White light emission in Tm/Er/Yb tri-doped Y2O3 transparent ceramic

Tm³⁺/Er³⁺/Yb³⁺ triply doped Y₂O₃ transparent ceramics were fabricated by solid state reaction and characterized from the point of view of white light upconversion luminescence. All the samples exhibited high transparency not only in near-infrared band but also in visible region. Strong red (Er³⁺: ⁴F_9/2 → ⁴I_15/2), green (Er³⁺: ²H_11/2, ⁴S_3/2 → ⁴I_15/2) and blue (Tm³⁺: ¹G₄ → ³H₆) upconversion emissions have been observed under 980 nm excitation at room temperature. By varying the concentration of Er³⁺ ion, various colors of upconversion luminescence (pure blue, bluish green, pure green and yellowish green), including white light with CIE-X = 0.295 and CIE-Y = 0.312, can be easily achieved.     

Upconversion luminescence in Er/Yb codoped Y2CaGe4O12

Calcium yttrium tetrametagermanates Y₂CaGe₄O₁₂ doped with Er³⁺ and Er³⁺/Yb³⁺ reveal upconversion emission in visible spectral range under near-infrared excitation, λ_ex = 980 nm. For the solid solution Er_xY_2−xCaGe₄O₁₂ concentration dependencies for the green and red lines of the visible emission around 526 nm (²H_11/2 → ⁴I_15/2), 545 nm (⁴S_3/2 → ⁴I_15/2) and 670 nm (⁴F_9/2 → ⁴I_15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y₂CaGe₄O₁₂:Er³⁺ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y₂CaGe₄O₁₂:Er³⁺ and Y₂CaGe₄O₁₂:Er³⁺, Yb³⁺ has been proposed. Conditions for visible emission occurrence under quasi-resonance λ_ex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition ⁴I_13/2 → ⁴I_15/2 in erbium ions has been detected.     

Enhancement of upconversion luminescence of Y2O3:Er nanocrystals by codoping Li-Zn

The upconversion (UC) luminescence in sol-gel synthesized Li⁺, Zn²⁺, or Li⁺-Zn²⁺ codoped Y₂O₃:Er³⁺ nanocrystals were investigated under the excitation of a 970 nm diode laser. Compared to undoped Y₂O₃:Er³⁺ samples, proper doping of Li⁺-Zn²⁺ leads to an drastic increase of the UC luminescence centered at 560 nm by a factor of 28. The UC luminescence enhancement is a result of the increased lifetime of the intermediate state ⁴I_11/2 (Er). The intensity ratio of the green over red emissions (green/red) is also affected by the codoping of Zn²⁺, Li⁺ and Li⁺-Zn²⁺ ions. Our results demonstrated that the Li⁺-Zn²⁺ codoping in Y₂O₃:Er³⁺ phosphors produced remarkable enhancement of the UC luminescence and green/red ratio, making this nanocrystal a promising candidate for photonic and biological applications.     

Structure and upconversion luminescence properties of BaYF5:Yb, Er nanoparticles prepared by different methods

BaYF₅:Yb³⁺, Er³⁺ (BYF) upconversion (UC) luminescence nanoparticles have been prepared using co-precipitation and hydrothermal techniques, respectively. Two different fluoride sources were used to synthesize BYF by the hydrothermal method, and the sizes of the as-prepared spherical particles were about 30 nm (NH₄BF₄ as a fluoride source) and 100 nm (NH₄HF₂ as a fluoride source), respectively. While the nanoparticles prepared by the co-precipitation method are irregular, many clusters and agglomerates can be seen. The UC fluorescence has been realized in all the as-prepared BYF samples upon 980 nm excitation. It is found that their luminescence spectra depend strongly upon the preparation method. Factors affecting the upconversion fluorescent intensity have been also studied. The UC emission transitions for ⁴F_9/2-⁴I_15/2 (red), ²H_11/2-⁴I_15/2 (green) and ⁴S_3/2-⁴I_15/2 (green) in the Yb³⁺/Er³⁺ codoped BYF nanoparticles depending on pumping power have also been discussed.     

Upconversion luminescence of Yb/Tb/Er-doped fluorosilicate glass ceramics containing SrF2 nanocrystals

Yb³⁺/Tb³⁺/Er³⁺-doped transparent alkaline earth fluorosilicate glass ceramics containing SrF₂ nanocrystals were prepared and their microstructures and spectroscopic properties were investigated. The formation of SrF₂ nanocrystals enriched with Yb³⁺/Tb³⁺/Er³⁺ in the glass ceramics was confirmed by XRD, HRTEM and SAED. The glass ceramics exhibited intense upconversion luminescence behaviors, owing to efficient energy transfers from Yb³⁺ to Er³⁺ and Tb³⁺ and low phonon assistant non-radiative transition probabilities of excited Er³⁺ and Tb³⁺. The upconversion could be enhanced significantly by increasing the annealing temperature and the luminescence color could be adjusted by varying the Yb³⁺/Tb³⁺/Er³⁺ ratio. Upconversion of Er³⁺ could be described as sequential energy transfer from single Yb³⁺ to single Er³⁺, while upconversion of Tb³⁺ described as cooperative energy transfer from double Yb³⁺ to single Tb³⁺.     

Synthesis and upconversion luminescent properties of Er doped and Er-Yb codoped GdOCl powders

Er³⁺ doped and Er³⁺-Yb³⁺ codoped GdOCl phosphors were prepared by modified solid state reaction. X-ray diffraction, scanning electron microscope, and Raman spectrum of the samples were studied. The result of Raman spectrum shows that the cutoff phonon energy of GdOCl is only 505 cm⁻¹, which is beneficial for upconversion luminescence. Infrared-to-visible upconversion emissions were observed under 980 nm diode laser excitation. It was found that the ratio of green to red upconversion emission intensity varies with concentration of the Er³⁺ or Yb³⁺ ion. Laser power and doping concentration dependence of upconversion luminescence were studied to understand the upconversion mechanisms. Excited state absorption and energy-transfer processes are proposed to be the possible mechanisms for the visible emissions.     

Spectroscopic properties of Yb and Er ions in heavy metal glasses

Selected heavy metal glasses containing Yb³⁺ and Er³⁺ ions have been studied. Near-infrared luminescence spectra at 1.53 μm and up-conversion spectra of Er³⁺ ions were registered under excitation of Yb³⁺ ions by 975 nm diode laser line. The luminescence bands correspond to ⁴I_13/2-⁴I_15/2 (NIR), ⁴S_3/2-⁴I_15/2 (green) and ⁴F_9/2-⁴I_15/2 (red) transitions of Er³⁺, respectively. The optical transitions of rare earth ions have been examined as a function of glass host. The unusual large spectral linewidth nearly close to 110 nm for ⁴I_13/2-⁴I_15/2 transition of Er³⁺ ions in Yb-Er co-doped lead borate glass was obtained, whereas long-lived NIR luminescence at 1.53 μm was detected in lead germanate glass. The NIR luminescence and up-conversion phenomena strongly depend on stretching vibrations of glass host, which was confirmed by FT-IR spectroscopy.     

Blue and green emissions with high color purity from nanocrystalline Ca2Gd8Si6O26:Ln (Ln = Tm or Er) phosphors

Blue and green light emissive nanocrystalline Ca₂Gd₈Si₆O₂₆ (CGS):Tm³⁺ and CGS:Er³⁺ phosphors with high color purity were prepared by solvothermal reaction method. The structural and morphological properties of these phosphors were evaluated by the powder X-ray diffraction (XRD) and scanning electron microscopy, respectively. From the XRD results, Tm³⁺:CGS and Er³⁺:CGS phosphors had the characteristic peaks of oxyapatite in the hexagonal lattice structure. The visible luminescence properties of phosphors were obtained by ultraviolet (UV) or near-UV light and low voltage electron beam (0.5-5 kV) excitation. The photoluminescence and cathodoluminescence properties were investigated by changing the variation of Tm³⁺ or Er³⁺ concentrations and the acceleration voltage, respectively. The CGS:Tm³⁺ phosphors exhibited the blue emission due to ¹D₂→³F₄ transition, while the CGS:Er³⁺ phosphors showed the green emission due to ⁴S_3/2→⁴I_15/2 transition. The color purity and chromaticity coordinates of the fabricated phosphors are comparable to or better than those of standard phosphors for lighting or imaging devices.     

Effect of Yb3+ concentration on the structures and upconversion luminescence properties of Y2O3:Er3+ ultrafine phosphors

Y2O3:Er3+ ultrafine phosphors with a varying Yb3+ ion concentration were prepared by a urea homogeneous precipitation method.The results of XRD show that all the samples are of a pure cubic structure and the average crystallite sizes can be calculated as 45,34,and 28 nm for Y2O3:Er3+ ultrafine phosphors with Yb3+ ion concentrations of 0,10%,and 20%,respectively.The lattice constant and cell volume of the ultrafine phosphors decrease with enhancing Yb3+ ion concentration.The upconversion luminescence spectra of all the samples were studied under 980 nm laser excitation.The strong green and red upconversion emission were observed,and attributed to the 2H11/2→4I15/2,4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+,respectively.The intensity of red emission increases with increasing Yb3+ ion concentration.The effect of Yb3+ ion concentration on the structures and upconversion luminescence mechanism were discussed.     

Upconversion luminescence properties of Er<Superscript>3+</Superscript> doped GeS<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>-KCl chalcohalide glasses

Er3+ ions doped chalcohalide glasses with the composition of 56GeS2-24Ga2S3-20KCl were fabricated by a melt-quenching method.Under 800 nm laser excitation,strong green emissions centered at 525 nm and 550 nm and weak red emission centered at 660 nm were observed,which were assigned to 2H11/2→4I15/2,4S3/2→4I15/2,and 4F9/2→4I15/2 transitions,respectively.The intensity reached maximum when the Er3+ ions concentration was 0.1 mol%.The possible upconversion luminescence mechanism was proposed from the discussion...     

Comparative studies of spectroscopic properties in Er–Yb codoped phosphate glasses

A spectroscopic investigation of an extensive series of commercially available Er³⁺–Yb³⁺ codoped phosphate glasses (IOG-1) with different Er–Yb concentrations have been presented based upon spectroscopic measurements and Judd–Ofelt theory. The composition and the atomic concentration of the respective elements of the glass samples were analyzed. Various spectroscopic parameters were obtained to evaluate the performances of these glasses as a laser material in the eye-safe laser wavelength of 1.53 μm. Using the measured fluorescence lifetimes the radiative quantum efficiency of the ⁴I_13/2 → ⁴I_15/2 transition is estimated to be 100% for the glass substrates with an Er³⁺ concentration of lower than 2.0 × 10²⁰ ions/cm³. The stimulated emission and absorption cross-sections were also determined and compared. The infrared and upconversion fluorescence were studied, and under 975 nm excitation the dominant upconversion mechanisms are excited state absorption (ESA) and energy transfer (ET) for the green emission, and ET for the red emission. The data obtained here provide useful guidelines on the choice of IOG-1 glasses with the fixed Er–Yb concentrations.     

Spectroscopic properties of Er-doped and Er/Yb-codoped PbF2–MOx (M = Te, Ge, B) oxyfluoride glasses

A series of Er³⁺-doped and Er³⁺/Yb³⁺-codoped lead germanate (GP), lead tellurite (TP) and lead borate (BP) transparent oxyfluoride glasses were synthesized. Spectroscopic characteristics of these materials were systemically measured. The Judd–Ofelt parameters were calculated according to absorption spectra. F⁻ ions have a strong effect on refractive index (n) and phonon energy of the host. Intense red and green upconversion is closely related to phonon energy, J–O parameters and sensitizer. The GPEY-2 glass (53GeO₂–43PbF₂–0.5Er₂O₃–2Yb₂O₃) shows good transparency and fairly strong upconversion emissions.     

Enhancement of the upconversion luminescence intensity in Er doped BaTiO3 nanocrystals by codoping with Li ions

Infrared-to-visible upconversion luminescence spectra were investigated in Er³⁺ doped and Er³⁺-Li⁺ codoped BaTiO₃ nanocrystals following excitation with 976 nm. By introducing Li⁺ ions, the upconverted emission intensity is found to be greatly enhanced compared to that of the nanocrystals without Li⁺ ions. The enhanced luminescence might be attributed to the oxygen vacancy generated by Li⁺ ion incorporation in the lattices and the distortion of the local asymmetry around Er³⁺. We observe that excitation power dependence and decay time are increased by the incorporation of Li⁺ ions. Li⁺ ions also can reduce the OH groups in specimen, which decrease nonradiative decay from the ⁴S_3/2 to ⁴F_9/2, enhancing the upconversion emission intensities.     

Effect of Bi doping on the quenching concentration of H11/2/S3/2 level of Er

Bi³⁺ and Er³⁺ codoped Y₂O₃ was prepared by sol-gel method. The upconversion emission was investigated under 980 nm excitation. For samples without Bi³⁺, the quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺ is 3.0 mol%. However, by 1.5 mol% Bi³⁺ doping the quenching concentration increases to 5.0 mol%; meanwhile, the green emission is enhanced 1.9 times. The results indicate that both the quenching concentration and the emission intensity of ²H_11/2/⁴S_3/2 level can be increased by Bi³⁺ doping.     

Morphology and phase evolvement of Yb/Er co-doped NaYF4 microtubes prepared with YF3 submicrospindles as precursor

Hexagonal phase NaYF₄ microtubes co-doped with Yb³⁺ and Er³⁺ were synthesized through a hydrothermal process with YF₃ submicrospindles as precursor. The X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were utilized to characterize the structure and morphology of the as-prepared products. XRD results show that pure cubic NaYF₄ crystals can be obtained when reaction time is 2 h. While the product is mixture of cubic and hexagonal phase NaYF₄ when reaction time is from 7 to 20 h. Continuing to increase the reaction time to 24 h, the pure hexagonal NaYF₄ crystals were formed. The FE-SEM and TEM results show that the morphology of pure cubic NaYF₄ is spherical clusters composed of spherical nanoparticles with average diameter of about 100 nm and the pure hexagonal NaYF₄ crystals have tubular structure with out diameter of about 0.3-0.5 μm, inner diameter of about 0.5-1 μm and length ranging from 3 to 12 μm. The luminescence properties of Yb³⁺/Er³⁺ co-doped cubic and hexagonal phase NaYF₄ microcrystals were also studied. Under 980-nm excitation, the upconversion luminescence (UCL) intensity of hexagonal phase NaYF₄ microtubes is much stronger than that of cubic phase clusters. Moreover, both red and green upconversion are ascribed to the two-photon process. Therefore, hexagonal phase NaYF₄ microtubes with high UCL efficiency may have a potential application in photonic device.     

Controllable synthesis of bifunctional NaYF4:Yb/Ho@SiO2/Au nanoparticles with upconversion luminescence and high X-ray attenuation

A novel type of bifunctional water-soluble NaYF₄:Yb³⁺/Ho³⁺@SiO₂/Au nanocomposite is fabricated by a facile layer-by-layer technology in which the mercapto-silica shell is used as the functional layer coating on the central NaYF₄:Yb³⁺/Ho³⁺ nanocrystals. Then by adjusting the mole ratio of the Au nanoparticles to the NaYF₄:Yb³⁺/Ho³⁺@SiO₂ nanoparticles, control of the gold loading on the upconversion nanocrystal surface is achieved. The fabricated nanocomposites inherit the excellent physical and chemical properties from their building blocks, simultaneously exhibiting upconversion luminescence and high X-ray attenuation and as well are easily modified with various molecules. These properties render the synthesized NaYF₄:Yb³⁺/Ho³⁺@SiO₂/Au nanocomposite not only useful as a multimodality contrast agent to increase the efficiency of molecular imaging but also has the potential of in situ curing of diseases.     

Spectroscopic investigation of Er-doped (Gd0.7Y0.3)2SiO5 single crystal for potential application in 1.5 μm laser

The Er³⁺:(Gd_0.7Y_0.3)₂SiO₅ (hereafter abbreviated as Er³⁺:G_0.7Y_0.3SO) single crystals were grown by the Czochralski technique. The unpolarized absorption spectrum and photoluminescence (PL) spectra were measured at room temperature. The Judd-Ofelt phenomenological method was used to estimate the intensity parameters, radiative lifetimes and branching ratios of luminescence. The stimulated emission cross-section of the ⁴I_13/2 → ⁴I_15/2 transition of special interest of ∼1.5 μm laser was calculated by the Fuchtbauer-Ladenburg equation, and the relevant gain cross-sections were also estimated for several inverse-population parameters to evaluate a potential laser activity of the Er³⁺:G_0.7Y_0.3SO system. Also, the potential range of the optical pumping was assessed based on the absorption cross-section at room temperature. Finally the excited state dynamics of the Er³⁺:G_0.7Y_0.3SO system was investigated and experimental lifetimes of ⁴I_13/2 and ⁴I_11/2 levels were measured. Taking into account the satisfactory absorption near 971 and 1529 nm, and the high gain cross-section in the range of 1530-1620 nm, the Er³⁺:G_0.7Y_0.3SO crystal can be considered as a promising active material for laser operation near 1.5 μm.     

Relationship between microstructure and the achieving of the single-band red upconversion fluorescence of Er3+/Yb3+ codoped crystallites

The as-cast and the heat-treated samples with the primary mole composition of (59.4 ? x)ZnF₂–40PbF₂–0.1B₂O₃–xYb₂O₃–0.5Er₂O₃ (x = 2, 4, 7, 10, 13) were synthesized. In the as-cast and the heat-treated samples with 13 mol% Yb₂O₃, the multi-band and the single red band upconversion (UC) lights were observed, respectively, under the excitation of a laser diode (LD) with excitation wavelength of 980 nm. For this interesting phenomenon, microstructure analysis revealed that the crystal phases and the distribution of Er³⁺ ion has been changed by the heat-treatment and the mole ratio of Yb³⁺ to Er³⁺ ion in the neighbor environment of Er³⁺ ion is increased from 0.74 to 16.96 which brought about the populating behavior variation of ⁴F_7/2 and ⁴F_9/2 levels of Er³⁺ ions, to which the achieving of the dazzlingly single-narrow-band red light was ascribed.     

Sensitized high-order ultraviolet upconversion emissions of Gd by Er in NaYF4 microcrystals

High-order ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ and Er³⁺ ions were observed in NaYF₄:Yb³⁺/Gd³⁺/Er³⁺ microcrystals under 980 nm excitation. These UC emissions came from six- and five-photon UC processes at low pump power range, which were confirmed by the pumping power dependences of UC fluorescence intensities. In these high-order UC processes, energy transfer (ET) processes of Er³⁺ → Gd³⁺ played crucial roles in populating the excited states of Gd³⁺ ions. Experiments on concentration variation and dynamic analysis revealed the ET processes between Er³⁺ and Gd³⁺ in detail. Some of possible population routes for populating excited Gd³⁺ ions were proposed based on spectral and dynamic analysis.     

Preparation and luminescent characteristics of Sr3RE2(BO3)4:Dy (RE = Y, La, Gd) phosphors for white LED

Dysprosium-activated Sr₃RE₂(BO₃)₄ (RE = Y, La, Gd) phosphors were synthesized by a high temperature solid-state reaction method. The phase uniformity of the phosphors was characterized by X-ray powder diffraction (XRD) and the luminescence characteristics were investigated. The excitation spectra at 575 nm emission show strong spectral bands in the region of 300-500 nm. The emission spectra of the phosphors with 365 nm excitation show three bands centered at 484 nm, 575 nm and 680 nm, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. The effect of Dy³⁺ concentration on the emission intensity of the phosphors was investigated. The fluorescence decay curves for ⁴F_9/2 → ⁶H_13/2 excited at 365 nm and monitored at λ_em of 575 nm were measured. The decay times decreased slowly with increasing Dy³⁺ doping concentration due to a trap capturing to resonance fluorescence transfer of the activated ions and due to the exchange interactions between activated ion pairs. In order to determine the type of interaction between activated ions, the concentration dependence curves (lg(I/x) versus lg x) of Sr₃RE₂(BO₃)₄:Dy³⁺ (RE = Y, La, Gd) were plotted. The concentration quenching mechanism of the ⁴F_9/2 → ⁶H_13/2 (575 nm) transition of Dy³⁺ is the d-d interaction. All results indicate these phosphors are promising white-color luminescent materials.