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.     

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.     

Influence of Yb concentration on upconversion luminescence of Ho

Upconversion (UC) emissions at 360 ((⁵F, ³F, ⁵G)₂ → ⁵I₈), 392 (³K₇/⁵G₄ → ⁵I₈), 428 (⁵G₅ → ⁵I₈), 554 (⁵S₂/⁵F₄ → ⁵I₈), 667 (⁵F₅ → ⁵I₈) and 754 (⁵S₂/⁵F₄ → ⁵I₇) nm were obtained in 0.1 mol% Ho³⁺/x mol% Yb³⁺:Y₂O₃ (x = 2, 5, 8, 11, 15) bulk ceramics under infrared (IR) excitation at 976 nm. The intensity of the UC luminescence centered at 554 and 754 nm increased with Yb³⁺ concentration from 2 to 5 mol% and decreased from 5 to 15 mol%, while the UC luminescence centered at 392, 428 and 667 nm increased with Yb³⁺ concentration from 2 to 11 mol%, then started to reduce with Yb³⁺ concentration until 15 mol%. This comes from the competition between the energy back transfer (EBT) process [⁵S₂/⁵F₄(Ho) + ²F_7/2(Yb) → ⁵I₆(Ho) + ²F_5/2(Yb) as well as ⁵F₅(Ho) + ²F_7/2(Yb) → ⁵I₇(Ho) + ²F_5/2(Yb)] and spontaneous radiation process. The intensity of the UC luminescence centered at 360 nm always increases with Yb³⁺ concentration from 2 to 15 mol%. We believe that it may come from the cooperation of energy transfer process from Yb³⁺ ions in the ²F_5/2 state and the cross energy transfer process ⁵S₂/⁵F₄ + ⁵I₆ → (⁵F, ³F, ⁵G)₂ + ⁵I₈.     

Eu/Tb ions co-doped white light luminescence Y2O3 phosphors

Y₂O₃:Eu³⁺, Tb³⁺ phosphors with white emission are prepared with different doping concentration of Eu³⁺ and Tb³⁺ ions and synthesizing temperatures from 750 to 950 °C by the co-precipitation method. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the crystallinity of the synthesized samples increases with enhancing the firing temperature. The photoluminescence spectra indicate the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors show five main emission peaks: three at 590, 611 and 629 nm originate from Eu³⁺ and two at 481 and 541 nm originate from Tb³⁺, under excitation of 250-320 nm irradition. The white light luminescence color could be changed by varying the excitation wavelength. Different concentrations of Eu³⁺ and Tb³⁺ ions were induced into the Y₂O₃ lattice and the energy transfer from Tb³⁺→Eu³⁺ ions in these phosphors was found. The Commission International de l’Eclairage (CIE) chromaticity shows that the Y₂O₃:Eu³⁺, Tb³⁺ phosphors can obtain an intense white emission.     

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.     

Enhanced near-infrared luminescence in Y2O3:Yb nanocrystals by codoping with Li ion

Y₂O₃:Yb³⁺ nanocrystals codoping with Li⁺ ions were synthesized by glycine combustion method. Codoping with Li⁺ ions leads to about 12 times enhancement of the photoluminescence (PL) intensity around 1 μm, in terms of the increased lifetimes at 1026 nm from 0.384 ms to 1.42 ms at room temperature. The enhancement in the PL intensity could be mostly attributed to the modification of the local symmetry around Yb³⁺ ions by codoping with Li⁺ ions.     

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.     

Probing of surface Eu ions present in ZnO:Eu nanoparticles by covering ZnO:Eu core with Y2O3 shell: Luminescence study

Eu³⁺ doped ZnO nanoparticles are known to have significance extent of surface Eu³⁺ ions due to a large difference in ionic radii. Effect of such Eu³⁺ ions on the luminescence properties of ZnO:Eu nanoparticles has been understood from the luminescence studies of ZnO:Eu nanoparticles covered with Y₂O₃ shell. Based on the asymmetric ratio of luminescence and extent of energy transfer, it is established that when ZnO:Eu nanoparticles are covered with Y₂O₃ shell, a part of Eu³⁺ ions present with ZnO:Eu core migrate to Y₂O₃ shell and occupy Y³⁺ lattice positions.     

Enhanced white light emission in Er/Tm/Yb/Li codoped Y2O3 nanocrystals

Er/Tm/Yb codoped Y₂O₃ nanocrystals and Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals have been synthesized by sol-gel method, bright white light emission has been observed at 976 nm excitation. The blue, green, and red emissions, respectively, arise from the transitions ¹G₄ → ³H₆ of Tm³⁺, ²H_11/2/⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ion. Moreover, after doping Li⁺ ions into Er/Tm/Yb codoped Y₂O₃ nanocrystals, the white light emission increase greatly. CIE coordinate of Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals is X = 0.32 and Y = 0.36 at 10 W/cm² excitation, which is very close to the standard equal energy white light illuminate (X = 0.33, Y = 0.33).     

Synthesis and luminescence properties of Gd6MoO12:Yb3+, Er3+ phosphor with enhanced photoluminescence by Li+ doping

Yb³⁺/Er³⁺ co-doped Gd₆MoO₁₂ and Yb³⁺/Er³⁺/Li⁺ tri-doped Gd₆MoO₁₂ phosphors were prepared by adjusting the annealing temperature via the high temperature solid-state method. Under the excitation of 980 nm semiconductor, the upconversion luminescence properties were investigated and discussed. In the experimental process, we get the optimum Yb³⁺ concentration and the concentration quench effect will happen while the concentration extends the given region. According to the Yb³⁺ concentration quenching effects, the critical distance between Yb³⁺ ions had been calculated. The measured UC luminescence exhibited a strong red emission near 660 nm and green emission at 530 nm and 550 nm, which are due to the transitions of Er³⁺(⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺(⁴I_15/2). Then the effect of excitation power density in different regions on the upconversion mechanisms was investigated and the calculated results demonstrate that the green and red upconversion is a two-photon process. A possible mechanism was discussed. After Li⁺ ions mixing, the upconversion emission enhanced largely, and the optimum Li⁺ concentration was obtained while fixed the Yb³⁺ and Er³⁺ on the above optimum concentration. This enhancement owns to the decrease of the local symmetry around Er³⁺ after Li⁺ ions doping into the system. This result indicates that Li⁺ is a promising candidate for improving luminescence in some case.     

Photoluminescence analysis of α-Al2O3 powders doped with Eu and Eu ions

Alumina (Al₂O₃) powders doped with europium trivalent (Eu³⁺) were prepared by a low-temperature (∼280 °C) combustion synthesis technique. When the powder was heat treated at 1200 °C for 2 h in the presence of flowing ammonia (NH₃), α-Al₂O₃ crystalline ceramic powders was obtained. The analysis of the luminescence showed that Eu³⁺ was reduced to europium divalent (Eu²⁺) after the heat-treatment process. Under ultraviolet (UV) lamp excitation (λ=254 nm) these powders containing sub-microcrystalline structures present bright red (Al₂O₃:Eu³⁺) and green (Al₂O₃:Eu²⁺) luminescence indicating that this material is a potential candidate for applications in phosphor technology.     

Enhancement of the upconversion luminescence in Y2O3:Er3+ powders by codoping with La3+ ions

Infrared-to-visible upconverted luminescent spectra of Er3 t and La3 t codoped Y2O3 powders are investigated.By introducing La3 t ions, the upconversion green radiation is found to be greatly enhanced when compared with the powders with La3 t absent. Such enhancement can be attributed to the modification of the local symmetry surrounding the Er3 t ion, which benefits the intra-4f transitions of Er3 t ion, and the decreasing interaction between Er3 t ions, which suppresses the energy transfer process4F7∕2t 4I11∕2→ 4F9∕2t 4F9∕2.     

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 luminescent properties of cubic Eu:Y2O3 nanocrystals and comparison to bulk Eu:Y2O3

Y₂O₃:Eu³⁺ nanocrystals were prepared by combustion synthesis. The particle size estimated by X-ray powder diffraction (XRD) was about 10 nm. A blue-shift of the charge-transfer (CT) band in excitation spectra was observed in Y₂O₃:Eu³⁺ nanocrystals compared with bulk Y₂O₃:Eu³⁺. The electronic structure of Y₂O₃ is calculated by density functional method and exchange and correlation have been treated by the generalized gradient approximation (GGA) within the scheme due to Perdew-Burke-Ernzerhof (PBE). The calculated results show that the energy centroid of 5d orbital in nanocrystal has increasing trend compared with that in the bulk material. The bond length and bond covalency are calculated by chemical bond theory. The bond lengths of Y₂O₃:Eu³⁺ nanocrystal are shorter than those of the bulk counterpart and the bond covalency of Y₂O₃:Eu³⁺ nanocrystal also has an increasing trend. By combining centroid shift and crystal-field splitting, the blue-shift of the CT band is interpreted.     

Down-conversion near infrared emission in Pr, Yb co-doped Y2O3 transparent ceramics

Pr³⁺, Yb³⁺ co-doped Y₂O₃ transparent ceramics have been prepared by the solid state reaction and vacuum sintering. Down-conversion near infrared emission has been demonstrated upon a 482 nm excitation. The energy of the 482 nm blue photon was first absorbed by Pr³⁺ and then delivered to Yb³⁺. Possible energy transfer mechanisms from Pr³⁺ to Yb³⁺ have been discussed. Under the 482 nm excitation, the Pr⁴⁺-Yb²⁺ charge transfer state would not seriously influence the energy transfer process. The dominant one should be either the cooperative down-conversion or the two-step photon emission. The efficient down-conversion near infrared emission has potential application in enhancing the conversion efficiency of crystalline silicon solar cells.     

Yb3+/Ho3+/Tm3+共掺碲酸盐玻璃的上转换白光发射及其机理研究

采用高温熔融法分别制备了Yb3+/Ho3+,Yb3+/Tm3+和Yb3+/Ho3+/Tm3+共掺的碲酸盐玻璃。在980nm红外激光激发下,Yb3+/Ho3+/Tm3+共掺的玻璃样品显示了强的蓝光、绿光和红光发射,分别对应于Tm3+的1 G4→3 H6跃迁、Ho3+的5 F4(5 S2)→5I 8跃迁以及Ho3+的5 F5→5I 8和Tm3+的1 G4→3 F4跃迁。通过对比发现,Yb3+/Ho3+/Tm3+共掺样品中的红、绿光积分发射强度比值(3.95)明显大于Yb3+/Ho3+共掺样品(1.69),这是由于Ho3+和Tm3+间存在交叉弛豫过程3 H4(Tm3+)+5I 6(Ho3+)→3 F4(Tm3+)+5 F5(Ho3+)和3 F4(Tm3+)+5I 8(Ho3+)→3 H6(Tm3+)+5I 7(Ho3+)所致。在激发功率密度为8.2 W.cm-2时,Yb3+/Ho3+/Tm3+共掺样品的上转换发光色坐标值为x=0.345,y=0.338,非常接近于等能白光(x=0.333,y=0.333)。     

尺寸效应对Er3+掺杂纳米Y2O3的发光特性的影响

采用燃烧法制备了不同尺寸的Er³⁺掺杂Y₂O₃粉体材料,研究了尺寸效应对Er³⁺掺杂纳米Y₂O₃材料发光特性的影响.光声光谱显示,对于不同晶粒尺寸的样品,Er³⁺离子光声峰位置几乎保持不变.这表明小尺寸效应对稀土离子能级位置影响很小.对488nm激光激发下的发射谱的分析发现,随着样品颗粒尺寸的减小,4S_3/2能级和关键词： 3+离子')" href="#">Er³⁺离子 纳米 发光 能量传递     

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.     

Preparation and characterization of upconversion luminescent LaF3:Yb,Er/LaF3 core/shell nanocrystals

LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals were successfully synthesized using solvothermal method. The crystal structure, morphology and photoluminescence properties of as-prepared nanocrystals were investigated in detail. XRD patterns show that the obtained LaF₃:Yb³⁺,Er³⁺ core and LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals exhibit hexagonal structure. The average particle size is about 9.3 nm and 11.4 nm for core and core/shell nanocrystals, respectively. Compared with LaF₃:Yb³⁺,Er³⁺ nanocrystals, both the upconversion emission intensity and the lifetime increase in LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals. The enhancement can be attributed to the LaF₃ shell which can eliminate the nonradiative centers on the surface of LaF₃:Yb³⁺,Er³⁺ nanocrystals.