Optical Properties of Luminescent Materials

Sidney J.L Ribeiro, Moliria V. dos Santos, Robson R. Silva, Edison Pecoraro, Rogeria R. Congalves, and Jose Mauricio A. Caiut 

Sol-gel methods (SGMs) are not that new, as the interested reader must be aware. In fact from the eighteenth century it is known that the hydrolysis of tetraethyl orthosilicate (TEOS-Si(OC2Hs) under acidic conditions, yielck Si02 in the form of a glass-like material in a typical sol-gel procedure [1,2]. However, it took almost 100 years for the method to get mature. SGMs are nowadays well established and developed giving birth to a wonderful collection of new multifunctional materials only possible to be obtained due to the intrinsic flexibility and low temperatures involved. This handbook shows several examples indeed. 

This chapter concerns luminescence or in the general sense the emission of light under appropriate excitation. The observation of luminescence properties of materials obtained by the SGM is much more recent than the SGM itself. More precisely, 30 years ago the first observations of photoluminescence from Eu and rhodamine 6G containing silica obtained by the SGM were reported [3,4]. 

The sensitivity of Eu photoluminescence properties to structural changes of the host was explored as a function of time and temperature of dehydration of silica-based gels. Similarities of the Eu final local structure with the one known for glasses obtained by classical melting of oxides were put forward [3]. In fact, the SGM became an optional strategy in the preparation of glasses or when low temperatures are required [5]. 

The Sol-Gel Handbook Synthesis, Characterization, and Applications, First Edition. 

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The photoluminescence of these nanoparticles has very different causes, depending on the type of nanomaterial semiconductor QDs luminescence by recombination of excitons, rare-earth doped nanoparticles photoluminescence by atom orbital (AO) transitions within the rare-earth ions acting as luminescent centers, and metallic nanoparticles emit light by various mechanisms. Consequently, the optical properties of luminescent nanoparticles can be very different, depending on the material they consist of. 

In addition, the optical properties of these materials, which include luminescence and thermochromism, are promising for optoelectronic applications, specifically the TCNQ species as a candidate for new semiconducting materials in view of the clear scanning electron microscope (SEM) image observed for crystals of this product.173... 

Another class of materials, alkanethiol-stabilised metal nanoparticles, display electronic, optical and structural features that are tunable via particle size [67]. The theme of this section is to demonstrate the effects of interfacial chemistry and material heterogeneity on electronic and optical properties of luminescent conjugated polymers at metal interfaces. 

The application of semiconductor luminescence to chemical sensing can rely on the chemical, electrical, and optical properties of II-VI and III-V semiconductors [1]. These properties provide the binding capability, transducing mechanism, and signal required for chemical sensing. The diverse chemical compositions of semiconductor materials provide a range of surfaces for molecular binding. 

Rare earth silicates exhibit potential applications as stable luminescent materials for phosphors, scintillators, and detectors. Silica and silicon substrates are frequently used for thin films fabrication, and their nanostructures including monodisperse sphere, NWs are also reliable templates and substrates. However, the composition, structure, and phase of rare earth silicates are rather complex, for example, there are many phases like silicate R2SiOs, disilicate R2Si207 (A-type, tetragonal), hexagonal Rx(Si04)602 oxyapatite, etc. The controlled synthesis of single-phase rare earth silicate nanomateriais can only be reached with precisely controlled experimental conditions. A number of heat treatment based routes, such as solid state reaction of rare earth oxides with silica/silicon substrate, sol-gel methods, and combustion method, as well as physical routes like pulsed laser ablation, have been applied to prepare various rare earth silicate powders and films. The optical properties of rare earth silicate nanocrystalline films and powders have been studied. 

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