Partition, luminescence and energy transfer of Er/Yb ions in oxyfluoride glass ceramic containing CaF2 nano-crystals

A complete spectroscopic investigation of energy transfer processes in oxyfluoride glass ceramics containing CaF₂ nano-crystals doped with various amounts of Er³⁺ and Yb³⁺ was reported. An enhancement of the 1.53 μm emission and infrared to visible up-conversion fluorescence was confirmed experimentally due to efficient non-radiative energy transfer from Yb³⁺ to Er³⁺ ions concentrated in CaF₂ nano-crystals. The efficiency of Yb³⁺ to Er³⁺ energy transfer in excess of 85% was obtained for 0.5 mol% Er³⁺/2.0 mol% Yb³⁺ co-doped glass ceramic. Using rate equation formulism, the coefficient of Yb³⁺ to Er³⁺ energy transfer was determined to be about 3.5 times higher than that of Er³⁺ to Yb³⁺ energy back transfer, which is sufficient to provide high ⁴I_11/2 population of Er³⁺ to improve the fluorescence of the co-doped glass ceramics.     

Upconversion emission of a novel glass ceramic containing Er: BaYF5 nano-crystals

In order to develop highly efficient upconversion (UC) material, Er³⁺-doped oxyfluoride glass ceramics containing BaYF₅ nano-crystals were fabricated for the first time. The randomly distributed BaYF₅ nano-particles with the size of nearly 25 nm precipitated from the glass matrix at about 700 °C, then tended to aggregate slightly with the increase of heating temperature, resulting in the enhancement of Er³⁺ UC emission in the wavelength ranging from 500 nm to 700 nm. The UC process was found very sensitive to Er³⁺ content: with the increase of Er³⁺ content from 0.5% to 1.0%, the UC intensity behaved with drastic exaltation; while further increasing Er³⁺ content to 2.0% resulted in a notable fluorescence quenching. Significantly, the UC efficiency of present glass ceramic preponderates notably over that of the previously reported glass ceramic containing LaF₃, implying its superior UC potentiality.     

Electrodeposition and characterization of CaF2 and rare earth doped CaF2 films

Calcium fluoride (CaF₂) and the rare earth doped CaF₂ films have been electrodeposited on indium tin oxide (ITO) electrode through electrochemical generation of acid in aqueous solutions near room temperature. For CaF₂ film electrodeposition, the local pH at ITO surface is lowered by the electrochemical oxidation of ascorbate anion, and then the Ca-ethylene diamine tetraacetic acid disodium complexes which are close to the ITO electrode are decomposed to release free Ca²⁺ to react with F⁻to form CaF₂ deposit onto ITO. In the same way, RE-doped CaF₂ films have been electrodeposited onto ITO electrode. The morphology of films studied by scanning electron microscopy revealed that they are agglomerated and dense. The films showed a little [111] preferred orientation by X-ray diffraction. It was also proved that doped RE ions were distributed homogeneously in the film by energy dispersive spectroscopy mapping. The optical properties of the electrodeposited CaF₂, CaF₂:Eu and CaF₂:Tb films were studied by photoluminescence, and all films exhibit intense emission peaks.     

Yb:CaF2 grown by liquid phase epitaxy

Ytterbium doped CaF₂ crystalline layers have been grown for the first time from high temperature solutions at controlled atmosphere by using the liquid phase epitaxy technique. Doped layers having thicknesses between a few microns to a hundred of microns have been grown onto non-oriented and (1 1 1) oriented CaF₂ substrates. The Yb³⁺:CaF₂ layers show structural properties very close to the undoped substrate without any further thermal treatment. Registration of room temperature emission spectra and fluorescence lifetime measurements performed with epitaxial layers corresponding to different ytterbium concentrations show similar spectroscopic properties as in the bulk crystals.     

Optical properties of transparent Dy doped Ba2TiSi2O8 glass ceramic

The Ba₂TiSi₂O₈ is a well known piezoelectric, ferroelectric and non-linear crystal. Nanocrystals of Ba₂TiSi₂O₈ doped with 1.5 Dy³⁺ have been obtained by thermal treatment of a precursor glass and their optical properties have been studied. X-ray diffraction patterns and optical measurements have been carried out on the precursor glass and glass ceramic samples. The emission spectra corresponding to the Dy³⁺: ⁴F_9/2 → ⁶H_13/2 (575 nm), ⁴F_9/2 → ⁶H_11/2 (670 nm) and ⁴F_9/2 → ⁶H_9/2 (757 nm) transitions have been obtained under laser excitation at 473 nm. These measurements confirm the incorporation of the Dy³⁺ ions into the Ba₂TiSi₂O₈ nanocrystals which produces an enhancement of luminescence at 575 nm. At this wavelength has been demonstrated a maximum optical amplification around 1.9 cm⁻¹ (∼8.2 dB/cm).     

Wetting induced by near-surface Ti-enrichment in the CaF2/In–Ti and CaF2/Cu–Ti systems

This paper is concerned with the wetting of CaF₂ by liquid Cu and In and with the effect of Ti additions to the melt. According to thermodynamic analysis and to the experimental observations, the significantly decreased contact angle following the addition of Ti to the molten metals is not due to the formation of interfacial fluoride phases, in contrast to previously reported results. Ab initio density functional calculations indicate that preferential Ti adsorption takes place at the near CaF₂ surface. It is suggested that the presence of a Ti-enriched liquid, adjacent to the substrate, gives rise, by means of heterogeneous nucleation, to the formation of a thin intermetallic compound layer that stands behind the experimentally observed enhanced wetting. The suggested wetting mechanism is supported by the notable correlation that has been observed between the temperature dependence of the contact angle and the temperature domains, associated with the presence of intermetallic compounds in both Me–Ti (Me = Cu, In) binary systems.     

Structural, thermal and optical characterization of TiO2:ZrO2 thin films prepared by sol-gel method

We have studied the structural and optical properties of thin films of TiO₂, doped with 5% ZrO₂ and deposited on glass substrate (by the sol-gel method). The dip-coated thin films have been examined at different annealing temperatures (350 to 450 °C) and for various layer thicknesses (63-286 nm). Refractive index and porosity were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.62-2.29 and the porosity is in the range of 0.21-0.70. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZrO₂, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 14.8 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range (8.58-20.56 nm).     

Epitaxial growth of Fe3Si/CaF2/Fe3Si magnetic tunnel junction structures on CaF2/Si(111) by molecular beam epitaxy

The Fe₃Si(24 nm)/CaF₂(2 nm)/Fe₃Si(12 nm) magnetic tunnel junction (MTJ) structures were grown epitaxially on CaF₂/Si(111) by molecular beam epitaxy (MBE). The 12-nm-thick Fe₃Si underlayer was grown epitaxially on CaF_2/Si(111) at approximately 400 °C; however, the surface of the Fe₃Si film was very rough, and thus a lot of pinholes are considered to exist in the 2-nm-thick CaF₂ barrier layer. The average roughness (Ra) of the CaF₂ barrier layer was 7.8 nm. This problem was overcome by low-temperature deposition of Fe and Si at 80 °C on CaF₂/Si(111), followed by annealing at 250 °C for 30 min to form the Fe₃Si layer. The Ra roughness was significantly reduced down to approximately 0.26 nm. A hysteresis loop with coercive field H_c of approximately 25 Oe was obtained in the magnetic field dependence of Kerr rotation at room temperature (RT).     

Electrochromic properties of TiO2 anatase thin films prepared by a dipping sol-gel method

Electrochromic TiO₂ anatase thin films on ITO were prepared by the sol-gel dipping method using a solution of titanium tetraisopropoxide, diethanolamine and ethanol. The films were transparent in the visible range and can be colored in a solution of LiClO₄ in propylene carbonate. The transmittances of the colored films were found to be strongly dependent on the Li⁺ inserted charge. Combining the experimental data obtained from in situ Raman and in situ transmittance spectra with the data from chronoamperometic measurements, it was demonstrated that the fully colorated state of the TiO₂ anatase films is Li_0.5TiO₂ with a crystalline structure of Imma space group symmetry. In the Raman spectra this coloration state exhibits five characteristic bands at 176, 224, 316, 531 and 629 cm⁻¹.     

Crack formation in TiO2 films prepared by sol-gel processing: Quantification and characterization

TiO₂ thin films were prepared by dip-coating from soluble precursor powders using different solvent mixtures. Under certain conditions cracking occurs during drying and thermal treatment. Both extent of damage and time of formation were evaluated from micrographs using image processing software. Different approaches for data evaluation are applied and compared. It is shown that reproducible measurements of defect densities are a useful tool for the optimization of process parameters.Data indicate that the presence of 1,5-pentanediol retards the formation of a rigid xerogel network that is responsible for crack formation during film drying. The air moisture during dip-coating also turns out to have a systematic effect on the microstructure of the densifying film as demonstrated by quantitative defect analysis. This method has to be used carefully, though, that a qualitative change of the damage pattern does not lead to a misinterpretation of the data.     

Study and fabrication of ZnNb2O6 thin films by sol-gel method

Zinc niobium oxide (ZnNb₂O₆) thin films were grown on ITO/glass substrate by sol-gel process. Microstructure and surface morphology of the ZnNb₂O₆ thin films have been studied by X-ray diffraction and scanning electron microscopy. Optical properties of the ZnNb₂O₆ thin films were obtained by UV-visible recording spectrophotometer. The dependence of the microstructure, optical transmittance spectra, optical band gap on annealing temperature was also investigated.     

Effect of Yb concentration on optical properties of Yb:CaF2 transparent ceramics

In Yb:CaF₂, the coordination of Yb³⁺ in the CaF₂ lattice determines the spectroscopic properties that make Yb:CaF₂ a good candidate for high power laser applications. In this work, we measure the optical absorption, emission, and fluorescence lifetime of 0.1, 1, 5, and 10 at% Yb:CaF₂ ceramics to determine whether Yb³⁺ substitutes as hexamer clusters giving rise to the tenability and long fluorescent lifetime observed in Yb:CaF₂ single crystals. Absorption and emission spectra show that the concentration of Yb³⁺ present in hexamer clusters, as opposed to isolated ions, increases with increasing Yb³⁺ content. Fluorescence lifetime also increases with increasing Yb³⁺ content. Laser testing on a 1 at% Yb:CaF₂ transparent ceramic demonstrates that these materials are viable laser gain media.     

Evaluation of coated and uncoated CaF2 optics by variable angle spectroscopic ellipsometry

CaF₂ is one of the dominating optical materials used for ArF excimer laser optics. Surface quality of optically finished CaF₂ plays an important role in the components' lifetime. A variable angle spectroscopic ellipsometry was employed to evaluate surface quality of optically finished CaF₂ in terms of top surface and subsurface damage. The subsurface damage was revealed by removing the top surface. Combining plasma ion assisted deposition and ellipsometric measurement, a dense smooth F-SiO₂ film was developed to prevent fluorine loss of CaF₂ optics under ArF excimer laser irradiation, leading to an extended lifetime. In addition, an integrated protective coating approach was established on fluoride multilayer coated CaF₂ optics, resulting in environmentally stable optical performance.     

Preparation of CuAlO2 and CuCrO2 thin films by sol-gel processing

CuAlO₂ and CuCrO₂ thin films were prepared by sol-gel processing and subsequent thermal treatment in air and inert gas atmosphere. Resistivities of 700 Ω cm and 60 Ω cm with optical transmissions of 65% and 32% were achieved respectively. The crystallization temperature of 700 °C allows the preparation of CuCrO₂ on borosilicate glass. P-type conductivity was verified by Seebeck measurements and a transparent heterostructure including p-CuCrO₂ showed rectifying behavior.     

CaF2对CAMS系矿渣微晶玻璃析晶特性及抗腐蚀性能的影响

以白云鄂博二次选后尾矿、高炉渣和粉煤灰为主要原料,采用熔融法制得不同CaF_2含量的CaOAl_2O_3-MgO-SiO_2(CAMS)系微晶玻璃,利用DSC、XRD、SEM、EDS等测试手段,研究了CaF_2对微晶玻璃析晶行为、显微结构及抗盐酸腐蚀性能的影响。结果表明:氟化钙添加量由2%增加至8%(质量分数),基础玻璃的析晶温度降低,析晶活化能由416.28kJ/mol降为309.02kJ/mol,但微晶玻璃的主晶相仍为透辉石相。氟化钙的添加在一定程度上降低了微晶玻璃的抗盐酸腐蚀性能,当氟化钙添加量为2%时,微晶玻璃经盐酸溶液浸泡21d,腐蚀层厚度为240μm;随着CaF_2添加量继续增大,微晶玻璃抗盐酸腐蚀性能急剧下降,当氟化钙添加量为8%时,盐酸腐蚀21d后,腐蚀层厚度为743μm。     

Spectroscopic properties of Er in sol-gel derived ZrO2 films

Films of (x/2)Er₂O₃-100ZrO₂ (x=0, 0.1, 1 and 5) were prepared by the sol-gel and dip-coating methods, and the influences of the type of ZrO₂ phase on Er³⁺ spectroscopic properties were investigated. X-ray diffraction patterns show the existence of metastable tetragonal phase at low temperatures, which are probably caused by both the excess surface energy due to the small crystallite-size effect (J. Phys. Chem. 4 (1965) 1238) and the stabilization of ZrO₂ with the addition of Er₂O₃ (J. Phys. Chem. B 106 (2002) 1909; J. Ceram. Soc. Jpn. 107 (1999) 1111). Under excitation at 488 nm, the green (525+545 nm) downconversion fluorescence due to the (²H_11/2+⁴S_3/2)→⁴I_15/2 transition and the red (670 nm) downconversion fluorescence due to the ⁴F_9/2→⁴I_15/2 transition were observed, and their intensities increased with increasing the heat-treatment temperature regardless of the type of the ZrO₂ phase. On the other hand, the intensity of red upconversion fluorescence also increased monotonically with the heat-treatment temperature under excitation at 780 nm, while the green upconversion fluorescence was hardly observed in the tetragonal phase (for the film with x=1, below 800 °C; x=5, below 700 °C), but was observed in the cubic (x=1, at 800 °C or above; x=5, at 700 °C or above) and monoclinic (x=0.1, at 600 °C or above) phases under excitations at 647 and 780 nm. The reason is probably that the f-f transition probability concerning the ⁴I_13/2, from which the excited state absorption occurs, varied sensitively with the local structure of Er³⁺ ions in the different phase of ZrO₂.     

Investigations on B-doped SiO2 thermal protective coatings by hybrid sol-gel method

This study first prepared B-doped SiO₂ coating by hybrid sol-gel method. SiO₂ coatings had been successfully used for thermal protection, but its toughness, oxidation resistance and cost are weak points. Hybrid sol-gel method could prepare ultra thin and low cost SiO₂ coatings on nickel alloys. And doping B could improve the coatings’ toughness and oxidation resistance. Thermogravimetry, differential thermal analysis and Fourier transform infrared spectroscopy were used to investigate the additives’ effect on coatings’ formations at different temperatures. X-ray photoelectron spectroscopy and X-ray diffraction were used to investigate the coatings’ element distribution and phase. The results showed that B-doped SiO₂ thermal protective coatings could withstand 1050 °C, and Si-O-B bond was generated to improve the coatings’ toughness. A crystal mullite phase formed on the coatings’ surface at high temperature by thermal diffusion of Al, which significantly improved B-doped SiO₂ thermal protective coatings’ oxidation resistance.     

Ab initio modeling of Al adsorption on CaF2 surfaces

Ab initio simulations of the adsorption of Al atoms on CaF₂ (0 0 1) and (1 1 1) surfaces have been performed for supercells with 7 different atomic configurations, using density functional theory. For (1 1 1) surfaces, a repulsive interaction was observed for most configurations, while a weak attraction was obtained when the Al atom was placed above F atoms. For the Ca-terminated (0 0 1) surface, the adsorption energy was about 5 times larger, whereas for the F-terminated (0 0 1) surface it was about 20 times greater. The comparative analysis indicates that the (0 0 1) surfaces are reactive and have a strong Al adatom bonding (chemisorption), especially for the F-terminated substrate. On the contrary, the (1 1 1) plane may be considered as non-reactive (physisorption), having a weak bonding of the Al adatom above the F site.     

Synthesis of CaCu3Ti4O12 ceramic via a sol-gel method

The novel nano-ultrafine powders for the preparation of CaCu₃Ti₄O₁₂ ceramic were prepared by the sol-gel method and citrate auto-ignition method. The obtained precursor powders were pressed, sintered at 1000 °C to fabricate microcrystal CaCu₃Ti₄O₁₂ ceramic. The microcrystalline phase of CaCu₃Ti₄O₁₂ was confirmed by X-ray powder diffraction (XRD). The morphology and size of the grains of the powders and ceramics under different heat treatments were observed using scanning electron microscopy (SEM). The relative dielectric constant of the ceramic sintered at 1000 °C was measured with a magnitude of more than 10⁴ at room temperature, which was approaching to those of Pb-containing complex perovskite ceramics, and the loss tangent was less than 0.20 in a broad frequency region. The relative dielectric constant and loss tangent were also compared with that of CaCu₃Ti₄O₁₂ ceramic prepared by other reported methods.