Emission analysis of RE (Dy or Tb):Ca3Ln(Y,Gd)(VO4)3 powder phosphors

This paper reports on the photoluminescence results of Dy³⁺ or Tb³⁺ ions in Ca₃Ln(Y,Gd)(VO₄)₃ powder phosphors. Emission spectra of Dy³⁺:Ca₃Ln(Y,Gd)(VO₄)₃ powder phosphors have shown blue emissions (⁴F_9/2 → ⁶H_15/2) at 481 nm, yellow emissions (⁴F_9/2 → ⁶H_13/2) at 572 nm and a weak red emissions (⁴F_9/2 → ⁶H_11/2) at 661 nm upon excitation with λ_exci = 310 nm (⁶H_15/2 → ⁴L_19/2). Similarly photoluminescence spectra of Tb³⁺:Ca₃Ln(Y,Gd)(VO₄)₃ have shown green emissions (⁵D₄ → ⁷F₅) at 545 nm with λ_exci = 312 nm. For these phosphors XRD, FTIR, SEM and EDAX measurements have also been carried out.     

Structural evolution and its influence on luminescence of SiO2–SrF2–ErF3 glass ceramics prepared by sol–gel method

Er³⁺ doped SrF₂–SiO₂ transparent glass ceramics were prepared by sol–gel method and heat treatment. The decomposition of Sr²⁺–CF₃COO⁻ and the formation of SrF₂ nano-crystals were found to proceed synchronously in the xerogel. After crystallization of the xerogel, SrF₂ nano-crystals with 8–10 nm in size distributed homogenously among the glassy matrix, and the microstructure of the glass ceramic was stable under and at the temperature of 800 °C probably due to interfacial interaction between nano-crystals and glassy matrix. When heat-treated at 800 °C, the chemically bonded water in the sample was eliminated, resulting in the appearance of the visible luminescence bands of ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions.     

Up-conversion luminescence properties of Y2O2S:Yb,Er nanophosphors

Up-converting yttrium oxysulfide nanomaterials doped with ytterbium and erbium (Y₂O₂S:Yb³⁺,Er³⁺) were prepared with the flux method. The precursor oxide materials were prepared using the combustion synthesis. The morphology of the oxysulfides was characterized with transmission electron microscopy (TEM). The particle size distribution was 10–110 nm, depending on the heating temperature. According to the X-ray powder diffraction (XPD), the crystal structure was found hexagonal and the particle sizes estimated with the Scherrer equation agreeded with the TEM images. Upon the 970 nm infrared (IR) laser excitation, the materials yield moderate green ((²H_11/2, ⁴S_3/2) → ⁴I_15/2 transition) and strong red (⁴F_9/2 → ⁴I_15/2) luminescence. The green luminescence was enhanced with respect to the red one by an increase in both the crystallite size and erbium concentration due to the cross-relaxation (CR) processes. The most intense up-conversion luminescence was achieved with x_Yb and x_Er equal to 0.10 and 0.005, respectively. Above these concentrations, concentration quenching occurred.     

Effect of Mg and Ti doping on the luminescence of Y2O2S:Eu

The Y₂O₂S:Eu³⁺,Mg²⁺,Ti^IV (x_Eu = 0.028, x_Mg = 0.086, x_Ti = 0.03) materials were prepared with the flux fusion method. According to X-ray powder diffraction, the materials had the hexagonal crystal structure. The emission of Y₂O₂S:Eu³⁺,Mg²⁺,Ti^IV was centered at 627 nm (λ_exc : 250 nm) due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The excitation spectra (λ_em : 627 nm) showed broad bands at 240 and 320 nm due to the O²⁻ → Eu³⁺ and S²⁻ → Eu³⁺ charge transfer transitions, respectively. The latter band can also overlap with the Ti → Eu³⁺ energy transfer. In the excitation spectra with synchrotron radiation, in addition to the O²⁻ → Eu³⁺ and S²⁻ → Eu³⁺ charge transfer transitions, excitation over the band gap was observed at 4.8 eV (258 nm). The red persistent luminescence due to the ⁵D₀ → ⁷F₂ emission from Eu³⁺ residing in the regular Y³⁺ site of the host was ca. 10 min with 1 min fluorescent lamp irradiation. In addition, a very broad band was observed at 600 nm probably due to the Ti³⁺ emission.     

Compositional dependence of spectroscopic parameters of Dy ions in Ge–Ga–Se glasses

Dy³⁺ doped chalcogenide glasses from Ge–Ga–Se system were prepared and spectroscopic parameters of Dy³⁺ ions in these glasses were studied on the basis of Judd–Ofelt theory. We have found that the values of Judd–Ofelt intensity parameters Ω_t (t = 2, 4, 6) have maximal values at the GeSe₂–Ga₂Se₃ tie-line. Largest values of spectroscopic parameters of 1.3 µm Dy³⁺ transition (⁶F_11/2, ⁶H_9/2 → ⁶H_15/2), localized again at the GeSe₂–Ga₂Se₃ tie-line, confirm the importance of the stoichiometry of amorphous matrix for optical properties of doping rare-earth ions.     

Dependence of optical properties on the composition in Er-doped xNaPO3–(80 − x)TeO2–10ZnO–10Na2O glasses

Transparent and uniform tellurite–phosphate glasses were prepared and the reason why the substitution of NaPO₃ for P₂O₅ can eliminate the coloration of tellurite–phosphate was discussed. The result of TDA indicated that introducing NaPO₃ into tellurite glasses can improve thermal stability of glass hosts. The compositional dependence of absorption cross-sections of ⁴I_13/2, ⁴I_11/2 and ²H_11/2 level, emission cross-section of ⁴I_13/2 level, host phonon energy, up-conversion and 1.5 μm optical emission intensity as well as and quantum yield for ⁴I_13/2 level in PTEr glasses were investigated too. By analyzing obtained data, authors believe that tellurite–phosphate glasses can be used as potential host material for developing optical amplifiers.     

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.     

Emissions properties of Ho: I7 → I8 transition sensitized by Er and Yb in fluorophosphate glasses

Emission properties of Ho³⁺ at 2.0 μm and the energy transfer mechanism between Yb³⁺, Er³⁺ and Ho³⁺ ions in fluorophosphate glasses are investigated. The measured emission spectra show that the ⁵I₇ → ⁵I₈ transition of Ho³⁺ upon 980 nm laser diode excitation is strong. Judd–Ofelt intensity parameters (Ω_λ, λ = 2, 4, 6), spontaneous transition probability (A_rad), radiative lifetime (τ_r), absorption cross section (σ_a), stimulated emission cross section (σ_e) and FWHM ×  for the transition of Ho³⁺: ⁵I₇ → ⁵I₈ are calculated and discussed. The obtained results show that the present Yb³⁺/Er³⁺/Ho³⁺ triply-doped fluorophosphate glass can be identified to be a promising material at 2.0 μm emission.     

Magneto-optical activity of f–f transitions in elpasolite Rb2NaDyF6

Absorption and magnetic circular dichroism (MCD) spectra of f–f transitions ⁶H_15/2 → ⁶F_3/2, ⁶F_5/2, ⁶(F_7/2 + H_5/2) have been measured in cubic crystal Rb₂NaDyF₆ with Dy³⁺ ions in centrosymmetrical O_h positions. Magneto-optical activities (MOA) of the transitions (the ratio of zero moments of the MCD and absorption bands) have been obtained from the corresponding spectra. Origins of the transitions MOA have been analyzed and theoretical estimations of the MOA values have been made. It turned out, that MOA of the transitions in the centrosymmetrical crystal Rb₂NaDyF₆ (being allowed by odd vibrations) are noticeably larger than those in non-centrosymmetrical compounds, where f–f transitions are allowed by static odd components of crystal field.     

Fabrication and optical properties of NUV-emitting SiO2–SrB4O7:Eu glass–ceramic thin films

SiO₂–SrB₄O₇:Eu²⁺ glass–ceramic thin films were fabricated for possible application in near ultraviolet (NUV) emitting devices. Nano-sized SrB₄O₇:Eu²⁺ powders were prepared by a Pechini-type sol–gel method and a subsequent ball-milling treatment. The powders showed NUV emissions centered at 367 nm, upon irradiation with UV of shorter wavelengths, due to an allowed 4f⁶5d¹ → 4f⁷ electronic transition of Eu²⁺ ions. The glass–ceramic thin films were prepared by dip-coating of tetraethylorthosilicate (TEOS) solutions dispersed with the nano-sized SrB₄O₇:Eu²⁺ powders and a subsequent heat-treatment. It was found that the glass–ceramic thin films had relatively high thermal stability up to 800 °C in terms of the Eu²⁺ emissions. SiO₂ layers surrounding SrB₄O₇:Eu²⁺ appeared to be effective for the surface passivation of the phosphor particles.     

Dielectric, magnetic and spectroscopic properties of Li2O–WO3–P2O5 glass system with Ag2O as additive

Li₂O–WO₃–P₂O₅ glasses containing small concentrations of Ag₂O from 0 to 1 mol% were prepared. A number of studies viz., chemical durability, dielectric studies (constant ′, loss tan δ, a.c. conductivity σ_ac over a range of frequency and temperature), spectroscopic (infrared, optical absorption ESR spectra) and magnetic susceptibility studies of these glasses, have been carried out. The interesting variations observed in all these properties with the concentration of Ag⁺ ions have been analyzed in the light of different oxidation states and environment of tungsten ions in the glass network.     

Fabrication and photoluminescence of thin films of lanthanide–polyoxometalates grafted on colloid silica sphere

In this paper, two lanthanide–polyoxometalates Na₉LnW₁₀O₃₆ (Ln=Eu, Dy, LnW₁₀) were grafted on 400 nm amine-functionalized spherical Stöber silica. The monolayer thin films of hybrid particles were fabricated on quartz by spin-coating method. The hybrid particles and thin films obtained were characterized by IR, UV–vis spectra, scanning electron microscopy, transmission electron microscope and luminescent spectra, respectively. The hybrid particles show strong luminescence which could be seen by naked eyes. The excitation spectra of hybrid particles and thin films show both abroad ligand to metal charge band and the excitation lines of rare earth ions. The transition ⁵D₀→⁷F₀ could be seen in the emission of hybrid EuW₁₀/SiO₂ spheres and thin films, which could not be found in spectrum of EuW₁₀ solid. It is noticed that the intensity ratio of red (⁵D₀→⁷F₂) to orange (⁵D₀→⁷F₁) of Eu³⁺ and the yellow to blue (⁴F_9/2→⁶H_13/2: ⁴F_9/2→⁶H_15/2) of Dy³⁺ in LnW₁₀/SiO₂ particles and thin films are quite different from those of LnW₁₀ solids. The different shapes and ratios between characteristic emissions of hybrid particles and thin films indicated the various symmetry of sites occupied by Ln³⁺ ions.     

A novel green emitting phosphor Ca1.5Y1.5Al3.5Si1.5O12:Tb

Vacuum ultraviolet (VUV) excitation and emission properties of Tb³⁺ ion doped silico-aluminate phosphor Ca_1.5Y_1.5Al_3.5Si_1.5O₁₂:Tb³⁺ was studied. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphor showed strong green-emission peaked at 545 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 545 nm was found at a content of about 14 mol% Tb³⁺. The 4f–5d transition absorption of Tb³⁺ is in the range from 150 nm to 260 nm, and there is an energy transfer from the host to the rare earth ions. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.     

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.     

Optical properties of transparent Dy doped Ba2TiSi2O8 glass ceramic

The Ba₂TiSi₂O₈ is a well known piezoelectric, ferroelectric and non-linear crystal. Nanocrystals of Ba₂TiSi₂O₈ doped with 1.5 Dy³⁺ have been obtained by thermal treatment of a precursor glass and their optical properties have been studied. X-ray diffraction patterns and optical measurements have been carried out on the precursor glass and glass ceramic samples. The emission spectra corresponding to the Dy³⁺: ⁴F_9/2 → ⁶H_13/2 (575 nm), ⁴F_9/2 → ⁶H_11/2 (670 nm) and ⁴F_9/2 → ⁶H_9/2 (757 nm) transitions have been obtained under laser excitation at 473 nm. These measurements confirm the incorporation of the Dy³⁺ ions into the Ba₂TiSi₂O₈ nanocrystals which produces an enhancement of luminescence at 575 nm. At this wavelength has been demonstrated a maximum optical amplification around 1.9 cm⁻¹ (∼8.2 dB/cm).     

VUV–UV spectroscopic properties of RE (RE = Ce, Eu and Tb)-doped KMLn(PO4)2 (M = Ca, Sr; Ln = Y, La, Lu)

The VUV excited luminescent properties of Ce³⁺, Eu³⁺ and Tb³⁺ in the matrices of KMLn(PO₄)₂ (M²⁺ = Ca, Sr; Ln³⁺ = Y, La, Lu) were investigated. The bands at about 155 nm in the VUV–UV excitation spectra are attributed to the host lattice absorption, which indicates that the optical band gap of KMLn(PO₄)₂ is about 8.0 eV. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 300–450 nm, and the energy transfer from host lattice to Ce³⁺ is efficient. For Eu³⁺-doped samples, the O²⁻–Eu³⁺ CTBs are observed to be at about 228 nm except KSrLu(PO₄)₂:Eu³⁺ (247 nm). As for Tb³⁺-doped samples, typical 4f → 5d absorption bands in the region of 175–250 nm were observed.     

Preparation, structure analysis and photoluminescence properties of MgGa2O4:Mn

In this work the photoluminescence and excitation spectra at room temperature of the spinel-type MgGa₂O₄ with 0.5% and 10.0% of Mn²⁺ have been studied. The polycrystalline samples were synthesized by standard solid-state reaction methods at high temperature. The photoluminescence spectra exhibit green and red emissions for both samples, attributed to ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn²⁺ ion in tetrahedral and octahedral sites of oxygen, respectively. The excitation spectra exhibit features unambiguously assigned to d–d transitions of Mn²⁺ in those kinds of sites. From the excitation spectra and Tanabe–Sugano matrices the crystal field Dq and Racah B parameters were obtained.     

Ultrafast third-order optical non-linearity of 0.56GeS2–0.24Ga2S3–0.2KX(X = Cl, Br, I) chalcohalide glasses by femtosecond Optical Kerr Effect

0.56GeS₂–0.24Ga₂S₃–0.2KX(X = Cl, Br, I) chalcohalide glasses were prepared and their third-order optical non-linearities χ⁽³⁾ have been studied systematically using the femtosecond time-resolved Optical Kerr Effect technique at wavelength of 800 nm. In this system, 0.56GeS₂–0.24Ga₂S₃–0.2KCl glass shows the largest χ⁽³⁾ (1.82 × 10⁻¹³ esu), but 0.56GeS₂–0.24Ga₂S₃–0.2KBr glass has the fastest optical non-linear response time in subpicosecond domain (about 340 fs), which is due to the ultrafast distortion of the electron clouds surrounding the balanced position of Ge, Ga, S, K and Br atoms. The local hyperpolarizability determining non-linear optical response are determined by the partially ionic bonds originating from Ge(Ga)–S bonds and halogen valence bonds. They show great potential applications on the glass-based optoelectronic devices like optical switching.     

Influence of calcination temperature on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol–gel-combustion processing

The detailed preparation process of Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ phosphor powders with red long afterglow by sol–gel-combustion method in the reducing atmosphere is reported. X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the effects of synthesis temperature on the crystal characteristics, morphology and luminescent properties of the as-synthesized Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors. The results reveal that Sr₃Al₂O₆ crystallizes completely when the combustion ash is sintered at 1200 °C. The excitation and the emission spectra indicate that the excitation broad-band lies chiefly in visible range and the phosphor powders emit strong light at 618 nm under the excitation of 472 nm. The light intensity and the light-lasting time of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors are increased when increasing the calcination temperatures from 1050 to 1200 °C. The afterglow of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors sintered at 1200 °C lasts for over 600 s when the excited source is cut off. The red emission mechanism is discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions.