Crystalline-structure-dependent green and red upconversion emissions of Er-Yb-Li codoped TiO2

Crystalline structures and infrared-to-visible upconversion luminescence spectra have been investigated in 1 mol% Er³⁺, 10 mol% Yb³⁺ and 0-20 mol% Li⁺ codoped TiO₂ [1Er10Yb(0-20)Li:TiO₂] nanocrystals. The crystalline structures of 1Er10Yb(0-20)Li:TiO₂ were divided into three parts by the addition of Yb³⁺ and Li⁺. Both green and red upconversion emissions were observed from the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ in Er³⁺-Yb³⁺-Li⁺ codoped TiO₂, respectively. The green and red upconversion emissions of 1Er:TiO₂ were enhanced significantly by Yb³⁺ and Li⁺ codoping, in which the intensities of green and red emissions and the intensity ratio of green to red emissions (I_green/I_red) were highly dependent on the crystalline structures. The significant enhanced upconversion emissions resulted from the energy migration between Er³⁺ and Yb³⁺ as well as the distortion of crystal field symmetry of Er³⁺ caused by the dissolving of Li⁺ at lower Li⁺ codoping concentration and the phase transformation at higher Li⁺ concentration. It is concluded that codoping with ions of smaller ionic radius like Li⁺ can efficiently improve the upconversion emissions of Er³⁺ or other rare-earth ions doped luminsecence materials.     

The effects of Li on the near-infrared luminescence properties of Nd/ Li codoped Y2O3 nanocrystals

Nd³⁺/ Li⁺ codoped Y₂O₃ nanocrystals were synthesized by glycine combustion method. The codoping of Li⁺ ions can lead to about twice enhancement of the near-infrared luminescence for the three spectral regions, which correspond to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 channels of Nd³⁺. The enhancement could be attributed to the improved morphology, the modification of the local symmetry around Nd³⁺ ions and the reducing number of OH⁻ groups by codoping with Li⁺ ions.     

Investigation of up-conversion luminescence properties of RE/Yb co-doped Y2O3 transparent ceramic (RE=Er, Ho, Pr, and Tm)

RE/Yb co-doped Y₂O₃ transparent ceramics (RE=Er, Ho, Pr, Tm) were fabricated and characterized from the point of up-conversion luminescence. All the samples exhibit high transparence not only in near-infrared band (NIR) band but also in visible region, which ensures the output of the up-conversion luminescence. Under 980 nm excitation, green and red emissions were observed in Er, Yb:Y₂O₃ transparent ceramic, while green emission was detected in Ho, Yb and Pr, Yb co-doped Y₂O₃ transparent ceramics. In Tm, Yb co-doped Y₂O₃ ceramic, very intense blue up-conversion luminescence was detected. The dependence of up-conversion emission intensity on the pumping power was measured for each RE/Yb co-doped Y₂O₃ transparent ceramic, and the up-conversion mechanism was discussed in detail. Meanwhile, the energy transfer efficiency was calculated.     

Temperature-dependent photoluminescence spectra of Er-Tm-codoped Al2O3 thin film

Er-Tm-codoped Al₂O₃ thin films with different Tm to Er concentration ratios were synthesized by cosputtering from separated Er, Tm, Si, and Al₂O₃ targets. The temperature dependence of photoluminescence (PL) spectra was studied. A flat and broad emission band was achieved in the 1.4-1.7 μm and the observed 1470, 1533 and 1800 nm emission bands were attributed to the transitions of Tm³⁺: ³H₄ → ³F₄, Er³⁺: ⁴I_13/2 → ⁴I_15/2 and Tm³⁺: ³F₄ → ³H₆, respectively. The temperature dependence is rather complicated. With increasing measuring temperature, the peak intensity related to Er³⁺ ions increases by a factor of five, while the Tm³⁺ PL intensity at 1800 nm decreases by one order of magnitude. This phenomenon is attributed to a complicated energy transfer (ET) processes involving both Er³⁺ and Tm³⁺ and increase of phonon-assisted ET rate with temperature as well. It should be helpful to fully understand ET processes between Er and Tm and achieve flat and broad emission band at different operating temperatures.     

White-light long-lasting phosphorescence from Tb-activated Y2O2S phosphor

The white-light long-lasting phosphors Y₂O₂S:Tb³⁺, Sr²⁺ or/and Zr⁴⁺ were prepared and studied. The white-light afterglow emission after the irradiation with 254 nm UV are composed of the blue light emission and the yellowish-green light emission, originating from the transitions of ⁵D₃→⁷F₅, ⁵D₄→⁷F₅ in Tb³⁺ when the Tb³⁺ concentration is not higher than 0.3 at%. The codoped Sr²⁺ and Zr⁴⁺ ions act as trap-creating ions. The afterglow can last over 21 min in the dark for Y₂O₂S:Tb³⁺0.3%, Sr²⁺4%, Zr⁴⁺4% after irradiation by 254 nm ultraviolet light. Y₂O₂S:Tb³⁺ may be a promising material for the development of white-light long-lasting phosphor since the Tb³⁺ has a high luminescent efficiency and the dominant excitation band of 4f →5d is located at 220-300 nm.     

Luminescence in Ln2CaGe4O12 under infrared laser excitation

Luminescent properties of the new group of crystalline optical materials Ln₂CaGe₄O₁₂, Ln=Dy, Ho, Er, Tm, Yb were studied under laser excitation at λ=976 nm in the stationary mode. Er₂CaGe₄O₁₂, Ho₂CaGe₄O₁₂ and also the solid solution Ln_xY_2−xCaGe₄O₁₂ (0?x?2) may be used in photonics as optical elements, such as converters and resonance amplifiers.     

Intense ultraviolet and blue upconversion emissions in Yb-Tm codoped stoichiometric Y7O6F9 powder

Stoichiometric Y₇O₆F₉ powder codoped with Yb³⁺-Tm³⁺ was synthesized via co-precipitation and subsequent calcining route. The results of X-ray diffraction and transmission electron microscopy reveal that when the calcining temperature is beyond 800 °C, orthorhombic YF₃ nanoparticles can be completely oxidized into orthorhombic Y₇O₆F₉ powder. Under the excitation of a 980 nm laser, Y₇O₆F₉ powder exhibits multicolor UC emission in regions spanning the UV to the NIR. In addition, the upconversion emission intensities of YF₃, Y₇O₆F₉ and Y₂O₃ powders were compared under the same dopant condition (Yb/Tm=5/0.5 mol%). The low phonon energy revealed by Raman spectra helped to understand the high efficient upconversion emission of Y₇O₆F₉ and the main phonon vibration of Y₇O₆F₉ lies at 472 cm⁻¹, which is far lower that of Y₂O₃ (at 708 cm⁻¹). Our results indicate that orthorhombic rare earth ions doped Y₇O₆F₉ is an efficient matrix for UV and blue UC emission, and has potential applications in color displays, anti-counterfeiting and multicolor fluorescent labels.     

Upconversion luminescence of colloidal solution of Y2O3 nano-particles doped with trivalent rare-earth ions

We have studied upconversion luminescence of colloidal solution of Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺. Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺ show sintering and agglomeration, because they are synthesized by firing a hydroxy carbonate precursor. Colloidal solution of Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺ is prepared through two-step dispersion process and the average diameter of the primary nano-particles is about 50 nm. Under excitation with 980-nm laser diode, upconversion luminescence of colloidal solution of the primary Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺ in methyl isobuthyl ketone strongly appeared near 660 nm and weakly near 550 nm.     

Experimental investigation on the upconversion mechanism of 754 nm NIR luminescence of Ho/Yb:Y2O3, Gd2O3 under 976 nm diode laserexcitation

Upconversion (UC) spectra of Ho³⁺/Yb³⁺ codoped Y₂O₃, Gd₂O₃ bulk ceramics were obtained under the excitation of a 976 nm diode laser. Systematic experimental studies, including power dependence, luminescence lifetime, and the intensity ratio σ for the green to NIR emissions, were carried out in order to confirm the UC mechanism of Ho³⁺ ions. Our results demonstrated that the NIR emission was associated with the ⁵F₄/⁵S₂→⁵I₇ transition of Ho³⁺ ions without the contribution of the ⁵I₄→⁵I₈ transition for Ho³⁺/Yb³⁺ codoped Y₂O₃ and Gd₂O₃ bulk ceramics. Additionally, population saturation in the ⁵I₇ energy level had been observed in Ho³⁺/Yb³⁺ codoped Y₂O₃, Gd₂O₃ bulk ceramics. All experimental observations can be well explained by the steady-state rate equations.     

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.     

Structure dependence of near-infrared stimulated blue emission in polycrystalline Ln2(WO4)3 (Ln=Gd and Lu) doped with Tm and Yb

Tm- and Yb-doped gadolinium tungstate, (Gd_xTm_yYb_1−x−y)₂(WO₄)₃ (x=0.7-0.9; y=0.001-0.01), have been prepared by the polymerized complex method to achieve a homogeneous dispersion of dopants and to stabilize the host structure. Decomposition (900 °C 5 h) of the precursors with x=0.8-0.9 yielded a pure monoclinic phase, while that of x=0.7 resulted in formation of an orthorhombic impurity. The monoclinic phase exhibits bright up-converted blue emission due to the ¹G₄→³H₆ transition of Tm³⁺ (472 nm) upon excitation into the Yb³⁺:²F_7/2→²F_5/2 absorption band as a result of energy transfer from Yb to Tm. The orthorhombic impurity acts as a strong quencher of emission, and the quenching mechanism has been discussed on the basis of structural and spectroscopic properties of orthorhombic Lu₂(WO₄)₃:Tm,Yb prepared by the same method.     

Preparation and photoluminescence characteristics of Eu-doped MgAl1.8Y0.2O4 nanocrystals

A simple combustion route was employed for the preparation of Eu³⁺-doped MgAl_1.8Y_0.2−xO₄ nanocrystals using metal nitrates as precursors and urea as a fuel in a preheated furnace at 500 °C. The powders thus obtained were then fired at 1000 °C for 3 h to get better luminescent properties. The incorporation of Eu³⁺ activator in these nanocrystals was checked by luminescence characteristics. These nanocrystals displayed bright red color on excitation under 254 nm UV source. The main emission peak was assigned to the transition [⁵D₀→⁷F₂] at 615 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out to understand surface morphological features and the particle size. Crystal structures of the nanocrystals were investigated by the X-ray diffraction (XRD) technique. The crystallite size of the as-prepared nanocrystals was around 29 nm, which was evaluated from the broad XRD peaks. The crystallite size increased to ∼45 nm on further heat treatment at 1000 °C.     

Enhanced luminescence of Ca3B2O6:Dy phosphors by Na-codoping for LED applications

The Ca_2.95−yDy_0.05B₂O₆:yNa⁺ (0≤y≤0.20) phosphors were synthesized at 1100 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE), photoluminescence (PL) spectra and thermoluminescence (TL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions on 350 nm excitation was observed at 480 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions and 660 nm (red) due to weak ⁴F_9/2→⁶H_11/2 emissions. The PL results from the investigated Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors show that Dy³⁺ emissions increase with the increase of the Na⁺ codoping ions. The integral intensity of yellow to blue (Y/B) can be tuned by controlling Na⁺ content. By the simulation of white light, the optimal CIE value (0.328, 0.334) can be achieved when the content of Na⁺-codoping ions is y=0.2. The results imply that the Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors could be potentially used as white LEDs.     

Synthesis and luminescent properties of Y6W2O15:Eu phosphors

In this paper, a novel phosphor, Y₆W₂O₁₅:Eu³⁺ was synthesized by thermal decomposition and phase transition of its decatungstate gel precursor. With stepwise increase of temperature to 750 °C, a crystalline phase of Y₆W₂O₁₅:Eu³⁺forms that gives intense red emission when excited at 466 nm, the emission is attributed to the Eu³⁺ ions transitions from ⁵D₀ excited states to ⁷F_J (J=0-4) ground states. The long excitation wavelength proves the Eu³⁺ transition follows the photoexcitation of the oxygen-metal (O→W lmct) charge transfer bands in yttrium tungstate. Some structural information regarding Y₆W₂O₁₅ provided by luminescence is in accord with that characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The long-wavelength excitation properties of this material may find application in the production of red phosphors for white light-emitting diodes (LEDs).     

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.     

Trapping mechanism in the afterglow process of the rare-earth activated Y2O2S phosphors

Phosphorescence properties are investigated in Y₂O₂S phosphors doped with rare-earth (lanthanoid, Ln) ions. Luminescence afterglow with a decay time of several ten milliseconds is observed at room temperature in the phosphors activated by Nd, Sm, Eu, Dy, Ho, Tm, Er, and Yb. The depths (thermal activation energies) of the traps causing the afterglow are measured with the transient luminescence method.It is concluded that the excited electron and the hole in the conduction and valence bands are trapped separately in the states (impurity levels) located in the vicinity of the Ln³⁺ ion. The trapping depths of the level range from 0.3 to 1.1 eV and are dependent on the electron affinity of the Ln³⁺ ion estimated from the energy difference between the 4fⁿ⁺¹ and the 4fⁿ configurations in the 4f shell of the ion.     

Yb and Yb-Eu luminescent properties of the Li2Lu5O4(BO3)3 phase

The luminescence of Yb³⁺ in the oxyborate Li₂Lu₅O₄(BO₃)₃ is reported. At low temperature, in addition to the usual ytterbium infrared emission, this phosphor presents an emission in the ultraviolet () which corresponds to the transition from the charge transfer state (O-Yb) to the 4f levels of Yb³⁺. The temperature quenching Tq50% is equal to 120 K. The infrared emission studied at room temperature is located between 950 and 1100 nm.Europium emission quenching in the Li₂Yb₅O₄(BO₃)₃ phase is related to Eu→Yb transfer by cross-relaxation. The reverse Yb→Eu transfer by cooperative sensitization is highlighted in the codoped Li₂Lu₅O₄(BO₃)₃ compound.     

Structural and optical properties of Er, Yb co-doped Y2O3 thin films

Thin Er³⁺, Yb³⁺ co-doped Y₂O₃ films were grown on (1 0 0) YAG substrates by pulsed laser deposition. Ceramic targets having different active ion concentration were used for ablation. The influence of the rare-earth content and oxygen pressure applied during the deposition on the structural, morphological and optical properties of the films were investigated. The films deposited at the lower pressure, 1 Pa, and at 1/10 Er to Yb doping ratio are highly textured along the (1 1 1) direction of the Y₂O₃ cubic phase. In addition to the crystalline structure, these films possess smoother surface compared to those prepared at the higher pressure, 10 Pa. All other films are polycrystalline, consisting of cubic and monoclinic phases of Y₂O₃. The rougher surface of the films produced at the higher-pressure leads to higher scattering losses and different behavior of the reflectivity spectra. Optical anisotropy in the films of less than 0.004 was measured regardless of the monoclinic structure obtained. Waveguide losses of about 1 dB/cm at 633 nm were obtained for the films produced at the lower oxygen pressure.     

Structural characterization and ytterbium spectroscopy in Sc2O3 nanocrystals

Ytterbium-doped scandium oxide nanocrystals measuring less than 25 nm with compositions of Sc_2−xYb_xO₃ (x=0.001-1) were prepared using the modified Pechini method. The Yb:Sc₂O₃ nanocrystals were obtained by calcination at low temperature such as 1073 K for 2 h. X-ray powder diffraction (XRD) and transmission electronic microscopy (TEM) were used to perform the structural characterization of nanocrystals; these studies indicated that the nanocrystals have high crystalline quality with cubic structure and Ia3¯ space group. The morphology and particle size were studied using electron microscopy. A detailed study of the effect of the nanodimension and the ytterbium concentration on the spectroscopic characteristics of Yb³⁺ as an active ion was carried out in terms of optical absorption, optical emission and fluorescence decay time at room and low temperature.     

Enhanced optical activation of Er in Er-doped Al2O3 powders by Y codoping in a sol-gel method

Enhanced photoluminescence (PL) mechanism of Er³⁺-doped Al₂O₃ powders by Y³⁺ codoping at wavelength 1.53 μm has been investigated through PL measurements of 0.1 mol% Er³⁺- and 0-20 mol% Y³⁺-codoped Al₂O₃ powders prepared at a sintering temperature of 900 °C in a non-aqueous sol-gel method. PL intensity and lifetime of Er³⁺-Y³⁺-codoped Al₂O₃ powders composed of γ-(Al,Er,Y)₂O₃ and θ-(Al,Er,Y)₂O₃ phases increased with increasing Y³⁺-codoping concentration. The 10-20 mol% Y³⁺ codoping in 0.1 mol% Er³⁺-doped Al₂O₃ powders intensified the PL intensity by about 20 times, with a PL lifetime prolonged from 3.5 to 5.8 ms. A maximal increase of the optical activity of Er³⁺ in 0.1 mol% Er³⁺-Y³⁺-codoped Al₂O₃ powders about one order was achieved by 10-20 mol% Y³⁺ codoping. It is found that the improved PL properties for Er³⁺-Y³⁺-codoped Al₂O₃ powders are mainly attributed to enhanced optical activation of Er³⁺ in the Al₂O₃ by Y³⁺ codoping, and to the slightly increased radiative quantum efficiency of Er³⁺ in the Al₂O₃.