Integration of equation 7 yields:. Therefore, equation 8 can be written as:. Equation 9 illustrates that the depopulation of the excited state in a two level system follows a first order exponential decay. However, if the emitting center occupies different chemical environments, the depopulation can deviate from the monoexponential behavior. Nevertheless, in complex level structures and systems in which energy transfer processes between excited states take place, the relationship between the population of the emitting state and the emission intensity is not straightforward.
Another important concept involved in the description of luminescent materials is the energy difference between emission and excitation maxima, which is usually termed Stokes shift. As a consequence, a transition between two levels of a 4f n configuration occurs practically without vibrational progression, which virtually gives rise to no shifts between absorption and emission Figure 5b. However, such a process would involve several different electronic states and should not be characterized as a Stokes shift according to the IUPAC definition.
In addition, other situations can be pointed out, notably the emission of high energy configurations such as 5d states, Figure 5c within the same chromophore, and the emissions arising from antenna sensitizers or charge transfer states Figure 5d. This process comprises transitions between different sets of crystal field levels, so that it cannot be characterized as a Stokes shift. In these examples, it is more obvious that the differences occur because absorption and emission processes take place in states that originally belong to different chemical species.
Although the loss of the excitation energy through the emission of electromagnetic radiation is the most evident pathway for excited state deactivation and return to the ground state, there are several competing mechanisms acting after the incidence of light by the studied material. These possibilities can both restrict the yield of light emission or reduce the amount of energy that is effectively available for the formation of excited states.
In the case of solid state luminescent processes, such mechanisms can be summarized by three situations. For example, if the absorbed energy does not reach the luminescent centers, the emission process will be limited by the generation of the excited states. In a second situation, the absorbed energy reaches luminescent centers, but effective emissions are reduced by the occurrence of non-radiative pathways for the return to the ground state. Finally, in a third situation, the emitted radiation is absorbed by the components of the luminescent material.
In the case of lanthanoid ions, concentration quenching mechanisms are particularly important, where the absorbed excitation energy migrates between identical ions in the lattice reducing the probability radiative deactivation.
Series: Handbook on the Physics and Chemistry of Rare Earths
The distance between identical centers become sufficiently small for occurrence of effective energy transfer interactions above a characteristic concentration value, which depends on the nature of the lanthanoid ion and on the lattice properties. Therefore, the formerly excited ion decays non-radiatively, whereas the neighbor ion is no longer available to absorb the excitation energy, thus reducing the global light generation in the system. This process is known as cross-relaxation, being normally assisted by the absorption or emission of phonons for the compensation of low energy differences between the electronic states.
Moreover, high concentrations favor energy migration between identical species, which increases the probability of non-radiative energy loss in impurities or structure defects. However, even at low activator concentrations, the host lattice vibrational structure can also affect emission processes, since vibronic coupling between excited states and high energy oscillators of the lattice can lead to non-radiative decays by means of phonon emission. Different quenching mechanisms reduce the probability of radiative decays, which culminates in lower luminescence quantum yields.
Therefore, the excited state quantum efficiency becomes:. Since, in a given excited state the total decay rate can be considered as the reciprocal of the luminescence lifetime of equation 9 , quantum efficiency can also be written as:. Consequently, quantum efficiency is a ratio between decays rates of a particular emitting level of the activator species that lead to a measurable luminescence, thus being independent of the mechanism involved in the formation of the excited state.
Therefore, quantum yields do not involve the measurement of decay ratios, but the quantification of the number of photons which emerge from the sample instead. This is normally performed by the use of integrating spheres or by comparison with standard compounds with known quantum yield. Finally, an even wider property can be defined, since not all incident photons are effectively absorbed due to competing events such as transmission, scattering, and reflection. In other words, the external quantum yield corresponds to the ratio between the number of photons that are emitted by the sample and the number of photons that reach the sample per unit of area per unit of time.
The use of RE-based solid state phosphors started in the s with the purpose to generate the three primary colors red, green and blue in cathode ray tubes and fluorescent lamps. Since then, the evolution and diversification of inorganic phosphors containing RE followed the development of new technologies for lighting and visualization. As a result, RE elements are currently still the most important components in luminescent materials for such applications, as well as several other fields, as detailed in Table 6. Phosphors with practical industrial use are mostly synthesized via high temperature solid-fstate chemistry techniques, in which simple precursors e.
Such procedures are normally preferred due to the simple protocols and to the high crystallinity of the final materials. However, the recent advent of particular applications imposing limitations such as high surface areas and low sizes also results in the development of liquid phase synthesis methodologies, which raise a growing importance in different fields. The development of the final phosphors comprises a delicate balance among several physical and chemical aspects that govern the final efficiency.
For instance, solid state luminescent materials usually comprise a spectroscopically inert inorganic host that accommodates chromo- or fluorophore species. Besides this primary role, the inorganic host also must provide a high resistance against photo chemical degradation, since high operation lifetimes are usually required, as well as low reactivity in the case of biological applications.
In addition, the final emission behavior also depends on the host crystalline structure, since expected spectroscopic properties are governed by site symmetries and covalence provided by the inorganic lattice. In this regard, sensitizing groups are usually related to the design of luminescent materials, since only in a few cases are the low molar absorptivity of 4f-4f absorptions sufficiently high to provide useful luminescence yields.
The sensitizer characteristics must involve not only a broad and intense absorption, but also the ability to transfer the absorbed energy to emitting groups. This, in turn, refers to the spectral overlap between sensitizer and emitting centers, which depends on a critical distance between the involved species. Such characteristics depend on the volume of unit cells and molar fractions of optically active species, which makes energy transfer parameters mathematically accessible from experimental data.
Nevertheless, even if inorganic host and sensitizers provide good general characteristics, the light emission process by a phosphor is completely dependent on the adequacy of the activators regarding desired properties. In this sense, even occurring in low concentrations, activator centers control the major final properties in solid state phosphors.
The synergism between activators, sensitizers and host thereby governs the final spectroscopic properties and efficiencies, as well as the color associated with the light emitted by the luminescent material. Colorimetry is a crucial subject in the development of phosphors, since color is one of the major limiting aspects for the applicability of luminescent materials.
Handbook on the Physics and Chemistry of Rare Earths, Volume 14
As color is a human perception of chromaticity and brightness rather than a definite physical property, the accurate description of colors associated with emitting compounds is usually considerably complex, and readers are referred to a more specialized literature for further details. Besides host, sensitizers and activators, the design of solid-state phosphors may also comprise other physicochemical aspects in order to provide the highest adequacy between obtained properties and required applications.
For instance, the concentration of structure defects, which can be either intrinsic to the host lattice or introduced through impurities, is also a fundamental characteristic. Defects may induce quenching mechanisms that reduce the global efficiencies or lead to parallel chemical processes that diminish the chemical stability. However, the role of structure defects depends completely on the nature of the luminescent material.
For example, characteristic mechanisms and times of persistent luminescence processes are intimately related to defects. Surface modification on phosphor particles may also play an important role in the study of such compounds, since chemical processes comprising defect passivation, increase of colloidal stability or enhancement of energy transfer efficiencies can be highly important for the final applicability of inorganic solid state phosphors.
In summary, the development of phosphors involves a wide range of chemical composition, oxidation states, impurity concentration, homogeneity, morphology, particle size, surface charge, etc.
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However, the increasing development of lighting and visualization technologies and more restrictive energetic and environmental concerns still stimulate the search for a new generation of solid state phosphors. For instance, as commercial compact fluorescent lamps CFL operate near their technological limit of light output, no large improvements of performance are expected for this class of lamps in the next years.
This, in turn, indicates that the other current technologies are potential candidates to bring the next revolution in artificial lighting, which comprises, for instance, light emitting diode LED lamps and lighting systems based on VUV phosphors. Currently, LED lamp devices are the most investigated systems regarding the development of phosphors, since they consist in energetically advantageous solid state lighting sources with potentially improvable output characteristics.
As system i comprises a complex mixture of phosphors and induces a lower energetic efficiency due to UV excitation, systems ii and iii are preferred for the improvement of LED systems, which are currently limited by the lack of an efficient blue light excitable red phosphor. The search for good red phosphors for LED lighting is therefore a highly attractive topic in the RE solid state phosphors research.
However, such compositions are unstable to moisture and they are not adequate to operate in high power LED systems. Moreover, such sulfides react with silver and nickel from electrodes, thus generating the more stable NiS and Ag 2 S and the presence of these black compounds decreases the photon flux from the phosphors and reduces the arrangement efficiency. As a consequence, the final cost of this kind of phosphor is comparable to pure metallic gold, so that the investigation of new compositions and new synthesis methods are still required for the development of this field.
Despite the mentioned drawbacks, LED systems appear as the most promising technology for a new generation of lighting systems, with some unrivalled advantages such as high efficiency, long operation lifetimes, and emission directionality. However, some particular restrictions can also conduct to a parallel development of other lighting arrangements in order to fulfill specific requirements that can become decisive in the future.
For instance, LED bulbs are currently associated with a very high manufacturing cost, even considering final prices per lumen that are decreasing very fast and high durability.
For instance, energy consumption restrictions have practically withdrawn low power incandescent lamps from the market in the European Union , USA , and Brazil , and future legislations will probably prohibit the use of mercury and other toxic metals in fluorescent and LED lighting systems. In this sense, VUV phosphors Table 7 can be considered as potential candidates for the development of complementary lighting systems that fulfill these specific requirements. This class of luminescent materials involves compounds that are efficiently excited at wavelengths lower than nm, being potentially applicable in combination with noble gas discharges.
For instance, xenon and neon are the most used noble gases in discharge mixtures, which can provide exciting radiation at nm resonant emission line of Xe plus two continua centered at and nm molecular emissions of Xe 2 under electric discharge.
However, PDPs can generally result in larger image sizes as well as faster images i. The investigation of solid state luminescent materials applicable in such systems still is an essential prospect in this field. Regarding lighting technologies based on VUV phosphors, excimer lamps present remarkable advantages of temperature independent output, instantaneous operation i.
Intensities of 4f-4f transitions in glass materials
Therefore, VUV phosphors must be continuously improved in order to develop such application field, which can consist in an important technology to complement LED lighting. VUV phosphors must display some fundamental properties that enable their large scale application, while drawbacks related to currently used materials must be overcome Table 7. Moreover, some compounds are susceptible to photochemical degradation under VUV irradiation, requiring additional surface coating procedures with MgO or Al 2 O 3 , for instance. Emissions that persist for more than 10 ms until intensities are reduced to ca.
Furthermore, the most fundamental property required of VUV phosphors is a broad and intense absorption band between and nm, which normally is provided by the host lattice. The excitation mechanism in VUV comprises mainly band-gap excitation within the host lattice. As a consequence, not only crystallinity and defect density, but also particle size and surface area affect the VUV luminescence efficiency, while nanostructured solids are preferred in order to lead to high luminescence yields. In addition, one of the main drawbacks of VUV phosphors is the very large energy shift between excitation and emission, where a very high fraction of the excitation energy is dissipated in parallel non-radiative processes.
With the aim to overcome this problem, phosphors with quantum yields greater than unity can be designed based on the so called quantum cutting processes. In this sense, RE solid state compounds are completely inseparable from the field of optical materials. The unique properties of this group of elements still stimulates increasing efforts of fundamental and applied research involving lighting, visualization, biolabeling, photoprotection, and photocatalysis, for instance.
Therefore, this text reviewed fundamental concepts concerned in the use of RE-based solid state materials for lighting and photoprotection processes.
The elaboration of new applications and the development of tailored properties are completely dependent on a deeper knowledge regarding concepts of 4f spectroscopy. This is particularly important for the development of a new generation of photoprotective materials, with controlled photocatalytic activities and absorptivity, as well as the obtainment of improved energy saving lighting sources.
In this regard, investigation of solid state RE materials is crucial for the development of environmentally clean and low energy consumption lighting systems.
Furthermore, LED systems comprise the most promising and improvable technology for the next revolution of artificial lighting, whereas the fulfillment of specific environmental and performance requirements temperature independent output, arbitrary design also leads to a promising future for lighting systems based on VUV phosphors.
In conclusion, the rare earths are still the friendliest available elements for the construction of new and forthcoming solid state materials for illumination, displays and photoprotection. Virtual Quim. Nova , 37 , DOI: Policy , 38 , A , 44A , S2. D: Appl. The authors describe the fundamental chemistry that underpins contemporary analytical separation techniques for lanthanide separation and analysis. This is done after a description of the rich assortment of separation methods in use has been introduced.
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Table of contents Preface. Electronic excitation in atomic species J. Connerade, R. Simple and complex halides G. Meyer, M. Solid electrolytes R. Kumar, H. Activated thermoluminescent TL dosimeters and related radiation detectors A. Analytical separations of the lanthanides: basic chemistry and methods K. Nash, M. Author index. Zirngiebl and G.
The electron-phonon interaction in intermetallic compounds P. Thalmeier and B. Heavy fermions N. Grewe and F. Subject index. This volume differs somewhat from the previous volumes in the series in that there is a strong emphasis on the physical aspects and not so much on the chemical aspects of intermetallic compounds.
Two of the chapters are concerned with relatively new experimental methods of studying rare earth metallic phases - high energy neutron spectroscopy and light scattering. In these chapters the authors explain the new kinds of information one obtains from these techniques and how this complements the knowledge previously gleaned from the more common measurements - such as NMR, heat capacities, magnetic susceptibility, transport and elastic properties. One of the remaining three chapters deals with NMR studies of rare earth intermetallics and the final two chapters are concerned, not so much with a particular experimental technique, but with physical phenomena that occur in these compounds: the electron-phonon interaction and heavy fermion behavior.
Gschneidner has published over journal articles and chapters in books and edited or written 40 books on the chemistry, materials science, and physics or rare earth materials. He was the founder of the Rare-earth Information Center and served as its Director for 30 years. We are always looking for ways to improve customer experience on Elsevier. We would like to ask you for a moment of your time to fill in a short questionnaire, at the end of your visit.
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