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.     

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 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.     

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.     

Influence of carbon templates and Yb concentration on red and green luminescence of uniform Y2O3:Yb/Er hollow microspheres

Uniform Yb³⁺ and Er³⁺-codoped Y₂O₃ hollow microspheres were synthesized via urea co-precipitation using carbon spheres as templates. Intense red emission (⁴F_9/2→⁴I_15/2) and weak green emission (²H_11/2, ⁴S_3/2→⁴I_15/2) of Er³⁺ were observed for the Yb³⁺ and Er³⁺-codoped Y₂O₃ hollow microspheres under 980 nm infrared excitation. The integrated intensity of visible emission and the ratio of red to green were found to be strongly dependent on the amount of carbon sphere templates and the concentration of Yb³⁺ ions. The amount of carbon sphere templates also plays an important role in adjusting the size of crystallite. Multi-phonon relaxation resulted from the absorbents (OH⁻ and CO₃²⁻) on the surface of the crystallite, and efficient occur of energy transfer processes and cross-relaxation between Er³⁺ and Yb³⁺ are responsible for the enhancement of intensity ratio of red to green emission. Interestingly, for higher concentration of Yb³⁺ ions, the green emission is assigned to a three-phonon process in Y₂O₃:Yb/Er hollow microspheres, which also could result in the increase of the red to green emission ratio. An explanation to account for these behaviors was presented.     

Crystal field analysis of Er in Sc2O3 transparent ceramics

A crystal field investigation of Er³⁺-doped Sc₂O₃ transparent ceramics is presented.The established energy level diagram is analyzed with crystal-field Hamiltonian of C₂ symmetry including J-mixing effect. Very satisfactory correlations were obtained between 53 calculated and experimental Stark energy levels, with an rms of 7.7 cm⁻¹. These results are compared with those reported for other rare earth ions in the same host and with Er³⁺ ions in other sesquioxides. The concordance between the crystal-field strength parameters indicates the consistence of our analysis. Furthermore, based on the extended electrostatic point charge model, a theoretical trend is proposed for rare earth ions in Sc₂O₃ in comparison with Y₂O₃. Based on this trend, very satisfactory results are obtained for Yb³⁺ ion in Sc₂O₃.     

Effects of non-stoichiometry and substitution on photoluminescence and afterglow luminescence of Sr4Al14O25:Eu, Dy phosphor

The effects of nonstoichiometry and cationic substitution on photoluminescence and afterglow characteristics of strontium aluminate phosphor (Sr₄Al₁₄O₂₅:Eu²⁺, Dy³⁺) were investigated. Photoluminescence intensity of both the strontium-deficit and -rich phosphors was enhanced, but no definite correlation was observed between the afterglow intensity and non-stoichiometry. The photoluminescence emission maxima were either blue or green shifted in case of non-stoichiometric phosphors, whereas the afterglow emission maxima were not affected by the non-stoichiometry. Substitution of strontium by calcium resulted in white afterglow emission at higher calcium concentration. The emission centers in case of photoluminescence and afterglow emission appear to be different. Addition of silver significantly enhanced the afterglow intensity due to increased trap density.     

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.     

Investigation on upconversion photoluminescence of Bi3TiNbO9:Er:Yb thin films

Bi₃TiNbO₉:Er³⁺:Yb³⁺ (BTNEY) thin films were fabricated on fused silica by pulsed laser deposition. It was demonstrated that different laser fluence and substrate temperature during growth of BTNEY upconversion photoluminescence (UC-PL) samples control the film’s grain size and hence influences the UC-PL properties. The average grain size of BTNEY thin films deposited on fused silica substrates with laser fluence 4, 5, 6, and 7 J/cm² are 30.8, 35.9, 40.6, and 43.4 nm, respectively. The 525 nm emission intensities increase with the deposition laser fluence and the emission intensities of BTNEY thin film deposited under 700 and 600 °C are almost 24 and 4 times, respectively, as strong as those of samples under 500 °C. The grain size of BTNEY thin film increases with the increasing temperature. UC-PL of BTNEY films is enhanced by increasing grain size of the films.     

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.     

Synthesis and optical properties of Er and Eu doped SrAl2O4 phosphor ceramic

We report, for the first time on luminescence from a Er³⁺ doped SrAl₂O₄ phosphor. Effects of Eu³⁺ doping were also studied. The influence of rare-earth doping in crystal structure and its optical properties were analysed by means of X-ray diffraction (XRD), Raman scattering, optical absorption, excitation and emission (PL) spectroscopy, thermally stimulated luminescence (TSL) and scanning electron microscope (SEM). Luminescence spectra and luminescence decay curves for Er³⁺ transitions in the near infrared region were recorded. The PL maximum for Eu doped SrAl₂O₄ is obtained at 620 nm and corresponds to the orange region of the spectrum. Diffraction patterns reveal a dominant phase, characteristic of the monoclinic SrAl₂O₄ compound and the presence of dopants has no effect on the basic crystal structure of SrAl₂O₄. The shapes of the glow curves are different for each dopant irradiated with either a ⁹⁰Sr-⁹⁰Y beta source, or UV light at 311 nm, and in detail the TL signals differ somewhat between Er and Eu dopants.     

Infrared-to-visible upconversion luminescence of Er doped barium-natrium-yttrium-fluoride phosphor

Infrared-to-visible upconversion luminescence has been investigated in Er³⁺-doped barium-natrium-yttrium-fluoride phosphor (Ba_xNa_yY_zF_2x+y+3z+3m:Er_m) with different cation concentrations. Intense upconversion emissions around 530, 550, and 660 nm corresponding to the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 transitions, respectively to the ⁴I_15/2 ground state were observed when excited by CW laser radiation at 1550 nm. We adopted the low-temperature combustion synthesis (LCS) method to decrease the phosphor particle size to 40-70 nm in order to couple to the photosensitive surface of CCD. The effect of the amount of carbamide on the particle size and the upconversion luminescence intensity was analyzed. The upconversion luminescence mechanism was studied by the log-log plot of intensity-power.     

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).     

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.     

退火温度对Er/Yb共掺Al2O3薄膜的光致荧光光谱的影响

通过对不同退火条件下Er/Yb共掺Al_２O_３薄膜光致荧光(PL)光谱的 系统分析，研究了高Er/Yb掺杂浓度所导致的晶体场变化对薄膜PL光谱的影响，并结合薄膜结构分析，探讨了Al_２O_３薄膜的结晶状态在Er^３＋激活、PL光谱宽化 中的作用及可能的物理机理.研 究结果表明：退火处理所导致的Er^３＋ PL光谱的变化与薄膜的微观状态之 间有着密 切的联系.在600—750℃范围内，薄膜呈非晶态结构，薄膜荧光强度的增加主要是薄膜内缺 陷减少所致；在800—900℃范围内，γ-Al_２O_３相的出现是导致荧 光强度明显增加的主 要原因；当退火温度为1000℃时，Er，Yb的大量析出致使荧光强度的急剧下降.此外，对PL 光谱线形分析表明，各子能级跃迁的相对强度变化是导致荧光光谱宽化的主要因素. 关键词： Er/Yb共掺 ２O_３薄膜')" href="#">Al_２O_３薄膜 光致荧光     

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.     

Effect of Mg and Ti doping on the luminescence of Y2O3:Eu

The Y₂O₃:Eu³⁺,Mg²⁺,Ti^IV materials (x_Eu: 0.02, x_Mg: 0.08, x_Ti: 0.04) were prepared by solid state reaction. The purity and crystal structure of the material was studied with the X-ray powder diffraction. Luminescence properties were studied in the UV-VUV range with the aid of synchrotron radiation. The emission of Y₂O₃:Eu³⁺,Mg²⁺,Ti^IV had a maximum at 612 nm (λ_exc: 250 nm) due to the ⁵D₀→⁷F₂ transition of Eu³⁺. The excitation spectra (λ_em: 612 nm) showed a broad band at 233 nm, due to the charge transfer transition between O²⁻ and Eu³⁺, and at 297 nm due to the Ti→Eu³⁺ energy transfer. Only very weak persistent luminescence was discovered. In the room and 10 K temperature excitation spectra, the line at 208 nm is due to the formation of a free exciton (FE) and a broad band at 199 nm was related to the valence-to-conduction band absorption of the Y₂O₃ host lattice. The absorption edge was ca. 205 nm giving 6.1 eV as the energy gap of Y₂O₃.     

NIR to visible upconversion in Er/Yb co-doped CaYAl3O7 phosphor obtained by solution combustion process

Using the combustion synthesis, CaYAl₃O₇:Er³⁺ phosphor powders co-doped with Yb³⁺ have been prepared at low temperatures (550 °C) in a few minutes. Formation of the compound was confirmed by X-ray powder diffraction. Near-infrared to visible upconversion fluorescence emission in the Er³⁺ doped CaYAl₃O₇ phosphor powder has been observed. The effect of co-doping with triply ionized ytterbium in the CaYAl₃O₇:Er³⁺ phosphor has been studied and the process involved is discussed.     

Infrared-to-visible upconversion emission of Er3+/Yb3+-codoped SrMoO4 phosphors as wide-range temperature sensor

Er³⁺/Yb³⁺-codoped SrMoO₄ phosphors were prepared by a high-temperature solid-state reaction method. At room temperature, all the as-prepared samples exhibited strong upconversion properties and the emission intensity increased dramatically with the increase of Yb³⁺ ion concentration, reaching its maximum value when the concentration was 5 mol%. The dependence of emission intensity on the pump power suggested that the upconversion emission was a two-photon process. Furthermore, the optical temperature sensing properties based on green upconversion emissions of the SrMoO₄:0.01Er³⁺/0.05Yb³⁺ phosphor were studied. It is found that the SrMoO₄:0.01Er³⁺/0.05Yb³⁺ phosphor can be operated over a very wide temperature range of 93–773 K with a maximum sensitivity of ∼0.0128 K⁻¹, indicating that low- and high-temperature thermometry can be simultaneously realized in this phosphor.     

Hydrothermal synthesis of BaYF5:Yb/Er upconversion luminescence submicrospheres by a surfactant-free aqueous solution route

BaYF₅:Yb³⁺/Er³⁺ upconversion (UC) luminescence submicrospheres have been synthesized by the hydrothermal synthesis method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), scanning probe microscope (SPM), transmission electron microscope (TEM), laser diffraction particle analyzer (LDPA) and UC emission spectra. The as-prepared highly crystalline BaYF₅:Yb³⁺/Er³⁺ submicrospheres are of uniform size depending on different reaction temperatures and reaction times. It is found that the usage of fluoride source NaBF₄ plays the crucial key in the formation of submicrosphere. Under the 980 nm excitation, the UC emission transitions for ⁴F_9/2→⁴I_15/2 (red), ²H_11/2, ⁴S_3/2→⁴I_15/2 (green) in the BaYF₅:Yb³⁺/Er³⁺ submicrospheres came from two-, two-, and two-photon UC processes, respectively. Further, the effects of Yb³⁺ ion concentration, size and surface of as-prepared submicrospheres, and pumping power on the UC luminescence properties of BaYF₅:Yb³⁺/Er³⁺ have also been discussed.