Energy transfer in CaSiO3 phosphors doped with Ce3+ and Tb3+

Calcium metasilicate phosphors activated by Ce³⁺ and Tb³⁺ have been studied for their emission characteristics. In two series of phosphors, one activator was kept at its optimum value while the other was varied. In another two series, one activator was kept below its optimum value and the other was varied. Concentration quenching effects start when each activator gives its maximum emission. There is clear evidence of an energy transfer from Ce³⁺ to Tb³⁺ because the⁵ D ₃ lines appear on addition of Ce³⁺ while they were conspicuously absent when Tb³⁺ alone was present. Their absence in singly activated phosphors could not have been due to cross-relaxation. Obviously X-ray excitation does not lead to⁵ D ₃ transitions which are achieved only by energy transfer. Further, considering the features of the emission spectra and the concentrations of activators used, the transfer could only be of the dipole-dipole type.     

Small polaron hopping conduction mechanism in Ni-doped LaFeO3

The electrical transport properties of LaFe_{1? x} Ni _x O₃ (0.1 ≤ x ≤ 0.6) bulk samples were investigated over a wide temperature range, i.e. 9–300 K. Powder x-ray diffraction patterns at room temperature showed that all samples were formed in a single phase. However, a structural transformation was observed from orthorhombic (Pnma) to rhombohedral crystal symmetry at x > 0.5 in Ni-doped samples, which is supported by the electrical transport analysis. Temperature-dependent resistivity data were fitted using Mott's variable-range hopping model for a limited range of temperatures to calculate the hopping distance and the density of states at Fermi level. It was found that all parameters vary systematically with an increase in Ni concentration. Moreover, the resistivity data were also fitted using the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed up to 50% Ni concentration, whereas an adiabatic hopping conduction mechanism is active above it. Such a change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Fe³⁺–O–Fe³⁺ and Fe³⁺–O–Ni³⁺ bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Ni doping.     

Energy transfer in tetracene doped fluorine

Time-resolved spectroscopy measurements on tetracene doped fluorene provide a crucial test for models proposed to explain the observed time dependent energy transfer rates in similar doped organic crystals. The results verify the importance of activator induced host traps.     

Mid-infrared luminescence and energy transfer of Dy3+/Tm3+ doped fluorophosphate glass

This work reports the observation of emissions at 2.9 μm, 1.8 μm and 1.47 μm from Dy³⁺/Tm³⁺ codoped fluorophosphate glass upon excitation of a conventional 800 nm laser diode. Judd–Ofelt intensity parameters and radiative properties of Dy³⁺ ions in present glasses were calculated using the Judd–Ofelt theory. The mechanism and microparameters of energy transfer processes were investigated based on photoluminescence performance and lifetime measurements. The Dy³⁺/Tm³⁺ codoped fluorophosphate glass possessing advantageous spectroscopic characteristics as well as excellent thermal stability is a promising candidate for an efficient 2.9 μm laser.     

Interpretation of charge transfer bands in Fe doped SrTiO3 crystals

The electronic structures of SrTiO₃ crystals doped with Fe³⁺, Fe⁴⁺ and Fe⁵⁺ ions have been investigated using the Xα cluster approach. The ground-state eigenvalues show the lower Fe acceptor level, of t_2g↓ symmetry, localized inside the SrTiO₃ band gap, respectively at 2.8 eV $\text{(}\text{Fe}{}^{\mathrm{3+}}\text{, S =}\text{5}\text{2}\text{), 1.6}\text{eV}\text{(}\text{Fe}{}^{\mathrm{4+}}\text{, S = 1)}\text{or}\text{1.1}\text{eV}\text{(}\text{Fe}{}^{\mathrm{4+}}\text{, S = 0)}\text{and}\text{1.1}\text{eV}\text{(}\text{Fe}{}^{\mathrm{5+}}\text{, S =}\text{3}\text{2}\text{)}$ above the valence band edge. Other acceptor levels, with eg↓ and eg↑ symmetries, appear inside the gap when the Fe nominal ionicity increases.The theoretical Xα excitation energies of O 2p-Fe 3d transitions confirms the experimental interpretations of acceptor charge transfer bands for the optical absorption spectra of SrTiO₃:Fe⁴⁺ and SrTiO₃:Fe⁵⁺ crystals.The large optical excitation energies compared with the thermal transitions are partly due to the O 2p band width.     

Energy transfer in europium doped yttrium vanadate crystals

The absorption, fluorescence and excitation spectra and fluorescence lifetimes were obtained at temperatures from 8 K to room temperature, for single crystals of yttrium vanadate doped with several concentrations of europium. A model is proposed to explain the luminescence properties of YVO₄ crystals based on transitions within the VO^3-₄ ion. The host sensitized energy transfer in YVO₄: Eu³⁺ is explained by amodel based on thermally activated exciton migration. The model predicts the observed temperature and concentration dependences for the fluorescence intensities and lifetimes. The diffusion coefficient, diffusion length, exciton hopping time and trapping rate are determined and compared with similar data obtained on other systems.     

Sono-photo-Fenton oxidation of bisphenol-A over a LaFeO3 perovskite catalyst

In this study, oxidation of bisphenol-A (IUPAC name – 2,2-(4,4-dihydroxyphenyl, BPA), which is an endocrine disrupting phenolic compound used in the polycarbonate plastic and epoxy resin industry, was investigated using sono-photo-Fenton process under visible light irradiation in the presence of an iron containing perovskite catalyst, LaFeO₃. The catalyst prepared by sol–gel method, calcined at 500 °C showed a catalytic activity in BPA oxidation using sono-photo-Fenton process with a degradation degree and a chemical oxygen demand (COD) reduction of 21.8% and 11.2%, respectively. Degradation of BPA was studied by using individual and combined advanced oxidation techniques including sonication, heterogeneous Fenton reaction and photo oxidation over this catalyst to understand the effect of each process on degradation of BPA. It was seen, the role of sonication was very important in hybrid sono-photo-Fenton process due to the pyrolysis and sonoluminescence effects caused by ultrasonic irradiation. The prepared LaFeO₃ perovskite catalyst was a good sonocatalyst rather than a photocatalyst. Sonication was not only the effective process to degrade BPA but also it was the cost effective process in terms of energy consumption. The studies show that the energy consumption is lower in the sono-Fenton process than those in the photo-Fenton and sono-photo- Fenton processes.     

Mössbauer spectroscopic study on nanocrystalline LaFeO3 materials

Perovskite-type nanocrystalline materials LaFeO₃ with different crystal size were synthesized. These samples with different compacted pressure were investigated by Mössbauer spectroscopy at room temperature. The results showed that the LaFeO₃ crystal in size of 70nm possessed an antiferromagnetic structure, that of 12nm was the superposition of the weaker antiferromagnetic structure and the stronger superparamagnetic structure. The rate of interface atoms to crystal atoms and the hyperfine field in nanocrystals varied with the compacted pressure.     

Synthesis and formaldehyde sensing performance of LaFeO3 hollow nanospheres

Metal oxide semiconductors with hollow structure and morphology have attracted considerable attentions because of their promising application on gas sensors. In this paper, LaFeO₃ hollow nanospheres have been prepared by using carbon spheres as templates in combination with calcination. Based on the observation of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and transmission electron microscope (TEM), the structure and morphology of the products were characterized. It has been revealed that as-prepared LaFeO₃ samples have a uniform diameter of around 300 nm and hollow structures with thin shells of about 30 nm consisting of numerous nanocrystals and nanopores. Owing to the hollow and porous structure, large surface area and more surface active sites, the sensor based on LaFeO₃ hollow nanospheres exhibited high response, good selectivity and stability to formaldehyde gas (HCHO). It suggests that the as-prepared LaFeO₃ hollow nanospheres are promising candidates for good performance formaldehyde sensor.     

Cation self-diffusion in LaFeO3 measured by the solid state reaction method

Cation diffusion in LaFeO₃ has been studied using the solid state reaction between sintered bodies of La₂O₃ and Fe₂O₃ at 950–1350 °C in air or O₂–N₂ mixtures. LaFeO₃ was the only product formed. The growth was parabolic and demonstrated to take place predominantly by diffusion of Fe³⁺ through the LaFeO₃ layer. The self-diffusion coefficient of Fe³⁺ was accordingly calculated from the parabolic rate constant, and at constant activity of La₂O₃, a_La2O3 = 1, it shows Arrhenius-type behaviour with activation energy 320 ± 20 kJ/mol. It appeared to be independent of the surrounding pO₂, but this was ascribed to lack of equilibrium with the atmosphere during growth of the LaFeO₃ layer. Correspondingly, the product LaFeO₃ is probably stoichiometric, and differences between our diffusivity and activation energy and those in the literature are discussed in view of this.     

Fabrication of unique ribbon-like porous LaFeO3 nanofibers photocatalyst via electrospinning

Novel LaFeO₃ ribbon-like porous nanofibers have been fabricated by electrospinning utilizing sol–gel precursors, followed by heat treatment at 600 °C for 2 h. The crystal structure, surface morphology, and microstructure were investigated by thermogravimetry–differential thermal analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, respectively. The photocatalytic activity was investigated for the photodegradation of methylene blue aqueous solution. The result shows that the ribbon-like porous nanofibers exhibit excellent photocatalytic activity compared with ultrafine LaFeO₃ nanofibers and LaFeO₃ powders.     

Laser time-resolved spectroscopy: Investigations of energy transfer in Eu3+ and Er3+ doped YVO4

Host-sensitized energy transfer in YVO₄ crystals doped with Eu³⁺ and Er³⁺ ions was investigated using laser excited time-resolved spectroscopy techniques. The results are consistent with a model in which the energy is transferred by a single step process from self- trapped excitons to activators at low temperatures, and by thermally activated exciton hopping at high temperatures. The rate of the latter process is found to be strongly affected by the trapping step with activator-induced host traps playing an important part in the trapping process. Time-resolved spectroscopy techniques are shown to be expecially useful in separately determining the characteristics of migration and trapping processes in exciton energy transfer. The parameters describing these processes are determined for the systems investigated using both diffusion and generalized random walk theories.     

Spectral properties and energy transfer in YAlO3 crystals doped with Ce ions and Mn ions

<正>We report the luminescence properties and decay profiles of Ce:YAlO_3,Mn:YAlO_3,and Ce,Mn:YAlO_3 crystals grown by Czochralski method.The spectroscopic properties show that both Mn~(2+) ions and Mn~(4+) ions exist in Mn:YAlO_3 and Ce,Mn:YAlO_3 crystals.The Mn~(2+) ions have a broad emission band of 60 nm in Mn-doped YAlO_3 crystal at 530 nm.The luminescence spectra also indicate that there is significant energy transfer between Ce~(3+) and Mn~(4+) in Ce,Mn:YAlO_3 crystal.Because of the energy transfer,the first decay component in Ce,Mn:YAlO_3 decreases from 24.5 to 10.8 ns,which is much faster than that of Ce:YAlO_3.     

Electronic deactivation and energy transfer in doped oligophenylenevinylene nanoparticles

Suspensions of oligophenylenevinylene (nPV) nanoparticles withn = 2 vinylene units are doped with nPVs of longer chainlengths,n = 3–5. Absorption and fluorescence spectroscopy and steady-state and time-resolved fluorescence anisotropy measurements are used to determine the photo-physical properties of the suspensions. Undoped nanoparticles form highly oriented H-aggregates with low fluorescence quantum yields (Φ_F ≈ 0.1). Introduction of bulky substituents into the particle constituting molecules perturbs the intermolecular orientation. Upon doping, efficient energy transfer to the dopants is found, changing the color and leading to enhancement of the fluorescence quantum yields up to Φ_F = 0.6. The intermolecular orientation is not changed upon doping.     

掺Tm~(3+)和Tm~(3+)/Ho~(3+)共掺碲钨酸盐玻璃中能量转换过程和机理

本文研究了808nm激光二极管抽运下掺Tm3+和Tm3+/Ho3+共掺碲酸盐玻璃的能量转换过程和机理.利用Dexter模型计算了能量转换微观参数和临界半径,分析了两者的变化特性;利用Vila等修正的Inokuti-Hirayama模型拟合了Tm3+的荧光衰减曲线.结果表明:Tm3+/Ho3+共掺碲酸盐玻璃的能量转换过程中,不仅包含Inokuti-Hirayama作用方式,而且存在着局域性相互作用,并随着Tm3+和Ho3+的浓度增大,后者所占比例逐渐增大.     

Concentration quenched luminescence and energy transfer analysis of Nd3+ ion doped Ba-Al-metaphosphate laser glasses

This paper reports the dopant ion (Nd³⁺) concentration effects on its luminescence properties in a new glass system based on barium-alumino-metaphosphates. Amongst the studied concentrations range of 0.276–13.31×10²⁰ ions/cm³, the glass with 2.879×10²⁰ ions/cm³ (1 mol%) Nd³⁺ concentration shows intense NIR emission from ⁴F_3/2 excited state, followed by a decrease in emission intensity for further increase in Nd³⁺ ion concentration. The observed luminescence quenching is ascribed to Nd³⁺ self-quenching through the donor-donor migration assisted cross-relaxation mechanism. The microscopic energy transfer parameters for donor-acceptor energy transfer, C _DA, and donor-donor energy migration, C _DD, have been obtained from the theoretical fittings to experimental decay curves and the spectral overlap model respectively. The C _DD parameters (×10^?39 cm⁶/sec) are found to be about three orders greater than that of C _DA (×10^?42 cm⁶/sec) for Nd³⁺ self-quenching in this host, demonstrating that the excitation energy migration among donors is due to the hopping mechanism. The energy transfer micoparameters obtained in the present study are comparable to the values reported for commercially available laser glasses LHG-8 and Q-98.     

Microwave-assisted synthesis of nanoparticulate perovskite LaFeO3 as a high active visible-light photocatalyst

Single phase perovskite LaFeO₃ nanoparticles are directly synthesized by a one-step microwave-assisted route without additional high temperature calcination process. The prepared sample is characterized by X-ray diffraction (XRD), field emission scanning microscope (FE-SEM), Brunauer–Emmett–Teller (BET) specific surface area measurement, diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). XRD and SEM show that the prepared LaFeO₃ is single phase perovskite and sphere-like shape. The DRS result shows that the prepared LaFeO₃ has strong visible-light absorption and an optical absorption onset of 525 nm, corresponding to an optical band gap of 2.36 eV. The photocatalytic experiment demonstrates that LaFeO₃ is highly visible-light photocatalytic active for the degradation of methylene blue. It is suggested that the narrow band gap and the strong visible-light absorption are responsible for the high visible-light photocatalytic activity of the prepared LaFeO₃.