1,721,055 research outputs found
Unraveling the Mechanisms of Thermal Quenching of Luminescence in Ce3+-Doped Garnet Phosphors
No full textThe environmental and economic benefits of phosphor-converted white-light-emitting diodes (pc-WLEDs) have been increasingly appreciated in recent years. However, a significant challenge in this field pertains to a phenomenon known as thermal quenching, which takes place inside phosphors and leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation. The development of new, more thermally stable phosphors depends on a better understanding of the mechanisms underpinning thermal quenching in phosphors. Here we review the current understanding of thermal quenching mechanisms in Ce3+-doped garnet phosphors, which are widely considered one of the most important families of phosphors for application in pc-WLEDs. In particular, we highlight key structural and dynamical properties, such as the coordination environment of the Ce3+ ions, phonons and local vibrational modes, and structural and chemical defects, which are shown to correlate with phosphor performance. We also discuss the perspectives for future studies in this field in hopes of accelerating the development of new efficient phosphors featuring suppressed thermal quenching of luminescence
Green persistent luminescence excitable by multiple wavelengths in the CaSc2O4:Ce3+ phosphor co-doped with Mg2+
No full textCalcium scandate (CaSc 2 O 4 ), when substituted with a small amount of the activator ion Ce 3+ to replace Ca 2+ , is a recently discovered green-emitting phosphor promising for solid-state lighting applications. The co-doping with aliovalent ions to compensate for the net positive charge, as induced by the Ce 3+ dopants, is a common approach to change the defect structure and tune the performance of the phosphor, but the underlying mechanisms are unclear. Here we investigate the effect of co-doping with charge-compensating Mg 2+ to substitute for Sc 3+ , using photoluminescence and thermoluminescence techniques. In comparison to the defect structure in Ce 3+ doped CaSc 2 O 4 , the co-doping with charge-compensating Mg 2+ leads to the formation of new traps. These traps are featured by activation energies in the range 0.58–0.64 eV, can be emptied at room temperature and show green long-lasting phosphorescence after excitation at 454, 340, and 254 nm, respectively. Analysis of the spectroscopic results in terms of a vacuum referred binding energy diagram allows us to reach at a plausible luminescence mechanism in {Ce 3+ /Ce 3+ Mg 2+ }-doped CaSc 2 O 4
The effect of cation substitution on the local coordination of protons in Ba2In1.85M0.15O6H2 (M = In, Ga, Sc and Y)
Full text linkWe report on an investigation of the local structure and vibrational dynamics in the brownmillerite-based proton conductors Ba2In1.85M0.15O6H2 with M = In, Ga, Sc and Y. The aim is to determine the effect of the cation (M) substitution on the local coordination environment of the protons. The techniques used are infrared spectroscopy and inelastic neutron scattering. The materials are characterized by two main types of proton sites, denoted as H (1) and H(2), which are featured by different local structures. We establish that the relative population of these two proton sites varies as a function of M. Specifically, it is found that, with respect to Ba2In2O6H2, the relative population of H(1) protons increases upon the substitution of In with any of the three different cations. The strongest effect is observed for M = Ga and Sc, whereas the effect observed for M = Y is minor. This new insight motivates efforts to unravel the mobility of the two types of protons, since then cation modification would offer a rational route for improving the proton conductivity of these types of materials
Weak thermal quenching of the luminescence in the Ca3Sc2Si3O12:Ce3+ garnet phosphor
Full text linkWe report results of the luminescence properties of the three garnet type phosphors Ce3+-doped Ca3Sc2Si3O12(CSSO:Ce3+), Sr3Y2Ge3O12(SYG:Ce3+) and Y3Al5O12(YAG:Ce3+), investigated using optical spectroscopy techniques and vacuum referred binding energy (VRBE) diagram analysis. By monitoring the temperature dependence of the luminescence decay time we establish an excellent, intrinsic, thermal stability of luminescence in CSSO:Ce3+, with a nearly constant decay time (≈60 ns) up to, at least, T = 860 K. In comparison, SYG:Ce3+and YAG:Ce3+exhibit a significant reduction of the luminescence decay time upon heating, starting at around T = 280 K and T = 550 K, respectively, suggesting a lower internal thermal stability of luminescence in these two garnet phosphors. These findings are supported by the energy separation between the Ce3+5d1level and the conduction band (CB) of the respective hosts, which are found at 1.36 eV (CSSO:Ce3+), 0.45 eV (SYG:Ce3+), and 1.17 eV (YAG:Ce3+), respectively, as predicted by their VRBE diagrams. The performance of CSSO:Ce3+was evaluated by applying the phosphor on a blue InGaN LED. The system shows a luminous efficacy of optical radiation of 243 lm W-1and a linear response with increasing applied voltage, suggesting it is a highly promising phosphor for future technological applications, particularly at high temperature operating environments
Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y3-xCexAl5O12: Design Principles for Phosphors Based on 5 d-4 f Transitions
No full textThe oxide garnet Y 3 Al 5 O 12 (YAG), when a few percent of the activator ions Ce 3+ substitutes for Y 3+ , is a luminescent material widely used in phosphor-converted white lighting. However, fundamental questions surrounding the defect chemistry and luminescent per formance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce 3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y 3-x Ce x Al 5 O 12 . Increasing the Ce 3+ concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO 8 moieties. Decreasing the Ce 3+ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce 3+ -doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO 8 moieties and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence
Perspectives of neutron scattering on proton conducting oxides
No full textUnderstanding the fundamental properties of materials of relevance for alternative energy technologies is crucial in addressing the global challenge of cleaner sources of energy. This Perspective article aims to demonstrate the important role that neutron scattering now plays in advancing the state of the art of the basic understanding of proton conducting oxides, which show potential as electrolytes in next-generation intermediate temperature fuel cells. In particular, the breadth of neutron scattering work on perovskite structured oxides, which continue to be the most promising class of electrolytes for intermediate-temperature applications, is reviewed. Key fundamental properties that are addressed include structures, proton sites, hydrogen-bonding interactions, proton dynamics, and concentration of protons in materials. Furthermore, the perspectives for future neutron studies within this field of research are discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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