Preparation of Luminescent Materials

Traditionally, luminescent materials are prepared by mix and fire techniques small grains of reactants (generally with diameters in the pm range or smaller) are thoroughly mixed (either dry or in a suspension) and heated. The heating temperatures typically exceed 1000 °C. To facilitate the reaction and to obtain luminescent materials of sufficient crystallinity (which generally improves the efficiency of the luminescence 

A suitable flux promotes crystaUinity of the luminescent material being manufactured and also the reactivity, by dissolving at least one of the reactants. Fluxes can be divided into two types a pure non-volatile liquid, e.g., a molten salt or a molten oxide (melting salt), and a volatile liquid or gas (volatile flux). 

A flux often reacts with the starting material, sometimes a melt, when used in small amounts (e.g. less than 10% by weight of the phosphor) and always shows decomposition or evaporation during phosphor synthesis. Frequently used fluxes are NH4CI, NH4Br and AIF3. 

Reactivity can be improved by using starting materials which decompose during heating, like carbonates or hydroxides. After decomposition (in the examples given CO2 or H2O leave the reaction mixture), one ends up with a much more reactive mixture, as a result of the larger specific surface area. 

Moreover, co-precipitation can be used. Using co-precipitation, a reaction mixture where the starting materials are mixed on an atomic scale can be obtained. By dissolving the reactants and co-precipitating them one obtains an intimately mixed starting mixture. Just to mention an example Y(N03)3 and Tb(N03)3 are dissolved in H2O and co-precipitated by adding to excess oxalic acid (2 to 1 on a molar basis), dissolved in hot water. Alternatively, the oxides can be dissolved in hot dilute nitric acid (4 M). The oxalates obtained are converted into the oxides by heating at 800 °C. 

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Scheme 1. Preparation of luminescent hybrid mesoporous material.

In this section, historic aspects of luminescent materials will be discussed first, followed by a short treatment of luminescence mechanisms and luminescence excitation schemes. Thereafter, devices based on luminescent materials and the way in which luminescent materials determine their operational performance will be discussed. Preparational aspects of luminescent materials will be described and then this section will end with an outlook. 

The original volume of this book was written to introduce the reader to the science and art of preparing inorganic luminescent materials, namely "phosphors". In order to understand how and why luminescent materials exhibit such specific intrinsic properties, one needs to be thoroughly versed in the science of the soUd state. Since that time, I have published a separate volume entitled ... 

Part three consists of five chapters in which many of the applications are discussed, viz. lighting (chapter 6), cathode-ray tubes (chapter 7), X-ray phosphors and scintillators (chapters 8 and 9), and several other applications (chapter 10). These chapters discuss the luminescent materials which have been, are or may be used in the applications concerned. Their performance is discussed in terms of the theoretical models presented in earlier chapters. In addition, the principles of the application and the preparation of the materials are dealt with briefly. Appendices on some, often not-well-understood, issues follow (nomenclature, spectral units, literature, emission spectra). 

Cuba, V., Niki, M. 2011. Radiation-assisted preparation of powder materials and their exciton luminescence. In Exciton Quasiparticles, ed. R. M. Bergin, pp. 181-210. New York Nova Science. 

Lanthanides present another group of luminescent materials suitable for sensors. Dysprosium and samarium pyridinedicarboxylates prepared by hydrothermal synthesis [233] exhibited a set of 3 emission bands assigned to different transitions of Dy and Sm with maxima ranging from 480 to 623 nm for Dy and 560 to 710nm for Sm. The conclusion has been drawn that the energy transfer from the organic ligand to the lanthanide ion is effective. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

The understanding of phosphors and solid-state luminescence has matured to the point at which relatively rational design and preparation of new light-emitting materials can be achieved. This has resulted from advances in solid-state physics and optical spectroscopy coupled to the development of new chemical synthesis techniques. This has led to the rapid development of phosphors as important industrial/technological materials. Examples of the occurrence of phosphors in everyday use include ... 

Fonda F. Seitz, "Preparation and Characteristics of Solid Luminescent Materials", J, Wiley, NY (1948) 10) W.E. Forsyth E.Q. 

R. Ward Preparation and Characteristics of Solid Luminescence Materials, Cornell Symp. 1946, Wiley and Sons, New York 1948. 

In another approach, a fluorescent conjugated polymer was used as the material for the preparation of a chemosensor to detect 2,4,6-trinitrotoluene (TNT) and its related nitroaromatic compounds. To this end, microparticles, made of three-dimensionally cross-linked poly(l,4-phenylene vinylene) (PPV) via emulsion polymerization, were synthesized [61]. This material was chosen due to its high fluorescence intensity and sensitivity to changes in its microenvironment. The chemosensor was exposed to vapour containing different amounts of TNT and quenching of the polymer luminescence at 560 nm was observed after excitation at 430 nm. The dependence of the fluorescence signal in response to the analyte was described by a modified Stem-Volmer equation that assumes the existence of two different cavity types. The authors proposed the modified Stem-Volmer equation as follows ... 

The photochemistry of rhenium complexes occupies a prominent position in the photochemistry of transition metal complexes. Along with early preparative studies on photosubstitution of carbonyl species like Re(CO)sX, the preparation of the remarkably stable yellow complex /ac-Re(CO)3(phen)Cl foreshadowed the discovery of the a large class of related luminescent materials by Wrighton and co-workers in the 1970s [ 1 ]. As pointed out by Vogler and Kunkley, the current photochemistry of rhenium complexes is rich, spanning eight oxidation states from formal Re(0) (for example, Re2(CO)io) to formal Re(VII) (for example MeReOs) [2],... 

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