Nonradiative Probability Research Articles

Improving luminescence and thermometric performance of Ba2CaWO6:Er3+ by tri-doping with Yb3+ and Na+

The Er3+ doped double perovskite Ba2CaWO6 crystal is a promising ratiometric thermometer based on the fluorescence intensity ratio (FIR) of transitions from 2H11/2 and 4S3/2 to the lowered 4I15/2 level. However, the Ca2+ vacancy defect caused by the charge difference between rare-earth ions and the substituted alkaline-earth ions gives rise to the non-radiative probability and limits the thermal sensitivity. Here, the up-conversion luminescence and thermometric performance of Er3+, Yb3+ doped Ba2CaWO6 are tuned by tri-doping with alkaline ions. The Ca2+ vacancy defect can be eliminated by the introduction of Na+, which occupies the Ca2+ site when it is doped into Ba2CaWO6 with Er3+ and Yb3+. On the contrary, the doping of Cs+ into Ba2CaWO6 with Er3+ and Yb3+ enhances the defect concentration because it occupies the site of Ba2+. Thus, the tri-doping of Na+ reduces the non-radiative probability and enhances the quantum efficiency of Er3+, leading to the improvement of the thermometric sensitivity of Ba2CaWO6. As a result, we get an excellent thermometric Ba2CaWO6:8%Yb3+,3.5%Er3+,6%Na+ powder with a luminescence lifetime of 515 μs and maximum thermal sensitivity (Sr) of 1.45%/K, which is more than three times higher than that of the BCWO:Er3+ powder.

Transition probabilities of Er3+ ions in alumino-silicate glasses

The spectroscopic properties of Er-doped Ca3Al2Si3O12 and Mg3Al2Si3O12 glasses produced by the Laser Floating Zone technique are presented in this work. By measuring the absorption spectra and refractive index dispersion of the materials, JO parameters of the Er3+ ions have been obtained in both doped materials, where very good accordance between the experimental and theoretical electric dipole oscillator strengths is found. Using calculated electric and magnetic dipole strengths, the branching ratios of the different optical transition of Er3+ in Ca and Mg glasses have been obtained, where the radiative lifetime of each level has been also calculated. The experimental lifetimes of the 4S3/2, 4I11/2 and 4I13/2 manifolds have been measured under pulse excitation. These values have been related with the radiative lifetime calculated by the JO analysis, allowing to determine the non-radiative probabilities of the levels. In addition, a gap law has been established for both Ca and Mg alumino-silicate glasses. Finally, the covalency of the Er-O bond was studied in terms of the spectroscopic features of these glasses.

Site Occupation of Eu2+ in Ba2- xSr xSiO4 ( x = 0-1.9) and Origin of Improved Luminescence Thermal Stability in the Intermediate Composition.

Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu2+ for the two distinct types of alkaline earth (AE) sites in Ba1.9995- xSr xEu0.0005SiO4 ( x = 0-1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu2+ prefers to be at the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, while at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu2+ 4f7 ground level position, the improved thermal stability of Eu2+ luminescence in the intermediate composition ( x = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition.

YAG:Er3+, CaF2:Er3+, and Er2O3 Emission Spectra Under Laser and Laser Thermal Excitation

Experimental luminescence and selective-emission (SE) spectra of YAG:Er3+ (10 at.%) and CaF2:Er3+ (1 at.%) single crystals and Er2O3 polycrystal under laser and laser thermal excitation of the Er3+-ion multiplets are compared. Luminescence spectra under resonant excitation are determined by multiplet population relaxation with the corresponding radiative and nonradiative probabilities. The form of the SE spectra is determined by the thermal population of the multiplets and the probabilities of only radiative transitions. The SE band at 800 nm (4I9/2 → 4I15/2) is an indicator of high-temperature thermal emission of Er3+ ions. The absence of this band in luminescence spectra is explained by the short lifetime of the τ(4I9/2) level of 53 ns at T = 300 K.

Gamma-radiation effects on luminescence properties of Eu3+ activated LaPO4 phosphor

Abstract Eu3+ activated LaPO4 phosphors were prepared by a high-temperature solid-state method and irradiated to different high-doses gamma-radiation in the 0–4 MGy range. No effects of high-doses of high-energy radiation on phosphor’s morphology and structure were observed, as documented by electron microscopy and X-ray diffraction measurements. On the other hand, photoluminescence measurements showed that emission properties of phosphor were affected by gamma-radiation; changes in radiative properties being prominent for absorbed radiation doses up to 250 kGy after which no additional changes are observed. Judd-Ofelt analysis of emission spectra is performed to thoroughly investigate radiative properties of phosphors. Analysis showed that radiative transition probability of Eu3+ emission decreases while non-radiative probability increases upon gamma-irradiation. Quantum efficiency of emission is decreased from about 46% to 35% when Eu3+ doped LaPO4 powders are exposed to gamma-radiation of 250 kGy dose, showing no additional decrease for higher gamma-radiation doses.

Energy transfer efficiency in YF3 nanocrystals: Quantifying the Yb3+ to Tm3+ infrared dynamics

In this work, we report on the determination of the infrared Yb3+ → Tm3+ energy transfer efficiency in YF3:Yb3+/Tm3+ nanocrystals through the study of Yb3+ dynamics. The obtained results are compared to those previously reported in macrocrystals to analyze possible changes related to size reduction. Luminescence lifetimes are much shorter in the nanoparticles than in bulk samples, a behavior that can be related to Yb3+ → Yb3+ migration and the enhanced surface/volume ratio of the nanoparticles. On the other hand, Yb3+ → Tm3+ energy transfer macroparameter remains unaltered, demonstrating that spectroscopic intrinsic parameters such as radiative and non-radiative probabilities are not affected by size reduction. Finally, a formula that describes Yb3+ lifetime dependence with Yb3+ and Tm3+ concentration is proposed, considering both the effects produced by migration between Yb3+ ions and energy transfer from Yb3+ to Tm3+ ions.

Compositional dependence of the 1.8 μm emission properties of Tm3+ ions in silicate glass

The compositional dependence of the 1.8 μm emission properties of Tm3+ ion-doped lead silicate glasses is investigated. Judd–Ofelt parameters are calculated and their variation with different glass modifier ions is obtained. The Judd–Ofelt parameters increase with decreased modifier ionic radius. A large spontaneous emission probability and a large emission cross-section are found to be related with the strength of the modifier ion. Fluorescence spectra are analyzed using rate equations and compared with recorded data. The results are very close, indicating the reliability of this method. Non-radiative probability is deduced by fitting the fluorescence decay curve; it becomes smaller with increased ionic field strength. Energy transfer processes are studied using the extended overlap integral method.

Influence of microemulsion conditions on the VUV-excited luminescence and microstructures of Y 3Al 5O 12: Eu 3+ phosphors

The effects of preparation conditions on the microstructures and luminescence properties of microemulsion derived YAG: Eu 3+ phosphors are investigated in this study. Both the emission spectra and EXAFS results reveal that the local environment of Eu 3+ ions becomes more symmetric with increasing calcination temperatures. The symmetric environment of Eu 3+ ions in the host results in increased emission intensity of the 5D 0 → 7F 1 transition. The particle sizes of YAG: Eu 3+ phosphors markedly reduce with decreased water content in the microemulsion system. The powders with reduced particle size exhibit strong luminescence intensity because of reduction in the non-radiative probability. For preparing bright YAG: Eu 3+ phosphors via the microemulsion route, the water-to-oil volume ratio and calcination conditions need to be well controlled.

Optical properties and site distribution of Cr 3+ ions in alkali-disilicate glasses

The optical properties of the low-field sites of Cr 3+-doped alkali (Li, Na, K) disilicate glasses have been investigated using the single configurational coordinate model. The assumption of a Gaussian site distribution for the Cr 3+ ions taking as parameter the zero-phonon energy has been considered. For alkali disilicate glasses the inhomogeneous contribution to the broadening of the bands, associated to the site distribution, is lower than the homogeneous one. The electron–lattice coupling S and the mean phonon energy ℏ ω 0 have been obtained with results around 4 and 500 cm −1, respectively, similar to those obtained by other authors in oxide glasses. The site-resolved study of the emission and excitation spectra and the luminescence decay curves have been carried out as a function of temperature. On the one hand, there is evidence of a non-radiative de-excitation process that becomes important over 140 K. On the other hand, and related to the site dependence of the radiative and non-radiative probabilities, different results involving low values for the quantum efficiencies and blue shifts of the emission bands as temperature increases have been explained. Besides, the non-exponential luminescence decay curves have been fitted to an expression proposed by the authors, which takes into account non-coupled distributions for the radiative and non-radiative de-excitation probabilities for the range of temperature covering from 13 to 300 K. From the fits, the temperature dependence of the non-radiative de-excitation probability is obtained for each disilicate glass, the results are in good agreement with the expression obtained assuming the harmonic approximation in the single configurational coordinate model.

Pulsed, broadly tuneable, photoacoustic study of rare-earth doped LiNbO 3

Rare-earth (RE) doped solids are widely used as optical emitters from the near infrared to the ultraviolet range. A key measurement for such applications, the luminescent quantum efficiency, quantifies the comparison between radiative and non-radiative probabilities, which can be evaluated from luminescence and photoacoustic measurements, respectively. The photoacoustic and luminescence excitation signals can be obtained either under pulsed dual-wavelength excitation or CW tuneable excitation, by using Nd-YAG and Ti:Sapphire lasers. In the present work, we demonstrate the use of pulsed, broadly tuneable, excitation using an optical parametric oscillator (OPO), extending the spectral range of application covering practically all the near infrared to the ultraviolet range, and accessing any RE emitting level. The procedure is applied to RE-doped lithium niobate (LiNbO 3), a material of great interest for optoelectronic applications.

Cross-relaxation process between +3 rare-earth ions in LiYF4 crystals.

The cross-relaxation probability (${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$) involving the $^{5}$${\mathit{S}}_{2}$ and $^{5}$${\mathit{I}}_{8}$ levels of holmium ions in ${\mathrm{LiYF}}_{4}$ was investigated and its concentration dependence determined. By using a random distribution function of ions in the crystal, it was possible to determine the concentration dependence of the nonradiative probability for the ion-ion energy transfer by dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole electric interactions. The applied model shows that the ion-ion cross-relaxation transfer between ${\mathrm{Ho}}^{3+}$ ions in ${\mathrm{LiYF}}_{4}$ crystals, cannot account for the experimental results. At a high concentration level of activators (\ensuremath{\gtrsim}1%), the excited ion must interact with two neighbors of same specie competing with the single ion cross-relaxation transfer. \textcopyright{} 1996 The American Physical Society.

Temperature dependence of non-radiative relaxation probabilities of Cr3+ in BIGaZYT fluoride glass measured by a pulsed photoacoustic technique

The temperature dependence of the non-radiative relaxation probabilities of Cr3+ in fluoride BIGaZYT glass have been obtained by pulsed photoacoustic technique. The results are compared with nonradiative probabilities obtained from luminescence measurements.

Temperature dependence of photoluminescence in hydrogenated amorphous silicon.

The temperature dependence of the photoluminescence (PL) intensity in hydrogenated amorphous silicon (a-Si:H) has been studied. At temperatures below 100 K the nonradiative probability has a power-law behavior with a temperature dependence ${T}^{\ensuremath{\gamma}}$, where \ensuremath{\gamma} is in the range 4 to 1.5, which is quite different from that above \ensuremath{\approxeq}100 K. After prolonged illumination at room temperature, a considerable change of the temperature dependence of the photoluminescence in undoped a-Si:H has been observed below about 100 K. The PL quenching begins at temperatures below 50 K for samples with a high defect density, while the quenching occurs at temperatures above 50 K for undoped a-Si:H with a low defect density. We propose a model assuming shallow radiative hole trap centers to explain our results. The temperature dependence of the photoluminescence below 100 K is probably due to the power-law temperature dependence of the capture cross sections for these radiative hole trap centers.

Multiphonon non-radiative relaxation rates and Judd–Ofelt parameters of lanthanide ions in various solid hosts

The pre-exponential factor β of the near exponential energy-gap law, which has previously been treated as a phenomenological parameter and recently as an electronic factor, is shown to be directly related, through the squared maximum phonon energy, to the sum of products of the intensity parameters τλ by the squared matrix elements ‖Uλ‖2 of the Judd–Ofelt tensor. Calculated β for several lanthanide ions Ln3+ in six crystalline and six glass hosts are in very good agreement with the experimental values. A modified Franck–Condon factor F expression is derived for slightly displaced undistorted parabolae using van Dijk's and Schuurmans' ν=[(ΔE/ħωM)– 1] argument (where ΔE/ħωM is the original phonon number). Among fifty-eight calculated non-radiative probabilities knr(=βF), fifteen differed from the experimental knr by more than one order of magnitude, but by less than a factor of 50, thus promising a theoretical evaluation of any unknown knr of an Ln3+ transition for which the Huang–Rhys parameter S is known or can be determined spectroscopically. It can also be shown that the vanishing of the proposed β expression when ΔE/ħωM= 1 is relaxed if vibrations lower than the most energetic one are also effective.

Optically stimulated luminescence and photoacoustic spectroscopy of NaCl, NaCl: Cu and NaCl: Ca, Cu crystals X-irradiated at room temperature

Recent development in the X-ray imaging method for the application of the two-dimensional dosimetry has invoked a new interest in the optically stimulated luminescence (OSL) in irradiated materials [1, 2]. The OSL emission is in many respects very similar to thermoluminescence (TL), because both are caused by the recombination of electrons and holes that have been isolated by the preceding irradiations. The difference is only in the manner of the stimulation: the OSL is stimulated optically and the TL thermally. We have recently shown that the OSL efficiency of the NaC1 crystals containing Cu + ions as the activator is markedly reduced by the addition of Ca 2+ ions in contrast to the case of TL [3]. For the decrease in efficiency of the OSL by Ca 2+ doping, two reasons may be considered. (a) The formation of Z~ centres by the optical illumination because of the presence of Ca 2+ ions. (It is well known that the Z~ centres are easily formed by bleaching the F-band when the crystals contain alkaline earth ions such as Ca2+). Therefore, some part of the energy of the illumination may be dissipated in the formation of Z~ centres instead of introducing the recombination. (b) The decrease or loss of the efficiency of luminescence of the Cu + ions because of their partial formation of some complex centres with the I-V (impurity-vacancy) dipoles which are formed by the Ca 2+ ions and positive ion vacancies. The purpose of the present report is to determine which item of the above two is responsible for the effect of the Ca 2+ co-doping to the NaC1 : Cu ÷ phosphor by using the photo-acoustic spectroscopy (PAS) technique, because PAS is a powerful tool for detecting the non-radiative probability of the relaxation process.

Interactions of aqueous 7 F and 5 D 1 europium(III) ions with bromide and inner-sphere coordination exciplex formation

The study of the europium(III) ion + water + bromide system shows totally outer-sphere interactions between the anion and the ground-state 7F octa-(E8) and nona-(E9) hydrated europium species. An appropriate analysis of these interactions, involving the corresponding (E8B) and (E9B) outer-sphere bromocomplexes or any other aqua-europium outer-space complexes, quantitatively rationalises the intriguing diminution of the europium-ion average hydration number, text-decoration:overlineN, with dilution reported in the literature from Raman and 5D0 lifetime measurements. The [E8]/[E9] concentration ratio is calculated and is ca. 10. Values of text-decoration:overlineN calculated for aqueous EuCl3 and EuBr3 solutions of various compositions indicate unambiguously a predominance of the octa-hydrated species. The projection of the solid-state coordination number of nine to aqueous solutions of middle-row lanthanide ions is unjustified. It is stressed that the symmetry-related number of components of the optical transitions is insufficient to establish the predominance of either the ennea- or octa-hydrated europium species in aqueous media, as previously claimed. Modifications of the 4ƒ and 5p radii of maximum density, accompanying the optical excitation to some (Smax– 1) state of europium(III), are shown to result in a value for the [*E8]/[*E9] ratio that is > 10. Optical absorption, steady-state and time-resolved emission measurements, stationary and transient photokinetics, as well as excited-state 5D1 energetics, show unambiguously the formation of an inner-sphere hepta-aquo-europium(III) bromoexciplex (*E7B), most probably occurring by a dissociative-interchange type of mechanism. Formation constants of *E8B and *E7B, the heat and entropy of the overall formation of *E7B, the radiative (5D1→7F) and non-radiative (5D1→5D0) probabilities of *E8 and *E7B, as well as the activation energies of their 5D1→5D0 relaxations, are determined. The 5D1→5D0 process is shown to occur by emission of one 1640 cm–1 phonon, corresponding to the H2O bending vibration, and the theoretically calculated non-radiative probabilities are in good agreement with the experimental values. Huang–Rhys numbers for the 5D1→5D0 transitions of *E8 and *E7B are evaluated and the dimensionless displacements, Δ, of the horizontal shift of the potentials are those expected from theory, if the degeneracy of the 1640 cm–1 mode is taken to be 8 and 7, respectively. The previously unclarified origin of the activation energy of the 5D1→5D0 process is identified as the point of minimum energy on the hypercurve of intersection of the two potential surfaces. The earlier claim, that the 5D1→5D0 process rate constant of the aquo-europium(III)ion is independent of isotopic substitution, is inexact. The experimental rate-constant ratio (H2O/D2O) is 2.2 and the calculated value is 2.7.

Nanosecond laser studies of photophysical processes in the benzene + tetracyanobenzene molecular complex

A 1 MW pulsed nitrogen gas laser was used to investigate the various photophysical processes operating in the 1, 2, 4, 5-tetracyanobenzene (TCNB)+ benzene molecular complex. These processes were found to be affected by increasing amounts of acetonitrile. In mixtures containing less than 20 % acetonitrile the fluorescent state of the EDA complex was formed with unit probability. However, the relative quantum yield and the lifetime of the fluorescent state was found to decrease as the amount of acetonitrile increased. This was attributed to a decrease in the radiative transition probability and an increase in non-radiative probabilities. At higher acetonitrile concentrations ionisation was the predominant decay mode of the excited state, and the quantum yields of ion production were estimated.