The kinetic limitation of anti-stokes luminescence of Er3+ and Yb3+ doped infrared upconversion materials

Infrared-to-visible wave-length conversion in the Yb³⁺−Er³⁺ doped phosphors system has been described by a simple three level model based on two ions mechanism. The excitation in the range of 900–1000 nm of an IR-photon is first absorbed by Yb³⁺ ion as a sensitizer attributed to the resonant energy transition in Er³⁺ ion from ⁴ I _3/2 → ⁴ S _15/2 and ¹ F _9/2 → ⁴ I _15/2, respectively for green and red emission. The essential energy transfer processes in this system i.e. upconversion from ⁴ I _11/2 and ¹ I _13/2, cross-relaxation from ⁴ S _3/2 and ¹ F _9/2 are taken into account. The limitations of the rate-equation approach are examined with a focus on the underlying dynamics of this rare-earth system.     

Effect of Yb3+ concentration on photoluminescence properties of cubic Gd2O3 phosphor

Yb³⁺ doped phosphor of Gd₂O₃ (Gd₂O₃:Yb³⁺) have been prepared by solid state reaction method. The structure and the particle size have been determined by X-ray powder diffraction measurements. The average particle size of the phosphor is in between 35 and 50 nm. The particle size and structure of the phosphor was further confirmed by TEM analysis. The visible and NIR luminescence spectra were recorded under the 980 nm laser excitation. The visible upconversion luminescence of Yb³⁺ ion was due to cooperative luminescence and the presence of rare earth impurity ions. The cooperative upconversion and NIR luminescence spectra as a function of Yb³⁺ ion concentration were measured and the emission intensity variation with Yb³⁺ ion concentration was discussed. Yb³⁺ energy migration quenched the cooperative luminescence of Gd₂O₃:Yb³⁺ phosphor with doping level over 5%, while the NIR emission luminescence continuously increases with increasing Yb³⁺ ion concentration.     

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.     

Preparation,characterization and electroluminescence studies of copper doped zinc sulfide nanocrystals

Copper doped zinc sulfide nanoparticles were prepared by chemical precipitation method. The size of the particles was varied by changing the concentration of capping agent. The XRD studies indicate that most of the samples are cubic in nature. The broadening of peaks tends to increase with increasing capping agent concentration showing decrease in particle size. The crystalline size computed using Scherrer formula is found to be in range of 3–10 nm. Absorption spectra show absorption edge in UV region. The edge was found to shift towards shorter wavelength as the capping agent concentration is increased. This indicates increased effective band gap and hence reduced particle size. The nanoparticle size has been estimated in the range 5–10 nm using effective mass approximation model. For electroluminescence (EL) study of ZnS:Cu nanocrystals, the EL cells were prepared by placing ZnS:Cu nanoparticles between SnO₂ coated conducting glass plate and aluminum foil. Alternating voltage of various frequencies was applied and EL brightness (B) at different voltages (V) was measured and reported in this paper.     

Yb3+ site occupation and host sensitization luminescence of a novel near‐infrared emitting Sr2CaMoO6:Yb3+ phosphor

The host sensitized near‐infrared (NIR) emitting phosphor Sr₂CaMoO₆:Yb³⁺ was fabricated by the solid state reaction method. The structural refinement and Raman spectra elucidate that Yb³⁺ ions preferentially occupy Ca²⁺ sites. The phosphor can harvest ultraviolet (UV)–blue photons and exhibits intense NIR emission at around 1012 nm with full‐width‐at‐half‐maximum of 1635 cm^–1. Moreover, the absolute NIR photoluminesence quantum yield (PLQY) is estimated to be about 9%. The Sr₂CaMoO₆:Yb³⁺ phosphor may be a promising luminescence downshifting material for improving the spectral response of solar cells in the UV region. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoluminescence and optically stimulated luminescence characteristics of Al2O3 doped with Tb

In the present work policrystals of α − Al₂O₃ doped with terbium were synthesized using the solvent evaporation method. The samples were prepared using Al(NO₃)₃·9H₂O and Tb(NO₃)₃·5H₂O reagents, with Tb concentrations between 1 and 5 mol% and thermally treated at high temperature above ∼1400 °C. X-ray diffraction measurements showed the α-phase formation of samples. TL glow curve presented an intense peak at ∼190 °C and two other with low intensity at 290 and 350 °C after gamma irradiation. The best doping concentration which presented high luminescence was the sample doped with 3 mol% of Tb. TL spectra and fluorescence measurements showed similar luminescence spectra with lines attribute to Tb³⁺ ions. A linear behavior to gamma dose between 1 and 20 Gy was observed in TL, using 190 °C peak as well as in OSL signal, this last carried out using 532 nm wavelength stimulation.     

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.     

Recombination processes in lanthanum beryllate single crystals doped with Ce3+ and Pr3+ ions

The paper presents experimental results on recombination processes in lanthanum beryllate (BLO) single crystals (undoped BLO, and doped with 0.5 at % by Ce³⁺ and Pr³⁺ ions) obtained using XRL-spectroscopy at T = 8, 80 and 290 K and thermoluminescence technique in the temperature range of 8–650 K. The paper discusses spectra of the steady-state XRL-luminescence recorded in the energy range from 1.5 to 6.2 eV at different temperatures between 8 and 290 K; temperature dependences of XRL intensity recorded in the temperature range from 8 to 650 K; thermoluminescence glow curves recorded in spectral-integrated regime after X-ray exposure at T₀ = 8 or 290 K.     

Synthesis and near-infrared luminescence properties of LaOCl:Nd3+/Yb3+

Near-infrared emitting phosphors LaOCl:Nd³⁺/Yb³⁺ were prepared by the solid-state method, and their structures and luminescent properties were investigated by using X-ray diffraction and photoluminescence analysis, respectively. The studies shows that tetragonal LaOCl:Nd³⁺/Yb³⁺ can be synthesized by the solid-state reaction at 600 °C for 3 h. Upon 353 nm UV excitation, LaOCl:Nd³⁺/Yb³⁺ sample shows strong near-infrared emission lines in the region of 1060–1150 nm (corresponding to ⁴F_3/2 → ⁴I_J′ transition of Nd³⁺, J′ = 9/2, 11/2, 13/2, 15/2) and 980–1050 nm (corresponding to ²F_5/2 → ²F_7/2 transition of Yb³⁺). The decreasing emission intensity of Nd³⁺ with increasing doping concentration of Yb³⁺ proved the energy transfer in LaOCl:Nd³⁺/Yb³⁺. The possible near-infrared emission and energy transfer mechanism between Nd³⁺ and Yb³⁺, as well as the energy transfer efficiency of LaOCl:Nd³⁺/Yb³⁺ were discussed.     

Comparative Investigation on Nanocrystal Structure and Luminescence Properties of Gadolinium Molybdates Codoped with Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript>

This paper reports on the comparative investigation of nanocrystal structure and luminescence properties of Er³⁺/Yb³⁺-codoped gadolinium molybdate nanocrystals Gd₂(MoO₄)₃ and Gd₂MoO₆ synthesized by the Pechini method with citric acid and ethylene glycol. Their crystallization, structure transformation, and morphologies have been investigated by X-ray diffraction, thermogravimetric/differential scanning calorimetry, and transmission electron microscopy. It is noticed that Er³⁺/Yb³⁺-codoped monoclinic Gd₂(MoO₄)₃ nanocrystals have shown an intense upconversion through a sintering of the organic complex precursor at 600°C. Furthermore, it transforms to orthorhombic Gd₂(MoO₄)₃ when the precursor is sintered at 900°C. In counterpart of monoclinic Gd₂MoO₆, however, the monoclinic structure remains unchanged when the precursor is sintered at a temperature ranging from 600°C to 900°C. Intense visible emissions of Er³⁺ attributed to the transitions of ²H_11/2, ⁴S_3/2–⁴I_15/2 at 520 and 550 nm, and ⁴F_9/2–⁴I_15/2 at 650 nm have been observed upon an excitation with a UV source and a 980 nm laser diode, and the involved mechanisms have been explained. It is quite interesting to observe obvious differences both in the excitation and the upconversion emission spectra of Er³⁺/Yb³⁺-codoped Gd₂(MoO₄)₃ respectively with monoclinic and orthorhombic structure. The quadratic dependence of fluorescence on excitation laser power has confirmed that two-photons contribute to upconversion of the green–red emissions.     

Yb:YAG晶体的光谱性能

系统地研究了不同掺杂浓度的Ｙｂ：ＹＡＧ晶体的光谱特性,通过吸收光谱的测量计算了晶体的吸收截面,用对易法计算了晶体的发射截面。在Ｙｂ：ＹＡＧ晶体毛坯中发现Ｙｂ＾２＋和色心,其浓度随Ｙｂ：ＹＡＧ晶体中Ｙｂ＾３＋的增加而增加。经１４００℃氧气氛退火后消失。首次用光子激发和Ｘ射线激发研究了Ｙｂ：ＹＡＧ晶体的荧光特性。     

Yb浓度对功率依赖的上转换荧光色彩的敏感度调控

控制激发光功率密度是一种调控红绿荧光比率的简单方法.然而,大多数上转换系统对功率的调控并不敏感.本文通过柠檬酸钠辅助的水热法,合成了一系列具有不同Yb浓度掺杂的NaYF₄：Yb/Ho微米棒.通过激光共聚焦显微镜系统,研究了Yb浓度和激发功率密度依赖的NaYF₄：Yb/Ho微米棒的上转换荧光特性.发射谱和同步荧光成像图案表明：荧光红绿比率不仅敏感于激发功率,而且敏感度依赖于Yb浓度.随着Yb浓度的增加,功率调控的红绿比率的敏感度增加,这暗示了功率调控的红绿比率的敏感度可以作为一种度量和评估Yb掺杂浓度的有效途径和方法.通过上/下转换发射谱、激发谱和功率依赖关系,揭示了功率调控红绿比率的机理,并提出了荧光色彩敏感于功率调控的上转换系统具有的特征和判据.本研究为设计和合成高敏感度的功率调控的上转换材料提供了理论基础和实验数据.     

Fabrication and luminescence behavior of phosphate glass ceramics co-doped with Er3+ and Yb3+

Transparent phosphate glass ceramics co-doped with Er³⁺ and Yb³⁺ in the system P₂O₅Li₂OCaF₂TiO₂ were successfully synthesized by melt-quenching and subsequent heating. Formation of the nanocrystals was confirmed by X-ray powder diffraction. Judd–Ofelt analyses of Er³⁺ ions in the precursor glasses and glass ceramics were performed to evaluate the intensity parameters Ω_2,4,6. Under 975 nm excitation, intense upconversion (UC) and infrared emission (1545 nm) were observed in the glass ceramics by efficient energy transfer from Yb³⁺ to Er³⁺. The luminescence processes were explained and the emission cross section was calculated by Fuchtbauer–Ladenburg (F–L) formula. The results confirm the potential applications of Er³⁺/Yb³⁺ co-doped glass ceramics as laser and fiber amplifier media.     

Influence of Yb~(3+) concentration on the upconversion luminescence of oxyfluoride material doped with Er~(3+)

In this paper,a new upconversion luminescent material co-doped with Erbium and Ytterbium is reported. The upconversion luminescence transition routes and related properties are studied. The results show that the absorption and emission intensities under excitation of 980 nm laser increase with the increase of the Yb3+ concentration. The red emission is stronger than the green emission. The energy transfer process plays an important role in the upconversion mechanism.     

Photoluminescence and photostimulated luminescence in the X-ray storage phosphor BaBr2 doped with cerium

In orthorhombic BaBr₂ : Ce³⁺ two kinds of luminescence bands at room temperature have been attributed to charge-compensated Ce³⁺ centres. One type was associated with potassium (or some other monovalent cation) on a neighbouring Ba site and another one associated with an unidentified defect. A third kind of emission, observed only as low temperature photoluminescence (PL), is ascribed to isolated Ce³⁺ ions. The charge-compensated Ce³⁺ complexes are active both in PL and photostimulated luminescence (PSL) following X-ray irradiation. The PSL is nearly as efficient as in the case of the commercially used X-ray storage phosphor BaFBr:Eu²⁺. The X-ray induced electrons are trapped in F-type centres whose band position is characteristic for the nearby Ce complex. As shown by the fingerprint character of the PSL itself, the hole partner in the recombination is also associated or identical with the same Ce complex.     

Ho3+离子掺杂单颗粒氟化物微米核壳结构的上转换发光特性

构建核壳结构可有效降低材料的表面缺陷及实现掺杂离子的可控区域分布,已成为目前增强及调控材料发光特性的有效手段之一.为此,本文以外延生长技术,构建了一系列NaLnF₄(Ln=Y,Yb,Ho)@NaLnF₄(Ln=Y,Yb)核壳微米结构,并实现了Ho³⁺离子上转换发光的增强及可控调节.借助共聚焦显微光谱测试系统,在980 nm近红外激光激发下,研究Ho³⁺离子在不同单颗粒核壳结构中的上转换发光特性.结果表明,当包覆NaYF₄惰性壳时,NaYF₄:Yb³⁺/Ho³⁺及NaYbF₄:Ho³⁺微米棒的上转换发射强度均得到了明显增强,而NaHoF₄@NaYF₄微米核壳结构的发射强度却没有发生明显的变化.当在其NaYF₄惰性壳中引入Yb³⁺离子时,NaYF₄:Yb³⁺/Ho³⁺,NaYbF₄:Ho³⁺及NaHoF₄微米核壳结构的发射强度及红绿比均再次得到了明显增强.基于对其光谱特性及动力学过程的研究,其发射增强主要由于壳层中的Yb³⁺离子通过能量迁移及传递过程有效地提高Ho³⁺离子激发,进而在双向协同的作用下实现其发光有效增强及色彩调控.由此可见,对于微米晶体而言,构建其不同的核壳结构不仅可实现其发光有效增强,且可根据掺杂离子的不同及其区域分布实现光谱的精准调控,为拓展高效发光特性的微米晶体在防伪、微纳光电器件等领域的应用提供新途径.     

A comparative analysis of infra-red luminescence spectra of ZnSe crystals doped with Yb,Gd or Cr impurities

Infra-red (IR) photoluminescence (PL) spectra of ZnSe crystals doped with Yb, Gd rare-earth impurities and Cr impurity are investigated. The influence of stoichiometric deviation on the spectra is studied and the structure of complex IR PL bands is analysed. The good coincidence between the structures of IR PL spectra of the samples doped with Yb, Gd, and Cr is shown. Correlation between the component parts of the bands at 1 and 2 μm is found and possibility to control the composition of IR PL spectra by enrichment of the samples with Zn or Se is discussed. The models that explain the formation of complexes based on rare-earth and background Cr and Cu impurities, responsible for IR PL bands, are proposed. Keywords: IR luminescence, ZnSe, Rare-earth impurities, Cr impurity.     

镱浓度对钬镱双掺二氧化钛上转换发光的影响

以钛酸丁酯为前躯体,乙醇为溶剂,冰乙酸为催化剂,用溶胶-凝胶法制备了TiO2∶Ho3+/Yb3+上转换发光纳米粉体,并研究其上转换发光机制.通过X射线衍射,能谱及扫描电镜对样品进行表征,结果表明样品是金红石型结构的粉体,样品中含有所掺杂的元素Ho和Yb,样品颗粒非常细小,直径约为100 nm,呈球形,并有明显的团聚现象.在常温下选用中心波长为980 nm、最大输出功率500 mV二极管激光器作激发光源测定样品的上转换发光性能,结果显示:在TiO2∶Ho3+/Yb3+样品的上转换荧光光谱中观察到红光和绿光,且绿光的强度比红光的强,绿光的强度约是红光强度的两倍,当Yb3+离子浓度为2.5 mol%时,位于543 nm处绿光的发射强度达到最大值.并详细研究了上转换发光机制.     

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)。