Hydrothermal Synthesis and Up‐conversion Luminescence of Ho3+/Yb3+ Co‐doped PbTiO3

Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals with different content of dopant were successfully prepared via a facile hydrothermal method. The purity, morphology, element distribution, chemical state and up‐conversion (UC) photoluminescence (PL) of PbTiO₃ nanocrystals affected by Ho³⁺ dopant are investigated systematically. X‐ray diffraction (XRD) results illustrate that PbTiO₃ samples with the doping Ho³⁺ concentration ranging from 0 to 5 mol‐% are perovskite structure. The doping Ho³⁺ ions have no change on the crystal structure of perovskite PbTiO₃. Owing to the non‐equivalent substitution of Ho³⁺ to Ti⁴⁺ in PbTiO₃, the particle size of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples is decreased as well as the particle agglomeration is detected. Moreover, Ho and Yb ions have uniform distributions in the PbTiO₃ nanoparticles as the presence of Ho³⁺ and Yb³⁺ cations. The up‐conversion spectra demonstrate that Ho/Yb co‐doped PbTiO₃ samples have up‐conversion emissions centered at 550 nm, 660 nm and 755 nm, corresponding to the transitions of ⁵F₄(⁵S₂)→⁵I₈, ⁵F₅→⁵I₈ and ⁵S₂(⁵F₄)→⁵I₇ of Ho³⁺ ions. Additionally, the effect of temperature on the UC PL property of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ system is further investigated. The sensitivity and the trend of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples in temperature from 298 k to 493K are calculated on the basis of fluorescence intensity ratio (FIR) method. Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals are verified the high potential in the optical temperature sensing.     

Fluorescence and lifetime studies of Ho3+: CaF2

Four fluorescence groups in the wavelength region 4775–7590 Å have been recorded in Ho³⁺: CaF₂ single crystals in the concentration range from 0.01 to 0.1 wt% of Ho³⁺. The observed variation of the relative intensities of various fluorescence lines with concentration indicate the presence of more than one Ho³⁺ site in CaF₂. This has been further substantiated by the observation of three different fluorescent decay times. The increase in the fluorescent decay times with concentration above 373 K is explained as probably being due to reabsorption of fluorescence radiation.     

Photoacoustic spectroscopic studies of Ho3+, Er3+, and Sm3+ doped polyvinyl alcohol films

Photoacoustic (PA) spectra of Ho³⁺, Er³⁺, and Sm³⁺ doped PVA films were obtained in 350–800 nm range. PA spectra were also obtained for the respective dopant oxides: Ho₂O₃, Er₂O₃, and Sm₂O₃ for comparison. It was found that in PVA the PA sensitivity has increased considerably compared to pure rare earth oxides. The relative intensities of absorption bands at 540 and 637 nm of Ho₃₊: PVA have shown distinct enhancement, indicating the increase in nonradiative relaxation at these excitations. Furthermore, the PA signals at wavelengths for different PA absorption bands were monitored as a function of chopping frequency. These experiments have shown that PA signal varies w⁻¹ both for oxides and PVA samples, suggesting that they behave as thermally thin samples. © 1997 John Wiley & Sons, Inc.     

Energy Levels and Band Intensities of Neodymium Organic Acid Complexes

Abstract

The optical absorption spectra of Nd³⁺ ion in glutamic, L-aspartic and maleic acids in UV-VIS and NIR regions have been recorded. From the spectral data thus obtained, various spectroscopic parameters are evaluated. The radiative lifetimes (γ_R) and branching ratios (β) for the excited states of Nd³⁺ ion in three acids have been computed. The relationship between the environment sensitive Judd-Ofelt parameter (ω₂) and the intensities of hypersensitive level (⁴G_5/2) is in good coincidence following the Judd-Ofelt theory.     

Visible Up-Conversion Luminescence in Ho3+: BaTiO3 Nano-Crystals Prepared by Sol Gel Technique

Visible up-conversion emissions at (435, 545, 580, 675 and 690 nm) and (437, 547 575 and 675 nm) have been observed from the sol-gel derived nano-crystalline Ho³⁺: BaTiO₃ powders and thin films respectively, under 808 nm laser diode excitation emissions. Combined with the energy level structure of Ho³⁺ ions and the kinetics of the visible emissions, the up-conversion mechanism has been analyzed and explained. The blue, green and red emissions of both samples has been attributed to the ground state-directed transition from (⁵F₁), (⁵S₂) and (⁵F₅), which are populated through excited state absorption (ESA) for 808 nm excitation. Nano-structure pure barium titanate and doped with different concentrations of Ho³⁺ ions in the from of powder and thin film have been prepared by sol-gel technique, using barium acetate (Ba(Ac)₂), and titanium butoxide (Ti(C₄H₉O)₄), as precursors. The thin films were prepared by sol-gel spin coating method. The as-grown thin films and powders were found to be amorphous, which crystallized to the tetragonal phase after heating at 750°C in air for 30 minutes. The crystallite sizes of the thin film and powder both doped with 4% Ho³⁺ ions was found to be equal to 11 and 16 nm, respectvely.     

Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1-xCaGd0.5Ho0.05Yb0.45(MoO4)3 Scheelite-Type Upconversion Phosphors

A set of new triple molybdates, Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45, was successfully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45 with variation of the Li_xNa_1-x (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho³⁺ = 0.05 and Yb³⁺ = 0.45 were obtained, and the effect of Li⁺ on their spectroscopic features was investigated. The crystal structures of Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I4₁/a by Rietveld analysis. Pure NaCaGd_0.5Ho_0.05Yb_0.45(MoO₄)₃ has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308.24 (3) ų, Z = 4. In Li-doped samples, big cation sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing Li doping level. The evaluated upconversion (UC) behavior and Raman spectroscopic results of the phosphors are discussed in detail. Under excitation at 980 nm, the phosphors provide yellow color emission based on the ⁵S₂/⁵F₄ → ⁵I₈ green emission and the ⁵F₅ → ⁵I₈ red emission. The incorporated Li⁺ ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho³⁺ and Yb³⁺ ions, and they further affected the UC transition probabilities in triple molybdates Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45. The complex UC intensity dependence on the Li content is explained by the specificity of unit cell distortion in a disordered large ion system within the scheelite crystal structure. The Raman spectra of Li_xNa_1-xCaGd_0.5(MoO₄)₃ doped with Ho³⁺ and Yb³⁺ ions were totally superimposed with the luminescence signal of Ho³⁺ ions in the range of Mo–O stretching vibrations, and increasing the Li⁺ content resulted in a change in the Ho³⁺ multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO₄)₃:Ho³⁺,Yb³⁺ phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.     

Intense Upconversion Luminescence of CaSc2O4:Ho3+/Yb3+ from Large Absorption Cross Section and Energy‐Transfer Rate of Yb3+

Concentration‐optimized CaSc₂O₄:0.2 % Ho³⁺/10 % Yb³⁺ shows stronger upconversion luminescence (UCL) than a typical concentration‐optimized upconverting phosphor Y₂O₃:0.2 % Ho³⁺/10 % Yb³⁺ upon excitation with a 980 nm laser diode pump. The ⁵F₄+⁵S₂→⁵I₈ green UCL around 545 nm and ⁵F₅→⁵I₈ red UCL around 660 nm of Ho³⁺ are enhanced by factors of 2.6 and 1.6, respectively. On analyzing the emission spectra and decay curves of Yb³⁺: ²F_5/2→²F_7/2 and Ho³⁺: ⁵I₆→⁵I₈, respectively, in the two hosts, we reveal that Yb³⁺ in CaSc₂O₄ exhibits a larger absorption cross section at 980 nm and subsequent larger Yb³⁺: ²F_5/2→Ho³⁺: ⁵I₆ energy‐transfer coefficient (8.55×10^?17 cm³ s^?1) compared to that (4.63×10^?17 cm³ s^?1) in Y₂O₃, indicating that CaSc₂O₄:Ho³⁺/Yb³⁺ is an excellent oxide upconverting material for achieving intense UCL.     

Strukturchemische und magnetische Untersuchungen an Ho3+‐β″‐Al2O3(Ho0, 5Mg0, 5Al10, 5O17)

Structural Chemistry and Magnetic Properties of Ho³⁺‐β″‐Al₂O₃(Ho_{0, 5}Mg_{0, 5}Al_{10, 5}O₁₇) The crystal structure of Ho³⁺‐β″‐Al₂O₃(Ho_{0, 5}Mg_{0, 5}Al_{10, 5}O₁₇) was determined by single crystal X‐ray diffraction methods at room temperature (trigonal, R3¯m, Z = 3, a = 561.43(12) pm, c = 3353.7(11) pm). The structural chemical results are correlated with magnetic measurements, where ligand field calculations applying the angular overlap model have been taken into account.     

Spectroscopic Properties and Upconversion Studies in Ho3+/Yb3+ Co‐doped Calcium Scandate with Spectrally Pure Green emission

The optical properties of a Ho³⁺/Yb³⁺ co‐doped CaSc₂O₄ oxide material are investigated in detail. The spectral properties are described as a function of doping concentrations. The efficient Yb³⁺→Ho³⁺ energy transfer is observed. The transfer efficiency approaches 50 % before concentration quenching. The concentration‐optimized sample exhibits a strong green emission accompanied with a weak red emission, showing perfect green monochromaticity. The results of the spectral distribution, power dependence, and lifetime measurements are presented. The green, red, and near‐infrared (NIR) emissions around 545, 660, and 759 nm are assigned to the ⁵F₄+⁵S₂→⁵I₈, ⁵F₅→⁵I₈, and ⁵F₄+⁵S₂→⁵I₇ transitions of Ho³⁺, respectively. The detailed study reveals the upconversion luminescence mechanism involved in a novel Ho³⁺/Yb³⁺ co‐doped CaSc₂O₄ oxide material.     

Absorption and fluorescence of Ho3+ IN La2S3·3Ga2S3

Absorption bands of Ho³⁺ in vitreous La₂S₃·3Ga₂S₃ in the range 500 to 2000 nm were assigned. Excitation spectra reveal additional levels ⁵G₆ and ⁵F₃ obscured by the intrinsic absorption of the glass. The Ho³⁺ emission in chalcogenide glasses is more intense than in oxide glasses due to smaller non radiative relaxation as predicted by the theory of multiphonon relaxation.     

Ho3+/Yb3+共掺ZnO-Al2O3-GeO2-SiO2玻璃陶瓷的制备和上转换发光性质

综合ZnO-Al₂O₃-SiO₂系和锗酸盐玻璃陶瓷的优点,采用熔融-晶化法首次制备了Ho³⁺/Yb³⁺共掺以ZnAl₂O₄为主晶相的ZnO-Al₂O₃-GeO₂-SiO₂系玻璃陶瓷。因[GeO₄]四面体和[SiO₄]四面体都是玻璃网络形成体,讨论了GeO₂取代SiO₂对玻璃陶瓷样品硬度及发光性能的影响,最终确定GeO₂的取代量为10.55%（w/w）时,玻璃陶瓷综合性能最佳。在980 nm泵浦光的激发下,发现强的绿色（546 nm）和弱的红色（650 nm）上转换发光,并研究了不同Ho³⁺/Yb³⁺掺杂比对样品上转换发光的影响,最终结果表明当Ho³⁺/Yb³⁺掺杂比为1：11（n/n）时样品荧光强度最强,在绿色上转换发光材料方面具有潜在的应用。     

Optical properties of Ho3+: Alkali mixed fluoride glasses

Optical absorption, fluorescence and laser characteristics of Ho³⁺ incorporated (1 % molar) glass systems of 48ZrF₄-23BaF₂-8A1F₃-20RF (RF = LiF-NaF or NaF-KF pair) were examined. Physical properties such as density, refractive index, mean atomic volume, molar refractivity and dielectric constant were measured. The measured absorption intensities were matched with the best fit Judd-Ofelt intensity parameters. The effects of the holmium glass chemical compositions were examined for properties such as spontaneous emission probabilities, branching ratios and radiative lifetimes of lasing transitions by the use of the Judd-Ofelt model. Studies were also carried out to understand the mixed alkali fluoride effects on the measured values of induced emission cross sections for the observed luminescent transitions ⁵I_7.6, ⁵F₅ and ⁵S₂ → ⁵I₈ of holmium glasses.     

Influence of Rare Earth Ho3+ Doping on Structural,Microstructure and Magnetic Properties of ZnO Bulk and Thin Film Systems

We have investigated the doping behavior of rare earth element holmium (Ho³⁺) in ZnO semiconductor. The structural, microstructure, and magnetic properties of Zn_1-xHo_xO (x=0.0, 0.04, and 0.05) thin films deposited on Si(100) substrate by thermal evaporation technique were studied. The ceramic targets were prepared by conventional solid state ceramic technique. The pallets used as target were final sintered at 900 oC in the presence of N2 atmosphere. The experimental results of X-ray diffraction (XRD) spectra, surface morphology, and magnetic properties show that the Ho³⁺ doped ZnO thin films has a strong influence on the materials properties. The higher angle shift in peak position and most preferred (101) orientation were observed in XRD pattern. These spectra confirmed the substitution of Ho3+ in ZnO lattice. The surface morphology and stoichiometry for both bulk and thin films were analyzed by scanning electron microscopy and energy dispersive spectroscopy. It was observed that grain size decreases with the increase of Ho³⁺. Room temperature ferromagnetism was observed for Zn_0.95Ho_0.05O films. The ferromagnetism might be attributed to the substitution of Ho ions for Zn²⁺ in ZnO lattices.     

Intensity parameters,radiative transitions and non-radiative relaxations of Ho3+ in various tellurite glasses

The intensity parameters of holmium in sodium, barium and zinc tellurite glasses were obtained from the absorption spectra. Using these parameters and the matrix elements of Ho³⁺, transition probabilities and branching ratios from the excited states (⁵F₄, ⁵S₂), ⁵F₅, ⁵I₄, ⁵I₅ and ⁵I₆ were calculated. Quantum efficiencies for the (⁵F₄, ⁵S₂) state were obtained and the multiphonon transition rates calculated at room temperature. Non-radiative multiphonon relaxations were obtained from the (⁵F₄, ⁵S₂) level. The relaxation increased in the order of zinc, barium, sodium, and is assumed to be dependent on the local site symmetry of Ho³⁺ in glass.     

Upconversion Luminescence from Ho3+ and Yb3+ Codoped α-NaYF4 Single Crystals

The Ho³⁺/Yb³⁺ co-doped α-NaYF₄ single crystal was grown successfully for the first time by a modified Bridgman method in which KF was used as assisting flux and a large temperature gradient (70-90 oC/cm) of solid-liquid interface was adopted. Upconversion emissions at green ～544 nm, red ～657 and ～751 nm were obtained under 980 nm laser diode excitation. The intensity at ～544 nm was much stronger than those of ～657 and ～751 nm. The mechanisms of the upconversion emissions were investigated by studying the relationship between the upconversion intensity and pump power. The optimized Yb³⁺ concentration was about 8.08 mol% when Ho³⁺ concentration was hold at about 1.0 mol%. The results showed that Ho³⁺/Yb³⁺+ doped α-NaYF₄ single crystal was a possible candidate upconversion material for the green solid-state laser.     

Preparations,Structures and Luminescence Properties of Penta‐rare‐earth Incorporated Tetravacant Dawson Selenotungstates and Their Ho3+/Tm3+ Co‐doped Derivatives

A family of penta‐rare‐earth incorporated tetravacant Dawson selenotungstates [H₂N(CH₃)₂]₁₀H₃[SeO₄RE₅(H₂O)₇(Se₂W₁₄O₅₂)₂] ? 40H₂O [RE=Ho³⁺ ( 1 ), Er³⁺ ( 2 ), Tm³⁺ ( 3 ), Tb³⁺ ( 4 )] were synthesized. It should be noted that a penta‐RE [SeO₄RE₅(H₂O)₇]¹¹⁺ central core connecting two tetra‐vacant Dawson‐type [Se₂W₁₄O₅₂]^12? subunits generates a dimeric assembly of [SeO₄RE₅ (H₂O)₇(Se₂W₁₄O₅₂)₂]^13? in the structures of 1 – 4 . Meanwhile, a class of Ho³⁺/Tm³⁺ co‐doped derivatives based on 1 with a Ho³⁺/Tm³⁺ molar ratio of 0.75:0.25–0.25:0.75 were also prepared and characterized by energy‐dispersive spectroscopy (EDS) analyses. Moreover, their luminescence properties were systematically investigated, which indicate that Tm³⁺ ions can sensitize the emission of Ho³⁺ ions in the visible region and prolong the fluorescence lifetime of Ho³⁺ ions to some extent. Energy transfer from Tm³⁺ ions to Ho³⁺ ions was probed by time‐resolved emission spectroscopy (TRES), and the CIE 1931 diagram has been applied to evaluate all possible luminescence colors.     

LiYF4晶体中Ho3+离子真空紫外吸收光谱的理论解析

运用配位场理论方法获得了Ho3+离子低自旋4f95d组态的谱项和J-光谱多重态,根据电偶极跃迁选律合理地解析了LiYF4晶体中Ho3+离子在真空紫外区(120～160 nm)的吸收光谱,4个主要的吸收峰分别被归属为从基态(5I8)到Ho3+离子的低自旋4f95d组态的自旋允许跃迁。     

Investigation on the upconversion luminescence of Sr3AlO4F:Yb3+, Er3+, Ho3+ phosphors

To develop new emission-tunable upconversion (UC) phosphors, the Sr₃AlO₄F:5%Yb³⁺, xEr³⁺, yHo³⁺ (0 ≤ x ≤ 1%, 0 ≤ y ≤ 1%) samples were prepared by conversional solid-state reaction method, and their luminescence properties upon 980 nm excitation were studied. Upon 980 nm excitation, Yb³⁺-Er³⁺ codoped Sr₃AlO₄F shows a predominant emission peak between 645 and 700 nm which is attributed to the ⁴F_9/2-⁴I_15/2 transition of Er³⁺, and the Er³⁺ green emissions have been almost quenched. In this case, the yellowish green emitting light is obtained. The possible reason was interpreted by the energy level diagram and the proposed UC mechanism. For Yb³⁺-Ho³⁺ codoped Sr₃AlO₄F, three emissions are observed obviously which are all derived from the Ho³⁺ ion. The corresponding chromaticity coordinates indicate a red emission has been gained. To realize the tunable emission, the typical Sr₃AlO₄F:5%Yb³⁺, 0.2%Er³⁺, 1%Ho³⁺ phosphor was developed, and its emission spectrum includes the emission peaks of both Er³⁺ and Ho³⁺. Correspondingly, the sample gives a yellow emission.     

Free ion and crystal field splitting of Tm3+ in single crystals of thulium sulfate octahydrate

We report a detailed analysis of the free ion centers of gravity and the crystalline electric field (CEF) splitting of the 4£¹²[SL]-J levels of Tm³⁺ in single crystals of Tm₂(SO₄)₃·8H₂O. A new interpretation of the ³P₀ and ¹l₆ centers of gravity in the sulfate lattice is given. A free ion calculation has been completed and the resulting intermediately coupled wavefunctions have been used to calculate the CEF splitting. Forty-three observed crystalline Stark level differences were used in the CEF analysis and could be accounted for to better than 5%. The CEF parameters are found to be very similar to those previously reported for Ho³⁺ in single crystals of Ho₂(SO₄)₃·8H₂O. The CEF splitting calculation predicts the presence of a pair of nearly twofold degenerate ground levels and the respective spectroscopic splitting factors in agreement with experiment.     

Quantifying Raman and emission gain coefficients of Ho3+ doped TeO2·ZnO·PbO·PbF2·Na2O (TZPPN) tellurite glass

Broadband amplifier emission near the second telecommunication window in Ho³⁺ doped 76TeO₂·10ZnO·9.0PbO·1.0PbF₂·3.0Na₂O was studied. The optical transition properties and radiative lifetimes of several excited states of Ho³⁺ have been predicted using intensity Judd–Ofelt parameters. The emission cross section for Ho³⁺ in this glass, around 2 μm, was calculated according to McCumber theory. The maximum stimulated emission cross section was calculated to be 0.9 × 10⁻²⁰ cm² for 2046 nm emissions. The theoretical gain cross sections was evaluated and positive gain bands was anticipated. Furthermore, the peak Raman gain coefficient in the present glass was around 250 times larger than that of SiO₂.