Host dependent frequency upconversion of Er-doped oxyfluoride germanate glasses

Er³⁺-doped oxyfluoride germanate glasses have been synthesized by the conventional melting and quenching method. The Judd-Ofelt intensity parameters were calculated based on the Judd-Ofelt theory and absorption spectra measurements. With the substitution of PbF₂ for PbO, the Ω₂ parameter decreases, while the Ω₆ parameter increases. These change trends indicate that fluoride anions come to coordinate erbium cations and the covalency of the Er-O bond decreases. Structural and thermal stability properties were obtained by Raman spectra and differential thermal analysis, indicating that PbF₂ plays an important role in the formation of glass network and has an important influence on the maximum phonon energy and thermal stability of host glasses. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were simultaneously observed at room temperature. With increasing PbF₂ content, the intensity of red (657 nm) emissions increases significantly, while that of the green (525 and 546 nm) emission increases slightly. The results indicate that PbF₂ has more influence on the red (657 nm) emission than the green (525 and 546 nm) emissions in oxyfluoride germanate glasses. The possible upconversion luminescence mechanisms have also been estimated and discussed.     

A new synthetic pathway of Sr2CeO4 phosphor and its characterization

A blue-white emitting Sr₂CeO₄ phosphor was synthesized via a simple sol-gel poly vinyl alcohol (PVA)-complexing process using strontium nitrate and cerium nitrate as raw materials. The samples were characterized by TG/DTA, XRD, FTIR, SEM and photoluminescence spectroscopy. The X-ray diffraction study confirms the structure of the system to be orthorhombic. The emission spectra when excited at 267 nm peaks at ∼470 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer state of the Ce⁴⁺ ion. The Commission International de l'Eclairage (CIE) co-ordinates for the Sr₂CeO₄ sample were also calculated.     

Sr2CeO4电荷迁移发光的光谱结构规律研究

利用高温固相反应法分别合成了不同物相形成机理的Sr₂CeO₄，Sr ₂CeO₄∶Ca ²⁺和Sr₂CeO₄∶Ba²⁺样品，并对其光谱特性进行 了研究.结果发现，对于由SrO和CeO₂直接反应生成的Sr₂CeO₄(Ⅰ)，激发主峰位于256nm 左右；而对于SrCeO₃和SrO反应生成的Sr₂CeO₄(Ⅱ)，激发主峰位于279nm左右.在Sr₂CeO₄(Ⅰ)中掺入Ca²⁺，其激发光谱随着Ca²⁺离子浓度的增加逐渐接近于Sr₂CeO_{4(Ⅱ)的激发光谱.激发主峰带应属于CeO6八面体终端Ce⁴⁺—O^2-键的电荷迁移 带.对于激发光谱中340nm左右的弱激发峰，其峰值波长不受形成机理及Ca²⁺掺杂的影响，只是其强度 随着 激发主峰的红移而增加，它可能属于CeO6八面体平面上Ce⁴⁺—O2-键的电荷 迁移带.形成机理及Ca²⁺掺杂对发射光谱没有影响.Ca²⁺在Sr 2CeO 4(Ⅱ)与Ba²⁺在Sr2CeO4(Ⅰ)和(Ⅱ)中均难 于替代Sr²⁺的位置.}     

Enhancement of photoluminescence in Sr2CeO4 phosphors by doping with non-rare earth impurities

Non-rare earth impurity doped Sr 2 CeO 4:X (X=Zn,Hg,Al,Ag,Cr) phosphors are prepared by using the combustion method.The structural and photoluminescent properties of the as-prepared phosphors are investigated by X-ray diffraction (XRD) and photoluminescence at room temperature.Experimental results show that zinc addition and firing processing can effectively enhance the photoluminescence of Sr 2 CeO 4 phosphors.     

Ultraviolet upconversion luminescence in Er-doped Y2O3 excited by 532 nm CW compact solid-state laser

Ultraviolet upconversion emissions at 262, 276, 308 and 320 nm were observed from Er³⁺-doped Y₂O₃ with a 532 nm continuous wave compact solid-state laser excitation. Power-dependence analysis demonstrates that two-photon upconversion process populates the ⁴D_5/2, ²H_9/2 and ²P_3/2 states. The energy transfer upconversion (ETU) plays an important role in populating ⁴D_5/2 and ²P_3/2 states. It appears that ²P_3/2 state population originates from ETU ²H_11/2+²H_11/2→⁴I_13/2+²P_3/2, moreover, a subsequent excited state absorption (ESA) from the ⁴I_9/2 level.     

Preparation and upconversion properties of Ba2ErF7 and Ba2ErF7:Yb powders

Novel Ba₂ErF₇ and Yb³⁺-doped Ba₂ErF₇ powders were synthesized by a coprecipitation method. In Ba₂ErF₇ sample, abundant upconverted emission bands from violet to infrared region are observed under 980 nm excitation, whereas only green and red emissions are observed under 812 nm excitation. Under the two excitations, the luminescence decay curves of the green and red emissions are measured and the quenching behaviors of Yb³⁺ doping are also explored. It is found that a suitable Yb³⁺ concentration can efficiently enhance the intensity ratio of the blue and violet emissions to the green and red ones, which may be due to the competition between the energy transfer process from Er³⁺ to Yb³⁺ and the sensitizing process from Yb³⁺ to Er³⁺ in Ba₂ErF₇:Yb³⁺. This indicates that the Yb³⁺-doped Ba₂ErF₇ might be a good candidate for blue and violet upconversion phosphor.     

Luminescent and morphological study of Sr2CeO4 blue phosphor prepared from oxalate precursors

Luminescent and morphological studies of Sr₂CeO₄ blue phosphor prepared from cerium-doped strontium oxalate precursor are reported. Powder samples were prepared from 5 and 25 mol% Ce³⁺-doped strontium oxalate as well as from a mechanical mixture of strontium oxalate and cerium oxalate at a 4:1 ratio, respectively. All the samples were characterized by XRD, IR, PLS, and SEM. The luminescent and structural properties of the Sr₂CeO₄ material are little affected by the SrCO₃ remaining from precursors. The Sr₂CeO₄ material consists in one-dimensional chains of edge-sharing CeO₆ octahedra that are linked together by Sr²⁺ ions. The carbonate ion might be associated with oxygen ions of the linear chain, and also with the oxygen atoms located in the equatorial position, which consequently affects the charge transfer bands between O²⁻ and Ce⁴⁺. As observed by SEM, the morphological changes are related to each kind of precursor and thermal treatment, along with irregular powder particles within the size range 0.5-2 μm.     

Synthesis and characterization of Sr2CeO4 phosphor: Positive features of sol-gel technique

Nano crystalline powder of Sr₂CeO₄ has been synthesized by sol-gel technique. The luminescence properties of the material were compared with the one synthesized by conventional solid state reaction technique. The homogeneity and uniformity of the particle prepared by this method is much better than that prepared by the solid state reaction technique. This blue emitting phosphor was characterized by X-ray diffraction, scanning electron microscopy images and luminescent measurements. The emission spectrum of the material exhibits a broad band around ∼480 nm. The photoluminescence spectra of the Sr₂CeO₄ reveals that the strong blue emission is assigned to the Ce⁴⁺-O²⁻ charge transfer transition (CTT) of Sr₂CeO₄ and not related to lattice defect. The Commission International de l’Eclairage coordinates are x=0.16 and y=0.25.     

Eu3+掺杂的Sr2CeO4发光材料的光致发光研究

利用高温固相反应法制备了Eu³⁺掺杂的Sr₂CeO₄样品,并对其吸附水前后的光谱特性进行了研究.结果发现,对于刚制备的Sr_2-xEu_xCeO_4+x/2样品, 在Ce⁴⁺—O^2-的电荷迁移激发中,只有强激发带(～35700cm^-1)与Eu³⁺离子间存在能量传递,而弱激发带 (～29400cm^-1)只是引起Ce⁴⁺—O^2-的电荷迁移发射;在Sr_2-xEu_xCeO_4+x/2样品吸附水后,Eu³⁺的线状吸收跃迁强度显著增加, Ce⁴⁺—O^2-两个激发带均向Eu³⁺离子传递能量. Ce⁴⁺—O^2-强激发带通过交换作用向Eu³⁺离子传递能量,而弱激发带与Eu³⁺离子间的能量传递机理是非辐射多极子近场力的相互作用. 关键词： 2-xEu_xCeO_4+x/2')" href="#">Sr_2-xEu_xCeO_4+x/2 发光性质 能量传递 吸附水     

Synthesis and photoluminescent characteristics of Sr2CeO4 phosphors prepared via a microwave-assisted solvothermal process

Blue-light emitting Sr₂CeO₄ phosphors were successfully prepared via a microwave-assisted solvothermal method employing ethylene glycol as a solvent. The formation of Sr₂CeO₄ phase began when the solvothermally derived precursors were heated at 800 °C. With increase in heating temperatures, significantly enhanced excitation and emission intensities were observed because of an increase in the amount of Sr₂CeO₄. Heating at 1200 °C led to a substantial decrease in mission intensity due to thermal decomposition of Sr₂CeO₄ at elevated temperatures. The solvothermally derived Sr₂CeO₄ was found to exhibit higher emission intensity than the solid-state-reaction-derived phosphors. According to the deconvoluted emission spectra, two emission peaks are attributed to two metal-to-ligand charge-transfer states. Based on the deconvoluted results, a qualitative energy-level diagram of Sr₂CeO₄ was proposed. VUV-excited luminescence studies for Sr₂CeO₄ indicate that one peak at 193 nm is assigned to the charge-transfer transition between Sr²⁺ and O²⁻.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.     

Competition between green and infrared emission in Er:YLiF4 upconversion lasers

The competition between two laser transitions in Er:YLiF₄ (⁴S_3/2 → ⁴I_15/2 at 551 nm and ⁴S_3/2 → ⁴I_13/2 at 850 nm) is studied using a model based on rate equations. The laser emission is pumped by upconversion at 795 nm; for comparison, we also discuss upconversion pumping by another mechanism, at 970 nm. The conditions that favor laser emission in various regimes on these two transitions are found.     

Influence of Yb doping concentration on upconversion luminescence of Er in Lu2O3 phosphor

Cubic phase Lu₂O₃:Er³⁺/Yb³⁺ nanocrystal phosphors were prepared by sol–gel method. Fourier transform infrared (FT-IR) spectra were measured to evaluate the vibrational feature of the samples. Green and red radiations were observed upon 980 nm diode laser excitation. Laser power and Er³⁺ or Yb³⁺ doping concentration dependence of upconversion luminescence were studied to understand upconversion mechanisms. Excited state absorption, cross relaxation and energy transfer processes are the possible mechanisms for the visible emissions.     

Ultraviolet upconversion luminescence enhancement in Yb/Er-codoped Y2O3 nanocrystals induced by tridoping with Li ions

Ultraviolet (UV) upconversion (UC) luminescence in Yb³⁺/Er³⁺-codoped yttrium oxide (Y₂O₃) nanocrystals can be enhanced by orders of magnitude via tridoping further with Li⁺ ions under diode laser excitation of 970 nm. Sensitized three-photon UC radiations at 390 and 409 nm, corresponding to the ⁴G_11/2→⁴I_15/2 and ⁴H_9/2→⁴I_15/2 of Er³⁺ ions, respectively, present an enhancement time of about 33 times, which is larger than the 24 times enhancement for the UC green radiation. The UV UC radiation at 320 nm that corresponds to the ²P_3/2→⁴I_15/2 of Er³⁺ ions has also been greatly enhanced. Theoretical calculations interpret that all the observed enhancement times of UV UC radiations arise from the prolonged lifetimes of their intermediate states.     

Facile synthesis of well-dispersed SrF2:Yb/Er upconversion nanocrystals in oleate complex systems

Well oil-dispersible SrF₂:Yb³⁺/Er³⁺ upconversion (UC) nanocrystals (NCs) were easily synthesized in the water-ethanol-oleic acid-sodium oleate complex systems. The as-prepared NCs all show size-uniformity, and their sizes, morphologies can be controlled by varying the solvent and reaction time, and rectangular SrF₂:Yb³⁺/Er³⁺ nanosheets with the sizes of 5-25 nm can be obtained. The possible mechanism on the nucleation and growth of nanocrystals occurred at the oleic acid/sodium oleate interface was also discussed. The size and morphology dependent UC luminescence behaviors have been observed in SrF₂:Yb³⁺/Er³⁺ NCs, and their UC luminescence transitions were proposed. The as-prepared UC nanocrystals are expected to fulfill the demand for biological applications.     

纳米纤维状Sr3Al2O5Cl2:Eu2+,Tm3+的白色长余辉性能

采用高温固相法在800℃下制备出系列白色长余辉荧光粉Sr₃Al₂O₅Cl₂：Eu²⁺,Tm³⁺,并研究了它们的结构、形貌及发光性能。样品Sr_2.91Al₂O₅Cl₂：0.04Eu²⁺,0.05 Tm³⁺具有单一晶相和纳米纤维结构。该样品在紫外光激发下表现出两个很强的宽带发射（分别位于~448 nm和~590 nm）。它的余辉寿命大约是20 min。利用此种荧光粉所制作出的白光LED器件表现出很强的白光发射。     

Tunable upconversion in Er-doped orthorhombic lead fluoride compound

Near infrared-visible upconversion in Er³⁺ doped orthorhombic PbF₂ compound is investigated. It is experimentally observed that the red emission intensity increases monotonously with Er³⁺ concentration increase, while the green emission intensity first increases and then decreases. Based on the rate-equation, the energy transfers involved in the upconversion processes have been explored. It is shown that due to the different multipolar nature for the energy transfer processes of ²H_11/2 (⁴S_3/2)+⁴I_15/2→⁴I_9/2+⁴I_13/2 and ⁴I_11/2+⁴I_11/2→⁴F_7/2+⁴I_15/2, the green and red upconversion emissions depend on Er³⁺ concentration in different ways. The theoretical results are in good agreement with the experimental observation. It is shown that the upconverted emission bands can be tuned by controlling Er³⁺ concentration in orthorhombic PbF₂ compound, which has many photonic applications under NIR excitation.     

Improving monochromaticity of upconversion luminescence by codoping Eu ions in Y2O3:Ho, Yb nanocrystals

Upconversion (UC) luminescence of Y₂O₃:Ho³⁺, Yb³⁺ nanocrystals codoped with different concentrations of Eu³⁺ ions were investigated to improve the monochromaticity of the UC emission. The results show that the monochromaticity, quantified by a parameter SR, increases as the concentration of Eu³⁺ ions becomes higher, which is due to the energy transfer between ⁵I₇ (Ho³⁺) and ⁷F₆ (Eu³⁺). The energy transfer accelerates the relaxation of Ho³⁺ ions from the ⁵I₇ to ⁵I₈ state and then quenches the red emission. The influence of the Eu³⁺ concentration on the pump power dependence of the red UC fluorescence in Y₂O₃:Ho³⁺, Yb³⁺, Eu³⁺ nanocrystals is verified using the steady-state rate equation theory.     

Infrared-to-visible upconversion in Er and Er/Yb activated Sb2O4 nanopowders prepared by sol-gel route

We prepared Er³⁺ doped and Er³⁺/Yb³⁺ codoped Sb₂O₄ nanocrystals by the sol-gel method. The Raman, X-ray diffraction (XRD), transmission electron microscope (TEM), and photoluminescence spectra of the samples were studied. The phonon energy of the Sb₂O₄ nanocrystals is very low (the maximum value being 461 cm⁻¹). The upconversion (UC) red emission of the Er³⁺/Yb³⁺ codoped sample is very strong at 975 nm laser diode excitation. The Sb₂O₄ nanocrystals will be a promising luminous material.     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.