

Spectroscopic investigations of Nd-, Er-, Er/Yb-, and Tm-ions doped SiO2-Al2O3-CaF2-GdF3 glasses

This paper reports on the absorption, visible and near-infrared luminescence properties of Nd³⁺, Er³⁺, Er³⁺/2Yb³⁺, and Tm³⁺ doped oxyfluoride aluminosilicate glasses. From the measured absorption spectra, Judd-Ofelt (J-O) intensity parameters (Ω₂, Ω₄ and Ω₆) have been calculated for all the studied ions. Decay lifetime curves were measured for the visible emissions of Er³⁺ (558 nm, green), and Tm³⁺ (650 and 795 nm), respectively. The near infrared emission spectrum of Nd³⁺ doped glass has shown full width at half maximum (FWHM) around 45 nm (for the ⁴F_3/2→⁴I_9/2 transition), 45 nm (for the ⁴F_3/2→⁴I_11/2 transition), and 60 nm (for the ⁴F_3/2→⁴I_13/2 transition), respectively, with 800 nm laser diode (LD) excitation. For Er³⁺, and Er³⁺/2Yb³⁺ co-doped glasses, the characteristic near infrared emission bands were spectrally centered at 1532 and 1544 nm, respectively, with 980 nm laser diode excitation, exhibiting full width at half maximum around 50 and 90 nm for the erbium ⁴I_13/2→⁴I_15/2 transition. The measured maximum decay times of ⁴I_13/2→⁴I_15/2 transition (at wavelength 1532 and 1544 nm) are about 5.280 and 5.719 ms for 1Er³⁺ and 1Er³⁺/2Yb³⁺ (mol%) co-doped glasses, respectively. The maximum stimulated emission cross sections for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ and Er³⁺/Yb³⁺ are 10.81×10⁻²¹ and 5.723×10^-21 cm². These glasses with better thermal stability, bright visible emissions and broad near-infrared emissions should have potential applications in broadly tunable laser sources, interesting optical luminescent materials and broadband optical amplification at low-loss telecommunication windows.     

NIR to visible upconversion emission from Dy: ZBLYAN glasses

This paper reports on the near infrared (730-783 nm) to the visible upconversion emissions at 482 nm (⁴F_9/2→⁶H_15/2), 576 nm (⁴F_9/2→⁶H_13/2) and 662 nm (⁴F_9/2→⁶H_11/2) from the Dy³⁺doped 53ZrF₄-20BaF₂-2LaF₃-2YF₃-3AlF₃-19NaF-1DyF₃ glasses. We have also carried out a systematic study on the normal emission properties of these glasses in order to understand their performance both as a NIR upconverted visible luminescent and as normal visible fluorescent optical systems of technical importance. With an increase in Dy³⁺ concentration beyond a particular value (1 mol%), activator-activator interaction becomes a significant cause of concentration quenching in the luminescence properties. The dependence of the emission spectra on the excitation wavelengths has also been examined and 451 nm was found to be the ideal excitation wavelength in the measurement of normal fluorescence spectra. In the case of NIR upconverted visible emission, we have observed that the NIR excitation at 783 nm as the suitable pump wavelength in demonstrating prominent visible emission colours from these glasses. The relevance in undertaking these optical materials lies in their potential for upconversion laser application in the visible wavelength region. The NIR upconversion phenomenon has been explained in terms of energy level schemes due to excited state absorption (ESA) and energy transfer upconversion (ETU) processes.     

Spectral performance and intensive green upconversion luminescence in Er/Yb-codoped NaY(WO4)2 crystal

Using Czochralski (CZ) pulling method, an Er³⁺/Yb³⁺-codoped NaY(WO₄)₂ crystal was prepared. Absorption spectra, emission spectra and excitation spectra of this crystal were measured at room temperature. Some optical parameters, such as intensity parameters, spontaneous emission probabilities and lifetimes, were calculated from absorption spectra with Judd-Ofelt (J-O) theory. Upconversion luminescence excited by a 970 nm diode laser was studied. In this crystal, green upconversion luminescence is particularly intensive. Energy transfer mechanisms that play an important role in upconversion processes were analyzed. Two cross-relaxation processes: ⁴G_11/2 + ⁴I_9/2 → ²H_11/2 (or ⁴S_3/2) + ²H_11/2 (or ⁴S_3/2), and ⁴G_11/2 + ⁴I_15/2 → ²H_11/2 (or ⁴S_3/2) + ²I_13/2, which contribute to the intensive green luminescence under 378 nm excitation, were put forward. Background energy transfer ⁴G_11/2(Er³⁺) + ²F_7/2(Yb³⁺) → ⁴F_9/2(Er³⁺) + ²F_5/2(Yb³⁺) was also demonstrated.     

Charge compensation on the luminescence properties of ZnWO4:Tb phosphors via hydrothermal synthesis

A phosphor Tb³⁺-doped ZnWO₄ (ZWO:Tb) phosphors were prepared by a hydrothermal method. X-ray powder diffraction (XRD) analysis revealed that the as-obtained sample is pure ZnWO₄ phase. The excitation and emission spectra indicated that the phosphor could be well excited by ultraviolet light (272 nm) and emit blue light at about 491 nm and green light at about 545 nm. Significant energy transfer from WO₄²⁻ groups to Tb³⁺ ions has been observed. Two approaches to charge compensation are investigated: (a) 2Zn²⁺ = Tb³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Zn²⁺ = 2Tb³⁺ + vacancy. Compared with two charge compensation patterns in the ZnWO₄:Tb³⁺, it has been found that ZnWO₄:Tb³⁺ phosphors used Li⁺ as charge compensation show greatly enhanced bluish-green emission under 272 nm excitation.     

Synthesis and optical characterization of PVP and SHMP-encapsulated Mn-doped ZnS nanocrystals

Zinc sulfide semiconductor nanocrystals doped Mn²⁺ have been synthesized via a solution-based method utilizing optimum dopant concentration (4%) and employing polyvinyl pyrrolidone (PVP) and sodium hexametapolyphosphate (SHMP) as capping agents. UV-vis absorbance spectra for all of the synthesized nanocrystals show an exitonic peak at around 310 nm. The particle size and morphology were characterized by scanning electron microscopy (SEM), FT-IR, X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectrum (PL). Diffraction data confirmed that the crystallite size is around 3-5 nm. Room temperature photoluminescence (PL) spectrum for the bare ZnS sample shows a strong band at ∼445 nm. The uncapped and capped(SHMP, PVP) ZnS:Mn²⁺ samples show a strong and broad band in the ∼580-585 nm range.     

Emission analysis of Dy and Pr:Bi2O3-ZnF2-B2O3-Li2O-Na2O glasses

In this paper, we present the spectral results of Dy³⁺ and Pr³⁺ (1.0 mol%) ions doped Bi₂O₃-ZnF₂-B₂O₃-Li₂O-Na₂O glasses. Measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC) profiles of these rare-earth ions doped glasses have been carried out. From the DSC thermograms, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The direct and indirect optical band gaps have been calculated based on the glasses UV absorption spectra. The emission spectrum of Dy³⁺:glass has shown two emission transitions ⁴F_7/2→⁶H_15/2 (482 nm) and ⁴F_7/2→⁶H_13/2 (576 nm) with an excitation at 390 nm wavelength and Pr³⁺:glass has shown a strong emission transition ¹D₂→³H₄ (610 nm) with an excitation at 445 nm. Upon exposure to UV radiation, Dy³⁺ and Pr³⁺ glasses have shown bright yellow and reddish colors, respectively, from their surfaces.     

Spectral-kinetic characteristics of Pr luminescence in LiLuF4 host upon excitation in the UV-VUV range

Spectral-kinetic study of Pr³⁺ luminescence has been performed for LiLuF₄:Pr(0.1 mol%) single crystal upon the excitation within 5-12 eV range at T=8 K. The fine-structure of Pr³⁺ 4f ²→4f 5d excitation spectra is shown for LiLuF₄:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr³⁺ ions into 4f 5d involving 4f ¹ core in the ground state. Favourable conditions have been revealed in LiLuF₄:Pr(0.1 mol%) for the transformation of UV-VUV excitation quanta into the visible range. Lightly doped LiLuF₄:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr³⁺³P₀ visible emission upon UV-VUV excitation. The mechanism of energy transfer between Lu³⁺ host ion and Pr³⁺ impurity is discussed.     

Photoluminescence characteristics of energy transfer between Er and Bi in Gd2O3: Er, Bi

The phosphors, Bi³⁺- activated Gd₂O₃:Er³⁺, were prepared by sol-gel combustion method, and their photoluminescent properties were investigated under ultraviolet light excitation. The emission spectrum exhibited sharp peaks at about 520, 535, 545, 550 and 559 nm due to (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺ ions. The luminescent intensity was remarkably improved by the incorporation of Bi³⁺ ions under 340 nm light excitation, which suggested very efficient energy transfer from Bi³⁺ ions to Er³⁺ions. The introducing of Bi³⁺ ions broadened the excitation band of the phosphor, of which a new strong peak occurred ranging from 320 to 360 nm due to the 6s²→6s6p transition of Bi³⁺ ions. There is significant energy overlap between the emission band of Bi³⁺ ions and the excitation band of Er³⁺ ions. Under 340 nm light excitation, Bi³⁺ absorbed most of the energy and transferred it to Er³⁺. The energy transfer probability from Bi³⁺ to Er³⁺ is strongly dependent on the Bi³⁺ ion concentration. Also, the sensitization effectiveness was studied and discussed in this paper.     

Preparation, characterization and study of optical properties of ZnS nanophosphor

In this work the preparation, characterization and photoluminescence studies of pure and copper-doped ZnS nanophosphors are reported, which are prepared by using solid-state reaction technique at a temperature of 100 °C. The as-obtained samples were characterized by X-ray diffraction (XRD) and UV-VIS Reflectance spectroscopy. The XRD analysis confirms the formation of cubic phase of undoped as well as Cu²⁺-doped ZnS nanoparticles. Furthermore it shows that the average size of pure as well as copper-doped samples ranges from 15 to 50 nm. The room-temperature PL spectra of the undoped ZnS sample showed two main peaks centered at around 421 and 450 nm, which are the characteristic emissions of interstitial zinc and sulfur vacancies, respectively. The PL of the doped sample showed a broad-band emission spectrum centered at 465 nm accompanied with shoulders at around 425, 450 and 510 nm, which are the characteristic emission peaks of interstitial zinc, sulfur vacancies and Cu²⁺ ions, respectively. Our experimental results indicate that the PL spectrum confirms the presence of Cu²⁺ ions in the ZnS nanoparticles as expected.     

Synthesis of high quality and monodisperse CdS:Mn/ZnS and CdS:Mn/CdS core–shell nanoparticles

CdS:Mn²⁺/ZnS and CdS:Mn²⁺/CdS core–shell nanoparticles were synthesized in aqueous medium via chemical precipitation method in an ambient atmosphere. Polyvinylpyrrolidone (PVP) was used as a capping agent. The effect of the shell (ZnS and CdS) thickness on CdS:Mn²⁺ nanoparticles was investigated. Inorganically passivated core/shell nanocrystals having a core (CdS:Mn²⁺) diameter of 4 nm and a ZnS-shell thickness of ∼0.5 nm exhibited improved PL intensity. Optimum concentration of doping ions (Mn²⁺) was selected through optical study. For all the core–shell samples two emission peaks were observed, the first one is band edge emission in the lower wavelength side due to energy transfer to the Mn²⁺ ions in the crystal lattice; the second emission is characteristic peak of Mn²⁺ ions (⁴T₁ → ⁶A₁). The XRD, TEM and PL results showed that the synthesized core–shell particles were of high quality and monodisperse.